The Future of Solar Power 9781685079505, 9798886972719, 9798886972320, 9798886973440, 9798886971903, 9798886972382, 9781685072148, 9781685072490, 9798886977097

Table of contents :
Contents
Preface
Chapter 1
The Assessment of a Solar Parabolic Trough Facility for Power Generation in Tropical Climates
Abstract
Nomenclature
Abbreviations
Greek Symbols
1. Introduction
2. Methodological Approach
2.1. Design Equations and Fundamentals
2.2. Design of the Solar Field
2.3. Design of the Power Block
2.4. The Operational Principle and Experimental Procedure
2.5. Measuring Instrumentations and Error Analysis
3. Results and Discussion
3.1. Potential Weather Assessment
3.2. Analysis of the PRC Thermal Performance
3.3. Analysis of the PTC Thermal Performance
3.4. Analysis of Standalone PTC Facility
Conclusion
References
Chapter 2
A Supplemental Solar Power System for Future Aircraft: Challenges and Opportunities
Abstract
1. Introduction: Photovoltaics in Aeronautical Applications. A Brief Review on PV Applications in Flying Vehicles
2. Challenges to Introducing Photovoltaics in Passenger Aircraft
2.1. Safety and Certification
2.2. Standardization
2.3. Aircraft Operation
2.4. Aircraft are Designed for a Large Customer Base Rather Than Optimized Operation
2.5. Structural Integration and Maintainability
2.6. Integration into the Aircraft Power System Architecture
Section Summary
3. A Conceptual Design Framework for Aircraft Featuring a Supplemental Solar Power System
3.1. The Supplemental Solar Power System Concept
3.2. Conceptual Design Framework for Aircraft Featuring an SPS
3.2.1. Aircraft Geometry and Usable Area for Solar Array Installation
3.2.2. Solar Cell Technology and Assumptions about Power Conversion Equipment
3.2.3. Absorbed Radiation Estimation (Module 1)
3.2.4. Effect of Flight Mission Pattern on Life-Cycle Power Yield of the Solar Power System (Module 2)
4. Which Aircraft Would Most Likely Benefit from a Supplemental Solar Power System?
4.1. Installed Power to Weight Ratio of the SPS
4.2. Usable Power Ratio
4.2.1. Aircraft Configuration Impact on Power Usage
4.2.2. Mission Pattern Analysis of Regional Aircraft and Impact on Solar Power Yield
Section Conclusion
5. Solar- Powered Air-Conditioning System for Aircraft
Conclusion
Acknowledgments
References
Chapter 3
Modeling, Conceptualizing, and Sizing Approach of Solar Farms for Powering of Drinking Water Supply and Irrigation Systems in Rural Zones: A Case Study from Tunisia
Abstract
1. Introduction
2. Conceptualization and Sizing of the Project Solar Energy-Based Irrigation System
2.1. Model Sections for Calculating the Consumer Water Requirement
2.2. Water Required for the Irrigation Network
2.3. Water Required for the Industrial Sectors
2.4. Water Required for the Domestic Sector
2.5. Model Part for Calculating the Energy Required to Compensate Drop Pressure
2.6. Design and Size the Solar Energy Farm with Photovoltaic Technology
3. Case Study from Tunisia
3.1. Conception and Sizing of a Solar Power Farm
3.2. Geographical, Demographical, and Water Consumption Characteristics in the Project Area
3.3. Hydraulic and Energetic Characteristics of the Drinking Water Network
Conclusion
Acknowledgments
References
Chapter 4
State-of-the-Art Analysis Methods of Solar Vortex Power Generation Technology
Abstract
Abbreviations
1. Introduction
2. Solar Vortex Power Generation Systems Considered in Past Studies
3. Analysis Methods
3.1. Experimental Studies
3.2. CFD Simulations
3.3. Mathematical Models
4. Recommendations for Further Analysis Methods to Study SVPG Systems
5. Turbine Unit Type and Location
Conclusion
References
Chapter 5
Comparative Evaluation of Computational Simulations of Solar Chimneys
Abstract
1. Introduction
2. Solar Chimney as an Energy Production Alternative
3. CFD Roles in the SCPP Research and Development
3.1. Targets of Computational Investigations of SCPP
3.2. Design Optimization by CFD
3.3. CFD Analysis Using Thermal Storage Enhancement
3.4. Turbine Configuration and Pressure Drop by CFD
3.5. Ambient Domain Influence on the Simulation Accuracy
3.6. Hybrid Solar Chimney Simulation
4. Modeling of the SCPP in Computational Simulations
4.1. Two- and Three-Dimensional Modelling
4.2. Steady-State and Transient Modelling
4.3. Solar Radiation Modelling
4.4. Turbulence Modelling
4.5. Computational Domain
4.6. Modeling of the Turbine Component
5. Simulations of the SCPP’s Collector
5.1. Enhanced Collectors by Design Modifications
5.2. Integrated Collectors with Thermal Energy Storage
6. Simulations of the SCPPs’ Tower
7. Simulations of Multi-Function SCPP
Conclusion
References
Chapter 6
Thin-Film Solar Cells for Vehicle Integrated Photovoltaic Technology: Integration Challenges and Opportunities in Hybrid Vehicles
Abstract
1. Introduction
1.1. An Overview of the Current Scenario
1.2. Importance of EV and the Aspects in Developing Economies
1.3. Modes of Energy Generation in Hybrid Passenger Vehicles
1.4. Hybridization in Vehicles
2. The Significant Scope of Energy Generation and Recovery in Hybrid Passenger Vehicles
2.1. Kinetic Energy Recovery System/Regenerative Braking Technology (RBT)
2.2. Energy Generation from Semiconductors
3. Vehicle Integrated Photovoltaics
3.1. Solar Powered Race Cars
3.2. Solar Assisted Passenger Electric Vehicle
3.3. Scope of SAPEV Energy Generation in German Geographical Conditions
3.4. Scope of SAPEV Energy Generation in South Asian Countries
4. Advanced Semiconductors for VIPV Technology
5. Computational Techniques Suited to Analyze VIPV Technology
5.1. MATLAB Simulink
5.2. Transient Simulation Systems - TRNSYS
5.3. PVsyst
5.4. Hybrid Optimization Model for Electric Renewables – HOMER
5.5. Geographical Information Systems (GIS)
6. Limitations of Current Energy Generation and Recovery Methods
Conclusion
References
Chapter 7
The Transition of Solar Thermal Energy in Recent Years and Future Directions for Industrial Utilization
Abstract
1. Introduction
2. Potential of Solar Energy for Water Heating
2.1. Solar Thermal Collectors (STCs)
2.2. Application of STCs in Industrial Processes
2.3. Types of Solar Thermal Collectors
3. Performance Assessment of STCs
3.1. Thermal or Energy Efficiency of STCs
3.2. Exergy Efficiency of STCs
3.3. Economic Analysis of Solar Thermal Collectors
4. Recent Advancements in Solar Thermal Collectors
4.1. Heat Transfer Fluids for Solar Thermal Collectors
5. Nano-Enhanced Heat Transfer Fluids
6. Prospects of Solar Thermal Energy
Conclusion
References
Chapter 8
Solar Thermal Energy Storage: Materials, Heat Transfer Analysis and Applications
Abstract
1. Introduction
2. Solar Thermal Technologies
3. Classification and Characteristics of Thermal Storage Systems
3.1. Sensible Heat Storage
3.2. Storage of Water Tanks
3.3. Underground Thermal Storage (UTS)
3.4. Packed-Bed Storage
3.5. Latent-Heat or Phase-Change Storage
3.5.1. Measurements and Capacity Prediction of PCM
3.5.2. Proprieties of PCMs
3.6. Organic Phase Change Materials Storage
3.7. Inorganic PCMs
3.7.1. Salt Hydrate
3.7.2. Metallic
3.8. Eutectics
3.9. PCM Containment
3.10. Thermal Properties of Various PCMs
4. Energy Storage in Buildings
4.1. Passive Solar Thermal Storage in Buildings
4.2. Active Solar Thermal Storage in Buildings
5. Thermal Energy Storage for Concentrated Solar Power Plants
6. Chemical Energy Storage
7. Cooling Thermal Energy Storage (CTES)
8. Performance and Cost of TES Systems
Conclusion
References
Index
About the Editor
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Renewable Energy: Research, Development and Policies

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Hussain H. Al-Kayiem Editor

The Future of Solar Power

Copyright © 2023 by Nova Science Publishers, Inc. https://doi.org/10.52305/VFIO3684 All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. We have partnered with Copyright Clearance Center to make it easy for you to obtain permissions to reuse content from this publication. Please visit copyright.com and search by Title, ISBN, or ISSN. For further questions about using the service on copyright.com, please contact:

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Library of Congress Cataloging-in-Publication Data

ISBN: 979-8-88697-709-7 (e-book)

Published by Nova Science Publishers, Inc. † New York

Contents

Preface

.......................................................................................... vii

Chapter 1

The Assessment of a Solar Parabolic Trough Facility for Power Generation in Tropical Climates ..............................................................................1 Hussain H. Al-Kayiem, Sanan T. Mohammad, Syed I. U. Gilani, Ayad K. Khlief and Mohammed A. Aurybi

Chapter 2

A Supplemental Solar Power System for Future Aircraft: Challenges and Opportunities ........................31 S. Liscouët-Hanke, N. Sadaka and R. El Chaar

Chapter 3

Modeling, Conceptualizing, and Sizing Approach of Solar Farms for Powering of Drinking Water Supply and Irrigation Systems in Rural Zones: A Case Study from Tunisia .................61 M. H. Sellami, H. Tiba, A. Nasri and H. BenAhmed

Chapter 4

State-of-the-Art Analysis Methods of Solar Vortex Power Generation Technology...........................83 Ali M. Tukkee, Hussain H. Al-Kayiem and Syed I. U. Gilani

Chapter 5

Comparative Evaluation of Computational Simulations of Solar Chimneys.....................................109 Iylia Elena Abdul Jamil and Hussain H. Al-Kayiem

vi

Contents

Chapter 6

Thin-Film Solar Cells for Vehicle Integrated Photovoltaic Technology: Integration Challenges and Opportunities in Hybrid Vehicles ...........................................................................153 Nitin Ralph Pochont, Y. Raja Sekhar and K. V. Sharma

Chapter 7

The Transition of Solar Thermal Energy in Recent Years and Future Directions for Industrial Utilization .....................................................189 Haris Naseer, Syed I.U. Gilani, Hussain H. Al-Kayiem and Muzaffar Ali

Chapter 8

Solar Thermal Energy Storage: Materials, Heat Transfer Analysis and Applications .............................217 Sulaiman Al-Hashmi

Index

.........................................................................................269

About the Editor .......................................................................................273

Preface

The future production, operation and management of energy in general, and solar energy in particular are major global matters. The energy crises faced by nations worldwide in the recent era demonstrated the need for changing from fossil to renewable. It is not only global warming and climate issues but also nations' economic and future security issues. The changes required to progress from energy marked based on hydrocarbons to sustainable energy resources, mainly solar-based energy market. The hydrocarbons such as oil, gas and diesel are imported/exported energy resources that are highly influenced by geopolitical statuses and create social and political instability. In a situation like this, the switch to locally produced energy is essential for the future safety of any nation with higher momentum towards the change from hydrocarbonbased imported exported energy to potential renewable-based energy. Solar is the largest energy source on earth and can be utilized in various methods. So far, there are two major directions in solar energy utilization: thermal and photovoltaic. Various technologies have been introduced to widen the utilization of solar energy, and they have not matured like PV and solar thermal. This book contains chapters that discuss the futuristic application of solar rather than the conventional photothermic and photovoltaic for heat and electricity generation. The book touches on new solar technologies that are not matured yet and are under research and development. Concentrated solar power has not been practically demonstrated in tropical areas. An experimentally based research demonstrated the possibility of power generation in tropical areas by a parabolic trough solar concentration. Solar transportation is another interesting solar utilization technology that can reduce fossil fuel dependence and GHG emissions. Another method of solar transportation is solar airplanes. There have been many successful attempts for airplanes to operate by installing PV panels on the airplane body. Cars could be powered by solar energy by either direct solar PV covered body or by electric power generation and powering the electric cars by solar PV stations.

viii

Hussain H. Al-Kayiem

Solar updraft power is a new direction of solar-sourced power generation. There are two methods reported on the topic of solar updraft power: the solar chimney power plant and the solar vortex power generator. Two chapters in this book reported a comprehensive outlook on the techniques. AgroPV is a technology for solar power generation to support agricultural activities. Such a direction is a good tool for developing rural areas and supporting poverty-stricken societies that are not connected to the national grids. Successful case studies presented in the book have been implemented in North Africa that can be adopted anywhere else to develop agricultural communities and secure better lifestyles by producing water for drinking and domestic use in addition to irrigation. As it is well known, solar energy suffers from interruption during the night and cloudy weather. This is a large setback in solar applications. One proposed solution to enhance solar utilization in the future is integrating solar systems with energy storage. This scheme is discussed in this book from the perspective of the futuristic impact of the TES on solar systems' performance and feasibility. In addition, the enhancement of solar systems by nanotechnologies is an important aspect of futuristic solar power. One chapter discusses the nanoenhanced solar system reported in the book. To conclude, solar is a promising energy resource with its largest available on the planet. The above-mentioned solar techniques for futuristic applications are in the development stages. Accordingly, a single reference that addresses the fundamentals and the state of the art of those technologies, which are not conventional, is vital for the academic and research communities. For that, this book is prepared to be a powerful reference tool to provide knowledge, data, and statistics to support the adoption of solar for future energy and power applications.

Chapter 1

The Assessment of a Solar Parabolic Trough Facility for Power Generation in Tropical Climates Hussain H. Al-Kayiem1, Sanan T. Mohammad2,3 Syed I. U. Gilani3 Ayad K. Khlief1 and Mohammed A. Aurybi4 1University

of Technology, Iraq Thermal Power Plant, Ministry of Electricity, Baghdad, Iraq 3Mechanical Engineering Department, Universiti Teknologi PETRONAS, Seri Iskandar, Perak, Malaysia 4SIER Company, Ministry of Industry and Mining, Iraq 2Dura

Abstract The solar parabolic trough power plant highly depends on geographic location and meteorological conditions. The absence of performance evaluation under tropical climatic conditions is a barrier to adopting this technology by local industries. As such, the foundation of this study is to provide successful evidence on the applicability of the solar parabolic trough concentration in the tropical environment. Local weather data, including solar radiation, ambient temperature, and humidity, have been acquired for around a year and used as input to design and fabricate an 

Corresponding Author’s Email: [email protected]; hhalkayiem51 @gmail.com.

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

2

Hussain H. Al-Kayiem, Sanan T. Mohammad, Syed I. U. Gilani et al. experimental solar parabolic trough collector facility. Five custom-made parabolic concentrating units coupled with evacuated tube receivers have been assessed. Experiments and performance evaluations have been carried out using the developed experimental model for five days. The maximum temperatures for preheating and superheater were 45.7°C and 165°C, respectively. The maximum average efficiency of the solar parabolic trough power plant was 6.5%, with a maximum net electrical power of 1083 W. The maximum instantaneous thermal efficiencies delivered by the facility reached 22.5%, while the maximum optical efficiency was around 27%.

Keywords: concentrated solar power, parabolic trough collector, solar thermal, standalone solar system, tropical climate

Nomenclature Ap h I Kair 𝐾𝜃 L m.l n Qgain UL V w x

aperture area (m2) Enthalpy of state (kJ/kg) aperture effective direct normal irradiance (W/m2) effective thermal conductivity of air (W/m·K) angle of incidence modifier length (m) fluid mass flow rate (kg/s) day of the year useful heat gain (W) overall heat-loss coefficient (W/m2·K) wind speed aperture width (m) the dryness fraction

Abbreviations CSP DSG PTC PTCPP DNI DHI

concentrating solar power direct steam generation parabolic trough collector parabolic trough concentrator power plant direct normal irradiance (W/m2) diffuse horizontal irradiance (W/m2)

The Assessment of a Solar Parabolic Trough Facility …

GHI HTF SM

3

global horizontal irradiance (W/m2) heat transfer fluid ratio of the thermal power generated through the solar field

Greek Symbols α γ τ 𝜌𝑐 θ ∅ η 𝜂𝑇 𝜂𝑔 𝜂𝑃 𝜇𝑜

absorbance of the receiver intercept factor Transmittance of the glass cover Reflectance of the mirror Angle of incidence Latitude of the place Efficiency of PTC Efficiency of turbine Efficiency of generator The overall efficiency of the parabolic trough power plant Optical efficiency

1. Introduction Several important energy-related concerns, such as increased demand for electricity, high carbon dioxide emissions, and fossil fuel depletion, exist in the current energy scenario. Renewable energy resources are essential in increasing energy demand and alleviating emissions and global warming problems. In particular, solar energy is an essential solution to energy shortage by providing effective, clean, and financially feasible solutions Khlief et al., 2010. Solar collectors capture solar energy and convert it into useful heat satisfactorily. For low temperatures up to 80°C, flat plate solar collectors are the most common type of assemblies. In contrast, evacuated tube solar collectors (ETRs) upgraded the operational temperature to produce working fluids above 100oC. With multiple ETRs arrangements and selective materials, ETR could operate at 200oC. Direct solar collectors are insufficient for highertemperature applications to produce steam or high-temperature HTF for power generation applications. Thus, concentrated solar thermal technologies have been developed to achieve medium and high-temperature working fluids that could be used for power generation. For medium-temperature levels up to

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Hussain H. Al-Kayiem, Sanan T. Mohammad, Syed I. U. Gilani et al.

200°C, low-quality concentration and evacuated tube collectors are the most common collectors. For high-temperature levels, parabolic trough collectors, solar towers with heliostat reflectors, Fresnel collectors, and solar dish collectors are the best solutions for satisfactory results (Iodice et al., 2016, AlKayiem, 2019). Despite its capability to produce a high-temperature range, concentrated solar thermal technologies still have technical problems. Indira et al., 2021 discussed some problems and briefly presented state-of-the-art solar concentration technologies and hybrid concentrated solar power systems. At the same time, Al-Kayiem, 2019 discussed the challenges facing concentrated solar techniques and the possible solutions for power generation. He proposed solutions for thermal energy storage, nano tactics, and hybrid concentrated solar power. Parabolic trough concentrators (PTCs) are efficient systems among other solar collection and conversion technologies because they offer good thermal efficiency at high temperatures of about 400°C–550°C. A solar PTC is a concentrated solar thermal technology for generating electricity and heat (Baharoon et al., 2015, Borunda et al., 2016). In this technology, the solar collector tube is the receiver or heat collector located at the focal line of the parabolic trough mirror. Solar radiation, which reaches a parabolic trough mirror, is concentrated on the surface of the collector tube. The process involves a combination that transfers radiative and convective energy and conductivity with fluid flow. Solar PTC with concentration ratios ranging from 15 to 30 provides temperatures in the range of 250°C–400°C, depending on the direct component of solar irradiance. With concentration ratios of 8– 15, PTCs may produce heat transfer fluid (HTF) between 90 and 250°C temperatures. Although the solar PTC is currently recognized as an efficient solar energy collection technology, it still has technical problems, such as heat loss problems around the solar collector, which dramatically decrease the thermal and electrical efficiency (Al-Kayiem, 2019; Kutscher et al., 2012; Fang et al., 2014 and Li et al., 2014). Several research works have been conducted to minimize the heat loss around the solar PTC by modifying the PTC to Compound Parabolic Collector (CPC) or by combining PTC with other solar technologies like PV. Solar CPC is an enhanced design of solar PTC. Solar CPCs can be smaller than solar PTC to produce similar thermal outputs. Design, performance evaluation and optical losses of the solar CPC systems are recently reported by, e.g., Indira et al., 2021, Hussain et al., 2021 and Akhter et al., 2021. Hybrid is achieved by Integrating conventional solar PTC with photovoltaic cells (PV) to develop a

The Assessment of a Solar Parabolic Trough Facility …

5

hybrid parabolic trough concentrator (CPV/T) or with thermoelectric modules (TEG) to develop a hybrid concentrator photovoltaic/thermal thermoelectric generator (CPV/T-TEG). Among them, Riahi et al., 2020 presented two hybrid parabolic trough concentrators (CPV/T and CPV/T-TEG), where the first system (CPV/T) was mainly composed of a parabolic trough concentrator, a hybrid collector, which contains photovoltaic cells and a heat exchanger as well as heat transfer fluid (liquid water). On the other hand, Indira et al., 2021 proposed a hybrid CPC and parabolic trough concentrator (CPC/PTC) system for concentrator photovoltaic/thermal (CPV/T) and hybrid concentrator photovoltaic/thermal thermoelectric generator (CPV/T-TEG) applications. The geometrical design and optical analysis of the hybrid solar CPC/PTC system are discussed in the study. Raytracing models were used to identify the different variables influencing the optical efficiency of both CPC and PTC. Kumaresan et al., 2012 experimented with a solar PTC integrated with TES. The study concluded that the PTC site should be as close as possible to the storage system, and the different components of the storage system must be properly isolated to reduce heat loss. However, multiple thermal studies have been conducted on solar collector tubes using parabolic trough technology. Ahmed, 2014 analyzed the thermal performance of a PTC through equation-solving software. Reddy et al., 2012 evaluated the energy and exergy of a PTC power plant by losses and efficiencies predictions. The operating pressure of a Rankine heat engine was optimized for maximized efficiency. Kearney et al., 2004 investigated molten salt as an HTF and thermal energy storage (TES) in a PTC solar plant. A feasibility study suggests using molten salt as an HTF to reduce the standard cost of electricity. Nouri et al., 2020 investigated the optimization of PTC power plant operations in variable irradiance conditions using all-sky imagers. Compared to a state-of-the-art reference controller, they found a relative increase in revenue of over 1.9% over a test period of 22 days. In Spain, Martin et al., 2013 used mathematical programming techniques to improve the operation of a concentrated solar power (CSP) plant using the Rankine regenerative cycle of a site. Garcia et al., 2011 developed a simulation model to determine the electric output of a 50 MW PTC plant and compared his simulation results with experimental measurements of a CSP power plant in Spain. A feasibility study of installing a PTC power plant on Cyprus Island was conducted by Poullikkas et al., 2009. The study used different operational and design parameters, such as plant capacity, capital investment, and

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Hussain H. Al-Kayiem, Sanan T. Mohammad, Syed I. U. Gilani et al.

operating hours. A thermodynamic model was developed by Larrain et al., 2010 to estimate the performance of a 100 MW hybrid PTC plant in Chile. The performance of various configurations of PTC was studied and compared by Boukelia et al., 2015 with two different HTFs. In another publication in 2015, the same research group, Boukelia et al. 2015, analyzed the feasibility and improvement of PTC stations equipped with integrated TES and FSS for Algerian climate conditions. The results recommend the installation of CSP stations in arid and semiarid regions. An experimental investigation on PTC performance under Tunisian weather was performed and reported by Chafie et al., 2016. Bishoyi et al., 2017 performed a performance analysis of a 100 MWe PTC plant located in Udaipur, India. In contrast, Shouman et al., 2015 described a strategic roadmap for introducing CSP technology in Egypt. Kordmahaleh et al., 2017 provided an economic model of a 25 MWe PTC solar power plant operating in Iran. The effect of the storage system on the factory operating hours was also examined in different loading conditions. Akana et al., 2021 simulated a PTC plant and predicted the possibility of steam production in Cameroon using PTCs. They considered two production modes: direct mode using water and indirect mode using TherminolVP1 as HTFs, based on the thermal energy balance between the receiver and the HTFs fluids. The impact of solar irradiation on the fluid outlet temperature was studied. The study recommended that the energy demand and supply are necessary to be predicted for the correct sizing of the PTCs. Most of the studies on PTC solar power plants' design, performance evaluation, and optimization were carried out for plant sites in the USA, Spain, Algeria, Tunisia, and India. The literature reveals a clear deficiency in the experimental data of the standalone PTC system's performance under tropical areas' real climatic. Operational investigations of PTC for a high direct normal irradiation (DNI) in tropical solar conditions are limited, and further supporting data is vital. Also, the applicable standalone CSP for the tropical climatic condition has not been comprehensively investigated. This research aims to provide experimental results of PTC technology and its performance in tropical climates. The specific objective is to present the design procedure and implementation of a solar PTC facility for electric power generation comprising five parabolic solar concentrators, evacuated tube receivers, and a power production block. The article presents experimental assessment results to demonstrate the thermal and optical performances of the implemented solar PTC facility. The chapter consisted of 4 sections. The first section reviews related literature and conclude the research gap which motivated the current work. In

The Assessment of a Solar Parabolic Trough Facility …

7

the second section, the materials and methods used in the research have been presented and discussed, including the methodological steps of the research, fundamental mathematics used in the design and evaluation, design procedure, experimental measurement procedure and instrumentations. Also, possible errors in the measurements are discussed. The results are presented in the third section, while section 4 presents the concluded achievements and findings.

2. Methodological Approach An experimental PTC testing facility has been built at Universiti Teknologi PETRONAS (UTP). The solar PTC facility and the measurements were carried out in the solar research site in UTP, located at 4°24' latitude, 100°58'E longitude, and 24 m altitude. The measurements were carried out over five days, starting on the last days of January and four days of February 2019. The flow chart of the implementation of the research is shown in Figure 1.

Design and implementatio n of the facility including the solar field and power block

performance Experimental analysis and Conceptual measurement evaluation design and theoretical predictions

Collection of meteorological data Figure 1. Methodological flow chart of the research.

The implemented facility consisted of three parts, the solar field part consisting of the PTCs, and the power block consisting of the power generation and circulation of the HTF. The third part is the measuring and

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Hussain H. Al-Kayiem, Sanan T. Mohammad, Syed I. U. Gilani et al.

control instrumentations. A schematic diagram of the hypothesized solar PTC facility is shown in Figure 2.

Figure 2. Simplified process flow diagram of the assessed PTC facility.

2.1. Design Equations and Fundamentals The yield of the PTCPP highly depends on climate conditions such as solar radiation, ambient temperature, and wind speed. Those parameters influence the system's heat loss (Benli, 2013; Gandhidasan and Mohandes, 2011; Purohit, 2010). The aperture effective direct normal irradiance falling perpendicularly on the plane is predicted using the DNI as: 𝐼 = 𝐷𝑁𝐼. cos 𝜃,

(1)

where 𝜃 is the incidence angle. The optical efficiency can be estimated using Yılmaz and Söylemez, 2014 approximation. 𝜂𝑜 = 𝜌𝑐 . 𝛾. 𝜏 . 𝛼 . 𝐾𝜃,

(2)

The Assessment of a Solar Parabolic Trough Facility …

9

where, 𝜌𝑐 is the reflectance of the mirror, γ is the intercept factor, τ is the transmittance of the glass cover, and α is the absorbance of the receiver. Kθ denotes the incidence angle modifier for the solar PTC system, which could be estimated by equation (3) proposed by Jradi and Riffat., 2014. 𝐾𝜃 = cos(𝜃) + 0.000884(𝜃) − 0.00005369(𝜃)2

(3)

The definition of the vital rate of energy from the collector is given by (Desai et al., 2014 and Mohammad et al., 2018): 𝑄𝑃𝑇𝐶 = 𝜂𝑡 . 𝐼. 𝐴𝑝

(4)

I, stand for direct normal incidence radiation on the PTC in W/m2. ηt is the thermal efficiency of the PTC, and AP, is the aperture area of the PTC in m2 predicted as below: 𝐴𝑝 = (𝑤 − 𝐷𝑐𝑜 )𝐿

(5)

Where, Dco is the outer diameter of the receiver glass tubes (= 0.1 m), L is the length of a single PTC unit equal to 6.4 m, and w is the width of the PTC, equal to 2.6 m. ∆𝑇

𝜂𝑡 = 𝜂𝑜 − 𝑈𝐿 . ( ) 𝐼

(6)

Where ηo is optical efficiency, UL is the overall heat-loss coefficient, Duffie and Beckman, 2013, and ∆T is: ∆ 𝑇 = 𝑇𝑚 – 𝑇𝑎

(7)

Where Ta is the ambient temperature; Tm is the average temperature with unit ˚C across the PTC unit predicted as: 𝑇𝑚 = (𝑇𝑖𝑛 + 𝑇𝑜𝑢𝑡 )/2

(8)

From the product specification documents of the selected steam turbine, values of the enthalpies required to solve equations are found in the IAPWSIF79 stem tables. Where subscripts 6 and 7 are denoted in Figure 2. The produced power by the steam turbine is:

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𝑊𝑠.𝑡 = 𝑚̇ 𝑠 (ℎ1 − ℎ2 )

(9)

Where h1 and h2 denote enthalpies of steam at locations 1 and 2, subscript s.t refers to the steam turbine, and subscript s refers to steam. The amount of heat gain from preheating collectors is predicted as below: 𝑄𝑔𝑎𝑖𝑛 𝑃𝑟ℎ𝑒𝑎𝑡𝑖𝑛𝑔 = 𝑚̇𝑡𝑜𝑡𝑎𝑙 (ℎ7 − ℎ6 )

(10)

𝑚̇𝑡𝑜𝑡𝑎𝑙 = 𝑚̇𝑤 + 𝑚̇𝑠

(11)

where 𝑚𝑡𝑜𝑡𝑎𝑙 ̇ is the mass flow rate of water passing from the collector in kg/s, 𝑚̇𝑤 is mass flow rate return from the steam separator to the mixing tank, 𝑚̇𝑠 is the mass flow rate of steam passing through the superheater. In the superheater section, which consists of two PTC units, steam gains more heat in the superheater collectors as: 𝑄𝑔𝑎𝑖𝑛 𝑠𝑢𝑝𝑒𝑟ℎ𝑒𝑎𝑡𝑒𝑟 = 𝑚̇ 𝑠 . (ℎ1 − ℎ9 )

(12)

h9 denotes enthalpies at station 9. The total heat gain from the solar field is predicted as below: 𝑄𝑔𝑎𝑖𝑛 𝑡𝑜𝑡𝑎𝑙 = 𝑚𝑡𝑜𝑡𝑎𝑙 ̇ . (ℎ7 − ℎ6 ) + 𝑚̇ 𝑠 . (ℎ1 − ℎ9 )

(13)

The mass flow rate inlet to the PTCs is normally steady in flow rate, temperature, and pressure. Hence, the dryness factor and the temperature would vary within the circulation network of the load part. The temperature difference could be determined by measurements and knowing the design dryness fraction at point 7 of the PTCs field, shown in Figure 2. The temperature at point 6 could be estimated by the enthalpy estimation at point 6, using equation 14. Thomas, 1996 implied a 0.8 as a typical value of the dryness fraction. ℎ6 = (1 − 𝑥7 ). ℎ8 + 𝑥7 . ℎ3

(14)

Finally, the overall solar PTCPP efficiency is predicted following the definition of Reddy and Kumar, 2012:

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𝜂𝑃 =

𝑄𝑔𝑎𝑖𝑛 𝑡𝑜𝑡𝑎𝑙 𝑆𝑀.𝐼.𝐿.𝑤

. 𝜂 𝑇 . 𝜂𝑔

11

(15)

Where ηT is the turbine's efficiency, assumed to be 60%. ηg is the efficiency of the generator assumed to be 70%. I, in W/m2, is the aperture effective direct normal irradiance determined by equation 1. L and w, in meters, are the total lengths and the width of the PTC system. The solar multiple (SM) is required to design the solar field. SM is defined as multiple aperture areas required to operate the power cycle at a predesigned design capacity. It should be more than unity. SM is defined as the ratio of the thermal power generated by the solar field to the thermal power needed by the power block at predesigned operational conditions. It is denoted mathematically as in equation 16. 𝑆𝑀 =

𝑝𝑜𝑤𝑒𝑟 𝑐𝑦𝑐𝑙𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑠𝑜𝑙𝑎𝑟 𝑓𝑖𝑒𝑙𝑑 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦

(16)

Cloudy weather and no solar insolation conditions should be counted during the design and sizing of the solar collector. In the current study, SM was taken as 1.16, according to the recommendations of Montes et al. 2009 and Al-Nasser, 2010.

2.2. Design of the Solar Field The solar parabolic trough part of the facility consists of five identical concentrator modules. Tracking the sun from East to West has been achieved by one-axis rotation. The trough collector concentrates, reflecting the solar radiation onto an absorber pipe located along its focal line. Each parabolic reflector has an aperture width of 2600 mm, an aperture area of 16.64 m2 and a focal length of 870 mm. The rim angle is 75.2o. Each evacuated tube receiver has a metallic tube of 40-mm-dia. And glass tube of 100-mm-dia. A schematic diagram of the absorber and receiver is shown in Figure 3. The solar absorption system is made of stainless steel covered with glass. It has a good connection to the heat by using thin material. Thereby, the absorption of heat by the HTF has a rapid response to changes in radiation intensity. The ETR has relatively a low emittance and consequently high optical quality material of the tube. The final reflectance is about 60%, a low value selected due to the stains in the mirrors. Table 1 shows the specifications

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of the ETR and the geometrical and optical specifications of the developed PTC facility.

Figure 3: Geometry of the parabolic trough collector.

Figure 3. Geometry of the parabolic trough collector.

Table 1. Specifications of the developed PTC facility Parameters Aperture area of PTC Length of the receiver (L) Width of the collector (W) Focal length (F) Receiver external diameter (Do) Receiver internal diameter (Di) Diameter of the glass tube Concentration ratio (C) Rim angle (°) Absorptance of the receiver (𝛼) Emittance of the receiver (𝜖) Glass envelope transmittance (𝜏) Glass envelope emittance (𝜖𝑐 ) Reflector surface reflectivity(𝜌) Intercept factor (𝛾)

Unit m2 m m m mm mm mm

Value 16.64 6.4 2.6 0.87 40 38 100 20.78 72.5 0.95 0.10 0.91 0.91 0.7 0.93

The position where the maximum solar concentration is achieved is consistent with the tube position. The pipe length is 2.136 m. The most important part of the absorption system is the evacuated envelope surrounding the heat pipes. The vacuum between glass and stainless steel tubes reduces the rate of heat loss. A flexible tube through which the HTF is connected to the stainless steel tube by flange fittings. The supporting mirror, discharge tube

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and temperature sensor system by the metal frame. This metal frame is designed to have the ability to track the sun's daily orbit. This goal can be achieved by rotating the thorough focus. These moves can be performed by adjusting the assembly window up and down manually. Figure 4 shows one of five trough concentrators, which are structures from the metal frame, parabolic mirror and evacuated tube.

Figure 4. Single unit of parabolic trough collector.

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2.3. Design of the Power Block The implemented solar PTC facility consists of several main parts: solar PTC, pumps, steam turbine, condenser, make-up tank, and measuring instruments. A schematic diagram of the proposed solar power plant is shown in Figure 5. The actual facility is shown in figures 6 a and b, including the solar field and the power block.

Figure 5. Schematic diagram of the hypothesized solar PTC facility, including (a) Layout of the solar field; (b) Layout of the power block.

Water extracted from the steam separator is directly mixed with feed water. At the outlet of the mixing tank, there is a feed pump that increases the fluid pressure to boiling pressure. The mixing tank has two inlets and a single outlet stream. The two inlets are the drain water from the steam separator and the feed water from the make-up tank. The total mass flow rate is the sum of the two inlets. A pump is attached to the solar test rig for suction and delivery of the HTF to a mixing tank. The second pump pushes water to a parabolic trough solar field. A pump with a small flow rate is required to obtain maximum heat absorption. The steam separator is a cast SG iron baffle-type separator used to remove entrained liquids in steam, compressed air and gas systems.

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Figure 6. Standalone solar parabolic trough concentrator facility at UTP, including (a) The solar field; and (b) The power block unit.

In this facility, the used green turbine is a small, lightweight turbo generator that delivers heat and electricity with relatively high efficiency. The turbine runs on some 160-200°C superheated, dried steam and operates in a condensing mode with a vacuum in a closed-loop system and a steam generator. The selected steam turbine does not have guide blades and gears; it has high reliability and efficiency. Losses are further reduced by using a near vacuum for the turbine wheels and generators. The fitting of insulation jackets would increase the performance of the separator. Also, installing a condenser ensued in a closed feedwater/steam circuit. The heat released from the

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condenser can be used for heating or cooling purposes. Figure 6 shows a photograph of the PTC facility. Solar energy is exploited with a parabolic trough concentrator, which is used for superheating the water in a Rankine cycle. This Rankine cycle produces electricity and simultaneously useful heat in the condenser.

2.4. The Operational Principle and Experimental Procedure The solar-array-power-plant parabolic trough is equipped with five parallel rows of parabolic reflectors. The ETR is placed over the length of the trough in the focus line to collect the reflected radiation, and the heat and liquid are then pumped through the tube. The evacuated tube receiver comprises a steel pipe surrounded by cermet paint and covered by glass protection with evacuated space between the steel pipe and the outer glass to reduce convective heat losses. The receiver tube is designed to accommodate high energy, and it can withstand the generated high temperatures. The feedwater enters the PTC preheater at point 6 to be preheated and evaporated, producing saturated steam with a low quality. However, the water and steam mixture is separated in the steam separator before directing the lowquality steam to the PTC superheater section. The liquid phase water is recirculated, and dry steam enters the superheater section to be superheated up to 165°C. Steam generation in the solar field is fed directly to a turbine then the electricity generation process takes place.

2.5. Measuring Instrumentations and Error Analysis Meteorological data of solar radiation, ambient temperature, and wind speed are necessary for the simulation, designing, modeling, and evaluating of the performance of a solar PTCPP. The DNI measurement instruments were set at 5 m off-ground level in the present assessment. An EKO Pyrheliometer instrument installed at the solar site of UTP was used to measure solar irradiance. The EKO Pyrheliometer can record maximum irradiance values at 2,000 W/m2 at a typical accuracy of ±0.005%. The mean, maximum, and minimum hourly values calculated from the raw data are stored every minute. Daily and monthly, the statistics were made for the solar radiation data from the hourly data set. According to the measurement of global solar radiation in

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17

the solar research center at UTP, it is realized that it receives 7.0 h of sunshine per day. Many sensors were fixed at various locations in the PTC facility to measure adequate operational variables. Flow meters were used to measure the steam and water flow rates. The variable area flowmeter comprises a conical measuring tube and a floater. The floater position is transmitted to an indicator dial via a magnetic transmission system, which indicates the flow value on-site and the output signal for remote display and control. Two thermometers were used to measure the flow inlet, Tin and outlet temperature, and Tout values from the power cycle. Thermocouples were attached at the outlet of the receiver evacuated tube to monitor the temperature profile of the parabolic trough during system operation. The thermocouple was connected to a digital data logger that would display the temperature of the absorber. Similarly, the ambient temperature, Tam and the air velocity, Vair, is measured near the collectors. All the thermocouples used are K- type thermocouples with a temperature measurement range from -40 to 750 with an accuracy of 0.6%. The accuracy of various measuring instruments used in the experiments is presented in Table 2. Table 2. Accuracy of various measuring instruments Instrument EKO Pyrheliometer Flow meter Thermocouple

Accuracy ±0.005 0.15 0.6%.

Range 0 – 2000 W/m2 0 – 5 m3/h -40 -750 ⁰C

% Error 0.02 0.2 0.6

3. Results and Discussion The experimental assessment results are analyzed and discussed in this section. The potential weather is essential to be described before presenting the solar system evaluation results.

3.1. Potential Weather Assessment The ambient temperature and the direct normal radiation measurements are shown in Figure 7 as the mean of the five experimental days. The collector's hourly maximum direct normal beam insolation was 914 W/m2 on February

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25 at 14:00, while the minimum was observed at around 400 W/m2 on February 15 at 14:00. However, many fluctuations occurred in the normal direct beam data because of clouds' presence over the experimental site. Malaysia is a tropical country with one-season weather all around the year. The measurements have been recorded for 12 hours of operation, from 7:00 to 19:00 daily, and the mean is presented in Figure 7. The variation in the hourly temperature over the year is slight. The daily average temperature is around 32°C during the day and around 25°C at night, with very small changes around the year. The site measurements show that the mean ambient temperature is around 33°C in the mid of the day, from 11:00 to 16:00. The mean measured temperature is around 25°C at 7:00 and increases sharply as the solar irradiation increases and stabilizes from 12:00 to 15:00. As the solar irradiation reduces after 15:00, the ambient temperature also reduces.

Figure 7. Hourly average ambient temperature and direct normal irradiation (DNI) during the experiment days.

Figure 8. Variation of an hour and zenith angles during the day.

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Variations of the hour and zenith angles are predicted and presented in Figure 8. The hour angle is negative from 7.00 until mid-themed day, and then the sign is positive until the evening. This trend exists throughout the year. Zenith angle varied from 80° in the morning to 120° in the evening and had a minimum value of 20° at noon in January.

3.2. Analysis of the PRC Thermal Performance

Preheated Steam Temperature (⁰C)

The aperture effective DNI values are above 400 W/m2 in all hours from 11:00 to 17:00 for all the measurement days. Figure 9 illustrates the hourly aperture effective DNI, I on the PTC mirrors. The maximum aperture effective DNI was approximately 900 W/m2 occurred on February 25 from 11:00 to 14:00. By contrast, February 15 was the lowest at approximately 379 W/m2. The steep reduction in the DNI happened between 14:00 to 16:00 due to cloudy and rainy weather. On most days, the DNI readings increase after the passing of clouds and rain. Effective direct normal irradiance is essential for properly designing and implementing concentrated solar power plants.

160 140 120 100 80 60 40 20 0 7 31-Jan

8

9

10 11 12 13 14 15 16 17 18 19 Time 4-Feb 11-Feb 15-Feb 25-Feb

Figure 9. Hourly aperture effective direct normal irradiance.

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3.3. Analysis of the PTC Thermal Performance The solar PTC, including preheating and superheating sections, has been subjected to experimental measurements for five days. The following figures illustrate the temperature, useful heat gains, and optical and thermal efficiency results and are discussed separately. Steam or steam-water mixture temperature is a critical parameter among the experimental results measurement parameters of the solar PTC facility. Outlet fluid temperature from the preheater helps in estimating energy and thermal efficiency. The experimental outlet temperatures from the solar field preheating for five days in 2019 are shown in Figure 10. The hourly maximum outlet temperature from preheating was recorded at around 145.7°C on February 25 at 14:00. The outlet temperatures fluctuated for all the days because of fluctuations in the direct normal irradiation. The steam outlet temperature was more stable between 11:00 and 15:00 than in the morning and after 15:00. This was realized in all five repeated measurement days.

Figure 10. Experimental measurement results of preheating outlet steam temperature.

The PTC superheater separately increases its flow temperature from saturation to the desired temperature and enhances the steam quality. Two PTC units are connected in parallel to perform as the superheating unit. A hand valve at the inlet of the loop controlled the steam flow distribution in the

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parallel loops. The hand valves must only be adjusted once during commissioning and checked regularly. The difference between the measured pressures of the evaporator outlet and turbine inlet estimated the pressure drop over the superheater. Figure 11 depicts the outlet temperature of the superheater. The superheater's hourly maximum experimental outlet temperatures were recorded at around 165°C on February 25 at 14:00; the lowest was approximately 142.6°C on February 15. The superheated steam temperature is highly dominated by solar irradiance. Steam, with over 100°C, was generated at 9:00, and its temperature increased rapidly. Within one hour, from 9:00 to 10:00, the steam temperature increased by 30 to 40°C. Between 11:00 and 15:00, the measured steam temperature stabilized at 150±10°C. On an average base, the measured steam temperature at the superheater outlet was higher on February 25 than on the other days. On this day, the measured solar irradiance is higher than on the other days (refer to Figure 11).

Figure 11. Experimental measurement results of superheated steam outlet temperature.

Figure 12 reveals the experimental heat gain results. The HTF absorbed the preheating PTCs against the hourly time five days after thermal losses from the receiver, and piping losses accounted for heat gain. On the other hand, the monthly mean daily heat gain is influenced by the climatic conditions of each site, like the DNI, the ambient temperature and the wind speed. It can reach a maximum of 13121 W at 14:00 on February 25 due to the high aperture

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effective direct normal irradiance of approximately 900 W/m2; it also reached a minimum of around 3989 W on February 15.

Figure 12. Hourly experimental useful heat gains in the preheating part of the PTCs.

Figure 13. Hourly experimental useful heat gain of superheater unit of the PTC.

Figure 13 shows the heat gained by steam in the superheater PTC units. The maximum heat gain was 300 W at 13:00 on February 25, whereas the lowest was approximately 50 W and around 11137.2 W on February 15. Figure 13 illustrates that the highest useful hourly heat transferred by the PTC considerably fluctuated for all five days. This fluctuation in useful heat gain

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occurred due to the cloudy and rainy weather. The higher values of useful heat gain were measured throughout the clear day than during the various cloudy days. There was a small increase in the estimated heat gain from 7:00 to 8:00 once the solar incidence started in the morning. The measured irradiation was still lower than 200 W/m2 before 8:00. A rapid increase in heat gain reached above 100 W at 9:00. After 11:00, the thermal heating of the working fluid stabilized until 14:00 and started to reduce rapidly after 15:00. It could be said that the system's response is slow in low solar irradiation. Optical efficiency, predictable by equation 6, is a vital parameter that affects the thermal efficiency of the PTC. Figure 14 illustrates the collector's optical efficiency. The optical efficiency gained on February 25 was 27%, whereas that gained on February 15 was 18%. As a result of the decrease in the incidence angle, the optical efficiency increased until morning and noon. As the incidence angle increased during the daytime, the optical efficiency decreased until evening.

Figure 14. Hourly collectors' optical efficiency.

Typically, the thermal efficiency of PTC relies on the optical efficiency of the collector system and aperture effective direct normal irradiance. The hour-based thermal efficiencies of the preheating and superheater of the solar PTC facility for five days are shown in Figure 15. The measurement results indicate that the collectors' thermal efficiency is proportional to the aperture's effective direct normal irradiance increases. Consequently, the performance of the collector increases. However, the preheater and superheater's maximum

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estimated instantaneous thermal efficiencies could reach 22.5% and 18.4%, respectively, on February 25 at 14:00. The variations in the experimentally estimated thermal efficiencies are due to several restrictions, such as the radiation incidence angle, the solar irradiation, and the heat losses from the absorber tube. Figure 15 shows that the deviation trend is similar to the useful heat gain, insolation, and aperture effective direct normal irradiance.

Figure 15. Hourly collectors' thermal efficiency of the solar PTC preheater and superheater in the tested facility by useful heat gained.

The efficiency was rapidly increased in the early hours of solar time, from 9:00 to 10:00. The efficiency increased at a lesser rate until 11:00 and stabilized until 15:00. After 15:00, there was a noticeable reduction in the efficiencies until 16:00, followed by high rapid reduction.

3.4. Analysis of Standalone PTC Facility Repeated measurements have assessed the developed standalone solar PTC facility for five days. The estimated electric power output is presented in Figure 16. The hourly variation of the net energy production for each day almost follows the hourly variation of the direct normal irradiation. The highest power values are reached on a clear day. The highest power output

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25

was 1083 W, achieved on February 25. The lowest electrical power output was 300 W, achieved on February 15.

Figure 16. Gross power output production by the power cycle.

The overall efficiency of the PTC facility is presented in Figure 17 for five measurement days. The efficiency increased from 0.0% to 5.5% within the morning hour, 8:00 to 9:00. The overall system efficiency reached a maximum of 6.5% at 14:00 on February 25 due to the high aperture effective DNI, which was approximately 900 W/m2. However, the efficiency started to reduce after 14:00 and reached 2.13% at 18:00 on the same day.

Figure 17. Overall efficiency of standalone parabolic trough power plant.

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Figure 18 shows the daily gross electrical power output and efficiency for five measurement days. The gross electric output of the tested PTC facility reached the maximum at high solar radiation. The maximum net power was 8.37 kWh/day on February 25, and the average efficiency of the PTC facility was 4.5% during a clear day. The lowest electrical power output was 4.80 kWh/day on January 31, and the average efficiency of 3.33% was obtained on a partly cloudy day.

Figure 18. The daily power output and average efficiency of the tested solar PTC facility.

Conclusion An experimental standalone solar PTC facility with direct steam generation is implemented and evaluated under a tropical climate. Assessment of the developed solar power facility demonstrated that direct steam could be feasibly generated in the tropical climate. The plant could reliably generate steam and electricity even with fluctuating weather conditions. The main outcomes of this assessment, which was carried out in 2019, can be summarized as follows: 

The maximum optical efficiency of the concentrators for preheating the water is 27%, and the peak value of the instantaneous thermal efficiency for the preheating PTC is 22.5%.

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



27

The experimental results showed that the maximum thermal efficiencies for the tested PTC solar preheater and superheater are around 22.5% and 18.4%, respectively. The average overall efficiency of the assessed PTC facility is 6.5%, and the maximum net produced electrical power is 1,083 W. Meanwhile, the minimum average efficiency is 4.6%, and the minimum net electrical power is 300 W.

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optic reflector: A case study. Journal of Cleaner Production 298. 126683. https://doi.org/10.1016/j.jclepro.2021.126683. Kordmahaleh, A. A., Naghashzadegan, M., Javaherdeh, K., Khoshgoftar, M. (2017). Design of a 25 MWe Solar Thermal Power Plant in Iran with Using Parabolic Trough Collectors and a Two-Tank Molten Salt Storage System. Int. J. Photoenergy. Article ID 4210184. https://doi.org/10.1155/2017/4210184. Kumaresan, G., Sridhar, R., Velraj, R. (2012). Performance studies of a solar parabolic trough collector with a thermal energy storage system. Energy 47: 395–402. https://doi.org/10.1016/j.energy.2012.09.036. Kutscher, C., Burkholder, F., Kathleen, S. J. (2012). Generation of a Parabolic Trough Collector Efficiency Curve From Separate Measurements of Outdoor Optical Efficiency and Indoor Receiver Heat Loss. Journal of Solar Energy Engineering 134: 011012. https://doi.org/10.1115/1.4005247. Larrain, T., Escobar, R., Vergara, J. (2010). Performance model to assist solar thermal power plant siting in northern Chile based on backup fuel consumption. Renew. Energy 35: 1632–1643. https://doi.org/10.1016/j.renene.2010.01.008. Li, C., Zhang, M., Miao, L., Zhou, J., Kang, Y. P., Fisher C. A. J. (2014). Effects of environmental factors on the conversion efficiency of solar thermoelectric cogenerators comprising parabola trough collectors and thermoelectric modules without evacuated tubular collector. Energy Conversion and Management 86: 944–51. https://doi.org/10.1016/j.enconman.2014.06.010. Martin, L., Mariano, M., Optimal year-round operation of a concentrated solar energy plant in the south of Europe. Appl. Therm. Eng. 2013; 59: 627–633. https://doi.org/10.1016/j.applthermaleng.2013.06.031. Mohammad, S. T., Al-Kayiem, H. H., Assadi, M. K., Sabir, O., Khlief. A. K. (2018). An integrated program of a standalone parabolic trough solar thermal power plant: Code description and test. Case Studies in Thermal Engineering 12: 26-37. https://doi.org/10.1016/j.csite.2018.02.006. Montes, M., Abánades, A., Martinez-Val, J., Valdés, M. (2009). Solar multiple optimizations for a solar-only thermal power plant, using oil as heat transfer fluid in the parabolic trough collectors. Solar Energy 83: 2165-2176. https://doi.org/ 10.1016/j.solener.2009.08.010. Nguimdo, Leonard Akana, N., Teka., J., Fopossie, F. (2021). Thermal Analysis of Parabolic Trough Solar Collector and Assessment of Steam Power Generation at Two Locations in Cameroon. International Journal of Renewable Energy Research 11(3). https://ijrer.org/ijrer/index.php/ijrer/article/view/11971/0. Nouri, B., Noureldin, K., Schlichting, Wilbert, S., T. Hirsch, T., Schroedter-Homscheidt, M., Kuhn, P., Kazantzidis, A., Zarzalejo, L. F., Blanc, P., Yasser, Z., Fernández, J., Pitz-Paal, R. (2020). Optimization of parabolic trough power plant operations in variable irradiance conditions using all sky imagers. Solar Energy 198: 434-453. https://doi.org/10.1016/j.solener.2020.01.045. Poullikkas, A. (2009). Economic analysis of power generation from parabolic trough solar thermal plants for the Mediterranean region-a case study for the island of Cyprus. Renew. Sustain. Energy Rev. 13:2474–2484. https://doi.org/10.1016/j.rser. 2009.03.014.

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Purohit. I. (2010). Testing of solar cookers and evaluation of instrumentation error, Renew. Energy 35 (9): 2053–2064. https://doi.org/10.1016/j.renene.2010.02.006. Reddy, V. S., Kaushik, S. C., Tyagi, S. K. (2012). Exergetic analysis and performance evaluation of parabolic trough concentrating solar thermal power plant (PTCSTPP). Energy 39: 258–273. https://doi.org/10.1016/j.energy.2012.01.023. Reddy, K., Kumar. K. R. (2012). Solar collector field design and viability analysis of standalone parabolic trough power plants for Indian conditions. Energy for Sustainable development 16: 456-470. https://doi.org/10.1016/j.esd.2012.09.003. Riahi, A., Ben Haj Ali, A., Fadhel, A., Guizani A. (2020). Balghouthi M. Performance investigation of a concentrating photovoltaic thermal hybrid solar system combined with thermoelectric generators. Energy Conversion and Management 205. 112377. https://doi.org/10.1016/j.enconman.2019.112377. Shouman, E. R., Khattab, N. M. (2015). Future economic of concentrating solar power (CSP) for electricity generation in Egypt. Renew. Sustain. Energy Rev. 2015; 41: 1119–1127. https://doi.org/10.1016/j.rser.2014.08.067. Thomas, A. (1996). Solar steam generating systems using parabolic trough concentrators, Energy conversion and management 37: 215-245. https://doi.org/10.1016/01968904(95)00162-7. Yılmaz, I. H., Söylemez, M. S. (2014). Thermo-mathematical modeling of parabolic trough collector. Energy Conversion and Management 88: 768-784. https://doi.org/10.1016/j.enconman.2014.09.031.

Chapter 2

A Supplemental Solar Power System for Future Aircraft: Challenges and Opportunities S. Liscouët-Hanke* N. Sadaka and R. El Chaar Department of Mechanical, Industrial and Aerospace Engineering, Concordia University, Montreal, Canada

Abstract A crucial objective for the aerospace industry is reducing its environmental footprint. Carbon-neutral growth via emissions reductions can only be achieved by investing in breakthrough technology development. An increasing interest in photovoltaics can be observed in aeronautics, either to increase the endurance of Unmanned Air Vehicles (UAVs) or for solar-powered flights for technology demonstration purposes, such as the Solar Impulse aircraft’s missions. Still, introducing solar power in commercial or recreational aircraft seems far in the future. However, with the recent trend of investing in technology development for hybrid-electric, all-electric, and hydrogen-powered aircraft, photovoltaics might also have their place as a supplemental power system. With today's solar cell efficiency, enough power can be generated from wing-mounted or fuselage-mounted solar panels to operate, for example, an autonomous air-conditioning system on the ground. Thus, a reduction in airport emissions and operational costs 

Corresponding Author’s Email: [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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S. Liscouët-Hanke, N. Sadaka and R. El Chaar could be achieved. In flight, fuel could be saved by powering various electrical systems with solar power. Therefore, in the context of the increasing electrification of air vehicles, and with the efficiency of photovoltaic cells continuously improving, it is essential and timely to explore avenues to integrate photovoltaics into the next generation of aircraft. This chapter presents some challenges of introducing photovoltaics into future passenger or transport aircraft. Furthermore, a conceptual design framework is presented that will help aircraft manufacturers evaluate the potential benefits of solar power systems on aircraft performance. Using this framework, we suggest possible avenues to integrate photovoltaics as a supplemental power system into future aircraft. Regional aircraft are shown to be a promising target platform for photovoltaics integration. In addition, the intelligent use of solar energy availability for an enhanced aircraft air-conditioning system is presented. In summary, this chapter gives avenues for the future of photovoltaics in the next generation of low-emission aircraft.

Keywords: solar flight, supplemental solar power system, aircraft conceptual design, air-conditioning system

1. Introduction: Photovoltaics in Aeronautical Applications. A Brief Review on PV Applications in Flying Vehicles Photovoltaics have been used successfully in space applications since the 1950s (Perlin 2002). The Vanguard 1 satellite was the first to have a dual power system of chemical batteries and silicon solar cells (Green McLaughlin and Lomask 1997). The Sunrise I (Boucher 1984) first flew in 1974 and was the first experimental solar-powered aircraft. In 1980, the world's first piloted solar-powered aircraft, the Gossamer Penguin, made its first flight. The Solar Challenger crossed the English Channel a year later, leading to the first international, piloted solar-powered flight. Many more prototype, research, and experimental flight vehicles followed in various configurations to address the technical challenges of solar-powered flight. More recently, Solar Impulse and Solar Impulse 2 made news by making piloted circumnavigations of the Earth without using a drop of fuel (Solar Impulse Foundation 2016). A complete review of solar-power aircraft can be found in refs. (Abbe and Smith 2016; Schoeberl 2008). Several successful attempts at solar-powered flying vehicles are listed in Table 1.

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Table 1. Overview of a sample of successful solar-powered flying vehicles and their solar power system characteristics Aircraft/ project name

Year of the first flight

Solar cell type

Solar cell efficiency

Sunrise I (II)

1974 (1975)

Spectrolab high-efficiency cell

14%

Gossamer Penguin

1980

Spectrolab HS213 cells

At least 12%

Solar Challenger

1981

HS208 cells

12.5%

Solar Impulse 2

2014

Monocrystalline solar cells

20%

Performance 560 W solar array; unmanned Demonstrated ability to fly a heavier-than-air, fixedwing vehicle on solar power alone First Human-crewed solarpowered flight (1 pilot) First International piloted solar-powered flight Human-crewed flight (1 pilot); >24h flight demonstrated (using solar cells and batteries)

In summary, looking at the history of solar flight, we see an effort to overcome the following challenges:  

Building an aircraft that can fly solely with solar power Sustaining long endurance

Achieving these two challenges requires balancing vehicle weight and size to accommodate solar panels. All solar flying vehicles have achieved this by combining the following aspects: using the most efficient solar power system available, designing lightweight aircraft, using the most straightforward aircraft systems (low power demand), and using the aircraft gliding capabilities (potential energy storage).

2. Challenges to Introducing Photovoltaics in Passenger Aircraft What is the challenge of scaling successful solar-powered flying vehicles to larger aircraft? One axis of current research and development is on giant, solarpowered airships. Several researchers have investigated the conceptual design and optimization of airships, e.g., (Zhang et al. 2017); (Pande and Verstraete, 2018). However, as the development of new airships provides more design

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freedom, this chapter will not cover this topic; we will examine the question of typical passenger or transport aircraft. One reason why design methods for previous solar aircraft are difficult to transition to larger, commercial aircraft is the relationship of usable area for solar cells to power required to sustain flight. This relationship is significantly smaller for larger passenger or cargo aircraft. One of the challenges lies in the demanding performance requirements for those larger passenger aircraft. This is illustrated through a selection of aircraft performance data provided in Table 2. Assuming the wing area is representative of the usable area for solar cells, the wing area compared to the power required by the aircraft to fly its mission (i.e., carrying the aircraft's weight) is an insightful metric. One can note a difference of two to four orders of magnitude between the research and commercial aircraft described in Table 2. In addition, several aspects distinguish the challenges of implementing solar power between demonstrator or research aircraft and passenger aircraft, general aviation or transport categories. These are discussed in the following subsections. Table 2. Comparison of aircraft design and performance characteristics between solar-powered research aircraft and typical certified passenger aircraft Aircraft

Engine Wing MTO Power area W (kg) (kW) (m2) Solar-powered research and demonstration aircraft Sunrise 2 0.6 8.36 10.21 Solair 1 2.2 22 200 Solar Challenger 1.86 21.83 153.77 Solar Impulse 2 30 200 2000 Certified passenger aircraft Cessna 172 120 16.2 1111 (light aircraft) Do228 1158 32 6575 (commuter aircraft) Q400 (regional 7562 64 30481 passenger aircraft)

Wing area/power (m2/kW)

Power/weigh t (kW/kg)

13.94 10 11.71 6.67

0.059 0.011 0.012 0.015

0.14

0.108

0.03

0.176

0.01

0.248

2.1. Safety and Certification Almost all aircraft need to follow some rules to be allowed to fly in public airspace. Passenger aircraft, general aviation or transport category aircraft

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must comply with numerous certification rules that ensure flight safety. These certification rules impact several aspects of aircraft design. Mostly, they limit the lightweight design methodology observed in extremis in experimental aircraft. Passenger aircraft require safety margins, fail-safe design, and redundancies. Many additional systems and components are usually required, often with multiple redundancies, which lead to a much higher empty weight of flying vehicles transporting passengers or goods in urban areas.

2.2. Standardization Many aircraft components are standardized to achieve an economically viable aircraft. As such, the electrical power system of most aircraft has a 28V DC network. The solar power system would need to connect to the aircraft standard systems, requiring step-up conversion. Any additional power conversion comes with further losses, which reduces the overall power-toweight ratio of the complete solar power system. There might be room for optimization with future, more-electric aircraft. However, as the solar power system will not be the primary power system, compromises are required, leading to suboptimal design.

2.3. Aircraft Operation Commercial aircraft must meet a specific range rather than achieving long endurance. At the same time, aircraft must be capable of operation during any hour of the day or night. However, daytime flights are most common for most regional flights due to night flight restrictions around metropolitan area airports. The operational analysis performed in the following section of this chapter will illustrate this aspect. While the constraint of long endurance can be lifted, the use of potential energy is unexplored for commercial aircraft. Some experimental aircraft use daylight excess power to climb to higher flight levels and perform a downward gliding flight during nighttime. This concept might be more challenging to implement due to aircraft traffic constraints (commercial aircraft usually fly on fixed flight levels). Still, it can be an interesting aspect to be explored for future air traffic management systems.

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Solar-powered aircraft are most efficient at high altitudes due to the increased solar cell efficiency at low temperatures and increased solar irradiation. The flight altitude is a parameter to consider in an aircraft optimized for solar power. High-altitude flight is favorable. However, most regional or smaller commuter aircraft fly at relatively low altitudes. On the other hand, business jets typically fly at high altitudes.

2.4. Aircraft are Designed for a Large Customer Base Rather Than Optimized Operation Aircraft are designed to maximize their reach in their market segment, making them less optimized for specific missions. However, more variety is required with the push for more efficient and environmentally friendly aircraft and the emergence of new aircraft manufacturers. A lower risk path for aircraft manufacturers would be to consider retrofitting existing aircraft into a low-emissions version. However, low emissions stem mainly from a low weight, which requires an optimized aircraft; optimizing an aircraft for weight while retrofitting it is a tremendous challenge. Once an aircraft is certified, modification to its structure and systems can only be done in a strictly regulated manner. Usually, improvements can lead to small changes in performance or weight but will not enable optimization of the aircraft.

2.5. Structural Integration and Maintainability Commercial aircraft and their structure are usually designed for around 20-30 years of service life. Solar cells degrade in performance over time due to solar radiation. If the solar cells are integrated with the structure, the installation concept needs to allow replacement, maintenance, and repair to optimize the weight. The load scenarios on aircraft skin-integrated solar cells are more complex on semi-monocoque structures and larger wings for high-speed aircraft than in rather stiff experimental planes. The fragility of the cells, required encapsulation, durability, bending, and temperature variations need to be considered carefully. All these aspects need to be well understood and impact the structural integration solution and, finally, the structural weight penalty or a penalty-neutral integration.

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2.6. Integration into the Aircraft Power System Architecture Aircraft require power to produce thrust – essential for flying – and for all systems the aircraft needs to function. In larger aircraft, it is impossible to get all the power required for thrust using solar power alone; for larger aircraft, the thrust required for a given wing area is too high to allow for a solarpowered flight. However, a substantial part of the so-called secondary power could be provided by solar power, depending on the type of system and aircraft. This book chapter will focus on this part. The so-called secondary power systems typically use pneumatic (bleedair) power, hydraulic power, or electrical power. In the past two decades, the so-called more-electric aircraft have been introduced in several aircraft (e.g., Airbus A380 and A350, Boeing B787, Gulfstream G650) to optimize the power system architecture and reduce the pneumatic and hydraulic power distribution systems' scope. As a supplemental solar power system would provide electric power, this trend towards electrification would make integrating such a system potentially easier. However, this supplemental electrical power stemming from the solar cells is not the same as the fuelsupplied electric generators. The details of the integration challenges are explained in more detail in the following section.

Section Summary The above-mentioned challenges make it difficult to imagine solar-powered flights for commercial or even general aviation aircraft. Still, those challenges do not preclude the potential benefit of introducing a supplemental solar power system that provides electricity whenever available and can help reduce the fuel burn and thus help reduce emissions. This is an exciting design challenge for future aircraft. With the introduction of more and more computer-based design tools, so-called multidisciplinary design analysis and optimization (MDAO), such scenarios might be better considered. The following section presents a conceptual design framework for a supplemental solar power system that can be introduced in typical aircraft conceptual design frameworks.

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3. A Conceptual Design Framework for Aircraft Featuring a Supplemental Solar Power System As the viability of integrating solar power is complex and multifaceted, the following section provides a conceptual design framework to analyze the benefits and manage the integration into flying vehicles. Before doing so, the supplemental solar power system concept is examined in more detail.

3.1. The Supplemental Solar Power System Concept As solar panel efficiency and the available space on aircraft surfaces are insufficient for solely solar-powered flight, the introduction is likely to occur through an auxiliary solar power system. The concept of a supplemental solar power system (SPS) is depicted in Figure 1.

Figure 1. Overview of the simplified Supplemental Solar Power System Architecture and its potential interface to other aircraft systems. Depending on the propulsion system architecture and the SPS concept, the battery for solar power storage is either optional or part of the electrical power system or part of the aircraft energy storage system.

Overall, there are three different scenarios of introducing an SPS in an aircraft:

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1. Concept 1: Drop-in power supply. The SPS is used as a drop-in power supply whenever the solar panels produce electric power. In this case, a power management system is required to prioritize the electricity coming from the SPS rather than from the electrical generators, usually mounted to the engine gearbox. This is the most straightforward system. The SPS can also recharge existing batteries in the aircraft (i.e., the APU start battery or the emergency or backup battery of the electrical system). However, this integration with the emergency power system needs to be carefully assessed from a safety point of view, depending on the applicable certification regulations. 2. Concept 2: SPS with energy storage. The SPS is connected to a dedicated battery that can be recharged, i.e., during ground operation. However, the sizing of these additional batteries needs to be performed carefully to not eradicate the fuel burn reduction benefit with the extra battery weight. Usually, aircraft with increased electrification feature several batteries in the power system architecture, which could be recharged by the solar power system. However, dedicated batteries might be suitable to optimize the electrical architecture. Combining the two concepts might be promising for more-electric, hybrid-electric, or all-electric aircraft. Rather complex analyses are required to determine whether an SPS is beneficial in an aircraft, i.e., if it leads to a net fuel burn reduction. To facilitate this analysis, we present a conceptual design framework in the following section.

3.2. Conceptual Design Framework for Aircraft Featuring an SPS The conceptual design framework depicted in Figure 2 focuses on analyzing the impact of an SPS on the aircraft. It is suited to integrate into typical MDAO frameworks for aircraft conceptual design. It does not yet allow for a complete life-cycle analysis. Details about this design framework and the underlying equations are provided in refs. (Liscouet-Hanke and Murugesan 2018; Liscouet-Hanke et al. 2018).

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Figure 2. Architecture of the Solar Power System integration framework for conceptual design studies.

In the following subsections, a brief description of the key features of this design framework is provided as guidance for a comprehensive analysis of SPS for future aircraft.

3.2.1. Aircraft Geometry and Usable Area for Solar Array Installation First, some geometrical information is required about the aircraft concept of interest. In particular, the conceptual designer needs to define the aircraft surfaces on which solar cells should be mounted. Figure 3 below shows some typical areas for solar cell installation. Each area has its own challenges, depending on the aircraft's geometry and configuration. A detailed analysis of all potentially usable areas should be considered. General guidelines are summarized in Table 3. One of the most important criteria is optimal exposure to sunlight. The usable area also depends on many factors, i.e., for smaller aircraft, installing solar cells on movable surfaces might not be possible due to the insufficient surface area to install a solar array. However, on larger aircraft, the surfaces like the elevator might be significant enough in size.

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Figure 3. Typical aircraft surfaces for installation of solar cells.

Table 3. Design consideration for placement of solar cells on aircraft surfaces Aircraft surface

Wing

Fuselage

Horizontal Tail

Vertical Tail

Engine Nacelles or Pylons

Design considerations Usable area is impacted by wing sweep angle (arrangement of rectangular cells), movable surfaces (slats, spoilers, flaps, ailerons), lightning strike zone at the wingtip Capture effectiveness is impacted by curvature due to aerodynamic profile, wing bending and twist and the shading effects (wing mounting with respect to the fuselage) Other aspects: bending and twist impact the solar cell durability; thermal interaction with systems/fuel stored in the wing will impact the efficiency Usable area is impacted by windows, access hatches and antenna placement, lightning strike zone on the front of the fuselage. Capture effectiveness is impacted by the curvature of the fuselage. Usable area is impacted by sweep angle (arrangement of rectangular cells), and movable surfaces (elevator). Capture effectiveness is impacted by curvature due to aerodynamic profile and the shading effects (horizontal tail mounting with respect to the vertical tail). Usable area is impacted by sweep angle (arrangement of rectangular cells), movable surfaces (elevator). Capture effectiveness is impacted by the vertical arrangement; typically, only one side of the vertical tail will be exposed to sunlight. Therefore, this area might not be recommended, as the usage factor of the solar power system is never above 50%. Usable area depends on the size and placement of the engines. Capture effectiveness is impacted by the curvature (which might be significant). Other aspects: challenging thermal and vibrational environment might reduce the durability of the solar cells and might impact the efficiency.

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The conceptual design framework includes an approximation to define the usable area on typical aircraft wings and fuselages, detailed in (LiscouetHanke and Murugesan 2018; Murugesan 2018). The implemented method facilitates estimating the usable area considering all the constraints above, based on empirical analysis of several aircraft. Roughly 40-50% of the total wing area can be used for solar cells. For the fuselage, the usable length is around 70-80% for larger aircraft and 50-60% for light aircraft. Depending on the fuselage curvature and window placement, about 30-40% of the upper circumferential length of the fuselage can be covered.

3.2.2. Solar Cell Technology and Assumptions about Power Conversion Equipment Information about the solar cells is required to analyze the output power of the solar cell and the weight of the solar cell system. Most of the recent successful solar-powered aircraft used mono-crystalline solar cells. The conceptual designer needs to have this data available and as an input for the analysis. In (Liscouet-Hanke et al. 2018), various solar cell manufacturer data samples are compared. In the examples in this chapter, we assume 20% efficiency for the solar cells and 90% efficiency for the power conversion equipment. For the unit weight of the solar cells, is it recommended to assume values around 0.30.8 kg/m2. However, as the technologies evolve fast, getting data from the solar cell manufacturer is recommended, i.e., to estimate the weight of the solar cell array, the conceptual design needs to consider additional weight due to the protection and installation of the solar cells. These aspects are also covered in more detail in Module 4 of the tool. 3.2.3. Absorbed Radiation Estimation (Module 1) Four aspects need to be considered to calculate the absorbed radiation by the solar cells on an aircraft: the aircraft's geographical location, the meteorological conditions, the orientation of the cells with respect to the sun, and the potential shading of the cells. All these aspects have been implemented in the framework in Module 1 (Liscouet-Hanke and Murugesan 2018; Murugesan 2018). The meteorological conditions are available as statistical data for different places in the world. They are also used for the flight analysis of the aircraft, which considers flight altitudes and changes in time zones. The aircraft configuration has a major impact on the occurring shading. The shading effect has also been quantified through an orientation analysis and implemented into a tool that adds a correction factor to the available output power. This shading analysis is illustrated in Figure 4.

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Figure 4. Illustration of shading analysis for the aircraft. Depending on the mounting of the wing, fuselage and tail, certain areas are shaded more frequently.

Depending on the aircraft's orientation with respect to the sun, a shading factor needs to be applied; it can range between a 12-40% reduction of the absorbed radiation. However, it is recommended to perform a detailed shading analysis for any particular aircraft configuration. In general, it is not recommended to consider the installation of solar cells on areas that will be shaded by adjacent aircraft areas for more than 50% of the aircraft operating time. Also, the shading analysis will give helpful information on how to

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segment and connect the solar cells to arrays and eventually install bypass diodes between the cells to minimize the impact of partial shading on the overall solar array power output.

3.2.4. Effect of Flight Mission Pattern on Life-Cycle Power Yield of the Solar Power System (Module 2) As will be illustrated in the subsequent section, it is important to have a complete understanding of the mission pattern the aircraft will perform during its operational life. Depending on the routes, operation time, and schedule, the overall power yield might vary significantly and lead to a non-viable solution. 





Module 3 summarizes the power yield analysis based on the inputs from Module 1 and Module 2, including the information about the technology of the solar cells and additional equipment of the SPS. Module 4 addresses the weight estimation of the SPS. A comprehensive example can be found in (Liscouet-Hanke et al. 2018). This module also includes a wiring weight estimation, impacted by the aircraft geometry. Module 5 is a placeholder for an impact analysis on any other aircraft system, depending on the SPS implementation concept. This chapter will provide a comprehensive example of the aircraft environmental control system and the auxiliary power unit.

The following section will expand on some interesting findings and conclusions that can be drawn from using the presented conceptual design framework.

4. Which Aircraft Would Most Likely Benefit from a Supplemental Solar Power System? The question is: At which point does an SPS benefit an aircraft? This depends on the following factors: (a) installed power to weight ratio of the SPS and (b) usable power ratio. Before we analyze these aspects more closely, we can look at some preliminary analyses of existing aircraft. Figure 5 ccompares the solar power available for the wing, tail and fuselage surfaces with the power installed for the secondary power system (the

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generator power per engine is used for comparison1). One can observe that for light aircraft, a significant portion of the secondary power can be provided by solar cells. Also, various medium-size aircraft (regional or business) could potentially supplement a sufficient amount of secondary power from solar cells.

Figure 5. Percentage of secondary power that can be provided by solar panels, on average over a year, for various aircraft types. The data provided in this chart is for ground conditions in Montreal, Canada, assuming 20% efficiency of the solar panels and 90% efficiency in the electrical components. Solar panels are assumed to be installed on the usable area of the wing, horizontal tail and fuselage. The solar output power is the yearly average between 5AM and Midnight.

1

We use the generator power per engine to account for the redundancy requirements. In most cases, when one generator is installed per engine, one of these generators need to be sized sufficiently to supply all necessary electrical systems. Sometimes, aircraft have two generators per engine; in this case two generators together need to provide sufficient power. This approximation allows for a conceptual comparison.

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4.1. Installed Power to Weight Ratio of the SPS All weight needs to be accounted for: the solar cells, the structural support and mounting, the wiring, connectors, control boxes and associated equipment. Even the integration into the aircraft control and data management needs to be accounted for in larger aircraft. It is crucial not to be too conservative or too optimistic at a conceptual level. The following aspects need to be considered for the design to reduce the effective power-to-weight ratio of the solar power system: 







Optimally design the solar array to minimize the need for power conversion (i.e., DC-DC boost converters). The solar cell needs to be connected and selected to meet the standard DC voltage in the network in an optimal manner. Integrate the solar cells in the structure; they should not be an add-on in terms of weight. The solar panel (cell integrated with back-panel and protective cover) should be designed with materials so it can replace the aircraft's skin locally. This, of course, poses several structural design challenges that need to be solved in the future. Another avenue is advances in photovoltaic materials, such as photovoltaic paints (e.g., quantum dot solar cells or perovskite solar paint), that could replace the traditional aircraft paint. Already the Solar One, in 1981, used a thin-film solar material. Researchers from the National Renewable Energy Laboratory (NREL) have achieved 13.4% efficiency for quantum dot solar cells in 2017, so-called photovoltaic paint, and efficiencies are still on the rise. Finally, solar cells with an optimal balance between efficiency and weight need to be considered.

All the aforementioned aspects require future research, needing a multidisciplinary approach to increase the benefit of prospective SPS for aircraft.

4.2. Usable Power Ratio It has been noted that the usable power ratio is a critical metric for SPS viability analysis. The usable power ratio depends on the following aspects:

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





47

Aircraft mission pattern: is the aircraft operating during daylight hours or not? For the mission analysis, it is also necessary to consider the operation throughout the year, not a single mission. Shading: which percentage of the solar arrays is exposed to the sun? This depends not only on the aircraft's orientation towards the sun but also on the aircraft's configuration. Solar array orientation and incident angle: in the past, airfoils have been designed with minimal upper surface curvature to improve the incident angle. More recently, high-speed aircraft use supercritical airfoils, which have the advantage of a relatively flat upper surface. In this way, the optimization benefits the installation of solar panels.

A previous study on a long-range business aircraft shows that the usable power needs to be above 50% to result in a potential net benefit of installing the solar system (Liscouet-Hanke et al. 2018). In the following subsections, the aircraft operations will be examined, and configurations will maximize the solar power usage factor.

4.2.1. Aircraft Configuration Impact on Power Usage Aircraft come in a variety of configurations. Figure 6 shows two typical aircraft configurations: (a) a high-wing aircraft with a T-tail and (b) a lowwing aircraft with a conventional (low) tail. If we install solar cells on the aircraft skin, the shading and orientation of the cells are a concern, as stated in the previous section.

Typical regional turboprop aircraft with a high wing and a T-tail

Figure 6. Typical aircraft configurations.

Typical single-aisle or wide-body commercial aircraft: low-wing and conventional (low) tail

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From a potential shading point of view, the high-wing configuration is clearly more beneficial for solar array installation on the wing. The area close to the fuselage provides the largest surface but is more prone to shading from the fuselage for low-wing aircraft.

4.2.2. Mission Pattern Analysis of Regional Aircraft and Impact on Solar Power Yield The authors have analyzed regional aircraft missions in various regions worldwide to identify the most suitable operation for aircraft featuring an SPS. As described in a survey on CO2 emissions stemming from aviation by ref. (Graver, Rutherford, and Sola Zheng 2020), air traffic routes that contribute the most to CO2 emissions are intra-Asia, intra-North America, intra-Europe, and between Europe and North America. These routes together are responsible for around 62% of the CO2 emissions. Hence, they are most likely the most traffic-intensive routes. In this chapter, we analyzed a selection of flying routes that illustrate the challenges of solar power systems’ effectiveness in reducing CO2 emissions. The here presented study covers flights in the following regions: 





Canada, including some Canada to USA flights. In this region, most regional travel is in the east-west or west-east direction (across longitudes) and has pronounced seasonal differences in daylight and hence solar irradiance exposure. Norway. Due to its geographic particularities, most regional travel is in a north-south or south-north direction (across latitudes). Like the Canadian routes, Norway has pronounced seasonal differences in daylight. Norway was also chosen due to its political push to make aviation emission-free by 2040 (Dowling 2018). East-Africa (Kenya, Ethiopia). This region is an emerging aviation region. Its proximity to the equator results in less pronounced seasonal changes in solar irradiation and daylight hours.

Previous literature also provides examples in other parts of the world, i.e., South America (Ramirez-Diaz, Nadal-Mora, and Piechocki 2015). The study here is not exhaustive but provides avenues for reflection on the viability of introducing SPS in aircraft fleets.

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The operation analysis has been performed using data from FlightRadar24 (https://www.flightradar24.com/), a web-based flight-tracking website. We analyzed data from several aircraft for the timeframe from 2019-2020. The first important aspect is to analyze the aircraft's typical daily operating schedule over a 24h timeframe to understand the portion of flight operation occurring in daylight conditions. From Figure 7, around 90% of the flight operation happens between 6 AM and 9 PM, with slight regional differences. Depending on the season, this timeframe coincidences with the main sunlight hours.

Figure 7. Regional aircraft flight operation in various regions.

The relatively short routes can explain the drop in mid-day operation. Most regional aircraft fly at least two daily missions, with a ground operation phase in between. With the operational and seasonal data, the information about the aircraft configuration and the SPS configuration, we can now analyze the potential power output of such an SPS. The authors developed a computer program (Module 3 in Figure 2) to analyze the captured irradiance, translated into potential solar panel output power for typical flights. Using this analysis tool, the seasonal averages for a typical regional aircraft, the Q400 (De Havilland Canada/Bombardier), are presented in Figure 8.

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Figure 8. Seasonal averages per region. The power yield from the aircraft-mounted solar panels are for a Q400 aircraft, using wing, horizontal tail and fuselage-mounted panels. The solar cell efficiency is assumed at 20% and 10% losses are assumed in the remaining power transmission and conversion chain.

For the same aircraft test case (Q400, solar cells with 20% efficiency), Figure 9 depicts yearly averages for various locations over a day. Due to the more considerable seasonal differences in the northern hemisphere, the annual averages are also lower.

Figure 9. Yearly averages for various airports worldwide - for a typical regional aircraft.

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The power yield from the aircraft-mounted solar panels for a Q400 aircraft, in Figure 9, using wing, horizontal tail and fuselage-mounted panels. The solar cell efficiency is 20%, and 10% losses are assumed in the remaining power transmission and conversion chain. The presented data here also assumes average ground weather conditions.

Section Conclusion The SPS would be most used throughout the year for aircraft operations around the equator and east-west operations. For operations in northern or southern hemispheres, seasonal differences prevail, but, on average, the implementations seem still beneficial. From the analysis presented above, aircraft flying on regional routes will be the best candidates. As for the aircraft configuration: high-wing, T-tail aircraft are the best option for aircraft featuring an SPS. Also, fuselage-mounted solar cells are best when placed on the top section of the fuselage. Non-circular, top-flattened surfaces increase the solar panel usage factor. Some futuristic designs (blended wing body, double-bubble fuselages) will bring larger surfaces potentially covered with solar panels. We can also note that for regional aircraft, around 5-18 kW of peak power can be generated by the SPS, considering various operation conditions. This amount of power is sufficient to power several aircraft systems. The following section will illustrate one example of matching power available from the SPS with the power required from aircraft systems in a promising way.

5. Solar-Powered Air-Conditioning System for Aircraft The section illustrates Modules 4 and 5 of the conceptual design framework (ref. to Figure 2) for the example of a solar-power-enhanced aircraft environmental control system (ECS). This system will be called a solarpowered air-conditioning system (SPACS) in the following. Aircraft ECS fulfill several functions. The most critical functions are temperature control, pressure control and air quality control. Here, we focus on temperature control. Aircraft, including their installed systems, are designed to sustain extreme environmental conditions, generally between 80°C to 55°C. In particular, aircraft can be parked on the ground for extended periods without an external power supply, leading to scorching conditions inside the aircraft (exceeding +55°C), like for a parked car. These extreme environmental conditions impact the longevity and reliability of electronic

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equipment. Also, the air-conditioning system must be designed to cool down the cabin rapidly once the aircraft is powered before passengers can enter the aircraft. A solar-powered air-conditioning system can help to improve the traditional ECS. This section shows the principle of a solar-powered ECS, explores the benefits on the aircraft level and provides an illustrative case study. Currently, aircraft are cooled on the ground using their environmental control system (ECS), powered with so-called bleed air from the auxiliary power unit (APU) and with electrical power for fans. The APU is a gas turbine engine that burns fuel and produces noise emissions. The APU sometimes runs continuously at hot airports to keep the aircraft cool (Aranjo, Hughes, and Chaudry 2012). The aircraft can also be cooled using a ground cart. However, these are not available at all airports or only at an airport's gates. Regional aircraft are often parked far away from the gate, without access to airport power. A modeling framework in Figure 10 shows the workflow, which required the creation of a model to estimate the potential benefits of such a system.

Figure 10. Analysis framework and information flow for the solar-powered ECS case study.

Several configurations in a solar-power enhanced ECS are possible: (1) using solar-powered additional ventilation; (2) using solar-power assisted cooling. Option 1 is considered here, assuming a solar-powered fan. The solarpowered fan helps circulate the air between the inside and outside, even when the aircraft is parked. Therefore, the aircraft's internal environment (cabin and equipment bays) stays at a lower temperature. Keeping a lower internal temperature helps with reducing the power required for the so-called “pulldown”. The pull-down is a transient operating case, often sizing the ECS cooling pack (typically an air-cycle machine, the heaviest and largest aircraft

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air conditioning system), in which the internal temperature is brought from the hot soaked temperature to around 28°C. This pull-down is required before letting crew and passengers enter the aircraft and should typically be achieved in 30 to 45 min. A transient thermal model is required to perform the pull-down analysis and estimate the potential size reduction of the APU and ECS. This thermal model was presented in (Sadaka, Liscouet-Hanke, and Sanchez 2019). Figure 11 shows the significant aspects of the transient thermal model in a typical commercial aircraft cabin. Heat enters the cabin through the windows (radiation) and the skin (conduction). Inside the cabin, the furniture (i.e., seats) and the cabin air act as heat capacities. Heat exchange also occurs through the underfloor, as conductive heat exchange or through the recirculation fan (sucking air from the cabin to the underfloor). The underfloor compartment has its own heat capacity and is influenced through thermal exchange with the tarmac. The traditional ECS or the solar-powered fan evacuates the underfloor air to the outside. Therefore, less heat needs to be evacuated during the so-called “pulldown” operation. This pull-down operation is the sizing case for the ECS (Zaparoli and de Andrade 2003). It is defined by the cooling energy required to reduce the cabin temperature of a heat-soaked aircraft on the ground to 28°C in a specific timeframe, usually between 30-60 min. Since the ECS requires bleed air from the APU, the pull-down loading also sizes the bleed air required by the APU. By implementing a solar-powered air-conditioning system (SPACS), the pull-down loading is reduced and results in the resizing of the ECS and APU, aiding in offsetting the added weight of the SPS. The proposed SPACS consists of fans that would reduce the rate at which the cabin and equipment bay heat up. The SPACS can increase aircraft independence by making airports in hot climates more accessible, particularly those without ground carts. By reducing the reliance on the ECS and APU, emissions and noise pollution would be reduced. The developed thermal model is used for a case study for the Q400. As discussed in the previous section, the Q400, a regional aircraft with a highwing and T-tail configuration, is a promising candidate aircraft configuration for the potential implementation of a supplemental solar power system. The power yield analysis shows that wing and tail-mounted solar cells with a 20% efficiency can provide up to a 25 kW peak on a sunny day in Montreal, Canada and around a 10 kW peak for the yearly average. However, the solar-powered ECS is only required to operate in sunny and hot conditions; therefore, the average daylight hours in the summer season would be a more meaningful

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timeframe for the average power output calculation. This is more than sufficient to power a fan of a typical air-conditioning system.

Figure 11. Overview of thermal environment for aircraft environmental control systems for an aircraft with a supplemental solar-powered system. The figure shows a typical commercial aircraft cabin cross-section.

The thermal model described above is used to determine the pull-down time for various heat-soak durations. Figure 12 shows the cabin temperature variation for a conventional Q400 (no solar-powered fan), with a 735W solarpowered fan and a 5kW solar-powered fan. The 735W solar-powered fan can provide a mass flow of around 0.38 kg/s, corresponding to the fan specification of the conventional Q400's ECS. Therefore, the SPACS could power the existing fans and ducting to minimize

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the weight impact and integration complexity. The 5kW solar-powered fan has a mass flow of 2.63 kg/s and would require the integration of a more complex system, but represents a benchmark for a higher ventilation capacity by the solar-powered system.

Figure 12. Pull-down results for a Q400 with a conventional ECS and a solarpowered air conditioning system. The pull-down is complete once the cabin temperature reaches 28°C.

The first observation from Figure 12 is that the cabin temperature rise is significantly higher for an aircraft without a solar-powered fan. A 3h hot soak can reach almost 60°C compared to 48°C with a small solar-powered fan. Following the simulation, the ECS was turned on after 1h, 3h, and 5 h of heat soak. The SPACS allowed the cabin to reach a lower steady-state temperature in less time than a conventional ECS. A reduced pull-down time was achieved, as shown in Table 4, as was a corresponding reduced pull-down load. The 5kW solar-powered fan reduced the steady-state temperature more than the 735W solar-powered fan. However, a more powerful fan becomes limited by the heat transfer through the floor and the amount of air transferred by the recirculation fan, resulting in diminishing returns for increased fan power.

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Table 4. Pull-down time reduction for Q400 with conventional ECS and a solar-powered air-conditioning system Hot soak duration

1h 3h Conventional System Pull down-time 62 min 79 min Hot-soaked cabin temperature 48°C 59°C System augmented with 735W solar-powered fan Pull down-time and relative 50 min -20% 55 min -30% improvement Cabin temperature and relative 45°C -6% 47°C -20% improvement System augmented with 5KW solar-powered fan Pull down-time and relative 50 min -20% 52 min -34% improvement Cabin temperature and relative 44°C -8% 45°C -24% improvement

5h 86 min 65°C 56 min

-35%

47°C

-27%

52 min

-40%

45°C

-31%

Based on the above-presented analysis, the potential effect on the ECS sizing was estimated. The ECS mass flow is adjusted to reach a pull-down to 28oC in 30 min. A detailed simulation performed in (Sadaka, Liscouet-Hanke, and Sanchez 2019) shows that an aircraft with a 735W fan requires about 55% less mass flow for the ECS to cool the cabin than an aircraft without a solarpowered fan. This reduced mass flow allows the ECS and APU to be smaller, ideally offsetting the additional weight of the SPS and SPACS. By reducing the required mass flow rate and the peak power demand of the ECS, the ECS pack will probably be smaller in size. However, most conceptual design weight estimation methods are not sensitive to the change in required mass flow rate or pull-down requirements. (Sforza 2014) summarizes several ECS weight estimation methods (Oman 1977, Torenbeek 1982, and Kroo 2008). All these methods are based on empirical data of traditional ECS. They are using the function of aircraft weight or passenger numbers. More work or assessment by an ECS manufacturer is required to estimate the potential weight benefit for such an optimized ECS. However, we can safely claim that a significant amount of weight can be saved for the cooling device (i.e., air-cycle machine) and the associated ducting and related components. In addition, the APU can also be reduced in size if the ECS design airflow is lower. The ECS pull-down is the sizing case for most APUs (i.e., for medium or smaller commercial aircraft). Therefore, the weight decrease due

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to the reduced pull-down loading would likely be proportional to that of the APU. In summary, using a solar-powered fan to stabilize the aircraft's internal temperature during hot-day parking has the benefit of improving the operation and reducing the weight of several aircraft systems. In addition, the improved internal temperature will also benefit sensitive electronic equipment, potentially reducing the cooling demand and ventilation system design for those systems. More detailed conceptual design tools for subsystem design aspects need to be developed to fully quantify the claims.

Conclusion So, will we see photovoltaics in future aircraft? Many design challenges presented in this chapter still need to be solved. However, there is an opportunity to introduce photovoltaics as a supplemental solar power system. With the help of the conceptual design framework presented in this chapter, aircraft designers should be able to identify aircraft concepts with promising system architectures that reduce fuel burn and CO2 emissions. The various design challenges have been illustrated and quantified. Overall, shorter-range aircraft (regional and commuter aircraft), particularly with daylight flying patterns, can harvest sufficient solar power to reduce aircraft fuel burn or optimize the operation of subsystems. Another example is an enhanced solar-powered air-conditioning system presented in a case study. In conclusion, in the current context of intense research in more-electric, hybrid-electric aircraft and all-electric aircraft, a supplemental solar power system might be a complementary enabling technology to mature, a promising technology for more environmentally friendly aircraft of the future.

Acknowledgments This book chapter would not have been possible without the work performed by several students in the Aircraft Systems Lab at Concordia University, namely Ezhil Shakti Murugesan (initial framework development and shading analysis), Noah Sadaka (solar-powered air-conditioning system modeling) and Richard El Chaar (flight mission analysis). The work was funded by the

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Concordia University Seed Funding Program and the Natural Sciences and Engineering Research Council of Canada Discovery Grants Program, grant number RGPIN-2019-05515.

References Abbe, G., and H. Smith. (2016). Technological Development Trends in Solar-Powered Aircraft Systems. Renewable and Sustainable Energy Reviews. https://doi.org/10. 1016/j.rser.2016.01.053 Air Transport Action Group. (2020). Facts and Figures. https://www.atag.org/factsfigures.html. Aranjo, Brendan Savio, Ben Richard Hughes, and Hassam Nasarullah Chaudry. (2012). Performance Investigation of Ground Cooling for the Airbus A380 in the United Arab Emirates. Applied Thermal Engineering 36 (1): 87–95. https://doi.org/10. 1016/j.applthermaleng.2011.12.002. Boucher, Robert J. (1984). History of Solar Flight. In AIAA//SAE/ASEE 20th Joint Propulsion Conference, 1984. https://doi.org/10.2514/6.1984-1429. Dowling, Stephen. (2018). Norway's Plan for a Fleet of Electric Planes. BBC Future, August 22, 2018. https://www.bbc.com/future/article/20180814-norways-plan-for-afleet-of-electric-planes. Graver, Brandon, Dan Rutherford, and Sola Zheng. (2020). CO2 Emissions from Commercial Aviation: 2013, 2018, and 2019. https://theicct.org/publications/co2emissions-commercial-aviation-2020. Green McLaughlin, Constance, and Milton Lomask. (1997). Vanguard, A History - NASA SP-4202. NASA Space Science Coordinated Archive. 1997. hhttps://www. hq.nasa.gov/office/pao/History/sputnik/TOC.html. Hupe, Jane. (2019). The Triennium in Review: Paving the Way to a Green Future. In The Next Chapter - 2019 Environmental Report, 9–11. ICAO. https://www.icao.int/ environmental-protection/Documents/ICAO-ENV-Report2019-F1-WEB (1).pdf. IATA. (2013). IATA Technology Roadmap. https://www.iata.org/contentassets/8d19e 716636a47c184e7221c77563c93/technology-roadmap-2013.pdf. Liscouet-Hanke, Susan, and Ezhil Shakti Murugesan. (2018). Conceptual Design Framework for an Aircraft Auxiliary Photovoltaic System. In AIAA Aviation Technology, Integration, and Operations Conference. https://doi.org/10.2514/ 6.2018-4137. Liscouet-Hanke, Susan, Arash Shafiei Shafiei, Luiz Lopes, and Sheldon Williamson. (2018). Proof-of-Concept Analysis of a Supplemental Solar Power System for Aircraft. Aircraft Engineering and Aerospace Technology. Mintsa, Mazarin. (2016). Solar-At-Gate Pilot Project – A Proven Energy-Saver. In 2019 Environmental Report - Destination Green - the Next Chapter, 330–32. ICAO. Murugesan, Ezhil Shakti. (2018). Conceptual Design Analysis for an Aircraft Auxiliary Photovoltaic System. Concordia University.

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Pande, Durlabh, and Dries Verstraete. (2018). Impact of Solar Cell Characteristics and Operating Conditions on the Sizing of a Solar Powered Nonrigid Airship. Aerospace Science and Technology 72: 353–63. Perlin, John. (2002). From Space to Earth - The Story of Solar Electricity. Cambridge: Harvard University Press. Ramirez-Diaz, Gabriel, Vicente Nadal-Mora, and Joaquin Piechocki. (2015). Descriptive Analysis of Viability of Fuel Saving in Commercial Aircraft through the Application of Photovoltaic Cells. Renewable and Sustainable Energy Reviews 51: 138–52. https://doi.org/10.1016/j.rser.2015.06.008. Sadaka, Noah, Susan Liscouet-Hanke, and Florian Sanchez. (2019). Model Development for the Conceptual Design of a Solar-Powered Aircraft Air-Conditioning System. In CASI AERO Conference. Laval, Canada. Schoeberl, Ernst. (2008). From Sunrise to Solar-Impulse 34 Years of Solar Powered Flight. Technical Soaring 32 (4): 115–21. Sforza, Pasquale. (2014). Refined Weight and Balance Estimate. In Commercial Airplane Design Principles, 301–47. Elsevier. https://doi.org/10.1016/B978-0-12-4199538.00008-5. Solar Impulse Foundation. (2016). Solar Impulse. https://aroundtheworld. solarimpulse.com/. Zaparoli, Edson Luiz, and Cláudia Regina de Andrade. (2003). Simulation of the Aircraft Cabin Cool-Down and Warm-Up. In Proceedings of the 17th International Congress of Mechanical Engineering. Sao Paulo. Zhang, Lanchuan, Mingyun Lv, Junhui Meng, and Huafei Du. (2017). Optimization of Solar-Powered Hybrid Airship Conceptual Design. Aerospace Science and Technology 65: 54–61.

Chapter 3

Modeling, Conceptualizing, and Sizing Approach of Solar Farms for Powering of Drinking Water Supply and Irrigation Systems in Rural Zones: A Case Study from Tunisia M. H. Sellami1,2, H. Tiba1 A. Nasri2 and H. BenAhmed3 1Laboratory

of Soft Material and Electromagnetic Modeling, Physics Department, Faculty of Science, University of Tunis El Manar, Rommana, Tunisia 2Hydraulic Department, High School of Engineering, Medjez El Bab IRESA, Jendouba University, Jendouba, Tunisia 3Laboratory of Plant, Soil and Environment Interactions (LR21ES01), Faculty of Sciences, University of Tunis EL Manar, Rommana, Tunisia

Abstract The effects of climatic changes are increasingly apparent. Worldwide, prolonged droughts are noted in some regions, while frequent and intensive rains are in other regions. As a result, rural zones are not on par with urban zones in water and energy supply areas. For that, experts and scientists are searching for relevant models to conceptualize and size solar power farms for drinking water supply and irrigation network systems in rural and mountainous areas. The fundamental equations and correlations to develop the models have been depicted and elaborated. 

Corresponding Author’s Email: [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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M. H. Sellami, H. Tiba, A. Nasri et al. These fundamentals have been applied to develop models for the sizing of solar power farms, which ensure the production of necessary electricity to operate an irrigation system inside the oasis and supply potable water for a rural agglomeration in Tunisia. Three separate models have been developed. The first model calculates the actual water required by vegetable canopies and rural agglomerations. The second model estimates the hydraulic components of the networks, including pipes, tanks, water organs, and pumps, and predicts the global energy needed for their running. The third model deals with the conception of solar power farms. Results from case studies for rural zones in Tunisia are presented using the three developed models.

Keywords: conception sizing, irrigation, modeling, potable water, rural, solar farm

1. Introduction Human life is at risk as infrastructures, buildings, and habitats can be damaged, and food security is threatened due to climate change. Principally, climatic change is due to the emission of greenhouse gases into the atmosphere. The enormous use of fossil-based energy is behind that emission. Renewable energy can be an appropriate alternative to replace fossil fuel usage over time. There is a direct relationship between renewable energy and sustainable development via its effect on human development and sustainable economic growth. Solar energy is energy transmitted by the sun through light and heat. This energy is virtually inexhaustible on the scale of human times, which is why it is classified among renewable energies. Solar energy also is the source of most renewable energies and fossil fuels. The energy recovered from the electricity produced in 20 days of sunshine is equivalent to the entire planet’s fossil reserve. One m² of land recovers as solar energy in 24 hours, the equivalent of a barrel of oil each year. Solar radiation is behind all phenomena of heat and mass transfer in the atmosphere. It causes water evaporation from soils, rivers, lakes, and oceans (Sellami 2008; Sellami 2020). Plant canopies grow, transpire water, and absorb carbon from the atmosphere through solar radiation. This balance will be altered when greenhouse gases and alveoli are released into the atmosphere resulting in climate change. The latter threatens food security, infrastructure, and human life through its effects. To minimize those effects, the main sources leading to climate change have to be battled.

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Many experts approve that the excessive use of fossil energy in all sectors of human development is principally the main cause of climate change. Renewable energy, in general, and solar power, in particular, could be an alternative to replacing fossil energy (Sellami, 2018; Rakhshani et al. 2019; Jurasz et al. 2020; Nicole et al. 2021). The energy recovered from the electricity produced in 20 days of sunshine is equivalent to the entire planet’s fossil reserve. 1.0 m² of land recovers as solar energy in 24 hours, the equivalent of a barrel of oil each year. Each year, the earth receives 1.070.000 Petawatt-hours (PWh equivalent to 1015 Wh), more than 8000 times the annual global energy consumption. Rural regions worldwide are the most vulnerable zones threatened due to the lack of water and energy sources. That is why decision-makers and experts are searching for models permitting to conceptualization and size of solar power farms, ensuring the functioning of drinking water supply networks and irrigation systems in rural and mountainous areas (Sellami 2015; Ramadoni and Indah 2020). This work proposes a modeling approach to conceptualize and sizing a solar unit assuming the electricity supply that water networks need for irrigation water and drinking water. The model presented is formed from three principal parts. The first part permits calculating the real amount of water needed by vegetable canopies and rural agglomerations. The second is reserved to model the hydraulic components of the networks (pipes, tanks, water organs, pumps...) and to evaluate the global energy needed for their running. The third part focus on the conception of solar power farms. Results from case studies for rural zones in Tunisia are presented.

2. Conceptualization and Sizing of the Project Solar Energy-Based Irrigation System To conceptualize and size a renewable energy system, the total energy needed to ensure the project’s running should be estimated. The first project consists of a potable water supply to a rural agglomeration in the Kasserine governorate. The second project is to irrigate the Nefzawa oasis in Kebili. The consumer, in our case, is a house or a factory or administration building for the potable water network and plants (date palms, fruit trees, market gardening) for the irrigation network. The geographic position of each consumer and its water requirement must be considered to select suitable

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pipes, canals, hydraulic components, water treatment, and pumping structures. Hence the project procedure is composed of three parts. The first is to determine water requirements for the consumers in the selected area. The second is to analyze the energy required to compensate for pressure drops inside the hydraulic networks and running of the equipment. The third is for the conceptualization and sizing of the solar power farm.

2.1. Model Sections for Calculating the Consumer Water Requirement The water debit to transfer through the hydraulic networks represents the water required for each consumer to supply. In our case, the consumer is a house, factory, or administration building for the potable water network and plants (date palms, fruit trees, market gardening) for the irrigation network. It is so important to estimate the water requirement to evaluate the total energy needed to compensate for the pressure drop inside the hydraulic networks and determine the total charge of the solar power farm.

2.2. Water Required for the Irrigation Network The amount of water needed by the plants in the field and by the field in the region for all vegetal speculations should be estimated. There are many models for estimating the water needed by plants as a function of the physiological characteristics of every specie (leaf area index, sap flow, stomata resistance), soil types (texture, structure, permeability, porosity), climatic parameters (solar radiation, temperature, precipitation, evaporation, transpiration, heat) and the economic and demographic evolution by region. Their use is possible and depends on the precision asked (Sellami 2015; Sahbani and Sellami 2020; Sellami 2020). Below is a simple and general formula to calculate the amount of water needed by plants in the field: 𝑄𝑝𝑙𝑎𝑛𝑡 = 𝐾𝑐 × 𝐸𝑇𝑃 Qplant: the amount of water needed. Kc: cultural coefficient that depends on the types of plants and soils. ETP: potential evapotranspiration.

(1)

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In the conception of a hydraulic network, the consumers should be identified. This means calculating the amount of water they need and the minimum pressure permitting lead water to the consumer at the highest manometric level now and in the future. For cultural activities, the consumers are the different vegetation species to irrigate. The required irrigation water must be precisely estimated as it would be used to conceptualize the efficient irrigation scheme, calculate the pumping station, size the different components of the desalination or treatment units, and determine the charge of the solar farm. The total amount of irrigation water to supply to the vegetation canopy depends on the plant’s physiological cycle, soil permeability, and local climatic conditions (evapotranspiration and rainfall) can be written as follows: 𝑄𝑖𝑟𝑟𝑖𝑔 = 𝑄𝑛𝑒𝑒𝑑−𝑝𝑙𝑎𝑛𝑡 + 𝑄𝑖𝑛𝑓−𝑠𝑜𝑖𝑙 + 𝑄𝐸𝑇𝑅 − 𝑄𝑝𝑙𝑢𝑣

(2)

Qirrig: Amount of irrigation water. Qneed-plant: Plants’ water needs. QETR: Amount of water evapotranspiration. Qpluv: Rainfall. The formula to estimate the potential evapotranspiration, is: 𝐸𝑇𝑃 =

𝛾 𝛾+𝛾∗

(∅𝑅𝑛𝑒𝑡 − ∅𝑐𝑠𝑜𝑙 )

1 𝜆

+

𝛾

900

𝛾+𝛾∗ 𝑇+275

𝑈2 (𝑒𝑎 − 𝑒𝑑 )

(3)

ed: Vapor pressure in the air in a saturated state, ea: Vapor pressure in air, γ: Psychrometric constant, γ*: Modified psychrometric constant, U2: Wind speed at m/s, ΦRne: Flux of net radiation, Φcsol: Heat flux released by the ground. The water needed by plants in the future could be evaluated as a function of demographic evolution, the sort of tolerant vegetation installed as food, the type of industrial culture to the implant, and the evolution of the agroalimentary industry and the climatic change. So, the economical and political orientations for durable development by the region should be considered. The agricultural maps, land-use plans, the data basis for vegetation characteristics, the gene banks, climatic data basis, models for plant transpiration, biosphere models, circulation, and climatic models should also be utilized.

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2.3. Water Required for the Industrial Sectors The industrial sector needs enormous amounts of water for all its activities to ensure its production continuity. The quantity of water needed depends on the type of product and the different processes used inside the industry. For example, water required for the textile industry (cotton tissue) is 4500 l/kg, for the dairy industry is 10 l/l milk, for the paper industry is 222 – 330 m/t, and for the sugar industry is 3000 l/t. The following relationships assist in estimating the amount of water needed for an industrial zone composed of different factories. If the type of factory is denoted by j and the industrial process is donated by i, then: 𝑄𝑧𝑜𝑛−𝑖𝑛𝑑𝑢𝑠 = ∑𝑗 ∑𝑖 𝑄𝑝𝑟𝑜𝑐

𝑖,𝑗

(4)

𝑄𝑧𝑜𝑛−𝑖𝑛𝑑𝑢𝑠 : Total water needed for the industrial zone. 𝑄𝑝𝑟𝑜𝑐 𝑖,𝑗 : Water required for process i in the industry j in the industrial zone. For the future, the water needed is evaluated as a function of the orientation of the durable development axis by region, census studies, economic plan, technology evolution, and market demand. According to the water advisers, an intervention to advise what type of industry could be installed in the region.

2.4. Water Required for the Domestic Sector The water needed for domestic here is potable water. It depends on the number of residents by region, their comfort requirement, and their future evolution. The census studies the statistical models should be used to evaluate the possible changes in the future. They are based on the following equations: For the demographic evolution, 𝑁𝑝𝑜𝑝−𝑦 = 𝑁𝑝𝑜𝑝−𝑦0 (1 + τ)𝑦−𝑦0) Npop-y: Population number for the year (y). Npop-yo: Population number for the reference year (y0). τ: Population evolution rate. For the consumption evolution.

(5)

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𝑉𝑐𝑜𝑛𝑠−(𝑦) = 𝑉𝑐𝑜𝑛𝑠−(𝑦−1) (1 + Γ)𝑛

67

(6)

Vcons-(y): Water consumption for the year (y). Vcons-(y-1): Water consumption for the year (y-1). n: Number of years for which to estimate the consumption. Γ: consumption evolution rate. For domestic usage, one must distinguish between the amount reserved for houses and that for public use (administration, collective valves). Also, the debit of water needed for a population depends on the region, climate, economic activities, and the comfort level asked by each person. The total debit of water is expressed by: 𝑄𝑢𝑠𝑎𝑔𝑒−𝑑𝑜𝑚𝑒𝑠 = 𝑁𝑝𝑒𝑟𝑠−ℎ𝑜𝑢𝑠𝑒 𝑁ℎ𝑜𝑢𝑠𝑒 𝑄𝑝𝑒𝑟𝑠 + 𝑄𝑝𝑢𝑏𝑙𝑖𝑐

(7)

Qusage-domes: Water debit for domestic usage. Qpublic: Water debit for public usage.

2.5. Model Part for Calculating the Energy Required to Compensate Drop Pressure Each hydraulic network’s functioning must begin by conceptualizing its scheme and all its components. The start is by evaluating the amount of water needed by each consumer. This means calculating the amount of water they need and the minimum necessary pressure that permits lead water to the consumer at the highest manometric level now and in the future. For agricultural activities, the consumers are the different vegetation species to irrigate. In contrast, the domestic and industrial sectors are houses (the number of persons inside) and factories (the type of products and the productivity). The evolution in the future of the amount of needed water must be precisely estimated to rehabilitate the efficient water schemes, calculate the pumping station, size the different components of the treatment units, and determine the charge of the renewable energy system. For all the consumers cited, the hydraulic schemes are defined as the course that the water flow follows from the source to the plant in the vegetation canopy or house in a rural agglomeration or a factory in an industrial zone (Sellami and Trabelsi 2009; Bermúdez et al. 2018; Abu-Mahfouz et al. 2019).

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It is formed from adduction/distribution networks. The differentiation between adduction and distribution is arbitrary. It depends on the sector to supply water and only facilitates the conception of the network when making hydraulic studies. The adduction part is from the sources to the tank or the series of tanks for water storage and treatment. The tanks (storage or treatment) are distributed to the consumer (Houses, factories, plants, vegetation canopies). In the current case, the adduction network is considered as the part from the water source (forages, deep boreholes, and wells) until the treatment stations or the storage tanks and the part from the storage tank up to the consumer is the distribution network. To ensure the water is needed at the time and to avoid its loss on the way, there is a tendency to conduct special canalizations (pipes, conduits), hydraulic accessories and apparatus (pump, tanks, treatment stations, floodgates, bends, diaphragm, flow meters, manometers, valves, sprinkler). Conceptualizing a water scheme and calculating its optimal size signifies determining the length and diameters of the pipes, fabrication materials, number of conduits sections, types and number of hydraulic accessories, their capacities (surface, volume, power, energy), and finally, the cost. Mathematical relationships, formulas, and models are used to perform hydraulic calculations. They are based on the energy balance between the network’s initial point (resource) and the endpoint (entrance to the sector). Comolet 1963, and Sellami 2015, using the theorem of Bernoulli, expressed this energetic balance as: 𝐻𝑖𝑛𝑖𝑡𝑖𝑎𝑙 = 𝐻𝑓𝑖𝑛𝑎𝑙 + 𝐽𝑖𝑛𝑖𝑡𝑖𝑎𝑙−𝑓𝑖𝑛𝑎𝑙

(8)

Hinitial is the energy at the initial point of the network. Hfinal is the energy at the last point of the network. Jinitial-final is the total loss of energy between the initial point and the last point of the network. The topographical status between the initial and final points of a water line, like the change of slope, change of direction, and/or obstacles. They are called knots. The distance between two successive particular points (knots) defines the length of a pipe section. For a hydraulic network with n sections and m particular points, the total pressure drop is expressed as follows: 𝐽𝑛−𝑚 = ∑𝑛 𝑗𝑛 𝐿𝑛 + ∑m Jm

(9)

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𝐽𝑛−𝑚 : Total pressure drops for a hydraulic network with n sections and m particular points, 𝑗 𝑛 : Linear pressure drop for the section n, 𝐿𝑛 : Length of the section n, 𝐽 𝑚 : Singular pressure drop for the particular point m. The linear energy loss by the water inside the pipes is a function of the diameter, the water flow, the pipe roughness, and the nature of the fabrication material. They are expressed in the following form: 𝑄 = 𝛽𝑗 𝑛 𝐷 𝑚 (10) Q: Water flow, j: Linear pressure drop, D: Diameter of the section, β, n, m: Adjustment parameters. The altitude of the particular points could be known using topographical measurements and level curve maps. The pressure needed to assure the water distribution to all the consumers must be superior to the highest manometric level at the streamside. Each hydraulic accessory in the network corresponds to a particular singular energy loss (that depends on the water velocity inside the pipes and the friction force via the knot (floodgates, bends, diaphragm). The singular pressure drops, which correspond to the energy loss when the water passes through a particular point in the network, are expressed as: 𝐽𝑖 = 𝛼𝑖

𝑉2𝑖 2𝑔

(11)

αi: Coefficient of singular pressure drop characterizing the particular point and the friction force, Vi: Flow velocity through the hydraulic attachment m/s, g: gravity (ms-2), d1: Diameter (m). By applying the Bernoulli theorem for every pipe section, all the needed parameters for the hydraulic network conception could be deduced. The total energy for running the water network is necessary to compensate for the total pressure drop inside the pipes and transport the water from the source up to the highest altitudes in the sector to water. To guarantee the arrival of water to all consumers, the pumping station must provide at least the following pressure: 𝐻𝑡𝑜𝑡−𝑝𝑒𝑒𝑠𝑠 = ∆𝐻𝑎𝑙𝑡 max + ∑𝑛 𝑗𝑛 𝐿𝑛 + ∑m Jm stor

(12)

∆𝐻𝑎𝑙𝑡 max : Difference in altitude between the water storage tank and the stor point with the highest altitude in the sector.

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Then, the total power that the pumping station must provide to deliver water for the irrigation system is expressed by: 𝑃𝑟𝑢𝑛−𝑖𝑟𝑟𝑖𝑔−𝑛𝑒𝑡 =

ρgQirrig (∆𝐻𝑎𝑙𝑡 max +∑𝑛 𝑗𝑛 𝐿𝑛 + ∑m Jm ) stor

Rpump

(13)

The total power that the pumping station must provide to deliver water for the rural agglomeration is expressed by: 𝑃𝑟𝑢𝑛−𝑑𝑟𝑖𝑛𝑘−𝑛𝑒𝑡 =

∑ ρgQdrink (∆𝐻𝑎𝑙𝑡 max stor +∑𝑛 𝑗𝑛 𝐿𝑛 + m Jm ) Rpump

(14)

The total power that the pumping station must provide to deliver water for the industrial sector is expressed by: 𝑃𝑟𝑢𝑛−𝑖𝑛𝑑𝑢𝑠−𝑛𝑒𝑡 =

∑ ρgQindus (∆𝐻𝑎𝑙𝑡 max stor +∑𝑛 𝑗𝑛 𝐿𝑛 + m Jm ) Rpump

(15)

Prun-irrig-net: Necessary power for the running of the irrigation network. ρ: density of the saltwater. Qirrig: Debit (m3/s), which represents the amount of water needed by plants, and Rpump: Pump yield.

2.6. Design and Size the Solar Energy Farm with Photovoltaic Technology The total energy that the solar power farm must produce represents the sum of power needed to operate each regional hydraulic network, including an irrigation scheme, potable water supply network, and industrial water supply network. In the preceding paragraph, each hydraulic network’s total energy is formulated. Different formulas to size all components of the solar farms have been proposed (Shinde and Wandre 2015; Yorkor and Leton 2017; Sharma et al. 2019; Tola1 et al. 2020). The solar power farm, formed from a photovoltaic panel, is characterized by its capacity to convert the solar radiation received by m² of surface to electrical power (Guzmán et al. 2018, Bermejo et al. 2019, Al-Ali et al. 2020, Bolwig et al. 2020). The power needed for each project could be calculated (irrigation system, drinking water network, industrial water network) after

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deducing the peak power and the panel surface (Kazem et al. 2015; Hossain et al. 2015; Kazem et al. 2017; Margeta and Durin 2017; Odesola and Bright 2019; Imjai et al. 2020). The solar radiation received in the zone of the project could be determined. For the irrigation network 𝑃𝑝𝑒𝑎𝑘−𝑖𝑟𝑖𝑔 =

Prun−irrig−net 𝐸𝑠𝑢𝑛 ×𝐹𝑐

(16)

For the drinking water network 𝑃𝑝𝑒𝑎𝑘−𝑑𝑟𝑖𝑛𝑘 =

Prun−drink−net 𝐸𝑠𝑢𝑛 ×𝐹𝑐

(17)

For the industrial water network 𝑃𝑝𝑒𝑎𝑘−𝑖𝑛𝑑𝑢𝑠 =

Prun−indus−net 𝐸𝑠𝑢𝑛 ×𝐹𝑐

(18)

Ppeak-irrig: Peak power for the irrigation system, Ppeak-drink: Peak power for the drinking water system, Ppeak-indus: Peak power for the industrial water system, Esun: Daily sunshine in the region, Fc is the Corrective factor of the system. The number of panels to install depends on the power delivered by one panel. For the irrigation system 𝑁𝑡𝑜𝑡−𝑝𝑎𝑛𝑒𝑙−𝑖𝑟𝑟𝑖𝑔 =

𝑃𝑝𝑒𝑎𝑘−𝑖𝑟𝑟𝑖𝑔 𝑃𝑜𝑛𝑒−𝑝𝑎𝑛𝑒𝑙

(19)

For the drinking water system 𝑁𝑡𝑜𝑡−𝑝𝑎𝑛𝑒𝑙−𝑑𝑟𝑖𝑛𝑘 =

𝑃𝑝𝑒𝑎𝑘−𝑑𝑟𝑖𝑛𝑘 𝑃𝑜𝑛𝑒−𝑝𝑎𝑛𝑒𝑙

(20)

For the industrial water system 𝑁𝑡𝑜𝑡−𝑝𝑎𝑛𝑒𝑙−𝑖𝑛𝑑𝑢𝑠 =

𝑃𝑝𝑒𝑎𝑘−𝑖𝑛𝑑𝑢𝑠 𝑃𝑜𝑛𝑒−𝑝𝑎𝑛𝑒𝑙

(21)

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Ntot-panel-irrig: Total number of panels to install for the irrigation system, Ntot-panel-drink: Total number of panels to install for the drinking system, Ntotpanel-indus: Total number of panels to install for the industrial system, P one-panel: Power of panel given by the manufacturer. The total surface occupied by solar farms is deduced from: 𝑆𝑡𝑜𝑡−𝑠𝑜𝑙−𝑓𝑎𝑟𝑚 = 𝑁𝑡𝑜𝑡−𝑝𝑎𝑛𝑒𝑙 𝑆𝑜𝑛𝑒−𝑝𝑎𝑛𝑒𝑙

(22)

Stot-sol-farm: Total surface occupied by the solar farm, Sone-panel: Surface of one panel given by the manufacturer.

3. Case Study from Tunisia The case study for the conceptualization and sizing of a solar power farm to supply electricity or the functioning of irrigation systems is performed in the south Tunisian Oasis: Nefzawa Oasis (Jemna and Rejim Maatoug). In the South of Tunisia, most of the oases in the south are from small private gardens considered familiar fishponds in which an essential diversity of original plants was found (Sellami 2008; Sellami 2020, Sellami and Nasri. 2021). The scarcity of water resources is menacing the plant productivity inside the oasis from the point of view of quality and quantity. That is why there is an orientation toward introducing localized irrigation to save water. The running of those systems needs an actual amount of energy, increasing their functioning costs. That is why using solar energy to provide the needed electricity could decrease the global cost of an irrigation project inside an oasis and increase farmer incomes. A case study’s results, by applying the developed approach presented above to conceptualize and size a solar farm, are presented. This will later supply the necessary energy to run a localized irrigation system inside the Nefzawa oasis (Tiba et al. 2020a; 2020b). A pilot station in Nefzawa oasis (Kibili) is considered for the project context “Sustainable development of oases.” The work has been performed in collaboration with the partners’ agricultural extension and training agency (AVFA), Development Office of Rjim Maatoug (ODRM), Regional Center of Oasis Agricultural (CRAO), and Institute for Italian University Cooperation (ICU) (Tiba et al. 2020a; 2020b, Tiba et al. 2022). Date palm farm is fitted with a localized irrigation system and installed the necessary probes to follow the humidity in the soil, the climatic parameters, and certain physiological indicators. After calculating the water

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needed by plants, the irrigation network and solar power farm have been sized. The evolution of the water required for the oasis canopy for the two pilot sites was calculated. The principal results are in the following figure. The need for irrigation for the two pilot plots is maximum during production. It varies from 15 mm in November to 178.6 mm in July. Based on the variation in the need for irrigation water, it is possible to size the irrigation system and conceptualize the solar power farm. The total pressure drop is about 35 m, the necessary pump debit is 4 l/s, and the pump power is 1.9 kW. By considering the power of one panel and its surface area, sixty panels with a total surface of about 100 m² are sufficient to meet the power requirement of the project. Figure 2 presents a photo of the solar power farm installed to run the irrigation system inside Nefzawa oasis.

Figure 1. Monthly evolution of the irrigation water need (mm/j) for the stations Jemna and Rjim Maàtoug Kébili.

Figure 2. Photo of solar power farm inside Nefzawa oasis.

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3.1. Conception and Sizing of a Solar Power Farm This section discusses the concept and size prediction of the solar farm to Supply Electricity for the Functioning of a Potable Water Supply System for a Rural Agglomeration in Kasserine Governorate in Tunisia. Internationally, the effects of the problem of scarcity of water sources are apparent and more glaring in rural zones. Slowing down in the speed of implementing new agricultural and industrial projects in these zones has been recorded. Also, there are no important hydraulic networks or structures. Hence, the lack of sanitation and hygiene in many rural agglomerations has been realized. Consequently, many sanitary and social impacts appear, accelerating the rate of both internal (from villages to towns in the same country) and external (between countries) immigration. Presenting methods and models to conceptualize and size efficient hydraulic networks using solar energy is extremely helpful in minimizing and working around the problems mentioned above. In this part, prediction results to supply potable water for a rural agglomeration in Tunisia using solar energy are presented.

3.2. Geographical, Demographical, and Water Consumption Characteristics in the Project Area The project zone is called (Ouled Halel). It is situated in the Midwest of Tunisia. It is part of the Wassaiya sector. It belongs to the Sbeitla Delegation of the Governorate of Kasserine. Access to this area is via the regional road GP13 connecting the town of Kasserine to the delegation of Sbeitla, up to the town of Charayea. The climate is cold in winter and hot in summer, particularly from mid-July to mid-August. The average monthly temperature recorded is about 16.4°C. The average of the hottest month (July) reaches 26.3 mm. In the coldest month (January) mean temperature is 8.4 °C. The annual precipitation is close to 297.5 mm. The relief of the project area is very rugged. The altitudes of the natural terrain of the different population groups vary between 675 and 787 m. There are fifteen population groups (locality) in the project zone. The total population in the project area is about 1831 inhabitants, spread over 416 homes and fifteen localities (Nasri 2019). The following table details the population distribution, their numbers, and the location’s altitude. The localities are too geographically dispersed. It is also clear that the altitudes are variable, and the natural terrain is very rugged. These findings are

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particularly important when searching the optimal direction of the hydraulic scheme to supply water to all houses with minimum energy cost. The statistical analysis shows that there is no evolution in the global population. Indeed, until the end of the project in 2039, and after the calculation, the total population will keep the value of 1831. The specific rural domestic water consumption is estimated at 50 l/day/inhabitant for the year 2019. An increase in this consumption of 1.0 l/year/inhabitant until the end of the project will be planned to consider the progress in hygiene. Hence, for the year 2024, the specific rural domestic water consumption is about 55 l/day/inhabitant, about 60 l/day/inhabitant for the year 2029, about 65 l/day/inhabitant for the year 2034 and about 70 l/day/inhabitant for the year 2039. The hourly peak water consumption for each location was calculated. It is changing from 5.4 l/s in 2019, to 5.94 l/s in 2024, to 6.48 l/s in 2029, to 7.03 l/s in 2034, and finally to 7.57 l/s in the year 2039. The above-mentioned values and the altitudes of each locality, the potable water network, and the total energy for its running could be predicted. Table 1. Population distribution, their numbers, and the altitude of each locality in the year 2018 Location El frahtiya Ennjajriya El abadliya Ennawaouiya El mahamdiya El hafainiya Eswadguiya El masaadiya El guawasmiya El abaidiya Jaouiethour Ejwamaaiya El marifag El barahmiya Ouledabbes TOTAL

Number of houses 62 35 28 58 13 31 17 21 9 26 43 14 21 16 22 416

Number of inhabitants 239 158 134 255 68 151 79 79 41 92 180 82 78 89 106 1831

Altitude in m 723-718 745-735 787-774 760-745 778-771 760-752 745-736 730-725 726-711 721-718 725-720 738-732 742-707 690-675 675-700

Two drillings are water resources in the study area (Ouled Halel delegation). They present the following characteristics:

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Drilling n° 1 in the year 2000. Geographical coordinates:   

X = 08° 57’ 40.13” Y = 35° 17’ 13.68” Z = 719 m NGT

Hydraulic characteristics are:    

Drilling flow: 10 L/s Drawdown: 4.2 m Static level: -170.15 m (548.85 m NGT) Dynamic level: -174.35 m (544.65 m NGT)

Drilling n° 2 in the year 2014 Geographical characteristics are:   

X = 08° 57’ 59” Y = 35° 17’ 1.7” Z = 702 m NGT

Hydraulic characteristics:    

Drilling flow: 5 l/s Drawdown: 5 m Static level: -156 m (546 m NGT) Dynamic level: -161m (541 m NGT)

The fictitious continuous flow for the project horizon is 3.15 l/s. The water flow is provided by the two. Boreholes are around 15 l/s. So, the water resources are sufficient to support the water needs of all localities in the project area.

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3.3. Hydraulic and Energetic Characteristics of the Drinking Water Network A seventy-five m3 capacity semi-buried tank is used to assure the water needed by the population in the project area up to the year 2039. The potable water network is formed into two parts. The first allows bringing water via discharge pipelines from the two boreholes to a semi-buried tank. The second is the distribution network. It permits to conduct of water from the semi-buried tank up to each house in each locality. Table 2 recapitulates the calculated hydraulic parameters for the drinking water network: Table 2. Hydraulic parameters calculated for the drinking water network

First discharge pipeline (from the first borehole up to the semi-buried tank) Second discharge pipeline (from the second borehole up to the semi-buried tank) Distribution pipelines

Diameter mm/bar 110 /10

Length (ml) 3485

110 /10

3770

160 / 10 110 / 10

4279 23750

The total length of discharge pipelines is about 7255 ml, while that of the distribution pipelines is about 28029 ml. The following figure presents the scheme of the potable water network for the project area.

Figure 3. Scheme of the hydraulic network to supply water to all localities.

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The total manometric head for all the networks is about 308 m, and the total pumping power for the two drillings is about 37.87 kW. Hence, about 378.7 kWh for a pumping time of 10 hr. The energy reserved for a 6 hr lighting is about 26.4 kWh. So, the total number of solar panels supplying the necessary energy is five hundred.

Conclusion With global environmental challenges (greenhouse gas emission, intensive inundation, prolonged drought, scarcity of water resources, exhausting fossil energy resources), specialists in many scientific fields, experts at the international level, and decision-makers are extremely worried about the water and food security of humankind. They are interested in proposing solutions based on scientific tools and mathematical models to conceptualize hybrid systems using renewable energy to supply water to all sectors (irrigation, domestic, industrial). Basic equations of the model for conception and the calculus of solar farms are presented, assuring the necessary energy for running hydraulic networks in rural zones (potable water supply and irrigation). The modeling approach is composed of three parts. The first model permits estimating the evolution of the amount of water the consumers need. The second is reserved for calculating the hydraulic parameters of the networks and the energy they need. The third model determines the size of the solar farms. The developed models are applied to conceptualizing a solar power farm destined to assure the functioning of a localized irrigation system for Nefzawa oasis (Kibili Governorate in the South of Tunisia). This oasis, like many regions, faces the effects of climatic change. It is concerned with global environmental challenges (greenhouse gas emissions, intensive inundation, long droughts, scarcity of water resources, and exhausting fossil energy resources). For each pilot site, the solar power farm occupies a total surface of about 100 m², which means sixty panels to compensate for a total pressure drop of about 35 m and to assure a pump power of about 1.9 kW for a pump debit of 4 l/s. A modeling approach is developed to conceptualize and size a solar power farm that runs a localized irrigation system inside Nefzawa oasis. The specificity of this oasis is that they use traditional irrigation systems for the loss of an essential amount of water by infiltration and evaporation. Also, the energy consumption for those kinds of systems is too high, which represents

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a handicap for farmers in facilitating their agricultural productivity. Pilot stations have been installed in Jemna and Rejim Maatoug as part of the oasis sustainable development project. Extending to other sites is a purpose, and the presented modeling approach permits the necessary analysis. The developed approach is used to size a solar power farm to provide the necessary energy for running a potable water network in a rural locality in the Governorate of Kasserine in the West Center of Tunisia. The total manometric head for all the networks is about 308 m, and the total pumping power for the two drillings is about 37. 87 kW. Hence, about 378.7 kWh for a pumping time of 10 h. The energy reserved for six hours of lighting is about 26.4 kWh. So, the total number of solar panels supplies the necessary energy is five hundred panels.

Acknowledgments For the case study concerning the potable water network, the authors would like to thank all the Director and Technicians of the National Society for Water Exploitation and Distribution of Tunisia for their collaboration. For the case study concerning the irrigation network inside the oasis, the work was conducted within the framework of the project “Sustainable development of oases” with the partners Agricultural Extension and Training Agency (AVFA), Development Office of Rjim Maatoug (ODRM), Regional Center of Oasis Agricultural (CRAO) and Institute for Italian University Cooperation (ICU). We would like to thank all of them.

References Abu-Mahfouz, M., Hamam, Y., Page, P. R., Adedeji, K. B., Anele, A. O. and Todini, E. (2019). Article Real-Time Dynamic Hydraulic Model of Water Distribution Networks. Water 2019, 11, 470; https://www.mdpi.com/2073-4441/11/3/470. Al-Ali, A. R., Al Nabulsi, A., Mukhopadhyay, S., Awal, M. S., Fernandes, S. and Ailabou, K. (2020). IoT-solar energy-powered smart farm irrigation system Journal of Electronic Science and Technology 17(4) 2020, 100017. Bermejo, J. F., Gómez, F. J. F., Olivencia, P. F. and Márquez, A. C. (2019). A Review of the Use of Artificial Neural Network Models for Energy and Reliability Prediction. A Study of the Solar PV, Hydraulic and Wind Energy Sources Appl. Sci. 2019, 9, 1844; https://www.mdpi.com/2076-3417/9/9/1844.

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Bermúdez, J.-R., López-Estrada, F.-R., Besançon, G., Guillermo, V.-P., Torres, L. and Hernández, H.-R. (2018). Modeling and Simulation of a Hydraulic Network for Leak Diagnosis. Math. Comput. Appl. 2018, 23, 70; https://www.mdpi.com/22978747/23/4/70. Bolwig, S., Baidoo, I., Danso, E. O., Rosati, F., Ninson, D., Hornum, S. T. and Sarpong D. B. (2020). Designing a sustainable business model for automated solar-PV drip irrigation for smallholders in Ghana. UNEP DTU Partnership Working paper series 2020 No. 1. Comolet, R. 1963 Experimental Fluids Mechanic. Real Fluids Dynamic., turbomachines. (Book in French language) Masson and CLE editors, French, p. 442 pages. Guzmán, A. B., Vicencio, R. B., Ardila-Rey, J. A., Ahumada, E. N., Araya, A. G., and Moreno, G. A. (2018) A Cost-Effective Methodology for Sizing Solar PV Systems for Existing Irrigation Facilities in Chile. Energies 2018, 11, 1853; https://www. mdpi.com/1996-1073/11/7/1853. Hossain, M. A., Hassan, M. S., Mottalib, M. A. and Hossain, M. (2015). Feasibility of solar pump for sustainable irrigation in Bangladesh. Int J Energy Environ Eng. 6:147-155. Available: http://www.sid.ir/en/VEWSSID/J_pdf/1025120150206.pdf. Imjai T., Thinsurat, K., Ditthakit, P., Wipulanusat, W., Setkit, M. and Garcia, R. (2020). Water Performance Study of an Integrated Solar Water Supply System for Isolated Agricultural Areas in Thailand: A Case-Study of the Royal Initiative Project Water 2020, 12(9), 2438; https://www.mdpi.com/2073-4441/12/9/2438. Jurasz, J., Canales, F. A., Kies, A., Guezgouz, M. and Beluco, A. (2020). A review on the complementarity of renewable energy sources: Concept, metrics, application, and future research directions. Solar Energy Volume 195, 1 January 2020, Pages 703-724. Kazem, H. A., Al-Waeli, A. H., Al-Kabi, A. H. and Al-Mamari, A. (2015). Technoeconomical assessment of optimum design for photovoltaic water pumping system for rural area in Oman. International Journal of Photoenergy, Vol. 2015, pp. 1-8, Article ID 514624 https://www.hindawi.com/journals/ijp/2015/514624/. Kazem, H. A., Al-Waeli, A. H., Chaichan, M. T., Al-Mamari, A. S., and Al-Kabi, A. H. (2017). Design, measurement, and evaluation of photovoltaic pumping system for rural areas in Oman. Environment, Development and Sustainability, 19(3), 1041-1053. Margeta, J., and Durin, B. (2017). Innovative approach for achieving sustainable urban water supply system by using solar photovoltaic energy. Ingeniería e Investigación, 37(1) 58-67. http://www.scielo.org.co/scielo.php?script=sci_arttext&pid=S012056092017000100008. Nasri, A. (2019) A study to conceptualize and to size a potable water supply network functioning with solar energy in Sbitla delegation (Ouled Hlel, Kasserine Governorate). Engineering End of Study Project, High School of Engineering, Medjez El Bab, IRESA, Jendoua University, Tunisia. 120 pages. Nicole, L., Alvar, C. and Petra, S. (2021) Solar for all: A framework to deliver inclusive and environmentally sustainable solar irrigation for smallholder agriculture. Energy Policy 154 (2021) 112313. Odesola, I. F. and Bright, S. (2019). Design of a small scale solar powered water pumping system. Int. J. Engr. Res. & Techn. (IJERT), 8(03): 471- 478, http://www.ijert.org.

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Rakhshani, E., Rouzbehi, K., Sánchez, A. J., Tobar, A. C. and Pouresmaei, E. (2019). Integration of Large Scale PV-Based Generation into Power Systems: A Survey. Energies 2019, 12, 1425; https://www.mdpi.com/1996-1073/12/8/1425. Ramadoni, S. and Indah S. (2020). Planning of Hybrid Micro-Hydro and Solar Photovoltaic Systems for Rural Areas of Central Java, Indonesia. Journal of Electrical and Computer Engineering 2020, Article ID 5972342, 16 p. Sahbani, F. and Sellami M. H. (2020). Experimental analysis of the effect of deficit irrigation on the productivity of a greenhouse tomato crop in Sejnane, Tunisia. International Journal Agriculture, Environment and Bio Research 5, No. 05 Pp: 3044, ISSN: 2456 8643 DOI:10.15413/ajsr.2019.0168. Sellami, M. H. (2008). A scientific guide for agricultural water management and biodiversity conservation inside the North African oasis. Chapter In: Agricultural Water Management Research Trends. ISBN 978-1-60456-159-3. Editor: Magnus L. Sorensen © 2008 Nova Science Publishers, Inc. pp.. 171-212. Sellami, M. H. and Trabelsi (2009). A modelling approach using the geographic information system for decision making in the water sector. Journal Geographia Technica issue 2/2009. Sellami, M. H. (2015) Thermo-economic modeling of renewable energy systems. Case study for wind farm. International Journal of Energy, Technology and Policy Vol. 11 N° 1 pp. 53-67. Sellami, M. H. (2015). Hydraulic : From theory to application. Agricultural Hydraulics: Equations, design and sizing. (Book written in French language) EdLivre, French, ISBN: 978-2-332-98572-9 142 pages. Sellami, M. H. (2018). A Thermo Economic Modeling Approach to Evaluate the Impacts of the Energy Sector: Theoretical Development and Case Study. Advance in Energy Research N° 29, Editors: Morena J. Acosta, Nova Science Publisher ISBN: 978-153613-076-8. Sellami, M. H. (2020). Quantitative and qualitative analysis of soil-plant-atmosphère interactions inside oases. Basic tools for modeling radiation, energy and mass exchanges inside mixed cultures. Case studies for the oases of North Africa. Book (written in French language), European University Edition., 145 pages, ISBN. 678620- 2-53578-6. Sellami M. H. and Nasri N. (2021) A quantitative analysis of evapotranspiration above and inside the traditional oasis in the South of Tunisia. Vie et milieu - Life and environment, 2021, 71: 19-27. Sharma, E., Khatiwada, N. R, and Ghimire, A. (2019). Design of Micro Water Supply System Using Solar Energy. 5. 8-17. https://www.researchgate.net/publication/ 338111842_Design_of_Micro_Water_Supply_System_Using_Solar_Energy. Shinde, V. B. and Wandre, S. S. (2015). Solar photovoltaic water pumping system for irrigation: A review. African Journal of Agricultural Research Vol. 10(22), pp. 22672273, https://academicjournals.org/journal/AJAR/article-abstract/18B317C53312. Tiba, H., Sellami, M. H., Dhaouadi, L.(2020a) Conception and sizing of a solar power farm for the running of localized irrigation systems inside Nefzawa oasis. The 11th International Renewable Energy Congress (IREC 2020)978-1-7281-55722/20/$31.00 ©2020 IEEE.

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Tiba H., Dhawadi, L. and Sellami, M. H. (2020b) Modeling approach for the analysis of the effectiveness of the micro-jet irrigation technique in the oases of Nefzawa. Equations and validation. International Journal of Water and Environmental Sciences and Techniques, Vol. 5 N°2 December – 2020, pp.55- 61; ISSN Online: 1737-9350 ISSN Print: 1737-6688. Tola1, A. F., Odugbose, B. D., Olatunde, O. B., Adeyemi, H. O., Adetifa, B. O. and Babalola, A. A. (2020). Design of solar powered water pumping system for irrigation in rural farm dwellings. Trends in Science & Technology Journal 5 No. 3 pp. 956 – 962. Tiba H, Sellami M. H., Dhaouadi L. 2022. A quantitative analysis of environmental and socio-economical impacts of introducing localized irrigation systems inside Nefzawa oasis. Agrociencia Journal Vol. 56 (no 7, Year 2022. Yorkor, B. and Leton T. G. (2017). Solar water supply for rural communities in rivers state, Niger Delta of Nigeria. International Journal of Energy and Environmental Research 5, No. 2, pp. 1-17.

Chapter 4

State-of-the-Art Analysis Methods of Solar Vortex Power Generation Technology Ali M. Tukkee1,2, Hussain H. Al-Kayiem3 and Syed I. U. Gilani2 1Air

Conditioning and Refrigeration Techniques Engineering Department, University of Warith Al-Anbiyaa, Karbala, Iraq 2Mechanical Engineering Department, Universiti Teknologi PETRONAS, Perak, Malaysia 3University of Technology, Iraq

Abstract Solar vortex power generation is a technology that uses a hot air stream produced from a solar collector to create a vortex field that can operate a wind turbine and generate electricity. The solar vortex power plant is a young technology requiring more investigation to understand fully. This paper reviews various methods used to study this technology and introduces possible methods that can be adopted in future studies. The paper reviews several proposed models of solar vortex systems and then presents the methods used in analyzing each system. The methods presented are experimental systems, CFD simulations and mathematical models. Certain unadopted methods are also proposed for future studies to help better understand the capability of these systems. Some methods that can improve our knowledge of solar vortex systems are highlighted by analyzing their outcome with a comparable technology: the solar chimney power plant. Discussions regarding the turbine unit type and 

Corresponding Author’s Email: [email protected]; [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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Keywords: solar thermal, solar updraft, exergy analysis, thermodynamic power cycle, turbine unit

Abbreviations AVE CFD SCPP SPD SVE SVPDS SVPG

Atmospheric vortex engine Computational fluid dynamics Solar chimney power plant Solar power plant with short diffuser Solar vortex engine Solar vortex power desalination system Solar vortex power generation.

1. Introduction Solar energy can be considered the major renewable energy source capable of replacing fossil fuels and fulfilling the world’s energy demand for the future. The technologies available for solar energy as a power generation source are numerous and continue to be developed. One such technology is solar updraft power generation. This technology can be divided into two methods depending on the same principle. The first method is the solar chimney power plant (SCPP), which has been studied extensively by many researchers in the past few decades. The validity of the SCPP has already been proved through a prototype power plant built in 1982 in Manzanares/Spain. Valuable data were collected from the Manzanares power plant, which set the ground for many researchers in this field (Haaf, 1984; Haaf et al., 1983). The second method is solar vortex power generation (SVPG) technology, which is still a young idea, and only a few studies are available. Both technologies use a solar collector to create an updraft stream of air, producing a pressure drop potential that can be used to operate a wind turbine. The SCPP suffers greatly from its dependence on the chimney’s height, representing technical and design challenges when constructing such a high structure. Also, the capital cost of the plant belongs to the chimney structure. The SVPG technology replaces the tall physical chimney structure with an air

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vortex field. Michaud, 1975 was the first to propose using artificial vortices generated from solar energy to produce electricity. After 25 years, Louat, 2000 presented his invention of the unbounded vortical chimney. This solar power plant design directly addresses replacing the solid chimney with a vortex field generated by solar energy. In 2001, one year after Louat presented his invention, Dunn, 2002 presented another solar vortex power plant design named the solar vortex electric power generator. A few years later, Michaud, 2006 presented his design for what he named the atmospheric vortex engine (AVE). The AVE does not necessarily consider using a solar collector as a thermal energy source. The heat source can be solar energy, warm seawater, warm, humid air or industrial waste heat. The solar collector can be the earth’s surface in the solar energy case. The AVE gained more recognition than previous designs and inspired other researchers to start working in the field of SVPG. Other researchers followed the idea presented by Michaud and proposed different vortex power generation systems integrated with solar collectors to be considered new, fully sustainable power generation methods. These systems are few, and publications in this field are still limited, which promotes an investigation of current and possible analysis methods. Some proposed systems were patented, while others were presented through published studies. Table 1 summarizes the patented inventions of solar vortex power plants. Table 1. Patented solar vortex power plant designs Name of invention / Patent Unbounded vortical chimney Solar vortex electric power generator Atmospheric vortex engine Solar power plant with short diffuser Air power generator tower Solar vortex engine

Registration Year 2000 2002 2006 2009 2010 2015

Inventor(s) Norman Louat Michael Dunn Louis Michaud Neven Ninic and Sandro Nizetic Alain Coustou and Paul Alary Hussain Al-Kayiem and Ayad Mustafa

Reviews on SVPG systems were presented, in which the authors focused on the proposed models and the possibility of such systems. Nizetic, 2011 discussed the available mechanical work from convective vortices from a theoretical point of view. He also discussed different experimental vortex generation systems that are not necessarily used for power generation. In the end, the review presented the concept of a solar power plant with a short diffuser (SPD) as a new method for using convective vortices in power

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generation. Nižetić, 2018 also presented a review that discussed the theoretical basis of using convective vortices for power generation. He then discussed the progress in the analysis of vortex power generation systems by reviewing only two of the proposed models, namely, the AVE and SPD. He also reviewed the progress in studying such flow fields from a meteorological point of view. Mustafa et al., 2015 presented another review of the SVPG technology. In their paper, the authors discussed different theories on convective vortices and their capability to produce mechanical work. The paper also proposed the idea of the solar vortex engine (SVE) as a novel renewable energy concept and compared it to the AVE and SPD. Ismaeel et al., 2017b, on the other hand, discussed the concept of convective vortices as heat engines and presented and evaluated the different available vortex power generation systems, whether utilizing solar energy or not. Table 2 summarizes the objectives of past reviews related to the field of SVPG. Table 2. Review papers concerned with the SVPG technology Authors Nizetic, 2011

Nižetić, 2018

Mustafa et al., 2015)

Ismaeel et al., 2017b

Objectives of the study Discussing the available mechanical work from convective vortices from a theoretical point of view. Reviewing the different experimental vortex generation systems but not necessarily used for power generation. Presenting the idea of a solar power plant with a short diffuser (SPD). Discussing the theoretical basis related to using convective vortices for power generation. Evaluating the progress in the analysis of the AVE and SPD. Discussing the progress in studying vortex flow fields from a meteorological point of view. Reviewing the different theories on convective vortices and discussing their capability to produce mechanical work. Presenting the idea of the SVE and comparing it to other vortex power generation systems. Discussing the concept of convective vortices as heat Engines. Presenting and evaluating the available vortex power generation systems, whether utilizing solar energy or not.

Different studies have been presented since the last conducted review. This paper discusses the current progress in the methods used to study SVPG systems so far. No past review has been presented that deals with these methods or the gaps in the field. The current proposed system designs are presented, and the methods used to analyze each design are discussed and evaluated. The lack of certain methods is presented as a proposal for future studies. It should also be mentioned that only studies dealing with vortex

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systems as a solar energy power generation technology are considered, so studies on artificial vortices for meteorological or other purposes are not included.

2. Solar Vortex Power Generation Systems Considered in Past Studies SVPG systems are versatile, and although the basic principle is the same, each design is different and unique compared to the others. This section presents the proposed systems considered in past studies to prepare the reader before introducing the adopted methods and gaps in the field. Figure 1 presents different views of the principle of the AVE presented by Michaud, 2006. The air enters tangentially into the base of a cylindrical structure, producing a vortex field that acts as a dynamic chimney. The vortex would be started by temporarily heating the air near the station’s center with fuel or steam. The pressure difference between the ambient air and the base of the vortex field drives multiple turbines situated at each inlet. The system utilizes heat exchangers at each inlet to help maintain the plant’s operation by sustaining the air vortex. Most of the systems following Michaud’s AVE consider using guide vanes inside the solar collector to help generate the vortex field. Ninic and Nizetic, 2009 proposed a solar vortex power plant design named the solar power plant with a short diffuser (SPD). This system uses guide vanes to generate the air vortex at the center of a large circular solar collector and underneath a transparent cover. The air vortex then leaves the collector tina short diffuser-like structure before exiting the atmosphere. Inside the diffuser structure, water nozzles would be fitted to start the plant and help maintain its stationary operation. A schematic of the system is shown in Figure 2. Another design with the same principle was presented by Zhang et al., 2015. The guide vanes in this system start at the inlet of the solar collector and do not reach the center. The authors also suggested covering the collector’s top surface with solar panels to create a hybrid solar power generation system. The generated vortex field from this system is similar in its properties to the natural phenomena of dust devils. The system mentioned above compared to dust devils is shown in Figure 3.

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Side view

Top view Figure 1. The atmospheric vortex engine (AVE) (Michaud, 2009).

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Figure 2. Solar power plant with a short diffuser (SPD) (Nižetić et al., 2017).

Figure 3. a. Dust devil occurring in nature and b. Configuration of a dust devil-like producing system. (Zhang et al., 2015).

Another SVPG concept presented by Simpson et al., 2012 is shown in Figure 4. This system has an open circular structure with guide vanes on its periphery. Inside the structure, a black-painted circular plate is located, which absorbs solar radiation to heat the air and generate the vortex field. This design is close to the AVE concept proposed by Michaud, stating that the overlying

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layer of air could act as the solar collector. This can reduce the cost of the system significantly, but with less solar energy to be collected.

Figure 4. Concept of the SVPG system presented by Simpson et al., 2012.

The previous systems inspired Mohiuddin, 2015 to present a similar device, as shown in Figure 5. Like the systems shown in Figures 3 and 4, it uses the energy at a certain height of the generated vortex to operate a wind turbine and generate electricity.

Figure 5. SVPG device (Mohiuddin and Uzgoren, 2016).

Zuo et al., 2020 presented the idea of a solar vortex power desalination system (SVPDS). The SVPDS principle is shown in Figure 6. The system’s design consists of a solar collector with guide vanes at the center, while seawater tanks are situated at the bottom of the collector. The seawater absorbs

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most of the solar radiation energy that passes through the collector cover, which raises the seawater temperature causing it to evaporate. The water vapor gets condensed into fresh water on the inner surface of the glass cover, which the water collectors gather. The latent condensation heat released in the condensation process is transferred to the air in the collector through the glass cover.

Figure 6. Solar vortex power desalination system (SVPDS). (Zuo et al., 2020).

A different SVPG system design was presented by Al-Kayiem and Mustafa, 2015, which was named the solar vortex engine (SVE). In this system, the guide vanes are not located inside the solar collector but within a small structure called the vortex generator. This structure replaces the tall chimney in SCPPs to generate a vortex field with a pressure drop potential. The principle of the SVE is shown in Figure 7.

The solar vortex engine

Sectional side view of the vortex generator

Figure 7. The solar vortex engine (SVE) principle (Al-Kayiem et al., 2018).

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Table 3 presents a summary of the SVPG as mentioned above systems. Table 3. A summary of the proposed SVPG systems Designers

System name

Michaud, 2006

Atmospheric vortex engine (AVE)

Ninic and Nizetic, 2009

Solar power Inside the solar plant with a short collector and close to diffuser (SPD) the center

Zhang et al., 2015

Unnamed

Simpson et al., 2012 Mohiuddin and Uzgoren, 2016 Zuo et al., 2020

Unnamed

Al-Kayiem and Mustafa, 2015

Unnamed

Solar vortex power desalination system (SVPDS) Solar vortex engine (SVE)

Location of guide vanes At the periphery of the cylindrical system

Inside, the solar collector starts from the inlet without reaching the center. At the periphery of the system At the periphery of the system Inside the solar collector and close to the center

Unique features The system has multiple inlets at the periphery followed by guide vanes and uses heat exchangers at each inlet to help maintain its operation. A short diffuser structure at the center of the solar collector as well as water sprayers to help generate and maintain the vortex field Solar cells at the collector cover create a hybrid solar system

The system utilizes a central plate with no cover to collect solar energy. The system utilizes a central plate with no cover to collect solar energy. Hybrid power generation and water desalination system

Inside a small structure Vortex generator structure at the located above the center of the system, which houses center of the solar the guide vanes and the wind turbine collector

3. Analysis Methods The SVPG technology is a new renewable energy method, and studies in this type of system are few. The methods adopted to study such systems are discussed in this section.

3.1. Experimental Studies There have been only two experimental setups presented in published studies that use solar energy as a heat source. The first experimental prototype was presented by Simpson et al., 2012, shown in Figure 8. The prototype has a 1.0-

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m-dia. and 0.61-m-height. A black-painted steel plate is located at the center, which acts as the solar collector of the system. Twelve adjustable wooden vanes of 40 cm × 60 cm distributed around the plate guide the air tangentially to generate the vortex column. A 6-bladed wind turbine connected to a generator was used to evaluate the system’s capabilities for power generation. The system was operated indoors and outdoors using a heater and solar energy. The indoor system differs slightly by having Aluminum vanes of 0.35 m × 0.6 m. Electric power is generated by using the vortex’s angular and axial momentum. Data from the small-scale experimental system were coupled with data reported from other studies concerned with convective vortices. Mathematical calculations showed that a system was producing a vortex of 5 meters in diameter with nominal tangential and axial wind speeds of 8 and 11 m/s coupled to a 10-m-dia. vertical-axis turbine could produce 63 kW. Al-Kayiem et al., 2018 presented an experimental model for the SVE discussed earlier. Figure 9 shows the experimental model of the SVE, which has an 8-m-diameter solar collector made from Perspex sheets and inclined by 8.8°. The vortex generator consists of two concentric cylinders; the outer cylinder has a height and diameter of 1 m, while the inner cylinder has a 0.6m-height and 0.9-m-dia. The experimental model was studied without the consideration of a power generation unit. However, it would be installed in the 0.4 m space below the inner cylinder if it is considered.

Figure 8. Solar vortex system prototype, a. Front view, b. Top view. (Simpson et al., 2012).

At the perimeter of the inner cylinder, there are eight entry slots 0.3-mhigh and 0.1-m-wide. A guide vane follows each slot tilted 25° to help generate the air vortex. A top plate with a hole of 0.3 m diameter covers both cylinders and allows the generated air vortex to exit the system into the atmosphere. Another hole of 0.1-m-diameter exists at the bottom of the inner cylinder to allow for an upward air stream to help raise the vortex field. The

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entire vortex generator structure is made from Aluminum, and an insulating layer covers the outer cylinder to prevent thermal energy from being lost to the surrounding atmosphere. The system showed that it could produce a vortex field using thermal energy from the sun, with a tangent-to-axial velocity ratio of 7.5 at the exit from the vortex generator. The SVE was further modified by Ismaeel, 2018 through experimental and numerical studies that changed the design of the vortex generator and the addition of thermal energy storage. Ismaeel removed the lower hole and extended the heights of the guide vanes and entry slots to the top of the vortex generator. This proved to be more efficient as it significantly increased the intensity of the generated air vortex. Other modifications were made by adding a conical structure at the center to help guide the air vortex and fabricating the vortex generator into a secondary solar collector. This was done by replacing the material of the top cover with a transparent Perspex plate and painting the interior of the inner cylinder with black paint. Experimental analysis was performed by Al-Kayiem et al., 2021 to evaluate the best coating material to enhance the heat transfer characteristics of the SVE. They found that using Al2O3 in black enamel paint enhances the photothermic conversion capabilities of the absorbing surface, which they used to enhance the SVE performance.

Figure 9. Experimental model of the SVE (Ismaeel, 2018).

Another small-scale experimental prototype was presented by Zhang et al., 2015 to validate the system presented earlier in Figure 3. It was an indoor system that used heaters to simulate the effect of solar energy. The experimental system and its dimensions are shown in Figure 10. The system consists entirely of a 4 m diameter solar collector, and the guide vanes are located inside the collector. The guide vanes extend to a distance of 1 m from

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the collector’s inlet. Only the middle of the collector’s cover is transparent and has a central hole of 0.1-m-dia. for the exiting air vortex. A cylindrical structure of 0.6-m-diameter and 0.5-m-height is located at the outlet to protect the base of the air vortex from the surrounding environment.

Figure 10. Experimental prototype of the SVPG system. (Zhang et al., 2015).

Table 4 summarizes the experimental prototypes of the SVPG systems. Table 4. Experimental prototypes of the SVPG systems Researchers (Simpson et al., 2012) (Al-Kayiem et al., 2018) (Zhang et al., 2015)

Size Diameter = 1 m Height = 0.61 m Diameter = 8.8 m Height = 1.8 m Diameter = 4 m Height = 0.65 m

Conditions of study Indoor and outdoor Outdoor Indoor

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Several small-scale experimental prototypes based on the concept of the AVE were developed. No published studies were presented for these prototypes, but information regarding these systems is available online (vortexengine.ca, 2022). These systems use thermal energy from burnt fuel to help generate the air vortex column. Some of the experimental prototypes based on the AVE are shown in Figure 11.

Figure 11. Experimental prototypes based on the concept of the AVE (vortexengine.ca, 2022).

3.2. CFD Simulations SVPG systems have been studied mostly by computational fluid dynamics (CFD) methods. CFD tools solve the continuity, momentum and energy equations, alongside any other scalar equations, if specified, to describe the flow field of the studied system. The flow field in these systems is complicated and cannot be evaluated with two-dimensional simulations as the vortex field moves in a rotational motion in a three-dimensional flow. Natarajan, 2011 presented a CFD simulation study for the AVE presented by Michaud. The study considered a small-scale indoor prototype and a largescale outdoor plant. The effect of crosswind was considered for the large-scale plant and was found to have no major effect on the generated air vortex. The study also confirmed the possibility of the vortex field replacing the role of

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the physical chimney, as the air temperature was maintained till the top of the computational domain. The SVE system was investigated by many researchers using CFD tools. Mustafa, 2015 used CFD simulation to investigate the vortex field generated inside the vortex generator, which would be hard to investigate experimentally. The effect of the upper cover hole diameter was also investigated by Al-Kayiem et al., 2016, considering different cases. Ismaeel et al. 2017; Ismaeel et al., 2017a presented CFD simulation studies for the same system considering the inner cylinder of the vortex generator with a small exterior structure. They focused on the system’s operating conditions, such as air velocity, temperature, and geometrical modifications. Das and Chandramohan (Das, 2020; Das and Chandramohan, 2020) presented more simulation studies for the SVE. They concentrated on further geometrical parameters of the vortex generator, such as the diameter of the upper hole and the number of entry slots. Like previous studies, they only considered the inner cylinder and added a 1-m-height cylindrical bounding structure to study the generated vortex extensively. They found that the original design of 8 entry slots and an upper hole of 0.3 m, proposed by AlKayiem et al., 2018, gives the optimum air velocity and pressure drop performance. Another CFD simulation for the SVE was presented by Tukkee et al., 2021, which considered both the inner and outer cylinders of the vortex generator. The authors investigated the effect of air humidity on the system’s performance. They advised that further studies on SVPG systems should consider air humidity as the system’s performance is affected by this factor. Its consideration might increase the accuracy of the results obtained from these studies. Zhang et al., 2015 presented a CFD study for their previously mentioned device, shown in Figure 3, which generates a dust devil-like vortex field. Mohiuddin and Uzgoren, 2016 presented a CFD simulation to investigate the effects of vane width, vane height, number of vanes and vane angle on the device performance shown in Figure 5. Another CFD simulation was presented by Nižetić et al., 2017 to prove the capability of the SPD system to produce a vortex field with a pressure drop potential that can be used to operate a wind turbine. Penga et al., 2019 evaluated the best guide vane topology for the SPD and its effect on the shape and stability of the vortex field. Both studies for the SPD did not consider the turbine unit or the water nozzles proposed in the original design. Finally, Zuo et al., 2020 used CFD simulation to study the SVPDS system without considering seawater tanks. Table 5 shows a summary of the CFD simulation conducted for SVPG systems.

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Table 5. A summary of the CFD simulations presented for SVPG systems Authors

System

Natarajan, 2011 AVE

Mustafa, 2015

SVE

Al-Kayiem et al., 2016 Ismaeel et al., 2017a Ismaeel, A. et al., 2017

SVE SVE SVE

Das and SVE Chandramohan, 2020 Das, 2020 SVE Tukkee et al., 2021 Zhang et al., 2015 Mohiuddin and Uzgoren, 2016

Nižetić et al., 2017 Penga et al., 2019 Zuo et al., 2020

Software used Fluent

ANSYS Fluent ANSYS Fluent ANSYS Fluent ANSYS Fluent ANSYS Fluent

ANSYS Fluent SVE ANSYS Fluent Unnamed OpenFOAM

Solution Objective of study type Transient Proving the concept of the AVE and the possibility of replacing the physical chimney with a vortex field SteadyFor a better understanding of the state properties of the generated air vortex field SteadyInvestigate the effect of the diameter of state the upper cover hole SteadyTo study the effect of operating conditions state such as inlet velocity and temperature. SteadyInvestigate the effect of the entry slots and state guide vanes’ height on the intensity of the air vortex SteadyStudy the effect of upper hole diameter state and operational conditions on the system Steadystate Steadystate Transient

Study the effect of the number of entry slots on the intensity of the air vortex Study the effect of air humidity on the performance of the system Show the capability of the proposed system in generating a vortex field. Unnamed OpenFOAM Transient To conduct a parametric study for investigating the effects of vane width, vane height, number of vanes and vane angle on the performance of the system SPD ANSYS SteadyTo present preliminary results for the CFX state system showing its capabilities. SPD ANSYS SteadyStudying the guide vane topology effect CFX state on the generated vortex field SVPDS ANSYS SteadyInvestigate the possibility of a hybrid Fluent state power generation and water desalination solar vortex system

The construction of a large-scale solar vortex power plant requires enormous effort and cost to be accomplished. CFD tools provide an easier and cheaper way to study a large-scale plant. Some of the simulation studies discussed earlier have been conducted for small-scale systems. In contrast, others considered a large-scale plant better to understand the power generation potential of such systems. Table 6 presents the general dimensions of the SVPG systems considered in each CFD simulation.

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Table 6. General dimensions of domains considered for CFD simulations of SVPG systems Studies Natarajan, 2011

System AVE

Description and dimensions of the computational domain Two systems were considered:  A small laboratory-size system with a 1.05-m-dia., 0.2-mheight and an exterior flow domain of 2.0 m.  A large-scale system with a diameter of 21.0 m, a height of 4.0 m and an exterior flow domain of 120.0 m. Al-Kayiem et al., SVE A small-scale system with dimensions similar to the 2016); Mustafa, experimental prototype, 1.0-m-dia. and 0.6-m-high, with an 2015 external computational domain of 1.5 m × 1.5 m × 3 m. The vortex arena of the generator is simulated without the solar collector or the lower part of the vortex generator. Ismaeel et al., 2017; SVE A small-scale system with the dimensions of the available Ismaeel et al., 2017a experimental prototype considering the 0.9 m diameter and 0.6m-height inner cylinder of the vortex generator without the solar collector and with an exterior vortex domain of 0.6-m-height. Das, 2020; Das and SVE A small-scale system with the dimensions of the available Chandramohan, 2020 experimental prototype considering the 0.9-m-dia. and 0.6-mhigh inner cylinder of the vortex generator without the solar collector and with a 1-m-diameter exterior vortex domain of 1 m height. Tukkee et al., 2021 SVE A small-scale system with dimensions similar to the experimental prototype. The entire vortex generator is simulated at 1.0-m-dia., 0.6-m-height of the arena and 0.4-m-height of the lower part. Also, an exterior vortex domain of 1.0-m-dia. and 2m-height. Zhang et al., 2015 Unnamed Three cases with solar collector diameters of 4, 40 and 400 m are simulated. Mohiuddin and Unnamed A small-scale system with a heated surface of 1.0-m-dia. and Uzgoren, 2016 upscaling and downscaling with factors ranging from 0.6 to 1.4. Nižetić et al., 2017 SPD Three large-scale cases with a 2025 m exterior domain and solar collector diameters of 200, 600 and 1000 m. Penga et al., 2019 SPD A large-scale system with a 250-m-height of the exterior domain and a solar collector diameter of 200 m.

It is noted that no CFD simulation has been conducted for SVPG systems that consider the influence of the power generation unit on the simulation results and flow field structure. It is an important factor that can be considered in future CFD simulation studies. The power generation unit in the SVPG is further discussed in section 5.

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3.3. Mathematical Models The principle of solar updraft power generation systems depends on fluid mechanics and thermodynamics basics. Only a few studies attempted to derive mathematical models describing the performance of convective vortices used for power generation. Ninic, 2006 analyzed the available power at the solar collector outlet in atmospheric conditions using thermodynamics. He showed that this power potential is larger than that obtained from SCPPs. The humidity of the air was also considered a factor that could increase the power potential of the air in such systems. A suggestion was made to replace the tall chimney of SCPPs with a gravitational vortex column that is not restricted by the construction capabilities of the solid chimney. The concept of the gravitational vortex column was compared to natural tornados to confirm the possibility of creating and maintaining such a flow field. Ninic and Nizetic, 2009 presented an analytical model to study gravitational vortex columns as an alternative to the physical chimney in updraft power plants. Their analysis assumed that the gravitational vortex column consists of three layers, with the third layer being formed by the descending air mass. The authors pointed out that the descending air is very important when studying gravitational vortex columns by comparing their assumptions to past studies on natural tornados and vortex phenomena. Boundary conditions were set for gravitational vortex columns on fluid mechanics and thermodynamic basis. (Nižetić, 2010) improved the elementary three-layer model by considering different previously neglected factors. The result of both works was a set of equations and boundary conditions solved for an analytical case. Nizetic, 2014 developed a mathematical equation to evaluate the pressure drop potential in convective vortices. The mathematical model was validated by comparing field data from previous studies concerned with natural vortex phenomena such as dust devils and tornados. The model showed a pressure potential in a generated air vortex that can generate electricity without destroying the vortex field. Mustafa et al., 2015 developed mathematical models that describe the performance and flow field of the SVE components. The conservation of mass, momentum and energy and the equation of state and vorticity equation were used to develop the mathematical models. Steady-state conditions were assumed for the solar collector, while the unsteady solution was considered for the vortex generator. The results obtained from the mathematical models were validated by comparison with results from the experimental system.

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4. Recommendations for Further Analysis Methods to Study SVPG Systems The SVPG technology is similar in its principle to the SCPP. That technology has been studied extensively and by many methods. One such method that could benefit the SVPG systems is thermodynamic analysis. Thermodynamic analysis for solar chimney systems has been conducted in two methods. First, energy and exergy analyses were used to understand the energy potential of such systems. The second method is developing power cycles that help understand the limitations and produce possible system design parameters. The exergy analysis complements the energy analysis by considering the irreversibilities of the system. This type of analysis can help evaluate the system’s actual efficiency, where it is lacking in performance and how to improve it. Petela, 2009 conducted a thermodynamic study for a simplified model of an SCPP. The study involved three thermodynamic analysis methods; the first method involves energy analysis, which predicts the system’s performance without considering the actual irreversibilities. The second method utilizes the use of exergy analysis. This method considers the effect of the system’s irreversibilities, giving more accurate results for the system’s performance. The exergy analysis requires the specification of a reference state, specified as the ambient conditions of the air surrounding the system. Another method presented by Petela, 2008 was also considered, which uses what is referred to as mechanical exergy or energy. This method theorizes that a substance’s maximum possible work, the exergy, could be achieved at a certain altitude instead of sea level. This occurs when the density of the working fluid differs from that at sea-level conditions. After conducting all these three analysis methods, a parametric study became possible to evaluate the effect of different design and operational parameters on the system’s performance. This outcome could be effective if applied to the earlier SVPG systems. It would require much data that can be gathered through experimental analysis or CFD simulations. No thermodynamic analysis has been conducted for SVPG systems. Yet, this method could solve the difficulty of specifying the best design parameters or the best condition in which the plant should be established. This is because SVPG systems are too complicated in design, and one cannot easily achieve this outcome by other methods. Another thermodynamic analysis that uses the models developed by Petela, 2009 was introduced by (Mehrpooya et al., 2016). In their study, the authors were concerned with energy and exergy efficiency in the climate of

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Tehran city in Iran. (Maia et al., 2013) also used energy and exergy analyses to study the airflow inside a solar chimney to dry agricultural products. This time, the effect of air humidity was considered, which is a significant factor if the method is to be adopted for SVPG systems. Power cycles are the second method for analyzing solar updraft systems with thermodynamic methods. Ideal thermodynamic power cycles have been developed for multiple energy conversion systems. These cycles provide insight into the limitation of certain energy systems and identify the parameters that affect the system’s efficiency. An ideal cycle involves many assumptions that help simplify the processes involved as much as possible and produce relations that cannot be developed otherwise. This makes the estimated values for the system efficiency using ideal cycles exaggerated compared to the actual values. Still, these cycles provide a good evaluation of the system’s limitations and identify significant design parameters. Updraft solar power systems can be compared to heat engines, as they transfer heat from a hot source to a cold sink and produce power. This makes them suitable to be analyzed by thermodynamic cycles. Backstrom and Gannon, 2000 proposed an ideal thermodynamic power cycle for SCPPs. The cycle is similar to the gas turbine Brayton cycle in most aspects, with differences in the compression and expansion processes. These processes involve shaft work in the Brayton cycle, while for the solar chimney, the transferred energy is equal to the potential energy required to raise or bring down the air stream. Also, the energy output from the expansion process is divided between operating the turbine and raising the air through the chimney. The efficiency of the cycle was found to be: 𝜂=

𝑔∆𝑧 𝐶𝑝 𝑇2

(1)

The elevation ∆z represents the chimney height, and T2 is the temperature at the inlet to the solar collector. This relation shows that the cycle efficiency is directly proportional to the chimney height and inversely proportional to the collector inlet temperature. Gannon and von Backstrom then modified the previous cycle, 2000, to include system losses. This allowed a more realistic prediction of the plant’s power output, resulting in a more efficient turbine design. Another thermodynamic power cycle was presented by Michaud, 2000 for atmospheric upward heat convection processes. First, he considered the processes inside a conventional piston-cylinder thermodynamic system. Then,

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the cycle was compared to the gas turbine Brayton cycle, but with replacing the mechanical devices used in the compression and expansion processes with what he called a riser and a downcomer. For each of the previous assumptions, three cases were studied, for which one case considers the working fluid to be moist air. The cycle was then assumed to occur inside a circular tube compared to the solar chimney system. Then, the normal tube was replaced with an annular tube to understand how a convective vortex works. Finally, the physical tube was replaced by centrifugal forces of the vortex field. Michaud, 1999 proposed a vortex power station to replace the solid chimney structure. Although the power cycle presented by Michaud mainly describes upward heat convection processes, it is the closest study to an SVPG thermodynamic power cycle. Yet the cycle did not present a relationship to describe the performance of such systems as the solar chimney power cycle discussed earlier did. Also, a general analysis of these systems cannot be considered as each previously proposed design is unique and could involve a different sequence of thermodynamic processes. It is suggested that attempts should be made to derive thermodynamic cycles for these different systems to investigate the parameters that affect the performance of each power plant design.

5. Turbine Unit Type and Location The turbine unit in the SCPP can consist of a single vertical-axis wind turbine, multiple vertical-axis wind turbines or multiple horizontal-axis wind turbines (Zhou and Xu, 2016). The location and type of the turbine unit have been discussed by researchers, with only one study to be presented with an actual turbine unit. For example, the turbine used in the well-known Manzanares SCPP is a particular type of wind turbine that is similar in its operation principle to the turbines used in hydroelectric power stations (Schlaich et al., 2005). Unlike conventional wind turbines that operate with staged velocity, this turbine converts static pressure to rotational energy. The blade pitch is adjusted during operation to regulate power output according to the variable flow of the air. For SVPG systems, the situation is more complicated as the location and type of the appropriate turbine vary from one system to another. As mentioned earlier, Simpson et al., 2012 added a turbine unit to their system. It comprises a 50-cm-diameter 6-bladed vertical axis single-rotor wind turbine integrated with a generator. The turbine blades are adjustable, and by

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decreasing the angle of the blades, additional power was captured from the axial flow alongside the tangential flow. The SVE design presented by Al-Kayiem et al., 2018 might be the closest to the SCPP in the turbine location. The turbine was proposed to be fitted in the lower part of the vortex generator, where a pressure drop potential could be used to operate it. Das 2020 and Das and Chandramohan, 2020 proposed that the turbine of the SVE should be installed at a certain height above the system where the highest vorticity is obtained. If this suggestion is valid, then a vertical axis wind turbine, like the one presented by Simpson et al., 2012, or a helical wind turbine might be a good choice. Zhang et al., 2015 suggested using a helical turbine at a certain vortex height. They pointed out that the vortex field generated from their system is similar to dust devils in which a low-velocity core region exists. This low-velocity region makes it possible to safely install a helical wind turbine at a certain height along the central axis of the vortex field. Finally, Nižetić et al., 2017 mentioned that the SPD system requires more investigation regarding the turbine unit’s location and the type and number of turbines to be used. If the turbine of the SPD is located inside the short diffuser, it will be bounded like in SCPPs and the original suggestion made for the SVE. A bounded wind turbine is not limited by Betz’s law and can capture more energy from the wind than an open-flow turbine. Aside from the study presented by Simpson et al., 2012, no actual turbine unit has been investigated for the other SVPG systems. It is suggested for researchers in this field concentrate on the turbine unit as there is a huge lack of data regarding this subject. Studying the properties of the generated vortex field can help determine the appropriate type and location of the turbine for each system.

Conclusion This study presented a review of the methods used in analyzing SVPG systems. SVPG systems vary significantly in their designs. Despite this, only two experimental systems that use solar energy were presented and investigated, as two indoor systems that simulate the effect of solar energy by heaters. Other systems based on the concept of the AVE were presented, but they used fossil fuel in their operation and were not included in published studies. Thermodynamic analysis methods were adopted for studying the SCPP, which can be very useful with SVPG systems. Exergy analysis can provide

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knowledge of the actual capabilities of SVPG systems and the parameters that affect the performance of these systems. Also, thermodynamic power cycles can help understand the limitations of these systems and produce expressions for the efficiency of each system. Only one research has been presented for SVPG systems that introduced a turbine unit. Researchers in this field are recommended to concentrate on the appropriate design and location of the turbine unit for each of the models presented by evaluating the properties of the generated vortex field. CFD simulation is a powerful, inexpensive tool that can add to the effect of the turbine unit.

References AI-Kayiem, H., Mustafa, A., 2015. Solar Vortex Engine. Malaysia Patent, PI2015702890. Al-Kayiem, H., Mustafa, A., Gilani, S., 2016. Vortex field simulation and analysis of a solar updraft power engine. WIT Trans. Ecol. Environ. 205, 193-202. Al-Kayiem, H. H., Ismaeel, A. A., Baheta, A. T., Aurybi, M. A., 2021. Performance enhancement of solar vortex power generator by Al2O3-in-black paint coating. Journal of Cleaner Production 316, 128303. Al-Kayiem, H. H., Mustafa, A. T., Gilani, S. I., 2018. Solar vortex engine: Experimental modelling and evaluation. Renewable Energy 121, 389-399. Backstr, V., Gannon, A., 2000. The solar chimney air standard thermodynamic cycle. Coustou, A., Alary, P, August 2010. Air power generator tower, in Patent (Ed.). US. Das, P., 2020. Performance evaluation of solar vortex engine and optimization of number of air entry slots and turbine location. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 1-17. Das, P., Chandramohan, V., 2020. Estimation of flow parameters and power potential of solar vortex engine (SVE) by varying its geometrical configurations: A numerical study. Energy Conversion and Management 223, 113272. Dunn, M., 2002. Solar vortex electric power generator. Google Patents. Gannon, A. J., von Backstro¨ m, T. W., 2000. Solar chimney cycle analysis with system loss and solar collector performance. J. Sol. Energy Eng. 122(3), 133-137. Haaf, W., 1984. Solar chimneys: part ii: preliminary test results from the Manzanares pilot plant. International Journal of Sustainable Energy 2(2), 141-161. Haaf, W., Friedrich, K., Mayr, G., Schlaich, J., 1983. Solar chimneys part I: principle and construction of the pilot plant in Manzanares. International Journal of Solar Energy 2(1), 3-20. Ismaeel, A., Al-Kayiem, H., Baheta, A., Aurybi, M., 2017. Numerical analysis on the influence of inflow guide vanes in A solar vortex power generator. WIT Transactions on Ecology and the Environment 224, 553-563. Ismaeel, A. A., 2018. Development and evaluation of artificial vortex power generator integrated with double solar absorber. Universiti Teknologi PETRONAS, Malaysia.

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Ismaeel, A. A., Al-Kayiem, H. H., Baheta, A. T., Aurybi, M. A., 2017a. CFD modeling of artificial vortex air generator for green electric power, MATEC Web of Conferences. EDP Sciences, p. 02009. Ismaeel, A. A., Al-Kayiem, H. H., Baheta, A. T., Aurybi, M. A., 2017b. Review and comparative analysis of vortex generation systems for sustainable electric power production. IET Renewable Power Generation 11(13), 1613-1624. Louat, N., 2000. Unbounded vortical chimney, in organization, W.i.p. (Ed.). The United States. Maia, C. B., Castro Silva, J. O., Cabezas-Gómez, L., Hanriot, S. M., Ferreira, A. G., 2013. Energy and exergy analysis of the airflow inside a solar chimney. Renewable and Sustainable Energy Reviews 27, 350-361. Mehrpooya, M., Shahsavan, M., Sharifzadeh, M. M. M., 2016. Modeling, energy and exergy analysis of solar chimney power plant-Tehran climate data case study. Energy 115, 257-273. Michaud, L. M., 1975. Proposal for the use of a controlled tornado-like vortex to capture the mechanical energy produced in the atmosphere from solar energy. Bulletin of the American Meteorological Society, 530-534. Michaud, L. M., 1999. Vortex process for capturing mechanical energy during upward heatconvection in the atmosphere. Applied Energy 62(4), 241-251. Michaud, L. M., 2000. Thermodynamic cycle of the atmospheric upward heat convection process. Meteorology and Atmospheric Physics 72(1), 29-46. Michaud, L. M., 2006. Atmospheric vortex engine. Google Patents. Michaud, L. M., 2009. The atmospheric vortex engine, 2009 IEEE Toronto International Conference Science and Technology for Humanity (TIC-STH). IEEE, pp. 971-975. Mohiuddin, A., 2015. Computational analysis of a solar vortex. Middle East Technical University. Mohiuddin, A., Uzgoren, E., 2016. Computational analysis of a solar energy induced vortex generator. Applied Thermal Engineering 98, 1036-1043. Mustafa, A., Al-Kayiem, H., Gilani, S., 2015. A review of convective and artificial vortices for power generation. International Journal of Sustainable Development and Planning 10(5), 650-665. Mustafa, A. T., 2015. Investigation And Evaluation Of Developed Solar Vortex Engine. Universiti Teknologi PETRONAS. Mustafa, A. T., Al-Kayiem, H. H., Gilani, S. I. U., 2015. Investigation and evaluation of the solar air collector model to support the solar vortex engine. ARPN Journal of Engineering and Applied Sciences 10(12), 5309-5319. Natarajan, D., 2011. Numerical simulation of tornado-like vortices. Ninic, N., 2006. Available energy of the air in solar chimneys and the possibility of its ground-level concentration. Solar Energy 80(7), 804-811. Ninic, N., Nizetic, S., 2009. Elementary theory of stationary vortex columns for solar chimney power plants. Solar Energy 83(4), 462-476. Ninic, N., Nizetic, S., 2009. Solar power plant with short diffuser. patent WO2009/060245 A 1. Nizetic, S., 2011. Technical utilisation of convective vortices for carbon-free electricity production: A review. Energy 36(2), 1236-1242.

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Nizetic, S., 2014. Analytical approach for estimating the pressure drop potential in convective vortex heat engines. Transactions of the Canadian Society for Mechanical Engineering 38(1), 81-91. Nižetić, S., 2010. An atmospheric gravitational vortex as a flow object: improvement of the three-layer model. Geofizika 27(I), 1. Nižetić, S., 2018. Carbon-free electricity production from the alternative energy concepts based on the utilization of the convective vortex systems as a heat engines: Review of the current status and perspective. Journal of cleaner production 170, 85-95. Nižetić, S., Penga, Ž., Arıcı, M., 2017. Contribution to the research of an alternative energy concept for carbon free electricity production: Concept of solar power plant with short diffuser. Energy Conversion and Management 148, 533-553. Penga, Ž., Nižetić, S., Arıcı, M., 2019. Solar plant with short diffuser concept: Further improvement of numerical model by included influence of guide vane topology on shape and stability of gravitational vortex. Journal of cleaner production 212, 353361. Petela, R., 2008. Influence of gravity on the exergy of substance. International Journal of Exergy 5(1), 1-17. Petela, R., 2009. Thermodynamic study of a simplified model of the solar chimney power plant. Solar Energy 83(1), 94-107. Schlaich, J. r., Bergermann, R., Schiel, W., Weinrebe, G., 2005. Design of commercial solar updraft tower systems—utilization of solar induced convective flows for power generation. J. Sol. Energy Eng. 127(1), 117-124. Simpson, M. W., Pearlstein, A. J., Glezer, A., 2012. Power generation from concentrated solar-heated air using buoyancy-induced vortices, Energy Sustainability. American Society of Mechanical Engineers, pp. 585-593. Tukkee, A. M., Al-Kayiem, H. H., Gilani, S. I., 2021. Humidity Effect on the Simulation Accuracy of Solar Vortex Engine Performance. Journal of Solar Energy Research Updates 8, 118-129. vortexengine.ca, 2022. Web Source: https://vortexengine.ca. Zhang, M., Luo, X., Li, T., Zhang, L., Meng, X., Kase, K., Wada, S., Yu, C. W., Gu, Z., 2015. From dust devil to sustainable swirling wind energy. Scientific reports 5(1), 15. Zhou, X., Xu, Y., 2016. Solar updraft tower power generation. Solar Energy 128, 95-125. Zuo, L., Qu, N., Ding, L., Dai, P., Liu, Z., Xu, B., Yuan, Y., 2020. A vortex-type solar updraft power-desalination integrated system. Energy Conversion and Management 222, 113216.

Chapter 5

Comparative Evaluation of Computational Simulations of Solar Chimneys Iylia Elena Abdul Jamil1,* and Hussain H. Al-Kayiem2 1Mechanical

Engineering Department, Universiti Teknologi PETRONAS, Seri Iskandar, Perak, Malaysia 2University of Technology, Iraq

Abstract Solar chimneys for power generation are a simple renewable energy technology. Computational methods allow a convenient yet effective way of studying various aspects of this promising green technology for clean and sustainable power generation. The large scale of an effective working Solar Chimney Power Plant (SCPP) has encouraged many researchers to conduct investigations using computational simulation methods. In this article, a comparative review is presented, discussing important computational work that has been carried out on the solar chimney for power generation that pioneered this research area and notable works in the past decade. The review is divided into several parts, including simulations of the overall SCPP system, individual components like the solar collector, and multipurpose SCPPs. Detailed comparisons of the simulation model development approach may be found in this review. The information presented here is a good reference point for existing and new researchers to work on improving the overall SCPP performance via computational simulations.

*

Corresponding Author’s Email: [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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Keywords: energy conversion, solar power production, solar chimney power plant, solar updraft power

1. Introduction Industrial growth within the last few decades has been rapid, which raises concerns about the high energy consumption that it entails worldwide. At the same time, keeping up with this high energy demand has become increasingly difficult. Environmental effects are a concern because fossil fuel, although readily available, has been associated with pollution and climate change. To balance energy demand and environmental concerns, the best possible solution is to reduce dependence on fossil fuels and turn to renewable resources. The solar chimney power plant (SCPP), sometimes known as the solar updraft tower (SUT) power plant, is one of the available renewable energy technologies which promotes the large-scale utilization of solar energy through a simple concept, as shown in Figure 1. Solar radiation passes through the transparent roof of the solar collector and is received by the natural ground or an additional absorber laid on the ground. The absorber heats the cool ambient air that enters the system through the greenhouse effect. Then, the warm air flows to the collector exit and the chimney’s entrance. The density difference between the warm air inside the chimney and the cool ambient air creates buoyancy that acts as the driving force. The updraft airflow is then generated in the chimney due to the buoyancy effect and is used to drive the pressure-staged turbine at the chimney base to generate electricity.

2. Solar Chimney as an Energy Production Alternative In 1982, the first pilot SCPP prototype was successfully constructed in Manzanares, Spain, by the German government and a Spanish utility company. Their goal was to determine the feasibility and verify the physical principle of the plant (Haaf, 1984; Haaf et al., 1983). As shown in Figure 2, the prototype had a 194-m high steel chimney, a 122-m radius collector, and a single vertical axis single-rotor turbine installed 9 meters above the ground level. Between 1986 and 1989, the plant automatically operated for almost 9 hours daily, with a peak power production of approximately 50 kW (Guo et al., 2019).

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Figure 1. Schematic of the solar chimney power plant (SCPP).

Since then, published work has focused on analyzing and enhancing the system’s performance through experiments, mathematical investigations, and computational simulations. Several publications can sum up the progress in this field so far. Too and Azwadi, 2016, give an overall summary of the works produced based on available publications. Their review includes experimental and numerical investigations, advantages and limitations of the SCPP, and touches on a few new proposed designs. Pradhan et al., 2021, focus on analyzing the design and performance of the system and provide insight into the mathematical modeling of each component of the system. Al-Kayiem and Aja, 2016, discuss the important historic progress of the SCPP and provide a comprehensive summary of progress in more recent years up until 2016. Chikere et al., 2011, zoom into the enhancement techniques that researchers have tested and propose their unique enhancement method. Focusing on the thermal energy storage component of the SCPP technology, Ismaeel et al., 2016 have written a comparative critique that serves as a good reference point for improving the thermal energy storage part of the system. More recently, Guo et al., 2019 answered several critical questions related to the performance, limitations, and concerns surrounding the SCPP based on its present research status.

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Figure 2. First SCPP prototype in Manzanares, Spain (Guo et al., 2019).

3. CFD Roles in the SCPP Research and Development Computational Fluid Dynamics (CFD) has become a widely utilized tool for resolving complex fluid flow problems in fluid mechanics and heat transfer. In the context of renewable energy, where much of the technology is still being researched and enhanced before being adopted on large scales, CFD is particularly essential as it allows researchers to develop novel approaches and recognize any potential issues in the early stages. The number of studies published on computational methods adopting CFD programs to predict SCPP performance has rapidly increased (Guo et al., 2014). However, to the author’s knowledge, no review has been carried out to focus solely on this scope. It is crucial that this gap is filled, as the development of numerical models differs from experimental and mathematical models, and the approach may vary greatly. This review aims to clarify, comment and

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compare different numerical simulation approaches found in publications. It also discusses the potential enhancement methods for the different components of the SCPP investigated numerically.

3.1. Targets of Computational Investigations of SCPP Due to the attractive potential of the solar updraft tower concept for power generation, increasing the total system efficiency has been the common goal among researchers in this area. In recent years, the computational investigation has found more work on proposed enhancement methods for the SCPP. Kazem and Chaichan, 2012 implemented the first CFD code simulation to solve the energy and Navier– Stokes equations in a steady state for natural convection using the finite volume method. Pastohr et al., 2004 then conducted one of the earliest investigations specific to the solar chimney using computational methods. They developed a 2D model of the SCPP to study the collector’s temperature distribution and flow fields. Xu et al., 2011 utilized a similar approach, with modifications to the turbine and energy storage layer settings. Ming et al., 2008 compared the performance of the three-blade turbine based on the Manzanares plant with a five-blade turbine. Although the modeling approach may differ (more of this in Section 3), the targets of all captured publications can be categorized into several themes described further in the following subsections. All published work reported on computational simulations of the SCPP, according to the author’s knowledge, can be classified as the following: Complete modeling of the SCPP, modeling of specific components only like the collector or turbine, and modeling of the solar chimney with an added function other than power generation. Earlier computational works were done mainly to analyze fluid behavior in the system strictly based on the Manzanares geometry with no modification, like Sangi et al., 2011. However, such analysis is presumably the base objective of all CFD simulations. Therefore, it is not considered one of the themes described in this section. It is essential to clarify that most published works can be found to target more than one of the objectives, especially as the field has gained more attention in the recent decade. Along with the themes discussed, several works also emphasize the geographical location’s influence on the system performance. However, this scope is not included in this review as it is primarily relevant to experimental investigations.

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3.2. Design Optimization by CFD In achieving higher system efficiency, computational investigations on the SCPP have focused on optimizing the design geometry of the chimney tower and collector (Too and Azwadi, 2016). A resounding conclusion in many publications is that the scale of the system directly impacts its performance (Guo et al., 2019). In other words, a larger collector diameter and a taller chimney will result in higher power output (Al-Azawiey et al., 2017; Hooi and Thangavelu, 2018; Huynh and Nguyen, 2021; Lal et al., 2016). On the other hand, this would translate to a very high cost where the construction is concerned, and therefore, the economic feasibility must be considered. Through a transient simulation, Cao et al., 2018 investigated the accumulated power generation of sloped SCPP with different solar collector angles. Ghalamchi and Ahanj, 2013 kept the ground absorber thickness constant in a 2D model while testing the effects of varied collector slope angle, chimney diameter, and collector inlet height. Ganguli and Deshpande, 2020 concluded that with varying chimney height and collector diameter combinations, the power increases exponentially as the collector diameter increases. Yapıcı et al., 2020 reported optimum chimney height and diameter values when considering both performance and economic factors. They also tested different collector slope angles, height, and diameter combinations. For sloped ground surfaces, Weli et al., 2021 proposed a few collector tilt angles while fixing the total plant height based on actual conditions in Northern Iraq. Other than optimizing the conventional design of the system, investigations have also been done to suggest different configurations of the SCPP that might yield higher performance. Yapıcı et al., 2020 included several different diverging chimney configurations as part of their study. Kebabsa et al., 2021 proposed a new concept of a diverging annular chimney for the SCPP, mainly to reduce tower surface in an effort to reduce cost. Too and Azwadi, 2016 compared the effects of a tapered chimney tower to the conventional design. More details on the enhanced design of the collector and chimney components are discussed in sections 4 and 5.

3.3. CFD Analysis Using Thermal Storage Enhancement By design, the basic SCPP principle relies heavily on the availability of solar energy. The ground absorbs and stores part of this energy as thermal energy. Heat is stored in the absorber when solar radiation is strong during the daytime

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and is released from the heat storage when the solar radiation is weak during the nighttime or on cloudy days. The medium where heat is stored is thermal energy storage (TES). At any location, the hours of low solar radiation, like during nighttime, the SCPP translates into an overall low power output from the system (Cao et al., 2018; Ganguli and Deshpande, 2020; Khidhir and Atrooshi, 2020; Larki et al., 2021). This setback has spurred researchers to enhance the methods of storing the energy from the sun so that the operation of the plant can be extended into the night. Al-Azawiey and Hassan, 2016 compared six different materials and their heat absorption abilities. The best materials were proposed based on their ability to enhance the SCPP performance during the day and extend its performance at night. For practicality, the materials were selected based on their availability in a specific local area, in this case, Malaysia, so they can be used as the ground material under the collector. A hybrid solar chimney was proposed by Cao et al. 2021 that utilizes phase change materials to provide simultaneous electricity and natural ventilation. They identified the most efficient characteristics of phase change material using a comprehensive 3D-validated numerical simulation, including conductivity, melting point, heat capacity, and enthalpy. Méndez and Bicer, 2021 investigated the effects of different heat storage systems on a solar chimney’s power production. They compared several solid and phase change materials in terms of the energy yield and capacity to prolong the system’s power production during the sun’s absence.

3.4. Turbine Configuration and Pressure Drop by CFD The turbine component of the SCPP is an integral part of the system since its performance directly affects thermal energy conversion into mechanical energy. Hence, modeling the turbine is an essential factor in computational investigations. Rabehi et al., 2018 used ANSYS Fluent software to design and simulate the SCPP with the reverse fan model. Mathematical models were written to explain the airflow and heat transfer inside the plant components, including the turbine. The Manzanares model was used as an example of a numerical simulation that includes a turbine. Based on the comparison of their results during turbine operation and in the absence of load, they reported that the effect of turbine pressure drop on the collector efficiency was marginal. However, it had a significant impact on the output of electricity. Guo et al.,

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2014 also confirmed that the effects of the turbine differential pressure on the system’s output were visible and vital. While not all simulation models include the turbine component, some works are dedicated to testing new turbine configurations. Zuo et al., 2021 proposed an air turbine design technique, considering an ideal turbine lowering in pressure for the SCPP, based on the lifting design method for the hydraulic turbine with an axial flow pump impeller. The reported results showed a 30.46% increase in maximum turbine power output compared to the original model and that the proposed turbine was less sensitive to rotational speed. Esmail et al., 2021 conducted a comparative study of three distinct models of the SCPP turbine, including the flow matrix (MTFM), the classic theory of the blade element (BET), and the change in the BET-based model to consider the turbine enclosure conduit. Kasaeian et al., 2017 modeled the SCPP in 3D based on the Manzanares model while considering the effects of turbine blades. Multiple cases were run to show the effects of the turbine rotational speed, turbine blade count, Figure 3, and other geometric parameters like collector diameter and chimney height. Hanna et al., 2016 performed numerical analysis using ANSYS CFX to simulate the flow through the SCPP system’s turbine with a single vertical axis. The publication detailed the turbine design parameters based on an experimental test rig discussed in the same work. Complete with a mathematical model for the turbine, their simulation results were analyzed in terms of pressure drop, turbine rotation speed, air velocity at the chimney, and average air temperature.

Figure 3. CAD models of turbines with 3, 4, and 5 blades (Esmail et al., 2021).

.

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3.5. Ambient Domain Influence on the Simulation Accuracy The SCPP performance is known to rely heavily, if not solely, on the availability of solar energy. To the author’s knowledge, solar irradiance is a key parameter being tested in most published works. Another important influencing parameter is the ambient crosswind, although this parameter is often not incorporated in computational simulations. The reason for this may be obvious: to consider the ambient crosswind effects, the computational domain for the simulation need to include the area surrounding the collector and chimney tower. This translates to a larger domain size and a higher number of cells which might require a high computational cost. However, several published works can be found which modeled the ambient domain surrounding the SCPP as part of their computational domain. Even when ambient crosswind effects are not considered, this approach may be beneficial in capturing the airflow characteristics directly outside the SCPP (Fasel et al., 2013). Shams et al., 2016 found that the flow inside the collector and chimney could be computed more accurately when it was coupled to the computation of the ambient flow outside. This conclusion was drawn when validating their simulations against experimental data from models of different scales. Similarly, Okada et al., 2015 defined the need for the ambient domain surrounding the system to have temperature-controlled surroundings, just like in their experimental setup. Ming et al., 2013 modeled the SCPP with an ambient domain surrounding the system in 3D. Their work aimed to analyze ambient crosswinds’ effects on their proposed modified design of SCPP. Jamil et al., 2020 emphasized that ambient domain inclusion is necessary to capture the natural phenomena more accurately and significantly when the ambient conditions are enhanced by design.

3.6. Hybrid Solar Chimney Simulation As discussed in section 2.2, research has pointed out the need for modifications and enhancements to the conventional SCPP model to ensure continuous operation, especially during nighttime. Apart from improving the energy storage capabilities of the ground layer, researchers have looked into the possibility of benefiting from other readily-available thermal sources to meet this goal. This integrated SCPP is often referred to as a hybrid solar

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chimney. Waste heat can be used to heat the base of the solar chimney’s collector, or it can be used to heat the air flowing inside the chimney. This approach not only enhances the SCPP performance but is a means for waste heat recovery from many industries. Aurybi et al. 2017 and Islamuddin et al. 2013 proposed a hybrid SCPP that benefits from flue or exhaust gases from a nearby gas turbine power plant and enhances the air kinetic energy at the collector and the chimney entrance. Modeled in 3D on ANSYS Fluent, the thermal energy of the airflow is increased in the solar collector of the plant by using hollow rectangular channels situated below the collector cover, as shown in Figure 4.

Figure 4. Proposed SCPP design with hollow rectangular channels under the collector cover for flue gas transport (Aurybi et al., 2017).

Al-Kayiem et al., 2012 proposed a technique dubbed the solar-flue chimney plant to recover the thermal energy of flue gases to enhance the performance of low-temperature SCPP. They developed a 2D model to simulate an inclined modified solar chimney’s thermal and fluid flow processes. An absorber plate is placed between the air and flue gas channels, Figure 5, to enhance heat absorption, while the flue channel is insulated on the other side. It was found that there is an optimum length for the absorber to achieve maximum heat collection efficiency. Pandey et al., 2021 used ANSYS 2019 R1 to analyze the solar chimney with a heat recovery heat exchanger. The heat exchanger design parameters were manipulated to test their influence on the performance system by modeling the heat transfer near the chimney base.

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Aliaga et al., 2021 proposed a modified solar chimney by replacing the classical solar collector array with a heat exchanger located in the middle section of the chimney, Figure 6. They utilized a CFD model scaled to the Manzanares plant dimensions and achieved a higher power density than was reported for that plant. The flat mirrors concentrate the solar energy and heat up a heat exchanger inside the tower. The heated air flows from the bottom to the top of the chimney and sets a wind turbine in motion inside the solar tower.

Figure 5. Proposed solar-flue chimney plant modified collector (Al-Kayiem et al., 2012).

Figure 6. Proposed SCPP design with no transparent collector (Aliaga et al., 2021).

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Table 1. Summary of the literature in this chapter based on basic simulation model geometry and research objectives grouped by chimney height (a) Chimney height of less than 1 m

0.66 0.63 0.914 1.5

 

Hybrid

0.4 0.7 0.741 0.77

Ambient

2015 2016 2011 2021

Turbine

Okada et al. Larki et al. Shams et al. Larki et al.

Simulation objective

TES

Year

Collector diameter (m)

Design

Author

Chimney height (m)

   



(b) Chimney height between 1 m to 20 m

3 3 3 4 6 6 6.3 6.5 8 9 10 10 12 14

6 2.75 3 3 9 6 10 12 7.14 8 10 10 6.6

   

Hybrid

2014 2018 2020 2021 2016 2020 2017 2017 2016 2020 2014 2016 2013 2020

Ambient

Kasaeian et al. Ayadi et al. Yapıcı et al. Aliaga et al. Hanna et al. Khidhir and Atrooshi Al-Azawiey et al. Aurybi et al. Lal et al. Lovchinov et al. Patel et al Too and Azwadi Ghalamchi and Ahanj Jamil et al.

Turbine

Year

TES

Author

Collector diameter (m)

Design

Simulation objective Chimney height (m)

 

  



    



(c) Chimney height between 20 m to 100 m

20

Rahdan et al. Hannun et al.

2021 2018

32.4 100

28.5 (side length) 30 75

Hybrid

2021

Ambient

Esmail et al.

Turbine

Year

TES

Author

Collector diameter (m)

Design

Simulation objective Chimney height (m)

 



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(d) Chimney height based on the Manzanares prototype (~194 m)



 

 







Hybrid

2008 2011 2011 2012 2013 2013 2014

Ambient

Tingzhen et al. Xu et al. Sangi et al. Fasel et al. Islamuddin et al. Ming et al. Gholamalizadeh and Kim Guo et al. Attig et al. Guo et al. Ming et al. Gholamalizadeh and Kim Guo et al. Kasaeian et al. Hu et al. Huang et al. Gholamalizadeh and Chung Allwörden et al. Hooi and Thangavelu Rabehi et al. Ganguli and Deshpande Méndez and Bicer Danook et al. Kebabsa et al. Pandey et al. Weli et al. Ganguli et al. Wang et al. Wang et al. Keshari et al. Zuo et al.

Turbine

Year

TES

Author

Collector diameter

Design

Simulation objective Chimney height

  

2014 2015 2015 2016 2016



The collector diameter is 244 m



2016 2017 2017 2017 2017 2018 2018 2018 2020 2020 2021 2021 2021 2021 2021 2021 2021 2021 2021



     



              

(e) Chimney height above 200 m

1214.4 2000 1100 504.4 2900

    

Hybrid

252.3 500 547 607.2 750

Ambient

2015 2017 2017 2018 2013

Turbine

Cao et al. Ming et al. Cao et al. Ayadi et al. Cao et al.

Simulation objective

TES

Year

Collector diameter (m)

Design

Author

Chimney height (m)

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The reported simulation investigations of the SCPP by CFD have been categorized and classified in table 1. The table is very informative, provides information, and compares the reported simulation works. The works are segregated into four different sections according to the investigated chimney height. It is worth expressing the authors’ opinion on the small models with less than 1 m height as there is a belief that such small models are insufficient to provide logic simulation to the actual thermofluid phenomena inside the SCPP.

4. Modeling of the SCPP in Computational Simulations Most publications on the SCPP computational investigations involve the complete model: collector, chimney and turbine. In this section, we describe in more detail a few key elements of the CFD model development and different approaches taken by researchers in modeling each one.

4.1. Two- and Three-Dimensional Modelling The selection between two- and three-dimensional modeling is one of the earliest steps in developing a computational simulation of the SCPP, which depends mainly on the scope, complexity, and objective of an investigation. The advantage of a two-dimensional model is that it requires much less computational effort to obtain satisfactory results, especially considering the fact that an actual solar chimney system like the Manzanares plant has large dimensions (Huang et al., 2017). Due to the simple geometry of an SCPP, twodimensional simulations are convenient with the assumption that the flow properties are constant in the radial direction. Often adopted with the 2D model is the typically axisymmetric design of the solar chimney, Figure 7, which further reduces the model size for simulation in half (Fasel et al., 2013; Ghalamchi and Ahanj, 2013; Lal et al., 2016). This simplicity of the 2D model also makes it very practical for transient simulations in terms of computing time (von Allwörden et al., 2018; Huang et al., 2017; Méndez and Bicer, 2021). In fact, due to its simplicity, the 2D approach is favored by researchers who look to develop new user-defined functions (UDFs). Huang et al., 2017 overcame the limitation of the standard radiation models available in 2D by developing new functions whose accuracy is comparable to that of the 3D approach. Fasel et al., 2013 simulated the SCPP based on the dimensions of

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the Manzanares tower with an ambient domain inclusion. The simple 2D model includes a large ambient domain (10 times the solar chimney dimensions) surrounding the system, allowing for a realistic flow into and out of the solar chimney system. Aliaga et al., 2021 simulated the natural convection in a solar chimney with a heat exchanger using a 2D model. Their methodology can be an efficient tool for evaluating the effect of modifying the design geometry of the chimney tower on the heat transfer inside the system.

Figure 7. Axisymmetric geometry of the model reduces the computational time (Ghalamchi and Ahanj, 2013).

On the other hand, a three-dimensional model allows for a more realistic simulation of the solar chimney. In terms of dealing with solar radiation, when using ANSYS Fluent, the solar load model’s ray tracing algorithm is only available in the 3D approach (more information on this is discussed in section 3.3). This model can be used to predict the direct illumination energy source that results from incident solar radiation (ANSYS FLUENT 13 User’s Guide 2013). Gholamalizadeh and Kim, 2016 utilized this approach to conduct a parametric study on the SCPP, with simulation results that satisfy experimental data. Attig et al., 2015 proposed an enhancement to the SCPP by adding an external heat source to improve the temperature and air velocity

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inside the system. The 3D model incorporated hollow channels under the collector for hot air to pass through, which causes a heat transfer process between the channel walls and air under the collector. Guo et al., 2014 investigated in detail the effects of solar radiation, turbine pressure drop, and ambient temperature effects on the SCPP with a 3D model based on the Manzanares plant. Validated against real data from the Spanish prototype, this numerical approach successfully modeled the radiation, solar load, and turbine function in the same solution, which can depict the actual performance of the plant closely. Published works have shown that both two- and three-dimensional modeling of the SCPP might have their own advantage. It should be noted that the selected approach affects not only the size, complexity, and results of a developed model but in commercial software like ANSYS. Both methods have their unique influence over the succeeding steps in the simulation.

4.2. Steady-State and Transient Modelling Steady-state analysis has been the preference over transient analysis in this field of research which can be attributed to a few factors. Tahar and Djezzar, 2012 stated that transient solutions in typical realistic situations converge rapidly to a stationary state. Apart from that, with reasonable assumptions made in most analyses, changes in some data, like solar radiation rate, are prolonged over a typical 24-hour cycle. This leads to most work being solved as a few stationary cases throughout a typical day instead of running a transient problem over a long period. A third reason a steady-state analysis may be preferred is that it allows studying the effects of variation in one parameter on overall performance at a specific time while other parameters are kept constant. Numerous published work on the SCPP based on steady-state analysis has proven that this method can provide reasonably accurate results given its practicality. Guo et al., 2013 confirmed that their simulation results done in steadystate were consistent with real data, although they tended to slightly overestimate the power output and the updraft velocity. The reason was that the simulation was conducted for a steady flow, and thermal equilibrium was achieved in the collector for each specified condition. In reality, the soil layer had thermal inertia. The usefulness of transient analysis on the solar chimney is not ignored. Cao et al., 2018 successfully conducted a transient analysis to compare the

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full-year performance of a conventional SCPP and a sloped SCPP. Transient mathematical models are built for the collector, chimney, and power conversion unit (PCU). These are then converted into codes in Fortran programming language and coupled into the commercial software TRNSYS. Here, user-supplied data on the SCPP configuration sizes and simulation period is required for the calculations, while the program will give the boundary conditions and the simulation time control. This work shows that the peak power capacity does not match the peak solar radiation in a typical summer week. Such analysis cannot be made using the steady-state assumption. This time-based analysis also shows the accumulated power generation for both systems throughout the year. Méndez and Bicer performed an investigation to compare the performance of different thermal storage mediums for the SCPP. Several solids and phase change materials were tested initially under steady-state conditions before four top options were tested through a time-based analysis. The results were compared in terms of the system’s power production and heat storage temperature conservation at the end of the simulation period. This work shows that more realistic results can be obtained through transient analysis as solar irradiance affects different thermal storage mediums in a 24hour cycle. Guo et al., 2016 conducted both steady and transient analyses of the SCPP based on the Manzanares plant geometry. The temperature rise at the collector exit between the two models differs mainly when solar irradiance is low. The steady-state model tends to overestimate the temperature in the morning while underestimating the temperature in the afternoon because of the soil’s thermal inertia. Similar trends can also be observed for power output, where the daily electricity generation from the transient model is 31.26% higher than in the steady-state model. This is attributed to soil heat storage’s energy storage and release processes. This significant difference between the performance estimations is of interest to the SCPP economic analysis. Considering the soil heat storage, the transient model can predict electricity production more accurately and provide better reference data for the economic analysis of the SCPP system than the steady model. Sheikhnejad and Nassab, 2021 studied the impact of a vortex generator in a solar chimney. The transient analysis showed that the model with the vortex generator could achieve a steady state faster than the model without the vortex generator. This transient analysis can also investigate natural convection over a time period and airflow patterns around the vortex generator inside the solar chimney.

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It is evident that although steady-state analysis allows for a simpler and cheaper analysis that can produce very good results, transient analysis has its advantages and is particularly important when investigating time-based processes.

4.3. Solar Radiation Modelling In the SCPP, conduction, convection, and radiation heat transfer models occur. Computational simulations of the flow in the system are the simplest when they only focus on conduction and convection. In contrast, those considering the buoyancy-driven flow and radiation models are more complex. Due to this factor, the radiation heat transfer inside the system has rarely been considered in previous numerical simulations (Kebabsa et al., 2021; Layeni et al., 2020; Lovchinov et al., 2020; Larki et al., 2021). In such studies, incident solar radiation on the ground through the semi-transparent collector is commonly treated as an internal heat source or heat flux (Ming et al., 2017; Larki et al., 2021). Hence, various heat flux is assigned to the surface ground as boundary conditions according to different solar radiation intensities. However, through experimentally validated work, Guo et al., 2014 stated that radiation heat transfer remains an essential factor in the greenhouse effect and should be investigated. This is to prevent the overestimation of energy absorbed by the SCPP. Radiation phenomena mainly occur in the collector, covered by different semi-transparent materials such as glass or plastic. The cover materials are nearly transparent for incident solar radiation but partly opaque for infrared radiation from the ground. (Danook et al., 2021). Cao et al., 2011 built a simplified mathematical model according to basic solar radiation, heat transfer and conduction heat theories. In this model, the radiation through the transparent collector is divided into three parts: reflected radiation, radiation transmitted through the glass, and radiation absorbed by the glass. REF 16 simulated the radiation heat transfer on ANSYS CFX by applying two radiation models: P-1 to solve the fluid domain and Monte Carlo, to solve the solid domain as a surface-to-surface configuration. To capture the radiation heat transfer phenomena in ANSYS Fluent, the Discrete Ordinates (DO) radiation model is commonly adopted to solve the radiative transfer equation. The reason for this is threefold: (1) only the DO model can be used to model semi-transparent walls of various types, (2) only the DO model can be used to compute non-gray radiation using a gray band

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model, and (3) the DO model can work well across a full range of optical thicknesses (Bernardes et al., 2003; Xu et al., 2011). The predictions of the maximum turbine pressure drop with the radiation model are more consistent with the experimental data than those neglecting the radiation phenomena in work reported by Guo et al., 2014. Pandey et al., 2021 used this model to investigate the potential power generation in nocturnal and cloudy conditions. Gholamalizadeh and Kim, 2016 utilized the DO model. Their results both show a decline in the updraft velocity and the full output power and are closer to the measured data than previous relative studies. About 96% of solar energy can arrive on the ground, and the ground radiation can barely transmit through the glass to the ambient (Huang et al., 2017). The DO model can capture the greenhouse effect in the collector resulting from the spectral radiative properties of the semi-transparent canopy and energy storage layers, making it a practical, helpful tool for an appropriate simulation of the SCPP. Usually combined with the DO radiation model is the solar load model. Abdeen et al., 2019 utilized the solar load model to simulate the direction and irradiance of the direct normal solar beam entering the computational domain. In ANSYS Fluent, the solar load model included a solar calculator with the option to control the sun’s location for a given time of day, date, and position. In this model, the solar ray tracing algorithm computed the heat flux on the boundary faces using the direction and magnitude of the incident solar radiation. This model is an efficient and practical approach to applying solar loads as heat sources in energy equations. The solar load can be used to model both steady and transient flows. However, it is only available for 3-D simulations. This could explain why many numerical investigations were done in 2-D to treat the incident solar radiation as the internal heat source or heat flux, as stated earlier in this section. However, to retain the simplicity of 2-D simulations and improve the radiation calculations, Huang et al., 2017 have proposed an improved solar radiation model that can be used for 2D simulations in ANSYS Fluent, coded as a User Defined Function (UDF). In their proposal, the collector and the ground are treated as two infinite parallel plates, which reduces the energy exchange in the region to two dimensions. Validated against data from the Manzanares plant, this method achieves similar results to the 3D method with the solar ray tracing model and the DO radiation model with reduced computational efforts.

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4.4. Turbulence Modelling The turbulence model must be considered when discussing SCPP modeling and simulation. Based on the investigation done by Ming et al., 2013, the Rayleigh number, a criterion that can measure the intensity of the buoyancyinduced flow, passed the upper limit transition value of natural convection from laminar to turbulent flow for their simulation based on the Manzanares plant geometry. The majority of previous computational simulations adopted the standard k-epsilon model (Al-Kayiem et al., 2012; Ghalamchi et al., 2013; Hanna et al., 2016; Hooi and Thangavelu, 2018; Huang et al., 2017; Kasaeian et al., 2017; Kasaeian et al., 2014; Layeni et al., 2020; Rabehi et al., 2018; Larki et al., 2021; Sangi et al., 2011; Too and Azwadi, 2016; Wang et al., 2021; Weli et al., 2021; Yapıcı et al., 2020) or RNG k-epsilon model (Abdeen et al., 2019; Attig et al., 2015; Cao et al., 2021; Danook et al., 2021; Gan, 2010; Ganguli and Pandit, 2021; Gholamalizadeh and Kim, 2016; Gu et al., 2014; Huynh and Nguyen, 2021; Layeni et al., 2020; Pandey et al., 2021). This popular selection is justified by some researchers, as can be found in the work of Nasraoui et al., 2018. Five different turbulence models were tested in 2D to study the impact of each model on the airflow characteristics inside the SCPP. Validated against real data (Ayadi et al., 2018; Kasaeian et al., 2014), all the k-epsilon models (standard, RNG, and realizable) show a good agreement in terms of air temperature distribution along the collector radius, with the standard model showing the most negligible error. On the other hand, the maximum error values are recorded for the k− ω model and the transition SST model. It should be noted that the experimental setups (Ayadi et al., 2018; Kasaeian, 2014) used to validate the simulation results are of the small-scale SCPP, with a chimney height between 2 to 3 m and collector diameter between 2.75 to 3 m. Nasraoui et al., 2022 reported a detailed comparison of the same five turbulence models on ANSYS Fluent. It concluded that the realizable k-epsilon is the one that gives the closes result to experimental data. By adopting the Manzanares plant geometry, Fasel et al., 2013 modeled the unresolved turbulence in the SCPP in 2D using the ANSYS Fluent Reynolds Stress Model (RSM). The RSM requires the solution of five additional transport equations: three Reynolds stress components, turbulent kinetic energy, and turbulent dissipation). The RSM provides better accuracy when compared to the other linear two-equation turbulence models (the k−epsilon and k−omega models) but requires a higher computational cost (Shams et al., 2016). The results show good agreement between the different

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models for parameters like air velocity, temperature, density, and mass flow rate. On the other hand, the results for turbulent kinetic energy, static pressure, and maximum available power show a significant difference. Lal et al., 2016 adopted the realizable k-epsilon turbulence model in ANSYS Fluent for a 2D simulation, although no justification was provided in the publication. Esmail et al., 2021 utilized the k-omega turbulence model to simulate a small-scale SCPP in 3D with a turbine. They justified this selection by stating that the k-omega turbulence model is more accurate and robust as it does not involve the complex nonlinear damping functions required for the k-ε model. However, the same authors cannot compare the k-omega and k-epsilon; therefore, this claim cannot be concluded. The same can be said for Aliaga et al., 2021. Kebabsa et al., 2021 simulated an annular SCPP in 3D using the SST komega turbulence model. When the radius of the external tower is large, there is a high potential for separating and secondary flows. The authors stated that this is a common selection for determining flow characteristics near the walls and boundary layer separation. Ganguli and Deshpande used the same model, 2020, which stated that adverse pressure gradients at the chimney entrance make the turbulence model selection critical for the energy dissipation predictions. The authors cited that although the RNG k-epsilon model is good for separated flows, the near wall predictions for adverse pressure gradients need attention in the problem at hand. Their comparison shows a 2-3% difference in air velocity values near the chimney wall between the RNG kepsilon and SST k-omega turbulence models. Table 2. Comparison of simulation models from selected literature adopting the Manzanares prototype geometry based on grid details and radiation and turbulence modeling Author

Year

CFD details Radiation Model -

Turbulence Model Standard k-ε

DO model

RSM Standard k-ε RNG k-ε

DO model DO model

RNG k-ε RNG k-ε

2D/3D

Number of cells

95780 elements Unstated 1732189 elements Fine mesh (0.1 m - 1 m) Unstated Fine mesh (0.05 m - 0.7 m)

Sangi et al.

2011

2D

Fasel et al. Ming et al. Gholamalizadeh and Kim Guo et al. Gholamalizadeh and Kim

2012 2013 2014

2D 3D 3D

2014 2016

3D 3D

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Table 2. (Continued) Author

Year

Sangi et al. Huang et al. Gholamalizadeh and Chung

2011 2017 2017

2D 2D 3D

Hooi and Thangavelu Rabehi et al. Ganguli and Deshpande Mendez and Bicer Danook et al. Kebabsa et al. Pandey et al Weli et al. Ganguli et al. Wang et al. Wang et al. Keshari et al.

2018

3D

CFD details Number of cells Radiation Model 95780 elements 116889 elements Variable >2.28 mil elements DO model (SC) 5.2 mil elements (turbine) Unstated DO model

2018 2020

3D 3D

1.7 mil elements 750000 elements

-

Standard k-ε SST k-ω

2020 2021 2021 2021 2021 2021 2021 2021 2021

2D 3D 3D 3D 3D 3D 3D 3D 3D

Unstated 1000000 elements 552000 elements 178607 elements 5027942 elements 481744 elements 345600 elements 2428738 elements 472000 elements

DO model

Standard k-ε RNG k-ε SST k-ω Standard k-ε Standard k-ε RNG k-ε Standard k-ε Standard k-ε RNG k-ε

2D/3D

Turbulence Model Standard k-ε Standard k-ε RNG k-ε

Standard k-ε

4.5. Computational Domain The ambient conditions around the SCPP are an important factor in its performance. As discussed in section 2.4, several reported investigations have been carried out by modeling the SCPP with a surrounding ambient domain for two reasons: to study ambient crosswind effects and to capture the airflow characteristics directly outside the SCPP. The ambient domain can be modeled in both 2D and 3D approaches. Ming et al., 2012 and Ming et al., 2013 investigated the ambient crosswind effects by placing the 3D SCPP model based on the Manzanares geometry in an imaginary box. Symmetric flow properties were assumed, allowing for a half reduction of the model size, Figure 8, saving computational time. The same method was adopted by Wang et al., 2021, whereby one of the vertical planes was set as an inlet boundary with a pre-set wind speed. At the same time, three outlet boundaries were prescribed with zero gauge pressure (two vertical planes and one plane parallel to the chimney exit) and one symmetry

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plane. Similarly, Larki et al., 2016 and Larki et al., 2021) modeled the SCPP with an ambient computational domain to test the effects of ambient crosswinds combined with a floating chimney. The simulation results were validated against experimental data of the same miniature scale.

Figure 8. A 3D model of the SCPP with the ambient domain included (Ming et al., 2012).

In the investigation based on a 2D model (Fasel et al., 2013; Shams et al., 2016), the authors stated that the ambient domain inclusion eliminates the need to specify boundary conditions at the collector inlet is crucial for the reliability of the results. Dimensions of the total computational domain are typically 10 times the collector radius and chimney height dimensions, which places the ambient boundaries far enough from the collector inlet and chimney outlet. The authors then prescribed zero velocity and a fixed temperature as the boundary conditions. To reduce computing time, refined structured grids are employed inside the chimney and on the collector surface, while coarse unstructured grids are employed in the ambient areas, as in Figure 9. Limited published work can be found on computational simulations of the complete SCPP that model the ambient domain. Shams et al., 2016 state that the ambient domain inclusion can show a more realistic flow pattern into the collector inlet and chimney outlet, even when ambient crosswinds are neglected. Furthermore, this approach estimates the convective cooling of the outside wall of the plant. They compared the same model with and without the ambient domain, and the results show an overestimation of almost 7.8% for mass flow rate and 9.7% for average air velocity.

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Figure 9. A 2D axisymmetric model of the SCPP with an ambient domain ten times larger than the plant size (Fasel et al., 2013).

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Table 3. Comparison between simulation attempts with ambient domain inclusion in the simulation domain Author Shams et al.

Year 2011

2D/3D 2D

Ming et al. Fasel et al.

2012 2012

3D 2D

Ming et al. Larki et al. Wang et al. Larki et al.

2013 2016 2021 2021

3D 3D 3D 3D

Boundary condition Zero gauge pressure and constant temperature Prescribed velocity at one inlet Zero velocity and constant temperature Prescribed velocity at one inlet Prescribed velocity at one inlet Prescribed velocity at one inlet Prescribed velocity at one inlet

Ambient crosswind Not considered Function Not considered Function Constant Constant Function

4.6. Modeling of the Turbine Component In many published works, the modeling of the turbine component has not been included. This is because the no-load condition, excluding the turbine component completely from the model, proves useful when evaluating the potential energy available and the feasibility of the modeled SCPP system. However, more details on the turbine model would be necessary to design a system under real operating conditions. The turbine can usually be treated in two ways in SCPP simulations. The first is by coupling the model with a real turbine model, and the second is by adopting the reversed fan model. The turbine’s detailed configuration is not considered with the reversed fan model. The turbine is treated as an infinitely thin disk, but the effect of pressure drop across the turbine can be conveniently considered using one of the two methods available (Rabehi et al., 2018). The first method is to specify the pressure drop using a constant value. Xu et al., 2011 used this method where the pressure drop across the turbine was between 0 Pa to 480 Pa. Guo et al., 2013 also used this method and set the turbine pressure drop as the independent control variable in the power output control strategies of their numerical simulation of the SCPP. The second method determines the pressure drop value as a function of velocity across the turbine. Guo et al., 2014 utilized this approach in their work and did a parametric study on the values of the pressure jump using the reversed fan model. Zhou et al., 2007 estimated the pressure drop values using the Betz limit. Koonsrisuk and Chitsomboon, 2009 calculated the pressure jump using an iterative approach.

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The above investigations indicated that the reverse fan model could adequately implement the pressure jump across a turbine cross-section at a low computational cost. However, in most cases, a parametric study or iterative calculations are needed to determine the pressure drop value. At this point, few attempts have been made to model the actual turbine in 3D through a computational simulation approach (Guo et al., 2015; Ming et al., 2008). Modeling of the physical turbine should have a considerable effect on the system’s fluid flow and heat transfer characteristics. Furthermore, from a design point of view, airflow distribution through an actual turbine is of great interest. Kasaeian et al. 2017 performed such a study, and they were able to report the influence of the turbine blade count and rotational speed on the mass flow rate or air and turbine torque and power generated. Gholamalizadeh and Chung, 2017 confirmed that details about the influence of a real turbine on the airflow through the SCPP system have not yet been widely reported. Their 3D simulation based on the Manzanares prototype compared the results with the simpler reverse fan model in terms of the pressure drop. They reported that the reverse fan model showed almost the same performance with a lower pressure drop of 43.7%. Based on this, they concluded that estimating a reliable pressure jump value for this model may be controversial since the system performance is significantly affected by this value. Table 4. Summary of selected literature with a focus on turbine modeling Author

Year

Pressure drop

2008

Turbine modeling approach Real model

Ming et al.

Xu et al. Guo et al. Guo et al.

2011 2013 2014

Reverse fan Reverse fan Reverse fan

Constant values Constant values Constant values, Linear polynomial of velocity (for validation)

Hanna et al. Gholamalizadeh and Chung Kasaeian et al. Rabehi et al.

2016 2017

Real model Real model

2017 2018

Real model Reverse fan

Configuration

3 – 5 blades pressure-staged turbine with CLARK Y aerofoil

Constant values

-

6 blades 4 blades (FX W151-A blade) 3 – 5 blades -

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Regardless of the turbine modeling approach, the Manzanares prototype proves to be the main source of reference for its design and dimensions. It is also most commonly utilized in many studies to validate the results of the numerical simulation (Bernardes n.d.; Chikere et al., 2011; de Richter et al., 2017; Zuo et al., 2021).

5. Simulations of the SCPP’s Collector Simulations related to the collector part have been presented in section 3 as part of the full SCPP model. However, the previous discussion is focused on bare collectors. The simulation attempts of enhanced collectors are discussed in this section. Compared with the bare collector, the enhanced models often require a slightly different approach to modeling as the physical changes may influence their computational models. Any changes in the thermodynamic properties will also influence how the heat transfer is modeled during computational simulation.

5.1. Enhanced Collectors by Design Modifications The Manzanares prototype was designed to have a horizontal transparent collector (collector slope angle, θ = 0). Many investigations have been done to test the effect of changing the collector slope angle on the system performance. Keshari et al., 2021 performed one such study, where they tested fifteen different models, each with a different collector slope angle. This modification, however, does not differ greatly from the conventional design where computational modeling is concerned, as the change can be captured in the geometry development and the rest of the simulation process is the same. Wang et al., 2021 modified the Manzanares plant geometry by adding a bank of baffles under the collector to improve the generating efficiency. For their simulation, they used ANSYS Fluent in 3D to compare the models of SCPP with and without baffles. Six new models were proposed, as shown in Figure 10, and the pressure fields, temperature fields, velocity fields, and power outputs of different models under the different baffles were discussed. For such modifications, the researchers performed a mesh dependency study for each model to find the most efficient number of cells that can produce accurate results. The baffles were set as a coupled wall for boundary conditions, while the rest of the model followed the typical setting.

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Figure 10. Different baffles configurations under the collector proposed by Wang et al., 2021.

Figure 11. Proposed blockage can influence the local velocity distribution at the collector inlet in the presence of ambient crosswinds (Ming et al., 2013).

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Figure 12. Instrumentation layout in the proposed model of SCPP with a tracking mirror reflector (Khidhir and Atrooshi, 2020).

Ming et al., 2013 modeled the SCPP with a blockage wall, Figure 11, to reduce the negative effect of ambient crosswind on the system. The wall, or blocker, is designed to be a few meters away from the collector inlet in case of the entrance of ambient crosswind. An ambient domain inclusion is required as part of the model to include the blocker feature in the simulation model. The actual inlet and outlet of the SCPP can no longer be referred to as the computational domain’s inlet and outlet. The researchers built an experimental setup on a small scale to verify their simulation results. More information on ambient domain modeling can be found in section 3.5. Khidhir and Atrooshi, 2020 proposed a modified collector with a tracking mirror reflector, Figure 12, to enhance the thermal concentration effect at the transition zone between the collector and the chimney base. Mathematical analysis of the thermal and flow behavior of the currents under the influence of the concentration effect from the reflector led to the development of a simulation algorithm to assist in the evaluation process. They applied a comparative analysis to investigate the effect of thermal concentration on the airflow velocity under the chimney.

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5.2. Integrated Collectors with Thermal Energy Storage A conventional SCPP relies solely on the energy from the sun. During the hours when solar energy is not available, the performance of the system is disrupted. Many works have been carried out to test the potential of extending the working hours of the SCPP through additional thermal energy storage. The idea is that the added energy storage can replace solar energy and create a continuous operation. The proposed methods might also enhance energy conversion efficiency even during the day.

Figure 13. Test section model to compare the characteristics of different ground materials (Al-Azawiey and Hassan, 2016).

Al-Azawiey and Hassan, 2016 tested different materials to be used as a heat absorption layer on the ground underneath the collector, Figure 13. Validated against an experiment of a similar design, as in Figure 14, performed in Malaysia, results showed that the different materials have different heat storage capacities. The numerical simulations enabled the authors to visualize the velocity field of the working fluid in the system and the temperature field at different times of the day, considering the available solar radiation intensity used in the study.

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Figure 14. Experimental setup used for validation of the simulation results (AlAzawiey and Hassan, 2016).

Méndez and Bicer, 2021 studied several solid and phase change materials as energy storage for the SCPP. It evaluated their effects on the energy yield and capacity to prolong the power output during the absence of the sun. Using COMSOL Multiphysics, they conducted the computational investigation by modeling the SCPP based on the Manzanares plant dimensions and swapping the selected materials as the ground material under the collector to compare the average temperature, power generation, and efficiency for ach one. For their hybrid dual-purpose solar chimney (more information is in section 6), Cao et al., 2021 identified the most efficient characteristics of phase change materials (PCM) using a comprehensive 3D validated numerical simulation. The phase change materials under their investigation were tested for their ability to enhance power generation and ventilation process efficiency.

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Table 5. Summary of selected literature with proposed enhancement technologies on the collector component Author Khidhir and Atrooshi

Year 2007

Enhancement type Design Geometry

Ming et al.

2013

Design Geometry

Al-Azawiey and Hassan

2016

TES

Keshari et al.

2021

Design Geometry

Wang et al.

2021

Design Geometry

Mendez and Bicer

2021

TES

Cao et al.

2021

TES

Details Tracking mirror reflector to enhance the temperature between collector and chimney base Blockage wall on the ground surrounding the collector to reduce adverse effects of ambient crosswinds Comparison between six different materials tested for their performance as thermal energy storage layers on the ground Varied collector slope angles in 15 different models Baffles under the collector roof in six different configurations Comparison between solids and phase change materials for thermal energy storage performance A hybrid and dual-purpose SC combining PCM and PV panel to enhance system performance

6. Simulations of the SCPPs’ Tower The chimney tower in the SCPP is one of the system’s main components. For a long period, the size of the system, which includes the chimney height, has been the determining factor in the system’s ability to produce (Khidhir and Atrooshi, 2020). More recently, optimization works have been done to find novel unconventional designs for the chimney tower. The conventional chimney model is treated as an adiabatic wall regardless of its scale. Yapıcı et al., 2020 tested several modifications to the SCPP design, including the chimney configuration. They compared different heights and diameters of the chimney, then investigated different geometries, Figure 15. The authors emphasized that analyzing the geometry of the SCPP’s tower is considered one of the best ways to increase the overall system’s efficiency. Similarly, Too and Azwadi, 2016 investigated several chimney geometry configurations in their computational simulation of the SCPP based on a 3D model. Patel et al., 2014 investigated different divergent chimney angles with various collector geometry on ANSYS CFX and concluded that an optimized

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configuration could be achieved. In a comprehensive study, Hu et al., 2017 tested the effect of the geometry of divergent chimneys on system performance while varying some geometric elements, including the area ratio for the air inlet section over the air outlet section, the change in the slope of the chimney walls and the scale of the plant. Okada et al., 2015 also developed a simulation model of the SCPP based on a laboratory scale model with a diverging chimney called a diffuser chimney.

Figure 15. Different chimney configurations proposed (Yapıcı et al., 2020).

Concerning diverging chimneys, Kebabsa et al., 2021 reported that the problem of boundary layer separation is not well investigated. The implication of this is a misleading comprehension of the actual effect of the diverging tower on SCPP performance when coupled with a turbine. The researchers investigated an annular, diverging tower as a new model that would overcome eddies creation in the conventional system while simultaneously considering a turbine presence. They modeled the 3D geometry with two symmetry surfaces to reduce computational time. This means only a quarter of the system is simulated, which helps to reduce computational effort.

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Besides the chimney geometry, several investigations have been reported on the floating solar chimney technology. Larki et al., 2016 and Larki et al., 2021 modeled a floating SCPP to investigate its performance at different tilt angles when exposed to external crosswinds, Figure 16. Their simulation results were verified against a small-scale experimental setup. For this purpose, ambient domain inclusion is required to capture the effects of ambient crosswinds on the chimney tilt angles.

Figure 16. Tilted solar chimney model with ambient domain (Larki et al., 2016).

Table 6. Summary of simulation attempts to analyze the system performance by proposed enhancement methods on the chimney component Author Patel et al.

Year 2014

Enhancement type Design modification

Okada et al. Yapıcı et al.

2015 2020

Design modification Design modification

Kebabsa et al. Larki et al. Ganguli et al.

2021 2021 2021

Design modification Design modification Design modification

Details Different divergent angles combined with varied collector slope Diffusor chimney New chimney configurations combining diverging angles with varied heights and diameter Annular and diverging chimney Floating solar chimney Gaps in the chimney as a path of recirculated air

Ganguli et al., 2021 proposed a way to use recirculated air to enhance the power output of the conventional SCPP through the design of two new chimney configurations. Both designs take advantage of the temperature difference between the hot air inside the chimney and the cold ambient to direct cold flow from the atmosphere from the top to the bottom of the

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chimney. They proposed to have this design coupled with other energyproducing devices to maximize its energy output.

7. Simulations of Multi-Function SCPP The workability of the SCPP has been expanded by researchers when the system is designed to have another purpose other than power generation. Layeni et al., 2020 attempted a dual-purpose solar chimney design that can provide ventilation and generate power simultaneously based on works by Barozzi and Imbabi, 1992. The proposed simulation model differs from the conventional SCPP in its fundamental design, where the solar chimney, in this case, is connected at one side to an enclosed space (house) through a cavity, as shown in Figure 17. The collector, which is made of glass, extends to the side of the chimney that faces out. Despite this difference in the geometry modeling, the other steps in the simulation model development follow that of the typical SCPP.

Figure 17. Dual-purpose solar chimney design for power generation and ventilation (Layeni et al., 2020).

Cao et al., 2021 proposed a hybrid solar chimney with a photovoltaic panel and phase change material designed to provide simultaneous natural ventilation and electricity effectively. Their 3D simulations investigate the impact of many effective parameters of phase change material, including

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conductivity, melting point, heat capacity, and its enthalpy, on the solar chimney’s ventilation features and the solar panel’s electrical efficiency. The simulation is performed under transient real weather conditions in Hong Kong for a specified period. Rahdan et al., 2021 combined the solar chimney for power generation with water desalination functions in a hybrid plant model. Adopting the 2D transient model for their simulation on ANSYS Fluent, they validated their model based on existing experimental data, with and without the proposed desalination functions. The authors used the volume of fluid (VOF) approach to model the multiphase flow, where three phases, including water, water vapor, and air, are considered for the desalination section of the plant. At the same time, the other components follow that of the typical SCPP design.

Conclusion Due to its large scale, a feasible solar chimney power plant requires a high capital cost. This has prompted many researchers to investigate the technology using computational simulations. This review discusses the different approaches used by researchers based on the information described in their publications. All simulation results discussed in published works have been validated through comparison with actual data from the Manzanares prototype or other small-scale plants. Computational simulations have been proven to be a handy tool for parametric studies on the SCPP. In most publications, there is an error of up to 10% in the results compared with actual data. However, this does not take away the advantages it provides as a tool to improve the SCPP technology as one of the future renewable energy resources. Apart from that, enhancement proposals have been widely done due to the convenience CFD provides.

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Buildings. Scientific African 8: e00298. DOI: https://doi.org/10.1016/j.sciaf. 2020.e00298 Lim, B. H., and Thangavelu, S. K. 2018. A Parametric Simulation of Solar Chimney Power Plant. IOP Conference Series: Materials Science and Engineering 297(1). DOI:10.1088/1757-899X/297/1/012057 Lovchinov, K., Nitchev, H., Petrov, M., Stoykov, R. and Tyutyundziev, N. 2020. Simulation and Modelling of Small Solar Thermal Plant. Measurement: Journal of the International Measurement Confederation 162: 107944. DOI: https://doi.org/10. 1016/j.measurement.2020.107944 Méndez, C. and Bicer, Y. 2021. Comparison of the Influence of Solid and Phase Change Materials as a Thermal Storage Medium on the Performance of a Solar Chimney. Energy Science and Engineering 9(8): 1274–88. DOI: https://doi.org/10.1002/ese3. 892 Ming, T., Gui, J., de Richter, R., Pan, Y. and Xu, G. 2013. Numerical Analysis on the Solar Updraft Power Plant System with a Blockage. Solar Energy 98: 58–69. DOI: http://dx.doi.org/10.1016/j.solener.2013.02.027. Ming, T., Liu, W., Xu, G., Xiong, Y., Guan, X. and Pan, Y. 2008. Numerical Simulation of the Solar Chimney Power Plant Systems Coupled with Turbine. Renewable Energy 33(5): 897–905.DOI: https://doi.org/10.1016/j.renene.2007.06.021 Ming, T., Wang, X., Richter, R., Liu, W., Wu, T. and Pan, Y. 2012. Numerical Analysis on the Influence of Ambient Crosswind on the Performance of Solar Updraft Power Plant System. Renewable and Sustainable Energy Reviews. DOI: 10.1016/J.RSER. 2012.04.055 Ming, T., Wu, Y., de Richter, R., Liu, W. and Sherif, S.A. 2017. Solar Updraft Power Plant System: A Brief Review and a Case Study on a New System with Radial Partition Walls in Its Collector. Renewable and Sustainable Energy Reviews 69: 472–87. DOI: https://doi.org/10.1016/j.rser.2016.11.135 Nasraoui, H., Moubarek B., and Driss, Z. 2022. Solar Chimney Power Plants: Numerical Investigations and Experimental Validation (Recent Advances in Renewable Energy). Bentham Science Publishers. Okada, S., Uchida, T., Karasudani, T. and Ohya, Y. 2015. Improvement in Solar Chimney Power Generation by Using a Diffuser Tower. Journal of Solar Energy Engineering, Transactions of the ASME 137(3). DOI: 10.1115/1.4029377 Pandey, M., Padhi, B. N. and Mishra, I. 2021. Performance Analysis of a Waste Heat Recovery Solar Chimney for Nocturnal Use. Engineering Science and Technology, an International Journal 24(1): 1–10. DOI: https://doi.org/10.1016/j.jestch.2020. 11.009. Pastohr, H., Kornadt, O. and Gürlebeck, K. 2004. Numerical and Analytical Calculations of the Temperature and Flow Field in the Upwind Power Plant. International Journal of Energy Research 28(6): 495–510. DOI: https://doi.org/10.1002/er.978 Patel, S. K., Prasad, D and Ahmed, M. R.. 2014. Computational Studies on the Effect of Geometric Parameters on the Performance of a Solar Chimney Power Plant. Energy Conversion and Management 77: 424–31. DOI: https://doi.org/10.1016/j. enconman.2013.09.056

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Pradhan, S., Chakraborty, R., Mandal, D.K., Barman, A. and Bose, P. 2021. Design and Performance Analysis of Solar Chimney Power Plant (SCPP): A Review. Sustainable Energy Technologies and Assessments 47(February): 101411. DOI: https://doi.org/10.1016/j.seta.2021.101411 Rabehi, R., Chaker, A., Ming, T. and Gong, T. 2018. Numerical Simulation of Solar Chimney Power Plant Adopting the Fan Model. Renewable Energy 126: 1093–1101. DOI: https://doi.org/10.1016/j.renene.2018.04.016. Rahdan, P., Kasaeian, A. and Yan, W. 2021. Simulation and Geometric Optimization of a Hybrid System of Solar Chimney and Water Desalination. Energy Conversion and Management 243: 114291. DOI: https://doi.org/10.1016/j.enconman.2021.114291 Sangi, R., Amidpour, M. and Hosseinizadeh, B. 2011. Modeling and Numerical Simulation of Solar Chimney Power Plants. Solar Energy. DOI: 10.1016/J. SOLENER.2011.01.011 Shams, E., Gross, A. and Fasel, H. 2011. Performance Analysis of Solar Chimneys of Different Physical Scales Using CFD. Proceedings of the ASME 2011 5th International Conference on Energy Sustainability. ASME 2011 5th International Conference on Energy Sustainability, Parts A, B, and C. Washington, DC, USA. August 7–10, 2011. pp. 2147-2156. ASME. DOI: https://doi.org/10.1115/ES201154537 Sheikhnejad, Y. and Nassab. S. A. 2021. Enhancement of Solar Chimney Performance by Passive Vortex Generator. Renewable Energy 169: 437–50. DOI: https://doi.org/10. 1016/j.renene.2021.01.026. Tahar, T. and Djezzar, M. 2012. Numerical Simulation of Natural Convection in a Solar Chimney. International Journal of Renewable Energy Research-IJRER. 2. 712-717. Too, J. H. Y. and Sidek, C. N. A. 2016. Numerical Analysis for Optimizing Solar Updraft Tower Design Using Computational Fluid Dynamics (CFD). Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 22(1): 8–36. Too, J. H. Y. and Sidek, C. N. A. 2016. A Brief Review on Solar Updraft Power Plant. Journal of Advanced Review on Scientific Research 18(1): 1–25. Wang, H., Chen, J., Dai, P., Zhang, F. and Li, Q. 2021a. Experimental and Numerical Study on the Performance of Solar Chimney under the Influence of Ambient Wind. International Journal of Energy Research. ER-21-19728. Wang, H., Chen, J., Dai, P., Zhang, F. and Li, Q. 2021. Simulation and Experimental Study of the Influence of the Baffles on Solar Chimney Power Plant System. Processes 9(5): 1–20. DOI: 10.3390/pr9050902 Weli, R. B., Atrooshi, S. A. and Schwarze, R. 2021. Investigation of the Performance Parameters of a Sloped Collector Solar Chimney Model – An Adaptation for the North of Iraq. Renewable Energy 176: 504–19. DOI: https://doi.org/10.1016/ j.renene.2021.05.075. Xu, G., Ming, T., Pan, Y., Meng, F. and Zhou, C. 2011. Numerical Analysis on the Performance of Solar Chimney Power Plant System. Energy Conversion and Management 52(2): 876–83. DOI: https://doi.org/10.1016/j.enconman.2010.08.014 Yapıcı, E. Ö., Ayli, E. and Nsaif, O. 2020. Numerical Investigation on the Performance of a Small Scale Solar Chimney Power Plant for Different Geometrical Parameters.

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Chapter 6

Thin-Film Solar Cells for Vehicle Integrated Photovoltaic Technology: Integration Challenges and Opportunities in Hybrid Vehicles Nitin Ralph Pochont1 Y. Raja Sekhar2, and K. V. Sharma3 1School

of Mechanical Engineering, Vellore Institute of Technology, Vellore, India for Disaster Mitigation and Management, Vellore Institute of Technology, Vellore, India 3Centre for Energy Studies, JNTUH College of Engineering, Hyderabad, India 2Centre

Abstract Passenger vehicles are becoming a common and essential mode of travel, even in rural households. These vehicles have limitations with energy conversion due to vehicle dynamics and varying operating scenarios. Hence, to achieve sustainable means of transport, energy recovery and generation in passenger cars are possible using advanced technologies. With the advent of hybrid and e-vehicles, technical challenges must be resolved to aspire to an emission-free travel mode. Solar photovoltaic (PV) charging and swapping stations are currently in a nascent stage to recharge and sustain the vehicle battery. Energy generation through the integration of photovoltaics within the vehicle to enhance drive range can reduce the dependency on charging infra and increase the driving range. Scientists worldwide devised technologies and methods for in-situ power 

Corresponding Author’s Email: [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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Nitin Ralph Pochont, Y. Raja Sekhar and K. V. Sharma production using Vehicle Integrated Photovoltaics (VIPV) to charge onboard batteries. Unlike braking recovery, which can produce a momentary output of less than 10% of the total energy required, VIPV renders a potential energy source with continuous power generation. This chapter presents an overview of technologies developed globally for different classes of passenger vehicles in recent times. Advanced photovoltaic thin-film materials that pose an advantage in-vehicle integration have been explored and discussed. Experimental and simulation research studies conducted by academia and industry to foresee the impact of energy recovery, generation, and solar PV integration in passenger vehicles are presented, along with different computational techniques. Recommendations and scope for technical limitations about VIPV passenger vehicles have been discussed in the conclusions.

Keywords: energy recovery and generation, vehicle integrated photovoltaics, braking recovery, thin-film materials, energy integration, limitations, computational techniques

1. Introduction 1.1. An Overview of the Current Scenario In today’s modern world, the way of urbanization and transportation is shifting by overseeing several technological and economic advancements. However, the aspect of vehicle emission is one of the crucial entities that significant countries are battling. An intentional focus on the automobile sector in reducing vehicle pollution has been a prime entity for the past few years. Concerning the upsurge in Green House Gases (GHGs) globally, the transportation sector corresponds to 14% of the total CO2 emission in 2010, while the proportion was observed to spike, shooting up to 28% by 2018. The emission percentage has doubled in 8 years, with transportation being the highest contributor to GHGs recently (US EPA, Transportation and Climate Change., 2020). Global warming caused due to the GHGs has an impact on the bio-diversity index and the lifestyle of humanity as well (Jacobson., 2009). Interestingly, the potential of renewable energy sources and advanced technology has abetted the market for e-mobility, which is forecasted to rise rapidly shortly. Through policies launched, significant countries have

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endorsed the idea of ultimately shifting to alternative energy sources for vehicles rather than conventional fuels.

1.2. Importance of EV and the Aspects in Developing Economies Electric Vehicles (EVs) have been around for a couple of decades. With fossil fuel prices skyrocketing in oil-importing nations, the need for alternate transport mechanisms such as EVs has increased substantially (Georg Bieker., 2021). India’s conventional transportation sector accounts for 18% of the country’s total energy consumption: nearly 94 million tonnes of oil equivalent energy, which is a very high proportion (E-mobility report, 2020). Through policies launched, significant countries have also endorsed the idea of ultimately shifting to alternative energy sources for vehicles rather than conventional fuels. Global ecologists and Auto-manufacturers look towards EVs and Hybrid Electric Vehicles (HEVs) to overcome the dearth. Major countries like Germany, France and England have announced the ban on conventional vehicles in 20-30 years. Whereas in India, policies enacted by the government through “FAME,” Faster Adoption and Manufacturing of (Hybrid &) Electric Vehicles, look forward to bringing in EVs with increasing sales to 30% for passenger cars, 40% for buses, 70% for commercial vehicles, and 80% for 2-wheelers and 3-wheelers by the end of 2030 (KPMG report., 2020). FAME policies have been launched in India to build and promote EVs in different categories (FAME India Scheme., 2019). India’s intermediate plan was to have 6 million EVs & HEVs on the roads by 2020, explicitly developing and promoting 55000 electric 4-wheeler passenger cars.

1.3. Modes of Energy Generation in Hybrid Passenger Vehicles Energy generation in passenger vehicles is essential for charge sustenance that improves the driving range and additional charge support in a vehicle. However, in the case of an electric vehicle and a hybrid vehicle, high traction energy released from the battery or the ultra-capacitor must overcome friction, acceleration, drag and hill climb forces (Where the energy goes - US EPA report). Henceforth, recharging and sustaining the battery charge is always considered a crucial entity that has adopted energy generation and recovery within the vehicle to attain a more extended driving range. The possible energy generation technologies embedded in a vehicle as illustrated in Figure 1.

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Table 1. Methods of energy recovery outlined for two vehicles variants Type of Source Turbo Compound (MGU-H) Piezo Electric Generator (PEG) Organic Rankine Cycle (ORC)

HEV ✔

EV x

✔

✔

✔

x

Thermal Electric Generator (TEG)

✔

x

Regenerative Damping

✔

✔

Rack-Pinion Damper ✔ (RPD)

✔

Hydraulic Electromagnetic Damper (HED)

✔

✔

Ball Screw Damper (BSD) Linear Motion Damper (LMD)

✔

✔

✔

✔

Output 5% saving in fuel consumption Produces up to 150 µW It saves about 10% of fuel

Remarks Requires Advanced Optimization Insufficient amount of energyoutput Complexity due to the large volume of the system Low efficiency and complexity

A maximum of 4W was obtainedwhen attached to the tailpipe silencer Extensive range from It depends on road 46 W to 7500 W conditions, and output is low on freeways Charge of 58Dynamics are 68W/damper at peak unpredictable with power road conditions Approximately Cost, complexness 33W/damper at a and unpredictable constant frequency of output due to mass compressionand variations expansion is applied. Around 12-15W per Low efficiency and damper on anaverage complexity An average of 40-43W Expensive and per shock absorber mainly used on high-end cars only

Reference [20] [21, 22] [23]

[24, 25]

[26]

[27]

[28, 31]

[29, 30] [32]

The idea of energy recovery is to convert lost energy from a different source and utilize it to propel or supplement the vehicle (Cundev et al., 2011). The areas which pose a potential for energy recovery in a car are the Engine, Chassis and Brakes domain. Table 1 describes the engine and chassis sources available for two-vehicle variants considering the output and limitations. In an EV, the chassis and brake domains are feasible to aid the notion of energy recovery. The horizontal and vertical motion of the vehicle fosters the ability to regenerate the lost energy. The vehicle executes a horizontal action during acceleration and deceleration, similarly executing vertical movement when encountering uneven roads. An extensive analysis comprising attained energy yield from strain energy recovery has been reported by Pan et al., 2021. Way back in 2015, researchers at Audi experimentally analyzed the possible application of regenerative damping in passenger ICVs and concluded that the

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potentiality for the same in EVs and HEVs is high in the future. But it is not easy to estimate the attainable energy yield from a damping regenerative system due to dynamics and road conditions (Zuo et al., 2013). Energy recovery through regenerative braking has shown high potential in EV drive range extension and ease through applicability. Perhaps, certain limitations are substantial in practical situations (Yoong et al., 2010).

Figure 1. Classification of Energy harvesting Methods for passenger vehicles (Pan et al., 2021; Ismail et al., 2015; Zhu et al., 2012; Glynne-Jones et al., 2004; Latz et al., 2013; Prakash et al., 2015; Jeng et al., 2013; Zuo et al., 2013; Li et al., 2012; Zhang et al., 2015; Kavianipour et al., 2014 and Khoshnoud et al., 2015).

Unlike Internal Combustion Vehicles and Hybrid Electric Vehicles (HEVs) that render the opportunity to recover lost energy from the enginealternator, exhaust recovery and damping effect, EVs are limited to this availability of sources. Pan et al., 2021 pointed out that kinetic energy harvesting technologies have a healthy slant in size and stability. The only dependency is upon the Kinetic Energy Recovery System (KERS) and the Strain energy through harnessing damping effect through the contingency of high-density batteries. Perhaps, for an EV that employs a single energy source, the achievability of drive range and dynamic performance is deprived (Jonathan et al., 2016). Moreover, recharging and charge sustainability for an

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EV are also influenced by external scenarios like Route Mapping, Supporting Infrastructure, and Charging Profile (Li et al., 2019).

1.4. Hybridization in Vehicles Hybridization (%) is a ratio between the total electric traction power by the sum of electric traction and engine power in a vehicle. It is a factor that determines the vehicle type and composition of electrical energy inherited within the vehicle. The relation (a) that defines vehicle hybridization was put forward by Allen Fuhs., 2008. Figure 2 illustrates the hybridization percentage for different passenger vehicles. 𝐻𝑦𝑏𝑟𝑖𝑑𝑖𝑧𝑎𝑡𝑖𝑜𝑛 =

Total electric traction power Sum of electric traction+Engine power

(a)

Figure 2. Degree of hybridization possible for different passenger vehicles.

2. The Significant Scope of Energy Generation and Recovery in Hybrid Passenger Vehicles 2.1. Kinetic Energy Recovery System/Regenerative Braking Technology (RBT) Specifically for urban driving conditions, decelerating a vehicle releases onethird to half of its energy in heat emitted through braking (Zhang et al., 2012).

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Hence, a regenerative/recovery system can harness the lost energy and enhance fuel economy (Pugi et al., 2017). An electromagnetic braking energy recovery system is one of the first technologies implemented. Researchers have developed several braking recovery technologies using Coil Springs, Triboelectric nanogenerators and Flywheels. These technologies have yielded low energy efficiency with complexity in design and cost (Jiang et al., 2013; Qi et al., 2020; Han et al., 2014; Šonský et al., 2019). Eventually, energy recovery through braking has gained a positive apprehension for EVs in driving range enhancement (Von Srbik., 2012). Coasting and braking the vehicle would produce a considerable charge that recharges the battery or the ultra-capacitor. The lost kinetic energy is captured when the vehicle decelerates, or braking force is applied to slow down and halt the vehicle. The Motor-Generator (M/G) unit uses the power generated for acceleration, Figure 4, else to supplement auxiliary loads when required. The M/G unit works symphony to ensure smooth drive and minimal turbulences. The RBT enhances the range for an EV by capturing energy with an efficiency varying from 16% to 70% (Boretti, 2013). Tawadros et al., 2012 postulated that the electric regenerative braking technology allows 45% of braking energy to be recovered. The US EPA’s case study stated that 17% of energy was recoverable through RBT technology on an EV. Depending on its driving cycle and driving pattern, Vorocky et al., 2011 articulate that an effective and efficient braking system could save up to 8% to 25% of the total energy used by the vehicle. Figure 3 illustrates the potential energy recoverable from different braking recovery systems based on several research studies. Henceforth, the usage of RBT has been commercially developed in vehicles made by Toyota, Tesla and Honda (Zhang et al., 2013). Based on the driving pattern, regenerative braking is classified into Serial regenerative braking (Category-A) and Parallel regenerative braking (Category-B). Serial braking systems work on the principle of retardation with an integrated control that estimates the deceleration and distributes the brake force. Similarly, parallel braking is allied with the brake pedal position depressed by the driver (Vorocky et al., 2011). Research reports that a serial braking system yields around 15-30% of fuel efficiency while a parallel braking system yields 9-18% fuel efficiency. The brake pedal feel is affected due to the modulation caused by wheel pressure and the driver’s improper brake utility (Qiu et al., 2016). Therefore, a brake control strategy is required to coordinate the friction brakes and RBT (Hui et al., 2011). The parallel regenerative braking system does not require a complex control system. An altered existing friction braking system is

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adequate (Zhang et al., 2013). An electro-hydraulic braking control strategy was developed for EVs by Pei et al., 2020. A genetic algorithm was induced to analyze the driver’s braking intention and optimize the brake force distribution.

Figure 3. Possible energy recovery for different passenger vehicles using the KERS system reported in the literature.

A similar method to generate a yield from kinetic energy is a Hydraulic Pump- Motor (P/M) unit and Hydraulic Accumulators (HACCs). The kinetic energy is converted and stored as hydraulic energy and released by the P/M unit when acting as a motor (KPMG report, 2020). Hydraulic systems offer reliability, a high-power ratio, and speed regulation, improving performance (Mathews et al., 2013). Research indicates that developments have been made in designing a parallel hydraulic-pneumatic and a combined regenerative dissipative system through which simulations estimated a yield of 70% to 14% braking energy recovery (Liu et al., 2020). The intricacy in design and maintenance poses a limitation in implementing the braking system.

Figure 4. Energy flow pattern in a regenerative braking system.

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As discussed in the previous session, the traction energy for a vehicle must overcome external forces acting on the vehicle. The equations relating to external forces such as wheel friction, potential force, aerodynamic drag, kinetic energy, and total energy have been outlined in the equation. (1-5) 𝐹 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 𝜇mg

(1)

𝐹 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 = mg. Sinθ

(2)

1

1

𝐹 𝑑𝑟𝑎𝑔 = 2 𝐶𝑑 𝐴𝑓 𝜌 𝜈 2 = 4 𝐶𝑑 𝐴𝑓 𝜌 (𝜈𝑓2+ 𝜈𝑖2) 𝐹 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 =

1 2

𝑚𝜈 2 =

1 4

(3)

𝑚(𝜈𝑓2- 𝜈𝑖2)

(4) 1

1

Etotal = {𝜇. mg Cos θ + mg Sin θ + 4 𝐶𝑑 𝐴𝑓 𝜌 (𝜈𝑓2+ 𝜈𝑖2) } ∆d + 4 𝑚(𝜈𝑓2- 𝜈𝑖2 ) (5) where, μ: Friction coefficient of the tire, m: Mass of the vehicle (kg), g: Gravitational energy (9.81 m/s2), θ: Road gradient angle, Cd: Coefficient of drag, Af: Frontal surface area of the vehicle (m2), ρ: Air Density (kg/m3), νf: Final velocity of the vehicle (m/s), νi: Initial velocity of the vehicle (m/s), ∆d: Distance traveled by the vehicle. Earlier researchers who developed and analyzed the regenerative braking system for EVs have experienced significant limitations in terms of practicality that are listed below; The yield from RBT depends on the drivetrain regenerative capacity and how the car is driven (Björnsson et al., 2016).    

The battery’s continuous charging and discharging would lead to overheating, destruction, and decreased life (Zeiaee, 2016). Electrical regeneration has a relatively poor round-trip efficiency due to many energy conversions (Cross and Brockbank, 2006). Regenerative braking is suitable for normal operations but inefficient during heavy or emergencies (Ehsani et al., 2018). The drivetrain requires a large battery and motor capacity to increase the energy recovery, complicating the design and cost (Yabe et al., 2012).

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







The rate of energy recovery from RBT is a function of physical parameters such as Motor Torque variation, Aerodynamics, Temperature Gradient, Vehicle Life, Type of terrain, and road resistance (Lei Pretto, 2017). The instantaneous high-power demand will damage the battery in urban driving conditions with frequent starting, acceleration and deceleration (Lam et al., 2012). The coordination between the frictional and regenerative brakes is challenging as regenerative braking cannot meet the complete braking demand, and control strategies are implemented (Lv et al., 2015). The recovered energy cannot be stored longer in a flywheel-based recovery system due to mechanical and fluid dynamics friction acting on the flywheel (Fathabadi, 2018).

2.2. Energy Generation from Semiconductors The photovoltaic effect through semiconductors has seen the face of the world since the 18th century. With advanced developments in solar photovoltaics, solar energy is expected to proliferate the use of conventional fuels in the future. Electric Vehicles certainly extract electricity from the grid for recharging the batteries. However, the dependency on the grid increases at peak hours in the day, thereby creating heavy loads and high tariffs. As a supplement to the grid, renewable energy sources such as solar and wind energy have shown a positive potential in charging EVs (Bhatti et al., 2016). Utilizing solar energy as an alternative source for vehicles was seen on the face of the world around 25 years ago. Interestingly, EVs charged through solar power emit 96% fewer pollutants when compared to grid-connected vehicle dependency. Comprehending the carbon footprint released by solar panels, carbon emissions neutralize in 2 years, with an average lifespan of 20 years for panels (Goldin et al., 2014). The flexibility in integrating renewable energy sources with an EV battery has an excellent competency (Hoarau and Perez, 2018). The synergy of these sources provides systematic and individual benefits (Chaouachi et al., 2016). PV systems in charging stations for an EV are most commonly found in developed countries through PV-grid connected and PV-standalone approaches (Li et al., 2019). This ideology gave rise to a concept entitled Charging while parking, Figure 5; the car park is roofed with PV modules that

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provide an electric charge to the car battery when parked and connected on a sunny day (Van Roy et al., 2013). The approach has an alternative terminology known as solar-powered charging stations (SPCS) (Robinson et al., 2014). The parking station roof can enhance the power output with a static and dynamic tracking mechanism (Hoffmann et al., 2018). Literature reports that the charging time for EVs can be reduced since SPCS enables two and three-level fast charging techniques (García-Triviño et al., 2016).

Figure 5. Charging station for EVs at a parking lot using Solar PV modules [76].

The concept of charging from shelters does provide advantages on the structural basis, space utility and reduction in grid-dependent electricity tariff during peak hours (Tulpule et al., 2013). Research reports that the grouping of PV-grid connections has more viability in charging EVs when parked, as the grid switches on to be the primary energy source when radiations are faint and during nights. PV-grid charging is efficient but requires a complex integration of energy sources from PV and grid connection, Figure 6. When the parking lot is vacant, the produced charge is diverted to the grid through a bidirectional source, facilitating the opportunity to charge a vehicle parked for nearly 8 hours a day in the SPCS. But there is always a chance of high grid dependency in unpredictable weather conditions. Unreliably, integrating renewables in a power mix would introduce practical challenges in supplydemand matching, i.e., the duck curve challenge (Kathare, 2018). Lathika, 2019 has put forward blockchain with big data and analytics to provide intelligent charging and load shifting systems to stabilize the power demand.

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Bhatti et al., 2016 studied the layouts, feasibility and reliability of gridconnected and standalone solar charging systems to conclude that PV-grid interlink is more viable due to its flexibility and interruption-less operation. Besides, PV standalone is completely grid-independent, relying on the output from incident solar radiation, Figure 7. The benefit is that it is reachable in remote locations where grid supply is unavailable or has high tariffs (Bhandari and Stadler, 2011; Tong et al., 2013). The system is accessible to charge the vehicle through solar power directly but is impractical when radiations are low during the day; hence a battery connection is feasible (Mossoba et al., 2012). Standalone charging also enables integrating a home erupted solar photovoltaic system into the charging station as an add-on energy source. The standalone charging stations will make the system fully grid-independent (Khan et al., 2015). The grid-connected PV charging system has progressed into a concept termed Vehicle to Grid Technology (V2G). It is a process that feeds electric charge from the vehicle back to the grid. V2G comprises a 2-way flow of charge from the gird and vehicle with sophisticated charging equipment (Celia Topping., 2021). Apart from the advantages, the drawbacks are more likely to arise in V2G technology like cost, compatibility difficulties, battery energy density, large scale of vehicles, and source integration (Divya and Sravani, 2019). In contrast, the Standalone PV charging system enables the concept of Vehicle to Home (V2H) technology (Khan et al., 2018). Secondly, implementing solar cells on the vehicle has exposed the possibility of generating energy on board. The concept sparked an insight to charge the vehicle battery with a terminology entitled Vehicle Integrated Photovoltaics.

Figure 6. Schematic diagram of Grid-Connected PV Charging System to charge an E-vehicle.

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Figure 7. Schematic diagram of Standalone PV Charging System to charge an e-vehicle.

3. Vehicle Integrated Photovoltaics Integrating solar photovoltaics on a vehicle is categorized into three broad areas in this paper.  

Solar Powered Race Car (SPRC): a solar power-driven noncommercial vehicle Solar Assisted Passenger Electric Vehicle (SAPEV): passenger evehicle incorporates solar PV as an energy source for the traction battery system.

3.1. Solar Powered Race Cars The deployment of solar-powered race cars has been around since the 1970s. SPRCs are not commercialized for public usage but are more intended for competitions such as the World Solar Challenge (WSC), which commenced in 1987 (Rizzo, 2010). The cars depend entirely on solar energy and are designed with specific shape and size considerations. The main challenge in an SPRC is space limitation to accommodate the PV cells (Tie and Tan, 2013). Henceforth, control algorithms are derived to improve the overall efficiency to nearly 60% (Schuss et al., 2012). Figure 8 portrays the standard schematic layout of the drivetrain designed for an SPRC. The aerodynamic design, weight, power electronics and vehicle dynamics for an SPRC are the crucial challenges in building the vehicle. Branko Davidovic et al., 2015 derived the

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relations used to obtain the power output from SPRC equations 6 to 9. The research states that the electrical component efficiency should be maximized to attain high power output, and the power needed to mobilize the vehicle should be minimal. Table 2 tabulates the most popular WSC solar race cars developed with their technical specifications and criteria. The powers required to propel the vehicle are, 𝑃𝜈 = 𝑃 𝑎𝑒𝑟𝑜𝑑𝑦𝑛𝑎𝑚𝑖𝑐 + 𝑃 𝑟𝑜𝑙𝑙𝑖𝑛𝑔 + 𝑃 𝑠𝑙𝑜𝑝𝑒 𝑃 𝑎𝑒𝑟𝑜𝑑𝑦𝑛𝑎𝑚𝑖𝑐 =

1 2

(6)

𝜌 𝐶𝑑 𝐴𝑓 𝜌 𝜈 3

(7)

𝑃 𝑟𝑜𝑙𝑙𝑖𝑛𝑔 = 𝑚. 𝑔. 𝑙. 𝐶𝑑

(8)

𝑃 𝑠𝑙𝑜𝑝𝑒 = 𝑚. 𝑔. 𝑙. 𝜈

(9)

Figure 8. Schematic illustration of solar-powered race car drivetrain.

Table 2. Solar-powered race cars reported in the literature S. Vehicle No.

1 2 3 4 5

Sunswift eVe Schulich Delta PowerCore SunCruiser ThyssenKrupp SunRiser Bocruiser

Panel Surface Area (m2) 4 2.7 3

Max Power (W)

Battery Capacity (kWh)

Seating Capacity

Reference

16 14 16

Max Range(km)/ Top Speed (km/h) 800/ 130 104/ 56 700/ 65

800 1000 8300

2 2 2

[91] [92] [93]

3

8700

14.8

600/ 120

2

[93]

6

1250

532 Li-Ion cells

120

4

[93]

Thin-Film Solar Cells for Vehicle Integrated Photovoltaic Technology S. Vehicle No.

Max Power (W)

Battery Capacity (kWh)

6 7 8 9 10 11 12 13

Panel Surface Area (m2) SERVe 6 Emilia 4 5 Astrolab 3.6 Stella Lux 5.8 SolUTra 9 Daedalus 391 cells Tokai Challenger 6 Solar World GT 6

1000 1200 1500 1300 1800 823

14 15

Nuna 9* Schulich Elysia*

6.5 16.1 7 15 5.5 16.2 14.9 638 Li-Ion cells 5.3 1335 Li-Ion cells

2.64 318 cells 1200

Max Range(km)/ Top Speed (km/h) 250/ 65 600/ 60 110/ 120 1000/ 125 1000/ 125 700/ 144 100 100

167

Seating Capacity

Reference

2 4 2 4 1 1 1 3

[94] [95] [96] [97] [98] [99] [100] [101]

1 2

[102] [103]

3.2. Solar Assisted Passenger Electric Vehicle Solar Assisted Passenger Electric Vehicles (SAPEV) reside in integrating solar PV in a vehicle to enhance the driving range or act as an alternative energy source which has been a well-established approach in the automotive sector since the 1990s. The photovoltaic effect supports auxiliary sources, maintains thermal management, or supplements a charge to propel the vehicle (Rizzo, 2010). Remarkably, solar energy creates a short supply chain by using primary energy through radiation to energize the vehicle. Unlike other sources like liquid fuels, hydrogen and wind, solar energy can be directly used for energy conversion, reducing energy consumption, cost, and emissions (Araki et al., 2018). Certain limitations arise for direct automotive usage in a vehicle, such as energy density and space. A suitable optimized methodology of integrating solar photovoltaics would gain enough yield that is not negligible for a vehicle for specific driving conditions. Solar PV embedded in an EV can improve the vehicle driving range efficiency to about 10-20% (Letendre, 2007). In comparison, Araki et al., 2018 estimated that nearly 70% of the energy required for a passenger vehicle could be achieved by PV energy. The designs have become more feasible for EVs concerning charging the vehicle battery, and supplementing the auxiliary electrical sources has increased concern. Kronthaler et al., 2014 described solar energy as a dynamic source with the potential to propel the electronic units, displays, and actuators in a vehicle. The roofs of electric vehicles are designed with solar panels as an integration source in conjunction with plug-in charging to reduce grid

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dependency and utilize green energy sources (Birnie, 2016). Apart from charging while parking, the design promotes increased yield when driven outdoors. However, external parameters like driving distance, duration, route and traffic conditions are vital aspects that influence the yield in a vehicle (Delta, 2013). Furthermore, Coraggio et al., 2010 proposed that solar photovoltaic incremental costs integrated into vehicles would have a shorter payback time than building-integrated photovoltaic systems. Unfortunately, the payback in EV mobility is more than five years. The schematic layout of a commonly used SAPEV is portrayed in Figure 9. Apart from passenger cars, manufacturers and researchers have developed solar mobility in vehicles like Urban Buses (Boonraksa et al., 2020), Trucks (Falkgrim, 2020), AutoRickshaws (Tuk-Tuk) (Saleh et al., 2016) and Minivans (Diverge vehicle innovations). This chapter focuses primarily on solar photovoltaics implemented on passenger cars specifically.

Figure 9. Powertrain of a solar assisted passenger electric vehicle.

3.3. Scope of SAPEV Energy Generation in German Geographical Conditions In a theoretical analysis conducted in 2020, the characteristic solar energy yield and potential range of popular EVs have been reported (Heinrich et al., 2020). The study estimates the solar potential and scope a photovoltaic roofintegrated concept could attain for an EV driven in Freiburg meteorological conditions, Figure 10. The country receives a long-term average global horizontal irradiance of 1022 to 1241 kWh/m2, with specific assumptions such

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as fixed module efficiency of 20% and un-shaded driving conditions. The theoretical potential yield estimates nearly 15,000 km, 13-23% of yearly driving distance. The intensive research and review indicate that the latent energy attained by solar photovoltaics is adequate to overcome the gap in energy recovery caused due to onboard braking recovery sources. Moreover, integrating an external 48V SLIPA battery or a lesser secondary battery is practically feasible in powering a vehicle’s auxiliaries while reducing the traction battery’s electrical load. The vehicles and their specifications are categorized in Table 3.

Figure 10. Global horizontal irradiation available in Germany (SOLARGIS).

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Table 3. Potential of energy generation by integrated solar roof PV on EVs S. Vehicle No. 1 2 3 4 5 6 7 8 9

Power consumption (kWh/100km) Audi e-tron 24 BYD e6 21.5 Chevrolet Bolt EV 16 Hyundai Ioniq Electric 13.8 Daimler EQC 23.5 Nissan Leaf 20 BMW i3 13.1 Renault ZOE 17.7 Volkswagen ID3 14

Battery Capacity (kWh) 95 80 60 38.2 80 40 37.9 52 77

Roof Area (m2) 2.3 2.2 2 2.2 2.1 1.8 1.9 1.7 1.9

Potential Solar Power, (W) 460 440 400 440 420 360 380 340 380

Solar range/year (km) 2063 2200 2688 3428 1923 1935 3122 2068 2921

3.4. Scope of SAPEV Energy Generation in South Asian Countries A similar ideology would yield higher results in South Asian counties like India, Bangladesh, Sri Lanka, and Pakistan. These countries are geographically equatorial-based and receive high solar radiation suitable for effective energy conversion and applications, Figure 11. Especially in India (MNRE report, 2021), a solar potential of nearly 5000 trillion kWh per day is received over land, making it highly viable for solar-embedded transportation.

Figure 11. Global Horizontal Irradiation potential available in South Asian countries (SOLARGIS).

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Figure 12. Average Global Horizontal Irradiation available in India (SOLARGIS).

India receives an average global horizontal irradiance ranging from 1550 to 2260 kWh/m2, Figure 12. Remarkably, India holds the second largest road network globally, with a span of 5.89 million kilometers (km) (Indian road industry report, 2021). While in 2021, the national highways in India will constitute nearly 151,019 km of distance and 2.7% of the total road network Figure 13 which can foster the viability of efficient transportation using solar energy.

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Figure 13. National Highway Road network of India (Indian road industry report).

4. Advanced Semiconductors for VIPV Technology Solar photovoltaic cells/modules with opaque structures developed significantly from crystalline and Amorphous Silicon. On the other hand, thinfilm cells made of Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS) and Gallium Arsenide (GaAs) have shown higher efficiencies in laboratory-based results. Practically, the efficiencies and output of the solar cells succumbed to lower values ranging from 15-20% on commercial bases. The major limitation is caused due to the Shockley Queisser Limit (SQL), which sets a theoretical limit on solar cells to produce less than 30% efficiency. The SQL is influenced by three primary parameters: black body radiation, solar spectrum and recombination in cells. However, research has proven that third-generation solar cells are heading to break the barrier caused due to SQL. Since it commenced in 2009, solar cell research on advanced cells has put forward tremendous results in this category.

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NREL reports that third-generation solar cells have higher efficiency than opaque cells with additional advantages such as semi-transparency and possible source integration (Rahmany and Etgar, 2020). Particularly, perovskite solar cells have undergone rapid development resulting in high power conversion efficiency attainable at lower manufacturing costs (Zhao et al., 2021). Perovskite solar cells have reported a steep rise in efficiency from 3.8% in 2009 to 25.2% in 2020 (Liu et al., 2021). With recent crystal engineering developments, perovskite solar cells’ mesoporous and planar structures offer both semi-transparent and flexible solar modules suitable for vehicle integration. Research towards increasing the power conversion efficiency of a solar photovoltaic cell is a steady progression. While incorporating solar cells onto the rooftop or the bonnet, the aerodynamic behavior and aesthetics are considered in deciding the integration mode. The solar cell sizing is determined based on the available solar energy and battery packs (Prasad et al., 2018). Different cells can be used for vehicles depending on the cost and efficiency. Figure 14 depicts the recent developments in solar PV with their efficiencies relating to surface integration. An analysis carried out by Yamaguchi et al., 2021 stated that solar cell efficiency should be nearly 30%, equivalent to driving a lightweight EV that consumes 642 kWh/year. The study concludes that 30% cell efficiency is the promising target for PVpowered passenger vehicles. Attaining an efficiency of 30% is highly difficult on low-cost solar modules or single junction Si-cells. Sasaki et al., 2013 report that III-V compound-based multijunction solar cells have a high conversion efficiency ranging from 37.9% to 39.2%. However, multijunction solar cells’ cost is far from the economic balance. Green et al., 2020 have graphed different solar cells’ efficiencies, clearly stating that cost and efficiency are inversely proportional. Seemingly, tandem cells combined with Silicon and III-V compound, or II-VI compound, are viable for producing high efficiencies at low cost (Essig et al., 2017). Research reports the roof shape as a threedimensional curved structure with a curve correction factor that proposes relative power generation. The curve correction factor must be considered, and an optimum cell or module should be implemented only after preliminary research on practical situations. Also, considering the aesthetics, appearance and dynamics, research at the Fraunhofer Institute for Solar Energy Systems (ISE) has taken a step toward developing invisible solar roofs integrated into vehicles. Solar cells were combined into a standard panoramic glass roof that would accommodate 366 solar cells enabling a nominal power of 210 W/m2 enhancing the driving range

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by 10 km (Steve Hanley., 2019). Outwardly, the output generated by solar photovoltaic cells on a moving vehicle varies with geographical location, irradiation condition and driving patterns. Preliminary research can be carried out using several computational tools.

Figure 14. The efficiencies of different solar cells suitable for surface integration in passenger vehicles (Tie et al., 2013; Nikolay Belyakov., 2019).

5. Computational Techniques Suited to Analyze VIPV Technology Computational tools are always an asset to any engineering problem or process. With advanced technology and tools, computing and simulating a solar vehicle is not an unachievable task nowadays. Computing techniques enhance the preliminary research with suitable models and analyses that foster the process in reality with comparative results. The computing technique proper for Vehicle Photovoltaic systems are discussed below;

5.1. MATLAB Simulink Simulink is one of the essential tools for simulating electromechanical components and layouts in engineering. Simulink enables a computer simulation model developed and tested with a Graphical User Interface (GUI). The model reduces time and eases the practical designing process with optimal

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theoretical results; most automobile companies rely on it (Akinnuli and Anjorin, 2016). In developing a solar-powered vehicle, models for Photovoltaic panel (I-V Curve), Current Block, SOC Block, Voltage Block, Resistor Block, and Exponential Block are efficiently achievable through Simulink (Sankar., 2016). Secondly, drive models such as PMS Motor drive system, Rotor Speed, Electromagnetic Torque, DC-DC converter current, PV performance and Output mechanical torque are theoretically attained from Simulink (Chowdhury et al., 2016). Satya Prakash et al., 2019 proposed a model on Simulink that charges the electric vehicle through a solar photovoltaic power plant. A solar EV model was developed to comprehend the enhancement attained in the operation time using a solar power source (Prasad and Krishnamoorthy, 2017).

5.2. Transient Simulation Systems - TRNSYS In 1995, a transportation article entitled “A solar-electric vehicle simulation code” was published by Craparo and Thacher, 1995. The simulation model was designed on the framework of TRNSYS. The model formulates SPC’s performance characteristics driven in different road and weather conditions and SHEVs in urban conditions. TRNSYS provides a weather incorporation system that can be embedded in the model. It enables submodules that can be grouped with programming languages like Python, Fortran and C. Alternatively, components can be written in the TRNSYS “W” language in the TRNSYS Simulation Studio, which does not require a compiler (Trnsys software). The flexibility in combining similar tools such as MATLAB for cosimulation and optimal outputs is feasible through the software (Bava and Furbo, 2017).

5.3. PVsyst PVsyst is modeling software for designers, researchers and engineers who simulate real-time environments virtually. PVsyst is mainly intended for the analysis carried out in photovoltaic systems. In a comparative study by Prasad et al., 2017, the developed Solar EV model using MATLAB was further analyzed using a PVsyst simulator that enables actual-time conditions. PVsyst assesses the solar energy potential that the photovoltaic system yields (Kandasamy et al., 2013).

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5.4. Hybrid Optimization Model for Electric Renewables – HOMER HOMER is commonly used in grid-connected and off-grid energy systems. The software also provides an optimization and sensitivity analysis for a system and operation & maintenance, capital and interest cost calculations (Huang et al., 2020). HOMER can be used in modeling the SPCS system in different locations along with validation for the system. In addition, it is accessible to find the best PV conditions for a particular site (Abdul-Wahab et al., 2019). In optimizing the solar photovoltaic system for an EV, modeling analysis of the green transportation system was simulated using HOMER. The study revealed that nearly 21% of the 13,792 kWh/year of energy could be produced by a PV system that can be used for charging the EV (Chowdhury et al., 2018). In 2020, HOMER Energy announced that HOMER Grid, software for designing grid-tied distributed energy projects, can model how hybrid systems power EV charging stations (Lili Francklyn., 2020). A solarwind hybrid electric charging station was designed and improved using HOMER, which resulted in estimating the annual electricity production of 843150 kWh, which is available for EV charging.

5.5. Geographical Information Systems (GIS) In mobility, assessing the amount of energy captured from a traveling vehicle is difficult to estimate and calculate unless the location and radiation statistics are attained. GIS aids in attaining raw data through maps and satellite images. Several researchers have accessed satellite images to study the solar radiation pattern for a specific place. The assessment gave way to developing models and estimating the yield of energy from the area and a solar vehicle if driven. Radiation potential can be analyzed and calculated from photogrammetric data with a developed 3D model at low costs (Fuentes et al., 2020). Choi et al., 2019 reviewed the GIS-Based Solar Radiation Mapping, Site Evaluation, and Potential Assessment for Korea. GIS provides a Spatiotemporal Resolution of an area that makes it much more accurate in assessing the solar radiation potential (Chow et al., 2014). Using a Multifunctional Transport Satellite (MTSAT) satellite, the spatial distribution of cloud coverage can be investigated (Wong et al., 2016). In a study made to estimate a solar bus photovoltaic potential in urban driving conditions, the location and aerial image were captured that described the solar irradiance and bus route for analysis (Oh et al., 2020).

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In assessing the solar potential for a bus route in New Zealand, GIS maps were implemented with source point collection (Rehman et al., 2020). In analyzing the influence of solar glare intensity on vehicular speed, a case study was conducted by considering a GIS route map between Deokcheon and Jinrye (Woo et al., 2021). Focussing on SCPS, GIS was used to estimate the charging opportunities for a solar vehicle in parking lots and roads by accounting for shadows cast by surrounding trees and buildings (Nanda et al., 2020). A case study was put forward in the US with a GIS application focused on investigating solar photovoltaic potential in large-scale retailers’ parking infrastructure. The results portray that nearly 3.1 MW of solar potential is accessible for EV charging (Deshmukh and Pearce, 2021). In an attempt to match the solar photovoltaic generation and electric vehicle charging in the Netherlands, a specific sample region was considered through GIS that deduced an ability to implement PV systems that could yield almost 71%-92% of the EV charging load in summer (Fretzen et al., 2021).

6. Limitations of Current Energy Generation and Recovery Methods Utilizing solar photovoltaic energy to energize the vehicle is a refreshing approach to transportation. The application of solar as an alternative source has advantages in power generation, efficient energy conversion and availability. But the benefits are countered by several practical limitations when implemented. The significant limitations identified in this research have been outlined below. 1. A high-voltage drivetrain battery is required to lead to conversion losses and safety aspects concerning the high energy conversion and utilization rate. 2. An intelligent battery control and charge management system is essential. 3. The roof and bonnet curvature may incur a mismatch in irradiance, causing a drop in cell power and the output yield. And also, the aesthetic appearance and durability are depreciated. 4. The yield is erratic due to uncertainty in radiation throughout the day. 5. The development of Thin-film, Perovskite and Quantum dot cells are still under research for automotive application.

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6. There is no specific standard to be followed when implementing solar PV onto vehicles. 7. Vehicle usage during rain and night conditions does not serve the purpose of solar energy. 8. Several manufacturers report that the energy density of solar power is low and insufficient. 9. Unlike buses and trucks, space constraints cars for panel integration. 10. Several countries are unaware of the feasibility of solar energy for transportation; hence, government policies do not exist.

Conclusion Implementing vehicle-integrated photovoltaics for passenger vehicles has been in the automotive sector for the past two decades. Research on passenger E-vehicles ensures achieving UN SDG 11 and 13, i.e., sustainable cities, communities, and climate action. This chapter highlights the importance of technologies and methods for energy generation, recovery and source integration to electric passenger vehicles. It is understood from various case studies carried out worldwide that passenger vehicles having VIPV can generate more energy than conventional braking recovery and other methods. Vehicle photovoltaics validates being a short-chain energy conversion phenomenon with a higher energy conversion rate and minimal energy losses. Electric vehicles have an advantage in achieving sustenance in improving the vehicle range through VIPV. The approach can be advantageous for business cabs, especially in urban regions, reducing GHG emissions. In tropical and sub-tropical regions such as South-East Asian countries, consistent inbound solar radiation is experienced for nearly eight months a year. Hence, for those regions, VIPV would positively impact energy generation in passenger vehicles. However, research studies conducted for South-East Asian regions on energy generation capability with VIPV are scarce. In India, the concept of VIPV has a prominent scope due to rapid urbanization and geographical conditions. With several computational tools accessible, the scope of simulating and analyzing preliminary working models is highly feasible. The PV technology for household applications is different from that used in vehicles. Solar cell development methods for commercial power production are entirely different from VIPV. 3rd generation solar cells and various source integration techniques would overcome the conventional

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drawbacks caused and attain high efficiencies that match modern transportation requirements. Similarly, the cell development method for VIPV must consider the vehicle’s surface curve factor, temperature limits of operation, and vehicle dynamics. The research groups can identify respective problem statements based on the type of vehicle, regional meteorological conditions and the semiconductor technology used for VIPV. Possible optimal solutions can be derived using various simulation models and experimental validation. Hybrid VIPV performance may be affected by the stability of the cell, regional operating conditions, and travel range area utilization for VIPV. However, the hybrid performance of VIPV passenger vehicles is not limited by road conditions.

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Chapter 7

The Transition of Solar Thermal Energy in Recent Years and Future Directions for Industrial Utilization Haris Naseer1,* Syed I.U. Gilani1 Hussain H. Al-Kayiem2 and Muzaffar Ali3 1Mechanical

Engineering Department, Universiti Teknologi PETRONAS, Seri Iskandar, Malaysia 2University of Technology, Iraq 3Department of Mechanical Engineering, University of Engineering and Technology, Taxila, Pakistan

Abstract Solar energy is an excellent technology due to its wide applications and viability. The enhancement in thermal power requirements due to population expansion and the post-global pandemic emphasized boosting solar thermal energy harvesting. In recent years, the footprints of carbon dioxide emissions have been treacherous. To curb the devastating conditions, scientists and investors put their efforts into maximizing the utilization of alternative energy resources. Thus, solar energy power generation reached up to 1032.5 terawatts, and still, a bundle of projects is running out to tackle the vicious gas emissions. Tech-enhanced solar thermal collectors are developed for utilizing the maximum solar thermal energy. The concentrated solar thermal collectors are constructed to *

Corresponding Author’s Email: [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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Haris Naseer, Syed I.U. Gilani, Hussain H. Al-Kayiem et al. concentrate the direct and diffused radiations towards the receiver tube. The focus was not only on the designing parameters of solar thermal collectors. Researchers also modified the heat transfer fluids via nanotechnology and improved the thermal energy storage and carrier capacity. Nano-enhanced solar thermal collectors outclassed conventional water and building heating technologies. Solar thermal production must increase substantially to reach Net Zero Emissions by 2050. Numerous methods and technologies are under-developed, focusing on improving the designing parameters of solar thermal collectors, thermal-integrated industrial processes, heat transfer fluids capacity and analysis methods.

Keywords: industrial solar, solar heating, solar thermal, solar concentrators, solar collectors, future of solar

1. Introduction The people of ancient times used solar energy for heating the water for their public baths in 200 B.C. In the early 19th and 20th centuries, the re-invention of solar water heaters commenced (Wang, Xu, and Ge 2016). Due to increased thermal energy demand and electricity prices after World War II, researchers started replacing electrical water heaters with solar water heaters (SWH). The researchers also modified the SWHs design to augment the thermal performance to fulfill the energy requirement for industrial sectors. Almost 40-60% of the energy consumption is shared by the industrial sector in many countries, increasing to 30% from 2018 to 2050 (Rezaei et al. 2018). Conventional energy resources are continuously depleting to meet energy requirements. The extensive usage of these resources causes air pollutants such as nitrogen oxide and carbon dioxide emissions. According to a report published by BP Statistical Review of World Energy 2022, carbon dioxide (CO2) emission increased exponentially as it shifted from 11185.9 MT in 1965 to 33884.1 MT in 2021 (“BP Statistical Review of World Energy 2022” 2022). The CO2 emission worldwide for the last five years is presented in Figure 1. However, despite the pandemic, solar power generation has risen and achieved a 127.67% enhancement since 2017. Solar power generation for the different regions in 2022 is presented in Figure 2, which signifies the potential of solar energy for thermal power generation.

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Figure 1. CO2 emission in the last five years. 450.0 400.0

Axis Title

350.0 300.0 250.0 200.0 150.0 100.0 50.0 0.0 North South Europe America and Central Aerica

CIS

Middle East

Afirca

Asia Pacific

Total

Figure 2. Enhancement in solar power generation in regions.

2. Potential of Solar Energy for Water Heating Earth receives an average of 1000 W/m2 that can be sourced in various applications. The most promising, abundant, sustainable, and clean energy

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resource is solar energy which can easily be harnessed and directly transformed into heat energy using solar thermal collectors. Sun emits solar radiation at 3.8 X 1023 kW (Thirugnanasambandam et al. 2010). Photovoltaic panels (PV) are utilized for electricity generation to alter solar energy into electricity directly. The sun's heat energy can be harnessed to preheat the feedwater of the power cycle, which leads to reduced vicious greenhouse gas emissions and improved efficacy. Systems that convert solar energy to thermal energy are named “solar collectors”. They are produced in various types according to the required hot water temperature.

2.1. Solar Thermal Collectors (STCs) STCs are devices that capture solar irradiance and convert them to heat energy by absorbing it on the receiver plate or tube. The heat transfer fluids (HTFs) flowing inside the receiver tube absorb the heat energy and transport the absorbed heat to the thermal energy storage tank or directly to the application. Nowadays, developed solar thermal collectors can easily be installed on-roof to heat the water for domestic and commercial use (Barber and Provy 2010). The applications of these solar thermal collectors can reduce CO2 emission by up to 18.5 MT per year and might be able to supply 60-70% of the energy requirement per annum for the domestic sector (Alim et al., 2013). Almost 1012% annual solar thermal energy growth rate is calculated between 1990 and 2016, indicating the potential of solar thermal collectors (Kalogirou 2004).

2.2. Application of STCs in Industrial Processes Solar thermal energy can be utilized in numerous industrial processes to fulfill the thermal energy demand. The output of solar thermal collectors, especially thermal energy, can be utilized in various industrial processes. Many industrial processes mainly operate in the medium-temperature range (90-250°C). Solar thermal collectors can be used directly to heat the working fluids or indirectly by using the heat exchanger. Indirect application of solar thermal collectors, heat energy cannot be stored in the thermal storage tank to be utilized when the solar radiations are minimum or in cloudy weather. In indirect application, heat energy can be supplied to various industrial processes working fluids via a heat exchanger or stored in a thermal storage tank. In indirect applications, the performance of solar thermal collectors depends on the thermal

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effectiveness of HTFs. Due to low thermal conductivity and viscosity, conventional fluids possess a low heat transfer coefficient. The dispersion of nano-sized particles in conventional fluids improves the heat transfer coefficient, which widens the application of solar thermal collectors in industrial sectors.

A few common industrial processes with different ranges of temperatures and their corresponding suitable solar thermal collectors are demonstrated in Table 1. Table 1. Industrial processes with temperature range and recommended STCs Industry Type Paper

Dairy

Textile

Oil Refinery/ Plastics Chemical

Processes Drying Boiler Feed Water Bleaching Pressurization Sterilization Drying Bleaching, Dyeing Degreasing Fixing Storing Preparation Distillation Separation Soap Synthetic Rubber

Temp. Range (°C) 60-80 60-90 130-150 60-80 100-120 120-180 60-90 100-130 160-180 50-90 120-140 140-150 200-220 200-260 150-200

FPSC √ √ √ √ √

√

Solar Collector Type ETC CPC LFR √ √ √ √ √ √ √ √ √ √ √

√

PTC

√ √ √ √ √ √ √ √ √ √ √ √ √

√ √

√ √

2.3. Types of Solar Thermal Collectors Solar thermal collectors can be categorized based on working temperature range, i.e., low temperature (˂ 90°C), medium temperature (90 - 250°C), and high temperature (>250°C) (Suman, Khan, and Pathak 2015). Evacuated tube solar collectors (ETSCs) and flat plate solar collectors (FPSCs) are low-temperature solar collectors. These solar collectors have gained popularity because they do not require complex fabrication and operating systems and can be considered the productive type of solar collector. Parabolic trough collectors (PTCs) and Linear Fresnel reflectors (LFRs) are high-temperature STCs. These types of solar collectors have drawbacks of

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very complex fabrication and continuous solar tracking throughout the operation, high operational and maintenance costs, and they also occupy a large space. Two main categories of STCs are concentrating and non-concentrating. Concentrating solar collectors receive the solar radiations by a larger aperture collector area and concentrate onto a smaller area of the receiver. While in non-concentrating, the absorber area is almost equal to the aperture area. Flat plate and evacuated tube solar collectors are examples of non-concentrating solar collectors and have been widely employed in low-temperature applications (Thirugnanasambandam et al., 2010). Types of various solar thermal collectors are provided in Figure 3.

Figure 3. Types of STCs.

In non-concentrating STCs, solar radiations are captured on the surface of the collector without any concentration. In FPSCs, metallic plates absorb the heat energy through the incident solar radiation and transfer the collected energy to the fluids flowing inside the tubes. This energy can be used for storage or directly for application (Shamshirgaran et al., 2020). Evacuated tube collectors (ETCs) consist of evacuated tubes and a storage tank. Various configurations such as heat pipes, water in a glass, and U-tube are used in ETCs. ETCs possess better efficiency and performance than FPSCs (Eltaweel

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et al., 2021). An apparent glass tube, i.e., borosilicate glass used for enveloping the absorber tube. A vacuum is generated to suppress the convection heat losses from the receiver, and metallic fins are applied to absorb the maximum heat energy by reducing the gap losses (Akhter et al., 2019). FPSC, ETC, and evacuated tube configuration are shown in Figure 4.

Figure 4. (Continued)

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Figure 4. (A) Flat plate solar collectors, (B) Compound parabolic concentrator, (C) Evacuated tube receiver, (C) Linear Fresnel reflector and (E) Parabolic trough collector.

ETCs are more efficient than the FPSCs due to fewer heat losses and better energy conversion between the receiver plate to the working medium. The FPSC and the ETC are similar in that they absorb direct and diffused solar radiation (Kumar et al., 2019). Various solar collectors are displayed in Figure 4. In the concentring STCs, solar radiations are focused onto the absorber plate of a very small area compared to the concentrator area, which leads to higher temperatures. Common examples of concentrating STCs are parabolic trough collectors (PTC), linear Fresnel collectors (LFC), and heliostat systems (Islam et al., 2019). Parabolic trough collectors have gained attention as the most reagent solar thermal technology for medium-high temperature applications (Hachicha et al., 2013). PTCs are known for their parabolic shape, constructed by twisting reflective material into a parabola with a centrally

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located absorber tube. PTCs concentrate the falling solar rays on their axis to the absorber tube, which is mounted at the focal line of the collector. PTCs are generally designed to achieve high temperatures with a high concentration ratio (Huang et al., 2012). Linear Fresnel collectors or reflectors (LFRs) are designed to arrange linear strips that focus the light on an installed receiver on a linear tower. LFRs consist of long flat mirrors in which one or more receivers carry heat transfer fluids (HTFs). They are usually designed for temperatures above 300 ⁰C (Morin et al., 2012). LFRs have advantages over PTCs, such as elimination of fluid couplings, simple structure, denser packing of mirrors, fixed receiver, and low system costs (Islam, Huda, and Saidur 2019). However, the drawback is that they occupy a larger space for the reflectors. These collectors are suitable for installation in desert areas (Mills 2004).

3. Performance Assessment of STCs The performance of solar collectors is associated with the energy absorbed by the heat transfer fluids. Conventional heat transfer fluids possess weak thermophysical properties, which degrade the overall thermal performance of the system. Nanofluids exhibit more thermophysical properties than conventional HTFs, making them feasible for employment in solar collector applications (Ahmadi et al., 2019). The utilization of nano additive suspension in energy-efficient applications possesses the advantage of a significant heat transfer coefficient (Bellos et al., 2018). Solar collectors' performance is calculated using the laws of thermodynamics and ASHRAE standards.

3.1. Thermal or Energy Efficiency of STCs A solar collector's thermal or energy efficiency is assessed based on the first law of thermodynamics. The efficiency of a solar collector is defined as the ratio of heat gain to incident energy, which can be calculated using equation 1 (Sahin et al., 2020). ηeng =

Qu Ac IT

(1)

Ac and IT are the collector area and total solar radiation on the collector aperture.

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The gained thermal energy by the working fluid could be predicted by the formula given in Equations 2-4 (Shamshirgaran et al., 2020). Q u = ṁCp (Tf,out − Tf,in )

(2)

Q u = Ac FR [I0 (τα) − UL (Tf,in − Ta )]

(3)

Q u = Ac [S−UL (Tpm − Ta )]

(4)

Here FR is the heat removal factor and can be estimated by the formula given in equation 5. IT is solar radiation. τ is the transmissivity of the glass cover, and α describes the absorptivity of the nanofluids. The optical efficiency of STCs is expressed by S. FR = Q

Qu

(5)

u(T=Ti )

FR, (τα), and UL are constant in most cases. Ui is the overall heat loss from the collector estimated by equation 6 (Deceased and Beckman 1982). UL = Ui + Ub + Ue

(6)

Therefore, the thermal efficiency can be estimated by introducing the heat removal factor, as shown in equation 7 (Klein 1975). ηeng = FR [(τα) −

UL (Tf,in −Ta ) IT

]

(7)

3.2. Exergy Efficiency of STCs Exergy efficiency is based on the second law of thermodynamics, which is the maximum output we can obtain through the environmental temperature. The exergy balance equation can be arranged in the steady-state conditions Equation 8 (Farahat et al., 2009; Suzuki, 1988). The pressure difference between the inlet, outlet, atmosphere, kinetic, and potential energy is insignificant. Ė𝑥𝑖𝑛 + Ė𝑥𝑠 + Ė𝑥𝑜𝑢𝑡 + Ė𝑥𝑙 + Ė𝑥𝑑 = 0

(8)

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The exergy at the inlet, Ėx𝑖𝑛 , is the sum of fluid flow Ėx𝑖𝑛,𝑓 and the solar radiation absorbed Ėx𝑄 . It can be written as Equation 9-11: Ėx𝑖𝑛 = Ėx𝑖𝑛,𝑓 + Ėx𝑄

(9) 𝑇

Ėx𝑖𝑛,𝑓 = ṁ𝐶𝑝 (𝑇𝑖 − 𝑇𝑎 − 𝑇𝑎 𝑙𝑛 ( 𝑖 )) + 𝑇 𝑎

4𝑇

ṁΔ𝑃𝑖

(10)

𝜌

4

1 𝑇

Ėx𝑄 = 𝜂𝑜𝑝𝑡 . 𝐼𝑇 . 𝐴𝑐𝑜𝑙 [1 − 3 𝑇𝑎 + 3 ( 𝑇𝑎 ) ] 𝑠

(11)

𝑠

Where 𝑇𝑎 is the ambient temperature, Δ𝑃𝑖 is the pressure difference between the fluid and the atmosphere, 𝜂𝑜𝑝𝑡 is the optical efficiency of the collector, and 𝑇𝑠 is the sun's temperature equal to 5770 K. In the steady state, the stored exergy rate is zero and can be written as Equation 12. Ė𝑥𝑠 = 0

(12)

If only the fluid flow is investigated at the outlet, then the exergy rate at the outlet Ė𝑥𝑜𝑢𝑡 is predicted using Equation 13: 𝑇

Ė𝑥𝑜𝑢𝑡,𝑓 = −ṁ𝐶𝑝 (𝑇𝑜 − 𝑇𝑎 − 𝑇𝑎 𝑙𝑛 (𝑇𝑜 )) − 𝑎

ṁΔ𝑃𝑖 𝜌

(13)

Ė𝑥𝑙 is the exergy leakage rate from the receiver to the atmosphere Equation 14: 𝑇

Ė𝑥𝑙 = −𝑈𝐿 𝐴𝑠 (𝑇𝑝 − 𝑇𝑎 ) (1 − 𝑎 ) 𝑇𝑝

(14)

Where 𝑈𝐿 is the overall heat transfer coefficient occurs due to the temperature gradient between the absorber plate surface and the sun, there would be a destroyed exergy rate Ė𝑥𝑑 . The destroyed energy will be due to the difference in surface temperature of the plate and the sun Ė𝑥𝑑,Δ𝑇𝑠 , the gradient in temperature between the absorber plate and the working fluid Ė𝑥𝑑,Δ𝑇𝑓 and due to pressure drop Ė𝑥𝑑,Δ𝑃 .

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The combined equation can be written as Equation 15-16. Ė𝑥𝑑 = Ė𝑥𝑑,Δ𝑇𝑠 + Ė𝑥𝑑,Δ𝑇𝑓 + Ė𝑥𝑑,Δ𝑃 1

1

𝑝

𝑠

(15) 𝑇

Ė𝑥𝑑 = −𝜂𝑜𝑝𝑡 . 𝐼𝑇 . 𝐴𝑐𝑜𝑙 . 𝑇𝑎 (𝑇 − 𝑇 ) − ṁ𝐶𝑝 𝑇𝑎 (𝑙𝑛 (𝑇𝑜) − ṁΔ𝑃

𝑇 𝑇𝑎 𝑙𝑛( 𝑜 ) 𝑇𝑎

𝜌

(𝑇𝑜 −𝑇𝑖 )

𝑖

(𝑇𝑜 −𝑇𝑖 ) 𝑇𝑝

)− (16)

By merging all the above equations, the exergy efficiency of the STC can be calculated by Equation 17 (Farahat, Sarhaddi, and Ajam 2009). 𝑇𝑜 Δ𝑃 ))− ] 𝑇𝑖 𝜌

ṁ[𝐶𝑝 (𝑇𝑜 −𝑇𝑖 −𝑇𝑎 𝑙𝑛(

𝜂𝑒𝑥𝑔 =

𝑇𝑎 ) 𝑇𝑠

𝐼𝑇 .𝐴𝑐𝑜𝑙 .(1−

(17)

3.3. Economic Analysis of Solar Thermal Collectors The economic analysis accounts for the dependent and independent costs associated with the STCs. The selection of suitable materials for an STC system is established on the payback period, internal rate of return, net present value, and performance-based cost. The sustainability of a solar thermal system is concerned with harvesting energy with minimized costs, including all fixed and operating expenditures. STCs with high efficiency while maintaining a lower expenditure would be technologically feasible. The cost associated with STC can be estimated by using equation 18. Cs = Cf + Ca A + Ch.np.

(18)

A describes the collector area and Ch.np. illustrates those costs for nanoadditives, i.e., purchase, preparation, etc. Cf is the independent cost, and Ca is the dependent cost. Several studies have been carried out on the economic analysis of STCs. The results indicate that the solar thermal energy system is cost-effective due to increased fuel prices (Kumar et al., 2019). PVT-CPC solar collectors' performance-based cost analyses using CNT-water-based nanofluids (Arora et al. 2020a). The production cost (Cp) is noticed to be lower with SWCNTs as compared to MWCNTs. A novel low-concentrated medium-temperature

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CPC solar collector is utilized to perform economic analysis (Bhusal et al., 2020). The economic analysis demonstrates that the CPC solar collector can be installed for $0.5/watt and yield a Levelized cost of heat (LCOH) of just about 2.9 cents/kWh for a 20-year lifetime, which signifies the potential of the CPC for utilize in decarbonization the industrial sector. The application of nanofluids enhances the performance of solar thermal collectors, which can help reduce the collector area, store and transfer more heat energy from the solar collector, and more CO2 reduction due to less burning of fuels and natural gases. Literature related to economic analysis indicates that exploiting nanofluids as working fluids in solar thermal collectors significantly reduces CO2 emission and is more cost-effective due to the surge in fuel and natural gas prices. However, it affects the life span of solar thermal collectors due to the continuously wearing absorber tubes, pumps, and pipes, increasing operational and maintenance costs. This detrimental impact on expenditure can be overcome by employing nanofluids as a working medium in solar thermal collectors.

4. Recent Advancements in Solar Thermal Collectors In recent years, modifications have been employed in the existing solar thermal collector to improve the thermal performance of collectors. Four essential factors affect the performance assessment of solar collectors. These factors are shown in Figure 5.

Figure 5. Factors influencing the performance of STCs.

Choosing the absorber material with higher absorptance is one renowned way to enhance thermal efficiency. The alternative method to augment the performance of the solar collector is related to heat transfer fluids (HTFs). Heat transfer fluids (HTFs) absorb solar thermal energy by flowing through the receiver tube and transporting it to the thermal storage tank or the place of application. This indicates the significance of the thermophysical behavior of

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heat transfer fluids as a working medium. The HTFs flowing inside the solar thermal collectors exhibit a vital role in enhancing the performance of the solar thermal collectors. A slight enhancement in the thermal effectiveness of these HTFs can substantially boost the thermal performance of the STCs.

4.1. Heat Transfer Fluids for Solar Thermal Collectors ` Various heat transfer fluids (HTFs) are employed as working fluids in solar thermal collectors. The primary function of HTFs is to absorb solar thermal energy by flowing inside the absorber tube and transporting this energy to the storage tank or directly to the application. Water is most commonly used as a working medium in heat transfer applications due to its availability at any location, high thermal properties, non-toxic and inexpensive. The most effective physical property of the water is the low viscosity among the other heat transfer fluids, i.e., thermal oils and glycols. The low viscosity of the water makes it feasible to be easily pumped, and relatively higher thermal conductivity enables the water to utilize in thermal energy transport (Loni et al. 2018). However, the low boiling point of water at normal pressure restricts it to use in medium-temperature application devices. Another main drawback is the corrosion formation in the pipes due to water usage as a working medium. Water is not feasible for mediumtemperature applications such as sterilizing, dyeing, printing, and water treatment as it converts to steam and loses its thermal efficiency (Akhter et al., 2019). The disadvantage of utilizing water is overcome by utilizing the mixture of water and different glycols to prepare nanofluids with a slight enhancement in the boiling point and decrement in the freezing point. Ethylene glycol (EG) is an anti-freezing reagent employed in many heating and cooling systems (Vafaei et al., 2017; Leong et al., 2018; Sundar et al., 2013). Thermal oils such as paraffinic, aromatic (refined mineral), and synthetic hydrocarbons have higher boiling points at normal atmospheric pressure than water and are used in high-temperature range applications. However, oils have high viscosity and low thermal conductivity, which impact the overall system performance (Asadi and Pourfattah 2019). HTFs employed in medium-temperature solar collectors must include the following characteristics. 1. High thermal conductivity allows the HTF to carry the thermal energy more rapidly and better heat transfer between the HTFs and the receiver.

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2. Low viscosity reduces the pressure drop in the thermal circuit lop and minimizes the required pumping power. 3. High specific heat capacity enables the HTFs to store heat energy. The literature on desired characteristics of HTFs for medium-temperature solar collectors illustrates that almost no conventional HTFs possess all the desirable properties. Thus, it urges the development of new fluids with enhanced thermophysical properties after introducing nanoparticles of metals, metal oxides, and carbon-based nanotubes that possess better thermophysical properties than conventional HTFs. Over the years, many efforts have been made to disperse these nanoparticles in conventional fluids and use these nanofluids for solar thermal harvesting.

5. Nano-Enhanced Heat Transfer Fluids Nanoparticles exhibit better thermophysical properties than conventional fluids and can be dispersed in fluids to boost the thermal effectiveness of the HTFs. The particles of the size of 1-100 nm are pioneered by (Choi 1995). There are many nanoparticles of metallic, metallic oxides, single-wall carbon nanotubes (SWCNTs), multiwall carbon nanotubes (MWCNTs), graphite, etc. (Naseer et al., 2021). These nanoparticles have been manufactured using mechanical and chemical techniques such as ball milling, catalytic chemical vapor deposition (CCVD), plasma treatment, and many more (Sundar et al., 2017). It seems reasonable to disperse certain nanoparticles into conventional HTFs to improve thermal conductivity. Before the introduction of nanoparticles, researchers suspended the particles of millimeters or micrometers, which led to problems such as poor stability, which resulted in clogging in the micro-channels of the heat exchangers. Due to these reasons, nanoparticles have gained researchers' attention worldwide due to their superior behavior. The thorough process of selecting nanoparticles for specific applications, the preparation of nanofluids with various concentrations and measurements of thermophysical properties are briefly discussed by various researchers, which is out of the scope of the chapter. The chapter summarizes the development of solar collectors, modification and applications of nanofluids.

Table 2. Development of Flat plate collector through design modification and adoption of nanoadditives in the working fluid Ref.

Investigation mode Experimental & Numerical

Parameters/ Modifications Spiral diameter, flow rate and pipe diameter

Nanoparticles+ Base Fluids Water

Flow range 0.5-2.5 lit/min

Kohol et al. 2022

Numerical

Solar radiations, flow rate, working fluid inlet and ambient temperature

Water

0-0.1 kg/s

Maio et al. 2022

Numerical

-

-

Verma et al. 2018

Experimental

Cr2O3/Cr-based coating was designed and considered for the collector's efficiency robustness analysis. Energetic and Exergetic Performance

0.5-2.0 LPM

Khin et al. 2017

Numerical

CuO-MWCNTs and MgOMWCNTs in Water CuO – Water

Hussein et al. 2020

Numerical

cf-MWCNTs-cfGNPs- Distilled water 0.05-0.1% wt.

2-4 LPM.

Aghakhani et al. 2022

MATLAB code calculating the efficiency considering weather conditions of Myanmar Thermal performance assessment

0.02 kg/s

Outcomes The pipe diameter and spiral's outer diameter increasing, along with the flow rate decreasing, raises the FPSC's outlet temperature. Increasing working fluid flow rate, temperature, ambient temperature, and insulation thickness positively affect the collector's performance. Increased solar radiation decreases the collector efficiency. Increases the exergy efficiency from 12.58% to 32.69%. The proposed SSAs in an EFPC were expected to have an overall efficiency that includes the cover glass optical losses and the substrate radiative losses of 0.84, 0.74, and 0.56 at 100, 200, and 300°C, respectively. The optimum volumetric fractions of nanoparticles are in the range of 0.75-1.0%. Almost 25.1% and 16.28% enhancement in thermal efficiency concerning the base fluid and MgObased mono nanofluid, respectively. Almost 5% enhancement is reported compared to the control fluid using CuO-based nanofluids.

Hybrid nanofluids perform better than mono nanofluids with the same nanoparticles. Thermal performance has an ascending relation with weight concentration. Almost 20% higher thermal efficiency is observed than in distilled water.

Table 3. Development of Compound Parabolic Concentrator through design modification and adoption of nanoadditives in the working fluid Ref. Lu et al. 2011) Liu et al. 2013 Q. Li et al. 2016

Investigation mode Experimental Experimental Experimental & Numerical

Chen and Liu 2022

Experimental & Numerical

(Khaledi et al., 2022)

Exp

Parameters/Modifications Thermosyphon and high temperature ETR Thermal performance assessment, Thermosyphon Comparison between the volumetric and conventional surface absorber Multi-section CPC with vacuum tube absorber, Rays incident angles 0, 20, 40 and 60.

Energy, hydraulic and exergy analysis

Nanoparticles+ Base Fluids CuO – DI water 0.8-1.5% wt. CuO – Water 1.2% wt. MWCNTs – Water 50 mg/L

Flow range -

-

-

MWCNTs-Silica (10:90)-EG-Water

1, 1.5, 2, 2.5 Lit/min

0.02 kg/sec

Outcomes CuO-based mono nanofluids significantly enhance collector performance by 30%. The dispersion of CuO nanoparticles in the base fluid enhances the collector efficacy MWCNTs-based nanofluids enhance efficiency. CPC is more suitable for providing thermal energy in the range of ≤ 250°C. The performance of solar radiation of M-CPC is significant, so that it would be a suitable solution for the application. The price and the resources are cut down, which makes it efficient in a wide range. Nanofluids improved the exergy efficiency with an average enhancement of 45%. Hybrid nanofluids outclassed the mono nanofluids and increased efficiency by 45% and 25%.

Table 4. Development of Direct Absorber Solar Collector through design modification and adoption of nanoadditives in the working fluid Ref.

Investigation mode Experimental & Numerical

Parameters/Modifications

X. Li, Zeng, and Lei 2020

Experimental

Menbari et al. 2016

Experimental & Numerical

Karami et al. 2016

Experimental

Delfani et al., 2016)

Nanoparticles+ Base Fluids f-MWCNTs – EG. 70:30 vol.

Flow range 0.0150.025 kg/sec

Optical and solar-thermal conversion performance

MWCNTs-SiC – EG. 0.01-1.0% wt

-

Direct Absorption PTC analysis by Radiative transfer and the energy equations Thermo-Optical analysis

CuO – Water 0.002-0.008% vol.

10-100 L/hr.

CuO – Water 0-100 ppm

-

Radiative transfer equation combined with the energy equation

Outcomes The collector's efficiency is enhanced by increasing the nanoparticle concentration and mass flow rate. The nanofluids improve thermal efficiency by up to 1029%. The solar thermal conversion increases with the nanoparticle fractions. The maximum 97.3% conversion efficiency is achieved at 1%, which is 48.6% higher than the base fluid. Mass flow rate and nanoparticle addition increase thermal efficiency. The thermal efficiency of the DASC increases from 18% to 52%. CuO performs better at higher temperatures and absorbs more heat than the base fluid.

Table 5. Development of Evacuated Tube Solar Collectors through design modification and adoption of nanoadditives in the working fluid Ref. Ghaderian et al., 2017 Eltaweel, et al. 2021

Investigation mode Experimental

Parameters/Modifications

Experimental

Comparative analysis between FPSC and ETSC in terms of energy efficiency

ET-based water heater coupled with internal coil and thermosyphon circulation.

Nanoparticles+ Base Fluids CuO – Distilled Water 0.03%-0.06% vol.

Flow range 20-60 L/hr.

MWCNTs – Water 0.005-0.05% wt.

0.00970.0388 kg/s m2

Outcomes The temperature of the nanofluid at the exit of the ETSC increases by up to 14%, and thermal efficiency enhances up to 51.4% with 0.6% of CuO. ETSCs perform better than FPSCs. Almost 59% energy efficiency is achieved when using MWCNTs nanofluids.

Table 6. Development of Parabolic Trough Collectors through design modification and adoption of nanoadditives in the working fluid Ref. Ghasemi and Ranjbar 2016 Menbari et al. 2017

Investigation mode Numerical

Experimental

Parameters/Modifications Forced convection turbulent flow inside receiver tube to calculate hydrodynamically and heat transfer coefficients Parabolic trough collector integrated with DASC

Nanoparticles+ Base Fluids CuO and Al2O3 – Water 0.5-3.0% vol

Flow range

Outcomes

-

CuO-based mono nanofluids exhibit better thermophysical properties than Al2O3. Heat transfer enhances up to 35% by using CuO.

CuO-Al2O3 - Water and water-EG

10-100 L/hr.

CuO shows better optical absorption properties and thermal conductivity. Thermal efficiency of 48.03% is achieved at 0.2% Al2O3-0.008%. CuO/water and 45.9% by 0.2% Al2O3-0.008% CuO/water-EG PTC outlet temperature has an ascending function with the volume fraction. MWCNTs possess higher thermal efficiency than SiO2, which is 30.4% higher than the base fluid. The thermal efficacy of Cu-EO over Fe3O4-EO is enhanced by 1.6% and 14.9%. Results revealed an increase in Reynolds number Re, Brinkmann number Br, and Eckert number. The highest exergy efficiency is attained for nanofluids, up to 28%. Inserting twisted perforated tape improves the (Nusselt number) compared to a tube without a tape. Compared to the plain tube condition, the Nusselt number increases by 5 to 40%, 11 to 101 percent, and 7 to 77 percent in conventional twisted tape ring attached to twisted tapes and modified rings attached to twisted tapes, respectively.

Kasaeian et al. 2017

Experimental

Glass-Glass Direct Absorption Solar collector tube mounted on PTC.

MWCNTs and SiO2 – EG. 0.1-0.3% vol.

1.15 kg/hr

Alkathiri et al. 2022

Numerical

Galerkin Finite Element method,

Copper/MagnetiteEngine Oil

-

Abidi et al. 2022

Numerical

Receiver tubes combined with twisted perforated tape

CuO-Water 1%, 2% and 4% vol.

Re=10,000, 20,000 and 30,000

Suresh Isravel et al. 2020

Experimental

Conventional Twisted Tape and Rings attachments

Water

-

Table 7. Development of Photo voltaic/Thermal Collectors through design modification and adoption of nanoadditives in the working fluid Ref.

Investigation mode Numerical

Parameters/Modifications

Roshdan et al. 2022

Numerical

PV/T solar collector using asymmetric CPC.

Arora et al. 2020

Numerical

N-PVT-CPC integrated with helically coiled heat exchanger

Kazemian et al. 2022

3-Dimensional Simulation and Eulerian-Eulerian models

Nanoparticles+ Base Fluids MWCNTs-Al2O3/ MWCNTsSiC/GrapheneAL2O3 and Graphene-SiC. Water

SWCNT/MWCNTWater

Flow range -

0.00820.1064 kg/s 0.03 kg/s

Outcomes MWCNTs-SiC improved the electrical and energy efficiency by 13.85% and 56.55%, respectively. There is a positive relationship between the volume concentration and the energy and exergy efficiency. The energy and electrical efficiency are the highest at the mass flow rate of 0.034 kg/s compared to the PV/T-CPC and flat PV/T solar collector. PV-T performs better by using CNTs nanoparticles. The self-sustained system with a distillation process makes it more compatible with fulfilling domestic requirements.

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Researchers have employed nanofluids for the performance enhancement of concentrating and non-concentrating solar collectors. The augmented thermophysical properties of nanofluids boost the heat transfer rate between the fluid and the receiver surface, improving the solar collectors' performance. Researchers have theoretically or experimentally investigated nanofluids' thermal effectiveness under various parameters' influence. Important findings of a few investigations research related to the development and nanotechnology applications in concentrating and non-concentrating SCs are tabulated in Table 2 to Table 7.

6. Prospects of Solar Thermal Energy Renewable energy must increase exponentially to attain the share of 60 percent anticipated by the IEA Net Zero Scenario by 2030. Solar thermal energy is the key player in attaining the Net Zero Scenario. Various scientific projects are running to develop the newly tech-enhanced solar thermal collectors and heat transfer mediums for energy conversion and thermal and storage management. Some of the future directions are given below in Figure 6.

Figure 6. Future directions of advancement in solar thermal collectors.

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Conclusion Solar energy has the potential to achieve the Net Zero Scenario by providing clean and sustainable energy and to fulfill the requirements of future power demand. Scientists have developed tech-effective STCs to utilize the maximum exergy of solar radiation. Concentrating STCs have outclassed conventional STCs. Compound parabolic and parabolic trough collectors integrated with evacuated tube receivers are the newly developed collectors. They utilized the maximum solar radiation incident on the aperture area and transferred the energy in heat transfer fluids. Nanotechnology has played a vital role in improving the energy and exergy efficiency of STCs by improving the heat transfer coefficient. Researchers are advancing solar thermal collectors' technology for prospects, including the parametric evaluation, design modifications, heat transfer fluids capability and analysis tools and techniques which make solar thermal energy more feasible and sustainable for domestic and industrial applications.

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Chapter 8

Solar Thermal Energy Storage: Materials, Heat Transfer Analysis and Applications Sulaiman Al-Hashmi* Center for Environmental Studies and Research, Sultan Qaboos University, Oman Energy Research Institute, University of Leeds, UK

Abstract Solar thermal energy storage (TES) is a system that collects and stocks thermal energy through heating/cooling in a storage medium. The stored energy can be used as the primary source later when there is no solar input. Solar TES is one of the promising approaches to encourage the adoption of solar energy in a broader range, as it recovers the setback of the interrupted solar process for heating-cooling sources and power generation. Solar TES technologies are used mainly in building and commercial processes. This chapter deals with solar TES technologies, which evaluate solar energy and decreeing the energy demand of a building, especially in hot regions. The principle and calculation of the storage capacity of several solar thermal energy storage systems are explained. Heat-sensitive storage technologies are briefly reviewed, including water tanks and underground and packed-bed storage methods. Moreover, it discusses a latent-heat storage system associated with phase-change materials for use in solar heating/cooling of buildings, solar water heating, heat-pump systems, and concentrating solar power plants, as well as thermo-chemical storage. Finally, cold thermal energy storage is also briefly reviewed, and unprecedented information on the efficiency and cost of solar TES systems is included.



Corresponding Author’s Email: [email protected].

In: The Future of Solar Power Editor: Hussain H. Al-Kayiem ISBN: 979-8-88697-709-7 © 2023 Nova Science Publishers, Inc.

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Keywords: solar thermal energy storage, energy performance, phase change materials, heating/cooling storage

1. Introduction Recent projections indicate that primary energy demand will rise by 48% in 2040 (European Union Energy in Figures, 2016). On the other hand, the switch to sustainable energy sources is accelerated because of the depletion of fossil fuel reserves and the environmental harm they cause. To restore the natural order and meet the demands of the growing population, renewable energy sources such as solar energy, ocean waves, wind, and biogas have proven essential (Twidell and Weir, 2015). Due to the environment’s unpredictable nature, techniques for storing these renewable energy sources are urgently needed (Iten et al., 2016). Systems using renewable energy technology now heavily rely on energy storage. A technique known as thermal energy storage (TES) uses heat or cold to store thermal energy that can later be used for power generation, heating and cooling systems, and other purposes. TES systems are primarily employed in construction and industrial processes. Using TES in an energy system has several benefits, including improved overall efficiency and higher reliability. It can also improve economics by lowering investment and operating costs and reducing environmental pollutants, i.e., carbon dioxide (CO2) emissions. Solar thermal systems use a significant portion of the Sun’s thermal energy during the day and are industrially mature, in contrast to photovoltaic systems with soaring efficiencies. But it lacks sufficient (thermal) backup to function even when there is little or no solar radiation. Combining concentrating solar power (CSP) plants with TES is becoming increasingly crucial for electricity storage since it allows solar heat to be stored for power generation when sunshine is scarce. To enable a 24-hour operation in a productive TES system, new materials are chosen, characterized, and improved in terms of their thermo-physical properties. Using heat and cold storage more extensively, it has been calculated that Europe could save about 1.4 million GWh/year and avoid 400 million tons of CO2 emissions from buildings and industrial sectors (The Energy Technology Systems Analysis Programmes, 2013). Storage density is energy per unit volume or mass of the TES. Optimizing solar ratio (the amount of solar radiation for heating and cooling), appliance efficiency (such as solar thermal collector and absorption chiller efficiency),

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and energy consumption for space heating and cooling consumption depend on storage density. Therefore, looking into the potential for utilizing phasechange materials (PCMs) in solar system applications would be wise. By decreasing the solar storage volume for a given solar fraction or raising the solar fraction for a given available volume, PCMs can boost the energy density of small-sized water storage tanks (Medrano et al., 2009). Thermal storage is possible on the plant’s hot or cold side. The former enables the supply of stored hot water from the collectors (and from the auxiliary heater) to the absorption chiller generator (in cooling mode) or straight to the consumers (in heating mode). With the latter, it is possible to supply the cooling terminals inside the building with the cold water that the absorption chiller produces and stores. The three storage conditions are typically classified as “hot,” “warm,” and “cold” based on the various temperature ranges. The typical operating range for a hot tank is 80–90°C, for a warm tank is 40–50°C, and for a cold tank is 7–15°C (Noro et al., 2014). Because heating and/or the production of domestic hot water (DHW) require heat storage on the hot side of solar plants, cold storage is acceptable in larger plants. In addition to gaining financial benefits from decreased electricity prices (in the case of electric compression chillers) depending on the time of day, cold storage is also utilized to reduce the installed cooling power and to enable the chiller to run more continuously (Khan et al., 2017). Thermal storage was initially unable to offer adequate thermal backup, although it did assist in the thermal stabilization of the system. As a result, solar-assisted thermal systems began to use thermal storage (Chidambaram et al., 2011). Since then, research into thermal energy storage technology has risen, as well as the usefulness and consequences of both sensible and latent heat storage in a variety of applications (Sharma et al., 2009; Zhou et al., 2012; Moreno et al., 2014; Moreno et al., 2012 and Zhai et al., 2013). These studies concentrated solely on one system component, side (cold or hot), or integral mechanism. These investigations focused on a single integral mechanism, side (cold or hot), or system component. Tian and Zhao, 2013 collated various studies on thermal energy storage systems and solar collectors in solar thermal applications. For example, Pintaldi et al., 2015 studied control strategies and thermal energy storage technologies for solar cooling systems. They primarily concentrated on different thermal storage technologies in sun-cooling applications, concentrating on higher temperatures (>100°C). A review of the PCM for cold storage for household refrigeration, i.e., only on the evaporator side, was put

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together by Joybari et al., 2015. Cold storage and air conditioning were covered separately in the study by Oró et al., 2012. Solar TES technologies that offer a means of valuing solar heat and lowering the energy demand of buildings are the primary subject of this chapter. The foundations of various energy storage techniques and predictions are presented. The utilization of water, subterranean, and packed-bed storage techniques are just a few examples of sensible heat storage (SHS) technologies that are briefly described here. Thermo-chemical storage (TCS) and latentheat storage (LHS) systems linked to PCMs for solar building heating and cooling, solar water heating, heat-pump systems, and CSP plants are also covered. Finally, a brief discussion of cooling thermal energy storage is given, along with essential details on the effectiveness and expense of TES systems.

2. Solar Thermal Technologies In solar thermal and PV (Photovoltaic) applications, a solar collector, a specialized energy exchanger, transfer solar radiation energy directly into electric energy or the thermal energy of the working fluid in solar thermal applications. A solar collector receives solar irradiation as heat, which is transmitted to its working fluid in thermal applications (air, water or oil). A thermal energy storage tank can be charged with the heat carried by the working fluid to utilize it later to supply household hot water or heating (at night or on cloudy days). A PV module for PV applications converts solar irradiation directly into electric energy (typically with relatively low efficiency) and generates a lot of waste heat. The waste heat produced by a PV module for PV applications can be recovered for thermal use by connecting the PV board with recuperating tubes filled with carrier fluids. Concentration ratios (Iten et al., 2016) show that solar collectors are typically divided into non-concentrating and concentrating collectors. A suntracking concentrating solar collector typically has concave reflecting surfaces to intercept and focus the solar irradiation to a much smaller receiving area, resulting in an increased heat flux so that the thermodynamic cycle can achieve higher efficiency. However, the non-concentrating collector has the same intercepting area as its absorbing area as the flat plate solar collector. Al-Kayiem 2019 classified the solar thermal technologies for power generation in Figure 1. His classification subdivided solar power technologies into high-temperature (HT) and low-temperature (LT) systems. He concluded

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that all those technologies require integration with TES to resolve the solar interruption problem and increase the systems’ reliabilities. Solar power systems can perform a more extended period of the day and have higher cumulative efficiency. The production scenario proposed by AlKayiem 2019 by integrated solar systems with TES is shown in Figure 2.

Figure 1. Classification of solar technologies for power generation based on temperature level. (Al-Kayiem 2019).

Figure 2. Production scenario of integrated solar plants with energy storage (Al-Kayiem 2019).

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3. Classification and Characteristics of Thermal Storage Systems TES has a place in thermodynamic systems because of intermittent availability and continual change in solar radiation. By conserving energy, TES increases the system’s performance and thermal reliability while reducing the gap between supply and demand. Therefore, it is crucial to create TES systems that are effective and affordable. However, only a small number of solar thermal plants have used TES extensively worldwide. In addition, research into developing TES systems for various household solar applications is ongoing (Joseph et al., 2016). A popular way to cut costs is to use computational fluid dynamics, and FLUENT software appears to be used well for various engineering applications (Al-Abidi et al., 2013). Figure 3 presents the primary solar thermal energy storage types. The following features can be used to characteristics an energy storage system (The Energy Technology Systems Analysis Programmes, 2013): 



Depending on the capacity of storage, the storage medium, and the size of the system. Capacity describes the amount of energy that can be held in the system; Power is a term used to describe how quickly a system’s energy may be released (and charged);

Figure 3. Types of solar thermal energy storage.

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



 

Efficiency is the ratio of energy delivered to users to the energy required to refuel the energy storage device. It takes into consideration the energy lost throughout the charging and draining cycles; Hours to months (i.e., hours, days, weeks, and months for seasonal storage) make up the storage period, which specifies how long the energy is kept in reserve; The amount of time required to charge and discharge the system is defined by the charge and discharge time; and Cost is expressed in terms of the storage system’s capacity (€/kWh) or power (€/kW), and it is influenced by the equipment’s lifetime, capital costs, and operating expenses (i.e., the number of cycles). Table 1. Comparison of TES system specifications (Hauer, 2011)

TES System Sensible storage (Hot water) Phase change material Chemical reactions

Capacity (kWh/t) 10-50

Power (MW) 0.001-10.0

Efficiency (%) 50-90

Storage Period day/months

Cost ($/kWh) 0.1-10

50-150

0.001-1.0

75-90

hours/months

10-50

120-250

0.01-1.0

75-100

hours/day

8-100

Figure 4. Three main processes of solar thermal energy storage: (a) Latent heat, (b) sensible heat, and (c) thermochemical method (De Garcia and Cabeza, 2015).

Power, capacity, and discharge time all depend on one another. Power and capacity may be interdependent in some storage systems. Any storage system

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should have a high power capacity for charging and discharging and a highenergy storage density. Table 1 (Hauer, 2011) displays typical TES system metrics such as capacity, power, efficiency, storage time, and cost. The three TES methods, sensible heat, latent, and thermo-chemical heat storage associated with chemical reactions, shown in Figure 4, are well established to work at temperatures between -40°C and more than 400°C (De Garcia and Cabeza, 2015). The three methods are based on the materials used in the TES system. Hence, the TES characteristics follow the thermophysical properties of the storage medium.

3.1. Sensible Heat Storage The most straightforward approach, known as SHS (Figure 4b), involves storing thermal energy by heating or cooling a solid or liquid medium (such as water, sand, molten salts, or rocks), with water being the least expensive choice. SHS has two key benefits: it is inexpensive and doesn’t come with the risks of using harmful ingredients. Water is the most well-liked and widely used heat storage medium and has many industrial and domestic uses. For typically large-scale applications, underground sensible heat storage in liquid and solid mediums is also utilized. The SHS system uses the storage medium’s heat capacity and temperature variation when charging and discharging. The specific heat of the medium, the temperature change, and the quantity of storage material all affect how much heat is stored (Kumar and Shukla, 2015). 𝑡

𝑄𝑆 = ∫𝑡 𝑓 𝑚𝐶𝑃 𝑑𝑡 = 𝑚𝐶𝑃 (𝑡𝑓 − 𝑡𝑖 ) 𝑖

(1)

where Qs is the quantity of heat stored in Joules; m is the mass of heat storage medium, in kg; Cp is the specific heat, in J/(kg·K); ti is the initial temperature, in °C; tf is the final temperature, in °C. Table 2 shows the popular SHS materials and their properties (Ayappan et al., 2016). Water appears to be the best SHS liquid because it is inexpensive and has high specific heat. However, above 100°C, oils, molten salts, and liquid metals are used. For air heating applications, rock bed storage materials are used. Water seems to be the best SHS liquid currently accessible due to its low cost and high specific heat. Oils, molten salts, and liquid metals are molten

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when temperatures exceed 100°C. Storage materials of the rock bed type are molten for air heating applications. Table 2. List of selected solid and liquid materials for sensible heat storage (Ayappan et al., 2016) Medium

Material type

Rock Brick Concrete Granite Aluminum Cast iron Water Calorie HT43 Engine oil Ethanol Propane Butane Isotunaol Isopentanol Octane

solid solid solid solid solid solid liquid oil oil Organic liquid Organic liquid Organic liquid Organic liquid Organic liquid Organic liquid

Temperature Range (°C) 20 20 20 20 20 20 0-100 12-260 ≤160 ≤78 ≤97 ≤118 ≤100 ≤148 ≤126

Density (kg/m3) 1555 2560 1600 2240 2640 2707 7900 1000 867 888 790 800 809 808 704

Specific heat (J/kg·K) 879 840 880 820 896 837 4190 2200 1880 2400 2500 2400 3000 2200 2400

Table 3. Solid-state sensible heat storage materials (Tian and Zhao, 2013) Storage Materials

Sand-rock minerals Reinforced concrete Cast iron NaCl Cast steel Silica fire bricks Magnesia fire bricks

Working Temperature (°C) 200-300 200-400 200-400 200-500 200-700 200-700 200-1200

Density (kg/m3) 1700 2200 7200 2160 7800 1800 3000

Thermal Conductivity (W/(m·K)) 1.0 1.5 37.0 7.0 40.0 1.5 5.0

Specific Heat (J/(kg·K)) 1.30 0.85 0.56 0.85 0.60 1.00 1.15

The primary properties of the most popular solid-state thermal storage materials (Tian and Zhao, 2013), such as sand-rock minerals, concrete, firebricks, and ferroalloy materials, are displayed in Table 3. These materials have working temperatures from 200 to 1200°C and have excellent thermal conductivities: 1.0–7.0 W/(m·K) for sand-rock minerals, concrete, and fire bricks; 37.0–40.0 W/(m·K) for ferroalloy materials. All the materials in Table 3 are inexpensive, with prices ranging from 0.05 to 5.00 $/kg. The sole

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drawback is that the storage unit may need to be unreasonably large due to its relatively low heat capacity, which ranges from 0.56 to 1.3 kJ/(kg∙°C).

3.2. Storage of Water Tanks Water is the most commonly used material in a sensible heat storage system. Hot-water tanks are a well-known thermal energy storage technology (Basecq, 2013). Hot-water tanks save energy in water heating systems by using solar energy and co-generation (i.e., heat and power) energy supply systems. As demonstrated by cutting-edge initiatives (Energy Conservation through Energy Storage Programme, 2016), water tank storage is a cost-efficient solution. Its efficiency can be further increased by guaranteeing proper water stratification in the tank and highly effective thermal insulation. Today’s R&D initiatives, for example, are focused on evacuated super-insulation with thermal conductivity of 0.01 W/(m∙K) at 90°C and 0.1 mbar and optimized system integration. Figure 5 depicts a typical system that employs a water tank.

Figure 5. A solar thermal system using water tank storage (Sarbu and Sebarchievici, 2016).

The energy storage capacity of a water (or other liquid) storage unit at a uniform temperature (i.e., thoroughly mixed or no stratified) functioning across a finite temperature difference is provided by Equation (2), rewritten as:

Solar Thermal Energy Storage

𝑄_𝑠 = 𝑚𝐶𝑝∆𝑡_𝑠

227

(2)

Qs is the total heat capacity for a cycle that works over the temperature range ts, and m and Cp are the mass and specific heat of the water in the unit. For most processes, the lowest temperature at which this kind of unit can work is limited by the needs of the process. The process, the vapor pressure of the liquid, or the collector heat loss may determine the upper limit. The balance of energy on the no-stratified tank is: 𝑄𝑆 = 𝑚𝐶𝑃

𝑑𝑡𝑠 𝑑𝜏

= Qu − 𝑄𝐿 − Us 𝐴𝑠 (𝑡𝑖 − 𝑡𝑎 )

(3)

where Qu and QL are rates of addition or removal of energy from the collector to the load. Us is the heat loss coefficient of the storage tank; As is the storage tank surface area; ti is the final temperature, in °C; ta is the ambient temperature for the tank; τ is the time. 𝑡𝑆 = 𝑡𝑖 +

𝛥𝜏 𝑚𝐶𝑃

[Qu − 𝑄𝐿 − Us 𝐴𝑠 (𝑡𝑖 − 𝑡𝑎 )]

(4)

Equation (3) is to be integrated over time to determine the long-term performance of the storage unit and the solar process. Practical long-term analytical solutions are not possible due to the complex time dependence of some of the terms. There are many possible numerical integration methods. Using simple Euler integration is usually satisfactory (i.e., rewriting the temperature derivative as (𝑡𝑠 −𝑡𝑖 )/∆τ and solving for the tank temperature at the end of a time increment): Equation (4) can be used to predict water storage temperature as a function of time. Once the tank temperature is known, other temperature-dependent quantities can be estimated. Hot water storage systems used as buffer storage for hot water supply are usually 500 L to several cubic meters (m3). This technology is also used in solar thermal installations for hot water and building heating systems (comb-systems). Large hot-water tanks are used for seasonal solar thermal heat storage in combination with small district heating systems. These systems can have a volume of up to several thousand cubic meters. Charging temperatures are in the range of (80–90°C). The usable temperature difference can be enhanced by using heat pumps for discharging (down to temperatures around 10°C) (Sarbu and Sebarchievici, 2016).

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Figure 6 shows a more complex solar thermal system with tank storage. The water store is the central part of the system. Solar collectors and a second heating source, such as a biofuel or gas boiler, charge it.

Figure 6. Schematic of a solar combisystem.

Heat is extracted from two heat sinks with very different characteristics: domestic hot water and space heating (Heier et al., 2015). Solar combisystems, like combistores, were also the focus of the European project Combisol, whose goal was to promote and standardize solar combisystems in Europe (Papillon et al., 2010). Water is a valuable storage medium appropriate for low-temperature sun cooling applications due to its high specific heat capacity, global availability, chemical stability, and inexpensive cost (e.g., single-stage absorption chillers and desiccant systems). Water’s boiling point of 1 bar limits its use as a sensible heat storage medium for high-temperature applications. Hence raising the system pressure is necessary (Pintaldi et al., 2015).

3.3. Underground Thermal Storage (UTS) Underground thermal energy storage is also a widely used storage technology, which uses the ground (e.g., the soil, sand, rocks, and clay) as a medium for heat and cold storage. Means must be provided to add energy to and remove it from the medium.

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Heat and mass transfer must be considered when analyzing a porous storage medium since energy can be transported through evaporation, condensation, and water movement. Pumping heat transfer fluids (HTFs) through buried pipe arrays accomplishes this. The pipes may be horizontal pipes buried in trenches or vertical U-tubes installed in wells (boreholes) spaced at suitable intervals in the storage field. The area of the pipe arrays and the rates of heat transmission through the pipes’ surroundings’ ground limit the charging and discharging rates. Most of the time, these storage systems are not insulated, though ground-level insulation may be present. Boreholes (ground heat exchangers), which draw low-temperature heat from the soil, are also widely utilized in conjunction with heat pumps. Aquifer storage is similar to ground storage, except water, which moves slowly through the earth, serves as the primary storage medium. Water is pushed out of and into the earth to heat and extract energy from it. A mechanism for heat exchange with the ground is provided by water flow. It is only possible to employ aquifers with modest natural flow rates through the storage field. The chemical reactions of hot water with earth materials could be another restriction. Aquifers function across narrower temperature ranges than water storage, similar to underground storage. Most applications are for storing winter cold to be used in the summer to cool massive office buildings and industrial activities. The aquifers’ heat capacity is 30-40 kWh/m3, and the storage volume for 1.0 m3 water equivalents is between 2-3 m3, according to Novo et al., 2010, discuss aquifer storage. The area susceptible to heat loss per volume reduced means that relative heat losses get reduced for increasing storage capacities. It is stressed. Paksoy et al., 2009 provided a state-of-the-art study of aquifer storage for the heating and cooling of buildings. It noted that, despite having numerous advantages over borehole storage, aquifer storage is much less suitable than borehole storage due to the underlying conditions required. The primary benefit indicated is that an aquifer can produce power at significantly higher rates since water can be drained out of it quickly. Cavern and pit storage is based on large underground water reservoirs created in the subsoil to serve as TES systems. Caverns are the same in their principles of operation as tanks. Energy is added to or removed from the store by pumping water into or out of the storage unit. The main difference is in how the heat is lost and how it might be transferred to the ground. These storage options are technically possible but aren’t very useful because they cost a lot to set up. Bauer et al., 2010 talk about a German central solar heating plant with seasonal storage. They also talk

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about how some installations lose heat. For example, in Friedrichshafen, seasonal water storage with a volume of 12,000 m3 lost between 32 -482 MWh of heat each year, giving it a storage utilization factor of about 60%. The solar fraction for this system is lower than expected, coming in at 21%-33% instead of the 43% that is predicted. This is because the buildings needed more heat, and the store’s return temperatures were higher than expected. Heier et al., 2011 looked at seasonal borehole storage and found that it loses about half of the solar energy that is put into it annually. The preferred approach for high-temperature, over 100°C, SHS uses thermal oil or molten salts. Solid materials, such as ceramics and concrete, are also considered for extremely high temperatures. However, most of these high-temperature-sensitive TES alternatives are still developing or demonstrating.

3.4. Packed-Bed Storage The heat capacity of a bed of loosely packed particles is used to store energy in a packed-bed (pebble-bed) storage unit. To add or take away energy from the bed, a fluid, usually air, is pumped through it. Many solids can be used, but rock and pebble are the most common. Figure 7 shows a storage unit with a pebble-bed floor. During operation, flow is kept through the bed in one direction (usually downward) when heat is added and in the other direction when heat is taken away. Heat can’t be added and taken away at the same time.

Figure 7. Schematic simple thermal heat with pebble bed storage (Sarbu and Sebarchievici, 2016).

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High stratification is one of the best things about a packed-bed storage unit. The pebbles near the entrance are heated, but the pebbles near the exit stay at the same temperature. The air temperature near the exit is very close to the bed’s temperature at the beginning. As time goes on, warmer air moves through the bed. When the bed is fully charged, the temperature is the same everywhere. A packed bed in a solar heating system does not normally operate with constant inlet temperature. During the day, the variable solar radiation, the ambient temperature, the collector inlet temperature, load requirements, and other time-dependent conditions result in a variable collector outlet temperature. Many studies on the heating and cooling of packed beds have been published. Schumann introduced the first analytical study in 1929. The basic assumptions leading to this model are a one-dimensional plug flow, no axial conduction or dispersion, constant properties, no mass transfer, no heat loss to the environment, and no temperature gradients within the solid particles. The differential equations for the fluid and bed temperatures (tb, tf) are as follows: 𝑃𝑓 𝑐𝑝,𝑓 ε

𝑑𝑡𝑓 𝑑𝜏

=−

𝑃𝑏 𝑐𝑝,𝑏 (1 − ε)

𝑑𝑡𝑏 𝑑𝜏

𝑚𝑓 𝑐𝑝,𝑓 𝑑𝑡𝑓 𝐴

𝑑𝑥

+ [𝑘𝑣 (𝑡𝑏 − 𝑡𝑓 )]

= 𝑘𝑣 (𝑡𝑏 − 𝑡𝑓 )

(5) (6)

where Pf is the fluid density; cp,f is the specific heat of fluid; ε is the bed void fraction; mf is the fluid mass; A is the bed cross-sectional area; kv is the volumetric (per unit bed volume) heat transfer coefficient between the bed and the fluid; τ is the time. For an air-based system, the first term on the left-hand side of Equation (5) can be neglected, and the equations can be written as follows (Duffie and Beckman, 2013): 𝑑𝑡𝑓 𝑥 𝐿

𝑑( ) 𝑑𝑡𝑓 𝑑𝛩

= NTU(𝑡𝑏 − 𝑡𝑓 )

(7)

= 𝑁𝑇𝑈(𝑡𝑏 − 𝑡𝑓 )

(8)

NTU =

𝑘𝑣 𝐴𝐿 𝑚𝑓 𝑐𝑝,𝑓

(9)

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And the dimensionless time is: 𝛩=

𝜏𝑚𝑓 𝑐𝑝,𝑓 𝑃𝑏 𝑐𝑝,𝑏 (1−ε)AL

(10)

where A is the bed cross-sectional area, L is the bed length, and NTU is the effectiveness. Analytical solutions to these equations exist for a step change in inlet conditions and cyclic operation. These analytical solutions are not helpful for the long-term study of solar energy systems, and numerical techniques such as the finite-difference and finite elements methods might be employed. Nems et al., 2017 present the results of a study in a packed bed filled with ceramic bricks. The designed storage installation is supposed to become part of a heating system installed in a single-family house and eventually be integrated with a concentrated solar collector adapted to climate conditions in Poland. The system’s working medium is air. The planned integration with a solar air heater dictated the ceramic bed’s investigated temperature ranges and air volume flow rates. The work concludes that the process of heat storage in ceramic brick has high efficiency. The experimental measurements showed that the efficiency was 72–93% for an airflow rate of 0.0050 m3/s and 74–96% for an airflow rate of 0.0068 m3/s. The choice of ceramic bricks as the filling material was dictated by several reasons. Structural stability can be provided more quickly to a large bed filled with bricks than to a bed filled with, e.g., crushed stone or pebbles. Bricks resist high temperatures and tolerate many charge/discharge cycles. Brick is also a readily available material and has good thermal properties. Brick does not emit harmful gases in high temperatures, which is essential, as the storage unit is to be located inside a residential house. Nems et al., 2017 recommended that a sensible heat storage material, such as ceramic brick, should be provided with a means to control airflow rate to maximize the effectiveness of the heat storage process if the heat comes from a source of variable intensity, such as a concentrated solar air collector. Al-Kayiem et al., 2022a used the black-painted pebble bed as an absorbing and storage medium on the ground of the solar vortex power generation plant. Using black painted pebbles, cheap and readily available, as sensible TES reduces the cost of electricity production by increasing the SVPG performance. An extension of the area covered by black painted pebbles, as extended sensible TES, enhances the SVPG performance by around 19.0%. In independent investigations, Al-Kayiem et al., 2022b tested pebble bed thermal storage enhancement by adding 40% of wasted tire crumbs. They found that

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233

adding 40%vol of used tire crumbs enhanced the solar thermal storage capacity of the pebbles bed and allowed the bed temperature to reach 112.5 C, with a 39.5% increment compared to pure pebbles. The 40%vol added tire crumbs to the 60%vol pebbles increased its charging capacity by 25% and the discharging capacity by 33%.

3.5. Latent-Heat or Phase-Change Storage LHS materials are known as PCMs because they release or absorb energy with a change in physical state. The heat is mainly stored in the phase-change process (at a pretty constant temperature) and is directly connected to the latent heat of the substance. The energy storage density increases, and hence the volume is reduced in the case of LHS (Figure 4b). Using an LHS system using PCMs is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process.

3.5.1. Measurements and Capacity Prediction of PCM The measurement techniques presently used for the latent heat of fusion and melting temperature of PCMs are differential thermal analysis (DTA) and differential scanning calorimeter (DSC) (Tian and Zhao, 2013). In DSC and DTA techniques, sample and reference materials are heated constantly. The temperature difference between them is proportional to the difference in heat flow between the two materials, and the record is the DSC curve. The recommended reference material is alumina (Al2O3). The latent heat of fusion is calculated using the area under the peak, and the tangent estimates the melting temperature at the point of the most significant slope on the face portion of the peak. The main advantage of using LHS over SHS is their capacity to store heat at similar temperature ranges. Initially, these materials act like SHS materials in that the temperature rises linearly with the system enthalpy; however, later, heat is absorbed or released at an almost constant temperature with a change in physical state. LHS is based on the heat absorption or release when a storage material undergoes a phase change from solid to liquid or liquid to gas or vice versa. The storage capacity Qs, in J, of the LHS system with a PCM medium (Tian and Zhao, 2013) is given by: 𝑡

𝑡

𝑖

𝑚

Q 𝑆 = ∫𝑡 𝑚 𝑚𝑐𝑝 𝑑𝑡 + 𝑚𝑓∆𝑞 + ∫𝑡 𝑓 𝑚𝑐𝑝 𝑑𝑡

(11)

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Q 𝑆 = 𝑚[ 𝑐𝑝,𝑠 (𝑡𝑚 − 𝑡𝑖 ) + 𝑚𝑓∆𝑞 + 𝑐𝑝 (𝑡𝑓 − 𝑡𝑚 )] )

(12)

where tm is the melting temperature, in °C; m is the mass of PCM medium, in kg; cp,s is the average specific heat of the solid phase between ti and tm, in kJ/(kg·K); cp,l is the average specific heat of the liquid phase between tm and tf, in J/(kg·K); f is the melt fraction; ∆q is the latent heat of fusion, in J/kg. For example, Glauber’s salt (Na2SO4·10H2O) has cp,s ≈ 1950 J/(kg°C), cp,l ≈ 3550 J/(kg·°C), and ∆q = 2.43 × 105 J/ kg at 34°C. Morrison and Abdel-Khalik, 1978 developed a model applicable to PCMs in small containers where the length in the flow direction is L, the crosssectional area of the material is A, and the wetted perimeter is P. The model is based on three assumptions: the heat transfer fluid passes through the storage unit in the x direction at the mass flow rate m and with inlet temperature, tf, i.   

During flow, axial conduction in the fluid is negligible. The Biot number is low enough that temperature gradients normal to the flow can be neglected. Heat losses from the bed are negligible.

An energy balance on the material gives: 𝑑𝑢 𝑑𝜏

=

𝜆𝑠 𝑑2 𝑡𝑠 𝑈𝑃 ⁄𝑃 2 + (𝑡 − 𝑡𝑠 ) 𝑃𝑠 𝐴 𝑓 𝑠 𝑑𝑥

(13)

where u, ts, λs and ρs are the PCM’s specific internal energy, temperature, thermal conductivity, and density; tf and U are the circulating fluid temperature and overall heat transfer coefficient between the fluid and PCM; τ is the time. An energy balance on the fluid is: 𝑑𝑡𝑓 𝑑𝜏

=

𝜆𝑠 𝑑2 𝑡𝑠 𝑈𝑃 ⁄𝑃 𝑑𝑥 2 + 𝑃 𝐴 (𝑡𝑓 − 𝑡𝑠 ) 𝑠 𝑠

(14)

where ρf, Af, and cp,f are the fluid’s density, area, and specific heat. The equation and boundary conditions for PCM storage can be simplified for particular cases. It has been shown that axial conduction during flow is negligible, and if the fluid capacitance is small, Equations (15) and (16) become (Duffie and Beckman, 2013):

Solar Thermal Energy Storage 𝑑𝑢 𝑑𝛩 𝑡𝑓 𝑑

𝑥 𝐿

235

= 𝑁𝑇𝑈(𝑡𝑠 − 𝑡𝑓 )

(15)

= 𝑁𝑇𝑈(𝑡𝑓 − 𝑡𝑠 )

(16)

where ratio Θ = τ.m.cp,f/ρsAL and effectiveness NTU = UPL/ (m.cp,f). As depicted in Figure 4, the phase change process occurs in different modes: solid–solid, liquid–gas, and solid-liquid. The transition between different crystallization forms is stored heat in the first case. For liquid–gas systems, latent heat is very high, but there are problems in storage control due to the high volume variations during phase change. The most widespread is the solid-liquid PCMs, which have a limited volume variation during latent heat exchange (generally less than 10%) and a reasonably high melting latent heat. Melting processes involve energy densities of 100 kWh/m3 (e.g., ice) compared to a typical 25 kWh/m3 for SHS options. PCMs can be used for short-term (daily) and long-term (seasonal) energy storage using various techniques and materials. Possible applications of PCMs are as follows: 

  

implementation in gypsum board, plaster, concrete, or other wall covering material being part of the building structure to enhance the thermal energy storage capacity, with primary utilization in peak-load shifting (and shaving) and solar energy (Kenisarin and Mahkamov, 2016) (in this application, typical operating temperature is 22–25°C, but it can vary as a function of climate and heating/cooling loads); cold storage for cooling plants (operating temperature 7–15°C) (Basecq et al., 2013); warm storage for heating plants (40–50°C) (Basecq et al., 2013); hot storage for solar cooling and heating (80–90°C) (Basecq et al., 2013).

Any latent heat energy storage system, therefore, possesses at least the following three components:   

a suitable PCM with a melting point in the desired temperature range, a suitable heat exchange surface, and a suitable container compatible with the PCM.

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3.5.2. Proprieties of PCMs PCMs have been used in thermal applications for a few decades. The characteristics of PCMs in the solar thermal application are: 





Thermo-physical properties: the latent heat of transition and thermal conductivity should be high, and density and volume variations during phase transition should be high and low, respectively. Kinetic and chemical properties: super-cooling should be limited to a few degrees, long-term chemical stability, compatibility with materials of construction, no toxicity, and no fire hazard. Economic advantages: low cost and large-scale availability.

PCMs are classified into different groups depending on the material nature (paraffin, fatty acids, salt hydrates, etc.). LHS materials are broadly classified based on their physical transformation for heat absorbing and desorbing capabilities. A few advantages and disadvantages of organic and inorganic PCMs and their influence on the solar cooling application are listed in Table 4 (Kasaeian et al., 2017). Table 4. Classification of phase-change materials (PCMs) (Kasaeian et al., 2017) Properties Density

Requirement High

Latent heat of fusion

High

Specific heat

High

Melting point

Relative

Super cooling

Minimal

Thermal conductivity Vapor pressure

High Low

Description High density improves energy storage density, which reduces the volume of the thermal energy storage system. Phase change materials should have very high latent heat of fusion. High latent heat of fusion improves the energy storage density of the system. High specific heat improves the energy storage density of the system. Phase change materials should have a melting point near the required operating temperature range of the thermal energy storage system. During the freezing process, super cooling should be minimal for phase change materials. Storage material should freeze completely as close as possible to its freezing temperature. The high thermal conductivity increases the thermal charging and discharging rate. Low vapor pressure decreases the need for pressure withstanding containment at high temperatures. It also decreases the cost of insulation.

Solar Thermal Energy Storage Properties Thermal stability

Requirement High

Chemical stability Volume change

High

Availability Toxicity

Abundant and easily available Non-toxic

Corrosiveness

Non-corrosive

Cost

Cheap

Minimal

237

Description The materials should not decompose at high temperatures. This gives wider operating temperature range and higher energy storage capacity for the material. Material properties should be stable even after extended thermal cycles of heating and cooling. The high chemical stability of storage materials increases the life of energy storage plants. For phase change materials, change in volume during the phase change process should be minimal. Significant changes in volume increase the required size of the container. The large density difference between two phases also causes phase segregation issue. Abundantly and easily available materials decreases the process cost. The materials should not be harmful to health of operators and environment. Corrosive thermal energy storage materials bring down the energy storage plant life drastically due to the corrosion of containers. The lower price of storage material reduces capital and process cost.

Considering actual application in thermal energy storage, the most widespread materials are paraffin (organics), hydrated salts (inorganic), and fatty acids (organics). In cold storage, ice water is often used as well. Table 5 shows some of the most relevant PCMs in different temperature ranges with their melting temperature, enthalpy, and density. An original composite PCM of caprylic-nonanoic acid/expanded graphite (CA-NA/EG) with an optimum absorption ratio (CA-NA/EG = 90:10, by mass) was prepared by Wang et al., 2017. The composite PCM has no supercooling, good melting point, acceptable latent heat and thermal conductivity, and excellent thermal reliability and stability. Table 6 presents the melting temperatures of common TES materials. In the decades 2000 to 2020, numerous attempts have been reported to enhance the thermal properties of PCMs for energy storage applications. A research team at Universiti Teknologi PETRONAS – Malaysia has developed a compacted flat plate solar collector integrated with nanocomposite TES. They developed a compacted solar water heater comprising FPC and TES, as shown in Figure 8.

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Table 5. Comparison of organic and inorganic materials for heat storage (Kasaeian et al., 2017) Organics

Noncorrosive Low or no undercooling Chemical and thermal stability Lower phase change enthalpy Low thermal conductivity Inflammability

Effect on Solar Inorganic Cooling System Advantages good Greater phase change crucial important Disadvantages bad Subcooling Corrosion crucial Phase separation undesirable Lack of thermal stability

Effect on Solar Cooling System

good

crucial undesirable undesirable crucial

Table 6. PCM properties PCMs Ice Na-acetate trihidrate Paraffin Erythritol

Melting Temperature (°C) 0 58 -5 - 120

Melting Enthalpy (KJ/kg) 333 250 150 -240 340

Density (g/cm3) 0.92 1.30 0.77 1.30

Figure 8. compacted solar water heater comprises flat plat collector and nanocomposite TES.

They investigated and reported the patented system performance in Saw et al., 2013 and Al-Kayiem and Lin 2014. They reported the development and

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239

characterization of Cu-Paraffin wax nanocomposite for thermal energy storage in Owolabi et al., 2014, and Owolabi et al., 2016.

3.6. Organic Phase Change Materials Storage Organic PCMs can melt and solidify many times without phase segregation. Because of their latent melting heat degradation, they crystallize with little or no super-cooling and are usually non-corrosive. The two main groups are: 



Paraffin waxes consist of a mixture of primarily straight chain n-alkenes CH3–(CH2)–CH3. The (CH3) – crystallization releases a large amount of latent heat. Both the melting point and latent heat of fusion increase with chain length. Due to cost considerations, however, only technical grade paraffin may be used as PCMs in latent heat storage systems. Paraffin is safe, reliable, predictable, less expensive, non-corrosive, and available in an extensive temperature range (5–80°C) (Sharma et al., 2009). Non-paraffin organic PCMs are the most numerous of the PCMs with highly varied properties. Several esters, fatty acids, alcohols, and glycols are suitable for energy storage (Abhat, 1981). The main features of these organic materials include high heat of fusion, inflammability, low thermal conductivity, low flash points, and instability at high temperatures.

Details of thermal properties, applications, and limitations of fatty acids are discussed by Yuan et al., 2014 and those of sugar alcohols/polyols (Del Barrio et al., 2017 and Gunasekara et al., 2016).

3.7. Inorganic PCMs Inorganic PCMs are mainly used in high-temperature solar applications, and one of the most reported challenges is their maintenance. These PCMs do not super-cool appreciably, and their melting enthalpies do not degrade with cycling. At lower temperatures, they freeze; at high temperatures, they are challenging to handle. The two main types are as follows:

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3.7.1. Salt Hydrate This is classified as a congruent, incongruent, and semi-congruent melting method (Sharma et al., 2009). They are alloys of inorganic salts (AB) and n kmol of water, forming a typical crystalline solid of the general formula AB·nH2O, whose solid-liquid transition is actually dehydration and hydration of the salt. A salt hydrate usually melts either to a salt hydrate with fewer moles of water, i.e.: AB · nH2O → AB · mH2O + (n − m) H2O or to its anhydrouform AB · nH2O → AB + nH2O Salt hydrates have been extensively studied in heat storage applications because of their favorable characteristics: high latent heat of fusion per unit volume, a relatively high thermal conductivity (almost double that of paraffin), low corrosiveness, and compatibility with plastics. The hydrate crystals break into anhydrous salt and water at the melting point or a lower hydrate and water. Due to the density difference, the lower hydrate (or anhydrous salt) settles down at the bottom of the container. For example, the main characteristics of some salts hydrate of the Phase Change Material Product Limited (UK) are depicted in Table 7 (Noro et al., 2014). One problem with most salt hydrates is that of incongruent melting caused by the fact that the released water of crystallization is not sufficient to dissolve all the solid phase present. Some disadvantages include incongruent melting and super-cooling, which can be tackled in different ways (by adding thickening agents, mechanical stirring, encapsulating the PCM to reduce separation, etc.). Other problems include the spontaneity of salt hydrates and a lower number of water moles during the discharge process. Adding chemicals can prevent the nucleation of lower salt hydrates, preferentially increasing the solubility of lower salt hydrates over the original salt hydrates with more water moles. Several research studies have shown the suitability of salt hydrates for thermal energy storage (Al-Abbasi et al., 2017). These materials dissociate into anhydrous salts, release water vapor when subjected to a heat source, and store energy supplied for dehydration upon heating. A numerical study was conducted to investigate the performance of three different salt hydrates, namely, magnesium sulfate (MgSO4·7H2O), cupric sulfate (CuSO4·5H2O), and gypsum (CaSO4·2H2O), to investigate their abilities to store thermo-

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chemical energy efficiently. It was shown that cupric sulfate had the highest efficiency and required the least heating time to initiate the chemical reaction. Table 7. Main thermo-physical characteristics of particular salt hydrates (Noro et al., 2014) PCM Type

S89 S44 S7

Phase Change Temp. (°C) 89 44 7

Density (kg/m3)

1550 1584 1700

Latent Heat Capacity (kJ/kg) 151 100 150

Volumetric Heat Capacity (MJ/m3) 234 158 255

Specific Heat Capacity (kJ/kg·K) 2.48 1.61 1.85

Thermal Conductivity (W/(m·K) 0.67 0.43 0.40

3.7.2. Metallic This category includes low melting point metals and their alloys. Such materials are used to help meet the high demands of large-capacity power plants. They are scarcely used in heat storage applications because of their low melting enthalpy per unit weight, even if they have high melting enthalpy per unit volume and high thermal conductivity (Al-Abbasi et al., 2013). Some of the features of these materials are as follows: (i) low heat of fusion per unit weight; (ii) high heat of fusion per unit volume; (iii) high thermal conductivity; (iv) low specific heat; (v) relatively low vapor pressure. A list of selected metallics is given in Table 8. Table 8. Melting point and latent heat of fusion for selected metallics (Al-Abbasi et al., 2013) Material

Melting Point (°C)

Gallium–gallium antimony Eutectic Gallium Cerro-low eutectic Bi–Cd–In eutectic Cerro-bend eutectic Bi–Pb–In eutectic Bi–In eutectic Bi–Pb eutectic

29.8 30 58 61 70 70 72 125

Latent Heat (kJ/kg) 80.3 90 25 32 29 25 -

An inorganic mixture based on an industrial by-product (bischofite) was developed and characterized for its application as a PCM for low-temperature thermal energy storage (Galazutdinova et al., 2017). Thermo-physical

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properties, specific heat capacity, cycling, and thermal stability were determined. In addition, it was shown that supercooling might be reduced by increasing the quantity of material. The most appropriate composition was 40 wt % bischofite and 60 wt % Mg(NO3)2·6H2O. Organic PCMs have many advantages over inorganic PCMs but are flammable and have low thermal conductivity. On the other hand, inorganic PCMs are cheaper, abundant, and nonflammable and have high heat storage capacity and thermal conductivities.

3.8. Eutectics Eutectic materials are a combination of two or more low melting materials with similar (congruent) melting and freezing points; eutectics nearly always melt and freeze without segregation and have high thermal conductivities and densities. The weight percentage of each material can be varied to obtain variations in the melting point of the resulting eutectic mixture (Sharma et al., 2009). For this reason, they are a suitable type of PCM, even if they are less diffused than the other groups. However, they have low latent and specific heat capacities (Su et al., 2015).

Figure 9. Categories of PCM based on melting point (Ge et al., 2013).

Numerous thermal energy storage applications use PCMs, which all fit a particular range suitable for their optimum thermal performance (Sharma et

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al., 2015). The temperature range is one of the main criteria for the suitability of a PCM in any application. Figure 9 presents a brief classification system based on melting temperatures that, depending on the application desired, can help one decide which PCM to use (Ge et al., 2013).

3.9. PCM Containment The PCMs are usually packed in different types of containers to increase their compatibility with other materials in the storage system, increase their handiness, and provide a suitable contact surface for heat transfer. Containment of PCMs helps contain the material in liquid and solid phases to prevent its possible chemical composition by interaction with its surroundings. Types of containment studied are bulk storage in tank heat exchangers, encapsulation macro-encapsulation, and micro-encapsulation. The main characteristic of PCM bulk systems is the need for more extensive heat transfer than in non-PCM tanks because the PCM’s heat storage density is higher than other storage media. The approaches extensively used are inserting fins and using high conductivity particles, metal structures, fibbers on the PCM side, direct contact heat exchangers, or the rolling cylinder method (Regin et al., 2008). An experimental and numerical analysis of paraffin wax, as PCM, was investigated by Al-Kayien and Al-Hamdo 2012. They encapsulated the paraffin wax in ping pong balls, arranged it in a cylindrical-shaped mesh container, and tested the charging and discharging behavior of the encapsulated PCM in hot and cold air streams. The other possibilities are macro- and micro-encapsulating (Cabeza et al., 2011). Macro-encapsulating includes a PCM in a tube, sphere, panel, cylinder, etc. and is the most widespread. Micro-encapsulating consists of a micro-sphere (diameter less than 1 mm) of PCM encapsulated in a very thin and high-molecular-weight polymer. The spheres are then incorporated into some compatible material. The choice of material (plastic or metallic-aluminum, or steel) and the geometry affect the thermal performance of the heat storage. The developed capsules can be used in one-tank TES systems, as shown in Figure 10. The PCM inside the capsules absorbs latent heat and melts. The heat is transferred to or from a heat transfer fluid as the heat transfer fluid flows through the space between the capsules. During the charging mode, the hot fluid from the solar field is circulated through the tank. Turing the discharging

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PCM encapsulation has been used extensively in building cooling systems where the air is passed through flat containers of PCM (Amaral et al., 2017). The effect of encapsulated PCM has a good scope in enhancing the performance of LHS systems (Liu et al., 2016) used in the solar absorption cooling system. Various approaches to preparing the encapsulated PCM (organic) as a new kind of TES medium have been extensively developed. Organic PCMs can be encapsulated physic-mechanically, chemically, and physic-chemically. They can be manufactured to suit the desired properties (Khadiran et al., 2015).

Figure 10. Direct contact TES system (Amaral et al., 2017).

3.10. Thermal Properties of Various PCMs The main advantages of PCM versus water SHSs are the possibility of reducing the tank volume for a given amount of energy stored. This can only be done if storage is operated in a very narrow temperature range around the phase-transition temperature and fewer on-off cycles of auxiliary heaters and chillers. The drawbacks of PCM versus water SHSs are higher costs and risks due to leaks of stability and erosion of PCMs encapsulation material. These materials use the latent heat between the solid- and liquid-phase change and must be encapsulated or stabilized for technical use in any building system, active or passive. It can be achieved via direct inclusion in the wall, by impregnation in a porous material such as gypsum (Nomura et al., 2009), via micro-encapsulation techniques (Tyagi et al., 2011), or using a shapestabilization or slurries of PCM suspended on a thermal fluid (Delgado et al., 2012).

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Material selection is the core and most important step in designing LHS. PCM is selected based on its melting temperature and heat of fusion. The parameters necessary to identify the material include the temperature required for the application and heat requirements. Thus, the PCM type is selected for its physical and chemical properties, considering its drawbacks. Heat flux DSC is one of the most reliable methods of laboratory thermal analysis for testing the heat storage capacity of PCM with a constant heating/cooling rate (Höhne et al., 2003). A visual combination of critical thermal properties of a few types of PCM was produced by Li et al., 2012 and displayed in Figure 11.

Figure 11. Thermal properties of various PCMs (Li et al., 2012).

4. Energy Storage in Buildings PCMs are considered potential energy-saving materials since, due to latent heat absorption, they can substantially increase the thermal mass of buildings

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compared to conventional building materials presented in Figure 12 (Hassan et al., 2016).

Figure 12. The maximum energy storage capacity of different 10-mm-thick building materials operates between 18 and 26 oC for 24 h (Hassan et al., 2016).

Storage concepts applied to the building sector have been classified as active or passive systems. Passive thermal energy systems can effectively enhance the naturally available heat energy sources to maintain the comfort conditions in buildings and minimize the use of mechanically assisted heating or cooling systems. Using active thermal energy systems provides a high degree of control of indoor conditions and improves heat storage. These systems are usually integrated with buildings to provide free cooling or to shift the thermal load from on-peak to off-peak conditions in several applications, such as DHW applications (De Gracia et al., 2011) and heating, ventilation, and airconditioning (HVAC) systems (Agyenim and Hewitt, 2010).

4.1. Passive Solar Thermal Storage in Buildings When high thermal-mass materials are used in buildings, sensible passive storage is the technology that allows for storing a high quantity of energy, providing thermal stability inside the building. As a passive technology, TES aims to provide thermal comfort with minimal use of HVAC energy. Materials typically used are rammed earth, alveolar bricks, concrete, or stone.

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Standard solar walls, also known as Trombe and solar water walls, also use sensible storage to achieve energy savings in buildings (Saadatian et al., 2012). A Trombe wall (Figure 13) (from the name of the French researcher who first proposed it in 1979) is a wall with high thermal capacity, shielded by a glass pane. A greenhouse effect is created, reducing thermal losses from the wall, heating the air between the wall and glass that can be introduced into the room with a natural draught due to the chimney effect of the heated air.

Figure 13. Schematics of a storage Trombe-wall (Saadatian et al., 2012).

The temperature of the wall increases as energy is absorbed, and timedependent temperature gradients are established in the wall. The energy lost through the glazing is transferred from the room side wall to the room by radiation and convection. This storage wall can be considered a set of N nodes connected by a thermal network, each with a temperature and capacitance (Ohanessian and Charters, 1978). Heat is transferred by radiation across the gap and convection between the air flowing in the gap, the absorbing surface, and the inner glazing. Energy balances are written for each node of thickness ∆x, resulting in a set of ordinary differential equations with terms that represent its time-dependent temperature and energy flows to all adjacent nodes. The general energy balance for any node i in the wall is:

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=

𝜆 𝑝𝑐𝑝 ∆𝑥 2

( 𝑐𝑖−1 + 𝑐𝑖+1 − 2𝑡𝑖 )

(17)

where (i = 2,..., N-1), λ is the thermal conductivity of the wall; ρ is the wall density; cp is the specific heat of the wall; τ is the time. Equations for nodes 1 and N must consider the node half thickness and the convection and radiation heat transfer. The set of N equations is simultaneously solved for the time-dependent temperatures at each node. The energy stored in the wall (relative to a base room temperature troom) can be calculated. If there is airflow through vents and to the room, the energy added to the room by this mechanism will be ma cp, a(to − troom), where to is the outer glazing temperature, and tr is the room temperature. PCM can be incorporated into construction materials using different methods, such as direct incorporation, immersion, encapsulation, microencapsulation, and shape stabilization. In direct incorporation and immersion, potential leakage has to be assessed. When the PCM is encapsulated or added in a shape-stabilized new material, a new layer appears in the construction system of the wall. Traditionally, wallboards have been studied as one of the best options for incorporating PCM into building walls. A new approach to PCM wallboards is the addition of an aluminum honeycomb containing a micro-encapsulated PCM wallboard (Lai and Hokoi, 2014). Similarly, PCM can also be impregnated or mixed with concrete or mortar. One of the objectives pursued here is maintaining the concrete’s mechanical properties while increasing its specific heat capacity. Another approach to incorporating PCM in building walls is to mix it with insulation materials. In a masonry wall, the PCM incorporation can be, for example, within clay bricks (Silva et al., 2012). In the PCM shutter concept, a shuttercontaining PCM is placed outside window areas. During the daytime, when they are opened to the outside, the exterior side is exposed to solar radiation, heat is absorbed, and the PCM melts. At night, we close the shutter and slide the windows, and heat from the PCM radiates into the rooms.

4.2. Active Solar Thermal Storage in Buildings The use of thermal energy storage in building active systems is an attractive and versatile solution for several applications for new or retrofitted buildings, such as the implementation of RES in the HVAC for space heating/cooling,

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improvement in the performance of the current installations, and the possible application of peak load-shifting strategies (Sun et al., 2013). One of the main applications of TES in active building systems is free cooling when the storage is charged with low night outdoor temperatures, and this stored cold is discharged when required by the cooling demand (Waqas and Din, 2013). The integration of TES in a building can be done in the building’s core (core, floor, walls), external solar façades, suspended ceilings, ventilation systems, PV systems, and water tanks. Furthermore, TES has been used in building solar systems to convert intermittent energy sources and meet heating and DHW demands. The most popular solar TES method of integrating solar air collectors in building walls or using PCM in ventilated façades is shown in Figure 14 (De Gracia et al., 2012).

Figure 14. Operational mode of the ventilated façade with PCM (De Gracia et al., 2012).

Within this context, heat pumps with TES systems are presented as a promising technology to shift electrical loads from high-peak to off-peak periods, thus serving as a powerful tool in demand-side management. TES has excellent potential as a pivotal technology to reduce the energy demand of buildings and/or to improve the energy efficiency of their energy systems. Like PCMs, nano-enhanced PCMs can be used in buildings for passive and active applications. In other words, they can be applied to building structures to enhance the overall thermal mass. Additionally, these structures can be used in heating, cooling, ventilation, and air-conditioning systems to

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reduce the incompatibility between the energy supply and the demand by shifting and reducing peak load. Many studies have been conducted on developing suitable nano-enhanced PCMs for building applications. A summary of the experimental study in this field is listed in Table 8 (Kasaeian et al., 2017). The combination of PCMs with solar and zero-energy buildings makes building research and energy management an excellent opportunity. The Summary of experimental studies on the applications of nanoenhanced PCMs in buildings by Constantinescu et al., 2010; Kalaiselvam et al., 2012; Kumaresan et al., 2013; Sayyar et al., 2014; Parameshwaran et al., 2014; Harikrishnan et al., 2014; Sarı et al., 2015; Amin et al., 2017 and Hussain et al., 2017, suggested that: 









 

Constantinescu et al., nano- composite building materials provide a nano-composite with appropriate thermo-physical properties for energy storage (Constantinescu et al., 2010). Kalaiselvam, a nano-enhanced PCM for cooling, improved the solidification and melting rate by increasing the number of nanoparticles in the PCM (2012). Kumaresan et al., PCM-based nano- fluids for cool storage reduced by about 14.0 and 20.1% at the ambient temperature of 90C and 120C, respectively (2013). Harikrishnan et al., composite PCMs for heating, providing a new composite PCM consisting of lauric acid (LA) and stearic acid (SA) with nano-particles (2014). Sari et al., Micro/nanocapsules of PCM, this nano PCM with melting latent heat of 116.25 J/g was suitable for free cooling applications (2015). Amin et al., 2016, beeswax/ graphene PCM, increasing the thermal conductivity to 2.8 W/m·K and the latent heat by 22.5%. (2016) Hassain et al., 2017, Activated carbon dispersed PCM, improvement in thermal conductivity of the pure PCM by 55% for 0.1 wt % nanodispersed PCM (2017).

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5. Thermal Energy Storage for Concentrated Solar Power Plants CSP systems use mirrors to concentrate sunlight from a large area to a small area, which is absorbed and converted to heat at high temperatures. The hightemperature heat is then used to drive a power block (usually a steam turbine connected to an electrical power generator) similar to the power block of a conventional thermal power plant. A significant advantage of CSP plants over solar photovoltaic power plants is that CSP plants may be coupled with conventional fuels and can utilize TES to overcome the intermittency of solar energy. Energy storage reduces the mismatch between supply and demand, improves the performance and reliability of energy systems, and plays a vital role in conserving energy (Garg et al., 2013). TES systems can collect energy during sunshine hours and store it to shift its delivery later or smooth out plant output during cloudy weather conditions. Hence, the operation of a solar thermal power plant can be extended beyond periods of no solar radiation without the need to burn fossil fuels. Several TES technologies implemented for CSP plants are mainly twotank and single-tank systems. In a two-tank system, the fluid is stored in two tanks, one at a high temperature and the other at a low temperature. Fluid from the low-temperature tank flows through the solar collector or receiver, where solar energy heats it to a high temperature, and it then flows to the hightemperature tank for storage. Fluid from the high-temperature tank flows through a heat exchanger, generating steam for electricity production. The fluid exits the heat exchanger at a low temperature and returns to the lowtemperature tank. These systems are called two-tank direct systems. On the other hand, an indirect system uses different fluids for heat transfer and storage. An indirect system is used in plants in which the heat-transfer fluid is too expensive or not suited for use as the storage fluid. The storage fluid from the low-temperature tank flows through an extra heat exchanger, where the high-temperature heat-transfer fluid heats it. The high-temperature storage fluid then flows back to the high-temperature storage tank. The fluid exits this heat exchanger at a low temperature and returns to the solar collector or receiver, where it is heated back to a high temperature. Storage fluid from the high-temperature tank generates steam in the same manner as the two-tank direct system. Figure 15 shows a two-tank thermal energy storage system integrated into a parabolic trough power plant (Herrmann et al., 2004).

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Single-tank systems, mostly thermocline systems, store thermal energy in a single tank in a solid medium, most commonly silica sand. At any time during operation, the top part of the medium is at a high temperature, and the bottom is at a low temperature. The hot- and cold-temperature regions are separated by a temperature gradient or thermocline. High-temperature heattransfer fluid flows into the top of the thermocline and exits the bottom at a low temperature. This process moves the thermocline downward and adds thermal energy to the system for storage. Reversing the flow moves the thermocline upward and removes thermal energy from the system to generate steam. Buoyancy effects create thermal stratification of the fluid within the tank, which helps to stabilize and maintain the thermocline. Using a solid storage medium and only needing one tank reduces the cost of this system relative to the two-tank systems. This system was demonstrated at the Solar One central receiver CSP system in California, where steam was used as the heat-transfer fluid, and mineral oil was used as the storage fluid.

Figure 15. Schematic of a parabolic trough power plant with two-tank molten salt storage (Herrmann et al., 2004).

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6. Chemical Energy Storage TCS uses thermochemical materials (TCM), which store and release heat by a reversible endothermic/exothermic reaction process (Figure 4c). During the charging process, heat is applied to material A, separating two parts B + C. The resulting reaction products can be easily separated and stored until the discharge process is required. Then, the two parts B + C, are mixed at suitable pressure and temperature conditions, and energy is released. Products B and C can be stored separately, and thermal losses from the storage units are restricted to sensible heat effects, usually minor compared to the heat of the reaction. The thermal decomposition of metal oxides for energy storage has been considered (Kerskes et al., 2012). These reactions may have an advantage because the oxygen evolved can be used for other purposes or discarded, and oxygen from the atmosphere can be used in reverse reactions. Two examples include the decomposition of potassium oxide 4KO2 ↔ 2K2O + 3O2

(18)

which occurs over a temperature range of 300–800 °C with the heat of decomposition of 2.1 MJ/kg, and that of lead oxide, 2PbO2 ↔ 2PbO + O2

(19)

which occurs over a temperature range of 300–350 °C with the heat of decomposition of 0.26 MJ/ kg. Many practical problems are yet to be faced in using these reactions. Energy storage by thermal decomposition of Ca(OH)2 has been extensively studied by Fujii et al., 1985. The forward reaction will proceed at temperatures above ~450 ◦C; adding zinc or aluminum can enhance the reaction rates. The product CaO is stored in the absence of water. The reverse exothermic reaction proceeds easily. An example of a photochemical decomposition reaction is the decomposition of nitrosyl chloride, which can be written as NOCl + photons → NO + Cl

(20)

The atomic chlorine produced forms chlorine gas, Cl2, with the release of a substantial part of the energy added to the NOCl in decomposition. Thus, the overall reaction is

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2NOCl + photons → 2NO + Cl2

(21)

The reverse reaction can be carried out to recover part of the energy of the photons entering the reaction. Processes that produce electrical energy may have storage provided as chemical energy in electrical storage batteries or their equivalent. Thermo-chemical reactions, such as adsorption (i.e., adhesion of a substance to the surface of another solid or liquid), can be used to store heat and cold and control humidity. The high storage capacity of sorption processes also allows thermal energy transportation. Table 9 lists some of TES’s most interesting chemical reactions (Garg et al., 2013). While sorption storage can only work at temperatures of up to ~350°C, temperatures of chemical reactions can go much higher. Table 9. Chemical reactions for thermal energy storage (Garg et al., 2013) Reaction

Chemical formal

Temperature (°C)

Methane steam reforming Ammonia dissociation Thermal dehydrogenation of metal hydrides Dehydration of metal hydroxides Catalytic dissociation

CH4+H2O=CO+3H2 2NH3=N2+3H2 MgH2=Mg+H2

480-1195 400-500 200-500

CA(OH)2=CAO+H2O SO3=SO2+1/2O2

402-572 520-960

Energy Density (kJ/Kg) 6053 3940 3079(Heat) 9000(H2) 1415 1235

7. Cooling Thermal Energy Storage (CTES) Cool thermal energy storage (CTES) has recently attracted interest for its industrial refrigeration applications, such as process cooling, food preservation, and building air-conditioning systems. PCMs and their thermal properties suitable for air-conditioning applications can be found in (Delgado et al., 2012). For air-conditioning and refrigeration (ice storage), temperatures from −5 to 150C are optimum for thermal storage (Noro et al., 2014; Azzouz et al., 2009; Marques et al., 2013, Marques et al., 2014), but at lower temperatures, latent heat storage materials are better than sensible heat storage materials (like water). Some of the commercially available PCMs for cold storage are shown in Table 10.

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Table 10. Commercially available PCM for cold storage (Chidambaram et al., 2011) Type(PCM)/Company

Name (PCM)

Melting Temperature (°C)

Hydrated salts – eutectic/ TEAP PCM

Freezer salt

S15 S13 S10 S8 S7 A15 A9 A8 A6 A4 A3 A2 E0 E-2 E-3 E-6 E-10 -

-16 4 7 7 10 15 15 13 10 8 7 15 9 8 6 4 3 2 0 -2 -3.7 -6 -10 -30 to 56 30 to 43

Latent Heat of Fusion (kJ/kg) 330 105 135 300 170 175 160 160 155 150 150 130 140 150 150 200 200 200 332 306 312 275 286 -

-

30 to 56

-

RT20 RT4 RT3 RT5 RT2HC HS7N

8 -4 4 9 -4 to 100 -7 to -5

140 179 198 205 230

SN06 SN03 STL-6 STL-3

-6 -3 -6 -3

284 328 284 328

Preservation

Hydrated salts/Plus ICE PCMProducts

Organic/Plus ICE PCMProducts

Eutectic/Plus ICE PCMProducts

Pure PCM/Microteklabs Micro-encapsulated PCM/Microteklabs Macro-encapsulated PCM/Microteklabs Paraffin/Rubitherm GmbH

Inorganic salts/ Rubitherm GmbH Salt solution/Cristopia Salt solution/Mitsubishi Chemical

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CTES appears to be one of the most appropriate methods for correcting the mismatch between the supply and demand of energy. Cold energy storage requires a better insulation tank, as the energy available in the cold state is expensive compared to the heat in a hot storage tank. Cheralathan et al., 2007 investigated the performance of an industrial refrigeration system integrated with CTES. The authors indicated significant capital and operating cost savings in thermal storage-integrated systems. The size of the PCM-based CTES system was also considerably reduced compared to that of a chilled water system. The application of CTES in buildings and energy systems has many advantages. CTES may play an essential role in managing peak loads and solving the intermittency problem of renewable energy sources, especially when cooling storage is integrated into district cooling systems (Ban et al., 2012). The sorption phenomenon can also be applied to TES. In that case, a heat source promotes the dissociation (endothermic process) of a working pair whose substances can be stored separately. Heat is released when they come into contact again (exothermic process). Therefore, the energy can then be stored with virtually no loss because the heat is not stored in a sensible or latent form but as potential energy as long as the substances are kept separate. Typical applications involve water vapor adsorption to silica gel or zeolites (i.e., microporous crystalline alumina-silicates). Open sorption systems based on lithium chloride to cool water and zeolites to control humidity are particularly important for use in hot/humid climates or confined spaces with high humidity. Adsorption TES is a promising technology that can provide a more compact and efficient solution for long-term TES. Solar thermal energy or waste heat from several processes can be used to regenerate the adsorbent and promote energy storage (Fernandes et al., 2014). Several research projects have already used the adsorption cycle to promote TES. In 1990, Kaubek and Maier-Laxhuber, 1990 patented an adsorption apparatus for electric heating storage, working with the zeolite/water pair and reporting 30% savings in energy consumption. The system can be used as an air-heating device or combined with a hot-water tank. In the first case, the adsorbent bed is heated by electric heating rods during the desorption phase, regenerating the adsorbent and releasing the condensation heat into the space to be heated. In the latter case, the condensation heat is released into a water tank during the desorption phase, while the adsorption heat is transferred to the water tank through a specific closed circuit in the adsorption stage. Hauer, 2007 presented a seasonal adsorption TES system

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working with the silica-gel/water pair, Figure 16. During the summer, while the system is charging, the heat from the solar collectors is conducted to three adsorbent beds, promoting the desorption stage. In the winter, the low temperatures in the solar collector promote the evaporation of the water in the evaporators/condensers, and the heat of adsorption is released to the building heating system.

Figure 16. Seasonal adsorption thermal storage system (Hauer, 2007).

A cascade storage system offers vast potential for improving solar cooling systems. In a cascaded storage system, PCMs with different melting temperatures are arranged in a series to store heat at different temperatures. Compared with a conventional single PCM-based storage system, cascaded multiple PCM-based storage systems would improve solar collecting efficiency, as the lower temperature at the bottom of the tank is connected to the inlet of the solar collector. The numerical results from the parametric study by Shaikh and Lafdi, 2006 indicated that the total energy charged rate could be significantly enhanced using composite PCMs compared to the single PCM. As shown in Figure 17, the different TES technologies: are sensible heat (i.e., water as an example), latent heat (i.e., different materials), and thermochemical (i.e., sorption and chemical reactions) for their working temperatures (Khan et al., 2007).

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Figure 17. Storage capacity of thermal energy storage with different temperatures (Khan et al., 2007).

8. Performance and Cost of TES Systems TES includes several technologies with specific performances, applications, and costs. Necessary fields of application for TES systems are in the building sector (e.g., DHW, space heating, and air-conditioning) and in the industrial sector (e.g., processes heating and cooling). TES systems can be installed as either centralized plants or distributed devices. Centralized plants are designed to store waste heat from large industrial processes, conventional power plants, combined heat and power plants, and renewable power plants, such as CSP. Their power capacity typically ranges from hundreds of kW to several MW. Distributed devices are usually buffered storage systems to accumulate solar heat for domestic and commercial buildings (e.g., hot water, heating, and appliances). Distributed systems are mainly in the range of a few to tens of kW. TES systems based on sensible heat storage offer a storage capacity ranging from 10 to 50 kWh/t and storage efficiencies between 50 and 90%, depending on the specific heat of the storage medium and thermal insulation technologies. PCMs can offer more storage capacity and efficiencies from 75

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to 90%. In most cases, storage is based on a solid–liquid phase change with energy densities of 100 kWh/m3 (e.g., ice). TCS systems can reach storage capacities of up to 250 kWh/t with more than 300 ◦C operating temperatures and efficiencies from 75% to nearly 100%. The cost of a complete system for SHS ranges between 0.1 and 10 €/kWh, depending on the size, application, and thermal insulation technology. The costs for PCM and TCS systems are, in general, higher. In these systems, high costs are associated with the heat (and mass) transfer technology, which has to be installed to achieve sufficient charging/discharging power. Costs of LHS systems based on PCMs range between 10 and 50 €/kWh, while TCS costs range between 8 and 100 €/kWh. The economic viability of a TES depends heavily on application and operation needs, including the number and frequency of the storage cycles. Cost estimated for TES systems includes storage materials, technical equipment for charging and discharging, and operation costs. Although economic analyses for conventional systems (without thermal storage) (Kizilkan et al., 2007) and systems with sensible storage (water) tanks are abundant (Al-Ugla et al., 2016; Calise et al., 2011; Calise, 2010; Eicker and Pietruschka, 2009; Al-Alili et al., 2012; Hang et al., 2011 and Tsoutsos et al., 2010), comparative cost analyses of using a PCM as a latent heat thermal storage unit in a solar absorption cooling system are rarely seen. Godarzi et al., 2013 designed a PCM storage system based on exergo-economic analysis and a genetic algorithm in a 45.4 kW LiBr/H2O system. Their analysis showed a payback period of 0.61 years without PCM storage to 1.13 years with PCM storage. Calise et al., 2011 numerically also investigated LiBr/H2O systems using thermo-economic and optimization techniques, and 64% of primary energy was saved with a payback period of 12 years. TES systems for sensible heat are relatively inexpensive as they consist of a simple tank for the storage medium and the equipment to charge/discharge. Storage media (e.g., water, soil, rocks, concrete or molten salts) are usually relatively cheap. However, the storage material container requires effective thermal insulation, which may be a vital element of the TES cost. In the case of UTES systems, boreholes and heat exchangers to activate the underground storage are the most critical cost elements. Specific costs range from 0.1 to 10 €/kWh (The Energy Technology Systems Analysis Programmes, 2015) and depend heavily on local conditions. PCM storage and TCS systems are significantly more complex and expensive than storage systems for sensible heat. In most cases (e.g., thermochemical reactors), they use enhanced heat and mass transfer technologies to achieve the required performance in terms of storage capacity and power, and

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the equipment cost is much higher than the cost of the storage material. The cost of systems using expensive micro-encapsulated PCMs, which avoid using heat exchange surfaces, can be even higher. TCS systems can be operated as either open systems (i.e., basically packed beds of pellets at ambient pressure) or closed systems. The difference between the pure PCM and the complete TES system is even higher for active PCM installations. As an example, the costs of a calcium–chloride storage system for heat rejected from a thermally driven absorption chiller includes the cost of calcium chloride, which is relatively inexpensive (0.3 €/kg), and the cost of a container, a heat exchanger, and other components around 65 €/kWh (The Energy Technology Systems Analysis Programmes, 2015). Materials for TCS are also expensive, as they have to be prepared (e.g., pelletized or layered over supporting structures). Also expensive are the containers and the auxiliary TCS equipment for both heat and mass transfer during energy charging and discharging. The overall economic evaluation of a TES system depends significantly on the specific application and operation needs, including the number and frequency of storage cycles. TES technologies face specific barriers to market entry, and the cost is a vital issue. Other barriers relate to material properties and stability, in particular for TCS. Each storage application needs a specific TES design to fit specific boundary conditions and requirements. R&D activities focus on all TES technologies. TES market development and penetration vary considerably, depending on the application fields and regions. Thus, TES’s potential for co-generation and district heating in Europe is associated with building stock. The implementation rate of co-generation is 10.2%, while the implementation of TES in these systems is assumed to be 15%. As far as TES for power applications is concerned, a driving sector is the CSP. Almost all new power plants in operation or under construction are equipped with TES systems, primarily based on molten salt. This is perhaps the most critical development field for large, centralized TES installations.

Conclusion Domestic systems, district heating, and industrial requirements can all benefit from SHS. Water is the most well-liked and widely used heat storage medium and has many industrial and domestic uses. For typically large-scale

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applications, underground sensible heat storage in liquid and solid mediums is also utilized. However, the specific heat of the storage medium limits the storage capacity of TES systems based on SHS. Additionally, good design is necessary for SHS systems to discharge thermal energy at steady temperatures. Due to the latent heat of the phase change, PCMs can have a larger storage capacity. The constant temperature of the phase transition in PCMs also enables a target-oriented discharging temperature. Three fundamental variables that affect the choice of PCMs in each application are melting temperature, latent heat of fusion, and PCM thermo-physical difficulties. The selection approach’s two main requirements are a high heat of fusion and a precise melting/solidification temperature (without subcooling). There have been numerous mechanical and nano-level improvements made to increase the rate of heat transmission is encouraging. Phase segregation in salt hydrates can be resolved by micro-encapsulation, which also enhances the heat transmission surface area. Most literature focuses on commonplace and widely used PCM materials like paraffin. We advise concentrating on specialized PCMs with a broad temperature range, like salt hydrates, and synthesizing PCMs tailored for particular building uses. TCS can provide even more storage space. Adsorption is one example of a thermochemical reaction that can manage humidity in various applications and store and release heat and cold on demand. Although materials with low melting points are detected in CTES, little is known about their thermal stability, phase separation, and subcooling problems. There aren’t many tests on thermal cold storage PCMs at the industrial (large scale) level. Sensible heat-based TES systems are currently on the market, whereas TCS- and PCM-based storage systems are mainly developing and demonstrated. Future greenhouses can integrate heating, cooling, and dehumidification functions with poly-generation options thanks to TES technologies. For this alternative to be widely embraced more cost-effectively, additional investigation is required into the potential for thermo-chemical energy storage and the continued development of PCMs. If its use is to be promoted, funding for research and development (R&D) of new storage materials is necessary, as are regulatory measures and investment incentives for TES integration in buildings, industrial applications, and variable renewable power generation.

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Index

# 5kW solar-powered fan, 54, 55 735W solar-powered fan, 54, 55, 56

A air-conditioning system, 31, 32, 51, 52, 53, 54, 56, 57, 59, 249, 254, 262, 266, 268 aircraft conceptual design, 32, 37, 39 atmospheric vortex engine (AVE), 84, 85, 86, 87, 88, 89, 92, 96, 98, 99, 104, 106 auxiliary power unit (APU), 39, 44, 52, 53, 56

B braking recovery, 154, 159, 169, 178

C charging, 153, 158, 161, 162, 163, 164, 165, 167, 176, 177, 179, 180, 181, 182, 183, 185, 186, 223, 224, 227, 229, 233, 236, 243, 253, 257, 259, 260 computational fluid dynamics (CFD), 83, 84, 96, 97, 98, 99, 101, 105, 106, 112, 113, 114, 115, 119, 122, 129, 130, 144, 145, 146, 147, 148, 150, 213, 222, 262 computational techniques, 154, 174 concentrated solar power, 2, 4, 5, 19, 27, 251 concentrating solar power (CSP), 2, 5, 6, 27, 30, 213, 217, 218, 220, 251, 252, 258, 260 conception sizing, 62

configuration, 40, 42, 43, 47, 48, 49, 51, 53, 89, 115, 125, 126, 133, 134, 140, 145, 195

D diffuse horizontal irradiance (W/m2) (DHI), 2 direct normal irradiance (W/m2) (DNI), 2, 6, 8, 16, 18, 19, 21, 25 direct steam generation (DSG), 2, 26

E electric vehicles (EVs), 155, 157, 159, 160, 161, 162, 163, 167, 168, 170, 179, 181, 183, 184, 185, 186 energy conversion, 29, 30, 102, 105, 107, 110, 115, 138, 146, 147, 148, 149, 150, 153, 161, 167, 170, 177, 178, 182, 183, 184, 196, 210, 214 energy integration, 154 energy performance, 218 energy recovery and generation, 153, 154 environmental control system (ECS), 44, 51, 52, 53, 54, 55, 56 evacuated tube collectors (ETCs), 4, 194, 196 evacuated tube solar collectors (ETSCs), 3, 193, 194, 207, 212 exergy analysis, 84, 101, 106, 205, 212, 213

270

Index

F

N

flat plate solar collectors (FPSCs), 3, 193, 194, 196, 207, 213 future of solar, 190

Net Zero Scenario, 210, 211

G

parabolic trough collector(s) (PTCs), 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 193, 196, 206, 208, 211, 213, 215 parabolic trough concentrator power plant (PTCPP), 2, 8, 10, 16 phase change materials, 115, 125, 139, 140, 149, 218, 236, 237, 239, 263, 264, 265, 266, 267, 268 photovoltaics, 31, 32, 33, 57, 153, 162, 165, 167, 168, 169, 178, 180, 181, 183, 186 potable water, 62, 63, 64, 66, 70, 74, 75, 77, 78, 79, 80

Geographical Information Systems (GIS), 176, 177, 180, 184, 186 global horizontal irradiance (W/m2) (GHI), 3

H heat transfer fluid(s) (HTFs), 3, 4, 5, 6, 7, 11, 12, 14, 21, 28, 29, 190, 192, 193, 197, 201, 202, 203, 211, 229, 234, 243 heating/cooling storage, 218 Hybrid Optimization Model for Electric Renewables (HOMER), 176, 183

I

P

R

industrial solar, 190 irrigation, v, viii, 61, 62, 63, 64, 65, 70, 71, 72, 73, 78, 79, 80, 81, 82

ratio of the thermal power generated through the solar field (SM), 3, 11 rural, v, viii, 61, 62, 63, 67, 70, 74, 75, 78, 79, 80, 81, 82, 153

L

S

limitations, 101, 102, 105, 111, 153, 154, 156, 161, 167, 177, 239 Linear Fresnel reflectors (LFRs), 193, 197

safety, vii, 34, 35, 39, 177, 264 shading, 41, 42, 43, 47, 48, 57 Solar Assisted Passenger Electric Vehicle (SAPEV), 165, 167, 168, 170 solar chimney power plant (SCPP), viii, 83, 84, 101, 103, 104, 106, 107, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151 solar collectors, 3, 85, 190, 192, 193, 194, 196, 197, 200, 201, 202, 203, 210, 211, 214, 215, 216, 219, 220, 257, 268 solar concentrators, 6, 190

M modeling, v, 16, 27, 30, 52, 57, 61, 62, 63, 78, 80, 81, 82, 106, 111, 113, 115, 118, 122, 124, 128, 129, 130, 133, 134, 135, 137, 139, 143, 145, 148, 150, 175, 176, 180, 182, 183, 215, 262

Index solar energy, vii, viii, 3, 4, 27, 28, 29, 32, 62, 63, 70, 72, 74, 79, 80, 81, 84, 85, 86, 87, 90, 92, 94, 104, 105, 106, 107, 110, 114, 117, 119, 127, 138, 144, 146, 147, 148, 149, 150, 162, 165, 167, 168, 171, 173, 175, 178, 179, 180, 181, 182, 183, 184, 185, 186, 189, 190, 191, 192, 211, 212, 213, 214, 215, 216, 217, 218, 226, 230, 232, 235, 251, 262, 267 solar farm, v, 61, 62, 65, 70, 72, 74, 78 solar flight, 32, 33, 58 solar heating, 190, 216, 217, 229, 231, 263, 266, 267 Solar Impulse, 31, 32, 33, 34, 59 Solar Impulse 2, 32, 33, 34 solar output power, 45 solar panels, 31, 33, 39, 45, 47, 50, 51, 78, 79, 87, 162, 167, 181 solar power plant with short diffuser (SPD), 84, 85, 86, 87, 89, 92, 97, 98, 99, 104, 107 solar power production, 110 Solar Powered Race Car (SPRC), 165 solar thermal, vi, vii, 2, 3, 4, 27, 28, 29, 30, 84, 149, 189, 190, 192, 193, 194, 196, 200, 201, 202, 203, 206, 210, 211, 213, 214, 215, 216, 217, 218, 219, 220, 222, 223, 226, 227, 228, 233, 236, 246, 248, 251, 262, 264, 265, 266, 268 solar thermal collectors (STCs), 189, 192, 193, 194, 196, 197, 198, 200, 201, 202, 210, 211, 213, 215 solar thermal energy storage, vi, 217, 218, 222, 223, 264, 265 solar updraft, viii, 84, 100, 102, 105, 107, 110, 113, 145, 146, 148, 149, 150 solar updraft power, viii, 84, 100, 105, 107, 110, 145, 149, 150 solar vortex engine (SVE), 84, 86, 91, 92, 93, 94, 97, 98, 99, 100, 104, 105, 106, 107 solar vortex power desalination system (SVPDS), 84, 90, 91, 92, 97, 98

271 solar vortex power generation (SVPG), v, 83, 84, 85, 86, 87, 89, 90, 91, 92, 95, 96, 97, 98, 99, 101, 102, 103, 104, 105, 232, 262 solar water heaters (SWH), 190 solar-powered flying vehicles, 32, 33 standalone solar system, 2 Sunrise I, 32, 33 supplemental solar power system (SPS), v, 31, 32, 37, 38, 39, 40, 44, 46, 48, 49, 51, 53, 56, 57, 58

T thermal energy storage (TES), viii, 4, 5, 6, 27, 29, 94, 111, 115, 120, 121, 138, 140, 148, 190, 192, 217, 218, 219, 220, 221, 222, 223, 224, 226, 228, 229, 230, 232, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 246, 248, 249, 251, 254, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268 thermodynamic power cycle, 84, 102, 103, 105 thin-film materials, 154 tropical climate, v, 1, 2, 6, 26 turbine unit, 83, 84, 97, 103, 104, 105

U usable power ratio, 44, 46

V Vanguard 1 satellite, 32 vehicle integrated photovoltaics, 154, 164, 165, 182

W water and energy supply areas, 61

About the Editor

Professor Hussain H. Al-Kayiem, PhD University of Technology, Iraq Email: [email protected]

Dr. Hussain H. Al-Kayiem completed his BSc, 1973; MSc, 1981 in Mech. Eng. from the University of Baghdad, and PHD from the University of Bradford-UK 1989. He headed the Mech. Eng. Dept. in the Military College of Engineering – Baghdad for 1990-1997 and Mech. Eng. Dept. in AlMustansirya University, Baghdad from 1999-2003. He joined Universiti Teknologi PETRONAS (UTP), Malaysia in 2006 till recently. As leader in Thermofluids, he published more than 300 papers and chapters, and 6 books.