Physics of Solar Energy 1774072815, 9781774072813

Table of contents :
Cover
Title Page
Copyright
DECLARATION
ABOUT THE EDITOR
TABLE OF CONTENTS
List of Contributors
List of Abbreviations
Preface
Chapter 1 Upconversion in Solar Cells
Abstract
Review
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 2 A Short Progress Report on High-Efficiency Perovskite Solar Cells
Abstract
Introduction
Principle And History of Perovskite Scs
High-Efficiency Perovskite Solar Cells
Stability of Perovskite Solar Cells
Interface Engineering
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 3 Colloidal Quantum Dot Based Solar Cells: From Materials to Devices
Abstract
Introduction
Size-Dependent Physical Properties of CQDs
Surface Modification and Characteristics Control
Enhancement of Power Conversion Efficiency Through Solar Cell Structure Design
Enhancement of Power Conversion Efficiency Through Surface Modification
Enhancement of Power Conversion Efficiency Using Optical Design
Conclusions
References
Chapter 4 Performance Evaluation of Nanofluids in Solar Energy: A Review of The Recent Literature
Abstract
Introduction
Literature Review of Recent Years
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 5 Solar to Fuels Conversion Technologies: A Perspective
Abstract
Introduction
Solar Alternative Fuels
Key Challenges and Opportunities
Acknowledgements
References
Chapter 6 Ab Initio Design of Nanostructures For Solar Energy Conversion: A Case Study on Silicon Nitride Nanowire
Abstract
Background
Methods
Results and Discussion
Conclusions
Acknowledgements
References
Chapter 7 An Efficient and Effective Design of InP Nanowires for Maximal Solar Energy Harvesting
Abstract
Background
Design For Maximal Light Harvesting of InP NWs
Results and Discussion
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 8 Optical Simulations of P3HT/Si Nanowire Array Hybrid Solar Cells
Abstract
Background
Methods
Results and Discussion
Conclusion
Acknowledgements
Authors’ Contributions
References
Chapter 9 Substantial Influence on Solar Energy Harnessing Ability by Geometries of Ordered Si Nanowire Array
Abstract
Background
Methods
Results and Discussion
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 10 Silicon Nanowires for Solar Thermal Energy Harvesting: an Experimental Evaluation on the Trade-off Effects of the Spectral Optical Properties
Abstract
Background
Methods
Results and Discussion
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 11 Enhanced Solar Energy Conversion in Au-Doped, Single-Wall Carbon Nanotube-Si Heterojunction Cells
Abstract
Background
Methods
Results and Discussion
Conclusions
Acknowledgments
Authors’ Contributions
References
Chapter 12 Dual Effect of TiO2 and CO3O4 Co-Semiconductors and Nanosensitizer on Dye-Sensitized Solar Cell Performance
Abstract
Background
Methods
Results and Discussion
Conclusion
Authors’ Contributions
References
Chapter 13 Effect of External Applied Electric Field on the Silicon Solar Cell’s Thermodynamic Efficiency
Abstract
Introduction
Studied Model
Thermodynamic Efficiency Calculation
Discussion
Conclusion
Acknowledgements
References
Chapter 14 Effect of Calcination Temperature on the Properties of Czts Absorber Layer Prepared by RF Sputtering for Solar Cell Applications .259
Abstract
Introduction
Experimental Details
Results and Discussion
Conclusion
Acknowledgement
References
Chapter 15 Enhancement of Energy Generation from Two Layer Solar Panels
Abstract
Background
Methods
Results and Discussion
Conclusions
Acknowledgements
Authors’ Contributions
References
Chapter 16 Spectrum Splitting For Efficient Utilization of Solar Radiation: A Novel Photovoltaic–Thermoelectric Power Generation System
Abstract
Background
Methods
Results and Discussion
Conclusion
References
Chapter 17 All Spray Pyrolysis-Coated CdTe–TiO2 Heterogeneous Films for Photo-Electrochemical Solar Cells
Abstract
Introduction
Experimental
Results and Discussion
Conclusion
Acknowledgements
References
Chapter 18 An Intelligent Solar Energy-Harvesting System For Wireless Sensor Networks
Abstract
Introduction
Related Work
Methodology
System Parameters Analysis
Simulation and Implementation
Conclusions
Acknowledgements
References
Chapter 19 Thermal Properties of Carbon Black Aqueous Nanofluids for Solar Absorption
Abstract
Introduction
Experiments
Results and Discussion
Conclusion
Authors’ Contributions
References
Index
Back Cover

Citation preview

PHYSICS OF SOLAR ENERGY

PHYSICS OF SOLAR ENERGY

Edited by:

Chenggui Sun

ARCLER

P

r

e

s

s

www.arclerpress.com

Physics of Solar Energy Chenggui Sun

Arcler Press 2010 Winston Park Drive, 2nd Floor Oakville, ON L6H 5R7 Canada www.arclerpress.com Tel: 001-289-291-7705 001-905-616-2116 Fax: 001-289-291-7601 Email: [email protected] e-book Edition 2020 ISBN: 978-1-77407-402-2 (e-book)

This book contains information obtained from highly regarded resources. Reprinted material sources are indicated. Copyright for individual articles remains with the authors as indicated and published under Creative Commons License. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data and views articulated in the chapters are those of the individual contributors, and not necessarily those of the editors or publishers. Editors or publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify. Notice: Registered trademark of products or corporate names are used only for explanation and identification without intent of infringement. © 2020 Arcler Press ISBN: 978-1-77407-281-3 (Hardcover)

Arcler Press publishes wide variety of books and eBooks. For more information about Arcler Press and its products, visit our website at www.arclerpress.com

DECLARATION Some content or chapters in this book are open access copyright free published research work, which is published under Creative Commons License and are indicated with the citation. We are thankful to the publishers and authors of the content and chapters as without them this book wouldn’t have been possible.

ABOUT THE EDITOR

Dr. Chenggui Sun obtained his Ph.D. degree in Chemical Engineering from the University of Waterloo, a public research university with a main campus in Waterloo, Ontario, Canada. His academic background is in the area of nanotechnology and membrane technology, which is at the interface between Chemical Engineering and Environmental Engineering. Dr. Sun enjoys the challenge of combining technology, engineering, and business expertise to contribute to organizational development in the context of a more sustainable, healthier world. He has proven ability to understand client needs and perspectives in order to formulate and implement strategic plans, and known networking and collaboration skills with a commitment to fostering growth in others. Dr. Sun is also reputable for approaching tasks with creativity, integrity, and drive to inspire cooperation and exceed expectations.

TABLE OF CONTENTS

List of Contributors .......................................................................................xv List of Abbreviations ................................................................................... xxv Preface.................................................................................................. ....xxix Chapter 1

Upconversion in Solar Cells ...................................................................... 1 Abstract ..................................................................................................... 2 Review ...................................................................................................... 2 Conclusions ............................................................................................. 19 Acknowledgements ................................................................................. 19 Authors’ Contributions ............................................................................. 19 References ............................................................................................... 20

Chapter 2

A Short Progress Report on High-Efficiency Perovskite Solar Cells ......... 27 Abstract ................................................................................................... 27 Introduction ............................................................................................. 28 Principle And History of Perovskite Scs .................................................... 29 High-Efficiency Perovskite Solar Cells ...................................................... 31 Stability of Perovskite Solar Cells ............................................................. 34 Interface Engineering ............................................................................... 37 Conclusions ............................................................................................. 38 Acknowledgements ................................................................................. 38 Authors’ Contributions ............................................................................. 38 References ............................................................................................... 39

Chapter 3

Colloidal Quantum Dot Based Solar Cells: From Materials to Devices ............................................................................................... 45 Abstract ................................................................................................... 45 Introduction ............................................................................................. 46 Size-Dependent Physical Properties of CQDs .......................................... 46

Surface Modification and Characteristics Control..................................... 48 Enhancement of Power Conversion Efficiency Through Solar Cell Structure Design............................................................. 50 Enhancement of Power Conversion Efficiency Through Surface Modification ...................................................................... 52 Enhancement of Power Conversion Efficiency Using Optical Design ............................................................................... 54 Conclusions ............................................................................................. 56 References ............................................................................................... 57 Chapter 4

Performance Evaluation of Nanofluids in Solar Energy: A Review of The Recent Literature .......................................................... 61 Abstract ................................................................................................... 61 Introduction ............................................................................................. 62 Literature Review of Recent Years............................................................. 63 Conclusions ............................................................................................. 85 Acknowledgements ................................................................................. 85 Authors’ Contributions ............................................................................. 85 References ............................................................................................... 86

Chapter 5

Solar to Fuels Conversion Technologies: A Perspective ........................... 89 Abstract ................................................................................................... 89 Introduction ............................................................................................. 90 Solar Alternative Fuels ............................................................................. 91 Key Challenges and Opportunities ......................................................... 112 Acknowledgements ............................................................................... 116 References ............................................................................................. 118

Chapter 6

Ab Initio Design of Nanostructures For Solar Energy Conversion: A Case Study on Silicon Nitride Nanowire ............................................ 127 Abstract ................................................................................................. 127 Background ........................................................................................... 128 Methods ................................................................................................ 130 Results and Discussion .......................................................................... 131 Conclusions ........................................................................................... 140 Acknowledgements ............................................................................... 141 References ............................................................................................. 142

x

Chapter 7

An Efficient and Effective Design of InP Nanowires for Maximal Solar Energy Harvesting ........................................................................ 147 Abstract ................................................................................................. 147 Background ........................................................................................... 148 Design For Maximal Light Harvesting of InP NWs.................................. 150 Results and Discussion .......................................................................... 155 Conclusions ........................................................................................... 162 Acknowledgements ............................................................................... 162 Authors’ Contributions ........................................................................... 163 References ............................................................................................. 164

Chapter 8

Optical Simulations of P3HT/Si Nanowire Array Hybrid Solar Cells ..... 167 Abstract ................................................................................................. 167 Background ........................................................................................... 168 Methods ................................................................................................ 169 Results and Discussion .......................................................................... 171 Conclusion ............................................................................................ 174 Acknowledgements ............................................................................... 175 Authors’ Contributions ........................................................................... 175 References ............................................................................................. 176

Chapter 9

Substantial Influence on Solar Energy Harnessing Ability by Geometries of Ordered Si Nanowire Array ........................................... 179 Abstract ................................................................................................. 179 Background ........................................................................................... 180 Methods ................................................................................................ 181 Results and Discussion .......................................................................... 182 Conclusions ........................................................................................... 188 Acknowledgements ............................................................................... 188 Authors’ Contributions ........................................................................... 188 References ............................................................................................. 189

Chapter 10 Silicon Nanowires for Solar Thermal Energy Harvesting: an Experimental Evaluation on the Trade-off Effects of the Spectral Optical Properties ................................................................................. 193 Abstract ................................................................................................. 193 Background ........................................................................................... 194 xi

Methods ................................................................................................ 195 Results and Discussion .......................................................................... 198 Conclusions ........................................................................................... 203 Acknowledgements ............................................................................... 203 Authors’ Contributions ........................................................................... 203 References ............................................................................................. 204 Chapter 11 Enhanced Solar Energy Conversion in Au-Doped, Single-Wall Carbon Nanotube-Si Heterojunction Cells ............................................ 209 Abstract ................................................................................................. 209 Background ........................................................................................... 210 Methods ................................................................................................ 211 Results and Discussion .......................................................................... 213 Conclusions ........................................................................................... 219 Acknowledgments ................................................................................. 219 Authors’ Contributions ........................................................................... 219 References ............................................................................................. 221 Chapter 12 Dual Effect of TiO2 and CO3O4 Co-Semiconductors and Nanosensitizer on Dye-Sensitized Solar Cell Performance .................... 227 Abstract ................................................................................................. 227 Background ........................................................................................... 228 Methods ................................................................................................ 229 Results and Discussion .......................................................................... 231 Conclusion ............................................................................................ 239 Authors’ Contributions ........................................................................... 239 References ............................................................................................. 240 Chapter 13 Effect of External Applied Electric Field on the Silicon Solar Cell’s Thermodynamic Efficiency .................................................................... 243 Abstract ................................................................................................. 244 Introduction ........................................................................................... 244 Studied Model ....................................................................................... 246 Thermodynamic Efficiency Calculation .................................................. 247 Discussion ............................................................................................. 253 Conclusion ............................................................................................ 255

xii

Acknowledgements ............................................................................... 255 References ............................................................................................. 256 Chapter 14 Effect of Calcination Temperature on the Properties of Czts Absorber Layer Prepared by RF Sputtering for Solar Cell Applications . 259 Abstract ................................................................................................. 259 Introduction ........................................................................................... 260 Experimental Details .............................................................................. 262 Results and Discussion .......................................................................... 264 Conclusion ............................................................................................ 273 Acknowledgement ................................................................................. 274 References ............................................................................................. 275 Chapter 15 Enhancement of Energy Generation from Two Layer Solar Panels ........ 279 Abstract ................................................................................................. 279 Background ........................................................................................... 280 Methods ................................................................................................ 281 Results and Discussion .......................................................................... 290 Conclusions ........................................................................................... 293 Acknowledgements ............................................................................... 293 Authors’ Contributions ........................................................................... 293 References ............................................................................................. 294 Chapter 16 Spectrum Splitting For Efficient Utilization of Solar Radiation: A Novel Photovoltaic–Thermoelectric Power Generation System ................................................................................ 297 Abstract ................................................................................................. 297 Background ........................................................................................... 298 Methods ................................................................................................ 300 Results and Discussion .......................................................................... 306 Conclusion ............................................................................................ 314 References ............................................................................................. 316 Chapter 17 All Spray Pyrolysis-Coated CdTe–TiO2 Heterogeneous Films for Photo-Electrochemical Solar Cells .......................................... 319 Abstract ................................................................................................. 319 Introduction ........................................................................................... 320 Experimental.......................................................................................... 322 xiii

Results and Discussion .......................................................................... 322 Conclusion ............................................................................................ 332 Acknowledgements ............................................................................... 332 References ............................................................................................. 333 Chapter 18 An Intelligent Solar Energy-Harvesting System For Wireless Sensor Networks............................................................................................... 337 Abstract ................................................................................................. 337 Introduction ........................................................................................... 338 Related Work ......................................................................................... 340 Methodology ......................................................................................... 341 System Parameters Analysis .................................................................... 345 Simulation and Implementation ............................................................. 348 Conclusions ........................................................................................... 358 Acknowledgements ............................................................................... 358 References ............................................................................................. 359 Chapter 19 Thermal Properties of Carbon Black Aqueous Nanofluids for Solar Absorption............................................................ 363 Abstract ................................................................................................. 363 Introduction ........................................................................................... 364 Experiments ........................................................................................... 366 Results and Discussion .......................................................................... 368 Conclusion ............................................................................................ 374 Authors’ Contributions ........................................................................... 375 References ............................................................................................. 376 Index ..................................................................................................... 381

xiv

LIST OF CONTRIBUTORS Wilfried GJHM van Sark Copernicus Institute, Utrecht University, Budapestlaan 6, Utrecht 3584 CD, The Netherlands Jessica de Wild Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus 5, Eindhoven 5656 AE, The Netherlands Jatin K Rath Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus 5, Eindhoven 5656 AE, The Netherlands Andries Meijerink Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht 3508 TA, The Netherlands Ruud EI Schropp Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus 5, Eindhoven 5656 AE, The Netherlands Present address: Solar Energy, Energy research Centre of the Netherlands (ECN), High Tech Campus Building 5, p-057 (WAY), Eindhoven 5656 AE, The Netherlands Present address: Plasma & Materials Processing, Department of Applied Physics, Eindhoven University of Technology (TU/e), Eindhoven 5600 MB, The Netherlands He Tang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, 610054 Chengdu, China School of Microelectronics and Solid-State Electronics,University of Electronic Science and Technology of China, 610054 Chengdu, China

xv

Shengsheng He School of Microelectronics and Solid-State Electronics,University of Electronic Science and Technology of China, 610054 Chengdu, China Chuangwei Peng School of Microelectronics and Solid-State Electronics,University of Electronic Science and Technology of China, 610054 Chengdu, China Jung Hoon Song Nano-Convergence Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon 34113, Republic of Korea Sohee Jeong Nano-Convergence Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon 34113, Republic of Korea Department of Nanomechatronics, University of Science and Technology (UST), Daejeon 34113, Republic of Korea Navid Bozorgan Mechanical Engineering Department, Abadan Branch, Islamic Azad University, Abadan, Iran Maryam Shafahi Mechanical Engineering Department, California State Polytechnic University, Pomona, California, USA Harry L. Tuller Department of Materials Science and Engineering, Massachusetts Institute of Technology and Materials Processing Center, Cambridge, MA 02139, USA International Institute of Carbon Neutral Energy Research, Kyushu University, Fukuoka, Japan Hui Pan Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, Avenida da Universidade, Taipa, Macao SAR, People’s Republic of China Dan Wu OPTIMUS, Photonics Centre of Excellence, School of Electrical and Electronic

xvi

Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Xiaohong Tang OPTIMUS, Photonics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Kai Wang Department of Electrical and Electronic Engineering, Southern University of Science and Technology, 1088 Xueyuan Avenue, 518055 Shenzhen, People’s Republic of China Zhubing He Department of Materials Science and Engineering, Southern University of Science and Technology, 1088 Xueyuan Avenue, 518055 Shenzhen, People’s Republic of China. Xianqiang Li OPTIMUS, Photonics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Wenbo Wang Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Xinhua Li Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Long Wen Key Laboratory of Nanodevices and Applications, Suzhou Institute of NanoTech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China Yufeng Zhao Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

xvii

Huahua Duan Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Bukang Zhou Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Tongfei Shi Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Xuesong Zeng Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Ning Li Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Yuqi Wang Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Zilong Wu State Key Laboratory of Surface Physics and Key Laboratory of Micro- and Nano-Photonic Structures (Ministry of Education) and Department of Physics, Fudan University, Shanghai 200433, China Ziyi Wang Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China Songyou Wang Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China xviii

Zhenyang Zhong State Key Laboratory of Surface Physics and Key Laboratory of Micro- and Nano-Photonic Structures (Ministry of Education) and Department of Physics, Fudan University, Shanghai 200433, China Abdoul Karim Sekone Department of Mechanical Engineering, National Chung Hsing University, Taichung, Taiwan, Republic of China Yu-Bin Chen Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China Ming-Chang Lu Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China Wen-Kai Chen Department of Mechanical Engineering, National Chung Hsing University, Taichung, Taiwan, Republic of China Chia-An Liu Department of Mechanical Engineering, National Chung Hsing University, Taichung, Taiwan, Republic of China Ming-Tsang Lee Department of Mechanical Engineering, National Chung Hsing University, Taichung, Taiwan, Republic of China Leifeng Chen State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China Hong He College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China

xix

Key Laboratory of Eco-Textiles, Ministry of Education, Jiangnan University, Wuxi 214122, Jiangsu, People’s Republic of China. Shijun Zhang State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Chen Xu State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Jianjiang Zhao State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Shichao Zhao College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China Yuhong Mi College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China Deren Yang State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China F. A. Taher Department of Physical Chemistry, Faculty of Science (Girls), Al-Azhar University, Youssif Abbas St., Nasr city, Cairo, Egypt Galila M. Elsayed Department of Physical Chemistry, Faculty of Science (Girls), Al-Azhar University, Youssif Abbas St., Nasr city, Cairo, Egypt

xx

N. M. Khattab Solar Energy Department, National Research Center, Elbohooth St., Dokki, Giza, Egypt N. Almohamady Faculty of Sci ence (Girls), Youssif Abbas St., Nasr city, Cairo, Egypt R. Zieba Falama Department of Renewable Energy, The Higher Institute of the Sahel, University of Maroua, Maroua, Cameroon Laboratory of Energy Research, Institute of Geological and Mining Research, Yaounde´, Cameroon Justin Mibaile Department of Physics, Higher Teachers’ Training College, University of Maroua, Maroua, Cameroon E. Guemene Dountio Laboratory of Energy Research, Institute of Geological and Mining Research, Yaounde´, Cameroon Noe¨l Djongyang Department of Renewable Energy, The Higher Institute of the Sahel, University of Maroua, Maroua, Cameroon Serge Y. Doka Department of Physics, Faculty of Science, University of Ngaounde´re´, Ngaounde´re´, Cameroon Timoleon C. Kofane Department of Physics, Faculty of Science, University of Yaounde´ I, Yaounde´, Cameroon Sachin Rondiya School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Avinash Rokade School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India xxi

Ashok Jadhavar School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Shruthi Nair School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Madhavi Chaudhari School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Rupali Kulkarni School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Azam Mayabadi School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Adinath Funde School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India Habib Pathan Department of Physics, Savitribai Phule Pune University, Pune 411 007, India Sandesh Jadkar Department of Physics, Savitribai Phule Pune University, Pune 411 007, India Pragya Sharma GERMI RIIC (Research and Innovation Centre), Gandhinagar, Gujarat 382007, India Tirumalachetty Harinarayana GERMI RIIC (Research and Innovation Centre), Gandhinagar, Gujarat 382007, India CSIR-NGRI (National Geophysical Research Institute), Hyderabad 500007, India

xxii

Esam Elsarrag Gulf Organisation for Research and Development, QSTP, Doha, Qatar Hans Pernau Department of Energy Systems, Fraunhofer IPM, Freiburg, Germany Jana Heuer Department of Energy Systems, Fraunhofer IPM, Freiburg, Germany Nibul Roshan Gulf Organisation for Research and Development, QSTP, Doha, Qatar Yousef Alhorr Gulf Organisation for Research and Development, QSTP, Doha, Qatar Kilian Bartholomé Department of Energy Systems, Fraunhofer IPM, Freiburg, Germany S. N. Vijayaraghavan Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala 682041, India Aditya Ashok Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala 682041, India Gopika Gopakumar Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala 682041, India Harigovind Menon Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala 682041, India Shantikumar V. Nair Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala 682041, India Mariyappan Shanmugam Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa xxiii

Vidyapeetham, Kochi, Kerala 682041, India Yin Li School of Information Science and Engineering, Central South University, Changsha 410083, China Ronghua Shi School of Information Science and Engineering, Central South University, Changsha 410083, China Dongxiao Han College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China Zhaoguo Meng College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China Daxiong Wu College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China Canying Zhang College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China Haitao Zhu College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China

xxiv

LIST OF ABBREVIATIONS AM

Air mass

APE

Average photon energy

BMPS

Battery as the main power supply

BHJ

Bulk heterojunction

CdTe

Cadmium telluride

CSS

Capture and storage

CQDs

Colloidal quantum dots

CSP

Concentrated solar power

CSP

Concentrating solar power

CBB

conduction band bottom

CIGS

Copper indium gallium diselenide

CZTS

Copper zinc tin sulfide

DFT

Density functional theory

DOS

Density of states

DMSs

Diluted magnetic semiconductors

DAC

Direct absorption solar collector

DASC

Direct Absorption Solar Collector

EML

Effective medium layer

EBL

Electron blocking layer

ETL

Electron transport layer

ECE

External collection efficiency

EQE

External quantum efficiency

FR

Filling ratio

FDTD

Finite-difference time-domain

FTO

Fluorine-doped tin oxide

FC-C

Front converter and cell

HPCC

High-Performance Computing Cluster

HTL

Hole transport layer

ICTO

Technology Office

IR

Infrared

ISEH

Intelligent solar energy-harvesting

IOT

Internet of Things

IEP

Isoelectric point

LSMO

lanthanum strontium manganese oxide

LBL

Layer-by-layer

LDV

Light-duty vehicle

MFCs

Mass flow controllers

MPPT

Maximum power point tracking

SPST

single-pole single-throw

MPA

Mercaptopropionic acid

MACE

Metal-assisted chemical etching

NREL

National renewable energy laboratory

NIR

Near-infrared

PML

Perfect match layer

PSCs

Perovskite solar cells

PCMs

Phase-change materials

PEC

Photoelectrochemical cells

PDS

Photothermal deflection spectroscopy

PV

Photovoltaics

PV/T

Photovoltaic thermal

PPSCs

Planar perovskite solar cells

PMMA

Polymethylmethacrylate

PCE

Power conversion efficiency

PCB

Printed circuit board

PEM

Proton exchange membrane

QDs

Quantum dots

RTE

Radiative transfer equation

RIE

Reactive ion etching

RL

Recombination layer

xxvi

SEM

Scanning electron microscopic

SOC

Self state of charge

SSP

Shallow solar pond

SWCNTs

Single wall carbon nanotubes

SCR)

Space charge region (

SAR

Specific absorption rate

SPD

Spray pyrolysis deposition

TMM

Transfer-matrix method

TEM

Transmission electron microscopy

TTA

Triplet-triplet annihilation

VBT

Valence band top

WEH

Wind energy harvester

WSN

Wireless sensor network

GIXRD

X-ray diffraction

XPS

X-ray photoelectron spectroscopy

xxvii

PREFACE

Due to the increasing energy demand of human society and the environmental issues caused by fossil fuel consumption, the development of renewable energy has become an imminent requirement. Among all the methods that generate renewable energy, solar systems play an important role in harvest energy from solar radiation for human activities. In this book, general reviews on solar energy conversion and utilization methods are provided and latest development in solar energy technologies are introduced. Progress reports and latest research on solar cells for converting energy captured from solar radiation to electricity are presented. The upconversion by lanthanide compounds in various host materials and the application of upconversion materials for thin silicon solar cells were reviewed and discussed. Because nanotechnology is deemed essential for the further development of solar cell technologies, research progresses and reviews regarding the latest development of nanomaterials for solar energy system are included. The recent development of cost effective and high-efficient perovskite solar cells (PSCs) was reported in another review, which introduced the history of PSCs and the key progress made in high-efficiency PSCs. The state-of-the-art results of PSC technologies were reported. Besides nanomaterials and PSCs, other development associated solar cell/energy system progresses are included in this book as well. In addition to the conversion of solar radiation to energy, heat transfer enhancement in solar devices is another significant issue in the context of energy saving and compact designs. One of the effective approaches to solve this issue is to replace the working fluid with nanofluids to improve heat transfer in solar devices. In this book, the latest research on the nanofluids’ applications in solar thermal engineering systems is reviewed, and comprehensive information for the design of a solar thermal system working at the optimum conditions is provided. A research on carbon black aqueous nanofluids for solar absorption is reported too. Besides the utilization of solar heat and electricity generated with solar cell, solar-to-fuel conversion has been attracting more and more attentions to satisfy the need to replace fossil fuels. The solar refinery concept, in which convert the chemical feedstocks CO2 and H2O into fuels with captured solar energy,

is considered as the key technology to produce cleaner fuels that remain easy to transport and store. In this book, recent advances and remaining challenges associated with solar-to-fuel conversion are discussed by researchers.

xxx

Upconversion in Solar Cells

1

Wilfried GJHM van Sark1, Jessica de Wild2 , Jatin K Rath2 , Andries Meijerink3 and Ruud EI Schropp2,4,5 1

Copernicus Institute, Utrecht University, Budapestlaan 6, Utrecht 3584 CD, The Netherlands

Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus 5, Eindhoven 5656 AE, The Netherlands 2

Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht 3508 TA, The Netherlands 3

Present address: Solar Energy, Energy research Centre of the Netherlands (ECN), High Tech Campus Building 5, p-057 (WAY), Eindhoven 5656 AE, The Netherlands 4

Present address: Plasma & Materials Processing, Department of Applied Physics, Eindhoven University of Technology (TU/e), Eindhoven 5600 MB, The Netherlands. 5

Citation: van Sark, W. GJHM, et al., “Upconversion in solar cells”. Nanoscale Research Letters (2013), https://doi.org/10.1186/1556-276X-8-81. Copyright: © van Sark et al.; licensee Springer, 2013. This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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ABSTRACT The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics. One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion. Modification of the spectrum requires down- and/or upconversion or downshifting of the spectrum, meaning that the energy of photons is modified to either lower (down) or higher (up) energy. Nanostructures such as quantum dots, luminescent dye molecules, and lanthanide-doped glasses are capable of absorbing photons at a certain wavelength and emitting photons at a different (shorter or longer) wavelength. We will discuss upconversion by lanthanide compounds in various host materials and will further demonstrate upconversion to work for thinfilm silicon solar cells. Keywords: Upconversion, Photovoltaics, Thin-film silicon, Spectral modification, Lanthanides

REVIEW INTRODUCTION Attaining high conversion efficiencies at low cost has been the key driver in photovoltaics (PV) research and development already for many decades, and this has resulted in a PV module cost of around US$0.5 per watt peak capacity today. Some commercially available modules have surpassed the 20% efficiency limit, and laboratory silicon solar cells are getting closer and closer [1] to the Shockley-Queisser limit of 31% for single-junction silicon cells [2]. However, a fundamental issue is that conventional single-junction semiconductor solar cells only effectively convert photons of energy close to the bandgap (Eg) as a result of the mismatch between the incident solar spectrum and the spectral absorption properties of the material [3]. Photons with energy (Eph) smaller than the bandgap are not absorbed, and their energy is not used for carrier generation. Photons with energy (Eph) larger than the bandgap are absorbed, but the excess energy Eph – Eg is lost due to thermalization of the generated electrons. These fundamental spectral losses are approximately 50% [4]. Several approaches have been suggested to overcome these losses, e.g., multiple stacked cells [5], intermediate bandgaps [6], multiple exciton generation [7], quantum dot concentrators [8, 9], and spectral converters, the latter being down- and upconverters [10,

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11] and downshifters [12, 13]. In these so-called third- or next-generation PV concepts [14, 15], nanotechnology is deemed essential in realizing most of these concepts [16].

Spectral Conversion Spectral conversion aims at modifying the incident solar spectrum such that a better match is obtained with the wavelength-dependent conversion efficiency of the solar cell. Its advantage is that it can be applied to existing solar cells and that optimization of the solar cell and spectral converter can be done separately. Different types of spectral conversion can be distinguished: (a) upconversion, in which two low-energy (sub-bandgap) photons are combined to give one high-energy photon; (b) downshifting or luminescence, in which one high-energy photon is transformed into one lower energy photon; and (c) downconversion or quantum cutting, in which one high-energy photon is transformed into two lower energy photons. Downshifting can give an efficiency increase by shifting photons to a spectral region where the solar cell has a higher quantum efficiency, i.e., basically improving the blue response of the solar cell, and improvements of up to 10% relative efficiency increase have been predicted [13]. Up- and downconversion, however, are predicted to be able to raise the efficiency above the SQ limit [10, 11]. For example, Richards [12] has shown for crystalline silicon (c-Si) that the potential relative gain in efficiency could be 32% and 35% for downconversion and upconversion, respectively, both calculated for the standard 1,000-W/m2 air mass (AM) 1.5 solar spectrum. Research on spectral conversion is focused on organic dyes, quantum dots, lanthanide ions, and transition metal ion systems for up- and downconversion [13, 17, 18]. An upconversion layer is to be placed at the back of the solar cells, and by converting part of the transmitted photons to wavelengths that can be absorbed, it is relatively easy to identify a positive contribution from the upconversion layer, even if the upconversion efficiency is low. In contrast, proof-of-principle experiments in solar cells are complicated for downconverters and downshifters because of the likelihood of competing non-radiative processes. These downconverters and downshifters have to be placed at the front of the solar cell, and any efficiency loss will reduce the overall efficiency of the system. Downconversion with close to 200% internal quantum efficiency has been demonstrated, but the actual quantum efficiency is lower due to concentration quenching and parasitic absorption processes [19, 20]. Even

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for a perfect 200% quantum yield system, a higher solar cell response requires a reflective coating to reflect the isotropically emitted photons from the downconversion layer back towards the solar cell. However, no proof-ofprinciple experiments have been reported to demonstrate an efficiency gain using downconversion materials. An upconverter also emits isotropically, but since it is placed at the back of the solar cells, the upconversion photons can easily be directed into the solar cell by placing a reflector behind the upconverter layer, as depicted in Figure 1.

Figure 1: Schematic view of solar cell with upconverter layer at the back. It is surrounded by a back reflector to ensure that upconverted radiation is directed towards the solar cell where it can be absorbed.

The usefulness of down- and upconversion and downshifting depends on the incident spectrum and intensity. While solar cells are designed and tested according to the ASTM standard [21], these conditions are rarely met outdoors. Spectral conditions for solar cells vary from AM0 (extraterrestrial) via AM1 (equator, summer and winter solstice) to AM10 (sunrise, sunset). The weighted average photon energy (APE) [22] can be used to parameterize this; the APE (using the range 300 to 1,400 nm) of AM1.5G is 1.674 eV, while the APE of AM0 and AM10 are 1.697 and 1.307 eV, respectively. Further, overcast skies cause higher scattering leading to diffuse spectra, which are blue-rich, e.g., the APE of the AM1.5 diffuse spectrum is calculated to be 2.005 eV, indeed much larger than the APE of the AM1.5 direct spectrum of 1.610 eV. As downconversion and downshifting effectively red-shift spectra, the more relative energy an incident spectrum contains in the blue part of the spectrum (high APE), the more gain can be expected [12, 23]. Application of downconversion layers will therefore be more beneficial for regions with high diffuse irradiation fraction, such as Northwestern Europe, where this fraction can be 50% or higher. In contrast, solar cells with upconversion (UC) layers will be performing well in countries with high direct irradiation

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fractions or in early morning and evening due to the high air mass resulting in low APE, albeit that the non-linear response to intensity may be limiting. Up- and downconversion layers could be combined on the same solar cell to overcome regionally dependent efficiencies. Optimization of either up- or downconversion layers could be very effective if the solar cell bandgap is a free design parameter. In this paper, we focus on upconversion materials for solar cells, in particular for thin-film silicon solar cells. We describe the present state of the art in upconversion materials and application in solar cells.

Upconversion Principles Upconversion was suggested by Bloembergen [24] and was related to the development of infrared (IR) detectors: IR photons would be detected through sequential absorption, as would be possible by the arrangement of energy levels of a solid. However, as Auzel pointed out, the essential role of energy transfer was only recognized nearly 20 years later [25]. Several types of upconversion mechanism exist, of which the addition de photon par transferts d’energie or, in English, energy transfer upconversion mechanism is the most efficient; it involves energy transfer from an excited ion, named sensitizer, to a neighboring ion, named activator [25]. Others are two-step absorption, being a ground-state absorption followed by an excited-state absorption, and second-harmonic generation. The latter mechanism requires extremely high intensities, of about 1010 times the sun’s intensity on a sunny day, to take place [26] and can therefore be ruled out as a viable mechanism for solar cell enhancement. Upconverters usually combine an active ion, of which the energy level scheme is employed for absorption, and a host material, in which the active ion is embedded. The most efficient upconversion has been reported for the lanthanide ion couples (Yb, Er) and (Yb, Tm) [27]. The first demonstration of such an upconversion layer was reported by Gibart et al. [28] who used a GaAs cell on top of a vitroceramic containing Yb3+ and Er3+: it showed 2.5% efficiency under very high excitation densities.

Upconverter Materials Lanthanides have been employed in upconverters attached to the back of bifacial silicon solar cells. Trivalent erbium is ideally suited for upconversion

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of near-infrared (NIR) light due to its ladder of nearly equally spaced energy levels that are multiples of the 4I15/2 to 4I13/2 transition (1,540 nm; see also Figure 2). Shalav et al. [29] have demonstrated a 2.5% increase of external quantum efficiency due to upconversion using NaYF4:20% Er3+. By depicting luminescent emission intensity as a function of incident monochromatic (1,523 nm) excitation power in a double-log plot, they showed that at low light intensities, a two-step upconversion process (4I15/2 → 4I13/2 → 4I11/2) dominates, while at higher intensities, a three-step upconversion process (4I15/2 → 4I13/2 → 4I11/2 → 4S3/2level) is involved.

Figure 2: Upconversion in the (Yb 3+ , Er 3+ ) couple. The dashed lines represent energy transfer, the full lines represent the radiative decay, and the curly lines indicate multi-phonon relaxation processes. The main route is a two-step energy transfer after excitation around 980 nm in the Yb3+ ion that leads to excitation to the 4F7/2 state of the Er3+ ion. After relaxation from this state, emission is observed from the 2H11/2 level, the 4S3/2 level (green), and the 4F9/2 level (red).

Strümpel et al. have identified the materials of possible use in up- (and down-) conversion for solar cells [26]. In addition to the NaYF4:(Er,Yb) phosphor, they suggest the use of BaCl2:(Er3+,Dy3+) [30], as chlorides were thought to be a better compromise between having a low phonon energy and a high-excitation spectrum, compared to the NaYF4[31, 32]. These lower phonon energies lead to lower non-radiative losses. In addition, the emission spectrum of dysprosium is similar to that of erbium, but the content of Dy3+ should be 700 nm could be more fully absorbed. These two effects can be achieved with the upconversion layer, combined with a highly reflecting back contact. While the upconversion

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layer converts sub-bandgap photons to ‘super’-bandgap photons that can thus be absorbed, a non-conductive reflector is a much better alternative than any metallic mirror, thus sending back both the unabsorbed super-bandgap photons as well as the upconverted super-bandgap photons into the cell. It is commonly estimated that the stabilized efficiency of the approximately 9% cell can be enhanced to approximately 12%. Besides a-Si, a material denoted as protocrystalline Si could be used; this is an amorphous material that is characterized by an enhanced medium-range structural order and a higher stability against light-induced degradation compared to standard amorphous silicon. The performance stability of protocrystalline silicon is within 10% of the initial performance; its bandgap is slightly higher than that of amorphous silicon. De Wild et al. [58] have demonstrated upconversion for a-Si cells with NaYF4 co-doped with (Er3+, Yb3+) as upconverter. The upconverter shows absorption at 980 nm (by the Yb3+ ion) leading to efficient emission of 653(red) and 520- to 540-nm (green) light (by the Er3+) after a two-step energy transfer process. The narrow absorption band around 980 nm for Yb3+ limits the spectral range of the IR light that can be used for upconversion. An external quantum efficiency of 0.02% at 980-nm laser irradiation was obtained. By using a third ion (for example, Ti3+) as a sensitizer, the full spectral range between 700 and 980 nm can be efficiently absorbed and converted to red and green light by the Yb-Er couple. A transition metal ion such as Ti3+incorporated in the host lattice absorbs over a broad spectral region and transfers the energy to a nearby Yb3+ ion through a dipole-dipole interaction [27, 31]. The resulting light emission in the green and red regions is very well absorbed by the cell with very good quantum efficiency for electron–hole generation.

Bifacial Solar Cells with Upconverter Concentrated broadband light excitation has recently been used to study two types of bifacial a-Si:H solar cells that were made with and without Gd2O2S:Er3+, Yb3+ upconverter attached at the back of the cells [59]. The upconverter powder mixture was applied to the rear of the solar cells by first dissolving it in a solution of PMMA in chloroform, after which it was drop cast. Two types of p-i-n a-Si:H solar cells were made: one on Asahi-textured SnO2:F glass and one on flat ZnO:Al 0.5% superstrate. The efficiency obtained for the cells is 8% for textured and 5% for flat solar cells, both without a back reflector. Backside illumination yields an efficiency of 5% for textured solar cells and 4% for flat solar cells. With illumination from

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the back, the efficiency is lower because the generation profile is reversed within the cell, and thus, the photogenerated minority carriers have to travel the largest mean distance, rather than the majority carriers. The spectral response measured through the n-layer shows a quantum efficiency of 0.7 for both textured and flat solar cells at 550 nm; the spectral response at 660 nm is lower, i.e., 0.4 for textured cells and 0.15 for flat cells. The transmission for 900 to 1,040 nm was 40% to 45% for the textured solar cells and between 60% and 80% for the flat solar cells. The thickness of the i-layer was chosen such that an interference maximum occurs at 950 nm, increasing the transmission at this wavelength. As a result, more light can be absorbed by the upconverter layer in the case of the flat solar cell configuration. Concentration levels of up to 25 times were reached using near-infrared light from a solar simulator. The absorption and emission spectra of the upconverter are shown in Figure 4. The absorption is highest around 950 nm. The upconverter was excited with filtered light of a xenon lamp at 950 ± 10 and 980 ± 10 nm. The 4 F7/2 state at 2.52 eV is reached after two energy transfer events from Yb to Er. The upconverter was already shown to be very efficient at low light intensities. Saturation was measured under light intensities of less than 1 W/ cm2. Although the absorption at 950 nm (1.31 eV) is higher, excitation at 980 nm (1.26 eV) leads to two times higher upconverted emission intensity. This may be attributed to the perfectly resonant energy transfer step of 980 nm (1.26 eV) since the 4F7/2 state is at 2.52 eV.

Figure 4: Upconverted emission and absorption spectra of the upconverter in PMMA layer. The emission spectrum is obtained when the upconverter shows

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no saturation and only emission peaks from the 4S3/2, 2H11/2 (510 to 560 nm), and 4 F9/2 (650 to 680 nm) states are observed.

For further experiments, the upconverter was excited at 980 nm with a pulsed Opotek Opolette laser. Because upconversion is a two-photon process, the efficiency should be quadratically dependent on the excitation power density. The intensity of the laser light was varied with neutral density filters. Upconversion spectra were recorded in the range of 400 to 850 nm under identical conditions with varying excitation power. Varying the intensity shows that for low light intensities, the red part is less than 6% of the total emission (see Figures 4 and 5). Only when the emission from the green-emitting states becomes saturated does the red emission become more significant and even blue emission from the 2H9/2 state is measured (see Figure 5). By comparing the emission intensities, it becomes clear that the emission intensity is not increasing quadratically with excitation power density. Instead, emissions from higher and lower energy states are visible. The inset in Figure 5 shows the integrated emission peaks for the green and total emissions, showing that at very high laser intensities, the total emission is saturated.

Figure 5: Upconverted emission spectra under low and high excitation density. For the low excitation power, the green state was not yet saturated. The intensities may be compared. New peaks (italic) are assigned: 2H9/2 → 4I15/2

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transition at 410 nm, 4I9/2 → 4I15/2 transition at 815 nm, and the intermediate transition 2H9/2 → 4I13/2 at 560 nm.

Sub-bandgap Response The sub-bandgap response in the near infrared due to the band tails of a-Si:H solar cells cannot be neglected [58]. To distinguish between upconverter response and sub-bandgap response, intensity-dependent current–voltage measurements are performed on solar cells with and without an upconverter at wavelengths longer than 900 nm using a solar simulator and a 900-nmlong pass filter. Intrinsic response of the band tails is linearly dependent on the light intensity, while response due to upconverted light is expected to be quadratically increasing with the concentration. Figure 6 shows the current measured for the different solar cells with different concentration factors of the sub-bandgap light. The slope of the line fitted to the data yields the value n, as given by Equation 2. As expected, the sub-bandgap response linearly increases with light intensity and values of n larger than 1 are measured for the upconversion solar cells. Note that the value is rather close to 1 because a large part of the total current is due to the sub-bandgap response (see Figure 6, upper graph). When the total current measured for the upconverter solar cells is corrected for the sub-bandgap response, the current due to upconversion only shows a higher value for n (see Figure 6, lower graph), i.e., a value of n = 1.5 and n = 1.8 is determined for textured and flat solar cells, respectively. Clearly, the current is not increasing quadratically with increasing concentration. It is unlikely that the upconverter is saturated because the power density is far below the saturation level of 0.6 W/cm2. It is therefore more likely that the deviations are due to decreasing carrier collection efficiency with increasing concentration. This effect would play a larger role in textured solar cells because they have a higher defect density than flat solar cells. This may explain why the value of n is closer to 2 for flat solar cells than for textured solar cells.

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Figure 6: Current measured in the solar cells under illumination of sub-bandgap light.In the upper graph, the total current of the reference and UC cells are plotted as a function of the concentration factor, while in the lower graph, the current generated by the upconverter is shown. The slope for sub-bandgap response is 1 for flat and textured solar cells. The contribution of the upconverter increases the slope slightly; when corrected for the sub-bandgap response, the slope is 1.5 for the textured solar cells and 1.8 for the flat solar cells.

Narrow and Broadband Light Comparison Monochromatic laser light with wavelength at 981 nm and a power density of 0.2 W/cm2 was used for textured solar cells and yielded a current density of 0.14 mA/cm2 for the upconverter solar cells and 0.04 mA/cm2 for the reference solar cells. Evidently, the contribution of sub-bandgap absorption is much smaller using monochromatic laser light. The current due to the upconverter is comparable to the current measured under 20 sun: approximately 0.1 mA/cm2(see Figure 6). This is remarkable in two

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ways. First, the results are in contrast with previously reported experiments with broadband excitation of c-Si solar cells [53], where the current under broadband excitation was much smaller than that under laser light excitation. However, in [53], another upconverter was applied (NaYF4) and different processes occur in the upconverter, namely excited state absorption. In the upconverter in this work (Gd2O2S), energy transfer upconversion is the main upconversion path, and the broadband absorption of Yb3+may increase the transfer between Yb3+ and Er3+. Second, the power that is absorbed by Yb3+ is 3.44 mW/cm2[37], which yields a broadband power density of 70 mW/cm2 under a concentration of 20 sun. This is three times less than the power density of the laser. A large difference here is that for broadband illumination, a 900-nm-long pass filter was used. Therefore, light of the solar simulator extends to further than 1,600 nm; thus, also the 4I13/2 state of Er3+ is excited directly. Addition of other paths that lead to upconverted light may contribute to the current. These paths may be non-resonant excited-state absorption between the energy levels of Er3+ or three-photon absorption around 1,540 nm at the 4I13/2 state of Er3+ (see Figure 2). Direct excitation of the 4I13/2 state of Er3+ followed by excited-state absorption from 4I13/2 to 2F9/2 results in a visible photon around 650 nm, while three-photon absorption around 1,540 nm results in emission from the 2F9/2 state too. Wavelengths required for these transitions are around 1,540 and 1,200 nm, which are present within the broad excitation spectrum. Contribution of these upconversion routes increases the emission and thereby the current in the solar cells.

Outlook Upconversion for solar cells is an emerging field, and the contribution of upconverter research to upconverter solar cell research increases rapidly. However, up to now, only proof-of-principle experiments have been performed on solar cells, mainly due to the high intensities that are deemed necessary. Some routes to enhance absorption are presently being developed, such as external sensitization and plasmonics. External sensitization can be achieved by, e.g., quantum dots or plasmons. Quantum dots (QDs) can be incorporated in a concentrator plate where the QDs absorb over a broad spectral range in the IR and emit in a narrow line, e.g., around 1,520 nm, resonant with the Er3+upconversion wavelength. Energy transfer from the QDs to Er3+ in this scheme is through radiative energy transfer. The viability of this concept was proven by Pan

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et al. [60] in c-Si solar cells, where a layer with QDs was placed below the upconverter layer. With the QDs, more light was absorbed and upconverted, which was proven by measuring the excitation spectra for the upconverted emission. The increased upconverted emission resulted in higher currents in the solar cell. More challenging are options to enhance upconversion efficiencies by manipulating emission and excitation processes through plasmonic coupling [61]. The use of plasmonic effects with upconverter materials is a new and emerging field, with many possibilities and challenges. In general, plasmonic resonance can be used in two ways to increase the upconversion efficiency: by enhancing either the absorption strength or the emission strength. When the absorption strength is enhanced, the emission increases with the square of the enhancement in the non-linear regime. In the case of resonance between the plasmon and the optical transition, strong enhancement can be achieved. Recently, Atre et al. [62] have modelled the effects of a spherical nanocresent consisting of a core of an upconverter material and a crescent-shaped Ag shell. A 10-fold increase in absorption as well as a 100fold increase in above-bandgap power emission toward the solar cell was calculated. A similar study has been performed using Au nanoparticles [63]. Experimental proof has recently been reported by Saboktakin et al. [64]. A related method is to enhance the absorption strength by nanofocusing of light in tapered metallic structures [65]. At the edges, enhancement has been reported due to focusing of the light in these areas. The other option is enhancing the emission. In this case, the emission of the upconverter is enhanced by nearby plasmon resonances [66]. Since the field enhancement decays away exponentially with the distance to metallic nanoparticle, the upconverter species have to be close to the surface of the nanoparticle to benefit from the field enhancement effects. For organic molecules, this presents no problem because the molecules are small enough to be placed in the field. For lanthanide upconverters, this is more difficult because the ions are typically contained in materials with grain sizes in the micrometer range. However, several groups have managed to make nanosized NaYF4 particles [67, 68]. This offers the possibility of plasmonic enhancement for lanthanide upconverters and decreases the light intensity required for efficient upconversion. Alternatively, upconversion using sensitized triplettriplet annihilation in organic molecules at moderate monochromatic excitation intensities increases the a-Si:H cell efficiency as well [46, 56].

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CONCLUSIONS In this paper, we have briefly reviewed upconversion for solar cells and have presented some relevant experimental results, focusing on the application of lanthanides in combination with wide-bandgap solar cells (a-Si:H). The proof-of-principle experiments that have been performed so far have shown that high intensities ar0e needed to demonstrate upconversion for solar cells. Within the lanthanid4es, large steps in decreasing the necessary intensity are not expected. In the organic field, there is a rapid decrease in intensity needed for efficient upconversion, while conversion wavelengths are not appropriate yet. External sensitization using quantum dots or options to enhance upconversion efficiencies by manipulating emission and excitation processes through plasmonic coupling may offer routes for successful upconversion deployment in solar cells. With further developments in these organic molecules, it remains to be seen if lanthanide upconverters, with plasmonic enhancement, or molecules in which TTA can be employed, will be the upconverter material for the future in wide-bandgap solar cells.

ACKNOWLEDGEMENTS The authors gratefully acknowledge Agentschap NL for the partial financial support within the framework of the EOS-NEO Programme as well as the Utrecht University Focus and Mass Programme, Karine van der Werf, Caspar van Bommel, Bart Sasbrink, Martin Huijzer, and Thijs Duindam for the sample preparation and characterization. AM acknowledges the support from the EU-FP7 NANOSPEC Programme (STREP 246200).

AUTHORS’ CONTRIBUTIONS RS, WvS, JR, and AM initiated and conceived this study. JdW, as a Ph.D. student in the groups of RS and AM under the cosupervision of JR and WvS, performed the experiments. WvS and JdW wrote the article. All authors read and approved the manuscript.

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A Short Progress Report on High-Efficiency Perovskite Solar Cells

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He Tang1,2 , Shengsheng He2, and Chuangwei Peng2 1 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, 610054 Chengdu, China

School of Microelectronics and Solid-State Electronics,University of Electronic Science and Technology of China, 610054 Chengdu, China 2

ABSTRACT Faced with the increasingly serious energy and environmental crisis in the world nowadays, the development of renewable energy has attracted increasingly more attention of all countries. Solar energy as an abundant

Citation: Tang, H., et al., “A short progress report on high-efficiency perovskite solar cells”, Nanoscale Research Letters (2017), https://doi.org/10.1186/s11671-017-2187-5, Copyright: © The Author(s). 2017, Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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and cheap energy is one of the most promising renewable energy sources. While high-performance solar cells have been well developed in the last couple of decades, the high module cost largely hinders wide deployment of photovoltaic devices. In the last 10 years, this urgent demand for costeffective solar cells greatly facilitates the research of solar cells. This paper reviews the recent development of cost-effective and high-efficient solar cell technologies. This report paper covers low-cost and highefficiency perovskite solar cells. The development and the state-of-theart results of perovskite solar cell technologies are also introduced. Keywords: Solar cells, Perovskites, Stability, Renewable energy

INTRODUCTION About 85% of the world’s energy requirements are currently satisfied by exhaustible fossil fuels that have detrimental consequences on human health and the environment. Moreover, the global energy demand is predicted to double by 2050 [1]. Therefore, the development of renewable energy, such as wind energy, water energy, and solar energy, becomes an imminent requirement. Renewable energy-based power generation capacity is estimated to be 128 GW in 2014, of which 37% is wind power, almost one third solar power, and more than a quarter from hydropower (Fig. 1 a). This amounted to more than 45% of world power generation capacity additions in 2014, consistent with the general upward trend in recent years.

Figure 1: a Global renewable-based power capacity additions by type and share of total capacity additions [60]. b Rapid PCE evolution of perovskite solar cells from 2009 to 2016.

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Due to abundance, low cost, and environmental friendliness, solar energy attracts increasingly more attention from all over the world, which makes the rapid development of solar cell research in recent years. In general, a commonly used classification divides the various PV technologies (in commercial as well as in R&D stage) into three generations [2]: first generation, G1: wafer-based; mainly mono c-Si and mc-Si ; second generation, G2: thin film; a-Si, CdTe, CIGS, CuGaSe; third generation, G3: multi-junction and organic photovoltaics (OPV), dye-sensitized solar cells (DSSCs), and solar cells based on quantum dots as well as other nanomaterials. The development of the three-generation solar cells produced a rich variety of solar cells, such as Si solar cells, III–V solar cells, perovskite solar cells (PSCs), thin film solar cells, dye-sensitized solar cells, and organic solar cells. However, practical, low-cost, and high-efficiency third-generation solar cells are yet to be demonstrated. Si solar cells are well developed and mature, but there is little room for further improvement [3, 4, 5, 6]. III–V solar cells have a very high efficiency; however, its weakness is the high cost, which limits its applications [7, 8, 9]. Quantum dot solar cells have been receiving significant attention because of their low cost and high efficiency, but most efficient devices have been prepared with toxic heavy metals of Cd or Pb [10, 11, 12]. Halide perovskites have recently emerged as promising materials for low-cost, high-efficiency solar cells. As the perovskite solar cell technology becomes more and more mature, the efficiency of perovskitebased solar cells has increased rapidly, from 3.8% in 2009 to 22.1% in 2016 [13, 14, 15, 16]. However, the stability issues still require further studies. To give an update of the field, this paper reviews the recent development of high-efficiency PSCs. This report briefly introduces the history of PSCs and then focuses on the key progress made in high-efficiency perovskite solar cells. Recent efforts on the stability of perovskite solar cells will also be discussed. At the end of the report,we also give a brief introduction to the interface engineering.

PRINCIPLE AND HISTORY OF PEROVSKITE SCS PSCs have recently become one of the hot spots owing to its low preparation cost and high-conversion efficiency in the fields of solar cell research. And it is regarded as a great potential material for its superiority (compared with other materials) that may assist perovskite with ultimate usurping of the reigning cell material.

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In 1991, inspired by the principle of photosynthesis, O’Regan and Gratzel reported a landmark construction of solar cell called dye-sensitized solar cell, which can cover the sun light energy into electricity energy with an efficiency about 7% [17]. Presenting numerous advantages such as abundant raw materials, facile processing, and low cost compared with conventional solar cells, these novel solar cells made itself investigated popularly rapidly after its arising. And it is this work that inspired the emergence of PSCs, a DSSC with perovskite compounds. Perovskite originally refers to a kind of ceramic oxides with general molecular formula ABY3 discovered by the German mineralogist Gustav Rose in 1839. It was named “perovskite” because it is a calcium titanate(CaTiO3) compounds exists in calcium titanium ore [18]. The crystal structure of a perovskite is showed in Fig. 2a. In 2009, perovskite structured materials were first utilized in solar cells by Miyasaka and his colleagues. They creatively replaced the dye pigment in DSSCs with two organicinorganic hybrid halide-based perovskites, CH3NH3PbBr3 and CH3NH3PbI3. And, eventually, they gained relatively not considerable power conversion efficiency (PCE) of 3.13 and 3.81%, respectively [13].

Figure 2: a Crystal structure of a perovskite [22]. b Schematic diagram of general device [23]. cCross-section scanning electron microscopy (SEM) images of a meso-superstructured perovskite solar cell (scale bar is 500 nm) [22]. d Cross-section SEM images of a normal planar perovskite solar cells with the presence of an HTL and an ETL [22].

However, the work did not gain much attention due to low efficiency and poor stability, which resulted from a hole transport layer (HTL) with liquid electrolyte. An evolutionary jump then happened in 2012 when Kim, Gratzel and Park et al. [14] used perovskite absorbers as the primary

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photoactive layer to fabricate solid-state meso-superstructured PSCs. Spiro-MeOTAD and mp-TiO2 were used as the hole transport and electron transport materials (HTM/ETM), respectively, in their work and resulted in a relatively high efficiency of 9.7% for the first reported perovskite-based solid-state mesoscopic heterojunction solar cell. After this breakthrough, the investigation of PSCs became hot gradually in photovoltaic (PV) research in the following years. Eventually, the efficiency of PSCs was promoted to 22.1% in early 2016 [1]. Since the maximum theoretical PCE of the PSCs employing CH3NH3PbI3−x Cl x is 31.4%, there is still enough space for development [19]. Figure 2 b shows the general configuration of PSCs, which usually comprises a tin-doped indium oxide (ITO)/fluorine-doped tin oxide (FTO) substrate, metal electrode, a perovskite photoactive layer, together with necessary charge transport layers (i.e., a hole transport layer (HTL) [20] and an electron transport layer (ETL) [21]) [22, 23]. Figure 2 c, d shows two main device architectures: meso-superstructured perovskite solar cells (MPSCs) [24], which incorporate a mesoporous layer, and planar perovskite solar cells (PPSCs) in which all layers are planar [25]. The working principle of these PSCs can be briefly summarized in the following ways: perovskite layer absorbs the incident light, generating electron and hole, which are extracted and transported by ETMs and HTMs, respectively. These charge carriers are finally collected by electrodes forming PSCs [23].

HIGH-EFFICIENCY PEROVSKITE SOLAR CELLS Intramolecular Exchange In June 2015, Woon Seok Yang and his colleagues report an approach for depositing high-quality FAPbI3 films with which they fabricated FAPbI3 PSCs with a PCE of 20.1% under AM 1.5 G full-sun illuminations [26]. On the road to enhance the efficiency of solar cells, the deposition of dense and uniform films is critical for optoelectronic properties of perovskite films and is an important research topic of highly efficient PSCs. Woon Seok Yang and his colleagues report an approach for depositing high-quality FAPbI3 films, involving FAPbI3 crystallization by the direct intramolecular exchange of dimethyl sulfoxide (DMSO) molecules intercalated in PbI2 with

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formamidinium iodide (Fig. 3). This process produces FAPbI3 films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using films prepared by this technique, they fabricated FAPbI3-based PSCs with maximum power conversion efficiency greater than 20%.

Figure 3: PbI2 complex formation and X-ray diffraction. a Schematics of FAPbI3 perovskite crystallization involving the direct intramolecular exchange of DMSO molecules intercalated in PbI2 with formamidinium iodide (FAI). The DMSO molecules are intercalated between edge-sharing [PbI6] octahedral layers. b Histogram of solar cell efficiencies for each 66 FAPbI3-based cells fabricated by IEP and conventional process [26].

Cesium-Containing Triple-Cation Perovskite Solar Cells Adding inorganic cesium to triple-cation perovskite compositions, Michael Saliba and his colleagues demonstrated a perovskite solar cell which not only possesses higher PCEs of 21.1% but also is more stable, contains less phase impurities, and is less sensitive to processing conditions [27, 28]. They investigated triple-cation perovskites of the generic form “ Cs x(MA0.17FA0.83)(100−x)Pb(I0.83Br0.17)3,” demonstrating that the use of all three cations, Cs, MA, and FA, provides additional versatility in fine-tuning highquality perovskite films (Fig. 4). They yielded stabilized PCEs exceeding 21 and 18% after 250 h under operational conditions. Even more, the triple-cation perovskite films are thermally more stable and less affected by fluctuating surrounding variables such as temperature, solvent vapors, or heating protocols. This robustness is important for reproducibility, which is one of the key requirements for cost-efficient large-scale manufacturing of PSCs.

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Figure 4: Cross-sectional SEM images of a Cs0M, b Cs5M, and c lowmagnification Cs5M devices [27].

Graded Bandgap Perovskite Solar Cells On November 7, 2016, scientists from University of California, Berkeley, and Lawrence Berkeley National Laboratory reported a new design that already achieved an average steady state efficiency of 18.4%, with a height of 21.7% and a peak efficiency of 26% [29, 30, 31]. They use a single-atom thick layer of hexagonal boron nitride to combine two materials into a tandem solar cell and, eventually, obtained high efficiency. The compositions of the perovskite materials are both the organic molecules methyl and ammonia, whereas one contains the metals tin and iodine, while the other contains lead and iodine doped with bromine. The former is tuned to preferentially absorb light with an energy of 1 eV—infrared or heat energy—while the latter absorbs photons of energy 2 eV, or an amber color. Prior to this attempt, the merging of two perovskite materials has failed because the materials degrade one another’s electronic performance. This new way to combine two perovskite solar cell materials into one “graded bandgap” solar cell demonstrated exciting results. The solar cell absorbs nearly the entire spectrum of visible light. This is very beneficial to improve efficiency. The structure is shown in Fig. 5. They found that freshly illuminated cells tend

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to have higher PCE than cells that have been illuminated for more than a few minutes. For example, for a given graded bandgap perovskite cell, the PCE is between 25 and 26% in the first 2 min of illumination while the cell reaches a “steady state” with a stable PCE of 20.8% after approximately 5 min. This result indicates that perovskite-based solar cells have timedependent performance characteristics. The measurement of 40 graded bandgap perovskite cells demonstrated that the average steady state PCE over all devices is 18.4% while the best graded bandgap cell in the steady state exhibited a PCE of 21.7%.

Figure 5: Cross-sectional schematic and SEM images of perovskite cell with integral monolayer h-BN and graphene aerogel. a Schematic of a graded bandgap perovskite solar cell. Gallium nitride (GaN), monolayer hexagonal boron nitride (h-BN), and graphene aerogel (GA) are key components of the high-efficiency cell architecture. b Cross-sectional SEM image of a representative perovskite device. The division between perovskite layers and the monolayer h-BN is not visible in this SEM image. The dashed lines indicate the approximate location of the perovskite layers and the monolayer h-BN as a guide to the eye. The location of perovskite layers and monolayer h-BN is extracted from the related EDAX analysis. Thickness of the CH3NH3SnI3 layer is 150 nm and that of the CH3NH_3PbI3−x Br x is 300 nm. Scale bar, 200 nm [29].

STABILITY OF PEROVSKITE SOLAR CELLS In recent years, the record efficiency of PSCs has been updated from 9.7 to 22.1%. However, the poor long-term device stability of PSCs is still a big remaining challenge for PSCs, which decide whether exciting achievements could be transferred from the laboratory to industry and outdoor applications. Therefore, long-term stability is an issue that needs to be addressed urgently for PSCs. Quite a number of people have shown interest in the issue of stability and given guiding opinions on improving stability [32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44].

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Multiple reports have suggested that moisture and oxygen, UV light, solution processing, and thermal stress are four key factors affecting the stability of PSCs. Observed (sometimes rapid) degradation occurs when devices are exposed to those environmental factors [22, 32, 45, 46]. Guangda Niu and his colleagues [32] expressed their views that in order to modulate the stability of PSCs, many factors should be taken into consideration, including the composition and crystal structure design of the perovskite; the preparation of the HTM layer and electrode materials; the thin film fabrication method, interfacial engineering, and encapsulation methods (multilayer encapsulation or helmet encapsulation); and the module technology. Their work verified that oxygen, together with moisture, could lead to the irreversible degradation of CH3NH3PbI3 which is always employed as sensitizers in PSCs. They expose TiO2/CH3NH3PbI3 film to air with a humidity of 60% at 35 °C for 18 h, and then, the absorption between 530 and 800 nm greatly decreased (Fig. 6 d).

Figure 6: a Proposed decomposition pathway of CH3NH3PbI3 in the presence of a water molecule. The main product of this pathway is PbI2 [48]. b Normalized absorbance measurements (taken at 410 nm) for CH3NH3PbI3 films exposed to

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different relative humidity [49]. c PDS spectra for CH3NH3PbI3 films before (initial state) and after exposure to a relative humidity in the range of 30–40% for different times. This clearly indicates a significant reduction in absorption in the range of 1.5–2.5 eV after exposure to humidity [1]. d Degradation of CH3NH3PbI3 in moisture and air atmosphere. UV-vis absorption spectra of TiO2/CH3NH3PbI3 film before and after degradation. The inset is a photograph of CH3NH3I exposed to different conditions: (1) CH3NH3I exposed to argon and without UV radiation; (2) CH3NH3I exposed to argon and with UV radiation; (3) CH3NH3I exposed to air and with UV radiation; and (4) CH3NH3I exposed to air and without UV radiation [32].

Especially, humidity is an indispensable factor when an experimental investigation on the issue of stability is conducted. Work lead by Kwon et al. shows that the hygroscopic nature of amine salts results from the origin of moisture instability [47]. Figure 6 a shows the likely process of CH3NH3PbI3 decomposition which was displayed by Frost et al. [48]. The process indicates that HI and MA are soluble in water, which directly leads to irreversible degradation of the perovskite layer. Yang et al. investigated this degradation process by performing in situ absorbance and grazing incidence X-ray diffraction (GIXRD) measurements [49]. To make a valid contrast in degradation, they carefully control the relative humidity (RH) in which the films were measured. Figure 6 b shows their research result of the influence of RH on the film degradation. The absorption reduced to half of its original value in only 4 h for the 98% RH case while this would take 10,000 h extrapolation of the degradation curve for a low RH of 20%. The result indicates expectedly that higher RH values cause a more rapid reduction in film absorption than a low RH. Moreover, further experiment demonstrates that varied carrier gases, N2 or air led to no significant change in the degradation of the absorbance, indicating that the main cause of degradation in the perovskite film, under normal atmosphere, is the presence of moisture. In 2014, De Wolf et al. used another powerful technique, photothermal deflection spectroscopy (PDS), to measure the moisture-induced decomposition of CH3NH3PbI3 [50]. They measured the PDS spectra of CH3NH3PbI3 layers after exposure to ambient air with 30–40% relative humidity during 1 and 20 h, respectively. Figure 6 c shows that the absorptance between photon energies of 1.5 and 2.5 eV drops by two orders of magnitude after exposure to humidity for 20 h. In addition, the absorption edge that occurs at 1.57 eV in its initial state shifts to 2.3 eV, an energy corresponding

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to the bandgap of PbI2 [51], which indicate that CH3NH3PbI3 can decompose into PbI2 in a humid ambient due to the dissolution of disordered CH3NH3I [35, 52]. Many methods are researched for stability enhancement of PSCs recent years. Xin Wang et al. successfully developed a simple solutionprocessed CeO x (x = 1.87) ETL at low temperature. According to their work, CeO x -based devices exhibit superior stability under light soaking compared to TiO2-based PSCs [53]. Zhiping Wang et al. presented the first longterm stability study of the new “mixed-cation mixed-halide” perovskite composition FA0.83Cs0.17Pb(I0.6Br0.4)3(FA = (HC(NH2)2)) and discover that the cells are remarkably stable when exposed to full-spectrum simulated sun light in ambient conditions without encapsulation [54]. Han et al. adopted thick carbon as the electrode and the device’s own hole transport layer; the cell was stable for >1000 h in ambient air under full sunlight while it achieved a PCE of 12.8% [55].

INTERFACE ENGINEERING The interface is vital to the performance of the devices, since it is not only critical to the exciton formation, dissociation, and recombination but also influences the degradation of devices [56]. As a result, the interface engineering for reduced recombination is extremely important to achieve high-performance and high-stability PSCs. Tan et al. reported a contact-passivation strategy using chlorine-capped TiO2colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. The PSCs achieved certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 cm2, respectively. Moreover, PSCs with efficiency greater than 20% retained 90% of their initial performance after 500 h of continuous room-temperature operation at their maximum power point under 1-sun illumination [57]. Wang and co-workers inserted an insulating tunneling layer between the perovskite and the electron transport layer. The thin insulating layer allowed the transport of photo-generated electrons from perovskite to C60 cathode through tunneling and blocked the photo-generated holes back into the perovskite. Devices with these insulating materials exhibited an increased PCE of 20.3% under 1-sun illumination [58]. Correa-Baena et al. provided some theoretical guidance by investigating in depth the recombination at the different interfaces in a PSC, including the chargeselective contacts and the effect of grain boundaries [59].

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CONCLUSIONS The development of PSCs in the last few years makes it a promising alternative for the next-generation, low-cost, and high-efficiency solar cell technology. Driven by the urgent need of cost-effective, high-efficient solar cells, PSCs have been intensively investigated in the recent years. Various kinds of methods are used to improve the performance. We summarize the recent development of high-efficiency PSCs. The recorded efficacy of single-junction PSCs has been increased by a few folds to over 22% in the last few years, approaching the best single crystalline silicon solar cells. Undoubtedly, halide perovskite materials have emerged as an attractive alternative to conventional silicon solar cells. However, the stability issue is still urgent to be solved. The recent progress made in the device architectures and new materials open new opportunities for highly stable PSCs.

ACKNOWLEDGEMENTS This work was supported in part by the Fundamental Research Funds for the Central Universities under Grant ZYGX2015Z004.

AUTHORS’ CONTRIBUTIONS HT provided the ideas and structure of the whole article and drafted the “Principle and History of Perovskite SCs” and “Intramolecular Exchange” sections. SSH mainly completed the “Cesium-Containing Triple-Cation Perovskite Solar Cells” and “Graded Bandgap Perovskite Solar Cells” sections. CWP wrote “Stability of perovskite solar cells” section. All authors read and approved the final manuscript.

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Colloidal Quantum Dot Based Solar Cells: From Materials to Devices

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Jung Hoon Song1 and Sohee Jeong1,2 Nano-Convergence Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon 34113, Republic of Korea

1

Department of Nanomechatronics, University of Science and Technology (UST), Daejeon 34113, Republic of Korea

2

ABSTRACT Colloidal quantum dots (CQDs) have attracted attention as a next-generation of photovoltaics (PVs) capable of a tunable band gap and low-cost solution process. Understanding and controlling the surface of CQDs lead to the Citation: Song, J. H. and S. Jeong, “Colloidal quantum dot based solar cells: from materials to devices”, Nano Convergence (2017), https://doi.org/10.1186/s40580-0170115-0. Copyright: © Korea Nano Technology Research Society 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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significant development in the performance of CQD PVs. Here we review recent progress in the realization of low-cost, efficient lead chalcogenide CQD PVs based on the surface investigation of CQDs. We focus on improving the electrical properties and air stability of the CQD achieved by material approaches and growing the power conversion efficiency (PCE) of the CQD PV obtained by structural approaches. Finally, we summarize the manners to improve the PCE of CQD PVs through optical design. The various issues mentioned in this review may provide insight into the commercialization of CQD PVs in the near future. Keywords: Colloidal quantum dots, Nanocrystals, Solar cells, Photovoltaics, Lead chalcogenides

INTRODUCTION Colloidal quantum dots (CQDs) are chemically-prepared semiconductor nanocrystals, which have diameter is less than twice the Bohr radius describing the spatial extension of exciton (electron–hole pair) in semiconductors. The CQDs have attracted attention over the past decade due to a solution based synthetic methods [1], easily tunable optoelectronic properties [2], and superior processing capabilities for optoelectronic applications [3]. Specifically, lead chalcogenide CQDs are considered as a prominent material for a next-generation photovoltaics (PVs) [4] owing to their wide tunable bandgaps covering from visible to near-infrared wavelength regime arising from a large Bohr exciton radius and narrow bulk bandgap. Also, the excitons in lead chalcogenide CQDs can be easily separated into electrons and holes because of their high dielectric constant and extinction coefficient. More importantly, because their material properties can be easily controlled using electron density functional design, lead chalcogenide CQDs are expected to exhibit efficient utilization of low-energy and low-intensity photons and efficient collection of high-energy charges, which are difficult to achieve in conventional PVs [5]. In this article, we discuss the current state of highefficiency CQD PV development.

SIZE-DEPENDENT PHYSICAL PROPERTIES OF CQDS When the size of a bulk semiconductor is reduced, discrete energy levels appear in the energy band owing to the quantum confinement effect, as

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shown in Fig. 1a. A CQD consists of a semiconductor core and surface ligands. In the case of a core, synthesis methods capable of controlling the size of binary or ternary compound semiconductors, such as II–VI (CdSe, CdS) compound semiconductors, III–V (InP, InAs), IV–VI (PbS, PbSe), and III–V (CuInS2, CuInSe2), have been developed and reported. As shown in Fig. 1a, the bandgap of IV–VI (PbS, PbSe) CQDs can be controlled to absorb light in the range of 600–3000 nm, which is suitable for solar cell materials. Additionally, most of the reported high-efficiency CQD PVs have been fabricated using IV–VI (PbS, PbSe) CQDs. Therefore, in this article, we mainly focus on the IV–VI (PbS, PbSe) CQDs. Typically, when the energy bandgap of a semiconductor decreases in a single-junction PV, the open-circuit voltage (VOC) decreases and the short-circuit current (JSC) increases by absorbing more light. Figure 1b shows that single-junction CQD PVs also exhibit the relationship between energy bandgaps and solar cell characteristics as described above [6]. In CQD solids, the hole mobility increases depending on the size of the CQDs [7], explained by the decrease in the total number of interparticle hops and the reduction of the coulombic charging energy of an individual particle.

Figure 1: a AM 1.5G solar spectrum (from ASTM G173-03 reference spectra), band diagram, and first exciton energy of PbS CQDs with various diameters. b Characteristics of Schottky junction PbS CQD PVs as a function of the first exciton energy (reprinted with permission from ref. 6, Copyright 2013 American Physical Society).

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SURFACE MODIFICATION AND CHARACTERISTICS CONTROL Nanomaterials such as CQDs generally have a high surface-area-to-volume ratio and the surfaces are prone to form dangling bonds, which causes defects. Especially, reducing surface defects is very important for obtaining high-efficiency CQD PVs. A CQD fabricated using wet-chemical synthesis consists of a semiconductor core and surface ligands, as shown in Fig. 2a [8]. The physical characteristics of the CQD can be controlled by the surface ligands owing to its high surface-area-to-volume ratio. Surface ligands generally have an amphiphilic structure consisting of a polar head group and a nonpolar aliphatic group. The rear portion of the ligand, which is composed of aliphatic groups, provides steric stabilization and dispersibility, which is the ability to dissolve in organic solvents. The head group typically contains amines, carboxylate, thiolate and phosphonate. These functional groups bind to the cationic metals on the CQD surfaces and produce nonstoichiometric CQDs, resulting in a doping effect. Typically, compound semiconductors exhibit p-type doping polarity under anion-rich conditions and n-type doping polarity under cation-rich conditions. Thus, doping can be controlled by using these ligands. When a CQD film is fabricated, the mobility of the charge carriers is determined by the length of ligands that passivate the surface of the CQDs. As shown in Fig. 2b, the mobility and coupling of the CQDs increase with the use of shorter-length ligands [9]. In addition, when surface ligands bind to the surface of CQDs, the electron distribution of the CQD surface and ligand changes, resulting in the formation of dipole moment on the CQD surface. The strength and direction of the surface dipole moment are determined by the surface ligands. Therefore, the position of the energy band is shifted by the surface dipole moment, as shown in Fig. 2c–d [10]. CQDs have many surface defects because of their high surfacearea-to-volume ratio. Therefore, the surface ligands play an important role in reducing these surface defects. Figure 2e–f show that additional passivation using halide atoms reduces the surface defects of CQDs and increases photoluminescence (PL) [11, 12]. Furthermore, as passivation prevents the oxidation of CQDs, their air stability can be improved [11, 12]. Consequently, the surface ligands of CQDs play an important role in controlling the dispersibility, air stability, and electrical properties (such as doping, mobility, electronic structure, and surface defects). Therefore, understanding the dependence of CQD characteristics on surface ligands is essential for obtaining high-efficiency CQD PVs.

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Figure 2: a Schematic diagram of a PbS CQD consisting of a semiconductor core and surface ligands (reprinted with permission from ref. 8, Copyright 2014 American Association for the Advancement of Science). b Charge carrier mobility of PbS CQD films with various surface ligand length (reprinted with permission from ref. 9, Copyright 2013 American Chemical Society). c Energy band position of PbS CQD films for d different surface ligands (reprinted with permission from ref. 10, Copyright 2013 American Chemical Society). Air stability analysis using e the absorbance spectrum and f quantum yield of PL as a function of storage time and its dependence on NH4Cl treatment on the surface of PbSe CQDs (reprinted with permission from ref. 11, Copyright 2014 American Chemical Society).

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ENHANCEMENT OF POWER CONVERSION EFFICIENCY THROUGH SOLAR CELL STRUCTURE DESIGN The power conversion efficiency (PCE) of CQD PVs has increased very rapidly since national renewable energy laboratory (NREL) certification began in 2010 and the highest PCE in the NREL chart is currently 13.4%. In the early stages of the development of CQD PVs, the PCE was increased in accordance with the structural changes of the devices. However, since 2012, the development of technologies that control the surface of CQDs has resulted in dramatic improvements of the performance of CQD PVs. In Sect. 4, we will describe the structural development of CQD PVs.

Figure 3: Schematic of photovoltaic architectures and flat-band diagrams at VOC of a CQD-sensitized solar cell, b Schottky junction solar cell, and c het-

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erojunction solar cell (reprinted with permission from ref. [13], Copyright 2010 American Chemical Society). d Schematic of CQD homojunction solar cell, energy band diagram, and cross-sectional image of scanning electron microscope image (reprinted with permission from ref. [19], Copyright 2012 American Chemical Society).

Initially, CQDs with high extinction coefficients were used instead of dyes in dye-sensitized solar cells. In this case, the CQD absorbs light to form excitons, and electrons and holes separated from the excitons are generally transferred through TiO2 and the electrolyte, respectively (Fig. 3a) [13]. Consequently, the electric characteristics of the CQDs have relatively low influence on the device in the dye-sensitized solar cell. However, the CQDs with uncontrolled electrical properties can be formed as a monolayer on TiO2 to absorb only a small amount of light. Therefore, mesoporous TiO2 is commonly used to overcome this problem. In recent years, new CQDs without heavy metals (such as Zn–Cu–In–Se CQDs) have been employed to produce a dye-sensitized solar cell structure that exhibits a relatively high PCE [14, 15, 16]. Currently, most of the high-efficiency CQD PVs use a thin film solar cell structure. For the PbS CQD solar cells, the excitons generated by light are easily separated by the internal field of the diode due to their high dielectric constant, and the separated electrons and holes move in the CQD thin film. Therefore, their electronic properties itself largely influence on the CQD solar cells. Such electronic properties can be controlled by chemically surface treatment. Initially, thin-film CQD PVs used a Schottky diode structure with a metal (Fig. 3b) [13, 17]. However, the QD film thickness cannot be increased, originated by the small built-in potential and narrow space charge region (SCR) in such Schottky junction CQD PVs. In addition, their charge carrier diffusion length is very short (several tens of nanometers) due to many surface trap states. For the high-efficiency CQD PVs, the SCR should be enlarged. To overcome this problem, heterojunction diodes have been fabricated using oxide semiconductors with n-type doping polarity such as ZnO and TiO2 (Fig. 3c) [13, 18].

Finally, a homojunction CQD PV with CQDs of n-type and p-type doping polarities has been successfully developed by stoichiometry control using surface ligands (Fig. 3d). The structure of the homojunction shows a lower efficiency than heterojunction structures. In a heterojunction using oxide semiconductors with relatively large energy bandgaps, such as ZnO and TiO2, the electrons move easily at the interface between the CQD

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and the oxide semiconductor; however, the holes cannot move across the interface and the recombination of electrons and holes decreases. Therefore, the homojunction CQD PVs show lower PCE than heterojunction ones [19].

ENHANCEMENT OF POWER CONVERSION EFFICIENCY THROUGH SURFACE MODIFICATION The surface modification of a CQD is an important factor that determines not only the characteristics of the CQD but also the characteristics of the resulting PV device. Figure 4 shows the efficiencies of CQD PVs, as reported by the NREL chart. As shown in the efficiency chart, there are significant developments of the CQD PVs efficiencies over the last 7 years as understanding and controlling the surface of CQDs. In the case of CQDs fabricated by wet-chemical synthesis, the surface is passivated by long organic ligands to reduce internal defects and improve size uniformity due to high synthesis temperatures. To fabricate a thin-film CQD PV, long organic ligands must be replaced with short ligands to improve carrier mobility. When forming a conductive CQD thin film, a layer-by-layer (LBL) process is used to repeatedly perform the deposition of the CQD film and the ligand exchange process to reduce cracks. After the synthesis, PbS CQDs consist of Pb-rich nonstoichiometric (111) surfaces passivated by ligands and stoichiometric (100) surfaces without ligands [20]. The long organic ligands on the non-stoichiometric (111) surface are replaced by mercaptopropionic acid (MPA) ligands containing sulfur (anion), leading to the p-type behavior in CQDs. Thus a heterojunction CQD PV with the p-type of CQDs and n-type of TiO2 or ZnO can be fabricated [21]. As mentioned in Sect. 4, the CQDs have many surface defects and a very short diffusion length; and thus charge carriers are mainly driven by the drift in the electric field inside the SCR. Therefore, to achieve high-efficiency CQD PVs, the CQD active layer should increase without decreasing the extraction efficiency of photoinduced carriers through improving the carrier diffusion length. The (100) surface has no surface ligands and is easily oxidized to generate defects. When producing CQD PVs with halide-treated CQDs (Fig. 2e–f), both the air stability can be improved and the efficiency can be increased by improving carrier diffusion length due to reduction of surface defects (Fig. 4a) [21].

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Figure 4: NREL efficiency chart of CQD PVs. a Schematic of a CQD treated with an MPA ligand and a chloride (inorganic) ligand; density of states in the energy bandgap obtained from a transient photo-voltage measurement (reprinted with permission from ref. 21, Copyright 2012 Macmillan Publishers Ltd.). b Band diagram of a CQD PV based on electron blocking with EDT-exchanged CQDs; energy band positions for various ligands obtained from ultraviolet photoelectron spectroscopy analysis (reprinted with permission from ref. 22, Copyright 2014 Macmillan Publishers Ltd.). c Schematic of CQD and CQD ink (reprinted with permission from ref. 25, Copyright 2016 Macmillan Publishers Ltd.).

In addition, the electron blocking layer (EBL) at the interface between the CQD thin film and the metal electrode is used in the same principle to prevent recombination by blocking the hole transport at the interface. In this case, EBL prevents the movement of electrons to the metal electrode through the energy band shift of ethanedithiol (EDT)-treated CQD film [22]. Therefore, high-efficiency CQD PVs can be achieved by using an EBL to reduce electron recombination. In this structure, the number of surface defects and the type of ligands vary depending on the acidity of the protonic solvent used in the ligand exchange process. An efficiency higher than 10% has been reported for CQD PVs produced by changing the electrical properties of CQD films with various protic solvents in the ligand exchange process (Fig. 4b) [23]. The fabrication of CQD PVs is based on the LBL process by employing the above ligand exchange process. This fabrication procedure is timeconsuming and the electrical properties of the CQD film are modified by the

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environment during the ligand exchange process. Therefore, the LBL process reduces the reproducibility of CQD PVs and inhibits their commercialization. To solve this problem, the surface of PbS CQDs has been completely modified with ammonium iodide (NH4I), which has the closest affinity with Pb, to allow the CQDs to be dissolved in N, N-dimethylformamide [24]. Using a technique similar to the above method and considering the dependence of the energy band shift on the ligand, CQDs with a PCE of 11.3% have been obtained (Fig. 4c) [25]. Ligand exchange performed in the liquid phase is more complete than solid ligand exchange because it increases the diffusion length by suppressing surface defects, resulting in increased thickness of the CQD film without reducing the efficiency.

ENHANCEMENT OF POWER CONVERSION EFFICIENCY USING OPTICAL DESIGN In CQDs, the quantum confinement effect separates the levels of the energy band of bulk semiconductors, resulting in an increase in the energy bandgap and a discrete energy level, as shown in Fig. 1a. This discrete energy level decreases the density of state and reduces the amount of absorbed light around the energy bandgap. Therefore, observations of the external quantum efficiency (EQE) spectrum of a CQD PV show a markedly decreased efficiency near the energy bandgap. This is a unique characteristic of the CQD. To solve this problem, surface plasmons of metal nanoparticles have been used to shift the spectrum of the incident sunlight to the region in which CQDs can absorb large amounts of light, thereby improving the PCE (Fig. 5a) [26]. Multiple junctions using CQDs improve the PCE because they provide easier control of the light absorption region of the CQDs, thus adjusting the energy bandgap. Although relatively few studies have been conducted on multi-junction CQD PVs, they have shown a possibility of maximized efficiency. The LBL process is performed to fabricate the conductive CQD film. In this case, because an organic ligand has a specific acidity and LBL process is performed using a polar solvent and a nonpolar solvent alternately, the pre-formed front sub-cell and intermediate recombination layer (RL) is damaged and the efficiency is lowered.

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Figure 5: a Absorption spectra of metal nanoparticles in TiO2 and PbS CQDs; EQE spectra with and without metal nanoparticles (reprinted with permission from ref. 26, Copyright 2015 Wiley–VCH Verlag GmbH & Co. KGaA). b Energy band diagram and J–V curves of multi-junction PVs using an organic PV and a CQD PV (reprinted with permission from ref. 27, Copyright 2016 Elsevier B.V.). c Energy band diagram and J–V curves of multi-junction PVs using CQD PVs (reprinted with permission from ref. 28, Copyright 2017 Wiley–VCH Verlag GmbH & Co. KGaA).

Therefore, as shown in Fig. 5b, a CQD PV with a smaller bandgap than that of the organic PV is placed in the front sub-cell, thereby minimizing the reduction in efficiency [27]. In addition, intermediate RLs have been

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developed that can induce sufficient electron and hole recombination without damage by using the solution process during the formation of the rear sub-cells to minimize the efficiency reduction (Fig. 5c) [28]. However, the current efficiency of multi-junction CQD PVs is lower than that of single-junction CQD PVs; therefore, further research is required to address these problems. One possible solution is to minimize the damage of the front sub-cell by using CQDs in which the ligand is exchanged in the liquid phase mentioned in the previous section.

CONCLUSIONS CQDs have been attracting much attention because they can absorb light above their energy bandgap with a high extinction coefficient and can be processed by using a solution process. Significant progress has been achieved in the development of CQD PVs by understanding their surface characteristics and using surface modification to obtain superior characteristics that distinguish them from general bulk semiconductors. In the future, we aim to address the various issues presented in this article and to commercialize CQD PVs along with the commercialization of CQD display fields.

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Performance Evaluation of Nanofluids in Solar Energy: A Review of The Recent Literature

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Navid Bozorgan1 and Maryam Shafahi2 Mechanical Engineering Department, Abadan Branch, Islamic Azad University, Abadan, Iran 1

2 Mechanical Engineering Department, California State Polytechnic University, Pomona, California, USA

ABSTRACT Utilizing nanofluid as an absorber fluid is an effective approach to enhance heat transfer in solar devices. The purpose of this review is to summarize the research done on the nanofluids’ applications in solar thermal engineering systems in recent years. This review article provides comprehensive

Citation: Bozorgan, N. and M. Shafahi, “Performance evaluation of nanofluids in solar energy: a review of the recent literature”, Micro and Nano Systems Letters (2015), https:// doi.org/10.1186/s40486-015-0014-2. Copyright: © Bozorgan and Shafahi; licensee Springer, 2015.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http:// creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/ zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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information for the design of a solar thermal system working at the optimum conditions. This paper identifies the opportunities for future research as well. Keywords: Nanofluids, Solar energy, Solar systems, Heat transfer enhancement

INTRODUCTION Energy is an important entity for the economic development of any country. On the other hand, fossil fuels meeting a great portion of the energy demand are scarce and their availability is decreasing continously. Nowadays, solar systems play an important role in the production of energy from renewable sources by converting solar radiation into useful heat or electricity. Considering the environmental protection and great uncertainty over future energy supplies, solar energy is a better alternative energy form in spite of its slightly higher operation costs. Heat transfer enhancement in solar devices is one of the significant issues in energy saving and compact designs. One of the effictive methods is to replace the working fluid with nanofluids as a novel strategy to improve heat transfer characteristics of the fluid. More recently reserachers have become interested in the use of nanofluids in collectors, water heaters, solar cooling systems, solar cells, solar stills, solar absorption refrigeration systems, and a combination of different solar devices due to higher thermal conductivity of nanofluids and the radiative properties of nanoparticle. How to select suitable nanofluids in solar applications is a key issue. The effectiveness of nanofluids as absorber fluids in a solar device strongly depends on the type of nanoparticles and base fluid, volume fraction of nanoparticles, radiative properties of nanofluids, temperature of the liquid, size and shape of the nanoparticles, pH values, and stability of the nanofluids [1]. It was found that only a few review papers have discussed the capability of nanofluids to enhance the performance of solar systems [2-5]. This paper compiles recent research in this field and identifies many issues that are open or even not commenced to investigate. It is authors’ hope that this review will be useful to determine the effectiveness of nanofluids in solar applications.

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LITERATURE REVIEW OF RECENT YEARS Using Nanofluids in Solar Collectors Role of Nanoparticles Gan et al. [6] experimently showed that the radiation absorption of Al2O3 nanofluids is less than Aluminuim nanofluids. For nanofluids with Al2O3 particles, the situation is different because of the different optical properties of Al2O3. The weak radiation absorption of Al2O3nanoparticles will not result in significant localized convective heat transfer from the particles to the base fluids. The use of Al2O3/water nanofluid as coolant was simulated for a silicon solar cell using the finite element method by Elmir et al. [7]. They considered the solar panel as an inclined cavity with a slope of 30°. Application of nanofluids increased the average Nusselt number and rate of cooling. They reported 27% enhancement in the heat transfer rate for 10% alumina nanofluid at Re = 5. Luo et al. [8] simulated the performance of a DAC solar collector with nanofluids using a 2D model by solving the radiative transport equations of particulate media and combining conduction and convection heat transfer equations. The nanofluid flows horizontally from right to left in a steadystate solar collector covered with a glass plate. A solar radiation simulator is used to validate their model. They prepared nanofluids by dispersing and oscillating TiO2, Al2O3, Ag, Cu, SiO2, graphite nanoparticles, and carbon nanotubes into Texatherm oil. Their results show that the use of nanofluid in solar collector can improve the outlet temperature and efficiency. They also found that the efficiency of most nanofluids are similar and larger than that of oil, except for TiO2.

Rahman et al. [9] performed a numerical study for a triangular shape solar collector with nanofluids by Galerkin weighted residual finite element method for a wide range of Grashof numbers (Gr). The corrugated bottom is kept at a constant high temperature and the side walls of the triangular enclosure are kept at a low temperature as seen in Figure 1. It is assumed that both fluid phase and nanoparticles are in thermal equilibrium and there is no slip between them. Nanofluid is Newtonian and incompressible, and flow is laminar and unsteady. Constant thermophysical properties are considered for the nanofluid except for the density variation in the buoyancy forces determined by using the Boussinesq approximation. Nevertheless, they have not mentioned the particle’s diameters. The authors concluded that high

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value of both Gr and solid volume fraction confirms better heat transfer through convection and conduction. Results showed 24.28% improvement for Gr = 106 at 10% volume fraction of copper particles. For lower values of Gr number, conduction is the primary mode of heat transfer for any value of solid volume fractions. The results showed that the convective heat transfer performance is better when the solid volume fraction is kept at 0.05 or 0.08. This study also showed that cu-water nanofluid is the best nanofluid for the augmentation of heat transfer.

Figure 1: (a) Schematic of the triangular shape collector (b) 3D view of a solar thermal collector filled with nanofluid [9].

Faizal et al. [10] investigated the thermal performance of nanofluid solar collector and its contribution size reduction to estimate the cost saving. Their findings indicated that efficiency of solar collector with nanofluids is calculated by the function of working fluid density, specific heat and mass flow rates. The results confirmed that higher density and lower specific heat of nanofluids offers higher thermal efficiency than water and can reduce the solar collector area about 25.6%, 21.6%, 22.1% and 21.5% for CuO, SiO2, TiO2 and Al2O3 nanofluids as seen in Figure 2. Hence, it will reduce the weight, energy and cost to manufacture the collector. The average value of

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220 MJ embodied energy can be saved for each collector, 2.4 years payback period can be achieved and around 170 kg less CO2 emissions will be the result of using nanofluid based solar collector compared to a conventional one. Environmental damage cost is also lower with the nanofluid based solar collector.

Figure 2: Percentage of size reduction for solar collector by applying different nanofluids.

Parvin et al. [11] numerically investigated the effects of the nanoparticle volume fraction (ϕ = 0%, 1%, 3%, 5% and 7%) and the Reynolds number (Re = 200, 400, 600, 800 and 1000) on the temperature distribution, rate of entropy generation, and collector efficiency.

Figure 3: Collector efficiency (η), mean entropy generation (S) and Bejan number (Be) at various concentrations.

The working fluid was incompressible Cu-water nanofluid under a laminar regime. Their findings were as follows: a) Increasing the particles concentration raises the fluid viscosity and decreases the Reynolds number

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and consequently decreases heat transfer. b) It is important to find the optimum volume fraction of nanoparticle for each application. c) The collector efficiency can be enhanced nearly 2 times by using Ag-water and Cu-water nanofluids with concentration of 3% as seen in Figure 3 d) The entropy generation is enhanced up to ϕ = 3% as seen in Figure 3. After this level, adding more nanoparticles makes no changes in mean entropy generation. Ladjevardi et al. [12] numerically studied the effects of using nanofluid on the performance of a solar collector as seen in Figure 4 considering the different diameter and volume fractions of graphite nanoparticles. They observed that in the infrared domain, the water optical characteristics are dominant while in the UV and visible ranges extinction coefficients are dependent on nanoparticle volume fractions. The extinction coefficient is calculated from the absorption and scattering efficiencies in this research. Their numerical results showed that nanofluid collector thermal efficiency increases about 88% compared with the one in pure water collector with the inlet temperature of 313 K. It also can be increased to 227% with the inlet temperature of 333 K.

Figure 4: Schematic of volumetric solar collector.

Filho et al. [13] studied silver nanoparticles as direct sunlight absorbers for solar thermal applications. Their results showed that the maximum

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stored thermal energy increases by 52%, 93% and 144% for silver particle concentration of 1.62, 3.25 and 6.5 ppm respectively due to the good photothermal conversion properties of silver nanoparticles. They also observed that the influence of particle concentration on the specific absorption rate (SAR) is only discernable at the initial heating period. It was concluded that reduction in the SAR at higher particle loadings (65 and 650 ppm) might be the result of: (i) The formation of agglomerates and reduction in the intensity of the sunlight into the fluid due to the deposited particles on the surface, (ii) The difference in the absorption efficiency of each particle at different fluid depth, (iii) The heat leak through radiation may become strengthened as the particle concentration exceeds a certain value as seen in Figures 5,6 and 7.

Figure 5: Experimental system: (a) a schematic illustration and (b) a snapshot of the system under direct sunlight on top of a roof.

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Figure 6: Comparison of the ratio of stored thermal energy at different concentrations (where b and u refer to thermocouples located at the bottom and upper positions respectively).

Figure 7: Specific absorption rate of silver nanoparticles (where b and u refer to thermocouples located at the bottom and upper positions respectively).

Karami et al. [14] experimentally showed that aqueous suspension based alkaline functionalized carbon nanotubes (f-CNT), 10 nm in diameter and 5-10 μm in length, has good stability as an absorber fluid in low-temperature

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Direct Absorption Solar Collector (DASC). The reason is associated with the hydrophilic nature of carboxylate groups. f-CNT considerably reduces the transmittance and enhances the thermal conductivity as seen in Figure 8. They recommended the use of this kind of nanofluids to absorb the light directly. In this study, f-CNTs was dispersed into the water by an ultrasonic instrument with the volume fractions less than 150 ppm. Higher concentrations produced a black solution which light was not able to pass through it.

Figure 8: Thermal conductivity of f-CNT/water NFs in ambient temperature and 60°C.

Said et al. [15] found that nanofluids with single wall carbon nanotubes (SWCNTs) in a flat plate solar collector showed the minimum entropy generation compared to the nanofluids prepared by suspending Al2O3, TiO2 and SiO2 nanoparticles in the same base fluid as seen in Figure 9. They attributed the decrease of the entropy generation to the increase in heat flux on the absorber plate due to the nanoparticles addition. Ultrasonicator and high pressure homogenizer (capacity of up to 2000 bar) is used to disperse the nanoparticles into the water. It was observed the SWCNTs nanofluids could reduce the entropy generation by 4.34% and enhance the heat transfer coefficient by 15.33%. It also had a small penalty in the pumping power by 1.2%.

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Figure 9: Change of entropy generation with volume fraction.

Tang et al. [16] prepared the carbon nanotube/PEG/SiO2 composites with high thermal conductivity from multiwall carbon nanotubes (MWCNTs), poly (ethylene glycol) (PEG) and inorganic SiO2. These composites had higher thermal conductivity than traditional phase-change materials (PCMs) because of the high thermal conductivity of MWCNTs. Their results clearly showed that PEG/ SiO2/MWCNT composites can effectively improve the efficiency of solar energy applications. Saidur et al. [17] investigated the effects of different parameters on the efficiency of a low-temperature nanofluid-based direct absorption solar collector (DAC) with water and aluminum nanoparticles. One big advantage of using low-temperature systems is that solar collectors can be relatively simple and inexpensive. Additionally, there are a number of working fluids suitable to low-temperature operation. Commonly used base liquids are water, oil, and ethylene glycol. They accounted for the effects of absorption and scattering within the nanofluid to evaluate the intensity distribution within the nanofluid by the radiative transfer equation (RTE). In order to calculate the spectral extinction coefficient of the nanofluid that is sum of scattering coefficient and absorption coefficient, they investigated the optical properties of the based fluid and nanoparticles separately. Their results revealed that Aluminum/water nanofluid with 1% volume fraction improves the solar absorption considerably. They found that the effect of particle size on the optical properties of nanofluid is minimal, but in order to have Rayleigh scattering the size of nanoparticles should be less than 20 nm. They also found that the extinction coefficient is linearly proportionate to volume

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fraction. Sokhansefat et al. [18] numerically investigated the heat transfer enhancement for Al2O3/synthetic oil nanofluid with concentrations up to 5% in a parabolic trough collector tube at different operational temperatures as seen in Figure 10. Nanofluid enhanced convective heat transfer coefficient as seen in Figure 11.

Figure 10: Schematic diagram of the parabolic trough collector and absorber tube.

Figure 11: Mean convective heat transfer coefficient vs.particle concentration at the operational temperatures of 300,400 and 500 K.

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Nasrin et al. [19] performed a numerical study to investigate the influence of Prandtl number on the flow, temperature fields, convective and radiated heat transfer rates, mean bulk temperature of the fluids and average velocity field in a solar with water- Al2O3 nanofluid collector as seen in Figure 12. The results showed that with increasing Pr from 1.73 to 6.62, the convective heat transfer enhances about 26% and 18% for nanofluid and base fluid respectively whereas the radiation enhances by 8%.

Figure 12: Schematic diagram of the solar collector.

Role of Base Fluid Colangelo et al. [20] experimently showed that the thermal conductivity improvement of the nanofluids with diathermic oil is greater than that with water in high temperature applications such as solar collectores. They observed that the thermal conductivity reduced with increasing the size of nanoparticles. Hordy et al. [21] made four different nanofluids by dispersing plasma functionalized multi-walled carbon nanotubes (MWCNTs) in water, ethylene glycol, propylene glycol and Therminol VP-1 heat transfer fluids with the aid of an ultrasonic bath. They examined both the long-term and hightemperature stability of CNT nanofluids for use in direct solar absorption. In this work plasma treatment applied to modify the surface of the MWCNTs to improve their dispersion property within the base fluid. This study reported a quantitative demonstration of the high temperature and long-term stability of ethylene glycol and propylene glycol-based MWCNT nanofluids for solar thermal collectors. Said et al. [22] experimentlly investigated the thermal conductivity, viscosity and pressure drop of water, ethylene glycol (EG) and EG + H2O (60:40)-based Al2O3 (13 nm) nanofluids prepared by using ultrasonic

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dispersion method in the operating temperature range of 25°C to 80°C at low range concentrations of 0.05% to 0.1% for. They observed that deviation of experimental values from estimated values of thermal conductivity of Al2O3/ water nanofluids is considerably high but the experimental values of Al2O3/ EG nanofluids are nearly similar to those of the model calculation as seen in Figure 13. Their results showd that nanofluids pressure drop at a low concentration flowing in a solar collector is slightly higher than the base fluid.

Figure 13: Thermal conductivity of Al2O3/EG (a) and Al2O3/water (b) nanofluids at different volume fractions and at 25°C.

Liu et al. [23] experimentally investigated the feasibility of using the graphene (GE)-dispersed nanofluids based on the ionic liquid 1-hexyl-3methylimidazolium tetrafluoroborate ([HMIM] BF4) in high-temperature heat transfer systems (such as solar collectors). Ionic liquids (ILs) are a group of molten salts with a melting below 100°C as well as a wide liquid temperature range from room temperature to a maximum temperature of 459°C. ILs have excellent thermophysical properties such as good thermal and chemical stability, high density and heat capacity and negligible vapor pressure. In this work, authors showed how to improve the performance of ILs for solar thermal systems. They observed 15.2%-22.9% enhancement in thermal conductivity using 0.06% graphene in the temperature range from 25 to 200°C as seen in Figure 14. Their results showed that GE is a better nanoadditive for nanofluids than other carbon materials and metal nanoparticles.

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Figure 14: Thermal conductivity of [HMIM]BF4 and the GE-dispersed Ionanofluids as a function of temperature.

Figure 15: Variation of specific heat capacity with temperature for the pure and the different nanoparticle concentration of Hitec nanofluid.

The authors attributed this reduction to the self-lubrication characteristic of GE. In addition, the results obtained from the thermogravimetric analysis showed the high thermal stability of GE/BF4 nanofluids. Their measurements showed that this novel class of nanofluids is very suitable for high temperature applications such as solar collectors.

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Ho et al. [24] found the optimal concentration of alumina nanoparticles in doped molten Hitec (a nitrate salt) by maximizing its specific heat capacity. High-temperature molten salt typically has a high heat capacity and is effective as a working fluid for concentrating solar power (CSP) systems. Their findings are as follows: 1- The addition of less than 2% Al2O3 nanoparticles significantly increases the specific heat of Hitec metal at low temperatures as seen in Figure 15, 2- For the volume fractions less than or equal to 0.5%, adding Al2O3 nanoparticles has a negative effect on the specific heat in temperature of 335°C, 3- At all temperatures, a concentration of 0.063 wt.% provides the maximum enhancement of specific heat about 19.9%, 4- The scanning electron microscopic (SEM) images show that, even at a relatively low concentration, nanoparticles aggregate as clusters with the size of 0.2 to 0.6 μm in the grain boundaries of Hitec, 5- The findings of this study suggest that the concentration that yields favorable uniform dispersion and optimal pattern of particles or clusters may maximize the specific heat. The simplified model of the solidfluid interfacial area demonstrates that interfacial area is maximal at a concentration of 0.023 wt.%. As the nanoparticle concentration increases above 0.023 wt. %, the formed clusters become larger and the interfacial area density between the solid clusters and the base fluid decreases which may reduce the increase in specific heat capacity. According to the results obtained from this study, the maximum enhancement of the specific heat capacity occurs at concentration of 0.063 wt.% instead of 0.023 wt.%. Indeed, some agglomeration of nanoparticles forming submicrometer clusters may be the best for the enhancement of specific heat capacity. However, the total interfacial area at concentration of 0.063 wt. % was slightly less than its value at concentration of 0.023 wt. %.

Role of Surfactants Singh et al. [25] added Cu to commercial solar heat transfer fluids (Therminol 59 (TH59) and Therminol 66 (TH66)) by the combination of temperature and ultrasonic ripening processes. They stated that surfactant selection has an important role in preparing stable nanofluids. Choosing the right surfactant is mainly dependent on the properties of the base fluids and particles. For example, silicon oxide nanoparticles were successfully dispersed in TH66 using benzalkonium chloride (BAC, Acros Organics) as a surfactant but the use of BAC surfactant with Cu nanoparticles did not provide sufficient stability of suspension due to the lack of specific interaction between the nanoparticles and the surfactant molecules. The bi-

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layer arrangement of surfactant molecules should provide good adhesion to the nanoparticle surface and miscibility with the aromatic solvent. In this work, authors used a combination of oleic acid and BAC and a mixture of octadecyl thiol (ODT) and BAC surfactants to disperse Cu nanoparticles in TH66 and TH59, respectively. They observed that 3D Cu nanoparticle agglomerates do not break by conventional sonication with ultrasound gun without temperature ripening. They showed that a sonication time of about 4 h leads to the effective breakup of Cu agglomerates into individual grains at a 120°C. They also concluded that Cu/TH66 nanofluids appear to be more stable than the Cu/TH59 nanofluids because of the higher dynamic viscosity. Yousefi et al. [26,27] studied the effect of Al2O3 (15 nm) and MWCNT (10-30 nm) water nanofluid on the efficiency of a flat plate solar collector experimentally. The weight fractions of the nanoparticles were 0.2% and 0.4%, and the experiments were performed with and without Triton X-100 as surfactant. Their findings showed that the surfactant presence in the nanofluid extremely affects solar collector’s efficiency. Lenert et al. [28] presented a combined modeling and experimental study to optimize the performance of a cylindrical nano-fluid volumetric receiver. They concluded that the efficiency is more than 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This study provides an important perspective in the use of nanofluids as volumetric receivers in concentrated solar applications. In this work, 28 nm carboncoated cobalt (C-Co) nanoparticles dispersed and suspended in Therminol VP-1 after 30 min in a sonication bath without any surfactant.

Role of the pH Yousefi et al. [29] investigated the effect of pH of MWCNT-H2O nanofluid on the efficiency of a flat-plate solar collector as seen in Figure 16. The experiments were carried out using 0.2 wt% MWCNT (10-30 nm) with various pH values (3.5, 6.5 and 9.5) and with Triton X-100 as an additive. They found that increasing or decreasing the pH with respect to the pH of the isoelectric point (IEP) would enhance the positive effect of nanofluids on the efficiency of the solar collector. The collector efficiency enhanced while the differences between the pH of nanofluids and that of isoelectric increased. As the nanofluids become more acidic (lower pH value), more charges are accumulated on the particle surface, leading to lower agglomeration of nanoparticles in the suspension. Consequently, the effective thermal

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conductivity of the nanofluid increases. In addition, with the increase in pH of the nanofluid, the surface charge of the CNT increases leading to the increase in thermal conductivity and stability of nanofluid.

Figure 16: The efficiency of the flat-plate solar collector with MWCNT nanofluid as base fluid at three pH values as compared with water in0.0333 kg/s mass flow rate.

Using Nanofluids in Photovoltaic/Thermal (PV/T) System Sardarabadi et al. [30] performed experiments to study the effects of using SiO2/water nanofluid as a coolant on the thermal and electrical efficiencies of a photovoltaic thermal (PV/T) system. A flat plate solar collector was attached to a PV panel. The tilt angle of the collector was set at a constant value of 32° to maximize the solar collecting area. It was observed that by adding a thermal collector to a PV system, the total exergy for the three cases with pure water, 1% silica/water nanofluid and 3% silica/water nanofluid increased by 19.36%, 22.61% and 24.31%, respectively as seen in Figure 17. Thermal efficiency of the PV/T collector for the two cases of 1 and 3 wt% of silica/water nanofluid increased 7.6% and 12.8%, respectively.

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Figure 17: Exergetic efficiency of the system for the three cases with pure water (a), 1% silica/water nanofluid (b) and 3% silica/water nanofluid (c) during the daily experiment.

Karami et al. [31] experimentlly investigated the cooling performance of water based Boehmite (AlOOH. xH2O) nanofluid in a hybrid photovoltaic (PV) cell. The PV cell is mono-crystalline silicon. Results showed that the nanofluid performed better than water and the average PV surface temperature decreased from 62.29°C to 32.5°C as seen in Figure 18. They reported that the electrical efficiency falls as the concentration of the nanofluid rises beyond a certain level. The authors attributed this reduction to the high surface activity of nanoparticles and their tendency to agglomeration/clustering at high particle loadings. Table 1 summarizes the results of nanofluids influence on different solar thermal applications

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Figure 18: Variation of the average temperatures of the PV surface at various flow rates for water and three different concentrations of nanofluid. Table 1: The influence of nanofluid on different solar thermal applications Author(s)

Nanofluid

Type of application

Observation

Luo et al. [8]

TiO2, Al2O3, Ag, Cu, SiO2, graphite, and carbon nanotubes in Texatherm oil

DAC solar collector

use of nanofluid in the solar collector can improve the outlet temperature and the efficiency

Rahman et al. [9]

Cu, Al2O3 and TiO2 triangular shape in water solar collector

Results showed 24.28% improvement for Gr=106 at 10% volume fraction of copper particles. the convective heat transfer performance is better when the solid volume fraction is kept at 0.05 or 0.08.

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80 Faizal et al. [10]

CuO, SiO2, TiO2 and Al2O3 in water

solar collector

results confirmed that higher density and lower specific heat of nanofluids offers higher thermal efficiency than water and therefore can reduce the solar collector area about 25.6%, 21.6%, 22.1% and 21.5% for CuO, SiO2, TiO2 and Al2O3 nanofluids. Environmental damage cost is also lower with the nanofluid based solar collector

Parvin et al. [11]

Cu/water

solar collector

Increasing the particles concentration raises the fluid viscosity and decreases the Reynolds number and consequently decreases heat transfer. There is a need to find the optimum volume fraction for each application

Ladjevardi et al. [12]

Graphite/water

solar collector

Their numerical results showed that nanofluid collector thermal efficiency increases about 88% compared with the pure water collector with the inlet temperature of 313 K. It also can be increased to 227% with the inlet temperature of 333 K.

Said et al. [15]

single wall carbon nanotubes, Al2O3, TiO2 and SiO2

flat plate solar collector

It was observed that SWCNTs nanofluids could reduce the entropy generation by 4.34% and enhance the heat transfer coefficient by 15.33%

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Saidur et al. [17]

Aluminum/water

direct absorption solar collector

Their results revealed that Aluminum/water nanofluid with 1% volume fraction improves the solar absorption considerably. They found that the effect of particle size on the optical properties of nanofluid is minimal, but in order to have Rayleigh scattering the size of nanoparticles should be less than 20 nm. They also found that the extinction coefficient is linearly proportionate to volume fraction

Sokhansefat et al. [18]

Al2O3/synthetic oil

parabolic trough collector tube

Nanofluid enhanced convective heat transfer coefficient.

Hordy et al. [21]

multi-walled carbon nanotubes/water ethylene glycol, propylene glycol

solar collector

quantitative demonstration of the high temperature and long-term stability of ethylene glycol and propylene glycol-based MWCNT nanofluids for solar thermal collectors

Said et al. [22]

Al2O3, water, ethylene glycol

Solar collector

Their results showed that nanofluids pressure drop at a low concentration flowing in a solar collector is slightly higher than the base fluid.

Liu et al. [23]

Grapheme/ ionic solar collectors liquid 1-hexyl-3-methylimidazolium tetrafluoroborate

They observed 15.2%22.9% enhancement in thermal conductivity using 0.06% volume graphene in the temperature range from 25 to 200°C. Their results showed that GE is a better nanoadditive for nanofluids than other carbon materials and metal nanoparticles

Ho et al. [24]

Alumina/ doped molten Hitec

The addition of less than 2% Al2O3nanoparticles significantly increases the specific heat of Hitec metal at low temperatures

concentrating solar power systems

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82 Singh et al. [25]

Cu/Therminol 59 (TH59) and Therminol 66 (TH66)

They stated that surfactant selection has an important role in preparing stable nanofluids. Choosing the right surfactant is mainly dependent on the properties of the base fluids and particles

Yousefi et al. [29]

MWCNT/water

flat plate solar collector

They found that increasing or decreasing the pH with respect to the pH of the isoelectric point (IEP) would enhance the positive effect of nanofluids on the efficiency of the solar collector

Sardarabadi et al. [30]

SiO2/water

PV/T

Thermal efficiency of the PV/T collector for the two cases of 1 and 3 wt% of silica/water nanofluid increased 7.6% and 12.8%, respectively.

Kabeel et al. [32]

Cu/water

water desalination unit

the water cost can be decreased from 16.43 to 11.68 $/m3 at ϕ=5%

Kabeel et al. [33]

Al2O3/water

solar still

using nanofluids improves the solar still water productivity by about 116% and 76% with and without operating the vacuum fan

Al-Nimr et al. [34]

silver-water

shallow solar pond energy stored in the nanofluid pond is about 216% more than the energy stored in the brine pond

Liu et al. [35]

CuO/water

maximum and mean values of the collecting efficiency of the collector with open thermosyphon using nanofluids increased 6.6% and 12.4%, respectively.

Using Nanofluids in Solar Stills Kabeel et al. [32] investigated a small unit for water desalination coupled with nano-fluid-based (Cu/water) solar collector as a heat source as seen in Figure 19. The system consists of a solar water heater (flat plate solar collector), a mixing tank and a flashing chamber plus a helical heat exchanger

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and a condenser. The desalination process is based on the evaporation of sea water under a very low pressure (vacuum). The evaporated water is then condensed to obtain fresh water. The simulation results showed that the nanoparticle concentration is an important factor on increasing the fresh water production and decreasing cost. Authors reported that the water cost can be decreased from 16.43 to 11.68 $/m3 at ϕ = 5% as seen in Figure 20.

Figure 19: Schematic diagram of single stage flash (SSF) system.

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Figure 20: Variations in system productivity and water cost as a function of nano-particle volume fraction.

Kabeel et al. [33] used Al2O3 nanoparticles with water inside a single basin solar still. Their results showed that using nanofluids improves the solar still water productivity by about 116% and 76% with and without operating the vacuum fan. The authors attributed this increment to the increase of evaporation rate inside the still. Utilizing nanofluid increases the rate of evaporation. In addition, due to this vacuum inside the still the evaporation rate increases further and the productivity increases compared with the still working at atmospheric conditions.

Using Nanofluids in Solar Pond Al-Nimr et al. [34] presented a mathematical model to describe the effects of using silver-water nanofluid on the thermal performance of a shallow solar pond (SSP) and showed that the energy stored in the nanofluid pond is about 216% more than the energy stored in the brine pond. The upper layer of the pond is made of mineral oil and the lower layer is made of silver (Ag) water-based nanofluid. Their results showed that for solar radiation of 1000 W/m2, the nanofluid pond required a depth less than 25 cm in order to absorb the light, while the brine pond depth must be more than 25 m to absorb the same amount of light. They attributed the increase of stored

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energy to the increase in thermal conductivity of the base fluid due to the nanoparticles addition that leads to uniform temperature distribution within the layer with reduction in heat losses.

Using Nanofluids in the Solar Collector Integrated with Open Thermosyphon Liu et al. [35] experimentally showed that the solar collector integrated with open thermosyphon has a much better collecting performance compared to the collector with concentric tube and its efficiency could be improved by using CuO/water nanofluid as the working fluid as well. Their results showed that the maximum and mean values of the collecting efficiency of the collector with open thermosyphon using nanofluids increased 6.6% and 12.4%, respectively.

CONCLUSIONS Nanofluids have been utilized to improve the efficiency of several solar thermal applications. Theoretical and experimental studies on solar systems proved that the system performance enhances noticeably by using nanofluids. A number of investigations presented the existence of an optimum concentration for nanoparticles in the base fluid. Adding nanoparticles beyond the optimum level no longer enhances the efficiency of the solar system. Optimal conditions are a function of nanoparticles size and concentration, base fluid, surfactant and pH as discussed throughout this article. Nanofluid utilization in the solar thermal systems is accompanied by important challenges including high cost of production, instability, agglomeration and erosion. This review article is an attempt to elucidate the advantages and disadvantages of nanofluids application in the solar system.

ACKNOWLEDGEMENTS The authors would like to express their appreciation to the Islamic Azad University of Abadan Branch for providing financial support.

AUTHORS’ CONTRIBUTIONS MS conducted the extensive literature review and NB wrote the article. Both authors read and approved the final manuscript.

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Solar to Fuels Conversion Technologies: A Perspective

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Harry L. Tuller1,2 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology and Materials Processing Center, Cambridge, MA 02139, USA

International Institute of Carbon Neutral Energy Research, Kyushu University, Fukuoka, Japan 2

ABSTRACT To meet increasing energy needs, while limiting greenhouse gas emissions over the coming decades, power capacity on a large scale will need to be provided from renewable sources, with solar expected to play a central role. While the focus to date has been on electricity generation via photovoltaic

Citation: Tuller, H. L., “Solar to fuels conversion technologies: a perspective”, Materials for Renewable and Sustainable Energy (2017), https://doi.org/10.1007/s40243-017-0088-2. Copyright: © The Author(s) 2017, Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons. org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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(PV) cells, electricity production currently accounts for only about onethird of total primary energy consumption. As a consequence, solar-to-fuel conversion will need to play an increasingly important role and, thereby, satisfy the need to replace high energy density fossil fuels with cleaner alternatives that remain easy to transport and store. The solar refinery concept (Herron et al. in Energy Environ Sci 8:126–157, 2015), in which captured solar radiation provides energy in the form of heat, electricity or photons, used to convert the basic chemical feedstocks CO2 and H2O into fuels, is reviewed as are the key conversion processes based on (1) combined PV and electrolysis, (2) photoelectrochemically driven electrolysis and (3) thermochemical processes, all focused on initially converting H2O and CO2 to H2 and CO. Recent advances, as well as remaining challenges, associated with solar-to-fuel conversion are discussed, as is the need for an intensive research and development effort to bring such processes to scale.

Keywords: Solar fuels, CO2, sequestration, Hydrogen economy, Photoelectrochemistry, Electrolysis, Fuel cell, Thermochemical processes, Solar energy

INTRODUCTION Fossil fuels are broadly used for transportation, electricity generation, industrial processes, and heating. Given their ready availability, high energy density,1 and ease of handling, storage, and transport, they supply more than 80% of the world’s overall energy needs, and 96% of the transportation sector’s energy demand, with much of the remaining 4% of transportation energy being electricity generated in plants that burn fossil fuels. At the same time, the combustion of fossil fuels to extract their stored chemical energy is a major source of greenhouse gas emissions, mostly carbon dioxide (CO2), and thus contributes to global warming. While interest in and utilization of solar energy as a key alternative clean energy source have grown rapidly in recent years, solar technology deployment has been largely directed to electricity generation. While important, recent advances in solar electricity generation do not address the continued need for high energy density fuels for transportation, heating, and industrial process uses, which together account for roughly 70% of overall energy requirements. This report discusses options for converting solar energy into fuels, largely through the solar-driven conversion of water and carbon dioxide into fuels and chemicals. This conversion would be achieved in a solar refinery [1],

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where solar energy acts on CO2 captured from flue gas emissions, together with water, to generate solar fuels. These fuels, which can be sustainably produced in liquid or gaseous form, offer multiple benefits in terms of grid stability, energy security, compatibility with existing infrastructure, and climate change mitigation. The opportunities and challenges associated with sourcing, producing, storing, and distributing solar fuels are the focus of this report.2

SOLAR ALTERNATIVE FUELS Increasingly, electricity, including the widespread electrification of transportation, is being seen as playing a pivotal role in achieving deep cuts in greenhouse gas emissions, such as the reductions—at 80% below 1990 levels—proposed for California by 2050 [2]. Even more aggressive goals recently articulated by the White House aim to derive 80% of electricity from clean energy sources by 2035 and reduce greenhouse gas emissions 17% by 2020 and 83% by 2050 (relative to a 2005 baseline) [3]. Given that electricity accounts for 30% of global energy consumption, and without an unexpected breakthrough in electricity storage,3 alternative, lowcarbon fuels will be needed to satisfy the remaining 70% of global energy requirements, particularly for transportation, manufacturing, and heating [4]. To get a sense of the magnitude of this challenge, one need only note that the U.S. registered light-duty vehicle (LDV) fleet of over 234 million vehicles consumes 8.4 million barrels of oil to travel 7.3 billion miles on a daily basis. This represents nearly 10% of total petroleum liquids consumption worldwide [5, 6, 7, 8]. Solar energy, among all carbon-free energy sources, is viewed by some experts, as the alternative with the greatest intermediate to long-term potential to replace fossil fuels [9]. For this to happen, however, two important challenges must be addressed. The first is providing adequate energy storage capabilities for solar-generated electricity, given the intermittent character of the solar resource. The second, perhaps more important challenge, is utilizing solar energy to aid in the production of clean alternative fuels for the transportation, industrial, and housing sectors [10]. Solar energy has, until now, accounted for a relatively small fraction of the overall energy supply, with its fluctuating contributions to the grid controlled and compensated by thermal generation (fossil-fuel combustion). As solar and wind penetration increases, however, the intermittency of these two energy sources seriously compromises the stability and quality

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of grid power. This issue has already begun to demand urgent attention in Germany where 36.8 terawatt-hours (TWh) of electricity (equal to 6.6% of total production)4 was generated by solar sources in 2015 [11]. During this same period, 87.1 TWh (equal to 15.6% of Germany’s total electricity production) was generated by nuclear plants, which are slated for shut down by 2021 [12]. While dependence on intermittent renewable energy sources is not yet quite so high in the United States, where solar and wind accounted for 0.6 and 4.7% of overall electricity production in 2015, respectively [13], the relentless decline in PV module prices has continued at a rate of 5–7% annually for the past decade [14]. The U.S. Energy Information Administration now expects utility-scale solar generating capacity to increase by more than 60% between the end of 2014 and the end of 2016, while wind capacity is expected to increase by about 23% over the same time period [15]. While these projections may be optimistic, and may assume the continued existence of various government subsidies, there is little doubt that generation from these intermittent energy sources will continue to show significant growth. This creates a strong incentive to bring into play, as quickly as possible, alternative storage systems that are both robust and carbon–neutral to ensure grid stability in the coming decades. A figure-of-merit for the storage of electrical energy generated by intermittent sources, defined as the ratio of the value of stored electricity to the cost of storage, is useful in comparing alternate storage technologies. For example, assuming a 1-day storage period, the figure-of-merit for electrical energy stored chemically via hydrogen produced by electrolysis is 12.7. While this is much higher—taking into account the cost, life, and efficiency of the process—than electrical battery storage, which has a figure-of-merit of 1.0 [16], these figures do not account for the efficiency of producing hydrogen or converting hydrogen back to electricity. If one combines the efficiency of electrolysis cells, at approximately 75%, with that of a combined cycle gas and steam turbine generator running on hydrogen (about 60%), the result is a full-cycle electricity–fuel–electricity efficiency of up to 45% [17]. More typical round trip efficiencies are reportedly closer to 30%, making the attraction of hydrogen vs battery storage less clear in the short term [18]. Hydrocarbon fuels with higher energy densities can also be synthesized by combining hydrogen with CO2 captured, for example, from coal-burning plants. Longer term, as fossil-fuel generating plants are replaced by renewable sources, CO2 could be captured from non-combustion sources such as cement plants. Siemens reports that synthetic natural gas (i.e., methane [CH4]) can be generated, on a pilot scale, from hydrogen and

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CO2 with up to 80% efficiency [17]. Synthetic natural gas has three times the energy density, on a volume basis, of hydrogen. Given the central role that chemical fuels already play in electricity generation, the conversion of solar energy into chemical fuels that are capable of being used in the existing distribution and end-use infrastructure would be highly desirable [9]. Synthetic gas, stored and distributed like conventional natural gas, could then be used to power vehicles or in heating systems—in addition to being used to generate electricity—on an as-needed basis. In Germany, for example, existing natural gas storage capacity—at more than 200 TWh—would be sufficient to satisfy consumption for several months [19]. Moreover, synthetic hydrocarbons can be used in a variety of additional ways, including in the production of fertilizer, plastics, and pharmaceuticals, as well as for transportation and heating [20]. Like most other countries, the United States is nearly completely dependent on petroleum for transportation; in fact, petroleum use for transportation accounts for about one-third of total annual U.S. CO2 emissions [21]. Worldwide, the transportation sector accounted for 19% of global energy demand in 2012 and oil supplied 96% of this demand, with the rest coming from natural gas, biofuels, and electricity [22]. Government regulations mandating improved vehicle fuel efficiency and the increasing electrification of transportation via the introduction of hybrid and plug-in vehicles will help reduce dependence on fossil fuels. However, many forms of transportation, including long-haul passenger vehicles, ships, trucks, and aircraft, will continue to require high energy density, but ideally carbon-free or carbon-neutral fuels.

Basic Solar Fuels Solar fuels are not new. The photo-assisted synthesis (photosynthesis) of chemical fuels, in the form of plant matter, is fundamental to life on Earth and supports all current biomass. The same process, over geological time, produced the fossil fuels on which human civilization has depended for the vast majority of its energy needs for the past century and earlier. Due to the relative inefficiency of natural photosynthesis, the use of all cultivatable land on Earth to produce biofuels would not satisfy humanity’s projected energy needs in the coming decades, particularly if one takes into account the energy needed to harvest, store, distribute, and convert biomass into useful chemical fuels [23]. An alternative approach that obviates the need to set aside vast tracts of arable land is to replicate the essential elements of photosynthesis found in natural organisms with artificial systems. On an industrial scale,

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one can visualize a solar refinery (see Fig. 1) that converts readily available sources of carbon and hydrogen, in the form of CO2 and water (H2O), to useful fuels, such as methanol (CH3OH), using energy sourced from a solar utility [1]. The solar utility, optimized to collect and concentrate solar energy and/or convert solar energy to electricity or heat, can be used to drive the electro-catalytic, photoelectrochemical, or thermochemical reactions needed for conversion processes. For example, electricity provided by PV cells can be used to generate hydrogen electrochemically from water via an electrolysis (electrocatalytic) cell.

Figure 1: Schematic of a Solar Refinery and solar fuel feedstocks (CO2, H2O, and solar energy) captured onsite or transported to the refinery. The Solar Utility provides energy in the form of heat, electricity or photons used to convert the CO2 and H2O into fuels either by direct CO2 reduction or solar activation of CO2/H2O to CO/H2 and subsequent catalytic conversion to fuels (e.g., via methanol synthesis or by the Fischer–Tropsch method. Color code: yellow— ambient; red—elevated temperatures.

(from Herron et al. [1]) Hydrogen, the most elemental fuel, has many attractive attributes—it is clean burning (water being the only by product of hydrogen combustion) and can be efficiently converted back to electricity via fuel cells. However, hydrogen lacks volumetric energy density, and cannot be easily stored and distributed like hydrocarbon fuels. Rather than utilizing solar-generated hydrogen directly and primarily as a fuel, its utility is much greater—at

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least in the short to intermediate term5—as an onsite fuel for converting CO2 to CH4 or for generating syngas, heat, or electricity [24]. Reacting CO2 with hydrogen (H2) not only provides an effective means for storing CO2 (in methane, for example), but also produces a fuel that is much easier to store, distribute, and utilize within the existing energy supply infrastructure. Thus, recycling CO2 to produce a hydrocarbon fuel would open the transportation sector to far greater reliance on renewable energy beyond what is currently feasible with rechargeable electric vehicles (at present, such vehicles comprise fewer than 3% of all vehicles sold in the United States) [8]. The idea of converting CO2, a product of combustion, to useful hydrocarbon fuels by harnessing solar energy is attractive in concept. However, significant reductions in CO2 capture costs and significant improvements in the efficiency with which solar energy is used to drive chemical conversions must be achieved to make the solar refinery a reality. We address these issues in greater detail below. Solar energy collected and concentrated within a solar utility (see Fig. 1) can be harnessed in different ways: (1) PV systems could convert sunlight into electricity which, in turn, could be used to drive electrochemical (electrolysis) cells that decompose inert chemical species such as H2O or CO2 into useful fuels; (2) photoelectrochemical or photocatalytic systems could be designed wherein electrochemical decomposition reactions (like the reactions in the previous example) are driven directly by light, without the need to separately generate electricity; and (3) photothermal systems could be used either to heat working fluids or help drive desired chemical reactions such as those connected with thermolysis, thermochemical cycles, etc. (see Fig. 1). Each of these approaches can, in principle, be used to generate environmentally friendly solar fuels that offer “efficient production, sufficient energy density, and flexible conversion into heat, electrical, or mechanical energy [25].” The energy stored in the chemical bonds of a solar fuel could be released via reaction with an oxidizer, typically air, either electrochemically (e.g., in fuel cells) or by combustion, as is usually the case with fossil fuels. Of the three approaches listed here, only the first (PV and electrolysis cells) can rely on infrastructure that is already installed today at a scale that would have the potential to significantly affect current energy needs. The photoelectrochemical and photothermal approaches, though they hold promise for achieving simplified assembly and/or high energy conversion efficiencies, require considerable development before moving from the laboratory into pilot scale and commercially viable assemblies. Remaining sections of this report discuss the status of these

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three approaches and the challenges that must be overcome to advance each of them. Given that, the contribution of artificially produced solar fuels, such as hydrogen and methane, remains extremely small at present, exceptional efforts—particularly to reduce costs—are needed to bring these clean and sustainable fuels up to meaningful levels. Critical challenges that will need to be overcome include improving the sourcing and collection of CO2, increasing the efficiency of solar-assisted catalytic conversion of CO2 and H2O into fuels, extending device lifetimes, reducing costs, and investing in infrastructure upgrades to reduce the large gap between current laboratory demonstrations and deployable technology. These challenges are discussed here in terms of key candidate solar fuels. Chapters 2 and 3 of the MIT Future of Solar Energy study [26] present a detailed analysis of options for generating electricity from sunlight via PV cells. A few highlights from that analysis are worth repeating here by way of providing context for this report. According to the Solar Electric Power Association, installed solar power generating capability in the United States totaled 10.7 GW as of 2013. PV accounted for most of this capability, roughly half of which (48%) was provided by utility-scale installations. Another 25 GW of solar generation capacity is projected to be installed by 2017 [27]. Solar thermal power, also referred to as concentrated solar power (CSP), represents a growing but much smaller component of installed solar power generating capacity. As of 2013, 926 MW of CSP capacity had been installed in the United States, but an additional 800 MW was anticipated in 2014 with a total of 3.2 GW of capacity projected by 2017 [27]. While generally more costly than PV, CSP enables lower cost thermal (rather than electrical) energy storage, which is key to overcoming issues related to solar energy’s intermittency. It is worth noting that of the approximately 1.06 TW of electrical power capacity in the United States in 2012 [28], 10.7 GW of solar power would represent only 1% of the total.

Hydrogen Production Hydrogen has been recognized for some time as providing a potential foundation for a clean, flexible, and secure energy future. The fact that it is accessible in the form of water makes hydrogen highly attractive. When hydrogen is used as a fuel, either by combustion or electrochemically in a fuel cell, the only byproduct is water—a feature that promises an emissionfree environment. While the combustion of hydrogen produces more energy on a mass basis (39.5 kWh/kg)6 than the combustion of any other fuel— e.g., 2.4 and 2.8 times the energy of methane and gasoline combustion,

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respectively [29]—hydrogen has low energy density by volume. In fact, at 2.8 kWh per liter, the energy density of hydrogen is 3.5 times lower than that of gasoline. Since liquefying hydrogen is highly energy intensive, and thus not practical, hydrogen is most effectively stored as a gas in high-pressure tanks. Given its simple chemical structure, it is one of a very small group of fuels capable of being used in low-temperature fuel cells, thereby making it the fuel of choice for fuel cell-powered vehicles [30]. Since hydrogen in its molecular form does not occur in nature, it is not an energy source and must be produced. In this sense, hydrogen is rather more like electricity, a convenient energy carrier, and, as will become evident, a strong synergy exists between electricity, hydrogen, and other renewable energy sources [31]. Hydrogen production today is actually a large net generator of CO2 emissions, with 13.7 kilograms (kg) of CO2 produced for every kg of H2, on average [32]. At present, approximately 96% of hydrogen is derived from fossil fuels and only 4% is produced via electrolysis [33]. Hydrogen is produced in high volumes (current global annual production exceeds 70 million metric tons, while annual U.S. production is projected to total 11 million metric tons in 2016) [34], largely via steam reforming of natural gas (methane)7for use in fertilizers and for use in the hydrocracking of heavy petroleum and the manufacture of methanol and hydrochloric acid. The value of hydrogen production worldwide is expected to reach $163 billion by 2015 [34, 35]. Hydrogen produced via water electrolysis is generally more expensive than by large-scale fuel processing techniques, although it becomes more attractive when produced onsite. However, if fossil fuels are used to generate the electricity that drives the electrolysis process, resulting emissions are actually higher than for natural gas reforming [36]. This points to the need and opportunity to harness renewable sources of energy, particularly intermittent sources such as solar and wind, for hydrogen production. The next sections review the two main options being considered for generating hydrogen using solar energy.

Water Electrolysis Water (H2O) can be decomposed into its elemental components hydrogen (H2) and oxygen (O2) by passing current between two electrodes immersed in an electrolysis cell. Oxygen is evolved at the positive electrode (anode), hydrogen is evolved at the negative electrode (cathode), and the two are separated from each other by an ionic conducting liquid or solid electrolyte that selectively transports H+, OH− or O2− ions across the cell, depending on the nature of the electrolysis cell. A cell voltage of at least 1.23 V is

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required. In practice, however, voltages closer to 1.9 V are needed to achieve reasonable current densities, and corresponding fluxes of generated hydrogen and oxygen gases. The need for higher voltage to overcome ohmic (resistive) losses and electrode over-potentials in turn reduces electricalto-chemical energy conversion efficiencies. There is thus great interest in identifying and optimizing catalysts that can accelerate the oxygen oxidation reaction at the anode and the hydrogen reduction reaction at the cathode, thereby reducing electrode over-potentials. Two approaches to harnessing solar energy to drive the electrolysis reaction are possible and are being pursued. The most obvious approach is to drive a conventional water electrolyzer using the electrical output of PV devices. Given that typical conversion efficiencies are 11.5–17.5% for commercial PV systems and 63–73% for electrolyzers, overall conversion efficiencies of approximately 12% can be expected and have been reported for optimized, combined PV–electrolyzer systems [1, 37]. The most obvious advantage of this approach is that both PV and electrolysis systems are commercially available, although large-scale electrolysis systems are not nearly as extensively available as PV systems. An alternative approach, still in the experimental stage, is the use of photoelectrolytic systems that combine the functions of light collection, charge separation, and electrolysis in a single cell. This is achieved by replacing one or both of the metallic electrodes in a conventional electrolysis cell with a semiconductor. The advantage of this approach is that it offers opportunities to minimize cost by eliminating redundant support structures and energy losses associated with cell interconnections. At the same time, it has been difficult to simultaneously achieve high conversion efficiencies and long-term operating stability, given that the semiconductors offering the highest efficiencies are susceptible to corrosion during cell operation. Several recent advances address these limitations. These include: (1) combining the PV and electrolysis cells into a single integrated tandem photoelectrochemical (PEC) cell, with theoretical solar-to-hydrogen conversion efficiencies of 31.1% at one Sun illumination [38], (2) protecting the semiconductors in PEC cells from corrosion [39], (3) introducing lowcost Earth-abundant catalysts [40], and (4) improving active area and optical absorptivity through the use of nanostructuring or nanowires [41]. These options are described in the next sections.

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Combined PV-electrolysis Systems The MIT Future of Solar Energy study [26] methodically compares the principal PV materials [e.g., crystalline and amorphous silicon (Si), cadmium telluride (CdTe), gallium arsenide (GaAs) and copper indium gallium diselenide (CIGS)] and device designs in terms of their relative costs, solar to electricity conversion efficiencies, long-term stability, and environmental impact. Overall, it is fair to say that a number of PV systems are now commercially available, with the option to trade off lifetime costs and efficiency depending on the particular application being considered. That being the case, it is more useful here to review options for electrolyzers that are generally less advanced in their development and commercialization. Water electrolysis is a relatively mature technology, with hydrogen production capacities ranging from a few cubic centimeters per minute (cm3/min) to thousands of cubic meters per hour (m3/h). Key performance parameters for electrolyzer systems are conversion efficiency [electrical to chemical energy (H2)]; current density (amps/unit area), which in turn determines the hydrogen flux density, durability, scalability, and cost; and, for some designs, reliance on noble metals such as platinum. The three major types of electrolyzers are based on aqueous alkaline (OH−), solid polymer (H+), and solid oxide (O2−) ionic conducting electrolytes. The most commercially developed option is the alkaline cell, which uses a 30% potassium hydroxide electrolyte solution that operates at 80–90 °C and pressures to 25–30 bar. Cathodes and anodes are porous nickel (Ni) coated, respectively, with platinum at the cathode and with metal oxides at the anode. Such cells exhibit good durability (10–20 years) and have efficiencies of 63–73%, but suffer from relatively low current densities, which means that larger systems are required to produce equivalent volumes of hydrogen [1, 31]. The proton exchange membrane (PEM) electrolyzer, which uses a polymer-based Nafion proton conducting membrane and has a working temperature of about 90 °C, can operate at considerably higher current densities than the alkaline cell and, therefore, can be more compact, but its conversion efficiency is lower—on the order of 56%. The PEM cell also suffers from reliance on precious metal electrocatalysts (typically platinum dispersed on carbon), a costly membrane (Nafion), and potential degradation in performance due to catalyst coarsening that reduces the active electrode area over time. The solid oxide electrolyzer cell utilizes a ceramic oxygen ion conducting electrolyte [typically yttria stabilized zirconia (YSZ) or Y0.1Zr0.9O2], and operates at much higher temperatures (500–850 °C) and at pressures of 30 bar.

700

2,200

25

25

500–350

104 cm−1 in the visible region indicating a direct bandgap characteristic of CZTS films.

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Figure 6: a UV–Visible NIR spectra recorded showing absorbance versus wavelength plot for CZTS thin films. b Plot of (αhυ)2 versus photon energy (hυ) to determine optical bandgap for as-deposited and calcinated CZTS films.

In the direct transition semiconductor, the optical energy bandgap (E opt) and the optical absorption coefficient (α) are related by [37], (4) where α is the absorption coefficient, B is the optical density of state and E is the photon energy. Therefore, optical bandgap can obtained by extrapolating the tangential line to the photon energy (E = hυ) axis in the plot of (αhυ)2 versus photon energy (hυ). Figure 6(b) shows plot of (αhυ)2 versus photon energy (hυ) (Tauc plot) of as-deposited and calcinated CZTS films. As seen from the figure with increase in calcination temperature optical bandgap of CZTS films decreases from 1.91 to 1.59 eV. The obtained bandgap values are consistent with the bulk value of CZTS (1.45–1.90 eV) [38]. The main factor that affect the band gap of CZTS films are the average grain size [39] and presence of multiple phases of CZTS in the films [40]. As revealed from our low angle-XRD (Fig. 1) and Raman spectroscopy (Fig. 2) analysis the existence of multiple phases CZTS in the films are ruled out. Therefore, decrease in optical bandgap of CZTS films can attribute to increase in average grain size. Graphical presentation of dependence bandgap on average grain size is shown in Fig. 7. The optical bandgap of CZTS film calcinated at 400 °C is ~1.59 eV which is quite close to the optimum value bandgap for photovoltaic solar conversion in the visible region of solar spectrum.

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Figure 7: Graphical presentation of dependence bandgap on average grain size of as-deposited and calcinated CZTS films.

Photo Response Measurement Figure 8 shows the current versus time (I–t) plot as-deposited and calcinated CZTS films at constant 0.2 V bias voltage under dark and illumination conditions. For electrical properties measurement we have used samples of area 0.5 cm2.

Figure 8: Current versus time (I–t) plot of as-deposited and calcinated CZTS films at constant 0.2 V bias voltage for 60 s illuminations and for 60 s dark condition.

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As seen there is significant improvement in the current with increase in calcinations temperature. The improvement in electrical properties may attribute to improvement in crystalline nature, texture, grain size of CZTS films with increase in calcinations temperature. Such larger grains CZTS thin films can be useful as an absorber layer for the improvement in photoelectric conversion efficiency because larger grain sized which can reduce the recombination rate of photo-generated charge carriers [41].

CONCLUSION In summary, nanocrystalline CZTS films have been prepared by home-made RF magnetron sputtering technique. Influence of calcination temperature in Ar atmosphere on structural, morphological, electrical and optical properties on CZTS films has been investigated. Formation of CZTS has been confirmed by x-ray photoelectron spectroscopy (XPS) whereas formation of KesteriteCZTS films has been confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. We found that the calcination process has a great influence on growth and nucleation of grains. XRD analysis revealed that the crystallinity and average grain size increases with increase in calcination temperature. Raman spectroscopy analysis show shifting of Raman peak shift towards lower wavenumber with increase in calcination temperature. The presence of internal compressive stress and shrinking of substrate during cooling may responsible for shifting of Raman peak towards lower wavenumber. However, shrinking of substrate while cooling has not been verified experimentally. Detail surface study (morphology and topology) reveal that CZTS thin films have densely packed and a highly interconnected network of grains with large area (4 cm2). AFM show significant difference in surface topography of CZTS films with change in calcination temperature. Increase in calcination temperature show increase in rms and average surface roughness of the CZTS films. UV–Visible spectroscopy analysis revealed that the absorption coefficient of as-deposited and calcinated CZTS films are in the range 104–105 cm−1in the visible region. The bandgap show decreasing trend with increase in calcination temperature (1.91–1.59 eV). The bandgap of CZTS film annealed at 400 °C was found ~1.59 eV which is quite close to the optimum value for photovoltaic solar conversion in the visible region of solar spectrum. It is found that the photo response depends upon the grain size effect, whereas photo response increases with the increase of the grain size. Employment these films as an absorber layer in CZTS solar cells can

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improve the conversion efficiency by reducing recombination rate of photogenerated charge carriers due to increased grain size.

ACKNOWLEDGEMENT Mr. Sachin Rondiya is grateful to Dr. Babasaheb Ambedkar Research and Training Institute (BARTI), Pune for research fellowship and financial assistance and INUP IITB project sponsored by DeitY, MCIT, Government of India. Mr. Avinash Rokade is grateful to MNRE, New Delhi for National Renewable Energy (NRE) fellowship. One of the authors Dr. Sandesh Jadkar is thankful to University Grants Commission, New Delhi for special financial support under UPE program. Mr. Ashok Jadhavar is thankful to BARC-SSPU program for financial support.

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Enhancement of Energy Generation from Two Layer Solar Panels

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Pragya Sharma1 and Tirumalachetty Harinarayana1,2 1

GERMI RIIC (Research and Innovation Centre), Gandhinagar, Gujarat 382007, India

2

CSIR-NGRI (National Geophysical Research Institute), Hyderabad 500007, India

ABSTRACT The enhancement of energy using solar photovoltaic in a limited space is important in urban areas due to increased land cost in the recent years. Although there exist different procedures and methodologies to focus the sunlight on solar panels, we have suggested a new approach to enhance the energy generation from the photovoltaic panels, i.e., by keeping the two layers of photovoltaic panels as collectors of energy one above the other

Citation: Sharma, P., and T. Harinarayana, “Enhancement of energy generation from two layer solar panels”, International Journal of Energy and Environmental Engineering (2012), https://doi.org/10.1186/2251-6832-3-12. Copyright: Sharma and Harinarayana; licensee Springer, 2012. This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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with the same size and orientation. Our results of two layer solar panels have shown about 75% increase in efficiency as compared to a single layer solar panel. This study can also be extended to n number of photovoltaic layers piled up one above the other, if the cost economics are justified with respect to the land cost.

Keywords: Simulation, PVSYST software program, Efficiency, Shade analysis, Land cost

BACKGROUND Among all possible alternative energy options, for example, wave energy, geothermal energy, solar energy, wind energy, and hydro energy, solar energy is becoming more popular in India. This is mainly due to (1) the availability of plenty of sunlight in all the seasons and also at all the locations of India and (2) the recent initiation of solar mission by the government of India with attractive incentives to the developers [1]. If we look at the world total renewable energy generation, which is around 5 × 1020 J per year, solar thermal contributes to 0.5%, wind 0.3%, geothermal 0.2%, biofuel 0.2%, and solar photovoltaic (PV) is only about 0.04% as per statistical review of world energy during 2007 [2]. In recent years, the technology upgradation has not only made solar photovoltaic technology price competitive but also as a viable technology. It is projected that by the year 2030, the solar PV electricity will also dominate compared to other sources of energy [3]. From the study growth of photovoltaic, an average about 45% annual increase is noticed during the years 2000 to 2009 [3, 4]. From the study of cost economics of a solar photovoltaic power plant, PV modules cost about 45% and the other 55% is due to components, like transformers, cables, inverters, and civil works [5]. Additionally, cost of the power plant also depends on the land value. If the solar power plant is close to the substation near the populated area, the transmission of energy losses will be minimum, but the cost of the land will be high [6]. If the power plant is at a remote location, the cost of the land is low but the energy losses will be high. On the other hand, with less population and in a remote location, the use of energy is limited to the local community. Ideally, the solar power plant needs to be located at a place where the energy generation from the plant can be connected directly to the power grid at an optimum distance from the plant. Apart from the government of India’s national solar mission program, the recent initiation by the government of Gujarat to establish the solar photovoltaic plants is

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commendable. While Gujarat alone crosses 600 MW power through solar, the rest of the country is far behind with only about 200 MW. The Gandhinagar Photovoltaic Rooftop Programme for solar energy generation using PV modules has set an example by government of Gujarat to save the land cost (see http://www.gpclindia.com/gpcl_rsg/index.html). Another way to save the land cost is to adopt a new methodology to get maximum output from the solar power plant in a limited area. In India, the cost of the land has grown up five to ten times for the last 10 years. This is true in all the urban and semi urban regions of India. In view of the above problem, an attempt has been made to study different configuration of solar panels to enhance the energy generation from a solar power plant. For this purpose, the PVSYST modeling software [7] has been used, and a design with a new concept for the solar PV module is suggested, and its advantages over conventional design are discussed. The rationale behind the present work is to enhance the energy generation for the limited space availability. In recent years several methods have been suggested. For example, in concentrating solar power technology [8], the lens or mirror for concentration of sunlight is used by refracting the rays and focusing them in a small area. In another recent study, a 3D type of solar panels is also reported [9]. Here we present another way of enhanced solar energy with two layers of solar panels as discussed in more detail below. Accordingly, the present study aimed to investigate the advantages of two layer solar panels with the same dimension and orientation lying one above the other. Additionally, the cost benefit analysis is also described to highlight the advantages of considering the suggested solar panel configuration from the present study.

METHODS The new design suggested in the present study is the result of several different design attempts using PVSYST software program. Before presenting the methodology, brief details of the software is presented in the following.

The Software Program Among the various software programs, PVSYST simulation software is the most popular to analyze the detailed performance of the plant in field conditions. It can be used for many ways, for example, to investigate different loads on the system, estimate the size of the system, determine the optimal size of the panel, and assess the energy production in the system

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Various other capabilities and options available in the PVSYST software simulation can be seen in [7]. PVSYST, a personal computer-based software package, can also be used to study the sizing and data analysis of complete PV system. It is used for different designs and sizes of the systems. It can evaluate monthly production and performance. It also performs economic evaluation of the PV system at the design stage itself. Its application performs a detailed simulation and also shading analysis according to several dozens of variables. PVSYST also considers the shading of a diffuse radiation [7, 10, 11, 12]. The limitation of the software is that it can compute only a single layer of PV module. This means that if there are two layers of PV modules, one above the other, the software has no provision or option to compute the solar energy. Apart from the PVSYST, there are about twelve other software tools currently in use for the simulation e.g., PV f-Chart, SOLCEL-II, PVSYSY, PVSIM, PVFORM, TRNSYS, ENERGY-10 PV, PVNet, PVSS, RETSCREEN, Renew, and SimPhoSys [10, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24].

The Data For the grid-connected system, the basic input and model parameters required for modeling are the following - PV component database, grid inverter database, geographical site information, and monthly meteorological data for horizontal global irradiance and temperature [7]. In the present study, the meteorological data is acquired from Meteonorm version 6.1.0.23 (see Table 1), a comprehensive climatological database for solar energy applications [7, 25, 26, 27]. Table 1: Radiation measurement details Name of site

Ahmedabad

Latitude (degrees)

23.067° N

Longitude (degrees)

72.633° E

Altitude (meter)

55 m

Radiation model

Default (hour)

Temperature model

Default (hour)

Radiation

1981-2000

Measured parameters (WMO nr: 426470)a

Gh, H_Dh, H_Gk, H_Dk, H_Bn, Ta

Azimuth

0°

Inclination

25°

World Meteorological Organisation - station code for Ahmedabad, India.

a

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In Table 1, the basic details of radiation measurement for Ahmedabad site are shown. The data have been measured and averaged over a period of 20 years. The radiation data is taken for 20 years period i.e., during 1981 to 2000. The meteorological data considered is given in Table 2. In this table, the information on the monthly average meteorological data of solar radiation, for Ahmedabad, is provided. The values provided are related to irradiation value of global radiation in horizontal direction (H_Gh), irradiation of diffuse radiation horizontal (H_Dh), global radiation in tilted plane (H_Gk), irradiation of diffuse radiation tilted plane (H_Dk), irradiation of the beam (H_Bn) and the air temperature (Ta). These values are used in our study to analyze the shading effects on the panels. Table 2: Monthly data from Meteonorm Month

H_Gh

H_Dh

H_Gk

H_Dk

H_Bn

Ta

(kWh/m2)

(kWh/m2)

(kWh/m2)

(kWh/m2)

(kWh/m2)

(°C)

January

147

32

201

42

211

19.6

February

157

36

195

44

193

22.4

March

203

50

227

57

225

27.9

April

214

64

215

67

205

31.6

May

225

78

208

77

202

33.0

June

184

93

165

88

125

31.7

July

139

97

128

92

56

29.1

August

137

92

131

89

63

28.1

September

163

71

171

74

131

28.9

October

171

57

201

65

180

27.9

November

144

39

188

48

181

24.1

December

137

34

188

43

188

20.3

Total

2022

742

2218

786

1961

27.1a

Annual average temperature; H_Gh, irradiation of global radiation, horizontal; H_Dh, irradiation of diffuse radiation, horizontal; H_Gk, irradiation of global radiation, tilted plane; H_Dk, irradiation of diffuse radiation, tilted plane; H_Bn, irradiation of beam; Ta, air temperature. a

In Figure 1, the solar panel design configuration considered for our model study is shown. It is a schematic diagram with two sets of layers, one lying above the other in such a way that the bottom layer is a solar panel and the top layer is a blank shade with a height separation of 10 m. Since the PVSYST software cannot compute solar energy using the two solar panels with one lying above the other, the top panel is a shade without solar panel but has the same dimension and same orientation of the solar

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panels in our present study. Later, we will compute the solar energy without shade and add the same with the solar energy with shade to get the total solar energy generated from the two panels. In our model, the DelSolar PV modules (DelSolar Co., Ltd., Miaoli County, Taiwan) have been selected. As a sample, 15 solar panel modules in Xdirection and a series of 15 rows of solar panels in another, say Y direction, are considered. Such a design is arbitrary and helps to compute parameters quickly. This configuration approximately provides about 50 kW of power output from the PV power plant. However, the same model can be extended to any length as required.

Figure 1: Schematic diagram showing the solar panel and above shade with 10 m height.

In Table 3, the information and details for the solar panels considered are shown. Details of the solar module and technology, power rating, and related module specifications are also provided. The technology considered is Sipolycrystalline DelSolar photovoltaic module which is available in PVSYST PV module library [7]. Each module can provide a maximum power output of 230.3 W. Accordingly, the 225 number of modules used in our study can provide a power output of about 50 kW. The modules are oriented in the south direction and accordingly, the azimuth angle is assumed as 0°. Both the modules and shade panels are tilted at the same angle of 23°. This tilt is chosen as the latitude (degrees) for the Ahmedabad site is 23.067°.

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Table 3: Details of the schematic model shown in Figure 1 and PV module specifications Number of modules

225

Area of the field (land)

770 m2 (approximately)

Active area of the modules (sensitive PV area)

372 m2 (Each module length 1.67 m and width 99 m)

Name of the manufacturer

DelSolar

Technology

Si-polycrystalline

Year

2010

P MPP

230.3 W

I MPP

7.72 A

V MPP

29.8 V

Module and shade tilt

23° N

Separation between rows of solar panel

2m

Height between solar panel and shade

10 m

Orientation

0° (exactly south)

Shading Factor Analysis The shading factor analysis provides the energy loss from photovoltaic panels due to near shading. Near shading means partial shading that affects a part of the panel(s) [7]. The shaded part changes during the day and also over a season. The shading factor is a ratio between the energy generated from the illuminated part and the total area of the field, or inversely, the energy loss [7]. In Table 4, the information of a single module mounting during no shade over the panels is provided. The shading loss is only a function of the sun’s height and azimuth for a near shading scene. The values in the table represent the shading factor defined above, and are the ratios of the illuminated part to the total area of the field as a function of height and azimuth of the sun position. The value varies as per the season and time of the day. For example, value 1.000 represents 100% illumination or available radiation over the panels during any particular time of the day and .961 represent 96.1% illumination and so on. In ‘no shade’ layout, the illumination over the panel is 100% most of the times except during morning and evening hours, when the height of the sun is 20° or below, with respect to site location, causes maximum shade.

1.000

1.000

1.000

1.000

1.000

1.000

1.000

Behind

Behind

80°

70°

60°

50°

40°

30°

20°

10°

0°

Behind

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−160

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−140

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−120

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−100

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−80

0.26

0.82

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−60

0.125

0.613

0.961

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−40

0.08

0.51

0.856

1.000

1.000

1.000

1.000

1.000

1.000

1.000

−20

0.067

0.479

0.824

1.000

1.000

1.000

1.000

1.000

1.000

1.000

0

0.08

0.51

0.856

1.000

1.000

1.000

1.000

1.000

1.000

1.000

20

0.125

0.613

0.961

1.000

1.000

1.000

1.000

1.000

1.000

1.000

40

0.260

0.821

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

60

0.675

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

80

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

100

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

120

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

140

Behind

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

160

Behind

Behind

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

180

Table 5 presents analysis for a shade at a height of 10 m above the photovoltaic panels. In our study, it is of the same dimension as of the bottom photovoltaic panel. Due to the presence of the shade, the shading factor in Table 5 showed lower value as compared to no shading scene in Table 4. Accordingly, the energy output reduces from the panels.

1.000

−180

90°

height

Azimuth

Table 4: Shading factor table for no shade over panels

286 Physics of Solar Energy

0.194

0.142

0.365

0.303

0.896

0.406

1.000

1.594

Behind

Behind

80°

70°

60°

50°

40°

30°

20°

10°

2°

−180

90°

Height

Azimuth

Behind

Behind

1.000

1.000

1.000

0.352

0.718

0.623

0.275

0.194

−160

Behind

1.000

1.000

1.000

1.000

0.899

0.826

1.000

0.602

0.194

−140

Behind

0.960

0.980

1.000

0.592

0.665

1.000

0.194

1.000

0.194

−120

1.000

1.000

1.000

1.000

0.643

0.642

0.957

0.919

0.566

0.194

−100

0.675

1.000

1.000

0.929

0.565

0.847

0.885

0.548

0.190

0.194

−80

0.26

0.821

0.986

0.797

0.638

0.390

0.748

0.492

0.747

0.194

−60

0.125

0.613

0.884

0.808

0.786

0.476

0.400

0.637

0.881

0.194

−40

0.084

0.512

0.778

0.648

0.720

0.538

0.783

0.793

0.585

0.194

−20

0.067

0.479

0.740

0.678

0.552

0.352

0.514

0.597

0.472

0.194

0

0.084

0.512

0.778

0.648

0.720

0.537

0.783

0.792

0.585

0.194

20

0.125

0.613

0.883

0.808

0.786

0.475

0.400

0.636

0.881

0.194

40

Table 5: Shading factor table for shade at a height 10 m above the solar panels

0.26

0.821

0.986

0.796

0.638

0.390

0.748

0.491

0.747

0.194

60

0.675

1.000

1.000

0.929

0.564

0.847

0.885

0.547

0.190

0.194

80

1.000

1.000

1.000

1.000

0.642

0.641

0.957

0.919

0.565

0.194

100

Behind

0.959

0.980

1.000

0.591

0.665

1.000

0.194

1.000

0.194

120

Behind

1.000

1.000

1.000

1.000

0.899

0.826

1.000

0.602

0.194

140

Behind

Behind

1.000

1.000

1.000

0.352

0.717

0.623

0.274

0.194

160

Behind

Behind

1.594

1.000

0.406

0.896

0.303

0.365

0.142

0.194

180

Enhancement of Energy Generation from Two Layer Solar Panels

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288

Annual Energy Yield In Table 6, the energy that can be supplied to the grid, for annual generation is shown. The results of the shading analysis of photovoltaic panels for the whole year is shown with no shade and shade at different heights; and the cumulative energy form the average radiation data for the years 1980 to 2000, supplied to the grid for two layer panel system, is shown. The annual total yield of energy supplied to the grid, when there is no shade over the panels, is given in column 2, and the energy generated by the single layer solar photovoltaic system with different shade heights 1, 3, 5, and 10 m is provided in column 3, 4, 5, 6, respectively. Similarly, energy generated by the two layer solar photovoltaic systems with separation values of 1, 3, 5, and 10 m is provided in column 3, 4, 5, 6 respectively. As can be seen, the amount of energy supplied to the grid varies with respect to different height separations 1, 3, 5, and 10 m between the panels. It is observed from the present study, the energy supplied to the grid is at maximum for the case of 10 m height separation. Table 6: Energy supplied to the grid by single layer with shade and two layer PV panel system Height between the panels

No shade (a)

Total annual energy gener- 77,980 ated by solar panels and shade both of equal dimensions (372 m2) (in kWh) Total annual energy gener- ated by two layer panels both of equal dimensions (372 m2) (in kWh)

1 m (b)

3 m (c)

5 m (d)

10 m (e)

28,100

40,887

45,775

55,942

106,080

118,867

123,755

133,922

(a + b)

(a + c)

(a + d)

(a + e)

In Figure 2, a comparative study is shown for the energy supplied to the grid in different months of the year for the radiation data averaged for the years 1980 to 2000. The vertical axis shows the energy supplied to the grid for each month, for example, column 1 in the figure shows the total energy supplied to the grid for different months of the year.

Enhancement of Energy Generation from Two Layer Solar Panels

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Figure 2: Monthly energy values supplied to the grid with or without the shade over the PV panels.

In Figure 3, a comparative study is shown for the two layer photovoltaic panels. The details of the average energy generated per day (December 20), for the radiation data averaged over a period of 20 years, are given in Table 7. It can be observed that the amount of energy enhanced with two layer photovoltaic panels increases with the increase in height between the panels. Obviously, the amount of energy supplied to the grid is higher for the two layer photovoltaic system as compared to single layer photovoltaic system. For example, in Figure 3 on X-axis, the histogram plot shows the amount of energy generated for the month of January to December. The cumulative energy yield for single and two layers system with different separations are presented as shown in the figure.

Figure 3: Monthly energy values supplied to the grid by two layer solar photovoltaic panels one above the other.

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290

Table 7: Result of single solar panel with shade and two layer solar panels one above the other Number

Height between the panels (meter)

Energy generated at the output of the array by single layer solar panels with shade (approximate kilowatthour per day)

Energy generated at the output of the array by two layer solar panels with different heights (approximate kilowatthour per day) (s. no.)

Increased in Efficiency for two layer solar panels with respect to single layer solar panel (approximate percentage)

1.

0a

252

-

-

2.

1

118

370(1 + 2)

46

3.

3

126

378(1 + 3)

50

4.

5

143

395(1 + 4)

56

5.

10

192

444(1 + 5)

76

Single layer solar panel system.

a

RESULTS AND DISCUSSION In the following, the details of important results derived from our study are discussed and can be seen in Table 7 and also in Figures 4 and 5. The power output from a single layer solar photovoltaic system, with and without shade, and also for the two layer photovoltaic system separated by 1, 3, 5 and 10 m, are compiled and shown in Table 7. The results shown are for a single day i.e., December 20 and were averaged for the years 1980 to 2000. The increase in efficiency is observed for the two layer solar panels for a configuration presented in Figure 1. The result of two layer solar panels, one above the other, with different height separation between them, showed enhancement of the energy. The energy generation for no shade over the panels is about 250 kWh/day. For a single layer solar panel with shade at 10 m of height, (maximum in our study) the energy generation is about 190 kWh/day. By combining the power from the two panels, the net result increases its efficiency by approximately 76% as compared to the power generated by the single layer solar panel without shade. Similarly, one can see that the resultant increase in the efficiency is around 56% for 5 m height, around 50% for 3 m height, and 46% for the 1 m height between the solar panels. The reason for choosing this day (December 20) for modeling from Meteonorm radiation data [25] is due to its clear day in the month of December.

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Figure 4: Effective energy at the output of array for single layer solar panel with shade at different heights.

Figure 5: Effective energy at the output of array for two layer solar panel.

Figure 4 is a graphical plot of the effective energy at the output of array as a function of the local time of a day. In our case, we considered December 20 of Meteonorm radiation data averaged over a period of 20 years, 1981 to

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2000. As can be observed from the figure, the obvious result that is closer the shade has less output. For example, for shade over the panel at a height of 1 m showed the lowest energy as compared to 3, 5, and 10 m. Figure 5 shows important result of enhancement in energy generation using two solar panels one above the other. As before, one can see that the enhancement increases with the height between the two layer solar panels. For 10 m height, considering a typical for a 50 + 50 kW photovoltaic system as an example, we observed as much as 65 kW peak around noon. In recent years, semi-transparent solar panels are also under way, and they pass on more solar energy to the bottom panels. Although our study clearly demonstrates the enhancement of energy generation for the two layer solar panel system as compared to single layer, one should be careful about the cost economics involved for such a system. In Table 8, the economics of the two layer solar panel system have been compiled. The monetary benefit for two layer solar panel system over a single layer solar panel system is shown. For a 50 kW of system, we assumed the land cost (e.g., Ahmedabad, Gandhinagar, Rajkot in Gujarat) as 20 million Indian rupees (INR), module cost of 4 million INR, and 1 million INR for other accessories. Accordingly, single layer solar panel system provides about 10 kWh of energy per day per million INR of investment. For the two layer of solar photovoltaic system, as the area remains same the land cost are zero. But for the two layer, the added expenditure are the solar panels and other mounting accessories. Thus for 10 m separation one can have 14.8 kWh/million (INR)/day, which is nearly 50% extra benefit for the one million INR investment. Table 8: Monetary benefits Number

Height between the panels (meter)

Land cost (in million INR)

Solar panel cost (in million INR)

1.

0a

20

4

2.

1

0

4

3.

3

0

4

Energy generation per day (kWh approximate)

Output of energy per million investment (kWh/million INR/day)

1

252

10.1

.5

370

12.5

.6

378

12.7

4.

5

0

4

.8

395

13.2

5.

10

0

4

1

444

14.8

Single layer solar panel system.

a

Accessories (in million INR)

Enhancement of Energy Generation from Two Layer Solar Panels

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CONCLUSIONS An attempt has been made in our study on near shade analysis of single and two layer solar panels through modeling for a limited dimension. The energy generation from a single layer solar panel system for a day (December 20 as a sample) is 252 kWh/day for 756 sq m area. It increases up to nearly 445 kWh/day with the two layer solar panels separated by 10 m in the same area. The output varies depending on the separation between the two layers of photovoltaic panels. Due to high land cost in urban areas, the present study is significant. We have shown an increase of over 70% in the output. The present modeling results are limited to the two layer PV system with opaque modules as solar energy collectors for small dimensions as shown in Figure 1. Our result is more applicable to roof tops of the houses or small scale plants. The study, however, can easily be extended to the n layer solar PV panel system of any dimension. However, the justification of the plant cost with respect to solar panels need to be considered. Thus, one needs to have an optimal cost in designing the number of solar panel layers. It should also be based on the foundations of the site location.

ACKNOWLEDGEMENTS PS is very much thankful and express sincere gratitude to Dr. Jayanta Deb Mondol, Ulster University, for providing his quick comments on the manuscript. PS and TH would like to acknowledge all the research technical and scientific staff of GERMI for their encouragement, motivation, and support.

AUTHORS’ CONTRIBUTIONS PS carried out all the computation, system designing, modeling analysis, software simulation, and drafted the manuscript. TH conceived of the study and participated in its design and coordination. Both authors read and approved the final manuscript.

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Spectrum Splitting For Efficient Utilization of Solar Radiation: A Novel Photovoltaic– Thermoelectric Power Generation System

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Esam Elsarrag1, Hans Pernau2 , Jana Heuer2 , Nibul Roshan1 , Yousef Alhorr1 and Kilian Bartholomé2 1

Gulf Organisation for Research and Development, QSTP, Doha, Qatar

2

Department of Energy Systems, Fraunhofer IPM, Freiburg, Germany

ABSTRACT Standard photovoltaic solar cells (PV cells) use only about half of the light spectrum provided by the sun. The infrared part is not utilized to produce electricity. Instead, the infrared light heats up the PV cells and thereby decreases the efficiency of the cell. Within this research project, a hybrid solar cell made of a standard PV cell and a thermally driven thermoelectric

Citation: Elsarrag, E., et al., “Spectrum splitting for efficient utilization of solar radiation: a novel photovoltaic–thermoelectric power generation system”, Renewables: Wind, Water, and Solar (2015), https://doi.org/10.1186/s40807-015-0016-y. Copyright: Elsarrag et al. 2015, Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons. org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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generator (TEG) is being developed. The light of the sun splits at about 800 nm. The visible and ultraviolet part is transferred to the PV cell; the infrared part illuminates the thermal TEG cell. With the hybrid solar cell, the full solar spectrum is exploited. In this paper, theoretical and experimental results for improving the performance of thermoelectric elements coupled with photovoltaic modules have been presented. The proposed concepts and the experimental results have provided a key input to develop a large scale of a hybrid PV-TE system. Keywords: Photovoltaic, Thermoelectric, Hybrid system, Renewable energy, Solar energy

BACKGROUND The basic idea for a combined PV and thermoelectric solar cell has been published in 2008 (Tritt et al. 2008). The history of thermoelectricity began in 1823 when Seebeck made his experiments about the conversion of a temperature gradient into an electrical current (Seebeck 1895). Especially within the last decade research on thermoelectric materials and systems has been intensified due to the awareness of the need to increase the efficiency of energy consumption. Thermoelectric devices can convert waste heat directly into electrical energy. Many efforts in this field have been made to implement thermoelectric generators (TEG) in automotive applications. The conversion efficiency of TE generators depends on the available temperatures and the material properties, namely the dimensionless figure of merit ZT:

T is the absolute temperature, α the Seebeck coefficient, σ the electrical conductivity and λ the thermal conductivity. Based on this value the conversion efficiency of the thermoelectric material using the temperature gradient between the hot side temperature T h and cold side temperature T c can be calculated as:

where η c is the Carnot efficiency. The higher the ZT value of the material the closer is the efficiency to the Carnot limit. Modern thermoelectric materials reach ZT values larger than 1 and with efficiencies more than

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4 % (Tritt et al. 2008). Using the technique of thermoelectric generators, to convert the infrared part of the sun spectrum into electrical energy, we could increase the overall performance of a combined PV and TE solar cell by approximately 10 % of the PV, thereby achieving around 20 % efficiency with the combined system rather than the 17–18 % efficiency from the PVonly setup. The basic idea of PV-TE was introduced by Tritt (2008) and Kraemer et al. (2011) who studied the utilization of both ultraviolet (UV) and infrared (IR) parts. Various papers have been published dealing with the combined use of thermoelectric and PV or solar thermal systems. Baranowski et al. (2012) claimed efficiencies of 15.9 % for concentrated solar thermoelectric generators (STEG) by developing a balance model and analyzing the present day materials under ideal conditions. A number of works on the STEG hybrids are based on concentrating solar power on to TEGs. Chávez Urbiola and Vorobiev (2013) designed and tested such a system with co-generation of hot water which was used as the coolant for the TEG hotside and achieving 5 % electrical efficiency. The studies conducted by Eswaramoorthy and Sanmugam (2013) and Kalogirou (2013) on the use of such systems in specific geographic locations gave more insight into the feasibility and possibility of large scale deployment of the systems. Leon et al. (2012) and Lertsatitthanakorn et al. (2013a, 2013b) evaluated the possibilities of concentrated solar power on hybrid systems using different strategies for TEG design and the cooling technique. Lippong et al. (2012) successfully implemented a cooling mechanism for solar TEG hybrid using phase change material and implied the possibility of using it as a sustainable system for independent operation. McEnany et al. (2011) developed an analysis model and denotes that, with the presently available materials and technology, efficiencies of more than 10 % can be achieved using solar TEG hybrid systems by the cascading of TEGs and under high temperature and optical irradiance operation. Meir et al. (2013) suggested controlled shaping of electric potential distribution in the thermoelectric converters for more efficient generation of thermoelectric energy, in theory. Mizoshiri et al. (2012) tested a hybrid system by implementing spectrum splitting on a thin-film TEG and focusing the near infra-red (NIR) radiation onto the TEG while the PV received the rest of the spectrum. The use of thin-film selective absorber coating for TEGs in the performance of hybrid systems was investigated by Ogbonnaya et al. (2013). Van Sark (2011) developed a model to analyse the feasibility of a PV-thermoelectric module in outdoor conditions and provided very optimistic results by considering

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ideal conditions of operation. Advances in the related fields such as: (1) the development of high-performance spectrally-selective solar absorber based on a yttria-stabilized zirconia cermet with high- temperature stability by Wang et al. (2011), (2) thin-film TEG model by Weinstein et al. (2013) which can be used in place of conventional TEGs with minimal losses, and (3) the multi-hybrid cell by Yang et al. (2013), which can harvest mechanical, solar and thermal energy at the same time, provided strength to the optimistic feasibility predictions of van Sark and Zhang et al. (2013) to come true. One such promising field is the solar spectrum splitting for energy co-generation. Within all these works, the splitting of the solar spectrum was discussed theoretically but not investigated in an extensive practical manner, except for Mizoshiri et al. (2012) who generated an open voltage of 79 mV. This study will investigate the performance of a thermoelectric generator by changing its material constitution and design features. The TEG is anticipated to be integrated with PV modules to form a hybrid photovoltaic– thermoelectric generator and increase the overall conversion efficiency from solar irradiance to electricity.

METHODS The First System Setup Figure 1 shows a simplified solar spectrum and the energy fractions which could be used by the PV cell and the TEG. Based on this concept, the first principal design was developed and implemented in a versatile test hybrid cell as shown in Fig. 2. This system consists of 15 cm × 15 cm monocrystalline PV cell, 1.5 cm × 1.5 cm TEG [Quickohm Model QC 31-1.0-3.9 M (Quick Cool Shop 2015)] and a beam splitter. For the first test setup, a so called “cold mirror” made by OpticBalzers (Datasheet: Cold Mirror 2015) was used to split the solar radiation. The beam splitter was placed at an angle of 45° to both the PV and TEG.

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Figure 1: Simplified solar spectrum and energy ratios to be used within the PV cell and the TEG (Tritt et al. 2008).

Figure 2: a Principal design idea; b versatile test hybrid cell.

The spectral characteristic of this mirror is shown in Fig. 3. As shown in the figure, the cut-off wavelength of this mirror is 700 nm, a little bit lower than the desired 800 nm, this leads to an approximate 50/50 splitting of the energy in the setup.

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Figure 3: Transmission and reflectance data of the used mirror by OpticBalzers (Product Page: Hi-Z 2015).

The test rig was designed to allow independent movements of the system components. It offers enough space to test different types of PV cells, absorbers and beam splitters. In the test rig both the PV cell and the TEG can be cooled, the input and output temperature of both coolers can be monitored. Both coolers use a liquid cooling media provided by a radiator cooling tower with an estimated cooling power of 1000 W. The lowest possible temperature depends on the surrounding temperature during measurements. It has to be evaluated within the project if a tailored mirror with 800 nm or another cut-off wavelength will achieve better performance or not. As the project aims to use commercially available parts to minimize system costs for the final hybrid module, the mirror from OpticBalzers was considered to initiate the tests. To verify the experimental data presented by Seebeck (1895), FEM simulation with Comsol Multiphysics was performed. The thermal absorber was simulated using the solar radiation tool enclosed in the heat transfer module. Two commercially available absorber materials were chosen for the simulations and lab tests. These are the “Metal Velvet™” absorber by Acktar Advanced Coatings (Website 2015) and the “Tinox® energy Al” by Almecosolar (Datasheet: Solare Absorberbeschichtungen 2015). The absorption data of both absorbers are shown in Fig. 4. The main difference between these two absorbers is the absorbance of IR light above 2.5 µm. “Metal Velvet™” is black up to very long wavelength as “Tinox® energy Al” is a so-called selective absorber and becomes transparent above 2.5 µm

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that leads to reduced emission losses. That is because the emissivity and the absorbance of an optically dense body are equal. If the absorber is heated by the sun, it will emit black body radiation in the range of about 5–8 µm wavelength depending on its temperature. This radiation loss reduces the maximum temperature which can be achieved in the system. The simulation results have shown that the “Metal Velvet™” absorber can reach up to 110 °C but the “Tinox® energy Al” can reach up to 345 °C in vacuum (the coating was in both cases attached to a 250 µm aluminum plate).

Figure 4: Absorption data of (black dots) the “Metal Velvet™” absorber by Acktar Advanced Coatings (Datasheet: Cold Mirror 2015) and (red circles) the “Tinox® energy Al” by Almecosolar (Website 2015).

In the next simulation step, a TEG and absorber were included in the model with a parameterized footprint area and height. The cold side of the TEG was attached to a 45 °C surface with a thermal conductivity of 1000 W/ mK. The thermal conductivity between TEG and absorber plate was set to infinite. Changing the footprint area from 2.5 × 2.5 to 50 × 50 mm2 the achievable hot and cold side temperatures as well as the heat flux through the TEG were simulated. The temperatures and an estimated generator power are plotted in Fig. 5. The conversion efficiency of the TEG was calculated from measured ZT data; together with the simulated heat flux the power was obtained. It can be seen that starting from the side length of around 18 mm of a square cut TEG; the TEG power output was proportional the temperature difference between the hot side and the cold side surfaces.

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Figure 5: TEG output power and temperatures of the TE-absorber depending on the side length (a).

For the next steps, the TEG model will be enhanced using the Comsol models developed by Jägle et al. (2008). Using the material data of the real TEG modules, the real performance of the system can be evaluated with a good accuracy.

The Second System Setup A second set of tests was conducted to compare the performance of the Hybrid PV system with a standard system. The Hybrid system consisted of a small size (15 cm × 15 cm) monocrystalline, custom made, low power PV Panel and a comparable sized TEG, Model HiZ-2, (2.9 × 2.9 cm). The setup makes use of the Bismuth Telluride based ‘HZ-2’ TEG Model from Hi Z (Product Page: Hi-Z 2015) which accommodates 97 thermocouples in 2.9 cm × 2.9 cm × 0.508 cm and has a conversion efficiency of 4.5 %. The TEG typically produces 2.5 Watts at 3.3 volts at Matched Load with a 200 °C temperature gradient between the surfaces at 30 °C ambient temperature. The standard system had a similar PV only setup. The testing was conducted in a solar simulator chamber (Model: SEC 1100, Manufacturer: Atlas) [Product Page: Atlas SEC 1100 (2015)]. The test setup was such that, both the benches were tested simultaneously inside the chamber. The Hybrid bench had the light falling on the cold mirror

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which was at 45° to the fixed light source to facilitate splitting of the light by the cold mirror (angle of incidence 45°). The mirror would split the incident light to the PV and TEG surfaces. The absorber surface with the TEG would get the IR radiation passing via the mirror. The PV surface, which is at 45° to the mirror and hence perpendicular to the light source, gets illuminated with the rest of the wavelength which is reflected by the mirror. The cold side of the TEG was cooled by an Aquaduct 360 Eco Mark II External water cooling tower (2015) and this temperature was dependent on the ambient temperature. The ambient temperature inside the chamber was kept at a constant maximum of 50 °C. The normal test bench had the similar PV facing the light at 45° and was parallel to the mirror in the Hybrid bench, to make sure that both test setups had the same amount of light incident on them.

Figure 6: Schematic of the measurement sensor connections to the setups.

The measurements were made using a purpose built microcontroller based embedded system using sensors to monitor the current, voltage and

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temperature levels of the different parts of the setup as shown in Fig. 6. The logging was done in real time high frequency samples and saved to a memory card in CSV format for easy analysis. The sensors used included LM35 precision IC for temperature sensing with a range of 0–100 °C and an accuracy of 0.25 °C at high ambient temperatures (Datasheet: LM35 sensor 2015). The PV and TEG currents were measured using INA219 based sensors, with a resolution of 0.8 mA and a maximum range of ±3.2 A measurement (Datasheet: INA219 sensor 2015). A 20 × 4 parallel interface graphical LCD provided real time data display for monitoring purposes. The data saving was done via a memory card shield and a DS1307 based Real Time Clock (RTC) module (Datasheet: RTC 2015). The loads used for both PVs were 8 Ohm independent resistive Loads and the TEG having a 1 Ohm resistive Load. The irradiance levels were gradually changed in 8 steps from 300 to 800 W/m2, which were the available steps in the SEC 1100 Model. The Output of the Normal system provides an output without the sunlight splitting, while the Hybrid System performance is after the sunlight splitting.

RESULTS AND DISCUSSION The First Setup Results To verify the simulation results first laboratory test have been done using the same 15 × 15 mm2TEG [Quickohm Model QC 31-1.0-3.9 M (2015)] under the absorber plate. Each absorber material is attached to a 250 µm Aluminum sheet. These absorber plates are interfaced with the TEG using a very thin layer (less than 1 mm) of Arctic Silver 5 thermal conductive paste which a Thermal conductivity of 8.9 W/mK (Product Page: Arctic Silver 2015). The measurements are performed in the solar simulator setup. The spectrum shape is similar to the AM1.5 standard and can be adjusted in its flux level from 0.12 to 1.1 suns. The output power of the TEG’s is measured via the voltage over a reference resistor of 1 Ω. The cold side is cooled using a radiator cool tower system. The temperatures of the cooler plate and the absorber plate are measured with PT100 thin film thermometers. The voltages, as well as the two resistances, are measured with a Keithley 2700/7700 multi-meter. The obtained data are plotted in Fig. 7 over the radiation flux of the solar simulator. Both Fig. 7a, b show that the temperature difference between two surfaces of the absorbers “Metal Velvet™” and “Tinox® energy Al”. The power output of the TEG(s) is proportional to the level of solar irradiations falling on the absorber’s surface.

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Figure 7: a Temperature of the absorber made from the “Metal Velvet™” and the cooler plate and the output power of the TEG plotted over the radiation flux of the solar simulator. bTemperature of the absorber made from the “Tinox® energy Al” and the cooler plate and the output power of the TEG plotted over the radiation flux of the solar simulato.

As revealed in Fig. 4, the absorbance of “Metal Velvet™” keeps at 100 % corresponding to any values of wavelength from 0 to 10,000 nm. The maximum temperature difference was around 18 K under the solar radiation level at 1.1 suns as shown in Fig. 7a. The corresponding power output at 1.1 suns was about 32 mW. However, due to the selective absorbance

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characteristic of “Tinox® energy Al”, the absorber greatly reduced the radiation emission heat loss and sustained a higher temperature between the hot and cold surface. As shown in Fig. 7a, b, the power output and the temperature difference between the two sides of “Tinox® energy Al” absorber-TEG assembly were always higher than the “Metal Velvet™” absorber-TEG assembly under different solar irradiation conditions of 500, 700 and 1000 W/m2. In order to achieve a higher temperature difference in the TEG, apart from changing the absorbance characteristic, another measure aimed to reduce the convective heat loss on the surface of the absorber by covering the absorber surface with the honeycomb was investigated. The experimental setup with honeycomb cover is shown in Fig. 8. At first, two different material absorbers had been cut into 7.5 × 7.5 cm2 pieces and had been placed on a thermally insulating Styropor block inside the solar simulator as shown in Fig. 9. The temperature of the absorber was measured with PT 100 sensor glued to the backside of the absorber plate covered inside the Styropor block. In the first experiment, the setups were covered with honeycomb. The purpose experiment only intends to compare the performance two different absorber materials.

Figure 8: Honeycomb structure on top of the absorber to reduce convection

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losses.

Figure 9: Setup to measure the different absorber materials in the solar simulator.

The two absorbers being compared in this part were “KG-1” and “Tinox”. The thickness of the heat absorber glass KG-1 was 3 mm. The thermal mass of “KG-1” is 40 times higher than the non-transparent absorber “Tinox” which was deposited on a 0.2 mm aluminum foil. Owing to the high thermal mass, the response time to a radiation change of “KG-1” absorber was much higher. Figure 10 shows the temperature profiles of the “KG-1” absorber and the “Tinox” absorber under the solar irradiation of 0.5, 0.8 and 1.1 suns against time. It is obvious to note that the temperature of the “Tinox” absorber was always much higher than the “KG-1” glass absorber. Because of the high thermal mass, the response time for a radiation change and the maximum temperature reached were low.

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Figure 10: Temperature of the KG-1 glass (red) and the Tinox absorber (black) as a function of time. Finally, the achieved temperatures with and without the honey-comb structure of three types of absorbers are shown in Fig. 11. The use of the honeycomb-structure leads to an increased temperature as the convective heat loss is reduced. Again, it is clear that minimizing the convective heat loss from the absorber surface enables to maximize the absorber’s temperature and potentially increases the electricity generation by the thermoelectric effect.

Figure 11: Maximum reached temperatures of the different absorber materials with and without the mounted honeycomb structure.

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The Second Setup Results The solar simulator was set to provide irradiance between 200 and 800 W/ m2, as shown in Fig. 12. Initially it is aimed to compare only the outputs the Normal (conventional) PV and the PV with the split mirror using two benches on the same time. It was noted that the PV with the splitted spectrum performed better than the Normal PV at low irradiance levels up to 700 W/ m2. Beyond this level the Normal PV produced more output than the PV with the split mirror see Fig. 13a. The power difference between the splitted spectrum PV and the full spectrum PV exceeded the 40 % at low irradiance as shown in Fig. 13b. The PVs temperatures during the test are shown in Fig. 14, which clearly shows that the split spectrum PV was cooler than the full spectrum PV at all times.

Figure 12: The different irradiance values on floor level used in the testing.

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Figure 13: Comparison of the power outputs of the PV panels only, in the normal (full spectrum) and hybrid (split spectrum) systems: a PV power outputs; b performance difference in percentage.

Figure 14: The temperatures of both the PVs during the testing periods.

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The performance comparison of the hybrid system (PV +TEG) and the Normal PV only system provided clear information on the difference in power that can be produced if the full spectrum of the sunlight is harnessed as shown in Fig. 15. The advantage of the power output of the hybrid system varied with the irradiance levels due to the factors mentioned above as well as the temperature difference between the TEG surfaces. The TEG power output is slightly better at higher irradiance levels; an average of around 10 % of the PV power output throughout the test with a constant ambient temperature of 50 °C. The efficiency curve of the TEG as calculated from the equations stated above along with information from the tests (Product Page: Hi-Z 2015) is provided in Fig. 16. The low TEG module efficiency is due to the lower temperature and heat absorbed by the TEG.

Figure 15: Power output comparison of the normal and hybrid systems.

Figure 16: Comparison between Carnot efficiency and calculated TEG efficiency.

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The hybrid system (PV + TEG) performed better than the Normal PV throughout the test period and the maximum difference achieved was nearly double that of the Normal (full spectrum) PV as shown in Fig. 17. The difference between the power outputs increased at lower irradiance levels (nearly 80 % difference at 300 W/m2) however; the difference reduced as the irradiance levels increased, (nearly around 5 % at 800 W/m2) Further studies based on change of the ambient temperature (instead of using the same ambient temperature at all irradiance levels) can lead to further information for controlling or tuning the system performance. This data can also be used to predict the performance of the hybrid system at different ambient climate conditions.

Figure 17: Advantage of hybrid system performance over the normal system.

CONCLUSION This study investigated the performance of a photovoltaic (PV) and thermoelectric generator (TEG) assembly by changing its material constitution and design features. The TEG is anticipated to be integrated with PV modules to form a hybrid photovoltaic along with a sunbeam splitter to increase the overall conversion efficiency from solar irradiance to electricity. The thermoelectric conversion efficiency is proportional to the temperature difference between the absorber’s hot and cold surfaces; however, the PV efficiency reduces with the increase of its temperature. The methods used to enhance the hybrid system performance were proposed. Their corresponding experiments were performed and the initial results were presented. Conclusively, proper selections of selective absorbance materials of the absorber

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are contributive to the thermoelectric generation. Alleviation of the convective heat loss from the surface of the absorber results in substantial positive impact to a TEG. The PV showed a better overall performance with the beam splitter. The proposed concepts and the positive experimental results provide useful information and reference for the further development of a hybrid PV-TE system for field testings.

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All Spray Pyrolysis-Coated CdTe–TiO2 Heterogeneous Films for PhotoElectrochemical Solar Cells

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S. N. Vijayaraghavan, Aditya Ashok, Gopika Gopakumar, Harigovind Menon, Shantikumar V. Nair, Mariyappan Shanmugam Amrita Center for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala 682041, India

ABSTRACT Cadmium telluride (CdTe) thin films of different thicknesses deposited onto titanium dioxide (TiO2) nanoparticle layer by spray pyrolysis deposition (SPD) are demonstrated as major photo-active semiconductor in photoelectrochemical solar cell configuration using iodide/triiodide (I−/I3−) redox couple as a hole transport layer. The CdTe–TiO2 heterogeneous films were Citation: Vijayaraghavan, S. N., et al., “All spray pyrolysis‑coated CdTe–TiO2 heterogeneous films for photo‑electrochemical solar cells”, Materials for Renewable and Sustainable Energy (2018), https://doi.org/10.1007/s40243-018-0120-1. Copyright: © The Author(s) 2018, Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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characterized by X-ray photoelectron spectroscopy which identified doublet split of Cd 3d and Ti 2p which confirms CdTe and TiO2. Optical absorbance and transmittance of CdTe and TiO2 films which were examined by UV–Vis spectroscopy confirm that the optical bandgap of CdTe is 1.5 eV with a dominant photo-absorption in the spectral window of 350–800 nm, while TiO2 showed a bandgap of 3.1 eV and is optically transparent in the visible spectral window. The present work examined photo-anodes comprising 1, 3, 5, and 10 SPD cycles of CdTe coated on TiO2 nanoparticle layer. The solar cell with 5 SPD cycles of CdTe resulting in 0.4% efficiency. Results can be articulated to the CdTe deposited by 5 SPD cycles provided an optimum surface coverage in the bulk of TiO2, while the higher SPD cycles leads to agglomeration which blocks the porosity of the heterogeneous films.

Keywords: Cadmium telluride, Titanium dioxide, Absorption, Spray pyrolysis deposition, Solar cell

INTRODUCTION Third generation energy-harvesting technology comprises all thin-film-based solar cells in addition to the excitonic photovoltaic devices which include dye and quantum-dot-sensitized solar cells [1, 2, 3]. Dye- and quantum-dotbased photovoltaic devices are important category in electrochemical solar cell technology due to their high performance and lower cost [4, 5]. Various dyes and quantum dots exhibiting optical absorption at different energy ranges were explored to develop electrochemical solar cell technology with different electrolytes [6, 7]. Various technical challenges including adsorption and stability of dyes onto electron acceptors, volatile, and corrosive nature of electrolytes are a few major hurdles in the progress roadmap of photoelectrochemical solar cell technology [8, 9]. Quantum dots of CdTe, CdS, and CdSe are being used as major photo-absorbing candidates in excitonic solar cells with wide bandgap semiconductors as electron acceptors such as TiO2 and ZnO [10, 11, 12]. CdTe has been realized as one of the potential energy-harvesting material candidate in thin-film solar cells due to its optimum bandgap energy and other attractive opto-electronic properties [13, 14]. CdTe is widely used as a major photo-active candidate (bandgap of 1.5 eV) along with CdS as a window layer (bandgap of 2.5 eV) in heterojunction thin-film solar cells [15]. It has been predicted that CdTe-based thin-film photovoltaic technology will enable better energy payback time compared to other thin-

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film-based solar cells [16]. It is an essential requirement to make energyharvesting technology more viable and affordable by employing suitable low-cost materials. While the other electrochemical solar cells experience major problems in terms of stability and photo-absorption, CdTe thin film can be considered as a highly stable which can be used to harvest photons in the visible spectral energy window. Various material processing techniques including close space sublimation, chemical bath deposition, and metal organic chemical vapor deposition are effectively utilized to deposit CdTe thin films for solar cell applications [17, 18, 19, 20]. CdTe solar cell module with 16.5% efficiency (η) with an active area of 1 cm2 and 10.5% in an area of 1400 cm2 was reported by Green et al. [21, 22]. Accelerated tests on CdTe-based thin-film solar cells under harsh environments, including high temperature and illumination, were performed and the materials have shown promising stability and lifetime except for few issues addressed by Corwine et al. [23, 24]. CdTe thin-film solar cells employ Cu as a back contact and it has been observed that Cu diffusion through the grain boundaries of CdTe is one of the major issue affecting its performance and stability to an extent [25]. Various deposition methods have been employed to coat CdTe thin films for energy-harvesting applications [17, 19]. Specifically, quantumconfined CdTe was coated on TiO2 by successive ionic layer adsorption and reaction and reasonable photovoltaic performance values were reported [10]. CdTe has been explored as a co-sensitizer along with traditional organic dyes on TiO2 and the solar cells have shown promising photovoltaic performance [26]. SPD is a versatile material processing technique, widely used in photovoltaic research to coat various functional materials including transparent conductor oxide and electron acceptor such as TiO2 [27]. It has been realized that SPD can be a potential technique for both thin- and thick-film production for energy-related applications [28]. SPD offers wide choices in choosing precursors and deposition temperature for desired material deposition [29]. The present work demonstrates the possibility of establishing photo-electrochemical solar cell technology based on all SPDbased photo-anodes comprising CdTe thin-film-coated TiO2nanoparticle layer and I−/I3− redox couple as a hole transport layer. Photo-electrochemical solar cells employing the heterogeneous CdTe–TiO2 heterogeneous photoanodes and I−/I3−electrolyte as a hole transporting layer were fabricated and characterized for 1, 3, 5, and 10 SPD cycles of CdTe to examine the effect of CdTe layer thickness on performance. Results show the possibility of establishing all SPD-based nanostructured photo-anodes (both electron acceptor and photo-active semiconductor) for photo-electrochemical solar cell technology.

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EXPERIMENTAL Both TiO2 and CdTe films reported in the present work were prepared by SPD technique (KM-150, SPD Laboratory Inc from Japan). For the TiO2 deposition, colloidal solution of TiO2 was prepared by mixing 5 mL of acetic acid and 50 mL of titanium iso-propoxide with 40 mL isopropyl alcohol. All these chemicals were used as purchased from Sigma-Aldrich without any further modifications. This solution was used as a precursor for SPDcoating process directly onto fluorine doped tin oxide (FTO)-coated glass substrates at 110 °C. The SPD-coated TiO2films were annealed at 450 °C for 1 h. For the deposition of CdTe on TiO2, 0.25 mM of cadmium nitrate tetrahydrate and 0.25 mM of tellurium dioxide were used as precursors. The SPD process for CdTe was carried out at 100 °C. Photo-electrochemical solar cells were fabricated using the CdTe-coated TiO2 on FTO as photoanodes, I−/I3− electrolyte as a hole transporting layer and Pt thin-film-coated glass substrates as counter electrode. X-ray photoelectron spectroscopy was used to confirm the SPD-processed CdTe and TiO2 using Kratos Analytical unit. Raman spectroscopy was carried out on TiO2 and CdTe samples using Witec Alpha confocal Raman-300 AR spectrometer with an excitation wavelength of 532 nm. Surface morphology of the TiO2- and CdTe-coated TiO2 films was studied by scanning electron microscope (SEM) using JEOL JSM-6490-LA. Perkin Elmer Lambda-750 was used to study the optical absorbance and transmittance characteristics of the SPD-processed TiO2 and CdTe films. Newport Oriel Class-A solar simulator was used to study the current density–voltage (J–V) measurements under AM 1.5 illumination using a Keithley 2400 digital source meter.

RESULTS AND DISCUSSION Figure 1a shows XPS survey scan obtained on the heterogeneous CdTe– TiO2 film used as a photo-anode in the solar cells. The wide survey spectrum confirmed cadmium, tellurium, and oxygen at 404.3, 575, and 531.3 eV, respectively. Carbon was spotted at 284.3 eV as an impurity present in the film. The characteristic peak corresponding to titanium (Ti 2p) did not show up in the survey spectrum due to the thickness of CdTe layer on TiO2 porous film. The particular sample used for XPS studies utilized 5 cycles of CdTe on TiO2 which covered the surface completely, and due to the limitation of XPS, the Ti 2p state was not found in the heterogeneous CdTe–TiO2 sample. High-resolution XPS scan was performed on the sample to examine the constituent elements in the heterogeneous film and Fig. 1b shows doublet

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split of Cd 3d state. Cd 3d5/2 and 3d3/2 states were observed at 404.9 and 411.6 eV, respectively. Figure 1c shows Te 3d5/2 and 3d3/2 at 575.9 and 586.3 eV, respectively.

Figure 1: a XPS survey spectrum showing Cd 3d, Te 3d, and O 1 s obtained from the heterogeneous film. b and c High-resolution scans of Cd 3d and Te 3d, respectively. dand e High-resolution XPS scans of Ti 2p (inset shows no Ti 2p due to CdTe on TiO2) and O 1s, respectively.

The major characteristic peaks corresponding to Cd 3d and Te 3d states assure the SPD-processed CdTe thin film. As shown in the survey spectrum obtained from the CdTe-coated TiO2 sample, the Ti 2p was not noticed due

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to the presence of CdTe on TiO2. Thus, we performed XPS on pure TiO2 with no CdTe layer on it to confirm the presence of Ti 2p, as shown in Fig. 1d. Ti 2p3/2 and Ti 2p1/2states were observed at the binding energy values of 459.3 and 465.1 eV, respectively. The inset shows the high-resolution XPS scan obtained from the CdTe-coated TiO2 film in the same binding energy range showing no Ti 2p feature which further assures proper surface coverage of CdTe film on TiO2. Figure 1e shows that O 1s high-resolution spectrum exhibiting two peaks at binding energy values of 530.5 and 532.5 eV represent oxygen with metal oxide and metal carbonate, respectively. These two peaks were further de-convoluted using two Gaussian peak fit which assures that the respective binding energy values correspond to the oxygens coordinate with metal and carbonate in the SPD-processed heterogeneous CdTe–TiO2 film.

Figure 2 shows Raman spectra obtained from the a TiO2 and b CdTe samples processed by SPD method. Four Raman active modes were observed at 157, 400, and 520 cm− 1 for TiO2, corresponding to bandgap and other modes, respectively. These four Raman active modes shown in Fig. 2a represent anatase nanocrystalline TiO2. The longitudinal optical (LO) mode and the second and third orders were observed at 166, 333, and 460 cm− 1, respectively. These Raman modes suggest that it can be either cubic or hexagonal phase of polycrystalline CdTe as both exhibit the LO modes at the same frequencies.

Figure 2: Raman spectrum obtained from a nanocrystalline anatase TiO2 and b CdTe thin film.

Figure 3 shows SEM images showing a cross-sectional view obtained from the CdTe-coated TiO2 sample. The film thickness is estimated to be around ~ 15 µm as shown in the cross-sectional view. Surface morphologies

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of the SPD-coated pure TiO2- and CdTe-covered TiO2samples were examined to understand the physical nature of the resultant heterogeneous film. Figure 3b shows surface morphology of the TiO2 film obtained at 10,000 × showing a highly porous nature in the bulk as can be viewed clearly through the clusters formed during the growth. The TiO2 film comprises particles of different sizes and agglomerates to form clusters and the clusters are of the order of microns in size. Furthermore, the morphology of the TiO2film asserts randomly distributed TiO2 particles formed the loosely packed clusters which are observed to be well connected to each other that can be considered as an essential requirement for an electron acceptor in nanoparticle based solar cells. Figure 3c, d shows surface morphology of CdTe-coated TiO2 film obtained at 10,000 × and 20,000 ×, respectively. Since the TiO2 film itself was observed as a porous nanoparticle network, CdTe deposition on porous TiO2 simply covered the surface and resulted in the mesoporous heterogeneous CdTe–TiO2photo-anode. Mesoporosity of CdTe-coated TiO2 can be confirmed through the surface morphology images, as shown in Fig. 3b–d. This can be viewed as randomly distributed mesoscopic clusters of SPD-processed CdTe/TiO2 heterogeneous film. Furthermore, the mesoscopic clusters of CdTe/TiO2 are, in general, expected to provide an efficient charge transport pathway for photo-generated electrons from CdTe to TiO2 to reach FTO electrode by diffusion-based transport process. The mesoporous nature of CdTe-coated TiO2 is a highly preferred morphology for photo-electrochemical solar cells due to an advantage in forming large surface area electrochemical junction between TiO2/CdTe and I−/I3− electrolyte. The mesoporous morphology observed in the SEM images asserts that I−/I3− electrolyte ions can diffuse into the bulk of CdTe covered TiO2 to make hole transport process. In the SPD process, initially, TiO2 layer was deposited onto FTO substrate and annealed at 450 °C for 4 h in atmospheric condition. The cross-sectional and surface morphologies of the CdTe-coated TiO2demonstrate that the bottom TiO2 layer was porous in nature through which the CdTe diffused into the bulk and get coated on the surface to form the heterogeneous photo-electrode stack comprising TiO2/ CdTe.

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Figure 3: SEM images show a cross-sectional view of heterogeneous CdTe– TiO2 film on FTO coated glass substrate, b surface morphology of the TiO2 with no CdTe layer on the surface c and d surface morphologies of CdTe-coated TiO2 at the magnifications of × 10,000 and × 20,000 exhibit the resulting porous surface.

CdTe and TiO2 samples were deposited by SPD onto bare glass substrates to study their optical properties using UV–Vis spectroscopy. Figure 4a shows digital photographs of the CdTe thin-film samples processed by SPD showing 1, 3, 5, and 10 cycles of deposition to examine the effect of thickness on photovoltaic performance of resulting photo-electrochemical solar cells. The images show variation in contrast due to the increasing thickness starting from 1 to 10 cycles having samples processed at 3 and 5 cycles in the middle range. While the contrast is exceptionally clear between 1 and 3 cycles, the higher number of cycles saturated the film color. All four CdTe films on glass substrates are observed to be highly smooth and continuous for naked eye, as can be seen in Fig. 4a. The films are observed to be pinhole free and highly uniform. These four films were subjected to UV–Vis spectroscopic studies to examine their optical properties. Figure 4b shows optical absorbance spectra of CdTe thin film of 1, 3, 5, and 10 cycles in a wavelength range of 350–800 nm. It is observed that optical absorbance of the films increased systematically with increasing thickness, as depicted in Fig. 4b. The characteristic feature of absorbance spectra for all four samples

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was similar but differed in magnitude due to the variation in thickness. Particularly, the 10 SPD cycles yield maximum possible absorbance among all four samples explicitly articulate the effect of thickness on the ability of trapping more photons. Figure 4c shows transmittance of the same four CdTe samples which further confirms the effect of sample thickness on transmittance. The variation in transmittance due to the difference in CdTe thickness is in good agreement with the absorbance spectra, as illustrated in Fig. 4b.

Figure 4: a Photographs of SPD-processed CdTe thin films, b optical absorbance of CdTe thin films of 1, 3, 5, and 10 cycles showing thickness dependent variation in absorbance, and ccorresponding transmittance spectra. d Comparative transmittance spectra of CdTe and TiO2 with an inset showing calculated optical bandgap for both materials.

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The CdTe sample of 1 SPD cycle showed maximum transmittance compared to other samples as expected. Figure 4d shows a comparative plot showing optical transmittance of CdTe and TiO2 films. The inset in Fig. 4d shows calculated optical bandgap values for both CdTe and TiO2 using Tauc plot technique, (αhν)1/2 vs. incident energy, showing 1.5 and 3.1 eV, respectively. It is important to note that TiO2 as a wide bandgap (3.1 eV) material does not interact with photons in the visible solar spectrum, while CdTe interact with visible energy photons as a narrow gap (1.5 eV) material. In this study, TiO2 is used as an electron acceptor, while CdTe is considered as a major photo-active semiconductor. The optical absorbance, transmittance characteristics along with the optical bandgap values of both layers assert the SPD-processed heterogeneous CdTe–TiO2 and can be a good choice for photo-electrochemical solar cell application. Semiconductors with optical bandgap close to 1.5 eV, such as CdTe, exhibit dominant photo-absorption in the visible spectral range. The present study shows absorption around 300 nm which could be due to the diffusion of TiO2which has optimum bandgap for high-energy photo-absorption. Moreover, morphology of the heterogeneous TiO2–CdTe film is randomly distributed nanoparticle network which may facilitate optical diffusion through which light scattering can be enhanced to efficient absorption. Figure 5 a–d shows J–V characteristics of the four sets of photoelectrochemical solar cells employed heterogeneous CdTe–TiO2 photoanodes in which thickness of CdTe layer on the surface of TiO2 was varied by varying the number of SPD cycles. Table 1 lists JSC, VOC, FF, and ηvalues of solar cells extracted from four different measurements performed to examine the consistency in their performance. Table 2 summarizes mean values and their standard deviations from four measurements showing JSC, VOC, FF, and η values of all four cells for 1, 3, 5, and 10 SPD cycles of CdTe extracted from the data set, as shown in Fig. 5a–d. The key parametric values reported in the present study indicate that some of the values are much better than previously reported, but some are not [4, 6, 24]. However, the present study explores the possibility of fabricating the complete photo-anode (TiO2– CdTe) by SPD method and the results showed the feasibility to achieve the solar cells with decent performance metrics. The cells used 1, 3, 5, and 10 SPD cycles of CdTe showed an average η values (obtained from four cells) of 0.12, 0.08, 0.15, and 0.04%, respectively. While JSC of the cells highly depends on number of SPD cycles, which determines thickness of CdTe, VOC in this cell design represents the energy difference between the quasi-fermi level of CdTe and the redox potential of electrolyte. It is expected that cells

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using 10 cycles of CdTe can yield higher JSC value compared to other cells only due to the increase in thickness of CdTe. Increasing thickness offers photons better chance to be trapped in the layer in which they can generate more photons which result in improved current in the cell. Results show that 1 SPD cycle of CdTe yields JSC of 613 mA cm− 2 in an average of four cells, while the average JSC value obtained from 10 SPD cycles of CdTe is 829 mA cm− 2. It is elucidated that by varying the CdTe coating from 1 SPD cycle to 10 cycles, 35% variation can be obtained in JSC values of the cell in an average. Particularly, 5 cycles of CdTe in the cell showed η of 0.4% with JSC, VOC, and FF values 686 mA cm− 2, 176 mV, and 33.2%, respectively. This is one of the relatively higher values obtained from four cells using 5 SPD cycles of CdTe on TiO2. It shows that there is an optimum CdTe thickness on the surface of TiO2 through which efficient photo-absorption, exciton generation, and charge injection can be achieved to get improved photovoltaic performance.

Figure 5: Illuminated J–V characteristics of the photo-electrochemical cells using heterogeneous CdTe–TiO2 as a photo-anode.

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Table 1: Photovoltaic parameters of solar cells used CdTe of 1, 3, 5, and 10 SPD cycles coated on TiO2 J SC (μA cm– 2)

V OC (mV)

FF (%)

η (%)

Meas. 1

581

469

42.69

0.117

Meas. 2

595

470

43.45

0.122

Meas. 3

619

444

44.07

0.122

Meas. 4

657

427

43.65

0.123

Meas. 1

552

364

47.42

0.096

Meas. 2

609

332

44.98

0.092

Meas. 3

667

301

39.67

0.080

Meas. 4

733

265

33.31

0.065

Meas. 1

533

347

50.47

0.094

Meas. 2

586

289

46.92

0.079

Meas. 3

629

236

38.93

0.058

Meas. 4

686

176

33.15

0.402

Meas. 1

657

320

29.49

0.062

Meas. 2

800

267

22.19

0.046

Meas. 3

895

211

14.76

0.028

Meas. 4

962

183

13.57

0.024

Cells CdTe-1 Cyl.

CdTe-3 Cyls.

CdTe-5 Cyls

CdTe-10 Cyls

Table 2: Mean and SD values of photovoltaic parameters obtained from four measurements JSC (µA cm− 2)

VOC (mV)

Mean

SD

Mean

SD

Mean

SD

Mean

SD

CdTe: 1 cyl.

613

28.80

453

18.28

43.47

0.50

0.12

0.002

CdTe: 3 cyls.

641

67.12

315

36.81

41.35

5.42

0.08

0.012

CdTe: 5 cyls.

609

56.19

262

63.38

42.37

6.77

0.15

0.141

CdTe: 10 cyls.

828

114.53

245

53.69

20.00

6.40

0.04

0.015

Cells

η (%)

FF (%)

Figure 6 shows energy band diagram of the solar cell employing FTO, Pt as contact layers, TiO2as an electron acceptor, electrolyte as a hole transport layer, and CdTe as a major photo-active semiconductor. Optical bandgap values for FTO and TiO2 are 3.6 and 3.1 eV, respectively. These two wide bandgap materials do not absorb photons in the visible spectral range and

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the incident light can pass through to reach CdTe coated on TiO2. Photoexcitation in CdTe creates electrons and holes in the conduction and valence bands, respectively. Photo-generated electrons from the conduction band of the CdTe are further injected into the conduction band of the TiO2 and transported to FTO by diffusion process. The free holes from the valence band of CdTe are transported to electrolyte. In the cell operation, exciton generation occurs at CdTe layer coated on TiO2. In general, electronic quality of CdTe, TiO2, and the interface formed by these two materials are considered to be major factors that determine electron transport. We believe that interfacial recombination loss at the heterogeneous CdTe–TiO2 films dominated which adversely affected the key photovoltaic parameters under illumination condition. We believe that the present work is an initial attempt to develop photo-electrochemical solar cell technology based on all SPDprocessed photo-anodes. Further improvisation in terms of performance is possible by improving the opto-electronic properties of CdTe and interfacial quality of heterogeneous CdTe–TiO2 photo-anodes in the solar cell.

Figure 6: Energy-level representation and the charge transport processes at the interfaces of FTO/TiO2/CdTe/electrolyte/Pt layers.

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CONCLUSION Heterogeneous CdTe–TiO2 films were prepared by SPD processing and demonstrated their applications in solar energy harvesting. The SPDprocessed CdTe exhibits an optical bandgap of 1.5 eV with a dominant photoabsorption in the spectral window of 350–800 nm. The presented results lead to the possibility of establishing all SPD-based material-processing scheme for photo-electrochemical solar cells.

ACKNOWLEDGEMENTS The authors would like to thank the Department of Science and Technology for financial support through Solar Energy Research Initiative program. Mr. Sarath and Sajin’s help in XPS and SEM studies on materials and is appreciated.

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An Intelligent Solar Energy-Harvesting System For Wireless Sensor Networks

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Yin Li and Ronghua Shi School of Information Science and Engineering, Central South University, Changsha 410083, China

ABSTRACT An intelligent solar energy-harvesting system for supplying a long term and stable power is proposed. The system is comprised of a solar panel, a lithium battery, and a control circuit. Hardware, instead of software, is used for charge management of the lithium battery, which improves the reliability and stability of the system. It prefers to use the solar energy whenever the sunshine is sufficient, and the lithium battery is a complementary power supply for conditions, such as overcast, rain, and night. The system adapts Citation: Li, Y. and R. Shi, “An intelligent solar energy-harvesting system for wireless sensor networks”, EURASIP Journal on Wireless Communications and Networking (2015), https://doi.org/10.1186/s13638-015-0414-2. Copyright: © Li and Shi. 2015. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

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a maximum power point tracking (MPPT) circuit to take full advantage of solar energy, and it ensures the lithium battery an extremely long life with an appropriate charging method, which shortens the frequency of the battery charge-discharge cycle. This system can be implemented with small power equipment which is especially suitable for outdoor-based wireless sensor nodes in the Internet of Things (IOT). Keywords: Solar energy harvesting, Bimodule power supply, MPPT, WSN, Internet of Things

INTRODUCTION Wireless sensor network (WSN) is the second largest network after the Internet in the world, and it ranks as the first of the next ten emerging technologies. Currently, it has been used widely in the Internet of Things (IOT), mainly for environmental parameter monitoring in various production circumstances, such as greenhouse [1, 2], water quality monitoring [3, 4] and so on. Conventionally, disposable batteries can be used for power supply in WSN, where researchers have made efforts to save the finite battery on power control by routing algorithm and topology optimization [5, 6, 7]. On the other hand, reducing the power consumption of the nodes always sacrifices performances like computing. The most up-to-date power density of available battery technology cannot match the needs of most WSN for long lifetime and small form factor, which limits the use of WSN due to the need for large batteries. It also has a slight possibility that the better batteries for small devices will become available in the next few years. Energy harvesting and management may be the most convenient ways to solve the problem of making WSN autonomous and enable widespread use of these systems in many applications [8]. The state-of-the-art energy-storage techniques for energy-harvesting systems in sustainable wireless sensor nodes can be classified into two technologies, i.e., supercapacitors and rechargeable batteries [9]. These two categories have their own advantages and disadvantages, involving energystorage density, lifetime, discharging, leakage, size and so on [10]. Since the supercapacitors have significantly lower power density and higher leakage overhead than rechargeable batteries [11], which makes them impractical for small-package WSN nodes, we employ an energy-harvesting system using a lithium battery as the storage.

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From the electrochemical theory, we may learn that the aging of the lithium battery is influenced greatly by the self state of charge (SOC) [12, 13, 14]. Data from [15] show that lithium batteries in high SOC are more vulnerable to environmental impacts of aging and SOC cycling the batteries enhances the resistive lives. Therefore, it must be avoided that the battery always be in high SOC to extend battery lifetime, and approaches, one example of which is recharging the battery until its voltage drops below a specific level, should be taken [16]. The charging managements are usually employed by microcontrollers for the flexibility of software designing and implementation [9, 17], but researchers [18] have proven that batterycharging controlled by software may have some problems in which charging logic could not work, and that the battery could not be charged under sufficient sunlight. In our system, the charging management is implemented by hardware instead of the codes running within the microcontroller for consideration of reliability. This paper focuses on an intelligent solar energy-harvesting (ISEH) system based on maximum power point tracking (MPPT) for wireless sensor nodes used in IOT, which prefers to use the solar power and takes the lithium battery as a supplementary under the condition of inadequate illumination. To prolong the lithium battery life, an intelligent circuit using RS triggers is proposed, which makes the lithium battery charge only when the battery voltage is lower than a specific value. The circuit can be divided into two main functional parts, i.e., the charging sub-circuit and the control sub-circuit. The sub-circuits are merged into one printed circuit board (PCB), and the whole system has been designed, built, and tested. Experimental results show that the system can work stably and quite fit the requirements of pre-designing. The contributions of this paper for the ISEH systems are as follows: •

•

Charging control of the lithium battery is implemented dexterously by RS triggers, which supports a reliable and stable operational status for the system contrasting the approach executed by software. We combine the advantages of other solar-harvesting systems, which are dominated by solar power using a lithium battery as an energy storage and only when the battery voltage drops below a specific level before charging it. This architecture will help extend the life of the battery and the system, and avoid wasting the solar energy.

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The construction of this paper is organized as follows. In Sec. 1, we give an introduction of our study. A brief review on the related work is outlined in Sec. 2. Then, we present the problems in solar energy-harvesting systems and the proposed system construction in Sec. 3. In Sec. 4, the corresponding calculations of some important parameters are shown. In Sec. 5, we provide the functional module implementation of the system; simulation and experimental results are also included in this section. In Sec. 6, a conclusion of this paper is made.

RELATED WORK Self-sustainable WSN systems are on the verge of being a broad requirement in many fields [19, 20], since most of WSN applications are difficult to maintain after their deployments. Researchers make great efforts to find out renewable energy resources from the environment for WSN usage, such as solar power, wind, vibration, heat, and RF [9]. How to collect and store energy effectively from the environment has been taken more and more seriously by researchers. Sharma et al. [21] studied a sensor node with an energy-harvesting source, and a buffer was used to store the generated energy. The sensor node periodically sensed a random field and generated packets. Only when the energy was available, the packets would be transmitted; otherwise, they were stored and waited upon. They also exploited throughput optimization, i.e., to obtain energy management policies for the largest possible data rate and the minimal mean delay in the queue. In order to increase the lifespan of WSN with powering-up methods, Ramasur et al. [22] took some efforts in a wind energy harvester (WEH) model. Their WEH consisted of a wind generator and a power management unit to store and condition the generated energy. The results showed that their aero-elastic flutter generator could produce more power compared with that of the other small-scale wind generators; however, the circuit was not equipped with maximum power point tracking (MPPT) resulting in poor efficiencies of the WEH. Besides harvesting the wind power, taking full advantage of the solar power may be more convenient in WSN usage. Although solar power is time- and season-dependent, it remains as one of the best choices by adapting a power management mechanism [23]. Yi et al. [24] put forward a wireless sensor node design based on a solar energy-power supply. They gave full consideration to energy-saving principles by adapting low-power consumption devices in every module and collected the solar energy to

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provide lasting power for the system. Another low-power solar energyharvesting system for WSN was put forward in [25], where Naveen et al. employed the system in an intelligent building. They adapted a solar energy harvester instead of an alkaline battery for the sensor nodes. They used a number of solar cells connected in series and parallel to each other to scavenge energy, and they applied a set of ultracapacitors to store up the energy. As a backup energy source, alkaline batteries were connected along with the capacitors. A solar-powered sensor module using low-cost capacitors as storage buffers was investigated in [26], and only capacitors were adapted. The advantage is that the energy-charging time is shortened within a second, although the module cannot work without light illumination. A batterysupercapacitor hybrid energy-storage module was proposed in [17], and an embedded processor was used to control the charging of the battery. Supercapacitors charge the battery only when their saved energy exceeds the peak requirements of processors running at full speed and ignores estimating the SOC of the battery. Taneja [27] et al. brought forward a kind of micro photovoltaic energy system. Alberola et al. [28] put forward another solar power system consisting of a supercapacitor and a lithium battery. To provide an uninterrupted power supply, the literature [29] exploited two battery groups for energy storage. In charging lithium batteries, some literatures have been presented [30, 31, 32]. The common method is that whenever the voltage of the solar panel is high enough, charging starts. According to the research of Ecker et al. [15], Takahashi et al. [33], and Liu et al. [34], the lithium battery capacity could fade rapidly when it is in high SOC for a long term; thereby, Jiang et al. [16] and Li et al. [35] have proposed a solution that the system charges the lithium battery only when the voltage is lower than a specific value. This strategy can solve the problem of charging too often and the lithium battery being in high SOC all the time; nevertheless, it may waste most of the solar power in that this system applied the lithium battery as the primary power source no matter how capable the solar panel was to supply the system or not.

METHODOLOGY In this section, to begin with, we analyze some shortages of the existing solar energy-harvesting systems and then present the methodology and the hardware components of our system.

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Problems to be considered Energy harvesting is one of the most promising technologies toward the goal of perpetual operation of WSN. Recent developments have allowed renewable energy sources such as solar or wind power to be used for wireless sensor nodes. Concerning the usage of solar energy, a lot of scholars have conducted considerable research works, while there are still some aspects which could take more optimization. •

Making full use of the solar energy is very important. The system employs the solar power as the preferential power source as long as the sunshine is available [27, 28, 29], rather than that which takes the rechargeable battery as the primary one and applies the solar energy only for charging. • To extend the rechargeable battery life as far as possible and keep the high performance of the battery, the charging process of the battery should be taken into control, avoiding too many chargedischarge cycles or the battery will always be in a high-charge state. • Designing a simple and ingenious control circuit can reduce the complexity of system development, decrease the power consumption, and increase the stability and reliability of the system. According to the survey of the related work, many researchers provide better solutions to the first two points mentioned above; however, the last point on how to improve the stability and reliability of the circuit needs to be researched further. Common energy management and charge control are always achieved by software, while in cases of extreme discharges, the microcontroller itself may be powered off and cannot be restarted even if there is sufficient illumination, leading to the question, how could the battery be charged? In this paper, we focus on application of hardware realization of charge management, which can greatly improve the robustness of the solarharvesting system.

System Construction The ISEH system is physically composed of a solar panel, a lithium battery, and a control circuit. The control circuit comprises a solar MPPT module, a charging sub-circuit, an over-discharged protection sub-circuit, and a boost DC/DC module for the lithium battery. The system schematic diagram is shown in Fig. 1.

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Figure 1: Scheme for the ISEH System.

As shown in Fig. 1, the system has three input branches, i.e., a solar panel, a lithium battery, and a mini-USB interface. It also has an output branch offered by an ordinary USB interface. In this paper, we use “bimodule” to emphasize that the output of the system may originate from the solar panel or the lithium battery, respectively. The standard mini-USB interface is a reservation to charge the lithium battery by an external power adapter if necessary. The functional components of the system are demonstrated as follows:

Solar power supply We propose a new solution for supplying the power to the sensor nodes, contrasting with that in [28] and [31]. In those two papers, the task of the solar power is only for charging the lithium battery and the super capacitor by a DC/DC converter, which may cause part of the solar energy to be wasted. While in our work, the solar branch has the priority to provide electrical power to make full use of its energy unless overcast, rainy days, or night etc. comes. The power generated via this branch flows to the MPPT module first. The MPPT guarantees that the system can utilize the almost peak power produced by the solar panel, and the principle of MPPT are shown in Fig. 2.

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Figure 2: Power, voltage and current of the solar panel.

When the solar panel meets a light load or comes very close to opening a circuit, the output voltage may approach the highest level. The power and the current are simultaneously very small. As the load becomes heavier, there are many changes that will be made until it reaches the peak value and then gradually declines. For example, the output voltage of the solar panel being decreased, the current being increased more rapidly, and the power being increased gradually as a synthesis. The MPPT circuit helps the output voltage of the solar panel stay around the peak power area. Therefore, the system can apply it with maximum efficiency [36, 37].

Lithium battery power supply As shown in Fig. 1, a mechanical single-pole single-throw (SPST) switch (3) is located following the lithium battery, which can deter the lithium battery from wasting energy in an open circuit and keep it safe in the case of transportation or stock. After system deployment, the switch should be switched on manually. If the power provided by the solar panel cannot satisfy the load, the voltage comparator (2) drives the switch (2) to connect to port B, which disconnects the solar branch and connects the lithium branch to the system output. A protection circuit is placed right after the battery to avoid overdischarge, followed by a DC/DC boost circuit to raise the voltage from battery nominal voltage to system output voltage. Most of the values used in this paper are listed in Table 1.

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Table 1: Summary of values Voltage/V Battery nominal voltage

3.7

System output voltage

5

Average operating current

Current /mA

16

Output voltage of the solar panel

0–23

Normal voltage

5

Voltage threshold

3.6

Protection voltage

3.3

Danger boundary

3

Charging circuit In Fig. 1, the switch (1) is an electronic SPST which controls the charging status of the lithium battery. It maintains the switch-off status as default, which means that the battery is not charged. If and only if the battery voltage is underneath the predefined level, the voltage comparator (1) triggers the switch (1) on, and the battery is charged at once if the solar panel could afford it. As soon as the charging starts for a few seconds, the battery voltage rises rapidly and changes the state of the comparator (1). That is why a RS trigger is required to keep the charging status. The control chip of the charging circuit is CN3063 [38]. As charging is terminated, the pin END of the chip changes to low level, which turns the switch (1) off and disconnects the charging circuit. The switch status is also maintained by the RS trigger.

SYSTEM PARAMETERS ANALYSIS Power Consumption of Node In applications of IOT, WSN nodes are generally divided into the following three categories; the sensor nodes, the routing nodes, and the sink node. The power consumption of the sink node is usually the largest, but it can be deployed indoors or easy to approach outdoors, so it is unnecessary to consider its power supply. In terms of hardware construction, the sensor nodes usually have more sensors than the routing nodes, for example, the dissolved oxygen sensor, PH sensor etc. The total power consumption of the

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sensor group is larger than that of the sensor node itself, consumed by data computing and radio transmission, so that the power consumption of the sensor nodes is larger than that of the routing nodes. In the system design, power consumption of the sensor nodes should be taken into the overriding consideration. In our research, the sensor nodes usually include the following components: • ARM Cortex-M0 • ZigBee module • Sensors (dissolved oxygen, PH, temperature etc.) The power consumption of all the components is calculated as shown in Table 2. Vin represents the input voltage, which is the ISEH system output voltage. The notation I stands for the average current. Especially, the data given in the table are the maximum or close to it. Each of the sensor’s current consumption is set to the average operating current. The notation T stands for the working hours in one day, 2 h in a day means the duty cycle of 8.33 %, that is to say, a data monitoring period lasts for 5 min every hour. As a kind of statistical result, the power consumption of one sensor node is about 0.92 Wh every day. Table 2: Power consumption of one sensor node

Cortex-m0 ZigBee Sensors Sum

V in / V 5 5 5

I/ mA

P/ mW

T/ h

W/ Wh

4 40 48

20 200 240

2 2 2

0.04 0.4 0.48 0.92

Battery Selection Comparison of common rechargeable batteries is shown in Table 3. Numbers 1–5 represent lead-acid battery, nickel-cadmium battery, nickel-hydrogen battery, lithium-ion battery, and lithium-polymer battery, respectively.

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Table 3: Comparison of rechargeable batteries Sequence number

Voltage/V

Volumetric energy density/ Wh/L

The number of cycles

Selfdischarge rate/% M-1

Memory effect

Environmental protection

1

2.0

60 ~ 75

250 ~ 300

5 ~ 15

No

Poisonous

2

1.2

110 ~ 130

300 ~ 700

15 ~ 30

Yes

Poisonous

3

1.2

140 ~ 300

400 ~ 1000

25 ~ 35

Little

Harmfulness

4

3.7

250 ~ 360

500 ~ 1000

5 ~ 10

No

Harmfulness

5

3.7

300 ~ 460

500 ~ 1000

2~5

No

Non-poisonous

As demonstrated in Table 3, the lithium-polymer battery is relatively satisfactory for the system usage, whose capacity depends on the capacity of the load and the longest rainy days of the system-deployed region.

Calculation of Battery Capacity The calculation formula of the battery capacity (BC) is given by (1) In this expression, A - Safety factor, between 1.1–1.4 QL - The average daily power consumption of the load, Wh NL - The longest continuous rainy days, set 7 according to the experience • TO - Temperature correction factor, in general, TO = 1 when temperature is above 0 °C, and TO = 1.1 above −10 °C and TO = 1.2 below −10 °C. • cc - The depth of battery discharge, generally speaking, if it is a lead-acid battery, the value is 0.75, if it is a nickel-cadmium battery, the value is 0.85, and if it is a lithium battery, the value is 0.80. In our research, the lithium battery would not be charged until its voltage dropped under a specific level, and it should provide the system with the rest energy just in case there is no sunshine for charging, namely, even when the battery voltage is reduced to the charging boundary, it still persists that appropriate energy as a backup should be considered, so the value of cc is set to 0.6. Combined with the data from above, the battery capacity is calculated to be about 3190−4061 mAh (for the nominal voltage of a lithium-polymer battery). Since the calculation is made with safety allowance, the battery capacity can be set to 4000 mAh. • • •

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SIMULATION AND IMPLEMENTATION In this section, the performance of the proposed ISEH system is simulated and compared with two types of existing solar energy-harvesting systems [16, 30, 31, 32, 35], then we gave the design details of each module in the ISEH system. Finally, we have implemented an experimental circuit, as shown in Fig. 9, which was tested with output characteristics of the system.

Performance Simulation Based on the analysis mentioned above, simulations are taken for demonstrating the differences between the ISEH system and the other systems. The parameters used in the simulations are shown in Table 4. Table 4: Simulation parameters Lithium battery Power consumption of node Duty cycle Total working hours

4000 mAh, 3.7 V 460 mW 1 40 h, 6 h for a loop, which includes 4 h of efficient sunshine and two overcast hours

It is worth noticing that the duty cycle is set to 1, which means that the node works all the time with full power consumption, and the sunshine performs a regular pattern like a square wave, which is unreal for the actual weather condition. All the settings are adapted only for the reason of facilitating the contrast effect. Results given in Fig. 3 are a comparison between the ISEH system and the previous system which could charge when the sunlight is sufficient (CWSS), which charges the battery as long as there is sufficient illumination.

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Figure 3: Simulation result between the ISEH and CWSS system.

In Fig. 3, the green line represents the lithium battery-voltage curve of the ISEH system and the red line represents that of the CWSS system; the dashed red line parallel to the horizontal axis is the charging threshold, and the dashed black square waves represent the intensity of the sunshine. The blue curve is a reference, one that represents the continuous discharge of the lithium battery voltage without charging. As shown in the figure, the lithium battery of the ISEH system discharges only when the sunshine is insufficient, and it is charged once during the whole 40 h. Relatively, the charge-discharge cycle of the CWSS system is six times, which is far more than that of the ISEH system. At the same time, it is SOC also in a high level most of the period. As a matter of fact, the real weather condition cannot be regular like that, so the number of the cycle is usually more than that shown in this simulation, which has a great influence on the life of lithium batteries. Figure 4 shows the comparison result between the ISEH system and the other system which uses the lithium battery as the main power supply (BMPS) and charges it only when its voltage is beneath a specific level. The parameters used in the simulations are also shown in Table 4. Except for the red curve with “o” which represents the battery voltage of the BMPS system, the rest of the curves are the same as that shown in Fig. 3.

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Figure 4: Simulation result between the ISEH and BMPS system.

As shown in Fig. 4, the BMPS system makes a little improvement, because the charge-discharge cycle is less than that of the CWSS system shown in Fig. 3. But whatever the weather is, the lithium battery is the main power source, which causes a waste of the solar energy and makes the discharge rate larger than that of the ISEH system, thus the number of charge-discharge cycles is also more than that of the ISEH system.

Detailed Design According to the principle of the system composition, each function module of the system is taken into detailed circuit design. The main functional module realization is described as follows.

MPPT Module The MPPT module is based on chip MP2307 which uses an autonomous tracking strategy to provide an upmost power supply for the system. The input of the module is the output voltage of the solar panel, and the output of it is the normal voltage.

Charging Module This module is based on the charging management chip CN3063 [38] which

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is produced by the Consonance Electronic Co. Ltd. The function of this module provides an intelligent method to charge the lithium battery. The input of this module is from the MPPT circuit, and the output is connected to the lithium battery. CN3063 is a single lithium battery-charging management chip, which can be easily powered by a solar panel with a wide range of input voltage. An on-chip 8-bit ADC can adjust the charging current automatically. Its regulation voltage can be adjusted by an external resistor, and the charging current can also be programmed externally with a single resistor. The charging process of CN3063 is drawn in Fig. 5 [38].

Figure 5: Charging profile of CN3063.

The lithium battery-charging process is generally divided into the following three phases: • Pre-charge phase • Constant current phase • Constant voltage phase The first phase is carried out only when the lithium battery voltage is very low. Usually, the second and the third phase occupy most of the charging process. The constant current of charging can be regulated externally, and the continuous programmable charge current can reach 600 mA. by

In the second step, the calculation formula of charging current is given (2)

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I CH - charging current, with unit A R ISET - the resistance from the ISET pin of the CN3063 to ground, with unit Ω For example, if the system requires a charging current of 500 mA, R ISET can be calculated as follows: • •

(3) To ensure good stability and temperature characteristics, R recommends the use of a metal film resistor with an accuracy of 1 %.

ISET

When charging continues, the level of the CHRG−CHRG− pin drops down by an internal switch, which indicates that the charging is in progress; otherwise, the pin is in a high-impedance state. This pin can be used to indicate whether the battery is being charged or not by an external LED. When the charging is terminated, the level of the DONE−DONE− pin drops down by an internal switch; otherwise, the pin is in a high-impedance state. The pin’s low level can be used as the input signal to the RS trigger, and it turns the trigger over and maintains the status, which ensures that the lithium battery cannot be charged until its voltage drops below the predefined level again.

Boost Module There are some differences between the battery nominal voltage and the system output voltage; therefore, it is necessary to use a DC/DC boost circuit to make the conversion. GS1661 is a current mode boost DC/DC converter, and it is available in SOT23-6 L package and provides space-saving PCB for applications. The chip can be implemented with fewer external components. The proposed module can raise voltage from 3.3–4.2 V to the system output voltage, and the output voltage can be determined by two external resistors (4) The peak current can also be determined by an external resistor R e3, the expression is (5)

Control Module of Branch Choosing Since the ISEH system has two branches for power supply, a switching control circuit as shown in Fig. 6 is needed.

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Figure 6: Power supply circuit switching module.

LM339 is a voltage comparator (U4A), and it has two inputs, the inverted input as a reference which is provided by a three-terminal regulator (OUTPUT) and the positive-going input which is provided by the MPPT output voltage. When the positive-going input voltage is higher than the reference, LM339 provides a high-level output, which makes Q1 (9013) switch on and drives the pin G of the Mosfet (U3) down; thus, U3 breaks over, and the system selects the solar panel for power support. At the same time, pin 1 of U4A is high, which makes ST2301 (U13) and Mosfet (U6) both go off and then the output of the lithium battery is closed. On the other hand, when the positive-going input voltage is beneath the reference, the output voltage of U4A should be low, and ST2301 (U13) switches on making U6 break over. As a matter of fact, the lithium battery provides the power instead of the solar panel.

Charge Control and Over Discharge Protection To prolong the life of the lithium battery, lengthening the charge-discharge cycle as long as possible is undoubtedly a good approach. Li et al. used a microcontroller to realize the charging progress, which is convenient and makes it easy to change some strategies, and it merely seems slightly more complicated for programming [35]. In this paper, we deploy a sophisticated method using RS triggers to implement this function with less cost. Consequently, it is more stable compared with the crash possibility of the microcontroller.

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The charge control and over discharge protection circuit is shown in Fig. 7.

Figure 7: Charge control and over discharge protection.

The lifetime of the lithium battery determines the life of the entire system. Based on the premise of not affecting the system work, reducing the charge-discharge cycles of the battery is very helpful. Experimental results show that the lithium battery can work for a long time when its voltage is between 3.7–3.9 V, but if the voltage is lower than the threshold, it drops rapidly with discharge going on. So the lithium battery should be charged at once when its voltage is under the threshold. Critically, if the battery voltage is lower than the danger boundary, it may be eternally damaged, so discharge should be stopped immediately, and the system has to do something to avoid this from happening [35]. According to the circuit in Fig. 7, when the battery voltage is lower than the threshold (whose value can be adjusted by the resistor R14 and R18), the output of U4B in LM339 and the pin 2R−2R− of RS trigger (U10) are both low. Since the battery is not fully charged, the pin DONE−DONE− of CN3063 is in a high-impedance state according to its data sheet. Therefore, the pin 2S−2S− of U10 is high. Based on the function table of RS trigger as shown in Table 5, the output Q of RS trigger should be low, thus the Mosfet U9 is conducted, and the system starts to charge the battery. Once the battery voltage is higher than the threshold with continuous charging, the level of 2R−2R− changes from low to high, and 2S−2S− remains at a high level at the same time because charging has not finished

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yet. According to Table 5, the output of the RS trigger maintains the former state, and the system still continues to charge the battery. Table 5: Function table of RS Trigger Inputs S--S-L L H H

R--R-L H L H

Output Q H H L Q0

When the battery has been charged, the pin DONE−DONE− of CN3063 converts to low, and the pin 2S−2S− is also being low. Simultaneously, the pin 2R−2R− remains high, so the RS trigger outputs high levels, and the charging circuit is disconnected. The pin DONE−DONE− turns to being in a high-impedance state once again, which makes the level of 2S−2S− high, and 2R−2R− also maintains a high state, thus the RS trigger remains in a Q0 state of high level, which means the circuit is out of the charging process. RS trigger realizes an intelligent charging strategy. When the lithium battery voltage falls below the threshold, the charging circuit will switch on. Subsequently, when the battery is fully charged, the circuit will disconnect at once. It would not be turned on until the battery voltage drops beneath the threshold again. All the descriptions in time sequence are shown in Fig. 8.

Figure 8: The RS trigger sequence diagram.

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In addition, the system also has an over-discharged protection module. When the lithium battery voltage is lower than the protection (the value can be adjusted by two resistors), U12 (ST2301) is disconnected, which cuts off the lithium battery power output to protect the battery from overdischarge.

Experimental Results A circuit board for testing is made as shown in Fig. 9. The ISEH system is constructed based on the board, which could provide uninterrupted power supply for the WSN nodes. The system is tested under daylight illumination, and the experimental results show that the system can switch the power supply branch automatically and work stably. When the lithium battery voltage drops underneath the predefined level, the lithium battery can be charged as soon as possible if the illumination is sufficient.

Figure 9: Circuit board.

As shown in Table 2, the power consumption of one sensor node is 460 mW. We can get two different operating points in the conditions of solar power or lithium battery-branch energy supply, as shown in Fig. 10a. The reason causing the operating points to be different is that the output voltage of the two branches have a slight difference in the circuit. Whether the power is provided by the solar or the lithium branch, the power efficiency (as shown in Fig. 10b) is higher than 60 %. It is worth mentioning that the power efficiency is up to 80 % with the lithium battery as a power supply. From Fig. 10b we can find that the efficiency of the solar power branch is lower than that of the lithium branch, and at the same time, the former is more sensitive with the load resistance. The reasons for this phenomenon can mainly be concluded in two parts. The first one is that the voltage

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difference between input and output of the solar branch is larger than that of the battery branch, and the second is that the former components cause more power loss than that of the latter.

Figure 10: Output characteristics of the ISEH system. a Power conversion efficiency versus load resistance. b Output power versus load resistance.

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CONCLUSIONS In this paper, a novel intelligent solar energy-harvesting system is designed by using an MPPT circuit. Hardware, instead of software, is used for charging management of the lithium battery, which can enhance the robustness of the system greatly. Analyses based on power supply requirements are made for WSN nodes in IOT. The system can afford a stable power supply with 5-V output voltage through a standard USB interface. Lithium batterycharging strategy can also ingeniously avoid the charge-discharge cycle a lot, and thus the lifetime of the lithium battery can be greatly extended. Experimental results demonstrate that the system can switch the power supply branch automatically. When the voltage of lithium battery drops below the predefined level, it can be charged properly. The system performs stably and safely with high reliability, high efficiency, low-power loss, and simple composition.

ACKNOWLEDGEMENTS This work was supported by the Hunan Provincial Natural Science Foundation of China under Grant 14JJ5009, the Post Doctoral Foundation of Central South University, China, and the National Natural Science Foundation of China (61272495, 61379153, and 61401519).

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Thermal Properties of Carbon Black Aqueous Nanofluids for Solar Absorption

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Dongxiao Han, Zhaoguo Meng, Daxiong Wu, Canying Zhang and Haitao Zhu

College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China

ABSTRACT In this article, carbon black nanofluids were prepared by dispersing the pretreated carbon black powder into distilled water. The size and morphology of the nanoparticles were explored. The photothermal properties, optical properties, rheological behaviors, and thermal conductivities of the

Citation: Han, D., et al., “Thermal properties of carbon black aqueous nanofluids for solar absorption”. Nanoscale Research Letters (2011), https://doi.org/10.1186/1556-276X-6-457. Copyright: © Han et al; licensee Springer, 2011. This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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nanofluids were also investigated. The results showed that the nanofluids of high-volume fraction had better photothermal properties. Both carbon black powder and nanofluids had good absorption in the whole wavelength ranging from 200 to 2,500 nm. The nanofluids exhibited a shear thinning behavior. The shear viscosity increased with the increasing volume fraction and decreased with the increasing temperature at the same shear rate. The thermal conductivity of carbon black nanofluids increased with the increase of volume fraction and temperature. Carbon black nanofluids had good absorption ability of solar energy and can effectively enhance the solar absorption efficiency. Keywords: Nanofluids, solar absorption, carbon black, photothermal properties, rheological behaviors, thermal conductivity

INTRODUCTION The major resource of renewable energy comes from the sun. Solar energy utilization is very important in the background of global warming and reduction of carbon dioxide emission. Solar energy has been explored through solar thermal utilization, photovoltaic power generation, and so on [1, 2, 3]. Solar thermal utilization is the most popular application among them. In conventional solar thermal collectors, plates or tubes coated with a layer of selectively absorbing material are used to absorb solar energy, and then energy is carried away by working fluids in the form of heat [4, 5]. This type of collector exhibits several shortcomings, such as limitations on incident flux density and relatively high heat losses [6]. In order to overcome these drawbacks, direct solar absorption collector has been used for solar thermal utilization. In this kind of collector, solar energy is directly absorbed by the working fluids meanwhile the generated heat is carried out by the working fluids [4]. In the last century, black liquids containing millimeter to micrometersized particle were used as working fluid in solar collectors due to their excellent photothermal properties [7]. However, the applications of these suspensions are limited because of severe abrasion, sedimentation, and plug problems of coarse particles. Recently, nanofluids have been applied as working fluids in direct solar collectors [5, 8, 9, 10, 11]. Nanofluid is a new class of heat transfer fluids containing stably suspended nano-sized particles, fibers, or tubes in the conventional heat transfer fluids such as water, ethylene glycol, engine oil, etc. [12, 13, 14, 15, 16]. Several researchers have reported that nanofluids could effectively improve the solar energy utilization [4,

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17, 18]. Taylor et al. found that nanofluids had excellent potential for solar thermal power plants. Efficiency improvement on the order of 5% to 10% was possible with a nanofluid receiver [19]. Shin et al. reported that the specific heat of a high temperature nanofluid (1 wt.% silica nanoparticles in a eutectic of lithium carbonate and potassium carbonate) enhanced by 25% compared with that of the pure eutectic [20]. The results of Tyagi et al. showed that the absolute efficiencies of the Al/water nanofluid-based direct absorption solar collectors were about 10% higher than that of the conventional flat-plate type collectors using pure water under similar operating conditions [6]. Mu et al. investigated the radiative properties of SiO2/water, TiO2/water, and ZrC/water nanofluids. They found that the ZrC nanofluid had the highest solar absorbance among the studied nanofluids [5]. However, the research on the solar energy utilization of nanofluids is only in the start stage, and the relative reports are scarce at present. When nanofluids are used as working fluids of the direct solar absorbers, the thermal properties of nanofluids are critical to the solar utilization. Photothermal property is very important to the assessment of solar energy absorption of nanofluids because it directly reflects the solar absorption ability of nanofluids. Viscosity and rheological behaviors not only are essential parameters for nanofluid stability and flow behaviors but also affect the heat transfer efficiency of direct solar absorbers. Thermal conductivity is an important parameter for heat transfer fluids. It also affects the collectors’ heat transfer efficiency. Great efforts have been made to the rheological behaviors and thermal conductivities of nanofluids [21, 22, 23, 24, 25, 26, 27], and these studies are helpful to the research of nanofluids as solar absorption working fluids. However, as mentioned above, there are only a few research committed to the photothermal properties [5, 18]. Therefore, more studies are essential to the photothermal property research. Carbon black is a kind of material that has very good absorption in the whole wavelength range of sunlight [18]. Carbon black nanofluids seem to have high potentials in the application of solar utilization. However, there are only a few researches on carbon black nanofluids [28, 29, 30, 31], which mainly concern about the viscosity, dispersion stability, and tribological behavior. In this study, carbon black nanofluids were prepared by dispersing the pretreated carbon black powder into distilled water. The size and morphology of the nanoparticles were explored. The photothermal properties, optical properties, rheological behaviors, and thermal conductivities of the nanofluids were also investigated.

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EXPERIMENTS Preparation of Nanofluids Commercial carbon black powder (N115) was supplied by Qingdao Degussa Company, Qingdao, China. To obtain stable nanofluids, the original carbon black powder was pretreated as follows: 15 g of original carbon black powder and 300 ml 30% H2O2 were added into a round-bottomed flask and heated to boiling under magnetic stirring. The reaction was carried out under stirring and boiling for 5 h. Then the mixture was filtrated at room temperature and dried at 100°C. Pretreated carbon black powder was obtained by repeating the process twice. Then the pretreated carbon black powder was ground and dispersed into distilled water under ultrasonic vibration for 1 h. Carbon black nanofluids of different particle volume fractions were prepared by adjusting the amount of carbon black and water.

Characterization of Carbon Black Nanofluids The transmission electron microscopy (TEM) images were captured on a JEM-2000EX (JEOL Ltd., Tokyo, Japan) transmission electron microscope with an acceleration voltage of 160 kV. The carbon black nanofluids were diluted with distilled water and one drop was placed on a carbon-coated copper grid and left to dry at room temperature. Particle size distributions of the nanoparticles in nanofluids were measured with a Zetasizer 3000HS (Malvern, Worcestershire, UK) particle size analyzer based on dynamic light scattering technology. The samples were also prepared by diluting the nanofluids with distilled water.

Measurements of Photothermal Properties of Carbon Black Nanofluids The schematic diagram of photothermal property test equipment was shown in Figure 1. Carbon black nanofluids were sealed in quartz tubes (d = 26 mm, h = 150 mm). The tubes were placed in an insulation box. Insulation materials were put under and between the tubes. Each tube was filled with nanofluids of the same amount, so that the experimental nanofluids had the same endothermic and heat transfer area. Temperatures of the nanofluids were measured and recorded in real time with thermocouples inserted in the nanofluids. The measurements were directly carried out in the sun and performed twice and averaged. The average atmospheric temperature is 24°C.

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Figure 1: Schematic diagram of the nanofluids photothermal property test equipment. 1, thermocouple; 2, quartz tube; 3, nanofluids; 4, insulation materials; 5, data acquisition device.

Measurements of optical properties of carbon black powder and nanofluids UV-Vis-NIR spectra of pretreated carbon black powder and nanofluids were recorded on a CARY-500 spectrophotometer (MedWOW, Necosia, Cyprus) at room temperature from 200 to 2,500 nm. The carbon black powder was put on a sample stage, and the absorption spectra were detected. The carbon black nanofluids of different volume fraction were put into quartz cuvettes, and the transmittance spectra were detected.

Measurements of Rheological Behaviors of Carbon Black Nanofluids The rheological behaviors of the carbon black nanofluids were investigated on a controlled stress viscometer (Physica MCR301, Anton Paar, Graz, Austria) with a cylindrical rotor. The shear rate and temperature ranged from 15 to 110 s-1 and 25°C to 50°C, respectively. A continuous reading of shear stress and shear rate was recorded automatically when the measurement process was stabilized after the nanofluids were transferred into a measurement chamber. The cylindrical sample cell was surrounded with a constant temperature water bath. The temperature measurement accuracy was 0.01°C.

Measurements of Thermal Conductivity of Carbon Black Nanofluids The thermal conductivity was measured on a KD2 Pro Thermal Property

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Analyzer (Decagon Inc., Pullman, WA, USA) using a single-needle sensor for heating and monitoring of the temperature, which is based on the transient hot wire method. The instrument’s probe (1.3 mm in diameter and 60-mm long) was vertically immersed in the center of nanofluids. The thermal conductivity range of the probe was 0.02 to approximately 2 Wm1 -1 K . The dimensions of cylindrical sample cell were 35 mm in diameter and 70 mm in length. Each measurement took 1 min. Calibration of the probe was carried out first by measuring the thermal conductivity of pure water, ethylene glycol, and glycerol. All our measurements were performed over ten times and averaged, and the time interval between the measurements was 15 min.

RESULTS AND DISCUSSION Characterization of Typical Sample Figure 2a shows the TEM image of the carbon black nanofluids. The primary nanoparticles are about 20 nm in diameter and aggregate to short clusters. Figure 2b shows the size distributions of carbon black nanofluids. The particle size of the carbon black nanofluid is about 50 to 500 nm and has a mean size of 190 nm. The agglomeration of the nanoparticles and the hydrodynamic diameter measured by the Malvern particle size analyzer are responsible for the larger particle size [21].

Figure 2: Characterization of the typical sample. (a) TEM image, (b) size distributions.

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Photothermal Properties of Carbon Black Nanofluids Figure 3a shows the temperatures of carbon black nanofluids and pure water as a function of the solar irradiation time. Figure 3b shows the temperature enhancement of nanofluids to pure water at the same irradiation time. It can be seen that the temperatures of the nanofluids increase more quickly than that of pure water. For example, within 42 min, the temperature of the 6.6 vol.% nanofluid increases from 24.4°C to 38.4°C while that of the pure water only increases to 31.2°C (Figure 3a). This indicates that carbon black nanofluids have good solar energy adsorption properties. It is clear that the nanofluids of high-volume fraction show higher temperatures, i.e., the solar adsorption ability enhances with the volume fraction in the experimental range (Figure 3). However, the temperature of 7.7 vol.% nanofluids is close to that of 6.6 vol.% sample, indicating that the photothermal properties will not change significantly when the volume fraction is higher than 6.6 vol.%. The temperature enhancements of carbon black nanofluids were higher than that of Mu’s TiO2/water, SiO2/water, and ZrC/water nanofluids (