Textbook of Renewable Energy 9385059955, 9789385059957

Renewable energies are sources of clean, inexhaustible and increasingly competitive energy. They differ from fossil fuel

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Textbook of Renewable Energy
 9385059955, 9789385059957

Table of contents :
Content: Cover
Textbook of Renewable Energy
Contents
Preface
1 Renewable Energy: An Overview
2 Solar Radiation and its Measurement
3 Solar Thermal Energy Conservation System
4 Solar Photovoltaic Systems
5 Biogas
6 Biomass
7 Biofuels
8 Wind Energy
9 Fuel Cells
10 Tidal Energy
11 Hydrogen Energy
12 Magneto Hydro Dynamic Power Generation
13 Geothermal Energy
14 Ocean Thermal Energy Conversion
15 Renewable Energy Applications in DevelopingSmart Cities
16 Environmental Aspects of Electrical EnergyGeneration
Glossary
References
Index

Citation preview

Textbook of Renewable Energy

Contents iii

Textbook of Renewable Energy

S. C. Bhatia BE (Chemical), BITS Pilani & MBA

R. K. Gupta B.Tech (Mech.) S. D. College of Engineering and Technology, Muzaffarpur.

WOODHEAD PUBLISHING INDIA PVT LTD New Delhi

Published by Woodhead Publishing India Pvt. Ltd. Woodhead Publishing India Pvt. Ltd., 303, Vardaan House, 7/28, Ansari Road, Daryaganj, New Delhi - 110002, India www.woodheadpublishingindia.com First published 2018, Woodhead Publishing India Pvt. Ltd. © Woodhead Publishing India Pvt. Ltd., 2018 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing India Pvt. Ltd. The consent of Woodhead Publishing India Pvt. Ltd. does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing India Pvt. Ltd. for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. Woodhead Publishing India Pvt. Ltd. ISBN: 978-81-936446-0-7 Woodhead Publishing India Pvt. Ltd. e-ISBN: 978-93-85059-95-7

Contents

v

Contents

Preface

xi

1. Renewable Energy: An Overview 1.1 1.2 1.3 1.4 1.5

Introduction Energy resources Indirect participation in sustainable energy solutions Barriers to the use of renewable energy technologies Comparison of various energy sources

2. Solar Radiation and its Measurement 2.1 2.2 2.3 2.4 2.5 2.6

Introduction Solar energy as a resource Solar angles Solar radiation Terrestrial irradiation Solar radiation measurement

3. Solar Thermal Energy Conversion System 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12

Introduction Solar water heating system Solar distillation Heating of swimming pool by solar energy Solar thermal power plants Solar ponds Solar PV pumping systems Solar crop drying Solar cookers Solar air conditioning and refrigeration Solar greenhouses Solar furnace and applications

1 1 2 7 7 12 15 15 16 18 19 21 21 25 25 25 28 31 35 38 40 42 42 44 46 51

vi Textbook of Renewable Energy

4. Solar Photovoltaic Systems 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13

Introduction Solar collectors Photovoltaic systems Photovoltaic effect Photovoltaic Cells Functions of the photovoltaic cells Types of PV cell materials PV modules and arrays PV system components Types of photovoltaic system Scale of PV systems Developing PV technologies Benefits and limitations of photovoltaic systems

53 53 54 54 55 56 56 59 61 62 62 66 67 68

5. Biogas

71

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11

71 71 72 73 75 75 77 78 78 79 82

Introduction Resource availability of biogas Types of biogas plants Principles of biogas technology Raw materials for biogas processes Technological and operational solutions for biogas plants Energy use of biogas Integrated approach Upgrading of biogas to natural gas quality Photosynthesis Producer gas

6. Biomass 6.1 6.2 6.3 6.4 6.5 6.6

Introduction Biomass conversion process Biochemical processes Biomass cogeneration Classification of cogeneration systems Applications of cogeneration

87 87 87 95 97 103 103

Contents

6.7 6.8

Advantages and disadvantages of various cogeneration systems Bio-based products as renewable energy sources

7. Biofuels 7.1 7.2 7.3 7.4 7.5 7.6

Introduction Classification of biofuels Perspectives on first and second generation biofuels Biofuels threats for economy and environment Biofuels can only contribute GHG savings Bio-based products as renewable energy sources

8. Wind Energy 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12

Introduction Characteristics of wind energy Technological aspects of wind energy Components of wind turbines Applications and efficiency of wind energy Wind turbine technology Offshore wind energy Wind farm Major failures in wind turbines Wind energy powering agriculture Carbon footprint of wind energy Barriers to wind energy

9. Fuel Cells 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Introduction Principles of a fuel cells Types of fuel cells Biochemical fuel cells (BFC) Classification of losses in an actual fuel cell Energy storage of fuel cell Electrical storage: Batteries and accumulators Comparison of fuel cells with batteries and internal combustion engines

vii

104 104 105 105 105 107 109 110 110 113 113 113 114 114 115 116 119 122 123 125 131 133 137 137 138 139 153 155 155 155 157

viii Textbook of Renewable Energy

9.9 9.10 10. Tidal 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

Applications of fuel cells Advantages and disadvantages of fuel cells

158 160

Energy Introduction Physical concepts of the tidal phenomena Components of a tidal power system Types of turbines Reciprocating devices Barriers and drivers in tidal energy Technology barriers Advantages and disadvantages of tidal energy

163 163 164 168 169 171 172 172 174

11. Hydrogen Energy 11.1 Introduction 11.2 Methods of production of hydrogen 11.3 Hydrogen production from biomass gasification 11.4 Hydrogen from partial oxidation of hydrocarbons 11.5 Hydrogen from electrolysis of water 11.6 Production of hydrogen from refinery off-gas stream 11.7 Production of hydrogen from steam methane reforming 11.8 Hydrogen from coal gasification 11.9 Hydrogen from thermochemical 11.10 Hydrogen from photosynthesis

175 175 176 179 182 182 183 186 188 189 189

12. Magneto Hydro Dynamic Power Generation 12.1 Introduction 12.2 Types of MHD Generators 12.3 Types of MHD Systems 12.4 Integration of MHD with conventional thermal systems 12.5 Selection of carrier gas and seed 12.6 Advantages and disadvantages of MHD power generator

197 197 198 201 206 207 212

13. Geothermal Energy 13.1 Introduction 13.2 Generating electricity from geothermal resources 13.3 Types of geothermal plants

217 217 218 219

Contents ix

13.4 13.5 13.6 13.7 13.8 13.9 13.10 13.11

Counter measures in operation of power plants Power house equipment Cooling tower system Comparison of geothermal power plants with other plants Market development Advantages and disadvantages of geothermal power plant Environmental issues EIA of geothermal projects

224 225 229 234 235 236 237 238

14. Ocean Thermal Energy Conversion 14.1 Introduction 14.2 Operating cycles 14.3 Description of cycles 14.4 Technology of various OTEC 14.5 Ocean energy devices 14.6 Multifunctionality of OTEC 14.7 Land, shelf and floating sites 14.8 Environmental considerations 14.9 Pros and cons of ocean energy

239 239 243 243 247 248 249 251 253 253

15. Renewable Energy Applications in Developing Smart Cities 15.1 Introduction 15.2 Solar smart cities 15.3 Details of smart cities 15.4 Solar cities programme 15.5 Statewise solar radiation in India 15.6 Financial support for designing smart cities 15.7 Applications of solar energy 15.8 Solar power plants 15.9 Benefits of solar energy over electricity 15.10 IITs Madras and Kanpur to coordinate smart city research 15.11 Summary of smart cities mission

255 255 256 257 257 259 259 260 262 264 265 266

16. Environmental Aspects of Electrical Energy Generation 16.1 Introduction 16.2 Environmental impacts of electricity generation and distribution

269 269 269

x Textbook of Renewable Energy

16.3 16.4 16.5 16.6 16.7 16.8 16.9

Global environmental awareness Paris agreement Difference between Paris agreement and Kyoto protocol Climate change and impact on environment Impact of renewable energy generation on environment Environmental impacts of different technologies Energy options for Indian economy

270 271 274 275 276 277 281

Glossary

283

References

311

Index

313

Contents xi

Preface

Renewable energies are sources of clean, inexhaustible and increasingly competitive energy. They differ from fossil fuels principally in their diversity, abundance and potential for use anywhere on the planet, but above all in that they produce neither greenhouse gases – which cause climate change – nor polluting emissions. There are many forms of renewable energy. Most of these renewable energies depend in one way or another on sunlight. Wind and hydroelectric power are the direct result of differential heating of the Earth’s surface which leads to air moving about (wind) and precipitation forming as the air is lifted. Solar energy is the direct conversion of sunlight using panels or collectors. Biomass energy is stored sunlight contained in plants. Other renewable energies that do not depend on sunlight are geothermal energy, which is a result of radioactive decay in the crust combined with the original heat of accreting the Earth, and tidal energy, which is a conversion of gravitational energy. Solar is the form of energy which relies on the nuclear fusion power from the core of the Sun. This energy can be collected and converted in a few different ways. The range is from solar water heating with solar collectors or attic cooling with solar attic fans for domestic use to the complex technologies of direct conversion of sunlight to electrical energy using mirrors and boilers or photovoltaic cells. Unfortunately these are currently insufficient to fully power our modern society. Wind power is the movement of the atmosphere is driven by differences of temperature at the Earth surface due to varying temperatures of the Earth’s surface when lit by sunlight. Wind energy can be used to pump water or generate electricity, but requires extensive areal coverage to produce significant amounts of energy. Hydroelectric energy is the form energy which uses the gravitational potential of elevated water that was lifted from the oceans by sunlight. It is not strictly speaking renewable since all reservoirs eventually fill up and require very expensive excavation to become useful again. At this time, most of the available locations for hydroelectric dams are already used in the developed world. Biomass is the term for energy from plants. Energy in this form is very commonly used throughout the world. Unfortunately the most popular is the burning of trees for cooking and warmth. This process releases copious amounts of carbon dioxide gases into the atmosphere and is a major contributor to unhealthy air in many areas.

xii Textbook of Renewable Energy

Some of the more modern forms of biomass energy are methane generation and production of alcohol for automobile fuel and fueling electric power plants. Hydrogen and fuel cells are also not strictly renewable energy resources but are very abundant in availability and are very low in pollution when utilised. Hydrogen can be burned as a fuel, typically in a vehicle, with only water as the combustion product. This clean burning fuel can mean a significant reduction of pollution in cities. Or the hydrogen can be used in fuel cells, which are similar to batteries, to power an electric motor. In either case significant production of hydrogen requires abundant power. Due to the need for energy to produce the initial hydrogen gas, the result is the relocation of pollution from the cities to the power plants. There are several promising methods to produce hydrogen, such as solar power, that may alter this picture drastically. Geothermal power energy left over from the original accretion of the planet and augmented by heat from radioactive decay seeps out slowly everywhere, everyday. In certain areas the geothermal gradient (increase in temperature with depth) is high enough to exploit to generate electricity. This possibility is limited to a few locations on Earth and many technical problems exist that limit its utility. Another form of geothermal energy is Earth energy, a result of the heat storage in the Earth's surface. Soil everywhere tends to stay at a relatively constant temperature, the yearly average, and can be used with heat pumps to heat a building in winter and cool a building in summer. This form of energy can lessen the need for other power to maintain comfortable temperatures in buildings, but cannot be used to produce electricity. Other forms of energy from tides, the oceans and hot hydrogen fusion can be used to generate electricity. Each of these is discussed in some detail with the final result being that each suffers from one or another significant drawback and cannot be relied upon at this time to solve the upcoming energy crunch. This Textbook of Renewable Energy contains 16 chapters. Chapter 1 is devoted to renewable energy: An overview. Chapter 2 focuses on solar radiation and its measurement. Solar radiation, often called the solar resource, is a general term for the electromagnetic radiation emitted by the Sun. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. Chapter 3 deals with solar thermal energy conservation system. Solar thermal technologies capture the heat energy from the Sun and use it for heating and/or the production of electricity. Chapter 4 concentrates on solar photovoltaic systems which convert sunlight directly into electricity without creating any air or water pollution. Chapter 5 focuses on biogas. Biogas is a combustible mixture of gases such as methane and carbon dioxide. Biogas is produced from the process of anaerobic digestion (AD) of wet organic waste, such as cattle and pig slurries, food wastes and

Contents Preface xiii

grown wet biomass. Chapter 6 deals with biomass which is fuel that is developed from organic materials, a renewable and sustainable source of energy used to creat electricity of other forms of power. Chapter 7 concentrates on biofuels. Biofuel is defined as any fuel whose energy is obtained through a process of biological carbon fixation. Chapter 8 focuses on wind energy. Wind energy is a form of renewable energy produced through machines that use wind as their power source. Wind energy is only possible because of the Sun. Chapter 9 is devoted to fuel cells. A fuel cell is an electrochemical device that produces electricity without combustion by combining hydrogen and oxygen to produce water and heat. Chapter 10 concentrates on tidal energy which is form of hydropower that utilises large amounts of energy within the oceans tides to generate electricity. Chapter 11 focuses on hydrogen energy as a energy source. Hydrogen can be considered as a clean energy carrier similar to electricity. Chapter 12 is devoted to magneto hydro dynamic power generation (MHD). MHD power generation is a direct energy conversion, which converts the heat energy directly into electrical energy without any intermediate mechanical energy conversion. Chapter 13 deals with geothermal energy. Geothermal energy, the heat of the Earth, provides continuous, 24 hr a day, clean, sustainable energy production. Geothermal energy is defined as heat from the Earth. It is a clean, renewable resource that provides energy around the world in a variety of applications and resources. Chapter 14 focuses on ocean thermal energy conversion which is obtained from oceans, via., waves, tidal ranges, tidal and ocean currents and thermal and saline gradients. Chapter 15 concentrates on renewable energy applications in developing smart cities. A smart city is an urban development to improve the quality of life using various types of technologies and to improve the efficiency of services. Smart city applications are developed with the goal of improving the management of urban flows and allowing for real time responses to challenges. Smart city technology includes government services like transport, traffic management, energy, health care, water, innovative urban agriculture and waste management. Chapter 16 deals with environmental aspects of electrical energy generation. All energy sources have some impacts on our enevironment. However renewable technologies are considered as clean sources of energy and optimal use of these resources minimise environmental impacts, produce minimum secondary wastes and are sustainable based on current and future economic and social societal needs. Such wide coverage makes this book a treatise on the subject. Diagrams, figures, tables, glossary and index supplement the text. All topics have been covered in a cogent and lucid style to help the reader grasp the information quickly and easily.

xiv Textbook of Renewable Energy

Appreciations are extended to Mr Harinder Singh, Senior DTP operator, who drew and labelled the flow diagrams and worked long hours to bring the book on time. I am also thankful to the editorial team of Woodhead Publishing India Pvt. for their wholehearted cooperation in bringing out the book in time. It may not be wrong to hold that this Textbook of Renewable Energy is essential reading for B.Tech./M.Tech. (Mechanical/Electrical/Chemical/ Environmental/Civil Engineering). Besides students, this book will prove useful to industrialists, consultants, research scholars, innovators and policy makers in the respective fields. It has been prepared with meticulous care, aiming at making the book errorfree. Constructive suggestions are always welcome from users of this book. S. C. Bhatia R. K. Gupta

1 Renewable Energy: An Overview

1.1

Introduction

In industrial revolution era the conventional energy sources such oil, coal, and natural gas have proven to be highly effective drivers of economic progress, but at the same time emissions from such sources have damaged our environment contributing to global warming and consequent climate change. The Inter Governmental Panel on Climate Change (IPCC) has been discussing and raising the issue of containing emissions from fossil fuels and other sources through available scientific and technological options to mitigate the problems of climate change due to gaseous emissions and rising CO2 levels on account of an oil and coal-powered global economy. Use of carbon sequestration measures and clean and green renewable energy has been persistently suggested. Renewable energy sources such as biomass, wind, solar, hydropower, and geothermal can provide sustainable energy services. Switching over to renewable-based energy systems is being increasingly considered by various countries globally. With refinements in technology the feasibility and cost of solar and wind power systems have become affordable. Also with the policy interventions and technology refinements, market systems are rapidly evolving in favour of renewable energy systems. The time of cheap oil and gas is over. Mankind can survive without globalisation, financial crises and flights to the Moon or Mars but not without adequate and affordable energy availability. Energy is directly related to the most critical economic and social issues which affect sustainable development such as water supply, sanitation, mobility, food production, environmental quality, education, job creation security and peace in regional and global context. Indeed the magnitude of change needed is immense, fundamental and directly related to the energy produced and consumed nationally and internationally. In addition, it is estimated that almost two billion people worldwide lack access to modern energy resources. Current approaches to energy are non-sustainable and non renewable. Today, the world’s energy supply is largely based on fossil fuels and nuclear power. These sources of energy will not last forever and have proven to be contributors to our environmental problems. In less than three centuries since the industrial revolution, mankind has already burned roughly half of the fossil fuels that accumulated under the Earth surface over hundreds of millions of years.

2 Textbook of Renewable Energy

Nuclear power is also based on a limited resource (uranium) and the use of nuclear power creates such incalculable risks that nuclear power plants cannot be insured. After 50 years of intensive research, no single safe long-term disposal site for radioactive waste has been found. Although some of the fossil energy resources might last a little longer than predicted, especially if additional reserves are discovered, the main problem of ‘scarcity’ will remain, and this represents the greatest challenge to humanity. Renewable energy offers our planet a chance to reduce carbon emissions, clean the air, and put our civilisation on a more sustainable footing. Renewable sources of energy are an essential part of an overall strategy of sustainable development. They help reduce dependence of energy imports, thereby ensuring a sustainable supply and climate protection. Furthermore renewable energy sources can help improve the competitiveness of industries over the long run and have a positive impact on regional development and employment. Renewable energies will provide a more diversified, balanced, and stable pool of energy sources.

1.2

Energy resources

Like other natural resources, energy resources are also renewable as well as non-renewable.

1.2.1

Renewable energy resources

Renewable energy resources are mostly biomass based and are available in unlimited amount in nature since these can be renewed (i.e., regenerated in natural process) over relatively short period of time. Renewable energy sources are inexhaustible, i.e., they can be replaced after we use them and can produce energy again and again. These include, solar energy, wind energy, water energy (hydro-electrical, ocean wave and tidal energy), and geothermal energy, etc. These can reproduce themselves in nature and can be harvested continuously through a sustained proper planning and management.

1.2.2

Non-renewable (exhaustible) energy resources

Non-renewable energy resources are available in limited amount and develop over a longer period of time. As a result of unlimited use, they are likely to be exhausted one day. These include various fossil fuels, firewood (or fuelwood) obtained from forest, petroplants, plant biomass (as agricultural wastes like bagasse), animal dung – including petroleum products, coal and natural gas and nuclear energy. Nuclear energy is mainly obtained from the nuclear fission of the uranium and thorium. The global resources of fossil fuel and uranium and thorium are limited and will be eventually be depleted. Moreover, use of

Renewable Energy: An Overview 3

fossil fuels for energy has negative environmental consequences, such as air pollution, global warming, acid rains and oil spills. Thus, it has become essential to minimise the use of fossil fuels and to replace them with renewable resources.

1.2.3

Conventional sources of energy

The conventional sources of energy are generally non-renewable sources of energy, which are being used since a long time. These sources of energy are being used extensively in such a way that their known reserves have been depleted to a great extent. At the same time it is becoming increasingly difficult to discover and exploit their new deposits. It is envisaged at known deposits of petroleum in various countries will get exhausted by the few decades and coal reserves are expected to last for another hundred years. Along with the coal, petroleum and natural gas, electricity is conventional source of energy, which is playing a barometer of a nation’s economic wellbeing. Availability of abundant electricity means unrestricted growth of industries, transport and agriculture. Depending upon raw material used, various types of electricity are hydroelectricity, thermal electricity (steam, gas, oil) and nuclear electricity. Conventional sources of energy are given below: Coal

Coal is one of the most important sources of energy and is being used for various purposes such as fuel for boilers and steam engines and for generation of electricity by thermal plants. Coal has also become a precious source of production of chemicals of industrial importance coal is and will continue to be the mainstay of power generation in India. It constitutes about 70% of total commercial energy consumed in the country. Crude oil or petroleum

It is believed that petroleum has been formed over a period of millions of years through conservation of remains of plants, animals and micro-organism, living in sea, in hydrocarbon by heat, pressure and catalytic action. Petroleum is a clear fuel as compared to coal as it burns completely and leaves no residue. It is also easier to transport and use. Petroleum is largely used in transportation, operating water lifting engines, generators, etc. Natural gas

Natural gas, a fossil fuel gift from nature, is composed of methane (96%) with small amounts of propane and ethane. Natural gas deposits often accompany

4 Textbook of Renewable Energy

oil deposits or may occur independently. It is the cleanest source of energy among fossil fuels. Nuclear energy Nuclear energy can be generated by nuclear fission in which nucleus of certain isotopes with large mass number is splitted in to lighter nuclei on bombardment of neutrons in order to release a huge amount of energy through a chain reaction or by nuclear fusion in which two isotopes of light elements are forced to form a heavier nucleus releasing enormous energy in the process. The heat energy produced as a result of either of the process is used to produce steam which runs electric turbine and generates electricity. The process of nuclear fusion is difficult to initiate but releases more energy then nuclear fission.

1.2.4

Non-conventional sources of energy

The increasing use of fossil fuels is leading to its shortage. It is estimated that if the present rate of consumption continues, the reserves of these fuel will get exhausted. Moreover, their use also causes environmental pollution. Therefore, there is need for using non-conventional sources such as solar, wind, tides, geothermal heat, and biomass including farm and animal waste as well as human excreta. All these sources are renewable or inexhaustible and do not cause environmental pollution. More over, they do not require heavy expenditure. They are capable of solving the twin problems of energy supply in decentralised manner and helping in sustaining cleaner environment. Non-conventional sources of energy are discussed below: Bioenergy

This is a type of renewable energy derived from biomass to create heat and electricity (or to produce liquid fuels used for transportation, like ethanol and biodiesel). Biomass refers to any organic matter coming from recently living plants or animals. Even though bioenergy generates about the same amount of carbon dioxide as fossil fuels, the replacement plants grown as biomass remove an equal amount of CO2 from the atmosphere, keeping the environmental impact relatively neutral. There are a variety of systems used to generate this type of electricity, ranging from directly burning biomass to capturing and using methane gas produced by the natural decomposition of organic material. Depending on the operation, there are many ways to incorporate bioenergy into sustainable energy plans: 1. Organisations can convert to fleet vehicles that use biofuels such as ethanol or biodiesel. 2. Manufacturing facilities can be equipped to burn biomass directly, producing steam captured by a turbine to generate electricity. In some

Renewable Energy: An Overview 5

cases, this process can power the facility as well as heating it. For example, paper mills can use wood waste to produce electricity and steam for heating. 3. Farm operations can convert waste from livestock into electricity using small, modular systems. 4. Towns can tap the methane gas created by the anaerobic digestion of organic waste in landfills and use it as fuel for generating electricity. Solar energy

Except for geothermal and hydrogen, the Sun plays a significant role in each of the other types of renewable energy listed here. The most direct use of this renewable energy source, however, is achieved by capturing the Sun energy directly. A variety of solar energy technologies are used to convert the Sun energy and light into heat, illumination, hot water, electricity and (paradoxically) cooling systems for businesses and industry. Photovoltaic (PV) systems use solar cells to convert sunlight into electricity. Solar hot water systems can be used to heat buildings by circulating water through flat-plate solar collectors. The Sun heat can be concentrated by mirrorcovered dishes that are focused to boil water in a conventional steam generator to produce electricity. Commercial and industrial buildings can also leverage the Sun power for larger-scale needs such as ventilation, heating and cooling. Finally, thoughtful architectural designs can passively take advantage of the Sun as a source of light and heating/cooling. Wind energy

Wind can be considered a form of solar energy because winds are caused by the uneven heating and cooling of the atmosphere by the Sun (as well as the rotation of the Earth and other topographical factors). Wind flow can be captured by turbines and converted into electricity. On a smaller scale, windmills are still used today to pump water on farms. Wind is one of the sustainability ideas for business that can be incorporated to cut business electricity costs. Commercial grade wind-powered generating systems are available to meet the renewable energy needs of many organisations: 1. Single wind turbines generate electricity as a supplement to an organisations existing electrical supply (when the wind blows, power generated by the system goes to offset the need for utility-supplied electricity). 2. Utility-scale wind farms generate electricity that can be purchased on the wholesale power market, either contractually or through a competitive bid process.

6 Textbook of Renewable Energy

Hydrogen

Hydrogen is the simplest (comprised of one proton and one electron) and most abundant element in the universe, yet it does not occur naturally as a gas on Earth. Instead, it is found in organic compounds (hydrocarbons such as gasoline, natural gas, methanol and propane) and water (H2O). Hydrogen can also be produced under certain conditions by some algae and bacteria using sunlight as an energy source. Hydrogen is high in energy, yet produces little or no pollution when burned. Hydrogen fuel cells convert the potential chemical energy of hydrogen into electricity, with pure water and heat as the only byproducts. However, practical and widespread commercialisation of these fuel cells will likely be limited until costs come down and durability improves. Other practical applications for this type of renewable energy include: 1. Large fuel cells providing emergency electricity for buildings and remote locations. 2. Marine vessels powered by hydrogen fuel cells. Ocean energy

There are two types of energy that can be produced by the ocean: thermal energy from the Sun heat and mechanical energy from the motion of tides and waves. Ocean thermal energy can be converted into electricity using a few different systems that rely on warm surface water temperatures. Ocean mechanical energy harnesses the ebbs and flows of tides caused by the rotation of the Earth and the gravitational influence of the Moon. Energy from winddriven waves can also be converted and used to cut business electricity costs. Practical uses for energy derived from the ocean include the following: 1. Cold ocean water from deep below the surface can be used to cool buildings (with desalinated water as a common by-product). 2. Seaside communities can employ the methods to tap natural ocean energy described above to supplement municipal power and energy needs. Geothermal energy

Geothermal energy, as the name implies, is derived from the heat of the Earth itself. This heat can be sourced close to the surface or from heated rock and reservoirs of hot water miles beneath our feet. Geothermal power plants harness these heat sources to generate electricity. On a much smaller scale, a geothermal heat pump system can leverage the constant temperature of the ground just ten feet under the surface to help supply heat to a nearby building in the winter, or help cool it in the summer. Geothermal energy can be part of a commercial utility energy solution on a large scale, or be part of a sustainable business practice on a local level.

Renewable Energy: An Overview 7

Direct use of geothermal energy may include: 1. Heating office buildings or manufacturing plants. 2. Helping to grow greenhouse plants. 3. Heating water at fish farms. 4. Aiding with various industrial processes (e.g., pasteurising milk).

1.3

Indirect participation in sustainable energy solutions

There is another way for organisation to embrace corporate social responsibility and invest in renewable energy that does not require the construction or maintenance of any equipment. Renewable Energy Certificates (RECs) are tradable, non-tangible energy commodities that confirm electricity was generated by a renewable energy resource and fed into a shared power grid. A certifying agency assigns a unique identification number to each REC produced by a green energy provider. The REC can then be sold on the open market. Electric utilities, businesses and public entities can purchase these certificates to fulfill clean energy regulatory requirements or to otherwise reduce their environmental impact. RECs allow buyers to support renewable energy initiatives while also allowing market forces to spur the further development of green energy. Depending on where you are located, you may also be able to purchase renewable energy directly from an offsite power generating facility.

1.4

Barriers to the use of renewable energy technologies

Renewable energy technologies have an enormous potential all over the world and that potential can be realised at a reasonable cost. Market research shows that many customers will purchase renewable power even if it costs somewhat more than conventional power. However, both economic theory and experience point to significant market barriers and market failures that will limit the development of renewables unless special policy measures are enacted to encourage that development. These hurdles can be grouped into four categories: 1. Commercialisation barriers faced by new technologies competing with mature technologies. 2. Price distortions from existing subsidies and unequal tax burdens between renewables and other energy sources. 3. Failure of the market to value the public benefits of renewables. 4. Market barriers such as inadequate information, lack of access to capital, ‘split incentives’ between building owners and tenants, and high transaction costs for making small purchases.

8 Textbook of Renewable Energy

1.4.1

Commercialisation barriers

To compete against mature fossil fuel and nuclear technologies renewables must overcome two major barriers to commercialisation: Undeveloped infrastructure and lack of economies of scale. Infrastructure

Developing new renewable resources will require large initial investments to build infrastructure. These investments increase the cost of providing renewable electricity, especially during early years. Examples include: Prospecting: Developers must find publicly acceptable sites with good resources and with access to transmission lines. Potential wind sites can require several years of monitoring to determine whether they are suitable. Permitting: Permitting issues for conventional energy technologies are generally well understood, and the process and standards for review are well defined. In contrast, renewables often involve new types of issues and ecosystem impacts. Marketing: In the past, individuals had no choices about the sources of their electricity. But electricity deregulation has opened the market so that customers have a variety of choices. Start-up companies must communicate the benefits of renewables to customers in order to persuade them to switch from traditional sources. Public education will be a critical part of a fully functioning market if renewables are to succeed. Installation, operation, and maintenance: Workers must be trained to install, operate, and maintain new technologies, as well as to grow and transport biomass fuels. Some renewables need operating experience in regional climate conditions before performance can be optimised. For example, the optimal spacing of wind turbines is likely to be different on New England ridgelines than on agricultural land in the Midwest. Economies of scale

Most renewable energy technologies are manufactured on assembly lines, where mass production can greatly reduce costs. Economies of scale are also likely to lead to cost reductions for wind, fuel cell, and biomass technologies. Unfortunately, as long as relatively few units are produced, prices will remain high. This leads to low demand, and therefore low production volumes. This chicken-and-egg problem is especially difficult with technologies that have long lives. However, scaling up manufacturing of new technologies too quickly can create its own problems, such as shortages of skilled labour and bottlenecks in parts supplies.

Renewable Energy: An Overview 9

1.4.2

Unequal government subsidies and taxes

Compared with renewables, nuclear and fossil fuel technologies enjoy a considerable advantage in government subsidies for research and development. In addition to receiving subsidies for research and development, conventional generating technologies have a lower tax burden. Fuel expenditures can be deducted from taxable income, but few renewables benefit from this deduction, since most do not use market supplied fuels. Income and property taxes are higher for renewables, which require large capital investments but have low fuel and operating expenses.

1.4.3

Market failure to value public benefits of renewables

Many of the benefits of renewables are public benefits that accrue to everyone—what economists call ‘public goods.’ For example, those who choose renewables reduce pollution for everyone and provide an environmental benefit to the public at large. A customer who is willing to pay more for electricity from renewables still has to breathe the same air as the neighbour who might choose not to pay more. Public goods do not motivate everyone who benefits to pay for them, if they can choose to be ‘free riders’ who benefit from the contributions of others. Employment, fuel diversity, price stability, and other indirect economic benefits of renewables also accrue to society as a whole. For example, for a large industrial customer, it may make more sense to risk moving to another region in response to increases in fuel prices rather than pay more for renewables to stabilise regional prices. While this strategy may benefit the individual firm, it is likely to hurt the region’s longterm economic competitiveness. In the same way, firms that can pass on increase in energy costs to customers may also lack an incentive to diversify fuel sources, even though investment in renewables would stabilise prices over the longer term. Research and development that produces societal benefits, but has little effect on a company bottom line, will be especially undervalued in restructured markets. Although R&D is likely to continue in a competitive electricity industry, and the desire to provide customer choice is likely to accelerate some innovations, research will probably shift to those areas with the fastest payback and those that allow companies to beat out competitors in the short term. Private funding is likely to dwindle for research with benefits that are primarily public or that do not result in a relatively quick payback, primarily to the under. Some research indicates that people will be willing to pay more for public benefits than economic theory would suggest. But investment in technologies where much of the payback does not accrue to the individual making the

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investment will always be less than the optimal investment for society. Twothirds of electricity produced is used by commercial and industrial customers. While some of these customers may also pay more for cleaner electricity sources, many will not. For these reasons, renewables will be unable to compete on a level playing field with conventional generation until new policies are adopted to internalise the public costs of these fossil fuel sources. Emission fees or caps on total pollution, with tradable emission permits, are examples of ways to internalise the costs of pollution, creating a more level arena for renewables.

1.4.4

Market barriers

Renewable energy technologies face considerable barriers in market transactions. Lack of information

Customers may have insufficient information to make informed choices. Most utilities provide little or no information about their emissions or the fuels they use. Because renewable technologies are relatively new, most customers know little about them. Many customers, for example, may think that solar and wind technologies are unreliable because they are available only when the Sun is shining or the wind is blowing. They are unlikely to be aware that these intermittent technologies can be highly reliable when combined with other options. Institutional barriers

Commercial and industrial customers are also generally unfamiliar with renewables and have institutional barriers to purchasing renewables. Industrial energy managers are trained only to find low-cost solutions. Industrial environmental managers look for ways to reduce in-house pollution and are unlikely to consider pollution associated with their electricity purchases. Even local electricity companies may be unfamiliar with renewables. Most utilities have not studied how renewable resources could fit into their systems or what local resources are available. For example, few have investigated how the output of solar and wind technologies matches their system peak load. Small size

Renewables projects and companies are generally small. Thus they have fewer resources than large generation companies or integrated utilities. These small companies are less able to communicate directly with large numbers of customers. They will have less clout negotiating favourable terms with larger market players. And they are less able to participate in regulatory or legislative proceedings, or in industry forums defining new electricity market rules.

Renewable Energy: An Overview 11

High transaction costs

Small projects have high transaction costs at many stages of the development cycle. For example, it costs more for financial institutions to evaluate the credit worthiness of many small projects than of one large project. It costs marketers more to negotiate contracts with many small projects, and to market to and sign up residential customers, who are the most likely segment to pay more for renewables. High financing costs

Renewables developers and customers may have difficulty obtaining financing at rates as low as may be available for conventional energy facilities. In addition to having higher transaction costs, financial institutions are generally unfamiliar with the new technologies and likely to perceive them as risky, so that they may lend money at higher rates. High financing costs are especially significant to the competitive position of renewables, since renewables generally require higher initial investments than fossil fuel plants, even though they have lower operating costs. Split incentives

When renewables are used locally to provide power to individual buildings and businesses through photovoltaics, fuel cells, or small wind turbines, they encounter additional market barriers. Landlords own some of the most costeffective building sites, but are unlikely to install equipment just so tenants can realise energy savings. And tenants may not have the right to modify the property or the interest in making a long-term investment. Few utilities consider the full value of distributed generating technologies. A small renewable energy system located in a neighbourhood with growing electricity use can help avoid investments to upgrade transmission or distribution lines to the neighbourhood. But utility generation planning departments generally consider only the cost of generating electricity with a distributed technology, not the potential savings in transmission and distribution costs. Transmission and distribution planners consider only the costs of alternative transmission and distribution technologies. Because planning is done in separate departments, no one looks at the potential integrated value of a solar module in avoiding all three: generation and transmission and distribution expenditures. Renewable technologies are sometimes cost-effective when this integrated value is considered. In a restructured industry where distribution, transmission, and generation are all in separate companies, planning for distributed generation may be even less likely than previously, unless policy makers provide significant incentives.

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Transmission costs

Renewables may also be charged higher transmission costs than conventional technologies or may be subject to other discriminatory grid policies. For example, a system that requires generators to reserve a block of capacity in advance may force an intermittent generator, like solar or wind, to pay for the maximum output they can generate at any moment. Most of the time, however, an intermittent resource generates at less than its maximum potential capacity. Since a wind farm produces, on average, only about a third of the time, it could have to pay three times more per kilowatt hour transmitted than a conventional plant designed to generate at full capacity all the time. Another problem is predicting the exact time and quantity of power for delivery, since wind speeds or sunshine can be difficult to predict more than a day or two in advance. Remotely located renewable resources may also have to pay heavily in transmission pricing schemes that charge according to distance or in those that charge ‘pancaked’ rates, which depend on the number of utility territories crossed. Green market limits

Given the numerous barriers facing renewables in the competitive market, how big the green electricity market is or could become is uncertain. Some initial signs are encouraging; others are less so. Survey after survey shows strong customer preference for green electricity. Market research also shows distinct market segments of customers interested in buying environmentally preferable products generally. Green markets for other products—including food, paper, cleaners, clothing, computers, furniture, and homes—are also emerging.

1.5

Comparison of various energy sources

A very brief summary of advantages and disadvantages of different types of energy sources are discussed in Table 1.1. Table 1.1: Advantages and disadvantages of various energy sources. Energy source Type of energy source

Advantages

Disadvantages

Coal

Conventional Non-renewable

Polluting source Bulky to transport

Oil and natural gas

Conventional Non-renewable

Extensively available Efficient conversion to electricity Easier to transport (tankers) Basis of petrochemical industry

Depletion of oxygen due to oil spillage and gas leakage Pollutants released (Cont’d…)

Renewable Energy: An Overview 13 Energy source Type of energy source

Fire wood

Conventional Non-renewable

Advantages

Disadvantages

Easier to transport (Pipelines) Cleaner than oil and coal Cheaper than oil Easy access

causes acid rain Exploration of new fuel is not easy

Provides energy to a large number of people

Hydro-power

Conventional Renewable

Nuclear energy Conventional Non-renewable

Non-polluting Promotes irrigation and fishing Cheap Emits large amount of energy

Solar energy

Non-conventional Renewable

Inexhaustible Non-polluting

Wind energy

Non-conventional Renewable

Non-polluting Low cost production of electricity once setup Safe and clean

Tidal energy

Non-conventional Renewable Non-conventional Renewable

Non-polluting Inexhaustible Clean eco-friendly and always available

Non-conventional Renewable

Low cost Easy to operate Make use of bio waste

Geothermal energy Biogas

Collection is time consuming Polluting Promoting green house effect Deforestation Displacement of local community Inundates low Expensive to setup Generates radioactive waste Expensive Expensive Diffused source, so gets wasted Noise pollution Wind mills costly to setup Disturbs radio and TV reception Harmful to birds Destroys wildlife habitat Difficult to harness Located far away from cities and so costly to transport the electricity Causes green house effect

2 Solar Radiation and its Measurement

2.1

Introduction

The Sun is a gaseous body composed mostly of hydrogen. Gravity causes intense pressure and heat at the core initiating nuclear fusing reactions. This means that atoms of lighter elements are combined into atoms of heavier elements, which releases enormous quantities of energy. Solar radiation, often called the solar resource, is a general term for the electromagnetic radiation emitted by the Sun. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. However, the technical feasibility and economical operation of these technologies at a specific location depends on the available solar resource. Every location on Earth receives sunlight at least part of the year. Because the Earth is round, the Sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). When the Sun rays are vertical, the Earth surface gets all the energy possible. The more slanted the Sun rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high Sun, and because of the tilted axis of rotation, these areas receive no Sun at all during part of the year. The Earth revolves around the Sun in an elliptical orbit and is closer to the Sun during part of the year. When the Sun is nearer the Earth, the Earth surface receives a little more solar energy. The Earth is nearer the Sun when it is summer in the southern hemisphere and winter in the northern hemisphere. However, the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference. The 23.5° tilt in the Earth axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other 6 months. Days and nights are both exactly 12 h long on the equinoxes, which occur each year on or around March 23 and September 22. Atmospheric effects: Solar radiation is absorbed, scattered and reflected by components of the atmosphere. Atmospheric effects of solar radiation are shown in Fig. 2.1.

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Atmospheric scattering

Absorbed Reflected Direct Diffuse Ground reflected

Figure 2.1: Atmospheric effects of solar radiation.

The amount of radiation reaching the Earth is less than what entered the top of the atmosphere. We classify it in two categories: 1. Direct radiation: Radiation from the Sun that reaches the Earth without scattering. 2. Diffuse radiation: Radiation that is scattered by the atmosphere and clouds. Air mass: Represents how much atmosphere the solar radiation has to pass through before reaching Earth surface pass the Earth surface. We are specifically concerned with terrestrial solar radiation – that is, the solar radiation reaching the surface of the Earth.

2.2

Solar energy as a resource

1. Basically all the forms of energy in the world as we know it are solar in origin. 2. Oil, coal, natural gas and woods were originally produced by photosynthetic processes, followed by complex chemical reactions in which

Solar Radiation and its Measurement 17

decaying vegetation was subjected to very high temperatures and pressures over a long period of time. 3. Even the wind and tide energy have a solar origin since they are caused by differences in temperature in various regions of the Earth.

2.2.1

Relation between the Earth and the Sun

1. It is very important to be able to know the exact position of the Sun on the sky (Fig. 2.2). 2. As we look from the Earth the Sun rotates around its axis once every four weeks. 9

Diameter = 1.39 × 10 m 7

Diameter = 1.27 × 10 m Sun

Earth

Angle = 32′

11

Distance = 1.496 × 10 m

Figure 2.2: Position of Sun and Earth.

2.2.2

Reckoning of time

1. In solar energy calculations, apparent solar time (AST) must be used to express the time of the day. 2. Apparent solar time is based on the apparent angular motion of the Sun across the sky. 3. The time when the Sun crosses the meridian of the observer is the local solar noon. This is the highest point the Sun can get in a day. 4. It usually does not coincide with the clock time of the locality. 5. It is necessary to convert local standard time (LST) to apparent solar time (AST) by applying two corrections: (a) equation of time and (b) longitude correction.

2.2.3

Equation of time

1. The Earth orbital velocity varies throughout the year so the apparent solar time varies slightly from the mean time kept by a clock running at a uniform rate. 2. The variation is called the equation of time (ET).

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3. The values of the equation of time as a function of the day of the year (N) can be obtained approximately from the following equation: ET = 9.87 sin(2B)-7.53 cos(B)-1.5 sin(B) [min] where, B = 360/364 (N – 81).

2.2.4

Apparent solar time

1. For the solar energy calculations Apparent Solar Time (AST) to specify the time of day must be used. 2. It is based on the relative motion of the Sun in the sky. 3. The time when Sun passes through meridian is 12.00 (solar noon). 4. At this time the Sun faces exactly south. 5. The general equation for calculating the apparent solar time (AST) is: AST = LST + ET ± 4 (SL – LL) where, LST = Local standard time. ET = Equation of time. SL = Standard longitude. LL = Local longitude. 6. If a location is east of Greenwich the sign is minus (–) and if it is west the sign is plus (+). 7. If a daylight saving time is used this must be subtracted from the local standard time. 8. For the locality of Germany the standard longitude (SL) is 15°E. If we consider a city which is at a local longitude (LL) of 17.3 east of Greenwich, the correction is: –4 × (15–17.3) = + 9.2 min. 9. Therefore AST equation can be written as: AST = LST + ET + 9.2 [min]

2.3

Solar angles

1. The Sun position in the sky changes from day to day and from hour to hour. 2. It is common knowledge that the Sun is higher in the sky in the summer than in winter. The relative motions of the Sun and Earth are not simple, but they are systematic and thus predictable. 3. Once a year the Earth moves around the Sun in an orbit that is elliptical in shape. 4. As the Earth makes its yearly revolution around the Sun it rotates every 24 hours about its axis which is tilted at an angle of 23 degrees 27.14 min (23.45°) to the plane of the elliptic which contains the Earth orbital plane and the Sun equator.

Solar Radiation and its Measurement 19

2.3.1

Solar geometry

1. The Earth rotates around its axis. 2. This axis is at a declination angle δ = 23.45° at the optical plane around the Sun (Fig. 2.3). 3. The movement around the Sun is elliptical.

δ

Earth

Sun

Figure 2.3: Axis of declination angle at optical plane around the Sun.

2.4

Solar radiation

1. All substances, solid bodies as well as liquids and gases above the absolute zero temperature, emit energy in the form of electromagnetic waves. 2. The radiation which is important to us is that emitted by the Sun which lies within the ultraviolet, visible and infrared regions. 3. The radiation wavelength which is important to solar energy application is between 0.15 and 3.0 µm. 4. The wavelengths in the visible region lie between 0.38 and 0.72 µm. The Sun is the central star of the solar system in which the Earth is. It has a form of a large glowing ball of gas, the chemical composition of mostly hydrogen and helium, but also other elements that are in it to a lesser extent, like oxygen, carbon, iron, neon, nitrogen, silicon, magnesium and sulphur. Energy from the Sun comes to the Earth in the form of solar radiation. Nuclear reactions take place in the interior of the Sun, during which hydrogen is transformed into helium by a fusion process, accompanied by the release of large amounts of energy, where the temperature reaches 15 million °C. Part of this energy comes to Earth in form of heat and light, and allows all processes, from photosynthesis to the production of electricity in photovoltaic systems. Under optimal conditions, the Earth surface can obtain 1.000 W/m2, while the actual value depends on the location, i.e., latitude, climatological location parameters such as frequency of cloud cover and haze, air pressure, etc.

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Considering the sunlight and the productivity of photovoltaic systems, it is necessary to understand the following concepts: 1. Irradiation, average density of the radiant solar radiation power, which is equal to the ratio of the solar radiation power and surface of the plane perpendicular to the direction of this radiation (W/m2). 2. Radiation, which represents the quantity of solar radiation that is radiated on the unit surface at a given time (Wh/m2) or (J/m2). Besides expressing it in hourly values, it is often expressed as daily, monthly or yearly radiation, depending on the time interval. The solar radiation weakens on its way through the Earth atmosphere due to the interaction with gases and vapours in the atmosphere and arrives at the Earth surface as direct and diffused. Direct sunlight comes directly from the Sun, while scattered or diffused radiation reaches the Earth from all directions. Considering direct and diffused radiation on a flat surface, we are talking about the total radiation. In case of an inclined surface, the rejected or reflected radiation has to be added to the direct and diffused radiation. Rejected radiation can be reflected from the ground or water. The largest component of solar radiation is direct, and the maximum radiation should be on a surface perpendicular to the direction of the Sun rays. The greatest radiation at any given moment is only possible if the plane is constantly referred to the movement of the Sun in the sky.

2.4.1

Solar constant

1. The amount of solar energy received per unit time per unit area at the mean distance of the Earth from the Sun on a surface normal to the Sun is called the solar constant Gsc. 2. This quantity is difficult to measure from the surface of the Earth because of the effect of the atmosphere. 3. When the Sun is closest to the Earth, on December 21 the solar heat on the outer edge of the Earth atmosphere is about 1400 W/m2 and when the Sun is farthest away on June 21 is about 1310 W/m2.

2.4.2

Solar radiation on Earth surface

1. The solar heat reaching the Earth surface is reduced below Gon because a large part of it is scattered, reflected back out into space and absorbed by the atmosphere. 2. The scattered radiation is termed diffuse or sky radiation. 3. The solar heat that comes directly through the atmosphere is termed direct or beam radiation.

Solar Radiation and its Measurement 21

4. The solar heat at any point on Earth depends on: (a) The ozone layer thickness. (b) The distance travelled through the atmosphere to reach that point. (c) The amount of haze in air (dust particles, water vapour, etc.). (d) The extent of the cloud cover.

2.5

Terrestrial irradiation

1. The relation between the diffuse and total radiation based on the monthly average clearness index K T is (empirical relation): 2 3 HD = 1.390 – 4.027 K T + 5.531 K T – 3.108 K T H

where, K T = H H0 2. H monthly average total insolation on a terrestrial horizontal surface (MJ/m2/day). 3. H0 monthly average daily total insolation on an extraterrestrial horizontal surface (MJ/m2) – estimated from the relation for a particular day of the month where solar radiation is equal to the mean monthly value.

2.6

Solar radiation measurement

The fraction of the energy flux emitted by the Sun and intercepted by the Earth is characterised by the solar constant. The solar constant is defined as essentially the measure of the solar energy flux density perpendicular to the ray direction per unit are per unit time. It is most precisely measured by satellites outside the Earth atmosphere. The solar constant is currently estimated at 1367 W/m2. This number actually varies by 3% because the orbit of the Earth is elliptical, and the distance from the Sun varies over the course of the year. Some small variation of the solar constant is also possible due to changes in Sun luminosity. This measured value includes all types of radiation, a substantial fraction of which is lost as the light passes through the atmosphere. As the solar radiation passes through the atmosphere, it gets absorbed, scattered, reflected, or transmitted. All these processes result in reduction of the energy flux density. Actually, the solar flux density is reduced by about 30% compared to extraterrestrial radiation flux on a sunny day and is reduced by as much as 90% on a cloudy day.

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The following main losses should be noted: 1. Absorbed by particles and molecules in the atmosphere is 10–30%. 2. Reflected and scattered back to space is 2–11%. 3. Scattered to Earth (direct radiation becomes diffuse) is 5–26%. As a result, the direct radiation reaching the Earth surface (or a device installed on the Earth surface) never exceeds 83% of the original extraterrestrial energy flux. This radiation that comes directly from the solar disk is defined as beam radiation. The scattered and reflected radiation that is sent to the Earth surface from all directions (reflected from other bodies, molecules, particles, droplets, etc.), is defined as diffuse radiation. The sum of the beam and diffuse components is defined as total (or global) radiation. It is important for us to differentiate between the beam radiation and diffuse radiation when talking about solar concentration in this section, because the beam radiation can be concentrated, while the diffuse radiation, in many cases, cannot. Short-wave radiation, in the wavelength range from 0.3 to 3 µm, comes directly from the Sun. It includes both beam and diffuse components. Long-wave radiation, with wavelength 3 µm or longer, originates from the sources at near-ambient temperatures - atmosphere, Earth surface, light collectors, other bodies. The solar radiation reaching the Earth is highly variable and depends on the state of the atmosphere at a specific locale. Two atmospheric processes can significantly affect the incident irradiation: scattering and absorption. Scattering is caused by interaction of the radiation with molecules, water, and dust particles in the air. How much light is scattered depends on the number of particles in the atmosphere, particle size, and the total air mass the radiation comes through. Different types of radiation at the Earth surface is shown in Fig. 2.4.

Beam solar

Diffuse solar

Reflected solar

Long wave sky

Reflected sky

Long wave surface

Figure 2.4: Different types of radiation at the Earth surface.

Solar Radiation and its Measurement 23

Absorption occurs upon interaction of the radiation with certain molecules, such as ozone (absorption of short-wave radiation - ultraviolet), water vapour and carbon dioxide (absorption of long-wave radiation - infrared). Due to these processes, out of the whole spectrum of solar radiation, only a small portion reaches the Earth surface. Thus most of X-rays and other shortwave radiation is absorbed by atmospheric components in the ionosphere, ultraviolet is absorbed by ozone, and not-so abundant long-wave radiation is absorbed by CO2. As a result, the main wavelength range to be considered for solar applications is from 0.29 to 2.5 m. The amount of solar radiation on the Earth surface can be instrumentally measured, and precise measurements are important for providing background solar data for solar energy conversion applications. There are two important types of instruments to measure solar radiation: 1. Pyrheliometer is used to measure direct beam radiation at normal incidence. There are different types of pyrheliometers. According to Duffie and Beckman, Abbot silver disc pyrheliometer and Angstrom compensation pyrheliometer are important primary standard instruments. Eppley normal incidence pyrheliometer (NIP) is a common instrument used for practical measurements in the US, and Kipp and Zonen actinometer is widely used in Europe. Both of these instruments are calibrated against the primary standard methods. Based on their design, the above listed instruments measure the beam radiation coming from the Sun and a small portion of the sky around the Sun. Based on the experimental studies involving various pyrheliometer design, the contribution of the circumsolar sky to the beam is relatively negligible on a sunny day with clear skies. However, a hazy sky or a uniform thin cloud cover redistributes the radiation so that contribution of the circumsolar sky to the measurement may become more significant. 2. Pyranometer is used to measure total hemispherical radiation - beam plus diffuse - on a horizontal surface. If shaded, a pyranometer measures diffuse radiation. Most of solar resource data come from pyranometers. The total irradiance (W/m2) measured on a horizontal surface by a pyranometer is expressed as follows: Itot = Ibeam cosθ + Idiffuse where, θ is the zenith angle (i.e., angle between the incident ray and the normal to the horizontal instrument plane. Examples of pyranometers are Eppley 180° or Eppley black-and-white pyranometers in the US and Moll-Gorczynsky pyranometer in Europe. These instruments are usually calibrated against standard pyrheliometers. There are pyranometers with thermocouple detectors and with photovoltaic

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detectors. The detectors ideally should be independent on the wavelength of the solar spectrum and angle of incidence. Pyranometers are also used to measure solar radiation on inclined surfaces, which is important for estimating input to collectors. Calibration of pyranometers depends on the inclination angle, so experimental data are needed to interpret the measurements. 3. Photoelectric sunshine recorder. The natural solar radiation is notoriously intermittent and varying in intensity. The most potent radiation that creates the highest potential for concentration and conversion is the bright sunshine, which has a large beam component. The duration of the bright sunshine at a locale is measured, for example, by a photoelectric sunshine recorder. The device has two selenium photovoltaic cells, one of which is shaded, and the other is exposed to the available solar radiation. When there is no beam radiation, the signal output from both cells is similar, while in bright sunshine, signal difference between the two cells is maximised. This technique can be used to monitor the bright sunshine hours. Solar radiation data collected through the above-mentioned instrumental methods provide the basis for development of any solar projects. Different type of solar radiation data is shown in Fig. 2.5. Pyrheliometric data

Pyranometric data

Solar radiation data

Types of radiation

Time resolved information Irradiance (instantaneous measurements)

Measurement setup

Irradiation (integrated over time) Averaged irradiation (over time)

Beam radiation

Diffuse radiation

Horizontal surface

Inclined surface

Total radiation

Figure 2.5: Different types of solar radiation data.

3 Solar Thermal Energy Conservation System

3.1

Introduction

Solar thermal technologies capture the heat energy from the Sun and use it for heating and/or the production of electricity. This is different from photovoltaic solar panels, which directly convert the Sun radiation to electricity. There are two main types of solar thermal systems for energy production– active and passive. Active systems require moving parts like fans or pumps to circulate heat-carrying fluids. Passive systems have no mechanical components and rely on design features only to capture heat (e.g., greenhouses). The technologies are also grouped by temperature – low, medium or high. Solar thermal systems have several advantages. The ‘fuel’ that powers them is free and renewable, so these systems are cheap to run and can replace some conventional fuel use. Solar thermal is an emission-free source of energy. Finally, solar thermal systems are relatively low maintenance because they use simpler technologies and passive systems that have no moving parts. In the case of concentrated solar power (CSP), the technology’s ability to produce large-scale generation is an advantage for regions that utilise a centralised electricity distribution system. Although abundant, many aspects of sunlight can cause problems for the use of solar thermal systems. Sunshine is not a very concentrated energy source, so it can take a large area to make a reasonable amount of energy, evoking land-use concerns. Sunshine is also intermittent and its availability is dependent on location. The location of CSP installations causes additional problems for the technology. Many are normally located in remote, desert areas, and, given that steam turbines produce electricity for CSP, water access and rapid evaporation are key concerns for the viability of the technology. In addition, transmission of electricity over large distances is expensive and can lead to distribution losses. Finally, practical challenges such as upfront capital costs and awareness of solar thermal technologies can also be barriers to implementation in some countries. Some of the important solar water heating system are discussed below.

3.2

Solar water heating system

A solar thermal device captures and transfers the heat energy available in solar radiation which can be used for meeting the requirements of heat in different temperature ranges.

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Three main temperature ranges used are: Low temperature Hot water 60°C to 80°C Medium temperature Drying 80°C to 140°C High temperature Cooking and power generation → 140°C Solar water heating system (SWHS) is a device which supplies hot water at 60°C to 80°C using only solar thermal energy without any other fuel. It has three main components, namely: 1. Solar collector. 2. Insulated hot water storage tank. 3. Cold water tank with required insulated hot water pipelines and accessories. In the case of smaller systems (100–2000 litres per day), the hot water reaches the user end, by natural (thermo–siphon) circulation for which the storage tank is located above the collectors. In higher capacity systems, a pump may be used for forced circulation of water.

3.2.1

Working principle

In a typical solar water heater, water is heated by the solar thermal energy absorbed by the collectors. The hot water with lower density moves upwards and cold water with higher density moves down from the tank due to gravity head. A bank of collectors can be arranged in a series – parallel combination to get higher quantity of hot water. A typical 100 litres insulated tank with a 2 m2 collector area, will supply water at a temperature of 60–80°C. Based on the collector system, solar water heaters can be of two types.

3.2.2

Flat plate collectors (FPC) based solar water heaters

The solar radiation is absorbed by flat plate collectors which consist of an insulated outer metallic box covered on the top with glass sheet. Inside there are blackened metallic absorber (selectively coated) sheets with built in channels or riser tubes to carry water. The absorber absorbs the solar radiation and transfers the heat to the flowing water.

3.2.3

Evacuated tube collectors (ETC) based solar water heaters

Evacuated tube collector is made of double layer borosilicate glass tubes evacuated for providing insulation. The outer wall of the inner tube is coated with selective absorbing material.

Solar Thermal Energy Conservation System 27

This helps absorption of solar radiation and transfers the heat to the water which flows through the inner tube. Selection of suitable solar water heating systems: 1. Flat plate collector (FPC) based systems are of metallic type and have longer life as compared to evacuated tube collector (ETC) based system as ETCs are made of glass which are of fragile in nature. 2. ETC based systems are 10 to 20% cheaper than FPC based system. They perform better in colder regions and avoid freezing problem during subzero temperature. FPC based systems also perform good with anti-freeze solution at sub zero temperature but their cost increases. 3. At places where water is hard and have larger chlorine content, FPC based system with heat exchanger must be installed as it will avoid scale deposition in copper tubes of solar collectors which can block the flow of water as well reduce its thermal performance. ETC based systems do not face such problem. 4. For a house with one bathroom and 3 to 4 members, 100 litre per day capacity system should be sufficient. For more numbers of bathrooms, the capacity will increase accordingly due to pipe losses and more number of family members. Generally the capacity is decided based on hot water required in mornings for bathing. If the usage is in evening and at other times also, the capacity is decided accordingly. 5. A 100 lpd capacity may cost Rs 20,000 to Rs 25,000 (as on year 2016) depending on type and location. The cost, however, does not increase linearly with increase in capacity, rather it comes down proportionately as we go for higher capacity system. The system cost does not include the cost of cold water tank, and its stand which is required if overhead tank is not installed in a house/building. Cost of hot water insulated pipe line also, may be extra if number of bathrooms are more than one. Additional cost towards all these components may increase by 5 to 10%. 6. Avoid putting of electricity back up in storage tank of solar system. If you have electric geyser of say less then 10 lpd capacity or an instant geyser it would be better if you connect the outlet line of solar system with inlet of geyser and set thermostat at 40°C. Your geyser will start only when you get water below 40°C from solar system and will switch off when temperature goes above say 42°C or so. This will save lot of electricity and heat water according to your requirement. However, if you have storage geyser of higher capacity, better to have a separate tap for solar system and use your electric geyser when you don’t get hot water from solar water heater.

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Hot water storage tank

The tanks are generally made of stainless steel to avoid corrosion and are insulated to reduce heat losses. They are also fitted with electrical heater as a backup during monsoon days. The tanks may also be made of Galvanised Iron (GI). Cold water tank and pipelines Cold water comes from the over head tank. Hot water from the system is transferred to various utility points through insulated pipelines. A heat exchanger may be provided when the water is hard. Usages

Domestic: Bathing, coffee/tea preparation, utensils cleaning, etc. Industrial: Pre-heating of boiler feed water, cooking/dishwashing in industrial canteens washing of milk canes in dairies, sterilisation of surgical instruments, etc.

3.3

Solar distillation

Solar distillation is a relatively simple treatment of brackish (i.e., contain dissolved salts) water supplies. Distillation is one of many processes that can be used for water purification and can use any heating source. Solar energy is a low tech option. In this process, water is evaporated, using the energy of the Sun then the vapour condenses as pure water. This process removes salts and other impurities. Solar distillation is used to produce drinking water or to produce pure water for lead acid batteries, laboratories, hospitals and in producing commercial products such as rose water. It is recommended that drinking water has 100 to 1000 mg/l of salt to maintain electrolyte levels and for taste. Some saline water may need to be added to the distilled water for acceptable drinking water. There are a number of other approaches to desalination, such as photovoltaic powered reverse-osmosis. Solar distillation has to be compared with these options to determine its appropriateness to any situation. If treatment of polluted water is required rather than desalination, slow sand filtration is a low cost option.

3.3.1

Energy requirement for water distillation

The energy required to evaporate water, called the latent heat of vapourisation of water, is 2.26 Megajouls per kilogram (MJ/kg). This means that to produce 1 litre (i.e., 1 kg as the density of water is 1 kg/l) of pure water by distilling brackish water requires a heat input of 2.26 MJ. This does not allow for the efficiency of the system used which will be less than 100%, or for any recovery of latent heat that is rejected when the water vapour is condensed.

Solar Thermal Energy Conservation System 29

It should be noted that, although 2.26 MJ/kg or 2260 kJ/kg is required to evaporate water, to pump water through 20 m head requires only 0.2 kJ/kg. Distillation is therefore normally considered only where there is no local source of fresh water that can be easily pumped or lifted.

3.3.2

Working of solar still

The main features are the same for all solar stills. The solar radiation is transmitted through the glass or plastic cover and captured by a black surface at the bottom of the still. A shallow layer of water absorbs the heat which then produces vapour within the chamber of the still. This layer should be 20 mm deep for best performance. The vapour condenses on the glass cover, which is at a lower temperature because it is in contact with the ambient air, and runs down into a gutter from where it is fed to a storage tank.

3.3.3

Design objectives for an efficient solar still

For high efficiency the solar still should maintain: 1. A high feed (undistilled) water temperature. 2. A large temperature difference between feed water and condensing surface. 3. Low vapour leakage. A high feed water temperature can be achieved if: 1. A high proportion of incoming radiation is absorbed by the feed water as heat. Hence low absorption glazing and a good radiation absorbing surface are required. 2. Heat losses from the floor and walls are kept low. 3. The water is shallow so there is not so much to heat. A large temperature difference can be achieved if: 1. The condensing surface absorbs little or none of the incoming radiation. 2. Condensing water dissipates heat which must be removed rapidly from the condensing surface by, for example, a second flow of water or air, or by condensing at night.

3.3.4

Stills

Single-basin stills (Fig. 3.1) have been much studied and their behaviour is well understood. The efficiency of solar stills which are well-constructed and maintained is about 50% although typical efficiencies can be 25%. Daily output as a function of solar irradiation is greatest in the early evening when the feed water is still hot but when outside temperatures are falling. At very high air temperatures such as over 45°C, the plate can become too warm and condensation on it can become problematic, leading to loss of efficiency.

30 Textbook of Renewable Energy Glass or plastic plate Condensate runoff Rainwater channel to outlet catchment channel Saline solution

Blackened surface

Overflow

Base incorporating insulation

Figure 3.1: Single-basin stills.

Some problems with solar stills which would reduce their efficiency include: 1. Poor fitting and joints, which increase colder air flow from outside into the still cracking, breakage or scratches on glass, which reduce solar transmission or let in air. 2. Growth of algae and deposition of dust, bird droppings, etc. To avoid this the stills need to be cleaned regularly every few days. 3. Damage over time to the blackened absorbing surface. 4. Accumulation of salt on the bottom, which needs to be removed periodically. 5. The saline water in the still is too deep, or dries out. The depth needs to be maintained at around 20 mm.

3.3.5

Multiple-effect basin stills

Multiple-effect basin stills have two or more compartments. The condensing surface of the lower compartment is the floor of the upper compartment. The heat given off by the condensing vapour provides energy to vapourise the feed water above. Efficiency is therefore greater than for a single-basin still typically being 35% or more but the cost and complexity are correspondingly higher.

3.3.6

Wick stills

In a wick still, the feed water flows slowly through a porous, radiationabsorbing pad (the wick). Two advantages are claimed over basin stills. First, the wick can be tilted so that the feed water presents a better angle to the Sun (reducing reflection and presenting a large effective area). Second, less feed water is in the still at any time and so the water is heated more quickly and to a higher temperature. Simple wick stills are more efficient than basin stills and some designs are claimed to cost less than a basin still of the same output.

Solar Thermal Energy Conservation System 31

3.3.7

Inverted absorber solar stills

Heat is absorbed from the underside of the still to improve efficiency. This allows that condenser plate and the collector plate to be separate. There are several designs of inverted absorber from the fairly simple to more complex designs.

3.3.8

Inclined stills

The aim of inclining a still is to increase the solar radiation, by catching it head on, rather than at an angle as with stills which lie flat. To do this constantly, as the Sun rises and sets, would need someone to monitor the Sun and turn the unit regularly, or a sophisticated automatic tracking and turning mechanism.

3.3.9

Condensate heat recovery

Heat recovery from the energy given out when water vapour condenses has generally not been attempted with small-scale solar distillation, unlike with larger-scale systems. In the simplest system, saline water is made to flow over the outside of the condensation plate before entering the still, but then this would reduce the amount of solar radiation passing through the plate.

3.3.10

Cost of pure water

The cost of pure water produced depends on: (i) the cost of making the still, (ii) the cost of the land, (iii) the life of the still, (iv) operating costs, (v) cost of the feed water, (vi) the discount rate adopted, and (vii) the amount of water produced. Rainwater collection is an even simpler technique than solar distillation and is preferable in area’s with 400 mm of rain annually, but requires a greater area and usually a larger storage tank. If ready-made collection surfaces exist (such as house roofs) these may provide a less expensive source for obtaining clean water.

3.4

Heating of swimming pool by solar energy

Swimming pools provide a great way to exercise and beat the summer heat. Building and maintaining a pool, however, also means relatively high costs added to your household’s budget. There are several ways that you can reduce operating and maintenance costs, lower water consumption, and conserve heat if you heat your pool. Many people heat their pools to extend the swimming season and/or to keep it at a temperature that they are personally comfortable with. This lets them enjoy the full value from their pool. Solar pool heaters are an option to heat the pool with ‘clean’ energy from the Sun, and can reduce heating costs.

32 Textbook of Renewable Energy

3.4.1

Sizing the pump right

Study shows that a 0.75 horsepower or smaller pump is generally sufficient for residential pools. Smaller pumps, which cost less, can be used if you decrease the pool circulation system’s hydraulic resistance. This can be done through one or more of the following ways: substituting a large filter (rated to at least 50% higher than the pool’s design flow rate), increasing the diameter or decreasing the length of the pipes, or replacing abrupt 90° elbows with 45° elbows or flexible pipe. These types of changes can slash up to 40% off the pump’s use of electricity.

3.4.2

Circulating the water

Circulating water keeps your pool’s chemicals mixed. However, as long as the water circulates while chemicals are added, they should remain evenly mixed with minimal daily circulation. Secondly, circulating the water keeps the pool free of debris by drawing water out and through the filter. It is not necessary to recirculate the water completely every day to remove debris and clean the water. One complete circulation usually takes between 6 and 12 h per day. But this may be longer than necessary since most debris either floats or sinks, and can be removed with a skimmer or vacuum. After about an hour, most of the pumping power is wasted by circulating clear water and does little to improve the water’s quality.

3.4.3

Reducing water and heat loss

Almost all of a pool’s heat loss about 95% occurs at the surface, mostly through evaporation to the air and radiation to the sky. A pool cover is an effective means to keep heat (and water) in a pool by reducing evaporation of water from the pool when it is not in use, and reduces radiant heat losses. Reducing water loss also reduces the amount of chemical water treatment required. Outdoor pools can gain a significant amount of heat from the Sun, absorbing 75 to 85% of the solar energy striking the pool surface. A bubble cover (sometimes called a solar cover) is one of the least expensive covers made specifically for swimming pools. It’s similar to bubble packing material except it has a thicker grade of plastic and ultraviolet (UV) inhibitors. Vinyl covers are made of a heavier material, which extends their use. You can also get vinyl covers with a thin layer of flexible insulation sandwiched between two layers of vinyl. A transparent bubble cover may reduce solar energy absorption by 5 to 15%, and an opaque cover may reduce it by 20 to 40%. However, the decrease in solar gain can be balanced or more than offset by the cover’s retention of the pool’s heat, which depends on the air temperature and humidity.

Solar Thermal Energy Conservation System 33

A cover also helps you keep the pool clean and extend the life of the chemicals in your pool. Figure 3.2 shows types of pool covers.

Bubble/solar

Vinyl

Insulated vinyl

Figure 3.2: Types of pool covers.

Covers should always be installed and used according to the manufacturers’ guidelines. A windbreak around the pool can also reduce evaporation, and make pool ‘lounging’ more comfortable and private. A 7-mph wind at the pool surface can increase heat loss by 300%. A windbreak could include a fence or vegetation. It needs to be high and close enough to the pool so that it effectively blocks wind from moving across the pool surface, but doesn’t block beneficial solar energy.

3.4.4

Heating the pool efficiently

The easiest way to save energy is to lower the thermostat on your pool’s heater so that it heats the pool no higher than the temperature that you feel comfortable with. Every 1°C reduction can cut your energy usage by between 5 and 10%. Once you have lowered the thermostat, keep the remaining heat from escaping by using a pool cover when the pool is not in use.

3.4.5

Solar pool heaters

Such heating systems are one of the most cost-effective applications of solar energy. It is relatively simple to integrate a solar water heater since most pools require a pump, filter, and plumbing. With a solar energy system, the pool’s water is pumped through the filter and then through a solar energy collector(s) instead of directly back to the pool. The Sun heats the water in the collector(s) before it returns to the pool.

34 Textbook of Renewable Energy

If you add a solar heater, you may need a pump larger than your present one, or a separate, smaller pump to pump the pool’s water to and through the solar collectors. Adding any heater, solar or otherwise, will preclude selecting the smallest pump. Nevertheless, you also may reduce pumping time to help cut costs. Solar collectors can also be used to cool the pool in hot climates or during peak summer months by circulating the water through the collectors at night. The collectors lose heat by radiation to the night sky.

3.4.6

Solar pool collectors

Collectors for heating a pool normally do not require glazing or insulation because they operate during warmer months when solar radiation and ambient temperatures are relatively high. This allows for a simpler design that is usually less expensive than collectors for domestic hot water. Many pool collectors are made of heavy duty rubber or plastic treated with a UV light inhibitor to extend the life of the panels. The advantages of plastic collectors are that they’re usually less expensive and weigh less than metal collectors. Metal collectors generally are made of copper tubing mounted on an aluminium plate. The disadvantages of metal collectors are that they are more susceptible to corrosion and freeze damage, and the copper tubes may react with your pool’s chlorine if the pH level falls below 7.2. Too many copper ions in pool water may form dark-coloured precipitates, which can coat the pool’s walls. This discolouration can only be removed by draining, cleaning, and repainting the pool. This problem can be reduced if the pH level is always kept above 7.2. The area needed for collectors to heat your pool depends on many factors. A general rule of thumb is that the collector surface area should equal at least one half of the pool’s surface area. In a relatively sunny climate, this additional heating helps extend the swimming season into spring and autumn. In cooler and cloudier areas, you may need to increase the collector’s surface area to equal the entire surface area of the pool. Collectors can be mounted on roofs or anywhere near the pool that provides the proper exposure, orientation, and tilt toward the Sun. The optimum collector orientation is south, but west-facing orientations are good if the collector’s surface area is increased to at least 75% of the pool’s surface area. East-facing orientations are marginally good. The tilt of the collector is as important as the orientation. For heating primarily in the summer, the tilt should equal the latitude where the pool is located minus 10 to 15°C are shown in Fig. 3.3. Where optimising the tilt is not possible, for example on an existing roof with a high slope or on a flat roof, increasing the collector area may be necessary to achieve the desired pool temperature. One potential benefit of roof installation is that it may reduce the cooling load of the building that it’s located on, since it puts the solar heat into the

Solar Thermal Energy Conservation System 35

Sun Latitude minus 10–15°C

South

Figure 3.3: For heating primarily in the summer, the tilt of a solar collector should equal the latitude where the pool is located minus 10 to 15°C.

pool water and keeps it from radiating into the attic and the conditioned space below. Because swimming pools include a pump and related plumbing, adding on a solar heater can be relatively simple. Unless you have experience with plumbing and electrical wiring, however, have a professional install your system. Often the pump circulates the pool water enough, but be sure it maintains a high flow rate to keep the panels operating at optimal efficiency. Your collector should require little maintenance if the pool’s chemical balance and filtering system are checked regularly.

3.5

Solar thermal power plants

Solar thermal power plants (STPT) produce electricity by converting the solar radiation into high temperature heat using mirrors and reflectors. The collectors are referred to as the solar-field. This energy is used to heat a working fluid and produce steam. Steam is then used to rotate a turbine or power an engine to drive a generator and produce electricity All STPT plants are based on four basic essential systems which are collector, receiver (absorber), transport/storage and power conversion. Parabolic trough, solar towers, parabolic dishes and linear fresnel reflectors are the four main technologies that are commercially available today.

3.5.1

Parabolic trough

Parabolic trough shaped mirrors collect and reflect the solar energy onto receiver tubes positioned along the focal line of parabolic mirrors. The troughs

36 Textbook of Renewable Energy

are usually designed to track the Sun along one axis, predominantly north– south. Heat transfer fluids, such as synthetic thermal oil suitable for temperatures up to 400°C, circulating through the tubes are used to generate steam through heat exchangers and steam generators and drive turbine to generate electricity through a steam cycle. This is a well established and proven CSP technology.

3.5.2

Solar towers

A circular array of heliostats concentrates sunlight on to a central receiver mounted at the top of a tower. The heliostats tack the Sun on two axis. The central receiver can achieve very high concentrations of solar irradiation thus resulting in extremely high temperature for the operating fluid. A heat-transfer medium in this central receiver absorbs the highly concentrated radiation reflected by the heliostats and converts it into thermal energy, which is used to generate superheated steam for the turbine through the Rankine cycle. Brayton cycle systems are also under testing because of the higher efficiencies. Spain has several solar tower systems operating or under construction, up to 20 MW capacity.

3.5.3

Parabolic dish

The parabolic shaped dish tracks the Sun, through a two axis movement, onto a thermal receiver mounted at the focal point. The concentrated beam radiation is absorbed into a receiver to heat a fluid or gas to approximately 750°C. This fluid or gas is then used to generate electricity in a small piston or Stirling engine or a micro turbine.

3.5.4

Linear fresnel reflectors

These reflectors are made of several slices of mirrors with small curvature approximating a parabola. Mirrors are mounted on trackers and configured to reflect sunlight onto elevated linear reflectors. Water flows through the receivers and is converted into steam and the intermediate heat transfer fluid is not required. These systems have lower investment costs and also lower optical performance as compared to parabolic trough collectors. This technology is still in the developmental stage.

3.5.5

Active solar plant

Low temperature – solar collector

The solar collector plate (Fig. 3.4) has four principal elements: 1. The transparent covert. 2. The absorbent layer.

Solar Thermal Energy Conservation System 37 Transparent covert Absorbent layer

Insulating Casting

Figure 3.4: Solar collector plate.

3. Insulating. 4. The casting. The transparent cover should have several characteristics for the appropriate performance of the collector: 1. Produce a green house effect and reduce the external losses. 2. Have a low coefficient of thermal conductivity. 3. The external surface has to be clean. 4. The collector must be sealed to prevent water and air to go into the system. The main materials used in the cover are: 1. Glass. 2. Transparent plastic. The absorbent layer receives the solar radiation that is transformed into heat and then it is transmitted to the fluid that will transport it. Two examples of these layers are: 1. Two metallic sheets separated by a few millimetres. 2. A metallic sheet that contain several tubes that carry the fluid.

3.5.6

Medium temperature and high temperature

In a solar electricity generation system, the rays of the Sun are used to generate heat. This systems use this energy to produce high temperatures that can boil water and drive steam machines to produce mechanical work or drive electrical generators in the same way as conventional oil, coal or nuclear power plants. We can differentiate basically two kinds of solar thermal systems: with and without concentration of solar rays. The systems that concentrate the solar rays use mirrors or lenses to focus the light into a specific zone to produce high temperatures, this allow the system to be very efficient energy conversion.

38 Textbook of Renewable Energy

3.6

Solar ponds

In this system, a large salty lake is used as a plate collector. With the right salt concentration in the water, the solar energy can be absorbed at the bottom of the lake. The heat is insulated by the different densities of the water and at the bottom the heat can reach 90°C, which is high enough to run a vapour cycle engine, at the top of the pond, the temperature can reach 30°C. There are three different layers of water in a solar pond, the top layer that has less concentration of salt; the intermediate layer, that acts as a thermal insulator and finally the bottom layer that has a high concentration of salt. These systems have a low solar to electricity conversion efficiency, less than 15% (having an ambient temperature of 20°C and a storage heat of 80°C). One advantage of this system is that because the heat is stored, it can run day and night if required. Also due to its simplicity, it can be constructed in rural areas in developing countries. Solar ponds are shown in Fig. 3.5. Table 3.1 shows the different characteristics of the solar electricity generators. Table 3.1: Different characteristics of the solar electricity generators.

Size Temperature °C (mean value) Peak efficiency Energy storage Annual efficiency

3.6.1

Parabolic trough

Solar tower

Parabolic dish

30–320 MW

10–200 MW

5–25 kW

390 20% Limited 16%

560 23% Yes 20%

750 30% Battery 25%

Solar ponds collector

1. Solar pond collector (Fig. 3.6) are either natural or artificial lakes, ponds or basins that act as a flatplate collector because of the different salt contents of water layers due to stratification. 2. The upper water layers of relatively low salt content are often provided with plastic covers to inhibit waves. 3. This upper mixing zone of such pond collectors usually is approximately 0.5 m thick. 4. The adjacent transition zone has a thickness of 1 to 2 m, and the lower storage zone is of 1.5 to 5 m thickness. Mechanism of solar ponds

1. If deeper layers of a common pond or lake are heated by the Sun, the heated water rises up to the surface since warm water has a lower density than cold water.

Sun

Figure 3.5: Solar ponds.

High salt content hot brine

Salt gradient layer

Low salt content cool water

Hot brine

Cold water

Earth

Evaporator

Pump

Generator

Electricity

Turbine Organic working fluid

Condenser

Solar Thermal Energy Conservation System 39

40 Textbook of Renewable Energy Solar radiation

Solar radiation

Solar pond

10

13

Solar pond

30

Density in g/cm

3

60

90

Temperature in °C

Figure 3.6: Solar ponds collector.

2. The heat supplied by the Sun is returned to the atmosphere at the water surface. 3. This is why, in most cases, the mean water temperature approximately equals ambient temperature. 4. In a solar pond, heat transmission to the atmosphere heat atmosphere is prevented by the salt dissolved in deeper layers, since, due to the salt, water density at the bottom of the pond is that high, that the water cannot rise to the surface, even if the Sun heats up the water to temperatures that are close to the boiling point. 5. The salt concentration of the different layers must thus increase with increasing depth. 6. In a first phase, this ensures stable water stratification. 7. The upper, almost salt-less layer only acts as transparent, heat-insulating cover for the cooling, heat-storing deeper layers at the pond bottom.

3.7

Solar PV pumping systems

Typically, diesel-powered pumps are used in areas where connecting to the electricity grid is difficult. Solar photovoltaic (PV) systems can be an attractive complementary energy source deployed along side diesel pumps in areas with plenty of sunshine and where the cost to run power lines is high. Photovoltaic systems have the benefit of being scalable, with capacity ranging from a few watts for applications such as automated farm gates or timers, to hundreds of kilowatts for the homestead and farm sheds. Rather than having one large centralised system, a number of distributed PV systems can be deployed at pump sites. A typical solar-powered pumping system consists of solar panels connected to an electric motor that runs a bore or surface electric pump. A solar pumping

Solar Thermal Energy Conservation System 41

solution available from your irrigation supplier will typically supply a DC (mains-powered) pump that is connected directly to the solar panel and does not require a DC/AC inverter. DC brushless motors also offer very high efficiency levels (over 90%). In cases where an AC (battery-powered) pump is already in place, an inverter is required between the PV panel and the motor to convert from the direct current generated by the solar panel to the alternate current required by the electric pump motor. In the case of a solar-diesel hybrid system, a solar pumping system (PV panel plus pump) is installed to complement the existing diesel pump operation. The solar pump can either pump directly into the system to offset diesel pump operation during daytime, or pump water to a storage tank or reservoir (which is part of the solar pumping solution) so that water is also available on cloudy days and at night. This is illustrated in the Fig. 3.7 Solar PV array

Pump controller

Storage tank DC bore pump Irrigation Dam

Diesel pump

Figure 3.7: Solar-diesel hybrid system with water storage.

3.7.1

Key steps in sizing a solar pumping system

1. Determine the total dynamic head (TDH) of the system using flow-rate requirements (L/min), pipe length and diameter, and height between suction and discharge points. TDH = static head + dynamic head (line friction). 2. Determine the daily flow (m3/day) requirement and the expected number of weeks per year of pumping.

42 Textbook of Renewable Energy

3. Depending on the water source, choose a surface or submersible pump. 4. Using manufacturer pump curves, select a pump of adequate size to meet head and flow requirements. 5. Knowing the power requirement and running time for the selected pump, determine the electrical load profile of the pumping operation to then size the solar PV system. 6. The capacity of the storage dam or tank is determined by the flow rate the process requires and the storage time, which can be equal to amount of time outside daylight hours for which the pump normally runs. 7. Consider using battery storage or combinations.

3.8

Solar crop drying

Controlled drying is required for various crops and products, such as grain, coffee, tobacco, fruits vegetables and fish. Their quality can be enhanced if the drying is properly carried out. Solar thermal technology can be used to assist with the drying of such products. The main principle of operation is to raise the heat of the product, which is usually held within a compartment or box, while at the same time passing air through the compartment to remove moisture. The flow of air is often promoted using the ‘stack’ effect which takes advantage of the fact that hot air rises and can therefore be drawn upwards through a chimney, while drawing in cooler air from below. Alternatively a fan can be used. The size and shape of the compartment varies depending on the product and the scale of the drying system. Large systems can use large barns while smaller systems may have a few trays in a small wooden housing. Solar crop drying technologies can help reduce environmental degradation caused by the use of fuel wood or fossil fuels for crop drying and can also help to reduce the costs associated with these fuels and hence the cost of the product. Helping to improve and protect crops also has beneficial effects on health and nutrition.

3.9

Solar cookers

Cooking is an activity that must be carried out almost on a daily basis for the sustenance of life. An enormous amount of energy is thus expended regularly on cooking. Cooking may be classified in four major categories based on the required range of temperature, viz., backing (85–90°C), boiling (100–130°C), frying (200–250°C) and roasting (more than 300°C).

3.9.1

Solar energy

As already discussed solar energy is considered a suitable alternative for variety of applications. It is a largest renewable resource, freely available everywhere

Solar Thermal Energy Conservation System 43

in adequate amounts, making it one of the most promising, clean, non-pollution sources. Solar energy devices hold a large potential for use in a variety of applications in developing countries. Presently a number of solar energy devices like solar cooker, solar water heater, solar photovoltaic pumps and solar photovoltaic lighting, etc. Solar cooking offers an effective method of utilising solar energy for meeting a considerable demand for cooking energy and hence, protecting the environment. Solar energy can make a major contribution to the energy needs for cooking food. Cooking with solar cookers is an energy-efficient, pollutionfree way to help fight global warming and take advantage of nature's free, inexhaustible energy supply. Concentrating solar cookers: Concentrating solar cookers can cook food for large number of people faster than box solar cookers. In a simple box type solar cooker one can bake, and boil, and in a cooker for domestic use, it is possible to cook food for four to five persons. Bigger size solar concentrator cookers are also available for cooking food up to 15 persons at a time. Modern technologies comprise solar kitchens and cooking plants for community applications using high temperature solar concentrators.

3.9.2

Box type solar cooker

The high energy photons of solar electromagnetic radiation absorb by a transparent glazing and the incident solar energy convert in to thermal heat by the process of photothermal effect. Thermodynamically the temperature of the exposed surface may achieve the temperature of Sun at ideal thermal equilibrium; but at higher temperatures the convective and radioactive losses becomes more dominant. In order to prevent the heat losses, insulation on sides and bottom is provided along with the double glazing over the black painted absorber. There are several designs of solar cookers, which were developed by researchers; however, box-type design has been most widely used world over and especially in India. A box type solar cooker is an alternative food cooking technology with sunlight as its only energy source, which essentially consists of a black painted metallic trapezoidal tray (cooking tray) and is usually covered with a double glass window. It is kept in a metal or fibreglass outer casing and the space between the cooking tray and outer casing is filled with the insulation like glass wool. The incoming solar radiation falls onto the double glass lid and passes through it to strike the blackened cooking pots and the cooking tray. The glass covers, while transmitting radiation of short wavelength which form major part of solar spectrum, is almost opaque to low temperature radiation emitted within the box. Thus, the temperature of the box rises until a balance is reached between the heat received through glazing and heat lost by

44 Textbook of Renewable Energy

exposed surface (greenhouse effect). In addition, a plane reflecting mirror (booster mirror) of about equal size as that the aperture area is used for augmentation of solar radiation on the aperture. The cooking tray is insulated on the sides and bottom. The heat is absorbed by the blackened surface and gets transferred to the food inside the pots to facilitate cooking. The mirror reflector is set in such a way to reflect the solar radiation falling on it to the cooker aperture. Usually four black painted cylindrical pots are placed inside the box as cooking vessels and the cooking load is kept usually 8 L/m2. The cooker takes 1/2 to 2 h to cook items such as rice, lentils and vegetables. The cooker may also be used to prepare simple cakes, roast cashew nuts, dry grapes, etc. In good solar radiation conditions, it is possible to achieve the temperature up to 150°C inside the hot box of this solar cooker, hence best suited for boiling kind of cooking. It is an ideal device for domestic cooking during most of the year, except for the monsoon season and cloudy days. A family size solar cooker is sufficient for 4 to 5 members and saves about 3 to 4 cylinders of LPG every year, while the life of this cooker is 10 to 20 years.

3.9.3

Benefits of solar cooking

Benefits of solar cooking are given below: 1. No smoke inhalation. 2. Minimal water requirement. 3. Reduced surveillance. 4. No risk of burning the food. 5. No soot accumulation on pots. 6. No worry about fire. 7. No burns from fire or coals.

3.10

Solar air conditioning and refrigeration

Air conditioning is one of the major consumers of electrical energy in many parts of the world today and already today air conditioning causes energy shortage in many parts of the world. Air conditioning systems in use are most often built around a vapour compression systems driven by grid-electricity. However, most ways of generating the electricity today, as well as the refrigerants being used in traditional vapour compression systems, have negative impact on the environment. Solar air conditioning might be a way to reduce the demand for electricity. In addition many solar air conditioning systems are constructed in ways that eliminate the need for CFC, HCFC or HFC refrigerants.

Solar Thermal Energy Conservation System 45

Solar panel: Solar panel refers either to a photovoltaic module, a solar thermal energy panel, or a set of solar photovoltaic (PV) modules electrically connected and mounted on a supporting structure. A PV module is a packaged, connected assembly of solar cells. Solar panels can be used as a component of a larger photovoltaic system to generate and supply electricity. Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired current capability. We need four solar panel each of 250 W. Battery: Battery store the electric power in the form of a chemical reaction. Without storage you would only have power when the Sun is shining or the generator is running. We need battery of 48 V. Invertor: The power invertor is the heart of the system. It makes 220 V AC from the 12 V DC stored in the batteries. It can also charge the batteries if connected to a generator or the AC line. For 12 V applications an invertor is not required. An invertor should only be required when it is necessary to convert the 12 V input to power a 220 V standard application. Charge controller: A charge controller is needed to prevent the overcharging of the battery. Proper charging of battery will prevent the damage and increase the life and performance of it. Refrigerants: Refrigerants can be defined as medium by which heat transfer in refrigeration system takes place. It absorbs latent heat at low temperature in evaporator and gets converted from liquid to vapour, thus producing cooling effect. It rejects latent heat at atmospheric temperature in condenser and it’s phase will be changed from vapour to liquid. Except air, all refrigerants change their phase during operation.

3.10.1

Solar refrigeration

Solar refrigeration may have applications in both developed and developing countries. Applications in developing countries such as vaccine storage or large scale food preservation have been the subject of much research. In developed countries the main area of interest is air conditioning. Research is underway on carbon - ammonia refrigerators driven by the heat of steam condensing in a thermosyphon heat pipe. The heat source can be solar energy, biomass, or some combination of the two. A new area of interest is the use of desiccant wheel technology for solar powered air conditioning. There is a demand for cooling in many parts of the world where there is no firm electricity supply and conventional fuels are difficult or expensive to obtain. Requirements tend to be either for medical uses where a high capital cost per kW of cooling is acceptable, or for food (especially fish) preservation where the cooling power required is much lower refrigerators have been sold

46 Textbook of Renewable Energy

at a higher cost. These high costs are considered acceptable since the application is related to medical provision.

3.10.2

Possible refrigeration cycles

Some of the possible refrigeration cycles are discussed below: 1. A standard mechanical vapour compression cycle, requiring an electrical input to a hermetically sealed compressor. The electricity is generated by photovoltaic panels. This advantage of using of the-shelf technology, but the disadvantages of high cost and the probable need for an electricity storage sub-system. 2. Intermittent adsorption cycles: Adsorption refrigeration cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating. The adsorbent acts as a ‘chemical compressor’ driven by heat. In its simplest form an adsorption refrigerator consists of two linked vessels, one of which contains adsorbent and both of which contain refrigerant. Initially the whole assembly is at low pressure and temperature, the adsorbent contains a large concentration of refrigerant within it and the other vessel contains refrigerant gas: 1. The adsorbent vessel (generator) is then heated, driving out the refrigerant and raising the system pressure. The desorbed refrigerant condenses as a liquid in the second vessel, rejecting heat. 2. Finally the generator is cooled back to ambient temperature, readsorbing the refrigerant and reducing the pressure. Because the liquid in the second vessel is depressurised and boils, it takes in heat and produces the required refrigeration effect. The cycle is discontinuous since useful cooling only occurs for one half of the cycle. Two such systems can be operated out of phase to provide continuous cooling. Such an arrangement has a comparatively low coefficient of performance (COP = Cooling/Heat Input). Also, the thermal conductivity of the bed is generally poor so the time taken for a cycle could be an hour or more and the cooling power per mass of adsorbent could be as low as 10 W/kg. This is not a problem with solar powered vaccine refrigerators which produce a few kg of ice each day and operate on a diurnal cycle.

3.11

Solar greenhouses

All greenhouses collect solar energy. Solar greenhouses are designed not only to collect solar energy during sunny days but also to store heat for use at night or during periods when it is cloudy. They can either stand alone or be attached to houses or barns.

Solar Thermal Energy Conservation System 47

A solar greenhouse may be an underground pit, a shed-type structure, or a quonset hut. Large-scale producers use free-standing solar greenhouses, while attached structures are primarily used by home-scale growers. Passive solar greenhouses are often good choices for small growers, because they are a cost-efficient way for farmers to extend the growing season. In colder climates or in areas with long periods of cloudy weather, solar heating may need to be supplemented with a gas or electric heating system to protect plants against extreme cold. Active solar greenhouses use supplemental energy to move solar heated air or water from storage or collection areas to other regions of the greenhouse. Use of solar electric (photovoltaic) heating systems for greenhouses is not cost-effective unless you are producing high-value crops.

3.11.1

Basic principles of solar greenhouse design

Solar greenhouses differ from conventional greenhouses in the following five ways: 1. Have glazing oriented to receive maximum solar heat during the winter. 2. Use heat storing materials to retain solar heat. 3. Have large amounts of insulation where there is little or no direct sunlight. 4. Use glazing material and glazing installation methods that minimise heat loss. 5. Rely primarily on natural ventilation for summer cooling. Understanding these basic principles of solar greenhouse design will assist us in designing, constructing, and maintaining an energy efficient structure.

3.11.2

Solar heat absorption

The two most critical factors affecting the amount of solar heat a greenhouse is able to absorb are: 1. The position or location of the greenhouse in relation to the Sun. 2. The type of glazing material used. Solar orientation

Since the Sun energy is strongest on the southern side of a building, glazing for solar greenhouses should ideally face true south. However, if trees, mountains, or other buildings block the path of the Sun when the greenhouse is in a true south orientation, an orientation within 15° to 20° of true south will provide about 90% of the solar capture of a true south orientation. The latitude of your location and the location of potential obstructions may also require that you adjust the orientation of your greenhouse slightly from true south to obtain optimal solar energy gain.

48 Textbook of Renewable Energy

Glazing

Glazing materials used in solar greenhouses should allow the greatest amount of solar energy to enter into the greenhouse while minimising energy loss. In addition, good plant growth requires that glazing materials allow a natural spectrum of photosynthetically active radiation (PAR) to enter. Many new greenhouse glazing materials have emerged in recent decades. Plastics now are the dominant type of glazing used in greenhouses, with the weatherability of these materials being enhanced by ultraviolet radiation degradation inhibitors, infrared radiation (IR) absorbency, anticondensation drip surfaces, and unique radiation transmission properties. The method used for mounting the glazing material affects the amount of heat loss. For example, cracks or holes caused by the mounting will allow heat to escape, while differences in the width of the air space between the two glazes will affect heat retention. Installation and framing for some glazing materials, such as acrylics, need to account for their expansion and contraction with hot and cold weather. As a general rule, a solar greenhouse should have approximately 0.75 to 1.5 square feet of glazing for each square foot of floor space.

3.11.3

Solar heat storage

For solar greenhouses to remain warm during cool nights or on cloudy days, solar heat that enters on sunny days must be stored within the greenhouse for later use. The most common method for storing solar energy is to place rocks, concrete, or water in direct line with the sunlight to absorb its heat. Brick or concrete-filled cinder block walls at the back (north side) of the greenhouse can also provide heat storage. However, only the outer four inches of thickness of this storage material effectively absorbs heat. Medium to darkcoloured ceramic tile flooring can also provide some heat storage. Storage materials

The amount of heat storage material required depends on your location. The amount of heat storage material required also depends on whether you intend to use your solar greenhouse for extending the growing season, or whether you want to grow plants in it year-round. Trombe walls are an innovative method for heat absorption and storage. These are low walls placed inside the greenhouse near the south-facing windows. They absorb heat on the front (south-facing) side of the wall and then radiate this heat into the greenhouse through the back (north-facing) side of the wall. A Trombe wall (Fig. 3.8) consists of an 8″ to 16″ thick masonry wall coated with a dark, heat absorbing material and faced with a single or double layer of glass placed from 3/4″ to 6″ away from the masonry wall to create a small airspace. Solar heat passes through the glass and is absorbed by

Solar Thermal Energy Conservation System 49

Sun

Warm air

Cool air

Figure 3.8: Trombe wall.

the dark surface. This heat is stored in the wall, where it is conducted slowly inward through the masonry. If you apply a sheet of metal foil or other reflective surface to the outer face of the wall, you can increase solar heat absorption by 30–60% (depending on your climate) while decreasing the potential for heat loss through outward radiation. ‘Water walls’ are a variation of the Trombe wall. Instead of a masonary wall, water-filled containers are placed in line with the Sun rays between the glazing and the greenhouse working space. The water can be in hard plastic tubes or other sturdy containers, and the top of the wall can serve as a bench. The Solviva solar greenhouse ‘water wall’ consists of two 2×4 stud walls, with the studs placed two feet on centers. A one-foot spacer connects the two walls. Plastic-covered horse fence wire was then fastened to each stud wall, and heavy duty, dark-coloured plastic water bags were inserted into the space between the two walls. The stud walls were positioned vertically in line with the Sun rays prior to the bags being filled with water. Active solar methods

An active method for solar heating greenhouses uses ‘subterranean heating’ or ‘Earth thermal storage solar heating.’ This method involves forcing solar heated air, water, or phase change materials through pipes buried in the floor. If you use hot air for subsurface heating, inexpensive flexible drainage or sewage piping about 10 centimeters (4 inches) in diameter can be used for the piping. Root-zone thermal heating with water is normally used in conjunction

50 Textbook of Renewable Energy

with gas-fired water heaters. This system can be readily adapted to solar and works well with both floor and bench heat. Greenhouse management practices also can affect heat storage. For example, a full greenhouse stores heat better than an empty one. However, almost half of the solar energy is used to evaporate water from leaf and soil surfaces and cannot be stored for future use. Besides adding some heat to the greenhouse, increased carbon dioxide in the greenhouse atmosphere, coming from the decomposition activities of the micro-organisms in the compost, can increase the efficiency of plant production.

3.11.4

Insulation

Wall and floor insulation

Good insulation helps to retain the solar energy absorbed by thermal mass materials. Keeping heat in requires you to insulate all areas of the greenhouse that are not glazed or used for heat absorption. Seal doors and vents with weather stripping. Install glazing snugly within casements. Polyurethane foams, polystyrene foams, and fibreglass batts are all good insulating materials. But these materials need to be kept dry to function effectively. A vapour barrier of heavy-duty polyethylene film placed between the greenhouse walls and the insulation will keep your greenhouse well insulated. Unglazed areas should be insulated to specifications of your region. On greenhouse floors, brick, masonry, or flagstone serves as a good heat sink. However, they can quickly lose heat to the ground if there is not an insulating barrier between the flooring and the soil. To protect against heat loss, insulate footings and the foundation with 1- to 2-inch sheets of rigid insulation or with a 4-inch-wide trench filled with pumice stone that extends to the bottom of the footings. You can also insulate flooring with four inches of pumice rock. Besides insulating the floor, it also allows water to drain through. External insulation

In external insulation you can insulate your greenhouse by burying part of the base in the ground or building it into the side of a south-facing hill. Straw bales or similar insulating material can also be placed along the unglazed outside walls to reduce heat loss from the greenhouse.

3.11.5

Ventilation

A building designed to collect heat when temperatures are cold also needs to be able to vent heat when temperatures are warm. Air exchange is also critical in providing plants with adequate levels of carbon dioxide and controlling

Solar Thermal Energy Conservation System 51

humidity. Thermal storage materials are effective in keeping a greenhouse cool in summer as well as keeping it warm in winter. Since these materials absorb heat during the day, less heat radiates within the greenhouse when the Sun is shining. When the Sun goes down, heat released from the thermal storage materials can be vented out of the greenhouse.

3.12

Solar furnace and applications

A solar furnace is a structure that uses concentrated solar power to produce high temperatures, usually for industry. Parabolic mirrors or heliostats concentrate light (Insolation) onto a focal point. The temperature at the focal point may reach 3500°C (6330°F), and this heat can be used to generate electricity, melt steel, make hydrogen fuel or nanomaterials. The solar furnace principle is being used to make inexpensive solar cookers and solar-powered barbecues, and for solar water pasteurisation. A prototype Scheffler reflector is being constructed in India for use in a solar crematorium. This 50 m² reflector will generate temperatures of 700°C (1292°F) and displace 200–300 kg of firewood used per cremation. Solar furnace is not exactly a furnace, but an optical system in which the solar radiation received by a collector is concentrated into a small area. If this highly concentrated radiant energy is received into a cavity, heat is generated and very high temperatures may be obtained. This cavity, which is really the furnace, is a minor part of the whole system, and solar furnaces should rather be called solar energy concentrators.

3.12.1

Advantages of the solar furnace

To anyone familiar with the field of very high temperatures, the performances of existing solar furnaces will not appear very spectacular. Temperatures of the order of 3000°C may be obtained through many different techniques: flames, electrical resistance heating (using graphite or tungsten elements), induction heating, arc melting in neutral atmosphere or levitation melting. All these techniques, however, have limitations-mostly because they require a certain type of atmosphere around the specimen under study. The flame heat source is always highly chemically reactive; the use of tungsten or graphite as an electrical heater, or as a susceptor in induction heating, requires a neutral or reducing atmosphere; arc melting and levitation are limited to electrical conductors. In a solar furnace, the heat source, in the form of a cone of radiation energy, may be called ‘pure heat,’ and does not impose any restriction to the kind of atmosphere which burrounds the specimen. It is indeed possible to place the object to be heated in a transparent vessel (made of glass or fused silica) and fill this vessel with any suitable gas. Operating under high vacuum is also a rather easy experimental technique.

52 Textbook of Renewable Energy

Another outstanding feature of solar furnaces is the very - concentration of heat obtainable within the focal area (the Sun image). Because of this high heat flux, heat is provided over a relatively small portion of a solid sample, and melting of the inside of a solid sample may be achieved so that the material under study serves as its own crucible. This possibility of heating a body from the inside out, which is unique to the solar furnace, is extremely useful in melting refractory substances which react very rapidly with any known crucible material at high temperature.

4 Solar Photovoltaic Systems

4.1

Introduction

Solar energy is radiant energy from the Sun. It is vital to us because it provides the world—directly or indirectly—with almost all of its energy. In addition to providing the energy that sustains the world, solar energy is stored in fossil fuels and biomass, and is responsible for powering the water cycle and producing wind. Every day the Sun radiates, or sends out, an enormous amount of energy. The Sun radiates more energy each day than the world uses in one year. Solar energy comes from within the Sun itself. Like other stars, the Sun is a big ball of gases—mostly hydrogen and helium. The hydrogen atoms in the Sun core combine to form helium and radiant energy in a process called nuclear fusion. Photovoltaic conversion is the direct conversion of sunlight into electricity without any heat engine to interfere. Photovoltaic devices are rugged and simple in design requiring very little maintenance and their biggest advantage being their construction as stand-alone systems to give outputs from microwatts to megawatts. Hence they are used for power source, water pumping, remote buildings, solar home systems, communications, satellites and space vehicles, reverse osmosis plants, and for even megawatt scale power plants. With such a vast array of applications, the demand for photovoltaics is increasing every year. Converting solar energy into electrical energy by PV installations is the most recognised way to use solar energy. Since solar photovoltaic cells are semiconductor devices, they have a lot in common with processing and production techniques of other semiconductor devices such as computers and memory chips. As it is well known, the requirements for purity and quality control of semiconductor devices are quite large. With today’s production, which reached a large scale, the whole industry production of solar cells has been developed and, due to low production cost, it is mostly located in the Far East. Photovoltaic cells produced by the majority of today’s most large producers are mainly made of crystalline silicon as semiconductor material. Solar photovoltaic modules, which are a result of combination of photovoltaic cells to increase their power, are highly reliable, durable and low noise devices to produce electricity. The fuel for the photovoltaic cell is free. The Sun is the only resource that is required for the operation of PV systems, and its energy is almost inexhaustible.

54 Textbook of Renewable Energy

A typical photovoltaic cell efficiency is about 15%, which means it can convert 1/6 of solar energy into electricity. Photovoltaic systems produce no noise, there are no moving parts and they do not emit pollutants into the environment. Taking into account the energy consumed in the production of photovoltaic cells, they produce several tens of times less carbon dioxide per unit in relation to the energy produced from fossil fuel technologies. Photovoltaic cell has a lifetime of more than thirty years and is one of the most reliable semiconductor products. Most solar cells are produced from silicon, which is non-toxic and is found in abundance in the Earth crust.

4.2

Solar collectors

Heating with solar energy is relatively easy—just look at a car parked in the Sun with its windows closed. Getting the right amount of heat in a desired location, however, requires more thought and careful design. Capturing sunlight and putting it to work effectively is difficult because the solar energy that reaches the Earth is spread out over a large area. The Sun does not deliver that much energy to any one place at any one time. How much solar energy a place receives depends on several conditions. These include the time of day, the season, the latitude of the area, the topography, and the amount of clouds in the sky. A solar collector is one way to collect heat from the Sun. A closed car on a sunny day is like a solar collector. As the sunlight passes through the car’s glass windows, it is absorbed by the seat covers, walls, and floor of the car. The light that is absorbed changes into heat. The car’s glass windows let light in, but do not let all the heat out. This is also how greenhouses are designed to stay warm year-round. A greenhouse or solar collector: 1. Allows sunlight in through the glass. 2. Absorbs the sunlight and changes it into heat. 3. Traps most of the heat inside.

4.3

Photovoltaic systems

Photovoltaic (PV) systems convert light directly into electricity. Photovoltaics literally means light–electricity. Commonly known as solar cells, PV cells are already an important part of our lives. The simplest PV systems power many of the small calculators, wrist watches, and outdoor lights we see every day. Larger PV systems generate electricity for factories and warehouses, provide electricity for pumping water, powering communications equipment, and lighting homes and running appliances. In certain applications and remote settings, such as motorist aid call boxes on highways and pumping water for livestock, PV power is the

Solar Photovoltaic Systems 55

cheapest form of electricity. Electric utility companies are building and including PV systems into their power supply networks. Photovoltaic systems (cell, module, network) require minimal maintenance. At the end of the life cycle, photovoltaic modules can almost be completely recycled. Photovoltaic modules bring electricity to rural areas where there is no electric power grid, and thus increase the life value of these areas. Photovoltaic systems will continue the future development in a direction to become a key factor in the production of electricity for households and buildings in general. The systems are installed on existing roofs and/or are integrated into the facade. These systems contribute to reducing energy consumption in buildings. A series of legislative acts of the European Union in the field of renewable energy and energy efficiency have been developed, particularly promoting photovoltaic technology for achieving the objectives of energy savings and CO2 reduction in public, private and commercial buildings. Also, photovoltaic technology, as a renewable energy source, contributes to power systems through diversification of energy sources and security of electricity supply. By the introduction of incentives for the energy produced by renewable sources in all developed countries, photovoltaic systems have become very affordable, and timely return of investment in photovoltaic systems has become short and constantly decreasing. In recent years, this industry is growing at a rate of 40% per year and the photovoltaic technology creates thousands of jobs at the local level.

4.4

Photovoltaic effect

The photovoltaic effect is the basic physical process through which a PV cell converts sunlight directly into electricity. PV technology works any time the Sun is shining, but more electricity is produced when the light is more intense and when it is striking the PV modules directly—when the rays of sunlight are perpendicular to the PV modules. Unlike solar systems for heating water, PV technology does not produce heat to make electricity. Instead, PV cells generate electricity directly from the electrons freed by the interaction of radiant energy with the semiconductor materials in the PV cells. Sunlight is composed of photons, or bundles of radiant energy. When photons strike a PV cell, they may be reflected, absorbed, or transmitted through the cell. Only the absorbed photons generate electricity. When the photons are absorbed, the energy of the photons is transferred to electrons in the atoms of the solar cell, which is actually a semiconductor. With their new-found energy, the electrons are able to escape from their normal positions associated with their atoms to become part of the current in

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an electrical circuit. By leaving their positions, the electrons cause holes to form in the atomic structure of the cell into which other electrons can move. Special electrical properties of the PV cell—a built-in electric field—provide the voltage needed to drive the current through a circuit and power an external load, such as a light bulb.

4.5

Photovoltaic Cells

The basic building block of PV technology is the photovoltaic cell. Different materials are used to produce PV cells, but silicon—the main ingredient in sand—is the most common basic material. Silicon, a common semiconductor material, is relatively cheap because it is widely available and used in other things, such as televisions, radios, and computers. PV cells, however, require very pure silicon, which can be expensive to produce. The amount of electricity a PV cell produces depends on its size, its conversion efficiency, and the intensity of the light source. Efficiency is a measure of the amount of electricity produced from the sunlight a cell receives. A typical PV cell produces 0.5 volts of electricity. It takes just a few PV cells to produce enough electricity to power a small watch or solar calculator. The most important parts of a PV cell are the semiconductor layers, where the electric current is created. There are a number of different materials suitable for making these semi-conducting layers, and each has benefits and drawbacks. Unfortunately, there is no one ideal material for all types of cells and applications.

4.6

Functions of the photovoltaic cells

The word ‘photovoltaic’ consists of two words: photo, a greek word for light, and voltaic, which defines the measurement value by which the activity of the electric field is expressed, i.e., the difference of potentials. Photovoltaic systems use cells to convert sunlight into electricity. Converting solar energy into electricity in a photovoltaic installation is the most known way of using solar energy. Functions of the photovoltaic cell is shown in Fig. 4.1. The light has a dual character according to quantum physics. Light is a particle and it is a wave. The particles of light are called photons. Photons are massless particles, moving at light speed. The energy of the photon depends on its wavelength and the frequency, and we can calculate it by the Einstein’s law, which is: E = hυ where: E = Photon energy h = Planck’s constant h = 6.626 × 10–34 Js υ = Photon frequency

Solar Photovoltaic Systems 57

Load

Current

Sunlight

n-type silicon Junction p-type silicon

Photons Electron flow – – +

+ Hole flow

Figure 4.1: Functions of the photovoltaic cell.

In metals and in the matter generally, electrons can exist as valence or as free. Valence electrons are associated with the atom, while the free electrons can move freely. In order for the valence electron to become free, we must get the energy that is greater than or equal to the energy of binding. Binding energy is the energy by which an electron is bound to an atom in one of the atomic bonds. In the case of photoelectric effect, the electron acquires the required energy by the collision with a photon. Part of the photon energy is consumed for the electron getting free from the influence of the atom which it is attached to, and the remaining energy is converted into kinetic energy of a now free electron. Free electrons obtained by the photoelectric effect are also called photoelectrons. The energy required to release a valence electron from the impact of an atom is called a ‘work out’ Wi, and it depends on the type of material in which the photoelectric effect has occurred.

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The equation that describes this process is as follows: hυ = Wi = Ekin where, hυ = Photon energy Wi = Work out Ekin = Kinetic energy of emitted electron The previous equation shows that the electron will be released if the photon energy is less than the work output. The photoelectric conversion in the PV junction. PV junction (diode) is a boundary between two differently doped semiconductor layers; one is a P-type layer (excess holes), and the second one is an N-type (excess electrons). At the boundary between the P and the N area, there is a spontaneous electric field, which affects the generated electrons and holes and determines the direction of the current. To obtain the energy by the photoelectric effect, there shall be a directed motion of photoelectrons, i.e., electricity. All charged particles, photoelectrons also, move in a directed motion under the influence of electric field. The electric field in the material itself is located in semiconductors, precisely in the impoverished area of PV junction (diode). It was pointed out for the semiconductors that, along with the free electrons in them, there are cavities as charge carriers, which are a sort of a by-product in the emergence of free electrons. Cavities occurs whenever the valence electron turns into a free electron, and this process is called the generation, while the reverse process, when the free electron fills the empty spaces - a cavity, is called recombination. If the electron-cavity pairs occur away from the impoverished areas it is possible to recombine before they are separated by the electric field. Photoelectrons and cavities in semiconductors are accumulated at opposite ends, thereby creating an electromotive force. If a consuming device is connected to such a system, the current will flow and we will get electricity. In this way, solar cells produce a voltage around 0.5–0.7 V, with a current density of about several tens of mA/cm2 depending on the solar radiation power as well as on the radiation spectrum. The usefulness of a photovoltaic solar cell is defined as the ratio of electric power provided by the PV solar cells and the solar radiation power. Mathematically, it can be presented in the following relation: η=

Pel U ⋅I = Psol E⋅A

where, Pel = Electrical output power Psol = Radiation power (Sun)

Solar Photovoltaic Systems 59

U = Effective value of output voltage I = Effective value of the electricity output E = Specific radiation power (for example W/m2) A = Area The usefulness of PV solar cells ranges from a few per cent to forty per cent. The remaining energy that is not converted into electrical energy is mainly converted into heat energy and thus warms the cell. Generally, the increase in solar cell temperature reduces the usefulness of PV cells. Standard calculations for the energy efficiency of solar photovoltaic cells are explained below. Energy conversion efficiency of a solar photovoltaic cell (η ‘ETA’) is the percentage of energy from the incident light that actually ends up as electricity. This is calculated at the point of maximum power, Pm, divided by the input light irradiation (E, in W/m2), all under standard test conditions (STC) and the surface of photovoltaic solar cells (AC in m2). η=

Pm E × Ac

Standard test conditions (STC), according to which the reference solar radiation is 1.000 W/m2, spectral distribution is 1.5 and cell temperature 25°C.

4.7

Types of PV cell materials

Let’s look more closely at how a PV cell is made and how it produces electricity which involves four step (Fig. 4.2) which are discussed below: Step 1: A slab (or wafer) of pure silicon is used to make a PV cell. The top of the slab is very thinly diffused with an ‘n’ dopant, such as phosphorous. On the base of the slab, a small amount of a ‘p’ dopant, typically boron, is diffused. The boron side of the slab is 1000 times thicker than the phosphorous side. Dopants are similar in atomic structure to the primary material. The phosphorous has one more electron in its outer shell than silicon, and the boron has one less. These dopants help create the electric field that motivates the energetic electrons out of the cell created when light strikes the PV cell. The phosphorous gives the wafer of silicon an excess of free electrons; it has a negative character. This is called the n-type silicon (n = negative). The n-type silicon is not charged it has an equal number of protons and electrons— but some of the electrons are not held tightly to the atoms. They are free to move to different locations within the layer. This silicon has a negative character, but not a negative charge. The boron gives the base of the silicon wafer a positive character, which will cause electrons to flow toward it. The base of the silicon is called p-type

60 Textbook of Renewable Energy Step 1 Negative character

+ –+ – + – + – +– + –

n-type silicon

Positive character

+ –

+ –

+ –

p-type silicon

+ –

Step 2 Positive character

+– +

Electric field

+

+ – +

+ –

Negative character

p-n junction p-type

+– – + – – + – – + – Step 3

n-type

Sun

Photons

Electric field

Positive character

+

+ – +

+ – +

n-type p-n junction

Negative character p-type

+ ––+ – – + – –+ – Step 4

Electric field

+

Sun









Free electron



Load



+ –

Proton Tightly-held electron A location that can accept an electron



Free electron

Figure 4.2: Various step involved in producing electricity.

Solar Photovoltaic Systems 61

silicon ( p = positive). The p-type silicon has an equal number of protons and electrons; it has a positive character, but not a positive charge. Step 2: Where the n-type silicon and p-type silicon meet, free electrons from the n-layer flow into the p-layer for a split second, then form a barrier to prevent more electrons from moving between the two sides. This point of contact and barrier is called the p-n junction. When both sides of the silicon slab are doped, there is now a negative charge in the p-type section of the junction and a positive charge in the n-type section of the junction due to movement of the electrons and ‘holes’ at the junction of the two types of materials. This imbalance in electrical charge at the p-n junction produces an electric field between the p-type and n-type. Step 3: If the PV cell is placed in the Sun, photons of light strike the electrons in the p-n junction and energise them, knocking them free of their atoms. These electrons are attracted to the positive charge in the n-type silicon and repelled by the negative charge in the p-type silicon. Most photon-electron collisions actually occur in the silicon base. Step 4: A conducting wire connects the p-type silicon to an external load such as a light or battery, and then back to the n-type silicon, forming a complete circuit. As the free electrons are pushed into the n-type silicon, they repel each other because they are of like charge. The wire provides a path for the electrons to move away from each other. This flow of electrons is an electric current that can power a load, such as a calculator or other device, as it travels through the circuit from the n-type to the p-type. In addition to the semiconducting materials, solar cells consist of a top metallic grid or other electrical contact to collect electrons from the semiconductor and transfer them to the external load, and a back contact layer to complete the electrical circuit.

4.8

PV modules and arrays

For more power, PV cells are connected together to form larger units called modules. Photovoltaic cells are connected in series and/or parallel circuits to produce higher voltages, currents, and power levels. A PV module is the smallest PV component sold commercially, and can range in power output from about 10 watts to 300 watts. A typical PV module consists of PV cells sandwiched between a clear front sheet, usually glass, and a backing sheet, usually glass or a type of tough plastic. This protects them from breakage and from the weather. An aluminum frame can be fitted around the PV module to enable easy affixing to a support structure. Photovoltaic arrays include two or more PV modules assembled as a pre-wired, field installable unit. A PV array is the complete power-generating unit, consisting of any number of modules and panels.

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4.9

PV system components

Although a PV module produces power when exposed to sunlight, a number of other components are required to properly conduct, control, convert, distribute, and store the energy produced by the array. Depending on the type of system, these components may include:

4.9.1

Power inverter

PV modules, because of their electrical properties, produce direct current rather than alternating current. Direct current (DC) is electric current that flows in a single direction. Many simple devices, such as those that run on batteries, use direct current. Alternating current (AC), in contrast, is electric current that reverses its direction of flow at regular intervals (120 times per second). This is the type of electricity provided by utilities, and the type required to run most modern appliances and electronic devices. In the simplest systems, DC current produced by PV modules is used directly. In applications where AC current is necessary, an inverter can be added to the system to convert DC to AC current.

4.9.2

Battery system

PV systems cannot store electricity, so batteries are often added. A PV system with a battery is configured by connecting the PV array to an inverter. The inverter is connected to a battery bank and to any load. During daylight hours, the PV array charges the battery bank. The battery bank supplies power to the load whenever it is needed. A device called a charge controller keeps the battery properly charged and prolongs its life by protecting it from being overcharged or completely discharged. PV systems with batteries can be designed to power DC or AC equipment. Systems operating only DC equipment do not need an inverter, only a charge controller. It is useful to remember that any time conversions are made in a system, there are associated losses. For example, when an inverter is used there is a small loss of power that can be described by the inverter’s conversion efficiency. Likewise, when batteries are used to store power, not only is there additional expense to purchase the batteries and associated equipment, but due to the internal resistance of the batteries there is a small loss of power as the charge is drawn out of the batteries.

4.10

Types of photovoltaic system

Photovoltaic systems can be generally divided into two basic groups: 1. Photovoltaic systems not connected to the network, stand-alone systems (off-grid). 2. Photovoltaic systems connected to public electricity network (on-grid).

Solar Photovoltaic Systems 63

There are lots of different subtypes of photovoltaic systems according to type and method of connecting to the network, or a way of storing energy on independent systems.

4.10.1

Network connected photovoltaic systems (ongrid)

The main components of PV systems are photovoltaic modules, photovoltaic inverter, mounting subframe and measuring cabinet with protective equipment and installation. Photovoltaic modules convert solar energy into DC current, while photovoltaic inverter adjusts the produced energy in a form which can be submitted to the public grid. The AC voltage is supplied to the electricity network through the protection and measuring equipment. Grid-connected systems is shown in Fig. 4.3. PV array

Inverter/power conditioning unit

Distribution panel

AC loads

Electric utility

Figure 4.3: Grid-connected systems.

Photovoltaic inverter is usually located indoors, although there are inverters for outdoor installation, where it must not be directly exposed to sunlight. Inverters produce high-quality AC current of corresponding voltage and are suitable for a network-connected photovoltaic systems. Network inverters operate like any other inverter, with the difference that the network inverters must ensure that the voltage they supply is in phase with the network voltage. This allows the photovoltaic systems to deliver the electricity to the electrical network. Electrical connection is usually located in the electrical control box, which is located in a separate room, but can also be placed in the measurement and terminal box, which then connects to the electrical control box. The meter is installed at the point of connection, a single phase, two-tariff, electronic system for single-phase, and a three phase, two-tariff, electronic system for two-phase and three phase systems. In such installations it is regularly proposed to setting

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up a fuse in front of and behind the counters in order to permit replacement of the meter at a no-load condition. The exact conditions of connection are synchronised with the local distributor of electric energy. Power OFF buttons must be provided both on the side of photovoltaic modules as well as on the side of network connection. The output voltage of the inverter must be in accordance with the Regulation on standardised voltages for low voltage electricity distribution network and electrical equipment. Standard sizes of the nominal voltage is 230 V, up to 400 V between phase and neutral conductor, between phase conductors, the quad-phase network nominal frequency of 50 Hz, and, under normal conditions, it should not differ from the nominal value by more than ±10%. Due to the large exposures to lightning, besides being connected to the lightning protection installation, the photovoltaic modules are protected by arresters and bias as well. Arresters are installed immediately after the module in order to prevent the impact of bias on the installation of the building.

4.10.2

Network connected home systems (possibility for own consumption)

These are the most popular types of solar photovoltaic systems that are suitable for home and commercial installations in developed and urban areas. Connection to the local electricity network allows selling to the local distributor of electric energy any excess of electricity generated and not used in the household consumption, because the PV system is connected to the network via a home installation in parallel operation with the distribution system. Also, the home is supplied with electricity from the grid when there is no sunny weather. The inverter, as already discussed, is used to convert direct current (DC) produced by the photovoltaic modules into alternating current (AC) located in the electrical grid and used to drive all the household appliances. This system gives two choices to the user: To sell the entire electricity produced to the local distributor, delivering all the electricity in the network (especially if there is a price incentive for electricity produced from renewable sources according to the status of eligible producer of electric energy - feed-in tariffs) or the electricity produced can be used to meet the current needs of households and sell any surplus in the electricity grid. The increase of interest in this type of connecting the photovoltaic system to the grid is expected to happen with convergence of prices of the electricity produced in a conventional way with the price of the electricity produced from renewable energy sources. So far, the incentive feed-in tariffs are favouring network installations only, although the photovoltaic system produces the most

Solar Photovoltaic Systems 65

electricity at mid-day, when the Sun is up, and can thus meet the energy needs and thereby relieve the power system.

4.10.3

Network connected solar power plants (farms)

These systems, also connected to the network, are generating large amounts of electricity by a photovoltaic installation on a localised area. The power of such photovoltaic power ranges from several hundred kilowatts to tens of megawatts, recently up to several hundred megawatts. Some of these installations can be located on large industrial facilities and terminals, but more often on large barren land surfaces. Such large installations are exploiting existing facilities to produce electricity at the location and thus compensate part of the electric energy demand in the area.

4.10.4

Standalone systems (offgrid) or isolated systems

These systems are used in rural areas where there is no electricity network and infrastructure. The systems are connected to a reservoir of energy (battery) by a control over the filling and emptying. The inverter can also be used to provide alternating current for standard electrical equipment and appliances. Stand-alone systems is shown in Fig. 4.4. PV array

Charge controller

Energy storage (battery)

DC load (energy use)

Inverter

Energy distribution

AC loads

Figure 4.4: Standalone photovoltaic system.

Typical stand-alone photovoltaic installations are used to ensure the availability of electricity in remote areas (mountain resorts, islands, rural areas in the developing areas). Rural electrification means either small home solar photovoltaic installations covering basic electricity needs of an individual household, or bigger solar photovoltaic network that provides enough electricity for several households.

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4.10.5

Hybrid systems

A solar photovoltaic system can be combined with other energy sources, such as biomass generator, wind turbines, diesel generator, all to ensure a constant and sufficient supply of electricity, since it is known that all renewable energy sources, including photovoltaic systems, are not constant in energy production. It means that, when there is no Sun, the system does not produce electricity, although the need for energy is constant, and therefore must be met from other sources. The hybrid system can be connected to a network, standalone or as a support network.

4.10.6

Independent systems for economic purposes

Use of electricity produced in solar photovoltaic systems in remote installations far from electrical networks is very common. An example is telecommunication equipment, especially for bridging the rural areas with the rest of the country with built electric grid. CDMA mobile stations are powered by photovoltaic or hybrid systems. Other photovoltaic installations, such as for traffic signs and lights, are today competitive because the cost of bringing electricity infrastructure in these remote place is quite high.

4.11

Scale of PV systems

There are three general scales at which photovoltaic systems are generally installed.

4.11.1

Residential

A residential system is designed to offset power usage at an individual residence. While usually unable to provide all power used by the homeowners, the system could help to offset the home’s electricity usage. This type of system might produce enough electricity to power part, or all, of one home’s electricity needs.

4.11.2

Commercial

A commercial system is designed to offset power usage at a business or industrial site. These systems are much larger than residential systems that can produce more power due to the often expansive roof-top space available for their installation. An example would be a grocery store that contracts with a company to place a solar array on their flat roof while simultaneously contracting to buy power from the installer at a fixed rate for many years. This type of system might produce enough electricity to operate all or part of the business or industrial site.

Solar Photovoltaic Systems 67

4.11.3

Utility

Utility systems are employed by energy companies to produce baseload or peak load power for sale to consumers. Large areas of land are typically required for their installation. An example would be a large PV array that is employed to produce power at peak usage times in the summer months when air conditioning accounts for a large part of the electrical usage. The array produces the most power when the Sun is at its peak and causing consumers to turn down their thermostats—requiring the extra electricity produced by the array.

4.12

Developing PV technologies

Today there are many new PV technologies either on the market, in the pipeline, or in the research phase. These technologies will have a direct effect on how much of our energy we derive from solar power in the future. Look for technologies that will make things less expensive or serve multiple purposes as they are applied to new designs.

4.12.1

Ribbon silicon

Thin crystalline silicon sheets are drawn out of molten silicon rather than being sawed from an ingot. This method is less expensive and less wasteful to produce silicon. However, the finished product is usually a lower quality material. In some cases, they will have cells of a higher conversion efficiency.

4.12.2

Thin-film technologies

This new class of materials allows the production of PV cells that are smaller and more flexible than the delicate silicon wafer technology that has dominated PV cell production in the past. These materials are not crystalline, but amorphous, in structure. This type of PV cell can actually be applied to a variety of materials to make any number of materials that you might use for another purpose—such as glazing for a window, or shingles for a roof. Imagine windows that produce electricity. Materials used for dual purposes (building material and PV cell) are called Building Integrated Photovoltaics (BIPV). CdTe: Cadmium telluride

This thin-film technology has higher solar spectrum absorption and lower costs to manufacture. It can have a conversion efficiency of up to 19%. There are concerns about the toxicity and scarcity of chemicals necessary for its production.

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CIGS: Copper indium gallium diselenide

The gallium is added to these thin-film cells to increase the energy absorption of the cells, which increases efficiency. This technology, although slightly more complicated, has a similar conversion efficiency of 20%. Earth abundant materials

Manufacturing PV cells from abundant, low cost resources is a research priority. One of the promising technologies is sulfoselenide or CZTS. The drawback to CZTS is a lower efficiency than other PV cells. Thin-film materials are much cheaper to produce and are lightweight. They are very versatile in how they can be applied to many structural materials. They can also be less efficient than current silicon crystal PV cells. However, what they lack in efficiency may be overcome by their flexibility of application and low cost.

4.12.3

Multijunction technologies

This category actually combines multiple layers of materials that are designed to absorb different wavelengths of solar energy—improving the efficiency of the cell by combining the output of the various layers. Multijunction cells are a high-cost PV technology, but can reach efficiencies of over 43%.

4.12.4

Dye sensitised solar cells

This organic-inorganic hybrid technology shows promise to be a very low cost technology. Using a small-molecule dye that absorbs photons, an accepting material such as zinc oxide, and an electrolyte, this technology is easy to manufacture from abundant materials. Research continues to improve durability and efficiency.

4.13

Benefits and limitations of photovoltaic systems

4.13.1

Benefits of photovoltaic systems

Photovoltaic systems offer many advantages: 1. They are safe, clean, and quiet to operate. 2. They are highly reliable. 3. They require virtually no maintenance. 4. They are cost-effective in remote areas and for some residential and commercial applications. 5. They are flexible and can be expanded to meet increasing electrical needs for homes and businesses. 6. They can provide independence from the grid or back-up during outages. 7. The fuel is renewable, domestically available, and free.

Solar Photovoltaic Systems 69

4.13.2

Limitations of photovoltaic systems

There are also some practical limitations to PV systems: 1. PV systems cannot operate all the time. 2. PV systems are not well suited for energy-intensive uses such as heating. 3. Grid-connected systems are becoming more economical, but can be expensive to buy and install. 4. Large amounts of land or space are required for utility or large scale generation. 5. The process to make PV technologies can have harmful effects on the environment.

5 Biogas

5.1

Introduction

Biogas is a combustible mixture of gases such as methane and carbon dioxide. Biogas is produced from the process of anaerobic digestion (AD) of wet organic waste, such as cattle and pig slurries, food wastes and grown wet biomass. The AD process produces CO2, methane and a digestate that can be used as fertiliser. The methane (biogas) can be burned to provide heat and/or electricity, or it can be used as a transport fuel in compressed form in the same way as compressed natural gas (CNG). Biogas can be generated from anything organic that contains carbon, it is the anaerobic digestion of biological material, ranging from cooking oils and crops to manure. Its composition is generally 60% methane and 25–39% carbon dioxide and other trace elements, though this mixture varies depending on the material used.

5.2

Resource availability of biogas

Biogas can be produced by the anaerobic digestion of a range of organic wastes, with the key wastes being: 1. Sewage sludge. 2. Wet manure slurries from intensive styles of agriculture. 3. Dry manures from animal beddings, known as farm yard manure (FYM). 4. Waste from food processing. 5. Food and organic waste from restaurants and other commercial operations. 6. Household kitchen and garden waste. 7. Leather industry wastes. 8. Abattoir industry wastes. 9. Pulp and paper industry wastewater. 10. Municipal wastewater/sewage. 11. Vegetable market yard wastes. 12. Animal/agro residue. All these wastes can be treated in other ways as well, such as landfill, direct spreading to land and composting. Sources of biogas production are shown in Fig. 5.1.

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Sewage

Compost/ other treatment

Household kitchen and garden waste Wet manure slurries

Digestate

Post treatment Heat Gas CHP

AD

Dry manure (FYM) Food processing waste

Spread to land

Methane

Biogas

Electric

Gas upgrade Vehicles

Landfill

Hydrogen

Commercial waste

Figure 5.1: Sources of biogas production.

5.3

Types of biogas plants

5.3.1

Fixed-dome plants

A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which sits on top of the digester. When gas production starts, the slurry is displaced into the compensation tank. Gas pressure increases with the volume of gas stored and the height difference between the slurry level in the digester and the slurry level in the compensation tank.

5.3.2

Floating drum plants

Floating-drum plants consist of an underground digester and a moving gasholder. The gas-holder floats either directly on the fermentation slurry or in a water jacket of its own. The gas is collected in the gas drum, which rises or moves down, according to the amount of gas stored. The gas drum is prevented from tilting by a guiding frame. If the drum floats in a water jacket, it cannot get stuck, even in substrate with high solid content.

5.3.3

Low cost polyethylene tube digester

In the case of the low-cost polyethylene tube digester model the tubular polyethylene film (two coats of 300 microns) is bended at each end around a

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6′′ PVC drainpipe and is wound with rubber strap of recycled tyre-tubes. With this system a hermetic isolated tank is obtained.

5.3.4

Balloon plants

A balloon plant consists of a heat-sealed plastic or rubber bag (balloon), combining digester and gas-holder. The gas is stored in the upper part of the balloon. The inlet and outlet are attached directly to the skin of the balloon. Gas pressure can be increased by placing weights on the balloon. If the gas pressure exceeds a limit that the balloon can withstand, it may damage the skin. Therefore, safety valves are required. If higher gas pressures are needed, a gas pump is required.

5.3.5

Horizontal plants

Horizontal biogas plants are usually chosen when shallow installation is called for (groundwater, rock). They are made of masonry or concrete stucture.

5.3.6

Earth pit plants

Masonry digesters are not necessary in stable soil (e.g., laterite). It is sufficient to line the pit with a thin layer of cement (wire-mesh fixed to the pit wall and plastered) in order to prevent seepage. The edge of the pit is reinforced with a ring of masonry that also serves as anchorage for the gas-holder. The gas holder can be made of metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden frame that extends down into the slurry and is anchored in place to counter its buoyancy. The requisite gas pressure is achieved by placing weights on the gasholder. An overflow point in the peripheral wall serves as the slurry outlet.

5.3.7

Ferrocement plants

The ferro-cement type of construction can be applied either as a self supporting shell or an Earth-pit lining. The vessel is usually cylindrical. Very small plants (Volume under 6 m3) can be prefabricated. As in the case of a fixed-dome plant, the ferrocement gasholder requires special sealing measures (proven reliability with cemented-on aluminium foil).

5.4

Principles of biogas technology

Biogas technology, i.e., anaerobic digestion is biological method for degrading and stabilising organic, biodegradable raw materials in special plants in a controlled manner. It is based on microbial activity in oxygen-free (anaerobic) conditions and results in two end-products: energy rich biogas and nutrientrich digestion residue, i.e., digestate. Anaerobic degradation of biodegradable

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materials also happens in nature, e.g., in swamps, soils, sediments and in ruminant metabolism.

5.4.1

Anaerobic degradation

During the anaerobic degradation process several different microbial consortia degrade the raw materials in parallel and/or subsequent degradation steps (Fig. 5.2). Hydrolysis occurs as complex organic molecules are broken down into their constituent parts, including fatty acids, amino acids, and simple sugars. Carbohydrates

Lipids

Proteins

Hydrolysis Simple sugars, amino acids

LCFA, alcohols

NH4

Acidogenesis

Intermediary products (VFA) Acetogenesis

Acetate

Hydrogen

Methanogenesis Methane

Figure 5.2: Anaerobic degradation of organic, biodegradable material.

In hydrolysis, the polymers (carbohydrates, proteins and lipids) are degraded into their monomers and dimers, via., hydrolytic enzymes excreted by acidogenic microbes. The higher the surface area of the raw materials, the more efficiently the hydrolytic enzymes can attack the material. Methanogenesis is the final stage, in which methane, analogous to natural gas, is formed, along with carbon dioxide and water.

5.4.2

Factors affecting anaerobic degradation

There are several factors which may affect the anaerobic degradation of biodegradable materials. Temperature and pH

Temperature influences the growth and survival of the micro-organisms. The lower the temperature, the slower the chemical and enzymatic reactions and microbial growth are.

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Inhibition and hydrogen partial pressure

Ammonification of organic nitrogen compounds produces ammonium nitrogen, part of which is present as its unionised form of ammonia (NH3). As ammonia is able to enter microbial cells rather freely due to having no electrical charge, it becomes toxic for the microbes at high concentrations. Technical and operational factors

Also technical and operational factors affect anaerobic degradation in biogas processes. For instance, mixing is important in all digester types. It is used to ensure good contact between the raw materials and the microbes, as well as constant temperature and homogenous quality throughout the digester contents.

5.5

Raw materials for biogas processes

Different raw materials will produce different amounts of biogas and methane depending on their content of carbohydrates, fats and proteins. In theory, all biodegradable materials with reasonable lignin content (i.e., not wood) are suitable raw materials for biogas processes. In agriculture, manure and most plant biomass can be directed to biogas plants, while from municipalities, food waste and sewage sludge are the most important material flows to biogas processes. Moreover, different industries produce biodegradable by-products which can be used in biogas plants.

5.6

Technological and operational solutions for biogas plants

Biogas plants are always case-specific. They are designed according to those particular conditions and characteristics and quantities of raw materials as intended when commissioning a plant. There are several technological and operational solutions to choose from and the length of the technology chain applied differ from smaller to larger scale according to factors, such as investment and operational cost, workload, the end-use of digestate intended, goals for energy production, etc. In small household plants very simple technological solutions are used. On farmscale the technology becomes somewhat more elaborate, but the aim is to still keep it simple and easy-to-use, while on large, centralised scale the biogas plant may consist of several different processing units the operation of which requires more monitoring and know how.

5.6.1

Different scales

Biogas is produced in biogas plants which differ in size (scale) and technology. Small and often self-made biogas plants are used in tropical countries for treating wastes from the household farming and cooking. In industrial countries

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with intensive agriculture the biogas plants are significantly bigger and more advanced, equipped with modern technology to increase digester capacity and to apply process control for stable operation.

5.6.2

Digester technologies

There are several different digester technologies used for anaerobic digestion. Olsson and others have divided biogas technology into three generations by level of technological approach and increase of bioconversion capacity, though not all of the technologies described are suitable for all types of raw materials. While continuously-stirred anaerobic digester to convert organic (CSTR) is still the most common and widely-used process for digestion of manure, energy crops and diverse municipal and industrial raw materials, upflow anaerobic sludge blanket (UASB), expanded bed (such as internal circulation reactor IC) and fluidised bed are only suitable for more dilute materials, i.e., mostly wastewaters. Agricultural digesters are usually described by construction of digester and process technology. A single (one-stage) CSTR is a common digester in agriculture with continuous feeding of manure and/or energy crops (e.g., maize and/or grass silage), with the process temperature in mesophilic range from 35 to 40°C. The plant may also be two- or multi-staged. In agricultural biogas plants this usually refers to the main reactor with temperature control and a separate post-digestion tank with biogas collection. Also a pre-reactor may be used for separating hydrolysis and acidification into one tank and the methane producing phase into another, the ultimate being optimisation of the conditions for the degradation phases.

5.6.3

Pre-treatments

Different pre-treatments may be used prior to the actual biogas reactor in order to: (i) improve the degradation of the raw materials (higher VS removal), (ii) increase biogas production, (iii) ensure the hygienic quality of the digestate, (iv) facilitate technical functions (e.g., prevent blockages), (v) ensure homogenous feed and reactor biomass (e.g., large particles of plant biomass float more easily than smaller ones), (vi) remove potentially inhibitive compounds, and/or (viii) enable process intensification (higher organic loading rate (OLR), shorter hydraulic retention time (HRT), smaller reactor size). Many of the pretreatments used facilitate more than one of the benefits mentioned simultaneously.

5.6.4

Post-treatments

Post-digestion tank

A post-digestion tank is an integral part of all biogas plants as it allows the feed to continue degradation and collects the remaining biogas potential in a

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controlled manner. This is not only important for minimising methane emissions of biogas plants, but also offers a significant increase in the overall biogas production of the plant. The remaining biogas potential of any digester residue is significant and may provide 10–30% of the overall biogas production in a biogas plant. Mechanical separation of digestate

The digestate can be mechanically separated into liquid and solid fractions which can then be utilised as fertilising and/or soil improving products as such or post-processed further. Mechanical separation can be performed with different technologies, and the technology choice affects the separation efficiency. Also, the characteristics of the digestate influence the end result. Post-processing of digestate solid fraction

The liquid and solid fractions from mechanical separation may be processed further in order to produce more targeted fertiliser products. Post-processing of digestate liquid fraction

In ammonia stripping, ammonia is stripped from the liquid fraction of the digestate into gaseous phase by increasing the pH (usually 11) and/or temperature (usually 70°C) to facilitate conversion of ammonium nitrogen to ammonia and then blowing the liquid with, e.g., air in order to make the easily evaporative ammonia shift into the gaseous phase.

5.7

Energy use of biogas

Biogas is a versatile, renewable fuel that can be used for production of heat, electricity and/or vehicle fuel. Biogas can be combusted in gas boilers to produce heat or in gas engines or turbines to produce both electricity and heat. It can also be upgraded to vehicle fuel quality by increasing the methane content through removal of most of the other compounds present.

5.7.1

Compounds in biogas that are disruptive to energy utilisation

In gas pipes and gas storage, the biogas temperature generally drops below the dew point, causing water to condense. The condensate must be drained off from the gas system through gas-tight piping and disposed off. Condensing can also be used as the drying method, i.e., the biogas is by design directed through a well/section in the pipeline in which the temperature is below the dew point and the condensed water is removed from the gas.

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5.7.2

Biogas in heat production

The easiest way of utilising biogas is to produce heat in a boiler. The technology has low investment and maintenance costs and is well-known and reliable. For small scale biogas plants located at a site with a high heat demand, it is probably the best alternative, at least in countries with rather low price for electricity produced with biogas.

5.7.3

Biogas in combined heat and power production (CHP)

CHP generation is the standard utilisation of biogas and considered very efficient for energy production. The technologies to convert biogas into electricity and heat are numerous and in general it is not complicated to connect to the electricity grid, although it is important to have a legislation promoting the connection to the electrical grid for small scale power plants. Most common CHPs are ordinary diesel or otto engines using biogas as fuel. Other methods are gas turbines, stirling motors, Rankine cycles and fuel cells.

5.8

Integrated approach

As organic matter is the raw material needed for biogas plants, an integrated biogas system should be promoted and banks should provide loans for the integration of biogas plants. Integrated programmes with energy sector, agriculture, livestock, forestry, irrigation, health, fishery and small industries help rural communities to raise their standard of living and this has a knock on effect, healthier people , sustainable energy, less deforestation. Biogas plants could be integrated with power pumps, treadle pumps, poultry, piggeries, fodder plants, milks and its products, toilet attachments, etc.

5.9

Upgrading of biogas to natural gas quality

Biogas has to be upgraded to natural gas quality in order to be used in normal vehicles designed to use natural gas. The most common technologies are the water scrubber technology and the PSA-technology. Gas upgrading is normally performed in two steps where the main step is the process that removes the CO2 from the gas. Minor contaminants are normally removed before the CO2removal and the water dew point can be adjusted before or after the upgrading (depending on the process).

5.9.1

Water scrubber technology

Two types of water absorption processes are commonly used for upgrading of gas from anaerobic digestion, single pass absorption and regenerative absorption. The major difference between the two processes is that the water in the single pass process is used only once. A typical installation is at a sewage water treatment

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plant. Water can also be recycled and in this case a stripper column has to be integrated in the process (regenerative absorption).

5.9.2

Pressure swing adsorption (PSA ) technology

Pressure Swing Adsorption, or PSA, is a method for the separation of carbon dioxide from methane by adsorption /desorption of carbon dioxide on zeolites or activated carbon at different pressure levels. The adsorption material adsorbs hydrogen sulphide irreversibly and thus is poisoned by hydrogen sulphide. For this reason a hydrogen sulphide removing step is often included in the PSA process.

5.9.3

Other technologies

In some cases membrane technologies have been used for gas upgrading. The membrane technology has a potential to be energy efficient but for the moment there is very limited experience in Sweden of this technology. Chemical adsorption technologies seem to be an attractive solution due to low methane losses and high selectivity. The process requires a rather high input of thermal energy for the regeneration of the chemical but can on the other hand be operated at low pressure that reduces the electrical energy demand of the process. Biogas vehicles

Biogas can be used in both heavy duty and light duty vehicles. Light duty vehicles can normally run both on natural gas and biogas without any modifications, whereas heavy duty vehicles without closed loop control may have to be adjusted, if they run alternately on biogas and natural gas. Biogas the pathway to hydrogen

Biogas can be regarded as one possible way to gradually change over to hydrogen as energy carrier. There are many similarities: 1. Hydrogen (if produced from sustainable sources) and biogas are both renewable fuels. 2. Hydrogen and biogas can both be distributed on the natural gas grid. 3. Hydrogen and biogas can be used in natural gas vehicles. The first European tests with hydrogen/natural gas mixtures in buses are now carried out in Malmö, Sweden.

5.10

Photosynthesis

Photosynthesis (Photo = light, synthesis = to join) is the single most important process on Earth on which depends the existence of human beings and almost

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all other living organisms. It is a process by which green plants, algae and chlorophyll containing bacteria utilise the energy of sunlight to synthesise their own food (organic matter) from simple inorganic molecules. Innumerable number of organic molecules which compose the living world are derived directly or indirectly from the photosynthetic organic matter. The oxidation of organic compounds releases stored energy to be utilised by the living organisms to carry out essential metabolic processes. It is important to note that photosynthesis is the only natural process which liberates oxygen to be used by all living forms for the process of aerobic respiration. Occurrence of photosynthesis: Photosynthesis occurs in green parts of the plant, mostly the leaves, sometimes the green stems and floral buds. The leaves contain specialised cells called mesophyll cells which contain the chloroplast– the pigment containing organelle. These are the actual sites for photosynthesis.

5.10.1

Significance of photosynthesis

Following points will justify the significance of the process of photosynthesis. 1. Green plants possess the green pigment, chlorophyll which can capture, transform, translocate and store energy which is readily available for all forms of life on this planet. 2. Photosynthesis is a process in which light energy is converted into chemical energy. 3. Except green plants, no other organism can directly utilise solar energy to synthesise food, hence they are dependent on green plants for their survival. 4. Green plants which can prepare organic food from simple inorganic elements are called autotrophic while all other organisms which cannot prepare their own food are called heterotrophic. 5. During photosynthesis, oxygen liberated into the atmosphere makes the environment livable for all aerobic organisms. 6. Simple carbohydrates produced in photosynthesis are transformed into lipids, proteins, nucleic acids and other organic molecules. 7. Plants and plant products are the major food sources of almost all organisms on the Earth. 8. Fossil fuels like coal, gas, and oil represent the photosynthetic products of the plants belonging to early geological periods. Photosynthesis is the process by which green plants, in the presence of light combine water and carbon dioxide to form carbohydrates. Oxygen is released as a by-product of photosynthesis. Current knowledge of photosynthesis has resulted from discoveries made over 300 years of work.

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Photosynthesis is represented by the following overall chemical equation: Chlorophyll

6CO 2 + 12H 2 O ⎯⎯⎯⎯⎯ → C6 H12 O6 + 6H 2 O + 6O2 Sunlight In photosynthesis, CO2 is fixed (or reduced) to carbohydrates (glucose C6H12O6). Water is split in the presence of light (called photolysis of water) to release O2. Note that O2 released comes from the water molecule and not from CO2. Role of sunlight in photosynthesis

Light consists of small particles or packages of energy called ‘photons’. A single photon is also called quantum. The chlorophyll will absorbs light energy. For investigating a process such as photosynthesis that is activated by light, it is important to establish the action spectrum for the process and to use this to identify the pigments involved.

5.10.2

Factors affecting rate of photosynthesis

Some of the factors affecting rate of photosynthesis are discussed below: Light: The rate of photosynthesis increases with increase of intensity of light within physiological limits or rate of photosynthesis is directly proportional to light intensity. Except on a cloudy day and at nights, light is never a limiting factor in photosynthesis in nature. Temperature: Very high and very low temperature affect the rate of photosynthesis plants and animals adversely. Rate of photosynthesis will rise with temperature from 5–37°C beyond which there is a rapid fall, as the enzymes involved in the process of the dark reaction are denatured at high temperature. Carbon dioxide: Since carbon dioxide being one of the raw materials for photosynthesis, its concentration affects the rate of photosynthesis markedly. Because of its very low concentration (0.03%) in the atmosphere, it acts as limiting factor in natural photosynthesis. Water: Water has an indirect effect on the rate of photosynthesis. Loss of water in the soil is immediately felt by the leaves, which get wilted and their stomata close down thus hampering the absorption of CO2 from the atmosphere. This causes decline in photosynthesis. Oxygen: Concentration of oxygen as an external factor, is never a limiting factor for photosynthesis because it is a by-product of photosynthesis, and it easily diffuses into the atmosphere from the photosynthesing organ, the leaf. However, excesss of O2 surrounding a green plant, reduces photosynthetic rate by promoting the rate of aerobic respiraiton.

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Mineral elements: Some mineral elements like magnesium, copper, manganese and chloride ions, which are components of photosynthetic enzymes, and magnesium as a component of chlorophylls are important, and their deficiency would affect the rate of photosynthesis indirectly by affecting the synthesis of photosynthetic enzymes and chlorophyll, respectively.

5.11

Producer gas

Producer gas is a low calorific value fuel gas comprising of mainly carbon monoxide and nitrogen. It is produced by passing air or a mixture of air and steam through a burning bed of solid fuel such as, coal, coke, wood or biomass. Hydrogen is also present in a significant amount in the producer gas if airsteam blast is used. The exact composition of producer gas depends on the type of fuel, composition of the blast and operating condition. Producer gas is formed in a gasifier, called gas producer.

5.11.1

Reactions involved in producer gas

The reactions involved in gas producer are as follows: When only air is used as blast through the fuel bed (air-blast)

C + O2 + N2 → CO2 + N2

…(5.1)

…(5.2) CO2 + C  2CO Reaction (5.1) is highly exothermic and occurs at temperature above 500°C. Due to heat generation by this reaction, the fuel bed temperature increases. CO2 formed in that reaction reacts with carbon of fuel to form CO. This reaction is named as Boudouard reaction and this reaction is important in the sense that the main component of producer gas, CO, is obtained in this reaction. This reaction is endothermic in nature and is favoured at temperatures above 500°C. As a whole the net process is exothermic. The overall reaction is: …(5.3) 2C + O2 + N2  2CO + N2 The favourable condition for reaction is high temperature, sufficient time of reaction and reactive fuel. If the fuel contains ash of low fusion point, such as below 1100°C, it melts and resolidifies into the cooler part of the fuel bed. This is called clinker and it disturbs the uniform burning of fuel and thus the overall efficiency decreases. The coals having big lumps are also not suitable as a good fuel due to non-uniformity of the bed. The lumps must be broken into small pieces for getting higher efficiency. After producer gas is formed in the bed, an opposite reaction to the Boudouard reaction may occur, which is called Neumann reversal reaction, to form CO2 and C. …(5.4) 2CO  CO2 + C

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When steam is used in admixture of air (steam blast)

At this condition, the above reactions occur (reactions 5.1, 5.2, 5.3, 5.4) and along with those, some other reactions also occur. Carbon reacts with steam to form carbon monoxide and carbon dioxide by the following reactions: …(5.5) C + H2O  CO + H2 …(5.6) C + 2H2O  CO2 + 2H2 Both the reactions (5.3) and (5.6) are endothermic. The reaction (5.5) is active at or above the temperature 1000°C but reaction (5.6) occurs at the temperature range of 500 to 600°C. The later reaction (5.6) is not desirable as it produces CO2, which is not a component of producer gas, hence, always the fuel bed temperature is kept high to avoid this reaction. The excess steam may also react with CO to form CO2 and H2 in water gas shift reaction as shown below. This is also an undesired side reaction. CO + H2O  CO2 + H2 …(5.7) Methanation or methane formation is another side reaction observed. …(5.8) C + 2H2  CH4 The above reaction is not favoured at high temperature. There are several advantages of using steam blast over air blast. In steam blast, hydrogen and methane are the two gaseous components formed which add more calorific value to producer gas. The endothermic reactions (5.5) and (5.6), which occur in steam blast do not allow the fuel bed temperature to shoot up very high, and this way they prevent clinkering of bed to a great extent. Clinkering reduces overall efficiency. The optimum temperature required for producer gas manufacture is within the range 1100–1300°C. There is some restriction on the amount of steam also. The endothermic reactions, (5.5) and (5.6) occur at a faster rate using large amount of steam, which thereby reduces the fuel bed temperature below 1100°C. Lower temperature of fuel bed encourages the carbon dioxide formation by the reactions (5.1), (5.6). Steam blast is formed either by injecting steam to the air or passing air through water. The first process is more convenient to use. The temperature of the air is raised by blowing steam into it upto a desired temperature. This temperature is called ‘blast saturation’ temperature.

5.11.2

Gas producer

In the gas producer, the fuel bed is set on a metallic grate. Figure 5.3 shows a fuel bed in a gas producer with different reaction zones. The zones are ash zone, oxidation zone, primary reduction zone, secondary reduction zone and drying zone. Different reactions occur at different zones of the bed. In a

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Gas space Decrease in calorofic value

Producer gas

Drying and carbonisation zone

Secondary reduction zone

Primary reduction zone

Oxidation zone Ash zone

Figure 5.3: Different reaction zones in a gas producer.

countercurrent movement of air-steam blast and solid fuel, the blast gets preheated at the ash zone. In the oxidation zone, carbon dioxide is formed by the reaction of carbon and oxygen of air. In the primary reduction zone, several reactions occur which produce carbon monoxide, carbon dioxide and hydrogen (reactions 5.2, 5.5 and 5.6). At this stage carbon monoxide formation is quite high. After this, secondary reduction zone starts where steam reacts with carbon monoxide to produce carbon dioxide. The topmost zone is drying zone where water vapour and volatile matter of the fuel are added to the gas. Hence, it is observed that, as the gas travels through the bed, its composition goes on changing at each point. The addition of volatile matter increases the calorific value of the exit gas. After the gas leaves the bed, it comes to the gas space above the bed. Here, water gas shift reaction (5.7) and Neumann reaction (5.4) occur, where, amount of carbon monoxide decreases. Hence, the composition of the producer gas is changed and a decrease in calorific value of the gas is observed. Different types of gas producers are used in industry. Depending on the direction of fuel and blast movement, the producers are broadly classified as, updraft, downdraft and cross draft producers. In these types of producers, the fuels move either countercurrent or concurrent to the flow of gasification

Biogas 85

medium (steam, air or oxygen) as the fuel is converted to fuel gas. They are relatively simple to operate in a fixed bed process. In an updraft fixed bed producer, the flows of the fuel and gases are countercurrent to each other. The reactive agent, i.e., air-steam blast is injected at the bottom of the reactor and ascends to the top while the fuel is introduced at the top and descends to the bottom through zones of progressively increasing temperatures (drying, secondary reduction, primary reduction and oxidation). Heat from the primary reduction and oxidation zones rises upward to provide energy for the next zones. Gases, tar and other volatile compounds are distributed at the top of the reactor and increase the calorific value of producer gas, while ash is removed from the bottom. In case of downdraft producers, the locations of the zones are reversed. The fuel is introduced at the top, and the reactive agent is introduced through a set of nozzles on the side of the reactor. Producer gas leaves from the bottom of the producer. Cross-draft producers exhibit many of the operating characteristics of downdraft gasifiers. Air or air/steam mixtures are introduced into the side of the gasifier near the bottom, while the producer gas is drawn off on the opposite side. Producer gas has a very low calorific value in the range of 1000 to 1200 Kcal/Nm3. Applications include the use of it as fuel for industrial kilns and heat treatment furnaces, such as those found in steel plants. Producer gas is also usable in plants that melt zinc for use in galvanising processes and for melting metals, such as aluminium and copper. It is used for heating open hearth furnaces in the manufacture of steel and glass. It is used for heating muffle furnaces and retorts in the manufacture of coke and coal gas.

6 Biomass

6.1

Introduction

Biomass includes all of the Earth living matter, plants and animals and the remaining of this living matter. Biomass resources include primary, secondary, and tertiary sources of biomass. Primary biomass resources are produced directly by photosynthesis and are taken directly from the land. They include perennial short-rotation woody crops and herbaceous crops, the seeds of oil crops, and residues resulting from the harvesting of agricultural crops and forest trees (e.g., wheat straw, corn stover, and the tops, limbs, and bark from trees). Bioenergy consists of solid, liquid, or gaseous fuels. Liquid fuels can be used directly in the existing road, railroad, and aviation transportation network stock, as well as in engine and turbine electrical power generators. Solid and gaseous fuels can be used for the production of electrical power from purposedesigned direct or indirect turbine-equipped power plants. Chemical products can also be obtained from all organic matter produced. Additionally power and chemicals can come from the use of plant-derived industrial, commercial, or urban wastes, or agricultural or forestry residues. Secondary biomass resources result from the processing of primary biomass resources either physically (e.g., the production of sawdust in mills), chemically (e.g., black liquor from pulping processes), or biologically (e.g., manure production by animals). Tertiary biomass resources are post-consumer residue streams including animal fats and greases, used vegetable oils, packaging wastes, and construction and demolition debris.

6.2

Biomass conversion process

There are various conversion technologies that can convert biomass resources into power, heat, and fuels for potential use in various countries. Figure 6.1 summarises the various biomass conversion processes. The various biomass conversion processes are discussed below: 1. Pyrolysis. 2. Carbonisation. 3. Biomass gasification. 4. Catalytic liquefaction.

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Biochemical platform (sugar)

Biogas platform (anaerobic digestion) Biomass feedstock Thermochemical platform • Pyrolysis • Gasification

Combined heat and power fuels, chemicals and materials

Carbon rich chains platform (biodiesel)

Figure 6.1: Biomass energy conversion process.

6.2.1

Pyrolysis

Pyrolysis is the application of heat to a feedstock in the absence of oxygen to break down the long chain molecules into short chain molecules. Typically the feedstock is biomass or waste, and the process is used to produce a syngas (a mixture of hydrogen, volatile organic compounds, and carbon monoxide). Varying the process conditions allows the production of fluids similar to diesel, and a variety of other products. A more detailed understanding of the physical and chemical properties governing the pyrolytic reactions has allowed the optimisation of reactor conditions necessary for these types of pyrolysis. Further work is now concentrating on the use of high pressure reactor conditions to produce hydrogen and on low pressure catalytic techniques (requiring zeolites) for alcohol production from the pyrolytic oil. The advantage of pyrolysis and gasification are that they convert solid material into gases and vapours which are less costly to handle, transport and store. The gases will burn in boilers, gas turbines and reciprocating engines increasing fuel flexibility and security. Capturing and combusting the methane and carbon monoxide in syngas makes use of the energy in the gas and produces carbon dioxide which is a less potent greenhouse gas than methane and offsets fossil fuel energy production.

6.2.2

Carbonisation

This is an age old pyrolytic process optimised for the production of charcoal. Traditional methods of charcoal production have centred on the use of Earth

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mounds or covered pits into which the wood is piled. Control of the reaction conditions is often crude and relies heavily on experience. The conversion efficiency using these traditional techniques is believed to be very low. During carbonisation most of the volatile components of the wood are eliminated; this process is also called ‘dry wood distillation.’ The modernisation of charcoal production has lead to large increases in production efficiencies with largescale industrial production in Brazil now achieving efficiencies of over 30% (by weight). Recirculating heated gas systems offer the potential to generate large quantities of charcoal and associated by-products, but are presently limited by high investment costs for large scale plant.

6.2.3

Biomass gasification

Biomass gasification, or producing gas from biomass, involves burning biomass under restricted air supply for the generation of producer gas. Producer gas is a mixture of gases: 18–22% carbon monoxide (CO), 8–12% hydrogen (H2), 8–12% carbon dioxide (CO2), 2–4% methane (CH4) and 45–50% nitrogen (N2) making up the rest. Gasification reactions

Producing gas from biomass consists of the following main reactions, which occur inside a biomass gasifier. 1. Drying: Biomass fuels usually contain 10–35% moisture. When biomass is heated to about 100°C, the moisture is converted into steam. 2. Pyrolysis: After drying, as heating continues, the biomass undergoes pyrolysis. Pyrolysis involves burning biomass completely without supplying any oxygen. As a result, the biomass is decomposed or separated into solids, liquids, and gases. Charcoal is the solid part, tar is the liquid part, and flue gases make up the gaseous part. 3. Oxidation: Air is introduced into the gasifier after the decomposition process. During oxidation, which takes place at about 700–1400°C, charcoal, or the solid carbonised fuel, reacts with the oxygen in the air to produce carbon dioxide and heat. C + O2 → CO2 + heat 4. Reduction: At higher temperatures and under reducing conditions, that is when not enough oxygen is available, the following reactions take place forming carbon dioxide, hydrogen, and methane. C + CO2 → 2 CO C + H2O → CO + H2 CO + H2O → CO2 + H2 C + 2H2 → CH4

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Advantages of biomass gasification technologies

1. Mature technology: Biomass gasifier technology is a mature technology and gasifiers are available in several designs and capacities to suit different requirements. 2. Small and modular: The technology is suitable and economical for small, decentralised applications, typically with capacities smaller than a megawatt. 3. Flexible operation: A gasifier based power system, unlike those based on other renewable sources such as the Sun and wind, can generate electricity when required and also wherever required. Whereas large thermal power plants and solar and wind based units are very location specific, biomass gasifier based systems can be set up at almost any place where biomass feedstock is available. 4. Economically viable: For small-scale systems, the cost of power generation by biomass gasification technology is far more reasonable than that of conventional diesel based power generation. 5. Socio-economically beneficial: Biomass gasifier based systems generate employment for local people. 6. Mitigate climate change: Biomass is a CO2 neutral fuel and, therefore, unlike fossil fuels such as diesel does not contribute to net CO2 emissions, which makes biomass based power generation systems an attractive option in mitigating the adverse effects of climate change. Types of gasifiers

Gasifiers can be classified based on the density factor, which is a ratio of the solid matter (the dense phase) a gasifier can burn to the total volume available. Gasifiers can be: (i) dense phase reactors, and (ii) lean phase reactors. Dense phase reactors: In dense phase reactors, the feedstock fills most of the space in the reactor. They are common, available in different designs depending upon the operating conditions, and are of three types: downdraft, updraft, and cross-draft. 1. Downdraft or co-current gasifiers: The downdraft (also known as cocurrent) gasifier is the most common type of gasifier. In downdraft gasifiers (Fig. 6.2), the pyrolysis zone is above the combustion zone and the reduction zone is below the combustion zone. Fuel is fed from the top. The flow of air and gas is downwards (hence the name) through the combustion and reduction zones. The term co-current is used because air moves in the same direction as that of fuel, downwards. A downdraft gasifier is so designed that tar, which is produced in the pyrolysis zone, travels through the combustion zone, where it is broken down or burnt.

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Gas

Drying zone

Pyrolysis zone

Reduction zone

Combustion zone Air

Ash pit

Figure 6.2: Downdraft gasifier.

As a result, the mixture of gases in the exit stream is relatively clean. The position of the combustion zone is thus a critical element in the downdraft gasifier, its main advantage being that it produces gas with low tar content, which is suitable for gas engines. 2. Updraft or counter-current gasifier: In updraft gasifiers (also known as counter-current), air enters from below the grate and flows upwards, whereas the fuel flows downwards. An updraft gasifier (Fig. 6.3) has distinctly defined zones for partial combustion, reduction, pyrolysis, and drying. The gas produced in the reduction zone leaves the gasifier reactor together with the products of pyrolysis from the pyrolysis zone and steam from the drying zone. The resulting combustible producer gas is rich in hydrocarbons (tars) and, therefore, has a higher calorific value, which makes updraft gasifiers more suitable where heat is needed, for example in industrial furnaces. The producer gas needs to be thoroughly cleaned if it is to be used for generating electricity. 3. Cross-draft gasifier: In a cross-draft gasifier (Fig. 6.4), air enters from one side of the gasifier reactor and leaves from the other. Cross-draft gasifiers have a few distinct advantages such as compact construction and low cleaning requirements. Also, cross-draft gasifiers do not need a grate; the ash falls to the bottom and does not come in the way of normal operation.

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Drying zone

Pyrolysis zone

Air

Combustion zone

Air

Reduction zone Ash pit

Gas

Figure 6.3: Updraft gasifier.

Drying zone

Pyrolysis zone Gas

Air Reduction zone Combustion zone Ash pit

Figure 6.4: Cross-draft gasifier.

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Lean phase reactors: Lean phase gasifiers lack separate zones for different reactions. All reactions – drying, combustion, pyrolysis, and reduction – occur in one large reactor chamber. Lean phase reactors are mostly of two types, fluidised bed gasifiers and entrained-flow gasifiers. 1. Fluidised bed gasifiers: In fluidised bed gasifiers (Fig. 6.5), the biomass is brought into an inert bed of fluidised material (e.g., sand, char, etc.). The fuel is fed into the fluidised system either above-bed or directly into the bed, depending upon the size and density of the fuel and how it is affected by the bed velocities. During normal operation, the bed media is maintained at a temperature between 550°C and 1000°C. When the fuel is introduced under such temperature conditions, its drying and pyrolysing reactions proceed rapidly, driving off all gaseous portions of the fuel at relatively low temperatures. The remaining char is oxidised within the bed to provide the heat source for the drying and devolatilising reactions to continue. Fluidised bed gasifiers are better than dense phase reactors in that they produce more heat in short time due to the abrasion phenomenon between inert bed material and biomass, giving a uniformly high (800–1000°C) bed temperature. A fluidised bed gasifier works as a hot bed of sand particles agitated constantly by air. Air is distributed through nozzles located at the bottom of the bed. Gas

Cyclone

Recirculation of the fines

Fuel

Fluidised bed

Distributor plate

Air/oxygen/steam

Ash

Figure 6.5: Fluidised bed gasifier.

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2. Entrained-flow gasifiers: In entrained-flow gasifiers, fuel and air are introduced from the top of the reactor, and fuel is carried by the air in the reactor. The operating temperatures are 1200–1600°C and the pressure is 20–80 bar. Entrained-flow gasifiers (Fig. 6.6) can be used for any type of fuel so long as it is dry (low moisture) and has low ash content. Due to the short residence time (0.5–4.0 seconds), high temperatures are required for such gasifiers. The advantage of entrained-flow gasifiers is that the gas contains very little tar. Coal/water slurry

Oxygen

Gas and slag Figure 6.6: Entrained-flow gasifier.

Advantages and disadvantages of different gasifier types are shown in Table 6.1.

6.2.4

Catalytic liquefaction

This technology has the potential to produce higher quality products of greater energy density. These products should also require less processing to produce marketable products. Catalytic liquefaction is a low temperature, high pressure thermochemical conversion process carried out in the liquid phase. It requires either a catalyst or a high hydrogen partial pressure. Technical problems have so far limited the opportunities of this technology.

Biomass 95 Table 6.1: Advantages and disadvantages of different gasifier types. Gasifier type

Advantages

Disadvantages

Updraft

Simple design

High amount of tar and pyrolysis products Extensive gas cleaning required if used for power application

High charcoal burn-out

High fuel to gas conversion efficiency Accepts fuels with higher moisture content Accepts fuels of different sizes Downdraft Low tar Limited scale-up Best option for usage in gas engines At low temperatures, more tar produced At lower loads, fewer particles in High amounts of ash and dust the gas Fuel requirements are strict Cross-draft Applicable for small-scale operations High amount of tar produced Due to high temperatures, gas cleaning requirements are low Fluidised bed Compact construction Gas stream contains fine particles of dust Uniform temperature profile Complex system due to low biomass hold up in the fuel bed Accepts fuel size variation Variety of biomass can be used but fuel flexibility is applicable for biomass of 0.1 cm to 1 cm size High ash melting point of biomass does not lead to clinker formation Entrained-flow Applicable to large systems High investment Short residence time for biomass Strict fuel requirements

6.3

Biochemical processes

The use of micro-organisms for the production of ethanol is an ancient art. However, in more recent times such organisms have become regarded as biochemical ‘factories’ for the treatment and conversion of most forms of human generated organic waste. Microbial engineering has encouraged the use of fermentation technologies (aerobic and anaerobic) for use in the production of energy (biogas) and fertiliser, and for the use in the removal of unwanted products from water and waste streams. 1. Anaerobic fermentation. 2. Methane production in landfills. 3. Ethanol fermentation.

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6.3.1

Anaerobic fermentation

Anaerobic digestion is the microbial digestion of feedstock releasing heat, methane, hydrogen sulphide, carbon dioxide and under specific conditions hydrogen gas. This process takes place over several days in large tanks where the ideal conditions are maintained. After the process the remaining solid digestate is suitable for use as fertiliser and the gases released are collectively referred to as biogas. This gas can be used as a fuel in a CHP scheme once the gas has been cleaned to remove acidic compounds by condensation. Anaerobic reactors are generally used for the production of methane rich biogas from manure (human and animal) and crop residues. They utilise mixed methanogenic bacterial cultures which are characterised by defined optimal temperature ranges for growth. These mixed cultures allow digesters to be operated over a wide temperature range, i.e., above 0°C up to 60°C. When functioning well, the bacteria convert about 90% of the feedstock energy content into biogas (containing about 55% methane), which is a readily useable energy source for cooking and lighting. The sludge produced after the manure has passed through the digester is non-toxic and odourless. Also, it has lost relatively little of its nitrogen or other nutrients during the digestion process thus, making a good fertiliser. In fact, compared to cattle manure left to dry in the field the digester sludge has a higher nitrogen content; many of the nitrogen compounds in fresh manure become volatised whilst drying in the Sun. The advantage of anaerobic digestion is that it naturally occurs to organic material and would release methane, a potent greenhouse gas, into the atmosphere. Capturing and combusting the methane makes use of the energy in the gas and produces carbon dioxide which is a less potent greenhouse gas than methane and offsets fossil fuel energy production. The disadvantages of anaerobic digestion are that the microbes required pose a health threat to people and livestock. The microbes are sensitive to changes in the feedstock, especially the presence of anti-microbial compounds, and changes in the reactor conditions: they require constant circulation of the reactor fluid, and a constant operating temperature and pH.

6.3.2

Methane production in landfills

Methane is a powerful greenhouse gas, with substantial amounts being derived from unutilised methane production from landfill sites. Its recovery therefore, not only results in the stabilisation of the landfill site, allowing faster reuse of the land, but also serves to lessen the impact of biospheric methane emissions on global warming. Anaerobic digestion in landfills is brought about by the microbial decomposition of the organic matter in refuse.

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6.3.3

Fermentation

Ethanol is mainly used as a substitute for imported oil in order to reduce their dependence on imported energy supplies. The substantial gains made in fermentation technologies now make the production of ethanol for use as a petroleum substitute and fuel enhancer, both economically competitive (given certain assumptions) and environmentally beneficial. The most commonly used feedstock in developing countries is sugarcane, due to its high productivity when supplied with sufficient water. Where water availability is limited, sweet sorghum or cassava may become the preferred feedstocks. Other advantages of sugarcane feedstock include the high residue energy potential and modern management practices which make sustainable and environmentally benign production possible whilst at the same time allowing continued production of sugar. Other feedstocks include saccharide-rich sugarbeet, and carbohydrate rich potatoes, wheat and maize. Recent advances in the use of cellulosic feedstock, may allow the competitive production of alcohol from woody agricultural residues and trees to become economically competitive in the medium term.

6.4

Biomass cogeneration

Cogeneration – ‘generating together’ – refers to the process wherein we obtain both heat and electricity from the same fuel at the same time. The process is also referred to as CHP, short for combined heat and power. A variety of fuels can be used for cogeneration including bagasse, natural gas, coal, and biomass. A cogeneration plant consists of four basic elements: A prime mover, an electricity generator, a heat extraction or recovery unit, and a control panel. Fuel is burnt in the system or prime mover to convert its chemical energy into heat energy, which, in turn, produces the mechanical energy to run a generator and ultimately produce electricity. Prime movers for CHP systems include steam turbines, gas turbines, reciprocating engines, micro-turbines, and fuel cells. The heat energy from the system is also used directly, as heat, or indirectly to produce steam, hot water, and hot air, thus making it a CHP or cogeneration system which is shown in Fig. 6.7.

6.4.1

Cogeneration technologies

Different types of cogeneration technologies are used depending upon the end use or purpose. Some commonly used cogeneration technologies are: 1. Steam turbines. 2. Gas turbines. 3. Reciprocating engines.

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Cogeneration

Gas turbine

Steam turbine

Back pressure

Open cycle

Extraction condensing

Closed cycle

Reciprocating engine

Figure 6.7: Cogeneration system.

Steam turbine cogeneration systems

Steam turbines work on the principle of the Rankine cycle, which consists of a heat source (boiler) that converts water into high-pressure steam. A multistage turbine allows the high pressure steam to expand, which lowers its pressure. The steam is then transported to a condenser, which is like a vacuum chamber and thus has negative pressure and converts, or condenses, the steam into water. Alternatively, the steam may be transported to a distribution system that delivers steam at intermediate temperatures for different applications. The condensate from the condenser or from the steam utilisation system returns to the feed water pump, and the cycle continues. These systems are suitable for capacities of 500 kW to 100 MW or even higher. Most common steam turbines used in a cogeneration system are the backpressure type or the extraction–condensing type. The choice between the two types depends on how much electricity and heat are required, steam pressure and temperature, and the economics of operation. Back-pressure steam turbines

Steam at a pressure higher or equal to atmospheric pressure is extracted from the turbine to the thermal load that is the point at which heat is required. At that point, the steam releases heat and gets condensed, or turns into water. The condensate (water) returns to the system at a flow rate that can be lower than the steam flow rate if some steam is used in the process. This loss of steam is then compensated for in the cycle in the form of ‘make-up’ water fed into the boiler. It has to be noted that this turbine system does not have a separate condenser. Back-pressure steam turbines (Fig. 6.8) are the most efficient among all cogeneration systems; their cogeneration efficiency ranges from 84% to 92%.

Biomass 99 HP steam

Boiler

Turbine

Fuel LP steam

Condensate

Process

Figure 6.8: Back-pressure steam turbine systems.

Extraction-condensing steam turbines

In extraction-condensing steam turbines (Fig. 6.9), steam is extracted at one or more intermediate stages at the required pressure and temperature. The remaining steam from the turbine is transported to the condenser at very low pressure, as low as 0.05 bar (5 kPa), corresponding to a condensing temperature of approximately 33°C. HP steam

Fuel

Turbine

Boiler

LP steam Condensate

Process

Figure 6.9: Extraction-condensing steam turbine.

Condenser

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6.4.2

Gas turbine cogeneration systems

Gas turbine cogeneration systems work on the principle of the Brayton cycle, in which atmospheric air is compressed, heated, and then expanded, producing more power than what is consumed by the compressor in compressing and heating the air. The capacity of gas turbines varies from a fraction of a megawatt to about 100 MW. A variety of fuels can be used: natural gas, light petroleum distillates such as gas oil and diesel oil, products of coal gasification, etc. Gas turbine cogeneration systems are often more useful than steam turbines because gas turbine systems are more flexible; they can operate at widely varying ratios of electrical output to thermal output as required by the intended use. Open cycle gas turbine cogeneration systems

Most of the currently available gas turbine systems work on the open Brayton cycle, in which the compressor takes in air from the atmosphere and sends the compressed air to the combustor. The air temperature also increases because of compression. Older and smaller units operate at a pressure ratio (ratio of outlet air pressure to inlet air pressure) of 15:1, whereas the newer and larger units operate at pressure ratios approaching 30:1. From the compressor, the air is delivered through a diffuser to a combustion chamber, where fuel is injected and burnt. Exhaust gases exit the combustor at high temperatures (about 600°C). The highest temperature in the cycle is reached at this point; the higher this temperature, the higher the cycle’s efficiency. This temperature is limited–currently to about 1300°C–by the ability of the gas turbine material to withstand high temperatures and the efficiency of the cooling blades. The exhaust gases at high pressure and temperature enter the gas turbine, supplying the mechanical energy to drive the compressor and the electric generator, which, in turn, produces electricity. The exhaust gases leave the turbine at considerably high temperatures (450–600°C), which are ideal for high-temperature heat recovery. The heat is recovered by a heat-recovery boiler for extracting heat more efficiently. Cogeneration system with open cycle gas turbine are shown in Fig. 6.10. The steam produced in the heat recovery boiler can be at high pressures and temperatures, which makes the steam suitable not only for thermal processes but also for running a steam turbine to produce additional power. The exhaust gases are finally released into the atmosphere, after extracting maximum heat in the various components of the cogeneration system. Closed cycle gas turbine cogeneration systems

In the closed cycle system, the working fluid (usually helium or air) circulates within a closed circuit. The heat is supplied to the closed cycle through a heat

Biomass 101 Exhaust gases

Condensate from process . Q Steam to process

Heat recovery steam generator

Fuel

Combustor

. W

G

Gas turbine

Compressor

Generator

Air

Figure 6.10: Cogeneration system with open cycle gas turbine.

exchanger, instead of direct combustion of the fuel in the working fluid circuit. This arrangement ensures that both the working fluid and the turbine machinery are isolated from both the combustion chambers (heat source). On exiting from the turbine, the working fluid cools down, releasing its useful heat in the form of mechanical energy to produce electricity. In the closed cycle gas turbine (Fig. 6.11), the gas turbine exhaust is recycled to the compressor after being cooled and thereby forms a closed working fluid circuit. The source of heat can be the external combustion of any fuel (e.g., industrial wastes, municipal waste, solar energy or nuclear energy). The capacities of such systems range from 2 to 50 MWe.

6.4.3

Reciprocating engine cogeneration systems

A reciprocating engine, such as a diesel engine, can be combined with a heatrecovery boiler that supplies heat to the steam turbine to generate both electricity and heat. Heat from reciprocating engines can be recovered from four potential sources: exhaust gases, water from the engine jacket used for cooling, lube oil used for cooling, and the turbocharger used for cooling. The first two are the major sources, which are also easy to use and hence more common. Of the total heat lost from an engine (depending on its operating efficiency), roughly half is in the form of exhaust gases (400–500°C), which can be utilised for

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Heat source

Heat exchanger

. W

G

Gas turbine

Condenser

Generator

Heat exchanger or boiler

Hot fluid to process . Q

Fluid from process

Figure 6.11: Cogeneration system with closed cycle gas turbine.

producing steam or for drying bricks, ceramics, animal feed, etc. The waste heat in the form of the water used for cooling the engine (20%–30%) can be utilised for pre-heating water or generating hot air. Heat from this source can be used for some industrial processes that require low-pressure steam, in hospitals for sterilising surgical equipment, garments, etc., and in food processing. Reciprocating engine cogeneration plants can attain overall efficiencies of more than 80%–90%, and their capacities span a wide range, from as little as a few kW to MW, depending on the capacity of the internal combustion engine (Fig. 6.12). Exhaust heat

–200°C Steam or hot water

–450°C Boiler

IC engine

Coolers Process Oil

Air Water

Figure 6.12: Reciprocating engine cogeneration systems.

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6.5

Classification of cogeneration systems

Based on priority in utilising the available energy, electricity or heat, cogeneration systems are classified as topping cycle (where priority is for generating electricity) and bottoming cycle (where heat takes priority over electricity).

6.5.1

Topping cycle

The topping cycle is the most commonly used method of cogeneration. In this cycle, fuel is used first for producing electricity and then for heat. Steam turbine topping cycles are commonly used in the pulp and paper industry; heat recovery and combined cycle systems are used in many chemical plants; and gas turbine cycles are useful in central heating or cooling systems.

6.5.2

Bottoming cycle

In a bottoming cycle, fuel is first used to produce thermal energy, and the heat rejected from the process is used for generating power through a heat-recovery boiler and a turbine generator. Bottoming cycles are suitable for manufacturing processes in which heat is rejected in large amounts and at high temperatures, typically in cement, steel, ceramic, gas, and petrochemical industries. Bottoming cycle plants are much less common than topping cycle plants.

6.6

Applications of cogeneration

1. Industrial applications of cogeneration are found mainly in sugar factories, food processing plants, pharmaceuticals, oil refineries, textile mills, and steel, cement, glass, and ceramics plants, which require both heat and electricity in substantial amounts. 2. Residential, commercial, and institutional applications tend to be found in smaller systems, often based on ‘packaged’ units. These systems are commonly used in hotels, leisure centres, offices, smaller hospitals, and residential complexes. Larger applications are based on a technology similar to the cogeneration systems used in industry, gas turbines, or larger reciprocating engines. Such systems are used in large hospitals, large office complexes, universities, and colleges. 3. District heating systems are used at airports, office and commercial buildings, and large housing complexes. The heat provided by cogeneration is ideal for space heating and for providing hot water for domestic, commercial, or industrial use. A feature of cogeneration-driven district heating systems is the option to use a variety of fuels to suit environmental, economic, or strategic priorities.

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6.7

Advantages and disadvantages of various cogeneration systems

The advantages and disadvantages of different types of cogeneration systems are shown in Table 6.2. Table 6.2: Advantages and disadvantages of different types of cogeneration systems. Type of cogeneration system

Advantages

Disadvantages

Back-pressure steam turbine

Simple configuration Low capital cost High cogeneration efficiency Flexible in design and operation Good fuel efficiency Relatively low investment cost per unit of electric output Short gestation period Low emissions High flexibility in operation Low civil construction cost due to block foundations and least number of auxiliaries High power efficiency Better suited as a standby power source

Low electrical efficiency Low part-load performance Limited flexibility in design and operation Cost intensive

Extraction-condensing steam turbine Gas turbine

Reciprocating engine cogeneration system

High fuel cost Poor efficiency at low loads Longer operation High maintenance cost

Low overall efficiency Availability of low temperature steam Highly maintenance prone

7 Biofuels

7.1

Introduction

Biofuels are drawing increasing attention worldwide as substitutes for petroleum derived transportation fuels to help address energy cost, energy security and global warming concerns associated with liquid fossil fuels. The term biofuel means any liquid fuel made from plant material that can be used as a substitute for petroleum-derived fuel. Biofuels can include relatively familiar ones, such as ethanol made from sugar cane or diesel-like fuel made from soyabean oil, to less familiar fuels such as dimethyl ether (DME) or Fischer-Tropsch liquids (FTL) made from lignocellulosic biomass. A relatively recently popularised classification for liquid biofuels includes ‘first generation’ and ‘second generation’ fuels. There are no strict technical definitions for these terms. The main distinction between them is the feedstock used. A first generation fuel is generally one made from sugars, grains, or seeds, i.e., one that uses only a specific (often edible) portion of the above ground biomass produced by a plant and relatively simple processing is required to produce a finished fuel. First generation fuels are already being produced in significant commercial quantities in a number of countries. Second generation fuels are generally those made from non-edible lignocellulosic biomass, either non-edible residues of food crop production (e.g., corn stalks or rice husks) or non-edible wholeplant biomass (e.g., grasses or trees grown specifically for energy).

7.2

Classification of biofuels

7.2.1

First generation biofuels

The most well-known first generation biofuel is ethanol made by fermenting sugar extracted from sugar cane or sugar beets, or sugar extracted from starch contained in maize kernels or other starch-laden crops. Similar processing, but with different fermentation organisms, can yield another alcohol, butanol. Commercialisation efforts for butanol are ongoing, while ethanol is already a well-established industry. Many countries are expanding or contemplating expanding their first generation ethanol production, with Brazil and the United States having by far the largest expansion plans. From the perspective of petroleum substitution or carbon emissions mitigation efficiencies, the potential for most first generation

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biofuels is limited. Since the significant amount of fossil fuel used to produce this ethanol substantially offsets the carbon emissions reductions from photosynthetic uptake of carbon by the corn plants. Table 7.1 shows the pros and cons of first generation biofuels. Table 7.1: Pros and cons of first generation biofuels. Pros

Cons

Simple and well-known production methods

Feedstocks compete directly with crops grown for food Production by-products need markets High-cost feedstocks lead to high-cost production (except Brazilian sugarcane ethanol) Low land-use efficiency

Familiar feedstocks Scalable to smaller production capacities

Fungibility with existing petroleum-derived fuels Experience with commercial production and use in several countries

7.2.2

Modest net reductions in fossil fuel use and greenhouse gas emissions with current processing methods (except Brazilian sugarcane ethanol)

Second generation biofuels

Second generation biofuels share the feature of being produced from lignocellulosic biomass, enabling the use of lower-cost, non-edible feedstocks, thereby limiting direct food vs. fuel competition. Second generation biofuels can be further classified in terms of the process used to convert the biomass to fuel: biochemical or thermochemical. Second generation ethanol or butanol would be made, via., biochemical processing, while all other second generation fuels discussed here would be made, via., thermochemical processing. Second generation thermochemical biofuels may be less familiar to most readers than second generation ethanol, because there are no first generation analogs. On the other hand, many second generation thermochemical fuels are fuels that are already being made commercially from fossil fuels using processing steps that in some cases are identical to those that would be used for biofuel production. These fuels include methanol, refined Fischer-Tropsch liquids (FTL) and dimethyl ether (DME). Mixed alcohols can also be made from fossil fuels, but there is no commercial production today due to the immature state of some components of systems for producing these. The other thermochemical biofuel is green diesel, for which there is no obvious fossil fuel analog. Unrefined fuels, such as pyrolysis oils, are also produced thermochemically, but these require considerable refining before they can be used in engines.

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7.2.3

Third generation biofuels

The third generation biofuel refers to biofuel derived from algal biomass. The diversity of fuel that algae can produce results from two characteristics of the microorganism. First, algae produce an oil that can easily be refined into diesel or even certain components of gasoline. More importantly, however, is a second property in it can be genetically manipulated to produce everything from ethanol and butanol to even gasoline and diesel fuel directly.

7.2.4

Fourth generation biofuels

Fourth generation biofuels are derived from specially engineered plants or biomass that may have higher energy yields or lower barriers to cellulosic breakdown or are able to be grown on non-agricultural land or bodies of water. In fourth generation production systems, biomass crops are seen as efficient carbon capturing machines that take CO2 out of the atmosphere and store it in their branches, trunks and leaves.

7.3

Perspectives on first and second generation biofuels

Metrics that can be useful for understanding and evaluating first and second generation biofuel systems include land use efficiency, net life cycle energy balance, net life cycle greenhouse gas balance and economics.

7.3.1

Land-use efficiency for providing transportation services

Land is ultimately the limiting resource for biofuels production. There is a wide variation in the total amount of biomass that can be produced on a unit area of land, depending on species chosen, soil and climate conditions and agronomic treatments. The high productivity per hectare of sugar cane, a first generation biofuel feedstock, rivals the highest productivities that have been achieved with plantations of eucalyptus, which could be a second generation biofuel feedstock. However, only a fraction of the sugar cane biomass is used for liquid fuel production in a first generation biofuel facility, whereas nearly all of the above-ground eucalyptus plant would be used for production of a second generation biofuel. (A second generation ethanol fuel could be made from the lignocellulosic fractions of the sugar cane, such as bagasse and other fibrous material, which would then make sugar cane ethanol one of the most land-efficient of all biofuels.) An informative measure of land-use efficiency is the level of transportation service that can be provided from a hectare of land. Taking into consideration the rate of biomass feedstock production per hectare, the efficiency of converting the feedstock into a biofuel and the efficiency of using the biofuel in a vehicle, one can estimate the vehicle-kilometres of travel that can be provided by a hectare of land. Among all biofuels, starch-

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based first generation fuels give the lowest yield of vehicle-kilometres/hectare/ year, since only a fraction of the above-ground biomass is used as input to a biofuel production facility. By this land-use efficiency measure, sugar-based first generation fuels are about twice as good as starch-based fuels. Second generation fuels, because they utilise much more of the available above ground biomass than first generation fuels, can provide an improvement of 50% or more in land-use efficiency over sugar-based first generation fuels.

7.3.2

Greenhouse gas emissions

The effectiveness with which greenhouse gas emissions (GHGs, including CO2, CH4 and others) can be avoided using biofuels is related to the amount and carbon intensity of the fossil fuel inputs needed to produce the biofuel, as well as to what fossil fuel is substituted by use of the biofuel. A proper GHG accounting considers the full life cycle of the biofuel, from planting and growing the biomass to conversion of the biomass to biofuel, to combustion of the biofuels at the point of use. (In the case of vehicle applications, this full life cycle analysis is sometimes referred to as a ‘well-towheels’ analysis.) If the harvested biomass is replaced by new biomass growing year-on-year at the same average rate at which it is harvested, then CO2 is being removed from the atmosphere by photosynthesis at the same rate at which the already-harvested biomass is releasing CO2 into the atmosphere – a carbon-neutral situation. However, typically some fossil fuel is consumed in the course of producing or converting the biomass or delivering the biofuels to the point of use, resulting in net positive GHG emissions on a life cycle basis. These emissions will offset to some degree the emissions that are avoided when the biofuel is used in place of a fossil fuel. There could also be net GHG emissions associated with converting land from its current use to use for biomass energy feedstock production. The net emissions might be positive if existing forests were to be removed to establish energy crops. The net emissions might be negative if perennial energy crops (which can build soil carbon) are established, replacing annual row crops that were being grown on carbon-depleted soil. There is a rich literature on GHG life cycle analyses (LCAs) of biofuels. Understanding such diversity in LCA results requires examining details of each analysis, including analytical boundaries, numerical input assumptions and calculation methodologies. However, without delving into that level of detail, it is possible to draw a few firm conclusions. Higher GHG savings with biofuels are more likely when sustainable biomass yields are high and fossil fuel inputs to achieve these are low, when biomass is converted to fuel efficiently and when the resulting biofuel is used efficiently. Conventional grain- and seed-based biofuels can provide only modest GHG mitigation benefits by any measure and will be able to provide only modest levels of fuel displacement in the long term in any case

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due to the relatively inefficient land use associated with these fuels. The fundamental reason for the poor performance of grains and seeds is that they represent only a portion (typically less than 50% of the dry mass) of the aboveground biomass, so they are disadvantaged from the yield point of view. Higher efficiency in converting seeds/grains to fuel compensates the lower biomass yield to some extent. For example, some 380 L of ethanol can be produced from a dry ton of corn grain using current technology. This compares to todays known technology for cellulosic biomass conversion to ethanol, which can only yield some 255 L/dry T (at least on paper – no commercial-scale plant has been built). Future improvements in cellulosic ethanol production are expected to eliminate the conversion efficiency advantage currently enjoyed by corn ethanol. More efficient land use in mitigating GHG impacts can be achieved in the longer term by dedicated high-yielding lignocellulosic energy crops. Decades of experience with development of food crop yields, together with recent experience with developing lignocellulosic energy crops, suggests that major yield gains can be expected (probably with lower inputs per ton of biomass produced) with concerted development efforts. While historically there have been relatively low levels of research and development support provided for energy crop development, recent major private sector investments in research and development are likely to accelerate the pace of progress in improving yields. Assuming high yields are sustainable and acceptable from biodiversity and other perspectives, land requirements to achieve GHG emission reductions with biofuels will be reduced. There is also the possibility for some by-product CO2 to be captured (for long-term underground storage) during the process of making biofuels, especially, via., thermochemical conversion, which could lead to negative GHG emissions for a biofuel system. Proposals have also been made for thermochemically co-processing coal and biomass to make carbon-neutral liquid fuels by capturing and storing some CO2 produced during the conversion process. Finally, it is worth noting that biomass can be converted into heat or electricity as well as into liquid fuel. GHG emissions per unit land area that are avoided in this way may be greater than when making liquid fuel. However, for electricity or heat production, a variety of renewable resources is available (hydro, solar, geothermal, wind, etc.). Biomass is the only renewable source of carbon, which makes it the only renewable resource for producing carbon-bearing liquid fuels.

7.4

Biofuels threats for economy and environment

Development of alternative energy sources, including biodiesel, leads to growth of threats for the economy end environment issues all over the world. The issue of alternative fuels has gained additional importance because of much environmental, financial and political matter. Access to energy and its pricing are the dominant factors which have been influencing economic development

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during the two centuries of industrial age. According to generally available data 35% of energy consumed globally is produced from oil, 29% from coal (and brown coal), 24% from gas, 6% by hydroelectric plants, 5% from nuclear power plants, while the remaining 1% can well be allocated to alternative sources of energy. From the point of view of balanced transport, a lot of attention has been devoted to the development of biofuels for diesel engines. However, development of alternative biofuel technologies, biodiesel included, leads to growth of threats for the economy and for socio-political relations. Heavy criticism was triggered by the disturbance on the food market caused by the 1st generation biodiesel technologies, growth of food prices as well as changes in use of agricultural fields – they have become yet another challenge for the biofuel industry and they have contributed to changes in many governments policies. The issue of alternative fuels has gained additional importance following the German government’s decision to accelerate the process of shutting down German nuclear power plants.

7.5

Biofuels can only contribute GHG savings

Biofuels can only contribute GHG savings if significant emissions from landuse change are avoided and appropriate production technologies employed: 1. There is a future for a sustainable biofuels industry. 2. Feedstock production must avoid agricultural land that would otherwise be used for food production: Current policies will reduce biodiversity and may even cause green-house gas emissions. 3. The introduction of biofuels should be significantly slowed until adequate controls to address displacement effects are implemented and are demonstrated to be effective. 4. A slowdown and shift in biofuel feedstock production will reduce the impact of biofuels on food commodity prices that have a detrimental effect upon the poorest people.

7.6

Bio-based products as renewable energy sources

Bioresource engineering is related to the applications of chemical engineering and agricultural engineering usually based on biological and/or agricultural feedstocks. Bioresource engineering is more general and encompasses a wider range of technologies and various elements such as food engineering and processing, biomass, biological waste treatment, bioenergy, biotransformations and bioresource systems analysis, and technologies including aerobic methods, anaerobic digestion, microbial growth processes, enzymatic methods associated with thermochemical conversion technologies like combustion, pyrolysis, gasification, catalysis, etc. The impact of urbanisation and increasing demand

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for land, food, and water presents engineers in a world with serious challenges. Little attention has been given to the interface between the biological world and traditional engineering in the past. It is the job of bioresource engineers to fill that gap. Agricultural and bioresource engineers develop efficient and environmentally-sensitive methods of producing food, fibre, timber, bio-based products and renewable energy sources for an ever-increasing world population.

7.6.1

Bioalcohols

Biologically produced alcohols most commonly ethanol and less commonly propanol and butanol are produced by the action of micro-organisms and enzymes through the fermentation of sugars or starches or cellulose. Biobutanol (also called biogasoline) is often claimed to provide a direct replacement for gasoline, because it can be used directly in a gasoline engine (in a similar way to biodiesel in diesel engines). Ethanol fuel is the most common biofuel worldwide, particularly in Brazil. Ethanol can be used in petrol engines as a replacement for gasoline; it can be mixed with gasoline to any percentage. In high altitude (thin air) locations, some states mandate a mix of gasoline and ethanol as a winter oxidiser to reduce atmospheric pollution emissions. Ethanol is also used to fuel bioethanol fireplaces. As they do not require a chimney and are ‘flueless’, bio ethanol fires are extremely useful for new build homes and apartments without a flue. Methanol: Methanol is currently produced from natural gas a non-renewable fossil fuel. It can also be produced from biomass as biomethanol. The methanol economy is an alternative to the hydrogen economy compared to today’s hydrogen production from natural gas. Butanol: Butanol is formed by ABE fermentation (acetone, butanol, ethanol) and experimental modifications of the process show potentially high net energy gains with butanol as the only liquid product. Butanol will produce more energy and allegedly can be burned ‘straight’ in existing gasoline engines and is less corrosive and less water soluble than ethanol, and could be distributed, via., existing infrastructures. DuPont and BP are working together to help develop Butanol. E. Coli has also been successfully engineered to produce butanol by hijacking their amino acid metabolism.

7.6.2

Biodiesel

Biodiesel is the most common biofuel used in various part of the world. It is produced from oils or fats using transesterification and is a liquid similar in composition to fossil/mineral diesel. Chemically, it consists mostly of fatty acid methyl (or ethyl) (FAMEs). Feedstocks for biodiesel include animal fats, vegetable oils, soya, rapeseed, jatropa, mahua, mustard, flax, sunflower, palm oil, hemp, field pennycress, pongamia, pinnata and algae. Pure biodiesel (B100)

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is the lowest emission diesel fuel. Biodiesel can be used in any diesel engine when mixed with mineral diesel. In some countries manufacturers cover their diesel engines under warranty for B100 use, although Volkswagen of Germany, for example, asks drivers to check by telephone with the VW environmental services department before switching to B100. B100 may become more viscous at lower temperatures, depending on the feedstock used.

7.6.3

Green diesel

Green diesel, also known as renewable diesel, is a form of diesel fuel which is derived from renewable feedstock rather than the fossil feedstock used in most diesel fuels. Green diesel feedstock can be sourced from a variety of oils including canola, algae, jatropa and salicornia in addition to tallow. Green diesel uses traditional fractional distillation to process the oils, not to be confused with biodiesel which is chemically quite different and processed using transesterification.

7.6.4

Vegetable oil

Straight unmodified edible vegetable oil is generally not used as fuel, but lower quality oil can and has been used for this purpose. Used vegetable oil is increasingly being processed into biodiesel, cleaned of water and particulates and used as a fuel. Oils and fats can be hydrogenated to give a diesel substitute. The resulting product is a straight chain hydrocarbon with a high cetane number, low in aromatics and sulphur and does not contain oxygen. Hydrogenated oils can be blended with diesel in all proportions.

7.6.5

Bioethers

Bioethers (also referred to as fuel ethers or oxygenated fuels) are cost-effective compounds that act as octane rating enhancers. Greatly reducing the amount of ground-level ozone, they contribute to the quality of the air we breathe.

7.6.6

Biogas

Biogas is methane produced by the process of anaerobic digestion of organic material by anaerobes. It can be produced either from biodegradable waste materials or by the use of energy crops fed into anaerobic digesters to supplement gas yields. The solid by-product, digestate, can be used as a biofuel or a fertiliser. Biogas can be recovered from mechanical biological treatment waste processing systems. Landfill gas is a less clean form of biogas which is produced in landfills through naturally occurring anaerobic digestion. If it escapes into the atmosphere it is a potential greenhouse gas. Farmers can produce biogas from manure from their cows by using an anaerobic digester. Biogas is also discussed in Chapter 5.

8 Wind Energy

8.1

Introduction

Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the Sun, the irregularities of the Earth surface, and rotation of the Earth. Wind flow patterns are modified by the Earth terrain, bodies of water and vegetative cover. This wind flow, or motion energy, when ‘harvested’ by modern wind turbines, can be used to generate electricity. The terms ‘wind energy’ or ‘wind power’ describe the process by which the wind is used to generate mechanical power or electricity. The urgent need to reduce the carbon footprint of human activities and the increased awareness of the consequences of climate destabilisation have rekindled interest in renewable energy sources as important elements to consider in the expansion or retrofitting of power systems. Wind energy is a form of renewable energy produced through machines that use wind as their power source. Wind energy is only possible because of the Sun. In recent years, wind energy has become one of the most economical renewable energy technology. Today, electricity generating wind turbines employ proven and tested technology and provide a secure and sustainable energy supply. At good, windy sites, wind energy can already successfully compete with conventional energy production. Many countries have considerable wind resources, which are still untapped.

8.2

Characteristics of wind energy

8.2.1

Wind distribution

Air in motion is what we call wind. The horizontal wind speed is usually much greater than the vertical wind speed. The total energy in the atmosphere is the result of conversion of potential energy of the atmosphere into kinetic energy. The ultimate energy source is of course the Sun. Since the Earth shape is a sphere the amount of solar energy reaching a horizontal Earth surface decreases towards the poles. Other factors affecting the energy absorbed by the Earth surface are cloudiness, albedo of the surface (i.e., the fraction of incoming energy reflected by a surface). Absorption by aerosols and scattering are also reducing factors.

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8.3

Technological aspects of wind energy

Wind power is the conversion of wind energy into electricity or mechanical energy using wind turbines. The power in the wind is extracted by allowing it to blow past moving blades that exert torque on a rotor. The amount of power transferred is dependent on the rotor size and the wind speed. Wind turbines range from small four hundred watt generators for residential use to several megawatt machines for wind farms and offshore. The small ones have direct drive generators, direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind; while the larger ones generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. Direct drive generators and aeroelastic blades for large wind turbines are being researched and direct current generators are sometimes used. Wind energy is plentiful, renewable, widely distributed, clean and reduces greenhouse gas emissions if used to replace fossil-fuel-derived electricity. The intermittency of wind does not create problems when using wind power at low to moderate penetration levels.

8.4

Components of wind turbines

Anemometer: Measures the wind speed and transmits wind speed data to the controller. Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to ‘lift’ and rotate. Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies. Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds. Gear box: Wind turbines rotate typically between 40 rpm and 400 rpm. Generators typically rotate at 1200 to 1800 rpm. Most wind turbines require a step-up gear-box for efficient generator operation (electricity production). Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 40 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring ‘direct-drive’ generators that operate at lower rotational speeds and don’t need gear boxes. Generator: Usually an off-the-shelf induction generator that produces 60cycle AC electricity.

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High-speed shaft: Drives the generator. Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute. Nacelle: The nacelle sits atop the tower and contains the gear box, lowand high-speed shafts, generator, controller and brake. Some nacelles are large enough for a helicopter to land on. Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity. Rotor: The blades and the hub together are called the rotor. Tower: Towers are made from tubular steel, concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity. Wind direction: This is an ‘upwind’ turbine, so-called because it operates facing into the wind. Other turbines are designed to run ‘downwind,’ facing away from the wind. Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind. Yaw drive: Upwind turbines face into the wind, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don’t require a yaw drive, the wind blows the rotor downwind.

8.5

Applications and efficiency of wind energy

Most modern wind power is generated in the form of electricity by converting the rotation of turbine blades into electrical current by means of an electrical generator. In windmills (a much older technology), wind energy is used to turn mechanical machinery to do physical work, such as crushing grain or pumping water. Recently, wind energy has also been used to desalinate water.

8.5.1

Wind electric systems

In wind electric systems, the rotor is coupled, via., a gearing or speed control system to a generator, which produces electricity. Wind power is used in large scale wind farms for national electrical grids as well as in small individual turbines for providing electricity to rural residences or grid-isolated locations.

8.5.2

Wind energy - water desalination

As wind energy converters supply mechanical or electrical energy, only vapour compression, reverse osmosis or electrodialysis come into consideration for wind-powered water desalination.

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Wind-powered water desalination plants can be operated in island mode (with or without an additional supply of electrical energy, for example from a diesel generator set) or in grid-parallel mode.

8.5.3

Wind pumps

With wind pumps, moving air turns a ‘rotor’ and the rotational motion of the blades is transferred to harmonic motion of the shaft, which is used to pump water or drive other mechanical devices such as grain mills. Water from wells as deep as 200 m can be pumped to the surface by wind pumps. To select a suitable wind pump, the following information is needed: mean wind speed, total pumping head, daily water requirement, well draw down, water quality and storage requirements.

8.6

Wind turbine technology

Wind turbine technology has developed rapidly in recent years and Europe is at the hub of this hightech industry. Wind turbines are becoming more powerful, with the latest turbine models having larger blade lengths which can utilise more wind and therefore produce more electricity, bringing down the cost of renewable energy generation.

8.6.1

Types of wind turbines

Wind turbines are classified into two general types: (i) horizontal axis and (ii) vertical axis. A horizontal axis machine has its blades rotating on an axis parallel to the ground. A vertical axis machine has its blades rotating on an axis perpendicular to the ground. There are a number of available designs for both and each type has certain advantages and disadvantages. However, compared with the horizontal axis type, very few vertical axis machines are available commercially.

8.6.2

Generating electricity from the wind turbines

Generating electricity from the wind is simple: Wind passes over the blades exerting a turning force. The rotating blades turn a shaft inside the nacelle, which goes into a gearbox. The gearbox increases the rotation speed for the generator, which uses magnetic fields to convert the rotational energy into electrical energy. The power output goes to a transformer, which converts the electricity from the generator at around 700 Volts (V) to the right voltage for the distribution system, typically between 11 kV and 132 kV. Offshore technology

Offshore wind farms are an exciting new area for the industry, largely due to the fact that there are higher wind speeds available offshore and economies of scale allow for the installation of larger size wind turbines offshore.

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Offshore wind turbine technology is based on the same principles as onshore technology. Foundations are constructed to hold the superstructure, of which there are a number of designs, but the most common is a driven pile. Operation and maintenance

Both onshore and offshore wind turbines have instruments on top of the nacelle, an anemometer and a wind vane, which respectively measure wind speed and direction. When the wind changes direction, motors turn the nacelle and the blades along with it, around to face into the wind. The blades also ‘pitch’ or angle to ensure that the optimum amount of power is extracted from the wind. All this information is recorded by computers and transmitted to a control centre, which can be many miles away. Wind turbines are not physically staffed, although each will have periodic mechanical checks, often carried out by local firms. The onboard computers also monitor the performance of each turbine component and will automatically shut the turbine down if any problems are detected, alerting an engineer that an onsite visit is required. The amount of electricity produced from a wind turbine depends on three factors: 1. Wind speed: The power available from the wind is a function of the cube of the wind speed. Therefore if the wind blows at twice the speed, its energy content will increase eight-fold. Turbines at a site where the wind speed averages 8 m/s produce around 75–100% more electricity than those where the average wind speed is 6 m/s. 2. Wind turbine availability: This is the capability to operate when the wind is blowing, i.e., when the wind turbine is not undergoing maintenance. This is typically 98% or above for modern European machines. 3. The way wind turbines are arranged: Wind farms are laid out so that one turbine does not take the wind away from another. However other factors such as environmental considerations, visibility and grid connection requirements often take precedence over the optimum wind capture layout. Grid-connected small wind turbines

Small scale wind turbines can be used in domestic, community and smaller wind energy projects and these can be either stand-alone or grid-connected systems. Stand alone systems are used to generate electricity for charging batteries to run small electrical applications, often in remote locations where it is expensive or not physically possible to connect to a mains power supply. Such examples include rural farms and island communities, with typical applications being water heating or pumping, electric livestock fencing, lighting or any kind of small electronic system needed to control or monitor remote

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equipment. With grid-connected turbines the output from the wind turbine is directly connected to the existing mains electricity supply. This type of system can be used both for individual wind turbines and for wind farms exporting electricity to the electricity network. A grid-connected wind turbine can be a good proposition if the consumption of electricity is high. Operating characteristics of wind mills

All wind machines share certain operating characteristics, such as cut-in, rated and cut-out wind speeds. Cut-in speed

1. Cut-in speed is the minimum wind speed at which the blades will turn and generate usable power. 2. This wind speed is typically between 10 and 16 kmph. Rated speed

1. The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power. For example, a ‘10 kilowatt’ wind turbine may not generate 10 kilowatts until wind speeds reach 40 kmph. 2. Rated speed for most machines is in the range of 40 to 55 kmph.

8.6.3

Horizontal axis wind turbines (HAWT)

Upwind turbine

The upwind turbine is a type of turbine in which the rotor faces the wind. A vast majority of wind turbines have this design. Its basic advantage is that it avoids the wind shade behind the tower. On the other hand, its basic drawback is that the rotor needs to be rather inflexible and placed at some distance from the tower. In addition, this kind of HAWT also needs a yaw mechanism to keep the rotor facing the wind. Downwind turbine

The downwind turbine is a turbine in which the rotor is on the downwind side of the tower. It has the theoretical advantage that they maybe built without a yaw mechanism, considering that their rotors and nacelles have the suitable design that makes the nacelle follow the wind passively. Another advantage is that the rotor may be made more flexible. Its basic drawback, on the other hand, is the fluctuation in the wind power due to the rotor passing through the wind shade of the tower.

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8.6.4

Advantages and disadvantages of (HAWT and VAWT)

Advantages of (HAWT and VAWT)

The advantages of the HAWT over the VAWT are given below: 1. Blades are to the side of the turbine’s center of gravity, helping stability. 2. The turbine collects the maximum amount of wind energy by allowing the angle of attack to be remotely adjusted. 3. The ability to pitch the rotor blades in a storm so that damage is minimised. 4. The tall tower allows the access to stronger wind in sites with wind shear and placement on uneven land or in offshore locations. 5. Most HAWTs are self-starting. 6. Can be cheaper because of higher production volume. Disadvantages of (HAWT and VAWT)

Disadvantages of the HAWT compared to the VAWT is that: 1. It has difficulties operating near the ground. 2. The tall towers and long blades are hard to transport from one place to another and they need a special installation procedure. 3. They can cause a navigation problem when placed offshore.

8.6.5

Vertical axis wind turbines

The vertical axis wind turbine is an old technology, dating back to almost 4000 years ago. Unlike the HAWT, the rotor of the VAWT rotates vertically around its axis instead of horizontally. Though it is not as efficient as a HAWT, it does offer benefits in low wind situations wherein HAWTs have a hard time operating.

8.7

Offshore wind energy

The potential for offshore wind is enormous in Europe and elsewhere, but the technical challenges are also great. The capital costs are higher than onshore, the risks are greater, the project sizes are greater and the costs of mistakes are greater. Offshore wind technology and practice has come a long way in a short time, but there is clearly much development still to be done. It also highlights the key differences associated with the assessment of the offshore wind resource and the energy production of offshore wind farms when compared with onshore wind farms. Many of the elements of the analyses are common to onshore and offshore projects.

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Onshore, topographic effects are one of the main driving forces of the wind regime. With no topographic effects offshore, other factors dominate the variation in wind speed with height.

8.7.1

Energy prediction for offshore wind turbines

The energy prediction step is essentially the same as for onshore predictions. There is generally only minor predicted variation in wind speed over a site. Given the absence of topography offshore, measurements from a mast can be considered representative of a much larger area than would be possible onshore. For large offshore sites, wake losses are likely to be higher than for many onshore wind farms. The wake losses are increased due to the size of the project and also due to lower ambient turbulence levels – the wind offshore is much smoother. There is therefore less mixing of the air behind the turbine, which results in a slower re-energising of the slow moving air and the wake lasts longer. Observations from the largest current offshore wind farms have identified shortcomings in the classic wind farm wake modelling techniques, due to the large size of the projects and perhaps due to specific aspects of the wind regime offshore.

8.7.2

Wind turbine technology for offshore locations

High availability is crucial for the economics of any wind farm. This depends primarily on high system reliability and adequate maintenance capability, with both being achieved within economic constraints on capital and operational costs. Key issues to be addressed for good economics of an offshore wind farm are: 1. Minimisation of maintenance requirements. 2. Maximisation of access feasibility.

8.7.3

Wind farm design offshore

Site selection

The selection of the site is the most important decision in the development of an offshore wind farm. It is best accomplished through a short-listing process that draws together all known information on the site options, with selection decisions driven by feasibility, economics and programme, taking account of information on consenting issues, grid connection and other technical issues. Wind turbine selection

Early selection of the wind turbine model for the project is typically necessary so that the design process for support structures (including site investigations),

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electrical system and grid connection can progress. Offshore projects require use of the larger wind turbines on the market, meaning that there is often limited choice, hence, securing the wind turbine model may be necessary to start up the project programme. Layout

The process for designing the layout of an offshore wind farm is similar to the process for an onshore wind farm, albeit with different drivers. Once the site is secured by a developer, the constraints and known data on the site are evaluated and input in the layout design, as shown in Fig. 8.1. Marine navigation Fishing areas Environmental impacts

Non-techanical constraints

Turbine model

Foundation design and cost

Wake losses

Previous design

Wind farm layout design

Site licence boundary

Maximum export capacity

Cable design and life-cycle costs

Installation impacts

Figure 8.1: Layout design process.

Electrical system

An offshore wind farm electrical system consists of six key elements: 1. Wind turbine generators. 2. Offshore inter-turbine cables (electrical collection system). 3. Offshore substation (if present). 4. Transmission cables to shore. 5. Onshore substation (and onshore cables). 6. Connection to the grid. Offshore substations: Offshore substations are used to reduce electrical losses by increasing the voltage and then exporting the power to shore.

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Onshore substations: Design of the onshore substation may be driven by the network operator, but there will be some choices to be made by the project developer. Generally, the onshore substation will consist of switchgear, metering, transformers and associated plant. Operations and maintenance

Successful performance of O&M is most critically dependent on service teams being able to access the wind farm as and when needed. Good progress has been made on this in recent years and accessibility has improved significantly. This has been achieved by incremental improvements in: 1. Vessels used. 2. Landing stages on the wind turbine structures. 3. Procedures. Floating systems

The main drivers for floating technology are: 1. Access to useful resource areas that are in deep water yet often near the shore. 2. Potential for standard equipment that is relatively independent of water depth and seabed conditions. 3. Easier installation and decommissioning. 4. The possibility of system retrieval as a maintenance option.

8.8

Wind farm

As in most other areas of power production, when it comes to capturing energy from the wind, efficiency comes in large numbers. Groups of large turbines, called wind farms or wind plants, are the most cost-efficient use of wind energy capacity.

8.8.1

Factors affecting turbine location

Once a site has been identified and the decision has been taken to invest in its development, the wind farm design process begins. The fundamental aim is to maximise energy production, minimise capital cost and operating costs and stay within the constraints imposed by the site. As the constraints and costs are all subject to some level of uncertainty, the optimisation process also seeks to minimise risk.

8.8.2

Optimisation of energy production

Once the wind farm constraints are defined, the layout of the wind farm can be optimised. This process is also called wind farm ‘micro-siting’. As noted

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above, the aim of such a process is to maximise the energy production of the wind farm whilst minimising the infrastructure and operating costs. For most projects, the economics are substantially more sensitive to changes in energy production than infrastructure costs. It is therefore appropriate to use energy production as the dominant layout design parameter. Visual influence of wind turbines

‘Visual influence’ is the term used for the visibility of the wind turbines from the surrounding area. In many countries the visual influence of a wind farm on the landscape is an important issue, especially in regions with high population density. Noise from operating wind turbines

In densely populated countries, noise can sometimes be a limiting factor for the generating capacity that can be installed on any particular site. The noise produced by operating turbines has been significantly reduced in recent years by turbine manufacturers, but is still a constraint.

8.9

Major failures in wind turbines

Reliability of wind turbines is a pre-requisite to ensure the healthy growth of wind energy. Even if new designs and prototypes performed by manufacturers and validated by certification bodies offer safer and more reliable wind turbines, their development and related improvement are based on the experience with turbines smaller than those currently being erected. Therefore the technology is still coming up against its limitations. To this end it has been recognised that there is a need for the continuous monitoring of major wind turbine components such as gear box, generator and rotor blades. These components are seen to require substantial maintenance and repair efforts or even retrofits. Hence, periodic inspection of these components by any independent third party to ensure the safe and efficient operation is also necessary. The wind turbines are designed for a life span of about 20 years. However, numerous of studies have shown that some equipments in the nacelle interior as such the electrical system, hydraulic systems or drive train present very high failure rates, requiring so frequently repairs or replacement.

8.9.1

Causes of wind turbine failures

The root causes of these failures are diverse. In principle, the root causes can be subdivided into two causes, external factors, such as icing, lightning or storm and internal factors, as for example the failure of components or of the control system. Two third of all wind turbine plant failures lead eventually to a downtime of the plant. In one third of the downtime case, the plant can be

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taken into operation within a short-term, sometimes after only half day. In other cases the downtime lasts longer, particularly, when there is a requirement for repairs or replacements. Wind turbine, which are equipped with a lightning protection can usually be taken into operation after a short-time. The most failures, accounting for more than 71%, are caused by internal factors. It is remarkable that the components defect, which mostly corresponds to technical components in the nacelle, accounts for almost 40% of the failure causes. The failures of the technical components can be considered as severe, since those mostly cause a plant downtime and require immediately repair or replacement. The wind turbine failures have different impacts on the wind turbine operation.

8.9.2

Mechanical failures

Micro-pitting

The phenomenon micro-pitting can be found on several components, such as on the bearing, main shaft and high-speed shaft and specially on the gears. Micro-pitting also known as grey staining or frosting is a wear and tear phenomenon, which occurred on the surface areas of heavily loaded metallic component. Corrosion

Corrosion is a chemical reaction or electrochemical reaction between a material and its environment, which can cause a remarkable alteration of its surface shape leading often to serious consequences. This phenomenon occurs usually on metallic material. Corrosion affects several components of the wind turbine, including the main shaft, high-speed shaft, generator, gearbox, as well the tower, which is often partly made of steel. However, specially affected from corrosion are offshore wind turbines, as the salt in the water and air increases the conductivity making the material more reactive. Misalignment

Misalignment is common problem found in the wind turbine drivetrain. Misalignment issues affect the main components within the nacelle, including the generator, gearbox, main shaft or high-speed shaft.

8.9.3

Inspections within the nacelle

In order to avoid all those mentioned failures and consequently downtimes and costs, it is necessary to perform regularly inspections. As already mentioned, due to an early, effective and accurate inspection, the maintenances operation, repairs and replacement can be exact planned in advance, reducing so the

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downtime period. The inspection requirements are different from component to component.

8.10

Wind energy powering agriculture

Wind, the result of global and local temperature difference, represents another source of renewable energy. The governing principle of wind energy is the transformation of wind flow into rotational movements. This is indeed the same principle as for hydropower systems. The power output of a wind energy system is generally estimated by multiplying the available wind speed by the swept area of the rotor. The rotational force can be used either directly (irrigation pumps, etc.), or to drive a generator and produce electricity (Find an animation of a wind pump here). Energy is one of the major parameters for establishing growth and progress of the country, rather the standard of living depends directly upon the per capita energy consumption. In agricultural systems, energy is available from different sources as human, animal, Sun, wind, biomass, coal, fertiliser, seed, agro-chemicals, petroleum products, electricity etc. Energy sources that release available energy directly to the system are classified as direct energy sources. Renewable energy and farming are a winning combination. Wind, solar and biomass energy can be harvested forever, providing farmers with a long-term source of income. Farmers and ranchers are in a unique position to benefit from the growth in the wind industry. To tap this market, farmers can lease land to wind developers, use the wind to generate power for their farms, or become wind power producers themselves. Farmers and ranchers can generate their own power from the wind. Small wind generators, ranging from 400 watts to 40 kilowatts or more, can meet the needs of an entire farm or can be targeted to specific applications. In Texas and the West, for example, many ranchers use wind generators to pump water for cattle. Electric wind generators are much more efficient and reliable than the old water-pumping fan-bladed windmills. They may also be cheaper than extending power lines and are more convenient and cheaper than diesel generators. ‘Net metering’ enables farmers to get the most out of their wind turbines. When a turbine produces more power than the farm needs at that moment, the extra power flows back into the electricity system for others to use, turning the electric meter backwards. When the turbine produces less than the farm is using, the meter spins forward, as it normally does. At the end of the month or year, the farmer pays for the net consumption or the electric company pays for the net production. Net metering rules and laws are in place in most states. Modern agriculture needs modern energy - the two are closely linked. For many developing countries, agriculture is the dominant sector in developing

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the economy. Increasing productivity and the modernisation of agricultural production systems are the primary drivers of global poverty reduction and energy plays a key role in achieving this. Energy input to modern and sustainable agricultural production and processing systems is a key factor in moving beyond subsistence farming towards food security, added value in rural areas and expansion into new agricultural markets. In many cases, renewable energy technologies and hybrid systems can provide energy services that neatly support the production process, e.g., by providing irrigation (pumps) or post harvest treatment (cooling) or processing (drying, milling, pressing). The requirements of mechanical energy in the agricultural production process are also of critical importance and include human and animal labour as well as fuels for mechanisation, pumping and other activities and indirectly the production of fertilisers and agrochemicals. Wind can be used to power both mechanical and electric pumps. Mechanical wind-powered pumps use reciprocal non-motorised submersible pumps and require wind speeds of 2.5 m/s minimum up to 4 m/s optimum. Capacity is much lower than for motorised centrifugal pumps, in the range of 1 m3/h at depths of 20 metres or more. Mechanical wind pumps require the availability of local maintenance and repair facilities to be able to respond quickly to mechanical failures. Adequate wind speeds must be present at the location of the wells. One advantage is that they can pump day or night as long as there is sufficient wind and can be used independently of electricity or fuel supplies. A disadvantage is that they must be located directly above the well, a location that may not be optimal in terms of local wind resources. Wind pumps are appropriate in windy areas without other sources of power and only for small irrigable areas. Wind electric turbines convert the kinetic energy of the wind into rotational mechanical energy that drives a generator to produce electricity, e.g., for pumping water for irrigation. Windmills are positioned for optimal wind conditions, providing greater site flexibility and in addition facilitating electricity production for other uses. Water-pumping applications generally make use of wind turbines with rated output between 1 kWe and 10 kWe. A wide variety of small wind electric turbines is commercially available, with rated outputs ranging from a few tens of watts to 100 kilowatts and is used worldwide to provide electricity in locations where alternatives are unavailable or are too expensive or difficult to provide.

8.10.1

Hybrid systems

Small-scale hybrid power systems, also a mature technology, are used worldwide. By combining different energy sources (solar-diesel, wind-diesel) hybrids can provide widespread and highly reliable electrical supply. These small hybrid

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systems are easy to install—no special tools or concrete are required. The term wind hybrid system describes any combination of wind energy with one or more additional sources of electricity generation (e.g., biomass, solar or a generator using fossil fuels). Hybrid system are very often used for standalone applications at remote sites. Energy storage

Hybrid systems contain an energy storage device to store the surplus energy during times of high energy production, which can be used for supply when production from renewable sources is low (e.g., no wind). For this reason, the size of the device is often described by the period of time in hours h0 the average load can be covered using the storage as the sole source of energy. Other important characteristics are the overall efficiency of the storage device (determined by the loss of energy during the charge and discharge-process), the output voltage Ub and the maximum permitted discharge. Lead-acid batteries today are the most common technology solution used in hybrid energy systems. There are several alternatives like flywheels, pumped hydro storage, hydraulic storage and fuel cells. Wind-diesel hybrid systems The combination of a diesel generator and a wind turbine in a hybrid system a very common and frequently used in remote areas. The following description includes considerations about system design and sizing of the components. Applications of wind hybrid systems

T/C stations: Stations for telecommunication in remote areas have to be supplied with power during long time periods. Extension of the electricity grid in most cases is a big financial effort, while the supply by sole diesel generators causes additional fuel- and maintainance-costs permanently. Small hybrid systems can be used to reduce fuel-consumption. A small wind turbine may be placed on the relay-mast of the T/C station, avoiding the additional installation costs of a turbine tower. As load variations of a T/C stations are rather low but a steady supply is needed, hybrid systems combining different RES-sources are preferable for this application. A battery storage for system back-up is necessary. Small desalination systems: Scarcity of potable water is often found in remote island situations, which at the same time often have a good wind potential. In some areas of water scarcity, water desalination contributes a considerable part to potable water supply. Implementing wind-based water desalination units in these areas is a cost-saving and emission-avoiding alternative to running the desalination units by fossil fuels. The techniques described as most efficient for water desalination, are reverse osmosis and mechanical vapour compression.

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Water pumping: There are many areas with scarce surface water availability but sufficient water resources in appropriate depth for pumping systems. In case of poor infrastructure for water supply, wind turbines can be used as electricity source for these systems. Common aims are providing water for domestic and community supply in remote locations, as well as cattle watering and irrigation. Agriculture still is the economic activity with the greatest water consumption (almost two thirds of worlwide water consumption). The government of India promoted the implementation of wind-pumping system very intensive. The high distribution of small systems has created a well established market for small and multi-bladed wind turbines in recent years. Choosing the right energy source

Grid electricity: Where grid connection is available an electric pump can be powered directly. However, the cost, availability, reliability and quality of local electricity supplies determine the use of this energy source. Fuels for combustion engines: Motorised pumps can be powered by fossil fuels (diesel, gasoline), either through generators that create electricity, or by transmitting power to the pump through a drive belt and vertical rotating shaft.

8.10.2

Wind-powered water pumping systems for livestock watering

Water supplies such as wells and dugouts can often be developed on the open range. However, the availability of power supplies on the open range is often limited, so some alternate form of energy is required to convey water from the source to a point of consumption. Wind energy is an abundant source of renewable energy that can be exploited for pumping water in remote locations and windmills are one of the oldest methods of harnessing the energy of the wind to pump water. Windmills generally consist of two types, with the classification depending on the orientation of the axis of rotation of the rotor. Vertical-axis wind turbines are efficient and can obtain power from wind blowing in any direction, whereas horizontal axis devices must be oriented facing the wind to extract power. Most windmills for water-pumping applications are of the horizontal-axis variety and have multi-bladed rotors that can supply the high torque required to initiate operation of a mechanical pump. Windmills can also be used to generate electricity, but electricity-generating units usually consist of vertical-axis rotors or highspeed propellor rotors, due to the requirement for low starting torques. Figure 8.2 shows a typical waterpumping windmill.

Wind Energy 129 130–180 m or 15 to 20 times height of obstruction Gear box

Multi-blade rotor

Tail

Mill swivel

Tower

Pump rod Well seal

Pump rod swivel

5–6 m minimum

Packer head Discharge pipe

Stock tank

Tower footing

Water level

Drop pipe

Pump cylinder Screen

Figure 8.2: Typical windmill water pump.

8.10.3

Wind energy based desalination processes and plants

Water is a valuable natural resource and its shortage is a serious problem being faced by many areas of the planet. Decision making on the water supply method includes technical and economic evaluation of various alternatives, taking into account the urgent character of the problem and the need for its sustainable solution. Desalination of brackish and sea water has become one of the most widely applicable methods to meet water demand and it is today widely applied in areas with limited water resources. One of the most promising desalination methods is based on reverse osmosis (RO) phenomenon. A critical issue in water desalination is the high energy demand and, more specifically,

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electricity for RO desalination units. RO is the desalination process with quite low energy requirements.

8.10.4

RES based desalination processes

The use of renewable energy sources (RES) for the operation of desalination plants is a feasible and environmentally compatible solution in areas with significant RES potential. The main driving forces for applying RES in desalination plants are the seasonal variability in water (and energy) demand, usually occurring when renewable energy availability is high, the limited availability of conventional energy supply in remote areas, the sufficiency of RES in islands, the technological advancements being achieved in desalination systems, the limitation of environmental impacts of conventional desalination systems and the relative easiness of the plant’s operation and maintenance compared to conventional energy ones. The best coupling of RES to desalination systems is determined from various criteria, such as the system’s efficiency, the investment and operational cost, availability of operational personnel, the suitability of the system to the characteristics of the location, the possibility for future increase of the system capacity, etc.). The selection of the appropriate RES desalination technology depends on a number of factors, including: 1. Required quantity of potable water (plant capacity). 2. Feed water salinity. 3. Remoteness. 4. Availability of grid electricity. 5. Technical infrastructure. 6. Type and potential of the local renewable energy resource. Various combinations of RES and desalination systems have been proposed and implemented, each one with its own characteristics and suitability under certain criteria. Desalination systems driven by wind power are the most frequent renewable energy desalination plants. Coastal areas have a high availability of wind power resources and wind power is the most competitive renewable energy technology in power generation. Therefore, wind powered desalination is a promising alternative. The idea to use wind power as an energy source for desalination is not new. Wind conditions, for example, in coastal areas are often in favour of this desalination system. Energy issues in desalination plants

All desalination systems use energy and, in fact, the energy consumption is one of the most important elements in determining water costs. About 0.7 kWh/m3 is theoretically the minimum energy required to obtain fresh water from seawater.

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For RO systems the energy consumption is in the range of 5–10 kWh/m3 without energy recovery (large production plants) and 3–4 kWh/m3 with energy recovery.

8.10.5

Environmental impacts of RES based desalination plants

Desalination plants cover the needs of remote areas in water. Usually they are implemented as a result of an analysis and alternative solutions evaluation amongst various possible solutions for water supply. For example, on several Greek islands, fresh water requirements are covered by the construction of large dams or ground reservoirs of desalination plants. In smaller islands, the only available solution is the transport of fresh water by ship, with high costs and improper hygienic conditions. All these water supply methods cause a spectrum of environmental impacts, more or less serious depending on the type of the project, its location and scale. The main environmental impacts of an RO desalination plant are the following: 1. Noise disturbance. 2. Optical disturbance. 3. Land use. 4. Interference with public access in the coast. 5. Abstraction of brackish groundwater. 6. Discharge of brine - a concentrated salt solution that may be hot and may contain various chemicals on coastal or marine eco-systems or, in the case of inland brackish water desalination, on rivers and aquifers. 7. The emission of greenhouse gases in the production of electricity and steam needed to power the desalination plants in case the energy provided is from the grid and fossil fuels are used to generate it.

8.11

Carbon footprint of wind energy

All electricity generation systems have a ‘carbon footprint’, that is, at some points during their construction and operation carbon dioxide (CO2) is emitted. To compare the impacts of these different technologies accurately, the total CO2 amounts emitted throughout a system’s life must be calculated. Emissions can be both direct – arising during operation of the power plant and indirect – arising during other non-operational phases of the life cycle. Fossil fuelled technologies (coal, oil, gas) have the largest carbon footprints, because they burn these fuels during operation. Non-fossil fuel based technologies such as wind, photovoltaics (solar), hydro, biomass, wave/tidal and nuclear are often referred to as ‘low carbon’ or ‘carbon neutral’ because they do not emit CO2 during their operation. However, they are not ‘carbon free’ forms of generation

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since CO2 emissions do arise in other phases of their life cycle such as during extraction, construction, maintenance and decommissioning. A ‘carbon footprint’ is the total amount of CO2 and other greenhouse gases, emitted over the full life cycle of a process or product. It is expressed as grams of CO2 equivalent per kilowatt hour of generation (gCO2eq/kWh), which accounts for the different global warming effects of other greenhouse gases. Carbon footprints are calculated using a method called Life Cycle Assessment (LCA) and is also referred to as the ‘cradle-to-grave’ approach. This method is used to analyse the cumulative environmental impacts of a process or product through all the stages of its life. It takes into account energy inputs and emission outputs throughout the whole production chain from exploration and extraction of raw materials to processing, transport and final use. The LCA method is internationally accredited by ISO 14000 standards. The robustness of the method means that although carbon footprints vary between individual power plants, the ranking of electricity generation technologies does not change with different sources of data.

8.11.1

Wind energy

Electricity generated from wind energy has one of the lowest carbon footprints. As with other low carbon technologies, nearly all the emissions occur during the manufacturing and construction phases, arising from the production of steel for the tower, concrete for the foundations and epoxy/fibreglass for the rotor blades. These account for 98% of the total life cycle CO2 emissions. Emissions generated during operation of wind turbines arise from routine maintenance inspection trips. This includes use of lubricants and transport. Onshore wind turbines are accessed by vehicle, while offshore turbines are maintained using boats and helicopters.

8.11.2

Carbon emissions displacement and payback of wind power

Estimates of the life cycle carbon emissions of wind farms are not, in themselves, particularly useful and are only really of interest for comparison with other forms of low-carbon generation. Further interpretation is required to calculate other values that may be more meaningful, such as the lifetime emissions reduction of a wind farm (the net reduction of greenhouse gas emissions taking into account both the life cycle carbon emissions and the lifetime emissions displacement) or the carbon payback period (the time for the emissions displacement to offset the life cycle carbon emissions). Estimates of carbon emissions displaced by wind power vary widely, as they are a measure of the displaced emissions resulting from wind power replacing other forms of generation. However, analysis of current research indicates that, while the carbon displacement

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of wind power generation can be approximated using such figures, this is likely to underestimate the positive impacts of wind power on carbon emissions. To sum up, for life cycle carbon emissions the most critical aspects are the wind farm capacity factor, the system lifetime, the approach to recycling credits and uncertainty in emissions factors. Additionally, the emissions associated with construction on peatlands are important for onshore farms, while vessel use for installation and maintenance is significant offshore. While there is modest uncertainty associated with estimates of lifecycle emissions, values for both on- and off-shore wind are substantially lower than unabated gas and coal generation and there are fewer inherent uncertainties than nuclear. Estimates of the lifecycle carbon emissions of offshore wind are generally lower than for onshore, due to better wind profiles and economies of scale.

8.11.3

Future aspects of carbon footprint reduction in electricity generation technologies

Carbon footprints could be further reduced in all electricity generation technologies if the manufacturing phase and other phases of their life cycles were fuelled by low carbon energy sources. For example, if steel for wind turbines were made using electricity generated by wind, solar or nuclear plants. Using less raw materials would also lower life cycle CO2 emissions, especially in emerging technologies such as marine and PV. New semi-conducting materials (organic cells and nano-rods), are being researched for PV as alternatives to energy and resource intensive silicon. Biomass has the potential to generate electricity with ‘negative’ CO2 emissions. Burning ‘carbon neutral’ biomass and capturing the emissions using carbon capture and storage (CCS) technologies would result in a net removal of CO2 from the atmosphere. Studies show that a ‘negative emission’ of up to –410gCO2/kwh can be achieved. However, some researchers suggest that CCS, intended for large fossil-fuelled plants (>1000MW), would not be adopted for smaller capacity biomass plants, typically