Design and Performance Optimization of Renewable Energy Systems

By Marc A. Rosen

Design and Performance Optimization of Renewable Energy Systems provides an integrated discussion of issues relating to renewable energy performance design and optimization using advanced thermodynamic analysis with modern methods to configure major renewable energy plant configurations (solar, geothermal, wind, hydro, PV). Vectors of performance enhancement reviewed include thermodynamics, heat transfer, exergoeconomics and neural network techniques. Source technologies studied range across geothermal power plants, hydroelectric power, solar power towers, linear concentrating PV, parabolic trough solar collectors, grid-tied hybrid solar PV/Fuel cell for freshwater production, and wind energy systems. Finally, nanofluids in renewable energy systems are reviewed and discussed from the heat transfer enhancement perspective.

• Reviews the fundamentals of thermodynamics and heat transfer concepts to help engineers overcome design challenges for performance maximization
• Explores advanced design and operating principles for solar, geothermal and wind energy systems with diagrams and examples
• Combines detailed mathematical modeling with relevant computational analyses, focusing on novel techniques such as artificial neural network analyses
• Demonstrates how to maximize overall system performance by achieving synergies in equipment and component efficiency

• Language: English
• Publisher: Academic Press
• Release date: Jan 12, 2021
• ISBN: 9780128232323

Book preview

Design and Performance Optimization of Renewable Energy Systems

Edited by

Mamdouh El Haj Assad

Sustainable and Renewable Energy Engineering Department,University of Sharjah, Sharjah, United Arab Emirates

Marc A. Rosen

Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON, Canada

Table of Contents

Cover image

Title page

Copyright

Dedication

List of contributors

Preface

Chapter 1. Applications of renewable energy sources

Abstract

Chapter Outline

1.1 Introduction

1.2 Solar energy

1.3 Wind energy

1.4 Geothermal energy

1.5 Hydro energy

1.6 Bioenergy

Conclusions

Acknowledgments

References

Chapter 2. Renewable energy and energy sustainability

Abstract

Chapter outline

2.1 Introduction

2.2 Sustainability

2.3 Energy

2.4 Societal energy use and energy sustainability

2.5 Energy sustainability: interpretations, definitions, and needs

2.6 Selected measures relating to renewable energy for enhancing energy sustainability

2.7 Illustrative example: net-zero energy buildings

2.8 Closure

References

Chapter 3. Heat exchangers and nanofluids

Abstract

Chapter outline

3.1 Introduction

3.2 Heat exchanger classification

3.3 Effectiveness concept

3.4 Nanofluids

3.5 Applications of nanofluids in heat exchangers used in renewable energy technologies

3.6 Exergy analysis of nanofluidic heat exchangers

Conclusions

Acknowledgement

References

Chapter 4. Exergy analysis

Abstract

Chapter Outline

4.1 Introduction

4.2 Exergy

4.3 Procedure for energy and exergy analyses

4.4 Conventional balances: mass, energy, and entropy

4.5 Exergy balance

4.6 Exergy consumption

4.7 Exergy of heat, work, and electricity interactions

4.8 Exergy of matter

4.9 Reference environment

4.10 Efficiencies and other measures of merit

4.11 Applications and implications

4.12 Illustrative examples

4.13 Closing remarks

Nomenclature

References

Chapter 5. Solar power tower system

Abstract

Chapter Outline

5.1 Introduction

5.2 Case study

5.3 Solar power tower direct steam system

5.4 Intelligent methods

5.5 Result and discussion

Conclusions

Acknowledgment

References

Chapter 6. Parabolic trough solar collectors

Abstract

Chapter Outline

6.1 Introduction

6.2 Parabolic trough solar collectors: a summary

6.3 Theoretical formulations

6.4 Parabolic trough solar collector analysis: a case study

Conclusions

References

Chapter 7. Benefit-cost analysis and parametric optimization using Taguchi method for a solar water heater

Abstract

Chapter Outline

7.1 Introduction

7.2 Economic analysis of solar water heating system

7.3 Results and discussion of economic analysis

7.4 Optimization of input parameters using Taguchi method

7.5 Signal-to-noise ratio

7.6 Data analysis and parameter optimization

Conclusions

Nomenclatures

Appendix

References

Chapter 8. Fundamentals and performance of solar photovoltaic systems

Abstract

Chapter outline

8.1 Introduction

8.2 The pn junction model for solar cells

8.3 Photovoltaic modules

8.4 Photovoltaic systems

Conclusion

References

Chapter 9. Cooling systems for linear concentrating photovoltaic (LCPV) system

Abstract

Chapter Outline

9.1 Introduction

9.2 Linear concentrating photovoltaic system

9.3 Cooling system

Conclusion

Acknowledgments

Nomenclatures

References

Chapter 10. Geothermal power plants

Abstract

Chapter Outline

10.1 Introduction

10.2 Dry steam power plant

10.3 Single-flash steam power plant

10.4 Double-flash steam power plant

10.5 Binary power plant (ORC)

10.6 Illustrative examples

10.7 Exercises

References

Chapter 11. Heat pumps and absorption chillers

Abstract

Chapter Outline

11.1 Introduction

11.2 Types of heat pumps and their advantages

11.3 Geothermal heat pumps

11.4 Conventional heat pump for cooling

11.5 Illustrative examples

11.6 Absorption chillers

11.7 Closing remarks

References

Chapter 12. Hydropower

Abstract

Chapter Outline

12.1 Introduction

12.2 Hydropower technology

12.3 Revaluation concepts for hydroelectric energy storage

12.4 Pumped storage

12.5 Modeling of micro hydroelectric power plants

12.6 Hydroelectric optimization problem

Conclusion

Acknowledgment

References

Chapter 13. Energy and exergy analyses of wind turbines

Abstract

Chapter Outline

13.1 Introduction

13.2 Energy analysis of wind turbines

13.3 Exergy analysis of wind turbines

13.4 Numerical example

Conclusions

References

Chapter 14. Energy storage

Abstract

Chapter Outline

14.1 Introduction

14.2 Electrochemical energy storage

14.3 Hydrogen energy storage

14.4 Mechanical energy storage

14.5 Electromagnetic energy storage

14.6 Fuel cells

14.7 Thermal energy storage

Conclusions

Acknowledgment

References

Chapter 15. Use of nanofluids in solar energy systems

Abstract

Chapter Outline

15.1 Nanofluid: a new generation of heat transfer fluids

15.2 Renewable energy versus nonrenewable energy

15.3 Solar energy

15.4 Simulation of nanofluid flow through solar absorbers

15.5 Solar stills

15.6 Concluding remarks

References

Chapter 16. Artificial Intelligence applications in renewable energy systems

Abstract

Chapter Outline

16.1 What is Artificial Intelligence?

16.2 Artificial Intelligence and renewable energy

16.3 Artificial Intelligence examples for a photovoltaic solar cell: case study

References

Index

Copyright

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Dedication

To our families and friends, sources of love, inspiration and joy.

–Editors

List of contributors

Mohammad Alhuyi Nazari,     Renewable Energy and Environmental Engineering Department, University of Tehran, Tehran, Iran

Anis Allagui

Sustainable and Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates

Center for Advanced Materials Research, Research Institute of Sciences and Engineering, University of Sharjah, Sharjah, United Arab Emirates

Department of Mechanical and Materials Engineering, Florida International University, Miami, FL, United States

Mohammad AlShabi,     Mechanical and Nuclear Engineering Department, University of Sharjah, Sharjah, United Arab Emirates

Mamdouh El Haj Assad,     Sustainable and Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates

Mohamed M. Awad,     Mechanical Power Engineering Department, Faculty of Engineering, Mansoura University, Mansoura, Egypt

Ashok K. Barik,     Department of Mechanical Engineering, College of Engineering and Technology, Bhubaneswar, India

Mehdi A. Ehyaei,     Department of Mechanical Engineering, Pardis Branch, Islamic Azad University, Pardis New City, Iran

Mohsen Izadi,     Department of Mechanical Engineering, Faculty of Engineering, Lorestan University, Khorramabad, Iran

Ali Khosravi,     Department of Mechanical Engineering, School of Engineering, Aalto University, Espoo, Finland

Mohammad Malekan,     Department of Mechanical and Production Engineering, Aarhus University, Aarhus, Denmark

Juan Jose Garcia Pabon,     Institute in Mechanical Engineering, Federal University of Itajubá (UNIFEI), Itajubá, Brazil

Sidhartha Pattnaik,     Department of Mechanical Engineering, College of Engineering and Technology, Bhubaneswar, India

Marc A. Rosen,     Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON, Canada

Auroshis Rout,     Department of Energy Science and Engineering, Indian Institute of Technology, Bombay, India

Sudhansu S. Sahoo,     Department of Mechanical Engineering, College of Engineering and Technology, Bhubaneswar, India

Tiia Sahrakorpi,     Department of Mechanical Engineering, School of Engineering, Aalto University, Espoo, Finland

Suneet Singh,     Department of Energy Science and Engineering, Indian Institute of Technology, Bombay, India

Di Zhang

Sustainable and Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates

Center for Advanced Materials Research, Research Institute of Sciences and Engineering, University of Sharjah, Sharjah, United Arab Emirates

Preface

Mamdouh El Haj Assad and Marc A. Rosen

The renewable energy sector is growing year by year, with increasingly broad applications in industry and other sectors of the economy. Still there is a lack of knowledge about this sector, in part due to the relatively small number of institutions that offer degrees in renewable energy. Furthermore, much renewable energy technology is relatively new and there often is a lack of expertise in the field.

Design and Performance Optimization of Renewable Energy Systems seeks to address this need, by providing an enhanced understanding of the potential and main challenges of this emerging field. Students, researchers, and engineers who are interested in renewable energy and associated technology often face challenges in finding suitable references that encapsulate or summarize the main renewable energy technology options as well as most relevant renewable energy materials and tools for their characterization. This book presents information on various renewable energy systems in addition to their applications—in such domains as space heating and cooling, power generation, and more—and means to determine their performances using advanced thermodynamics and neural network techniques. To ensure leading edge methods are covered, a detailed chapter is included on exergy analysis for renewable energy systems, complementing the coverage of fundamentals of thermodynamics, heat transfer, and neural network analysis. The book synthesizes and describes in detail the knowledge that currently is distributed across the literature for different types of power plants driven by renewable energy sources.

This material in the book is organized so as to provide an accessible and comprehensive source of information for students, engineers, and researchers interested in all aspects of renewable energy systems. The book thereby provides readers with an exhaustive guide to key concepts and the state-of-the-art of the numerous facets of renewable energy systems, as well as what is needed for advancing the field of renewable energy and the performance of renewable energy systems. The coverage of artificial neural network analysis is important for the optimization of these systems.

The main objective of the book is to provide a unique work that presents design concepts for solar, geothermal, hydro, and wind energy systems based on the concepts of thermodynamics, heat transfer, and artificial neural networks. To facilitate this objective, the book includes material on energy storage and heat pumps driven by renewable energy sources, as they help in realizing the full potential of renewable energy. An understanding of the operating principles of renewable energy systems is conveyed to the reader. The book includes assessments of geothermal power plants, hydroelectric power facilities, solar power towers, linear concentrating PV units, parabolic trough solar collectors, and wind turbines. The use of nanofluids in renewable energy systems is reviewed and discussed, especially from the perspective of heat transfer enhancement.

Design and Performance Optimization of Renewable Energy Systems contains 16 chapters, and serves as a reference for students, engineers, and researchers involved in the field renewable energy.

Chapter 1

Applications of renewable energy sources

Mamdouh El Haj Assad¹, Mohammad Alhuyi Nazari² and Marc A. Rosen³,    ¹1Sustainable and Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates,    ²2Renewable Energy and Environmental Engineering Department, University of Tehran, Tehran, Iran,    ³3Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON, Canada

Abstract

Renewable energy has been the focus point of study at both graduate and undergraduate levels and of increasing societal interest, and its utilization is growing at a fast pace over much of the world. Renewable energy is a highly advantageous option for society to provide energy, including electricity and heat, with low environment damage. Energy regulations in most countries are becoming increasingly strict, largely in order to limit emissions of greenhouse gases and other pollutants. This has increased activity with and support for the use of renewable energy sources. In addition, there is a need for engineers and technologists to design, install, and operate renewable energy systems. This chapter introduces the most common types of renewable energy sources and their applications, for residential, institutional, commercial, industrial, transportation, and other purposes.

Keywords

Solar; wind; geothermal; hydro; bioenergy; cooling; heating; electricity

Chapter Outline

Outline

1.1 Introduction 1

1.2 Solar energy 2

1.2.1 Solar electricity generation 2

1.2.2 Heating and cooling 4

1.2.3 Desalination 7

1.3 Wind energy 7

1.4 Geothermal energy 9

1.4.1 Geothermal electricity 9

1.4.2 Geothermal heating 11

1.4.3 Geothermal cooling 11

1.5 Hydro energy 12

1.6 Bioenergy 13

Conclusions 13

References 13

1.1 Introduction

World total primary energy consumption has exhibited an increasing trend over the last decade, as shown in Fig. 1.1. Fossil fuels such as oil, coal, and natural gas presently have the highest share in the electricity generation mix, as shown in Fig. 1.2. However, due to concerns over the environmental issues associated with fossil fuels and their finite nature and potential depletion in the future, renewable energy sources are gradually being substituted in place of them. There are various types of renewable energy sources such as wind, solar, geothermal, and biomass. These are used for variety of applications including heating, cooling, electricity generation, and desalination [1,2]. Renewable energy sources have a notable role in meeting worldwide heating and cooling demands, achieving approximately a 10% share in 2016 [3].

Figure 1.1 World total primary energy consumption for 2008–18, based on BP report in 2018 [4].

Figure 1.2 Breakdown of global share of energy sources in electricity generation in 2018 [4].

Generally, solar collectors or geothermal heat exchangers can be employed for heating purposes. In addition to heating, the thermal energy of renewable sources can be utilized for desalination units. In renewable desalination systems, the absorbed thermal energy is used for saline water evaporation and producing fresh water.

Despite the applicability of renewable sources for different purposes, developments in renewable energy sources have in recent years been mainly concentrated in the power generation area. According to the REN21 report [3], 181 GW of renewable power was added to the global capacity of renewable energy power plants in 2018. In this period, among the various types of global installed renewable energy systems, solar photovoltaic (PV) panels have the highest share of electrical generation capacity, with approximately 100 GW, and wind turbines rank second at about 50 GW.

Renewable energy systems can be used for electricity generation both directly and indirectly. For instance, PV panels are employed for direct conversion of solar energy to electricity while in some cases the thermal energy of the renewable sources are extracted to drive power plants based on such thermal processes as Brayton and Rankine cycles. The efficiency of renewable energy systems for electricity production depends on several factors, including the employed technology and geographical and ambient conditions. Details of the most commonly employed renewable energy technologies are given in the remainder of this chapter.

1.2 Solar energy

The most abundant energy resource on the Earth is solar energy. The received energy of the Earth from the sun in 1 hour is approximately equal to the required energy for 1 year of human activities [5]. As indicated earlier, solar energy is broadly applied for electricity generation. According to the International Energy Agency (IEA), solar technologies have the potential to contribute a 14% decrease in carbon dioxide emissions in the power sector by 2050, on the basis of the BLUE Map scenario [6]. In addition to electricity generation, solar energy can be used for other purposes such as heating and desalination. The main advantages of solar energy are its wide availability and accessibility, although its intermittency makes predictability somewhat challenging. Various technologies can be applied for harvesting solar energy and convert it to required types of energy.

1.2.1 Solar electricity generation

PV panels are applied for direct conversion of solar irradiance into electricity. In PV cells, special types of semiconductors are used. The solar irradiance on the semiconductor provides the energy for electron transfer, which gives rise to an electrical current [7]. The most conventional types of PV technologies are crystalline and thin film, although some innovative technologies such as organic cells are being investigated [5]. According to the IEA [5], crystalline silicone and thin film technologies account for approximately 85%–90% and 10%–15% of worldwide PV cell market, respectively. Efficiencies of PV cells based on their technology are presented in Table 1.1.

Table 1.1

As seen in Table 1.1, the efficiencies of PV cells are not very high; therefore, the generated electricity for each for a specific surface area is often low or inadequate. In order to overcome this problem, concentrators can be coupled with PV cells. In this configuration, the solar radiation is concentrated using optical tools, which boosts the supplied energy for a specific area. Due to the performance degradation of PV cells at high temperatures, it is necessary to reduce their temperatures when concentrators are used. With such approaches, thermal systems are integrated with the PV cells for cooling, by extracting the absorbed thermal energy, as represented in Fig. 1.3. Sometimes the extracted thermal energy is supplied as a coproduct.

Figure 1.3 Schematic of concentrated photovoltaic (PV) system with thermal system [9].

The performance of PV cells in generating electricity depends on several parameters including temperature, solar irradiance, dust accumulation, shading and soiling of PV panels. The produced electricity by a PV cell depends greatly on the solar irradiance (solar power per unit of area). An increase in the solar irradiance makes more energy available for conversion to electricity. In addition to the solar irradiance, the temperature of the PV cell affects the output power due to its impact on the efficiency of the cell. Lowering the temperature of the cell normally results in higher efficiency. Due to the importance of the PV cell in generating electricity, several approaches exist for thermal management of PV cells, such as employing water flow, phase change materials (PCMs), and heat pipes. The presence of dirt or dust on the surface of PV cells can block sunlight from the cell, which lowers the energy received; as a consequence, the output power is correspondingly decreased. The dust reduction factor is typically equal to 0.93, meaning a 7% reduction in the input solar irradiance for a cell [10]. Spraying water on the surface of PV cells is suggested to overcome problems related to dirt and dust accumulation, and it can sometimes assist in temperature control [11]. Shadowing is another unfavorable phenomenon which degrades the performance of PV cells. According to some studies [12,13], in cases where 5%–10% of the array of solar panels is shaded, the generated electricity can be reduced over 80%.

In addition to direct methods, some technologies can be applied to indirectly convert solar energy to electricity. In these types of technologies, solar energy is used to drive thermal power plants. In order to produce a high quantity of thermal energy in a limited space, concentrators must be used. Generally, there are three types of concentrated solar power technologies, including linear parabolic collector systems, solar towers, and parabolic dish collectors. Linear parabolic collectors consist of a linear concentrator which has parabolic cross-sectional shape. The surface of the concentrator follows the path of the sun on a single axis. This concentrator is installed on a support structure, which keeps it fixed and permits appropriate performance in unfavorable conditions such as windy weather. In these concentrators, the received sunlight is focused on a tube along the focal point. Inside the tube there is an operating fluid which receives heat from the concentrated solar irradiation. In parabolic dishes, the reflecting panels follow the sun’s path by rotating around two axes, which are orthogonal. The panels focus the sunlight on a receiver which is located at the focal point. By employing these concentrators, high-temperature thermal energy is transferred to the operating fluid. In solar tower systems, reflecting panels with flat surfaces, known as heliostats, are used for concentrating the sunlight. These panels rotate on two axes and focus the solar radiation on the receiver located at the top of the tower which is in the center of the system. The fluid inside the solar receiver absorbs the concentrated solar energy, raising its temperature and pressure. Various types of solar thermal systems are illustrated in Fig. 1.4.

Figure 1.4 Schematics of three types of concentrating solar collectors: (A) linear, (B) solar tower, and (C) parabolic dish [14, 15].

Solar thermal energy systems can be integrated with existing thermal power plants to improve the efficiency. In some cases, solar energy is applied for preheating the compressed air that enters the combustion chamber in a gas turbine cycle, as shown in Fig. 1.5. The existence of a thermal energy storage unit in these configurations can further improve the efficiency of the system and make it operable at nighttime.

Figure 1.5 Schematic of solar-assisted gas turbine [16].

In addition to hybrid systems that use both fossil fuels and solar thermal energy for power generation, solar energy can be used alone for electricity production. In these types of systems, solar energy is applied to increase the temperature and pressure of air (or another working fluid) to levels appropriate for input to a power generation turbine. Solar concentrators are employed to extract more energy per unit of area. The performance of these cycles can be improved by heat recovery. In Fig. 1.6, a schematic diagram of a Brayton cycle with heat recovery and intercooling units is presented. In addition to employing heat recovery units for improving the efficiency of these cycles, some other ideas have been applied for this purpose, such as using supercritical fluids and combining Brayton cycles with other cycles such as Rankine cycles. In these configurations, the outlet hot gases of the gas turbine provide the main thermal input to drive the Rankine cycle. The efficiencies are usually higher for these configurations than for simple Brayton cycles [17].

Figure 1.6 Schematic of solar-driven Brayton cycle [18].

1.2.2 Heating and cooling

The share of building sector energy consumption in worldwide final energy utilization, which refers to final energy consumption by end users, was 35.3% around 2010 [19]. Renewable energy systems, especially solar technologies, can be used in several sectors to provide the energy required for heating and cooling. Renewable-based heating technologies are applied in order to collect, store, and deliver thermal energy to buildings, while cooling systems are used to supply cooling capacity [19].

Various systems can be used for heating a building using solar energy, such as a Trombe wall, an unglazed transpired solar façade, and a solar chimney. Trombe walls consist of a large wall, an air channel and an outer glazing. A schematic of a Trombe wall is illustrated in Fig. 1.7. In these types of systems, the large wall is used for absorbing and storing the energy of the sun that passes through the glazing. A portion of the absorbed heat is transferred via conductive and convective heat transfer mechanisms to the interior space. In addition, cold air enters the channels through a lower vent, is heated and moves upwards due to the buoyancy effect and exits the channel via an upper vent. An unglazed transpired solar façade consists of metal sheet walls with some holes that are employed for capturing solar thermal energy and heating air. As shown in Fig. 1.8, a fan is used for circulating the air flow. A solar chimney operates based on the principle of converting thermal energy into kinetic energy for air circulation. In addition to these methods, there are other technologies for air heating in buildings, such as solar roofs.

Figure 1.7 Schematic of Trombe wall (central vertical thick dark bar), from a top view perspective [19].

Figure 1.8 Schematic of unglazed transpired solar façade [19].

Solar thermal energy utilization in buildings is not limited to air heating as it can also be used for water heating. In most cases, water flows through sun-facing collectors. Many configurations are proposed for solar water heating systems, and these are mainly categorized as direct and indirect water heating approaches. In direct water heating systems, water flows through a collector and absorbs thermal energy, while in indirect methods heat exchangers are employed for transferring the thermal energy of the applied collectors.

As noted earlier, solar energy can be applied for cooling purposes. Solar cooling systems utilize the absorbed heat from sunlight in the thermally driven cooling processes. Generally, two main processes occur in these systems. In closed cycles, sorption chillers that are thermally driven are used for producing chilled water for utilization in space conditioning facilities. In open cycle solar cooling systems, water is typically employed as the refrigerant and a desiccant as a sorbent for air treatment of the ventilation technology [20]. One of the main advantages of solar cooling systems compared with alternatives is related to its nature. Since the highest solar irradiation coincides with the maximum required cooling demand, employing these types of systems can lead to a reduction in peak electrical demands on the electrical network in comparison with conventional systems used for cooling. In addition, solar cooling technologies can be applied for heating purposes, including water heating, in cold seasons [20].

1.2.3 Desalination

The increasing trend in world population and consequent increase in the need for consumable freshwater necessitate the development and utilization of desalination systems. A desalination process removes salt from saline feed-water, providing useful water suitable for such purposes as drinking and agriculture [21]. Desalination systems are mainly categorized as thermal or membrane technology types. As of the end of 2016, approximately 73% of the world’s desalination units were based on membrane technology and the remaining ones were of the thermal types [21]. Renewable energy systems can be applied for desalination both directly and indirectly. Generally, in direct techniques, the thermal energy of renewable energy sources is used for water evaporation and salt removal, while in indirect methods the required electricity for membrane technologies is generated by renewable sources.

Solar energy is among the most attractive renewable energy sources for desalination units. In thermal desalination systems, the heat required for saline water evaporation is supplied by the sun. Using thermal energy storage units such as PCMs makes solar thermal desalination units capable for nighttime operation [22]. Despite the improvements made to the performance of solar thermal desalination units by employing thermal storage, which have raised their costs, they can be economically feasible for large-scale systems [21]. In addition to thermal desalination systems, solar energy can be employed for indirect water desalination. In these types of desalination technologies, the electricity generated by PV panels or solar thermal power plants is used in membrane-based desalination units.

1.3 Wind energy

The utilization of wind as an energy source dates back to antiquity. The wind was used for grinding grain by employing vertical axis windmills and applied for transportation through sailboats [23]. In recent decades, wind turbines are used increasingly for electricity generation. In order to generate electricity from wind energy, wind turbines are applied. Wind turbines coupled with generators convert the kinematic energy of the wind to the electricity. According to the IEA [6], wind energy is expected to be responsible for a 12% reduction in carbon dioxide emissions by 2050, based on the BLUE map scenario. In addition to the benefits of wind turbines in decreasing emissions of carbon dioxide, their utilization can reduce the production of other pollutants such as oxides of nitrogen and sulfur [6]. Wind energy has accounted for approximately half of the world renewable energy generation in 2017 and 2018 [4]. In 2018 worldwide wind generation increased by 32 Mtoe compared with 2017, which gave it the first ranking among renewable energy sources, and its growth rate that year (2018) was about 12.6% [4].

There are two major classes of wind turbines based on the orientation of their axes, as shown in Fig. 1.9. The main components of horizontal axis turbines are the electrical generator and the rotor shaft and blades. In these types of wind turbines, wind vanes are employed for small-scale turbines while sensors are usually needed for large-scale turbines. In vertical axis wind turbines, the shaft of the rotor is installed vertically. There is no requirement for the blades to be pointed into the direction of wind for effective performance, which is one of the most important advantages of these types of wind turbines [24]. Generally, the power coefficient, which is defined as the ratio of actual generated power to the total power of the wind flowing into the turbine blade for a specific speed, is lower for vertical rather than horizontal axis wind turbines. This constitutes one of the main reasons for the greater commercial availability and success of horizontal axis wind turbines [23]. In addition, horizontal axis wind turbines have higher aerodynamic yields, lower costs, lower mechanical stresses, autonomous startup, and fewer requirements for components at ground level [25]. The typical number of blades for horizontal axis turbines varies between 1 and 3.

Figure 1.9 Vertical and horizontal axis wind turbines [24].

In addition to the type of wind turbine axis, various other criteria are used for their classification. Based on standard 624001 of the International Electrotechnical Commission (IEC) [26], wind turbines can be divided into several classes on the basis of their allowable ranges for parameters such as extreme wind gust in last 50 years, turbulence, and mean annual velocity, as shown in Table 1.2.

Table 1.2

In offshore regions, wind has higher speeds and its power is more constant compared with onshore locations. Moreover, air turbulence is lower in offshore regions [27]. These features make offshore locations more suitable for installing wind turbines and generating electricity. In addition to these favorable features of offshore regions for wind turbines, there are some other benefits such as the possibility of employing wind turbines with larger dimensions, which provide greater electricity generation, fewer physical restrictions such as buildings which block the flow of wind, and prevention of unfavorable visual impacts compared with onshore regions. However, offshore wind turbines also have disadvantages, the main being their higher cost due to the required structures. In addition, based on an environmental assessment [28], using onshore wind turbines leads to lower life cycle emissions of greenhouse gases compared with offshore facilities due to the requirement for floating platforms fixed in the water, when employing offshore wind turbines.

1.4 Geothermal energy

Another renewable energy source which can be used for electricity generation and heating is geothermal energy. This type of energy is able to provide electricity and heat with low carbon emissions, from hydrothermal resources at high temperature, deep aquifer systems at low and moderate temperatures, and hot rock resources [29]. Relative to solar and wind energy, the stability in electrical power output of geothermal systems is higher. Moreover, geothermal power plants are not affected by variations in climate, which causes them to have higher capacity factors and makes them more appropriate for baseload electricity generation [30]. According to the IEA, worldwide electricity generation from geothermal systems in 2017 was approximately 84 TWh. The cumulative capacity of such geothermal systems was about 14 GW, and is expected to reach 17 GW by 2023 [30]. The IEA suggests that geothermal energy has the potential to account for a 3% reduction in emissions of carbon dioxide associated with electricity generation by 2050, based on the BLUE map scenario [6]. Geothermal energy sources can be used for heating and cooling [31], freshwater production [32], as well as electricity generation [33]. In most cases, heat exchangers are used to transfer heat from the high-temperature water extracted from geothermal resources to the working fluids used in heat pumps for heating/cooling purposes. Geothermal heat pumps are renewable energy-based units.

1.4.1 Geothermal electricity

Geothermal energy can be used as a reliable source for producing electricity. Generally, conventional Rankine or Kalina cycles are used for geothermal power generation. In order to generate electricity from geothermal resources, it is necessary to extract their thermal energy. In geothermal power plants, steam from the hot sublayers of the Earth are typically used for electricity production, in systems employing turbines and generators, as shown in Fig. 1.10.

Figure 1.10 Schematic of geothermal power plant [34].

Generally, there are three kinds of geothermal power plants: dry steam, flash steam (single and double flash), and binary cycle. In dry steam power plants, the steam extracted from sublayers is directly piped to the power plant to drive a steam turbine, as shown in Fig. 1.10. In single flash steam geothermal power plants, high-temperature saturated liquid water (>182°C) is extracted. Reducing the pressure of this water in this process leads to partial boiling of the water. The produced mixture in this stage is divided by a separator into liquid and vapor flows. The steam flow passes through the steam turbine while the liquid water exits the separator and is reinjected back into the ground. The double flash steam geothermal power plant is similar to single flash system, except in the