Conversion of Water and CO2 to Fuels using Solar Energy E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1315

Veröffentlichungsjahr: 2024

Beschreibung

Conversion of Water and CO2 to Fuels using Solar Energy Comprehensive Resource for Understanding the Emerging Solar Technologies for Hydrogen Generation via Water Splitting and Carbon-based Fuel Production via CO2 Recycling Fossil fuel burning is the primary source of carbon in the atmosphere. The realization that such burning can harm the life on our planet, has led to a surge in research activities that focus on the development of alternative strategies for energy conversion. Fuel generation using solar energy is one of the most promising approaches that has received widespread attention. The fuels produced using sunlight are commonly referred to as "solar fuels." This book provides researchers interested in solar fuel generation a comprehensive understanding of the emerging solar technologies for hydrogen generation via water splitting and carbon-based fuel production via CO2 recycling. The book presents the fundamental science, technologies, techno-economic analysis, and most importantly, the materials that are being explored to establish artificial methods of fuel production using solar energy. For the rapid advancement of the field, it is necessary for researchers, particularly for those who are new to the field, to have clear knowledge of various materials studied so far and their performance. For this reason, almost half of the book is dedicated to the discussions on materials and properties. Key topics discussed in the book include: * Photocatalytic/photoelectrochemical processes that use semiconductor photocatalysts, including both ceramic and non-ceramic materials * Photovoltaic assisted electrochemical processes * Solar thermochemical processes * Molecular photosynthesis Researchers and professionals in the fields of energy and materials and closely related science and engineering disciplines could use this book to acquire clear insights on both mainstream solar fuel technologies and those in the developmental stages.

Leseprobe

Conversion of Water and CO2 to Fuels using Solar Energy

Science, Technology and Materials

Edited by

Oomman K. Varghese

University of Houston

Houston

USA

Flavio L. Souza

Universidade Federal do ABC

Santo Andre

Brazil

Copyright © 2024 The American Ceramic Society and John Wiley & Sons, Inc. All rights reserved.

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List of Contributors

Kirk H. Bevan

Division of Materials Engineering

Faculty of Engineering

McGill University

Montreal

QC

Canada

Kamran Dastafkan

School of Chemistry

The University of New South Wales

Faculty of Science

Sydney

NSW

Australia

Wanjae Dong

Department of Electrical Engineering and Computer Science

University of Michigan

Ann Arbor

MI

USA

Christoph Falter

Synhelion SA

Zurich

Switzerland

Ke Fan

State Key Laboratory of Fine Chemicals

School of Chemical Engineering

Dalian University of Technology

Dalian

China

Liniquer A. Fontana

Institute of Chemistry

University of Campinas (UNICAMP)

Campinas

SP

Brazil

Nicolas Gaillard

Hawaii Natural Energy Institute

University of Hawaii at Manoa

Honolulu

HI

USA

Bingchan Hu

State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF)

School of Energy and Power Engineering

Xi'an Jiaotong University

Xi'an

Shaanxi

China

Asif Iqbal

Division of Materials Engineering

Faculty of Engineering

McGill University

Montreal

QC

Canada

Muhammad S. Khan

International Research Center for Renewable Energy (IRCRE)

State Key Laboratory of Multiphase Flow in Power Engineering (MPFE)

Xi'an Jiaotong University

Xi'an

China

Youjun Lu

State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF)

School of Energy and Power Engineering

Xi'an Jiaotong University

Xi'an

Shaanxi

China

Jackson D. Megiatto Jr.

Institute of Chemistry

University of Campinas (UNICAMP)

Campinas

SP

Brazil

Zetian Mi

Department of Electrical Engineering and Computer Science

University of Michigan

Ann Arbor

MI

USA

Botong Miao

Division of Materials Engineering

Faculty of Engineering

McGill University

Montreal

QC

Canada

Catia Ornelas

Institute of Chemistry

University of Campinas

Campinas

SP

Brazil

Heng Pan

State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF)

School of Energy and Power Engineering

Xi'an Jiaotong University

Xi'an

Shaanxi

China

Appu V. Raghu

Laser Molecular Photoscience Laboratory

Molecular Photoscience Research Center

Kobe University

Kobe

Japan

Vitor H. Rigolin

Institute of Chemistry

University of Campinas (UNICAMP)

Campinas

SP

Brazil

Ingrid Rodriguez‐Gutierrez

Brazilian Nanotechnology National Laboratory (LNNANO)

Brazilian Center for Research in Energy and Materials (CNPEM)

Campinas

Brazil

Centro de Ciências Naturais e Humanas (CCNH)

Federal University of ABC (UFABC)

Santo André

Brazil

Shaohua Shen

International Research Center for Renewable Energy (IRCRE)

State Key Laboratory of Multiphase Flow in Power Engineering (MPFE)

Xi'an Jiaotong University

Xi'an

China

Flavio L. Souza

Brazilian Nanotechnology National Laboratory (LNNANO)

Brazilian Center for Research in Energy and Materials (CNPEM)

Campinas

Brazil

Centro de Ciências Naturais e Humanas (CCNH)

Federal University of ABC (UFABC)

Santo André

Brazil

Mark T. Spitler

Department of Chemistry

University of North Carolina

Chapel Hill

NC

USA

Qian Sun

School of Chemistry

The University of New South Wales

Faculty of Science

Sydney

NSW

Australia

Takashi Tachikawa

Laser Molecular Photoscience Laboratory

Molecular Photoscience Research Center

Kobe University

Kobe

Japan

Department of Chemistry

Graduate School of Science

Kobe University

Kobe

Japan

Lianpeng Tong

School of Chemistry and Chemical Engineering

Guangzhou Key Laboratory for Clean Energy and Materials

Guangzhou University

Guangzhou

China

Oomman K. Varghese

Nanomaterials and Devices Laboratory

Department of Physics

University of Houston

Houston

TX

USA

Texas Center for Superconductivity

University of Houston

Houston

TX

USA

Lei Wang

Department of Chemical and Biomolecular Engineering

National University of Singapore

Singapore

Singapore

Chuan Zhao

School of Chemistry

The University of New South Wales

Faculty of Science

Sydney

NSW

Australia

Baowen Zhou

School of Mechanical Engineering

Research Center for Renewable Synthetic Fuel

Shanghai Jiao Tong University

Shanghai

China

Department of Electrical Engineering and Computer Science

University of Michigan

Ann Arbor

MI

USA

Peng Zhou

Department of Electrical Engineering and Computer Science

University of Michigan

Ann Arbor

MI

USA

Preface

The twenty‐first century world is experiencing planetary and societal challenges that are unprecedented in human history. The United Nations identifies climate breakdown, air pollution, and biodiversity loss as a triple planetary crisis threatening the life on Earth. It is an irrefutable fact that fossil fuels, which form the backbone of the global energy system, are the primary cause of this crisis. The global average temperature is rising dramatically due to the changes in the planet's energy budget caused by the atmospheric accumulation of greenhouse gases originating from fossil fuel burning. The impact on the life on the planet is multifaceted. The changes happening in the environment are becoming irreversible. The window of opportunity to limit warming is rapidly closing. Our actions in the next couple of decades are crucial to securing a sustainable future.

Under these circumstances, the development of a viable alternative energy system has become imperative. It is no longer simply a matter of choice, but rather a collective responsibility to explore and implement innovative approaches that can power our societies without further compromising the delicate balance of the planet. In this scenario, solar fuel generation stands out as a promising avenue toward a green and sustainable future. Harnessing the power of the Sun, the most abundant and renewable energy source available, holds tremendous potential for mitigating the adverse impacts of traditional energy generation methods.

By converting water and carbon dioxide (CO2) into fuels using energy from sunlight, the world can enjoy similar benefits as those provided by fossil fuels while minimizing their detrimental effects. The potential benefits of these synthetic fuels, called solar fuels, extend far beyond the realm of energy production and pollution control. The research, development, and implementation of solar fuel technologies stimulate innovation, drive technological advancements, and propel the growth of a new green economy. Moreover, incorporating solar fuel generation inspires a global shift towards a more harmonious relationship between humanity and the natural world, where environmental preservation and economic progress go hand in hand.

This book is a testament to the collective efforts of leading experts in the field who have devoted their time and energy over years and decades to unraveling the complexities of solar fuel generation. It gives the readers a ride through the historical, scientific, and environmental aspects of global warming, the establishment of an alternative energy system based on solar power and fuels, the primary approaches used for solar fuel generation, the scientific principles, technological and economic perspectives, and the key materials investigated for technology development and commercialization. By furnishing the fundamentals, discussing the latest advancements, and digging deep into the research on materials that enable solar fuel generation, this book is envisioned to be a resource for the knowledge accumulated through decades of research and a source of inspiration for further innovation in the quest for sustainable energy solutions.

The book is structured into an introductory chapter followed by two sections dealing with the techno‐scientific and material aspects separately. The introductory chapter (Chapter 1) provides perspectives on global warming and climate change as consequences of excessive fossil fuel burning. It discusses briefly the current energy situation and sustainable options. The focus of this chapter is on introducing the two artificial fuel generation pathways, water splitting and CO2 reduction, and the most promising approaches for accomplishing them. Section 1 (Chapters 2–7) entitled “Solar Fuel Generation Processes: Science and Technology,” introduces the three promising approaches to fuel generation, namely, photoelectrochemical (PEC)/photocatalytic, concentrated solar thermochemical, and photovoltaic‐assisted electrochemical. Certain unique perspectives on the natural and artificial photosynthetic systems are given in this section. In Section 2, entitled “Materials for Solar Fuel Generation,” (Chapters 8–12), emphasis is given on discussing the key materials explored in the three fuel generation approaches mentioned above. Development of catalysts for photo‐, thermo‐ and electro‐conversion forms the heart of the discussion.

Chapter 2 discusses the basic science of artificial photosynthesis. It first introduces the catalyst complexes performing natural photosynthesis. This is followed by a brief discussion of bioinspired artificial systems for fuel generation. The remainder of the chapter is targeted at PEC and photocatalytic systems for water splitting and CO2 reduction. The fundamental principles, device configurations, parameters of relevance for evaluating the performance of devices, and examples of materials used in these applications are given. Chapter 3 gives a comprehensive understanding of the scientific, technological, environmental, and economic aspects of the concentrated solar thermochemical fuel generation processes. It reviews the thermodynamics of the thermochemical cycles and the reactor designs for a two‐step process. By taking a two‐step thermochemical cycle as an example, the readers are taken deeper into a discussion of the energy and mass balance in the reactor and fuel plant scales, the techno‐economic and life cycle aspects, water and land requirements for commercially implementing the technology, and the geographic locations around the world appropriate for producing fuels via solar thermochemical processes. Photovoltaics (PV) can be used for powering water electrolyzers and providing external bias for PEC cells. Both these technologies, from basic concepts to techno‐economics, are discussed in detail in Chapter 4. Moreover, it covers the topic of monolithically stacked PV‐PEC devices for spontaneous water splitting under solar irradiation.

Two different perspectives on artificial photosynthesis are given in Chapters 5 and 6. Chapter 5 takes the readers on a captivating journey through the history of molecular photosynthesis. It gives an overview of the evolution of natural photosynthetic systems, emphasizing the relevance of the photo‐redox enzymes, and then connects the discussion to the development of molecular complexes and organic‐inorganic hybrid systems for artificial photosynthesis. Chapter 6 gives a unique perspective on the kinetic designs for the solar reduction of CO2. The catalytic reactions in natural photosynthesis and man‐made systems are systematically analyzed. It points out the issues with the current approaches and identifies potential opportunities for widening the exploration of efficient and durable systems for fuel generation using CO2 and water, following the pathway of natural photosynthesis. Chapter 7 presents the application of the band diagram approach to understanding the characteristics of PEC devices. Taking the case of photoanodes in PEC cells, both analytical and numerical modeling methods are discussed.

Section 2 begins with Chapter 8, which details the performance of redox pairs used in two‐step thermochemical cycles for the conversion of CO2 and water (H2O) to fuels, while touching on the basis of thermolysis (single step) and multistep cycles. It also discusses the catalysts investigated for sulfur‐iodine cycle, solar gasification, and reforming of methane and methanol. While PV‐assisted water electrolysis has become a mature technology for commercial production of hydrogen, the electrocatalytic conversion of CO2 to fuels and value‐added chemicals is an emerging technology in which significant research efforts are being invested. Chapter 9 gives the fundamentals of CO2 reduction reactions, electrochemical cell designs, electrolytes, and various catalysts, including compounds of metals, single‐atom catalysts, and molecular catalysts. A feature of this chapter is the discussion of the in‐situ characterization of electrocatalysts for CO2 reduction.

Another very promising approach that has not yet reached the technology commercialization stage is PEC and photocatalytic water splitting and CO2 conversion. Chapters 10–12 are dedicated to the materials researched for this approach. The semiconductors used for PEC fuel generation (called photocatalysts) are basically divided into oxides and non‐oxides. Chapter 10 covers the ceramic materials with an emphasis on the oxide photocatalysts used for PEC water splitting. It discusses the properties and performance of carbides and nitrides including two‐dimensional transition metal carbides and nitrides called MXenes. Among the non‐oxides, gallium nitride (GaN)‐based compounds form one of the most promising families of materials. Chapter 11 discusses GaN and its compounds used for artificial photosynthesis. Two dedicated sections focus on integrated devices for solar water splitting and CO2 reduction separately. Nanostructured semiconductors are the most investigated class of materials in the area of PEC and photocatalytic fuel generation. Chapter 12 exposes the readers to the unique electronic, plasmonic, structural, morphological, and surface properties of nanomaterials and their heterostructures used for PEC and photocatalytic applications. It provides a comprehensive view of the characteristics of zero, one, and two‐dimensional photocatalysts in performing solar water splitting and CO2 reduction.

Through its compilation of various perspectives and insights, this book is expected to facilitate further innovations in solar fuel generation technologies and help speed up our journey toward a cleaner and more sustainable energy future. Recognizing our shared responsibility, embracing renewable energy sources, and prioritizing sustainability and responsible care for the environment are essential. It is our hope that this book will serve as an inspiring catalyst, sparking our collective efforts toward a world where solar fuel generation powers our societies while preserving the integrity of our planet.

Oomman K. Varghese (Editor)

Flavio L. Souza (Editor)

Department of Physics & Texas Center for Superconductivity

                              Centro de Ciências Naturais e Humanas (CCNH)

University of Houston

                              Federal University of ABC (UFABC)

1Solar Fuel Generation: The Relevance and Approaches

Ingrid Rodriguez-Gutierrez1,2, Flavio L. Souza1,2, and Oomman K. Varghese3,4

1Brazilian Nanotechnology National Laboratory (LNNANO), Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, Brazil

2Centro de Ciências Naturais e Humanas (CCNH), Federal University of ABC (UFABC), Santo André, Brazil

3Nanomaterials and Devices Laboratory, Department of Physics, University of Houston, Houston, TX, USA

4Texas Center for Superconductivity, University of Houston, Houston, TX, USA

1.1 Introduction

“Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1 °C above [the average temperature in] 1850–1900 in 2011–2020”, declared United Nations Intergovernmental Panel on Climate Change (IPCC) in the Synthesis Report of its 6th Assessment Report (IPCC, 2023). The “human activity” mentioned in the report as the primary cause of greenhouse gas accumulation in the atmosphere was not hidden to great scientific minds of the nineteenth century even though the consequences of the new energy system that emerged in conjunction with industrial revolution were not apparent. Professor Svante Arrhenius, in his book Worlds in the Making: The Evolution of the Universe, asserted: “We thus recognize that the percentage of carbonic acid in the air must be increasing at a constant rate as long as the consumption of coal, petroleum, etc., is maintained at its present figure and at a still more rapid rate if this consumption should continue to increase as it does now” (Arrhenius, 1908). He wrote this statement based on his calculation published in 1896. According to this calculation, a 50% increase in carbonic acid (historically a synonym for carbon dioxide, CO2) concentration in the atmosphere would increase the global average temperature by about 4 °C (Arrhenius, 1896). Professor Arrhenius and many of his contemporary scientists knew that the blanket provided by the greenhouse gases, especially CO2, was keeping the planet warm and hoped that it would not send the planet into another ice age. Apparently, they did not anticipate that the temperature increase due to excessive usage of fossil fuels would really go beyond safe limits and make catastrophic, potentially irreversible, changes in the climate and ecosystem. Until the second decade of the twenty‐first century, the climate change due to anthropogenic global warming was a myth to many, including scientists, policy makers and corporate executives; however, now it is an undeniable truth.

It is clear that human beings have abused nature extensively. There is only a little time left to act prudently and reverse the changes to some extent, before the situation goes out of hand. Of course, going back to pre‐industrial revolution era, burning wood for heat and using horse for work, is pretty naïve to think of. There has been an exponential growth in energy demands due to technological advancements and population growth in the post‐industrial revolution period. According to the International Energy Agency (IEA) statistics (International Energy Agency, 2022), oil, coal, and natural gas currently account for roughly 80% of the global energy production. Hydro, biomass/biofuel, solar, and other renewables, and nuclear technologies bearing the remaining load are largely sustainable. The task of replacing fossil fuel‐based energy system with a sustainable one is humongous and exceedingly complex. Fossil fuels are naturally available, do not need conversion to other forms for energy production, and can be stored or transported to almost any location. Moreover, plastic and a myriad of other products are derived from these fuels. No single energy vector has all these qualities. For example, electric power generated from renewable sources involves an energy conversion process and has limited options for transportation and storage. Hydrogen is another energy carrier. It can be transported, stored, and used as a raw material for chemicals; however, its production requires renewable energy and a renewable precursor such as water (H2O). The same is true with other synthetic fuels such as ammonia and biomass products. Therefore, it is highly likely that the future energy system developed to replace the fossil fuel‐oriented one will have contributions from electricity and different synthetic fuels generated using a wide range of sustainable pathways.

The sustainable pathways for electricity and fuel production should use, in principle, recycling approaches involving renewable energy sources while maintaining carbon neutrality. For example, hydrogen fuel generated via water splitting using renewable energy recycles water. Similarly, biofuels enable recycling of CO2. A future energy system would quite likely use a range of renewable energy sources, especially solar, wind, hydro, geothermal, and tidal. Biomass has solar energy stored as potential energy in atomic and molecular bonds. Biomass and nuclear energy, along with a small fraction of fossil fuels, will continue contributing to the global energy supply for the foreseeable future. Among the renewable technologies, the solar power and fuel generation technologies are expected to have the highest share in the new energy system due to the abundance of solar energy.

In this rare time of energy system transition, it is important to accelerate the development of fuel generation technologies to make the transition fast and least disruptive, as renewable power generation alone cannot fill the gap created upon phasing out fossil fuels. There are significant challenges to overcome as the number of solar photons reaching Earth per unit area per second (called photon flux) is limited and the fuels should be produced within the limitations imposed by thermodynamics and kinetics. This chapter gives an introduction to the solar‐driven technologies for synthetic fuel generation using H2O and/or CO2 as precursors. Prior to discussing these technologies, we would provide an overview of the effects of greenhouse gas emissions on global warming and associated climate change.

1.2 The Nexus Between Fossil Fuels, Global Warming, and Climate Change

Even before the time of Svante Arrhenius, the scientific community was aware of the warming potentials of the constituents of the atmosphere. The seminal works of Foote and Tyndall published in 1856 and 1859, respectively, showing the infrared (IR) absorption ability of CO2 and the resulting temperature rise are examples (Foote, 1856; Tyndall, 1861). Prior to these experiments, Joseph Fourier conducted studies on the role of atmosphere in keeping the surface of Earth warm. In his article published in 1824, Fourier stated: “[Earth's] temperature can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in repassing into the air when converted into non‐luminous heat”(Fleming, 1999; Fourier, 1824). Although the exact cause of the opacity of the atmosphere for the rays emitted by Earth was not clear to the scientific community two hundred years ago, they had insights about the role of atmosphere in preserving warmth in the planet.

Sunlight reaching Earth consists of wavelengths in the ultraviolet (UV) to IR region (about 200–4,500 nm). The incident light is partly reflected (this portion is called albedo) and partly absorbed. The atmosphere is generally transparent to the reflected wavelengths as its components do not absorb (but scatter) relatively high‐energy radiations. On the other hand, Earth as a blackbody reemits the absorbed sunlight in the form of electromagnetic waves of longer wavelengths (mid‐to‐far IR region). In a steady state, the energy received by Earth and that released must be equal. Since nitrogen and oxygen are the primary constituents of the atmosphere (78.08% and 20.95% by volume, respectively, in dry air), the atmosphere must be transparent to IR rays. This is because N2 and O2 are homogenous diatomic molecules and, therefore, they are not IR‐active. If N2 and O2 were the only constituents of the atmosphere, global average temperature would have stayed at about −18 °C (255 K). Nevertheless, the atmosphere contains water vapor (up to about 4% by volume), CO2 and methane (CH4). These molecules have IR‐active modes of vibration and therefore, they absorb the IR waves reemitted by Earth causing the temperature to increase. This is termed the greenhouse effect because the glass enclosure of a greenhouse for growing plants traps outgoing long‐wavelength IR rays and warms the environment (Florence et al., 1950).

The CO2 level in the atmosphere has stayed between approximately 180 and 300 ppm since the last ice age about 800,000 years ago. The average CO2 concentration during the pre‐industrial revolution period was about 280 ppm. A blanket consisting primarily of CO2 and water vapor formed via natural processes kept the atmosphere warmer by around 31 °C compared to −18 °C (taking the average temperature to ∼13 °C), with minor warming and cooling periods (approximately ±1 °C variation for over 5,000 years of human history) lasting a few hundred years. This pattern has been disrupted since the beginning of the twentieth century. In 1880, the average CO2 concentration became 290 ppm and the global average temperature was about 13.6 °C. Since the middle of the twentieth century, both CO2 concentration and global average temperature have increased consistently, as obvious from Figure 1.1,1 which shows a graph between CO2 released into the atmosphere and the global average temperature change with time. The average temperature in the period 1960–1990 is taken as reference for temperature change. Such a steep increase in CO2 concentration has never been noticed since the last ice age. It has now been proven beyond doubt that increasing concentration of anthropogenic CO2 in the atmosphere is primarily responsible for the observed rise in the global average temperature.

Figure 1.1 The trend in global average temperature change relative to the 1961–1990 average temperature and CO2 emissions from fossil fuels and industry.

In addition to CO2 and water vapor, CH4, nitrous oxide (N2O), and chlorofluorocarbons in the atmosphere also absorb IR radiation and contribute to greenhouse effect. Nevertheless, the contribution from CO2 to global temperature rise is much higher than that from these compounds despite the fact that their global warming potential2 is many folds higher than that of CO2. This is because of the significantly higher amount of CO2 in the atmosphere (∼418 ppm of CO2 vs. 1.9 ppm of CH4 in 2022) and the associated higher radiative forcing.3 Per the National Oceanic and Atmospheric Administration (NOAA) data (https://gml.noaa.gov/aggi/aggi.html), the radiative forcing caused by CO2 was 2.17 W/m2 while that due to methane was 0.65 W/m2 in 2022. This means that the contribution of CO2 in preventing the energy from escaping the Earth's atmosphere is over three times higher than that of methane. Fossil fuels are the sources of about 75% of the global greenhouse gases (CO2 is ∼90% in this composition) emitted to the atmosphere.

Among the three phases (solid, liquid, and gas) of fossil fuels, coal releases maximum CO2 while burning. While some evidence of fossil fuel use is found in studies conducted on societies flourished before Christ, the modern era coal mining started in the sixteenth century, and drilling for gas and oil began in 1821 and 1859, respectively. Since the initiation of mechanization that occurred in conjunction with the invention of steam engine, coal has been used in place of wood due to its better heating value (two to three times). Coal consists of 70–90% carbon by mass and the rest hydrogen and impurities, including sulfur, oxygen, and oxides. The hydrogen‐to‐carbon ratio is lowest in coal. To compare, hydrogen‐to‐carbon molar ratio in anthracite coal is 0.5 : 1, whereas the ratio is 3 : 1 for methane, and this is the reason for higher CO2 emissions from coal. The emission is lower for natural gas and oil products such as gasoline, diesel, and kerosene, but not substantially. While burning 1 ton of coal releases about 3.2 tons of CO2, the corresponding numbers are 2.3 and 1.8 tons, respectively, for gasoline and natural gas. About 37 billion metric tons of CO2 is emitted per year globally. The average annual CO2 uptake by land is 25–30% and that by ocean is ∼25%. The atmosphere retains the rest. The average CO2 level in 2022 was 418 ppm, and it is increasing at a rate of around 3 ppm per year. Efforts are being carried out by several countries to control and reduce CO2 production from fossil fuels. In 2015, the Paris agreement was adopted by 196 parties, outlining strategies to limit global warming to 1.5 °C by achieving net zero by 2050 (Bhore, 2016). Nevertheless, according to the UN Emissions Gap Report 2022, there has been very limited progress in reducing the large emissions gap for 2030 (United Nations Environment Programme, 2022). The gap between promised measures and those needed to keep the emissions per the Paris agreement is huge. The IPCC warns that unless sufficient measures are implemented per the time line of the Paris agreement, it will be difficult to keep the CO2 concentration below 450 ppm and warming below 2 °C in this century (IPCC 2014, 2023). The temperature rise of such magnitude would trigger catastrophic and irreversible changes in the climate and ecosystems, as well as extreme hardships, to vulnerable people (Lenton et al., 2023; McKay et al., 2022; Tollefson, 2021).

Apart from CO2, release of CO produced due to incomplete combustion, unburnt hydrocarbons, oxides of nitrogen NOx (primarily NO and NO2), mercury (from coal), SO2 produced via oxidation of sulfur, and particulate matter are other problems with fossil fuel burning. While CO2 increases the temperature of the atmosphere, the small particulate matter dispersed in the atmosphere can scatter incoming light and cause cooling effect due to the reduction in energy reaching Earth. Although some particles absorb the infrared radiation from Earth and cause warming effect, global radiative forcing due to aerosol–radiation interactions and aerosol–cloud interactions is negative, indicating that their overall effect is cooling. Does this mean that atmospheric particulate concentrations should be increased to offset the global warming? An author of this chapter recalls the experience in traveling in coal‐fueled trains with steam engines. The carbon particles from coal firing cover the body and nostrils of the passengers soon after the beginning of the journey. The relationship between climate change and air quality is very complex. Pollution makes the human population vulnerable to many chronic and terminal diseases including cancer (Turner et al., 2020). Benzene and polycyclic aromatic hydrocarbons are examples of carcinogens released from oil combustion. Although the use of catalytic converters, particulate filters, and unleaded fuel could help reduce the emission of hazardous pollutants, their concentrations in the atmosphere are increasing. The World Health Organization statistics associate about 7 million deaths every year with air pollution.

Nevertheless, some people still believe that global warming and its effect are not real. Some of the common arguments are the following: (a) CO2 concentration has shown both increasing and decreasing characteristics several times in the history of the planet. In fact, it has been oscillating between about 280 and 180 ppm every 50,000–100,000 years for the past 800,000 years. Nevertheless, the steep increase in CO2 concentration that the globe is currently experiencing is unprecedented. (b) A higher CO2 level is desirable because plants would grow better and the food production would increase. While it is true that higher CO2 favors the growth of plants, the overall effect of the global warming on the plants is complex. The extreme climatic conditions could adversely affect their growth. Moreover, the food crops would lose key nutrients and proteins at higher CO2 levels. (c) The amount of anthropogenic greenhouse gas in the atmosphere is low compared to that from natural sources such as volcanos. The emission from natural sources are offset by the processes such as photosynthesis, absorption by ocean and creation of soil and peats. In the past, these processes prevented CO2 concentrations from going above 300 ppm. (d) The CO2 concentration in the atmosphere (0.042%) is too small to create an appreciable effect on the thermal budget of Earth. This is not correct. Every ppm of CO2 is equivalent to dumping over 12 billion tons of CO2 into the atmosphere (∼37 billion tons @ ∼3 ppm per year). The global warming potential of this amount of CO2 is high. (e) Studies such as those conducted by the IPCC do not take into account all aspects of development, and, therefore, their reports do not portray the realistic picture of the situation. Even if the number of deaths due to climate change increases, the overall life expectancy will increase due to advancements in healthcare, sanitation, and other sectors (O'Neill, 2023). This is equivalent to stating that let the weak die and the strong survive. (6) The unusual wildfires and extreme weather conditions occurring in many parts of the world cannot be related to climate change. While all wildfire incidents and other weather extremes cannot be attributed to climate change, some disasters are indeed driven by global warming, as determined by various scientific agencies around the world, including NASA (https://climate.nasa.gov/), through daily monitoring of the atmosphere using multiple technologies.

It is a fact that there is a close agreement between the predictions of most climatic models and the observed changes. Regardless of the difference in opinion, it is important to unite and utilize the options available now, which may disappear for ever after a short time if appropriate actions are not promptly taken.

1.3 The Energy System Transformation

Fossil fuels account for about 80% of world's energy production (see Figure 1.2). Among the three forms of fossil fuels, natural gas has the highest heating value (∼52 vs. 45 MJ/kg for crude oil and 24–30 MJ/kg for coal) and least CO2 emission (about half vs. coal). A strategy followed by many nations is to increase the share of natural gas in the energy production and reduce that of coal and oil. While this is desirable, the global warming trend has reached a stage where a reversal may not happen by just halving the CO2 emission. Moreover, there are concerns about using natural gas because natural gas typically contains 85–95% methane (the remaining consists of ethane, heavier hydrocarbons, and non‐hydrocarbons such as CO2), which is a greenhouse gas. Among the remaining major sources, nuclear, hydro, and biomass/biofuels are, in principle, carbon‐neutral technologies. According to the IEA World Energy Outlook 2022 (https://www.iea.org/reports/world-energy-outlook-2022), while the contributions from all these sources would increase in the future, the increase in the share of nuclear and biofuels in the energy production would be substantial by 2030; however, the growth is expected to slow down as 2050 approaches (International Energy Agency, 2022). Nuclear and biofuel technologies are mature, and these could help reduce the emissions rapidly. The concerns over radioactive waste produced during nuclear power production, food vs. fuel as well as low efficiency problems in biofuel‐based processes, and challenges related to loss of forests and wild life habitats in establishing hydroelectric power projects are some of the factors that would lower the interest in the expansion of these technologies. Therefore, meeting the future energy demand efficiently and securely requires extensive utilization of renewable energy sources.

Figure 1.2 The world's total primary energy supply by fuels in 2021.

The data are collected from the International Energy Agency.

The renewable energy conversion technologies such as solar photovoltaics (PV), wind energy, geothermal energy, and ocean energy are the primary contributors to the renewable energy market (∼3% of overall energy supply). A technology that is expected to replace the fossil fuel‐based power production is solar PV. The contribution to global electricity production (∼28,500 TWh) from solar PV reached 4.6% in 2022, and it is increasing consistently at a level envisioned for the period 2023–2030 under the net zero emission by 2050 scenario (International Energy Agency, 2022). The market share of solar PV is expected to grow at the rate of 30% in this period. Nevertheless, renewables still cannot replace fossil fuels completely unless radical improvements in establishing renewable source‐based power plants are achieved. The implementation of sustainable energy technologies requires overcoming challenges such as the establishment of long‐term contracts, ensuring priority grid access, and continually installing new renewable energy facilities. Other obstacles such as reduced electricity demand, supply chain disruptions, and construction delays in some regions, must be addressed for promoting the adoption of sustainable energy.

Despite the rapid growth of the renewable electric power market, the intermittency of the renewable energy supply related to diurnal cycles, seasons, and other weather‐related factors makes it highly unreliable. The present energy storing technologies fall short, necessitating the development of strategies to overcome the limitations of sustainable energy technologies. One of the most promising approaches is the efficient conversion of the intermittent energy into chemical fuels. The high energy density of these fuels (e.g., hydrogen, methane, and methanol) makes them a viable option for solar energy storage.

1.4 Solar Fuels