Focus

In natural photosynthesis, organisms optimize solar energy conversion through organized assemblies of photofunctional chromophores and catalysts within proteins that provide specifically tailored environments for chemical reactions. As with their natural counterparts, artificial photosynthetic systems for practical production of solar fuels must collect light energy, separate charge, and transport charge to catalytic sites where multielectron redox processes occur. Although encouraging progress has been made on each aspect of this complex problem, researchers have not yet developed self-ordering components and the tailored environments necessary to realize a fully functional artificial photosynthetic system.

Synopsis

Previously, researchers used complex, covalent molecular systems comprising chromophores, electron donors, and electron acceptors to mimic both the light-harvesting (antenna) and charge-separation functions of natural photosynthetic arrays. These systems allow one to derive fundamental insights into the dependences of electron-transfer rate constants on donor–acceptor distance and orientation, electronic interaction, and the free energy of the reaction. However, significantly more complex systems are required in order to achieve functions comparable to natural photosynthesis. Self-assembly provides a facile means for organizing large numbers of molecules into supramolecular structures that can bridge length scales from nanometers to macroscopic dimensions. To achieve an artificial photosynthetic system, the resulting structures must provide pathways for the migration of light excitation energy among antenna chromophores, and from antennas to reaction centers. They also must incorporate charge conduits, that is, molecular “wires” that can efficiently move electrons and holes between reaction centers and catalytic sites. The central challenge is to develop small, functional building blocks that have the appropriate molecular-recognition properties to facilitate self-assembly of complete, functional artificial photosynthetic systems.

Focus

Photosynthesis is the biological process that converts sunlight into chemical energy. It provides the basis for life on Earth and is the ultimate source of all fossil fuels and of the oxygen we breathe. The primary light reactions occur with high quantum yield and drive free-energy-demanding chemical reactions with unsurpassed efficiency. Coupling of photosynthesis to hydrogenases allows some organisms to evolve H2. Research into understanding and applying the molecular details and reaction mechanisms of the involved catalysts is well under way.

Synopsis

Life needs free energy. On our planet this free energy is mostly provided by the Sun. The sunlight is captured and converted into chemical energy by a process known as photosynthesis (from Greek, photo, “light,” and synthesis, “putting together”). This process occurs in plants and many bacteria. The “big bang” of evolution was the development of oxygenic photosynthesis. In this process sunlight is employed to split the abundant water into the molecular oxygen we breathe. The protons and electrons gained are employed by the organism within complex reaction sequences to reduce CO2 to carbohydrates. The widespread availability of the electron source water allowed oxygenic organisms to spread and diversify rapidly. The O2 produced was initially toxic for most species, but those which learned to cope with the emerging oxygen-rich atmosphere were able to gain additional energy by “burning” organic matter.

Focus

This chapter introduces a wide array of sustainable approaches that use solar energy technology to address challenges faced by communities in developing regions of the world. Special emphasis is placed on how rural electrification efforts can provide communities with an off-grid power supply that can stimulate technological development through the improvement of a wide range of resources including infrastructure, health care, and education.

Synopsis

In many areas of the developing world, even basic energy access is still a privilege, not a right. Excessive demand from rapid urbanization often leads to unreliable electricity supplies in the urban areas. However, the problem is most acute in rural and nomadic communities, where lower population densities and income levels make it less practical to establish the necessary infrastructure to connect these communities to the electrical grids that power the cities and megacities. In this context, there is a growing need for innovative energy solutions tailored to the particular needs and demands of these communities, if wide-scale rural electrification that extends energy access to rural regions is to be possible.

Focus

The technology of flight provides immeasurable benefits for today's society: promoting global trade and commerce, providing humanitarian relief, and connecting people. In the next millennium, progressive environmental considerations will play a key role in our ability to continue to provide these benefits seamlessly. As with other transport, the consumption of petroleum-based fuels and materials draws from the Earth's finite natural resources. To move toward fully sustainable aviation, there must be a continued focus on reducing the environmental footprint over the product life cycle.

Synopsis

To ensure a balance between the social and economic benefits of aviation and the energy and environmental impacts, the aviation industry is working on improvements across the entire life cycle of its products and services. Opportunities for environmental improvement lie in advanced materials and manufacturing technologies, improved aerodynamics systems and engine efficiency, alternative fuels, increased fleet operational efficiency, and aircraft recycling.

Throughout this book an effort has been made to use Si and SI-derived units, but the diversity of the field and the, often independent development of areas that comprise it, have led to well-established use of non-SI units in some cases.

Where that was the case, we provide conversion in the chapter text or figure captions, if the figures that are suitable for the text, use non-SI units. Here we summarize the SI, SI-derived and other units, to facilitate use of the various parts of the book.

Focus

The development of “carbon-neutral” biofuels and biomaterials is critical for stabilizing atmospheric carbon dioxide levels and reducing the current dependence on petroleum. Microbes are self-replicating “biocatalytic” systems that can convert solar energy and plant biomass into a wide range of molecules that can be used for biofuels and biomaterials. However, major technical challenges need to be addressed before the approaches become economically viable. Among these challenges are the recalcitrance of lignocellulosic feedstocks, the low cell density of algal production systems, and the scaling needed to minimize impacts on freshwater/arable land.

Synopsis

Biofuels and biomaterials are among the diverse portfolio of technologies considered essential to address concerns over an excessive dependence on fossil fuels and their impact on the environment, such as through CO2 emissions. Whereas the burning of fossil fuels increases the amount of CO2 in the atmosphere, biofuels and biomaterials have the potential to be carbon neutral or negative. Because the carbon sequestered in biofuels is eventually returned to the atmosphere upon burning, these are primarily carbon-neutral technologies, although some crops accumulate carbon in the soil and have the potential to be carbon-negative. The use of biomaterials, on the other hand, can decrease the atmospheric concentration of carbon dioxide by fixing it in useful materials.

Focus

Tar sands and oil shale are “unconventional” oil resources. Unconventional oil resources are characterized by their solid, or near-solid, state under reservoir conditions, which requires new, and sometimes unproven, technology for their recovery. For tar sands the hydrocarbon is a highly viscous bitumen; for oil shale, it is a solid hydrocarbon called “kerogen.” Unconventional oil resources are found in greater quantities than conventional petroleum, and will play an increasingly important role in liquid fuel supply as conventional petroleum becomes harder to produce. With the commercial success of Canadian tar-sand production, and the proving of technology, these unconventional resources are increasingly becoming “conventional.” This chapter focuses on the trends that drive increased production from tar sands and oil shale, and discusses the geological, technical, environmental, and fiscal issues governing their development.

Synopsis

Oil shale and tar sands occur in dozens of countries around the world. With in-place resources totaling at least 4 trillion barrels (bbl), they exceed the world's remaining petroleum reserves, which are probably less than 2 trillion bbl. As petroleum becomes harder to produce, oil shale and tar sands are finding economic and thermodynamic parity with petroleum. Thermodynamic parity, e.g., similarity in the energy cost of producing energy, is a key indicator of economic competitiveness.

Focus

We are all familiar with small-scale electrical energy storage in chemical batteries, from cars to cell phones. Batteries offer near-instant response time, but cost tends to scale linearly with size, making very large batteries or systems of batteries prohibitively expensive. Mechanical energy storage, in contrast, tends to be inexpensive at large scales due to the use of relatively low-cost materials (e.g., concrete and steel) and low-cost storage media (e.g., water, air), and due to long device lifetimes. The levelized cost of energy (LCOE), which is essentially the break-even selling price per kilowatt-hour (kWh) including all lifetime costs, for pumped-hydroelectric and compressed-air storage can be much less than for smaller-scale technologies such as batteries.

Synopsis

Electrical energy can be converted into any of the three forms of mechanical energy: gravitational potential, elastic potential, or kinetic. Each of these can, in turn, be converted back to electricity. In principle, though never in practice, interconversion can be 100% efficient. The most common mechanical energy-storage technologies are pumped-hydroelectric energy storage (PHES), which uses gravitational potential energy; compressed-air energy storage (CAES), which uses the elastic potential energy of pressurized air; and flywheels, which use rotational kinetic energy.

Focus

Utilization of solar energy on a terrawatt scale is a viable, environmentally conscious solution to the growing global demand for energy. Key solar technologies that can provide significant reductions in carbon emissions and environmental pollution, including conventional photovoltaics, concentrating photovoltaics, and solar thermal technologies, as well as passive solar technologies such as biofuels, biomass, and wind power, are highlighted here and discussed in detail in the following chapters.

Synopsis

As described in Chapter 3, energy, water, and food supply will all pose key challenges in the coming decades. Because of their ramifications in terms of socioeconomic development, quality of life, and political relations, these mounting concerns could result in more conflict and global instability if not addressed promptly and effectively. This chapter summarizes the contributions that active and passive solar energy technologies could make toward addressing this crisis. Solar energy is a viable solution to both energy demand and environmental issues; however, the mass deployment of some solar technologies faces some real challenges that are not necessarily only technological in nature but in many cases are also economic and sociopolitical. Today, coal, natural gas, oil derivatives, and nuclear energy are the most cost-effective routes to large-scale electricity generation. However, these traditional technologies have environmental impacts that, in some instances, have been catastrophic not only for the environment but also for the people and the economy of the areas affected (e.g., the Chernobyl nuclear disaster of 1986, the Gulf of Mexico oil spill of 2010).

Focus

Despite the Kyoto Protocol and a wealth of good intentions, emissions of greenhouse gases (GHGs) – the primary cause of climate change – have continued to increase, not decrease, in recent years.

We face a global environmental crisis that is expected to include increased temperatures over land and in oceans, rising sea levels, more acidification of the oceans, increased flooding as well as drought, and extinction of many species as a result. The climate–energy crisis could cause major disruptions to ecosystems, the availability of fresh water, farming, economic activity, and global political stability on many levels.

Focus

Human society requires increasing material inputs to sustain its production and reproduction. Many of the environmental problems we are currently facing can be attributed to this metabolism of society. This chapter explores the evolution of material use during human history and discusses current trends and patterns of global material consumption.

Synopsis

This chapter introduces the concept of social metabolism and the corresponding methodology of material flow accounting, which can be used to investigate exchange processes of materials and energy between societies and their natural environment and to address corresponding environmental problems. Against this background, the evolution of material use during human history from hunter-gatherers to current industrial societies is explored. The multiplication of global material extraction in the twentieth century, the shift from renewable biomass toward mineral and fossil materials and the growing share of materials used for non-energy applications are discussed in more detail. The chapter analyzes growth in material flows during industrialization and explores the interrelationships of material and energy use, population growth, economic development, and technological change. Inequalities in the global use of materials across countries and world regions are addressed. The chapter ends with a brief account of the challenges arising from the expected future growth of materials use.

Preface

Academically and industrially there is increasing awareness that energy and the environment present society with issues that are pressing and need to be approached globally. Many of the global effects are driven by two factors: the continuing increase in population as shown in Figure 1 from the IEA World Energy Outlook report for 2009 and the increasing demand for energy, both from the new developing and from the developed countries, as shown in Figure 2, which comes from the BP Energy Outlook 2030 Report page 8, published in January 2011, which summarizes global energy use over 60 years.

This growing global energy demand alone, independent of environmental concerns, is such a problem globally that innovative and creative solutions must be found to meet this demand. This is probably more challenging in the developing countries, where resource limitations may limit the number of options available, than in the developed ones. We note this, because as this text will emphasize, there is no obvious way to achieve this. In fact, it is clear that the demand will have to be met by a mosaic of current, as well as new, sources. Overall, the fact that a diversity of new energy sources is needed, will create new, large-scale industries, a development that may lead to significant changes in, or even the end of, some of our current established industries. If we now include the various environmental concerns that accompany modern society's functioning, this leads to the drive to achieve new energy-generating and -storage capacity via sustainable and clean technologies. At the same time, both these concerns, and the increasing energy demand, re-emphasize the need to accelerate the adoption of more efficient technologies worldwide. Underlying all of these changes is the basic understanding that the approaches that will be implemented must be ultimately sustainable not for tens or hundreds of years, but for thousands of years.

Focus

A new automotive “DNA” based on electrification and connectivity will be required in order to address the associated energy, environment, safety, and congestion challenges. What are the potential materials and design implications if future vehicles can sense and communicate with each other and the surroundings, drive autonomously, and do not crash?

Synopsis

Transformational change is coming to the automobile. Amid growing concerns about energy security, the environment, traffic safety, and congestion, there is an increasing realization that the 120-year-old foundational “DNA” of the automobile is not sustainable. In response, auto manufacturers are introducing a wide range of propulsion, electronics, and communications technologies. These will create a new automotive DNA, based on electrification and connectivity, and will profoundly affect personal mobility. Future vehicles are likely to be tailored to a range of specific uses, from short urban commutes to long-distance cargo hauling, and they will likely be energized by electricity and hydrogen. Unlike gasoline or diesel fuels, electricity and hydrogen can be made from primary energy sources that are diverse and can be made renewably. Future vehicles will be propelled with electric motors, perhaps in the wheels, and the braking, steering, and driving functions will be controlled electronically.

Focus

Energy use is inexorably woven into the fabric of modern civilization. Human well-being, economic productivity, and national security all depend on the availability of plentiful and affordable energy supplies. However, over the past half century, we have come to understand that continued growth of energy use along the lines of current energy systems will lead to unacceptable consequences for the Earth's climate and oceans. Maintaining and increasing the access to energy services to satisfy crucial societal needs requires the development of a sustainable global energy system that transitions away from energy supply options with high greenhouse gas (GHG) emissions and unhealthy air pollutants. Disparity in energy access is also not sustainable. We must provide sufficient energy for the estimated 1.6 billion people who do not have access to modern energy systems today. Fortunately, plentiful energy resources are available to meet our needs, and technology pathways for making this transition exist. Continuing to lower the cost and increase the reliability of energy from sustainable energy resources will facilitate this transition. Changing the world's energy systems to reduce GHG emissions is one of the critical challenges that humans must face in this century. The required transition can begin now with improvements in efficiency of energy conversion and use, and with continuing deployment over the coming decades of a variety of existing and innovative technologies. With continuing attention to energy conversions that minimize wastes, have low life-cycle impacts, and maximize recycling of materials, a set of sustainable energy systems can be created.

Synopsis

Feeding, clothing, and housing a growing world population will be a significant challenge in this century, as will supplying the fresh water, heat, lighting, and transportation we will need to live comfortable and productive lives. This chapter discusses energy sustainability, with emphasis on the requirement to reduce GHG emissions. A sustainable energy system is one in which energy is supplied and converted to energy services in ways that avoid unacceptable consequences for local, regional, and global natural systems that control climate and support ecosystems that provide essential services. Figure 3.1 illustrates typical conversions of a primary energy resource (solar, wind, geothermal energy, fossil or nuclear resources, etc.) into a product, such as a fuel or an energy carrier, like electricity, that then can be converted to a service like heat, light, or mechanical work. Sustainable processes and systems that convert some primary energy resource into energy services will be ones that are as efficient as possible – smaller quantities of the primary energy resource are needed and fewer waste materials are created if the conversions are efficient. Some have argued that only energy flows such as solar, wind, and wave power should be considered sustainable. Others note that any system of energy conversions has some footprint and impact, and that sustainability is necessarily a relative measure, not an absolute one. In any case, sustainable systems will have low impacts over the full life cycle of the conversions (see Chapter 41). Recycling of materials used in energy conversions will be maximized, and amounts of waste materials created in the chain of energy conversions to services will be minimized when the whole cycle from primary resource to services (such as mechanical work) to waste heat and products is considered.

Focus

Solar fuels are substances that store solar energy in the form of usable chemical energy. For them to be appropriate substances have to meet various requirements, which include the ability for their efficient production, sufficient energy density, and flexible conversion into heat, electrical, or mechanical energy. An essential requisite is environmental friendliness in order to sustainably incorporate conversion products into the global circulation of matter of the biosphere. This can be fulfilled now only by H2 and to some extent by some carbohydrates directly or indirectly produced by a solar energy source. Direct conversion of solar energy into free chemical energy – either by hydrogen or by hydrocarbon production – requires the development of efficient catalysts for the oxidation of water. This represents a huge materials design challenge, because multiple requirements for catalyst materials must be addressed simultaneously. After an introduction concerning the materials requirements for solar fuel production, storage, transport, and consumption, this chapter focuses on the topic of water-oxidation catalysis: what we can we learn from evolution for the development of an artificial oxygen-evolution center?

Synopsis

Solar fuels are substances that store solar energy as usable chemical energy at rates that allow sustainable conversion into other forms of energy. Oxidation of water driven by solar light, in order to gain a proton source for either hydrogen or hydrocarbon formation, seems to be the key issue. There are three fundamental steps in the conversion of solar energy into chemical energy (the first two of which also apply to the conversion of solar energy into electrical energy in photovoltaic cells; see Chapter 18). The first is light capture – absorbing the sunlight and transforming it into chemical energy of excited electron–hole pairs. The second is electron and hole transfer – separating and transporting sunlight-excited electrons and holes from their original sites in order to use them. The third is catalysis – the efficient generation and breaking of chemical bonds using the electrons and holes produced in this process to reduce and oxidize compounds, respectively. Although much progress has been made in catalytic control of reaction paths for the production of hydrocarbon and ammonia compounds, detailed understanding of the atomic processes occurring at catalysts and materials design of highly active and specific catalysts are still in their early stages. Here, the outstanding problem is the control of multielectron-transfer reactions, such as the evolution of oxygen from water, which requires four electrons for the liberation of one molecule of oxygen. Thermodynamics promise an energetically most favorable possibility for the oxidation of water when all four electrons are extracted in a correlated way with a minimum identical energy input (1.23 eV). Multielectron transfer requires a well-balanced coordinated chemical reaction involving multiple electron transfers through a sufficiently complex reacting chemical catalyst.

Focus

Energy efficiency has been recognized as the most effective near-term means to meet the energy and environmental crisis we face today. In the USA, buildings are the largest energy-consumption sector of the economy. Residential and commercial buildings combined consume over 40% of the primary energy and over two-thirds of the total electricity [1]. Heating and cooling are the largest portions of this. Demonstration homes have shown that the heating consumption can be reduced by as much as 90% by the proper application of very thick thermal insulation in the walls, roof, and windows [2]. One challenge is the development of very thin economical insulation materials that provide the same performance.

Synopsis

Thermal insulations comprise a wide variety of materials whose primary function is the reduction of heat and mass transfer. These insulations are made from foams, fibers, and other fine-structured solids that encapsulate a gas or are held in vacuum. In buildings, insulation improves energy efficiency by reducing heat loss in winter and heat gain in summer. Even modern windows have been designed to act as insulators to improve building performance. Appliances such as refrigerators and ovens use insulation to maintain temperature and to be more energy-efficient. Insulations are also used in industrial operations such as furnaces for metal and glass manufacture as well as as a means to control silicon-chip formation. In space and on Earth, insulations are used for protection in harsh environments. The development of the next generation of insulations requires an understanding of the physics of heat transfer and of the role advanced materials play in limiting heat transfer by the mechanisms of conduction, radiation, and convection.

Focus

The Sun's radiation can be modeled as a 6,000-K blackbody radiator. Whereas photovoltaics (PV) can convert the part of the Sun's spectrum to electrical energy, over 40% of that spectrum, namely, the infrared (IR) range, is lost as heat. In solar thermoelectrics (TE), the thermal energy from the IR range is converted directly into electricity. Therefore, a solar PV–TE hybrid system would have access to the entire spectrum of the Sun.

Synopsis

With respect to solar energy conversion, PV devices utilize the UV region, whereas TE devices utilize the IR region (which is waste heat with respect to the PV devices) to generate electricity. In a solar PV–TE hybrid system, a high-efficiency solar collector would turn the sunlight (from the IR spectrum) into heat that would then be transformed by TE devices into usable electricity. In addition, the solar thermal energy could be stored in a thermal bath, or TE devices could be used to charge batteries that could then provide electricity when the Sun was not shining. Such a TE system would need to operate at around 1,000 K (~700 °C), and the materials would need to exhibit high ZT values around this temperature.