Availability of affordable energy has enabled spectacular growth of industrialization and human development in all parts of the world. With growth now accelerating in developing countries, demands on energy sources and infrastructure are being stretched to new limits. Additional energy issues include the push for renewable resources with reduced greenhouse gas emissions and energy security affected by the uneven distribution of energy resources around the globe. Together, these issues present a field of opportunity for innovations to address energy challenges throughout the world and all along the energy flow. These energy challenges form the backdrop for this special expanded issue of MRS Bulletin on Harnessing Materials for Energy. This article introduces the global landscape of materials issues associated with energy. It examines the complex web of energy availability, production, storage, transmission, distribution, use, and efficiency. It focuses on the materials challenges that lie at the core of these areas and discusses how revolutionary concepts can address them. Cross-cutting topics are introduced and interrelationships between topics explored. Article topics are set in the context of the grand energy challenges that face the world into the middle of this century.

Increasing demand for energy, diminishing stocks of oil and natural gas, and the public's desire to enhance environmental quality, particularly by reducing greenhouse gas emissions, all point to the need for improved materials. For example, generating electricity from the most abundant fossil fuel, coal, efficiently and with no environmental damage, presents notable challenges to develop higher performance materials. Technologies exist to transform one fossil fuel to other uses, such as coal to a gas or liquid. New materials that increase the efficiency of the transformation and lower its cost would provide valuable flexibility. Materials should be evaluated in terms of their entire life-cycle in order to discern which will make the greatest contribution. Because society has many pressing needs, both commercial value and contribution to fundamental materials science should guide priorities in materials research.

A sustainable global energy system requires a transition away from energy sources with high greenhouse emissions. Vast energy resources are available to meet our needs, and technology pathways for making this transition exist. Lowering the cost and increasing the reliability and quality of energy from sustainable energy sources will facilitate this transition. Changing the world's energy systems is a huge challenge, but it is one that can be undertaken now with improvements in energy efficiency and with continuing deployment of a variety of technologies. Numerous opportunities exist for research in material sciences to contribute to this global-scale challenge.

Reducing CO2 emissions from the use of fossil fuel is the primary purpose of carbon dioxide capture and storage (CCS). Two basic approaches to CCS are available.1,2 In one approach, CO2 is captured directly from the industrial source, concentrated into a nearly pure form, and then pumped deep underground for long-term storage (see Figure 1). As an alternative to storage in underground geological formations, it has also been suggested that CO2 could be stored in the ocean. This could be done either by dissolving it in the mid-depth ocean (1–3 km) or by forming pools of CO2 on the sea bottom where the ocean is deeper than 3 km and, consequently, CO2 is denser than seawater. The second approach to CCS captures CO2directly from the atmosphere by enhancing natural biological processes that sequester CO2 in plants, soils, and marine sediments. All of these options for CCS have been investigated over the past decade, their potential to mitigate CO2 emissions has been evaluated,1 and several summaries are available.1,3,4

Coal is a critical component in the international energy portfolio, used extensively for electricity generation. Coal is also readily converted to liquid fuels and/or hydrogen for the transportation industry. However, energy extracted from coal comes at a large environmental price: coal combustion can produce large quantities of ash and CO2, as well as other pollutants. Advanced technologies can increase the efficiencies and decrease the emissions associated with burning coal and provide an opportunity for CO2 capture and sequestration. However, these advanced technologies increase the severity of plant operating conditions and thus require improved materials that can stand up to the harsh operating environments. The materials challenges offered by advanced coal conversion technologies must be solved in order to make burning coal an economically and environmentally sound choice for producing energy.

A recent report published by the National Petroleum Council (NPC) in the United States predicted a 50–60% growth in total global demand for energy by 2030. Because oil, gas, and coal will continue to be the primary energy sources during this time, the energy industry will have to continue increasing the supply of these fuels to meet this increasing demand. Achieving this goal will require the exploitation of both conventional and unconventional reservoirs of oil and gas in an environmentally acceptable manner. Such efforts will, in turn, require advancements in materials science, particularly in the development of materials that can withstand high-pressure, high-temperature, and high-stress conditions.

The discovery that gas hydrates (also called clathrate hydrates) can crystallize (Figure 1) as a solid by the combination of water and several types of gases exposed to low temperatures and elevated pressure goes back to the 1800s. French researchers were the frst to report the formation of methane, ethane, and propane hydrates.1 Results of these studies remained as scientifc novelties until the mid-1930s, when it was discovered in Germany that gas hydrates forming as solids above 0°C in gas pipelines blocked the fow of natural gas.2 This observation initiated a furry of activities both in Europe and in the United States to fnd various inhibitors to prevent hydrate formation in gas transmission lines. During the mid-1960s, it was recognized that nature, over millions of years, has deposited vast amounts of methane hydrates along most of the continental margins in the ocean sediments, as well as along the permafrost regions in Alaska, Canada, and Russia.3 Figure 2 shows the presence of methane hydrate deposits in the ocean sediments and in the permafrost regions of the world. These deposits are byproducts of microbial decomposition of organic matter or of Earth's geothermal heating distributed worldwide where temperature and pressure are suitable for hydrate formation. The distribution of organic carbon in Earth's crust as methane

Global energy demand is expected to increase steeply, creating an urgent need to evolve a judicious global energy policy, exploiting the potential of all available energy resources, including nuclear energy. With increasing awareness of environmental issues, nuclear energy is expected to play an important role on the energy scenario in the coming decades. The immediate thrust in the science and technology of nuclear materials is to realize a robust reactor technology with associated fuel cycle and ensure the cost competitiveness of nuclear power and to extend the service life of reactors to 100 years. Accordingly, the present-generation materials need to be modified to meet the demands of prolonged exposure to irradiation and extended service life for the reactor. Emerging nuclear systems incorporate features to ensure environmental friendliness, effective waste management, enhanced safety, and proliferation resistance and require development of high-temperature materials and the associated technologies. Fusion, on a longer horizon of about fve decades, also requires the development of a new spectrum of materials. The development of next-generation materials technology is expected to occur in short times and is likely to be further accelerated by strong international collaborations.

Every energy source has environmental impacts—positive and negative. Nuclear power is a carbon-free source of energy that can reduce CO2 emissions by displacing the use of fossil fuels. The present level of carbon displacement is approximately 0.5 gigatonnes of carbon per year (GtC/year), compared to the nearly 8 GtC/year emitted by the use of fossil fuels. However, there are three major negative environmental impacts of nuclear power: catastrophic accidents, nuclear weapons, and nuclear waste. The last two, weapons and waste, are directly tied to the type of nuclear fuel cycle (Figure 4 in the main nuclear article by Raj et al. in this issue). The different fuel cycles refect different strategies for the utilization of fssile nuclides, mainly 235U and 239Pu, and these different strategies have important implications for nuclear waste management and nuclear weapons proliferation.

Raj et al. describe the promise of nuclear energy as a sustainable, affordable, and carbon-free source available this century on a scale that can help meet the world's growing need for energy and help slow the pace of global climate change. However, the factor of millions gain in energy release from nuclear fssion compared to all conventional energy sources that tap the energy of electrons (Figure 1) has also been used to create explosives of unprecedented lethality and, hence, poses a serious challenge to the expansion of nuclear energy worldwide. Although the end of the cold war has eliminated the threat of annihilating humanity, the likelihood of a devastating nuclear attack has increased as more nations, subnational groups, and terrorists seek to acquire nuclear weapons.

The main concept currently in use in wind energy involves horizontal-axis wind turbines with blades of fiber composite materials. This turbine concept is expected to remain as the major provider of wind power in the foreseeable future. However, turbine sizes are increasing, and installation offshore means that wind turbines will be exposed to more demanding environmental conditions. Many challenges are posed by the use of fiber composites in increasingly large blades and increasingly hostile environments. Among these are achieving adequate stiffness to prevent excessive blade deflection, preventing buckling failure, ensuring adequate fatigue life under variable wind loading combined with gravitational loading, and minimizing the occurrence and consequences of production defects. A major challenge is to develop cost-effective ways to ensure that production defects do not cause unacceptable reductions in equipment strength and lifetime, given that inspection of large wind power structures is often problematic.

The direct conversion of solar energy to electricity by photovoltaic cells or thermal energy in concentrated solar power systems is emerging as a leading contender for next-generation green power production. The photovoltaics (PV) area is rapidly evolving based on new materials and deposition approaches. At present, PV is predominately based on crystalline and polycrystalline Si and is growing at >40% per year with production rapidly approaching 3 gigawatts/year with PV installations supplying <1% of energy used in the world. Increased cell efficiency and reduced manufacturing expenses are critical in achieving reasonable costs for PV and solarthermal. CdTe thin-film solar cells have reported a manufactured cost of $1.25/watt. There is also the promise of increased efficiency by use of multijunction cells or hybrid devices organized at the nanoscale. This could lead to conversion efficiencies of greater than 50%. Solar energy conversion increasingly represents one of the largest new businesses currently emerging in any sector of the economy.

Photovoltaics is not the only means of using sunlight to generate electricity. Another major solar technology is called “concentrating solar power” or CSP. CSP technologies use concentrating optics to generate high temperatures that are used to drive conventional steam or gas turbines. CSP is generally considered a central generation technology, rather than a source of distributed generation. That is, a large amount of power is generated in one location, with transmission and distribution to the various points of use, rather than generating small amounts of the power at numerous points of use. Because of this feature, CSP is predominantly a utility-scale source of power.

The field of thermoelectricity began in the early 1800s with the discovery of the thermoelectric effect by Thomas Seebeck. Seebeck found that, when the junctions of two dissimilar materials are held at different temperatures (ΔT), a voltage (V) is generated that is proportional to ΔT. The proportionality constant is the Seebeck coeffcient or thermopower: α = −δV/ΔT. When the circuit is closed, this couple allows for direct conversion of thermal energy (heat) to electrical energy. The conversion effciency, ηTE, is related to a quantity called the fgure of merit, ZT, that is determined by three main material parameters: the thermopower α, the electrical resistivity ρ, and the thermal conductivity κ.

The World Bank estimates that over two billion people on the planet live their daily lives without access to basic, reliable electric services. Rural populations in Africa, Latin America, Asia, and island nations need clean water, health services, communications, and light at night. Small, simple, solar electric systems are part of the solution—increasing the quality of life, often at a cost that is less than what is presently being spent for kerosene, dry-cell batteries, and the recharging of automotive batteries that must be lugged to the nearest town on a weekly basis (see Figure 1).

California continues its tradition of leading the United States in environmental stewardship through the California Solar Initiative (CSI), a $3.3 billion program established in January 2006. The goal is to generate 3 GW of electricity by 2017 through photovoltaic methods by installing solar cells on the roofs of existing and new residential and commercial buildings (see Figure 1). CSI will “reduce our output of greenhouse gases by 3 million tons,” California Governor Arnold Schwarzenegger said in a speech given in October 2006. “That is equivalent to taking one million cars off the road.”

Biomass remains a key energy source for several billion people living in developing countries, and the production of liquid biofuels for transportation is growing rapidly. However, both traditional biomass energy and crop-based biofuels technologies have negative environmental and social impacts. The overall research challenge for bioenergy is to develop the technologies to produce useful products at low costs while minimizing the use of scarce resources such as arable land and water. This requires substantial advancements in modern biomass power generation and the success of liquid biofuel technologies that permit the use of lignocellulosic feedstocks or possibly algae. With such technologies, biomass resources could meet a significant fraction (over 10%) of global energy demand. Both improved policies and technologies are needed to ensure that bioenergy contributes significantly to economic, social, and environmental goals.

Although ethanol is now made from the sugars in the starch fraction of corn and other crops and from the sugar in sugarcane, a much greater impact for ethanol in terms of fuel use could be realized if the sugars from more recalcitrant cellulosic biomass could be converted to ethanol. Cellulosic biomass is the structural portion of plants and includes agricultural (e.g., corn stover, which is all of the above-ground portion of the corn plant, excluding the grain) and forestry (e.g., sawdust) residues, major fractions of municipal solid waste (e.g., waste paper and yard waste), and herbaceous (e.g., switchgrass) and woody (e.g., poplar) crops grown as energy resources. Although distinctive in outward appearance, these materials all comprise about 40–50% cellulose and 20–30% hemicellulose, with lesser amounts of lignin and other compounds such as sugars, oils, and minerals. Cellulose is a polymer of glucose sugar molecules that are physically linked together in a crystalline structure to provide structural support for plants. Hemicellulose is also made up of sugars covalently joined together in long chains, but it generally includes fve different sugars: arabinose, galactose, glucose, mannose, and xylose. In addition, hemicellulose is an amorphous, branched material. Lignin is a phenylpropene compound that can be viewed as a low-sulfur, immature coal.

All oxygen-dependent life depends on photosynthesis. In addition to breathing the oxygen produced by photosynthesis, humans have been harnessing energy from photosynthesis for millennia. Since the beginning of human societal structures, human needs have driven the evolution of agricultural production, and they continue to do so. Recently, it has been suggested that agriculture can contribute substantially to human technological (nonnutritional) energy needs. This possibility raises concern because the projections of human energy needs argue convincingly that without large increases in energy conversion effciency (ECE), land-grown biofuel production and food production will compete for land, a largely untenable compromise given the current nutritional status of the world's underdeveloped societies.

Some forms of renewable energy have long contributed to electricity generation, whereas others are just emerging. For example, large-scale hydropower is a mature technology generating about 16% of global electricity, and many smaller scale systems are also being installed worldwide. Future opportunities to improve the technology are limited but include upgrading of existing plants to gain greater performance efficiencies and reduced maintenance. Geothermal energy, widely used for power generation and direct heat applications, is also mature, but new technologies could improve plant designs, extend their lifetimes, and improve reliability. By contrast, ocean energy is an emerging renewable energy technology. Design, development, and testing of a myriad of devices remain mainly in the research and development stage, with many opportunities for materials science to improve design and performance, reduce costly maintenance procedures, and extend plant operating lifetimes under the harsh marine environment.