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Sustainability and Energy PERSPECTIVE Don't Forget Long-Term Fundamental Research in Energy George M. Whitesides" and George W. Crabtree/ Achieving a fundamental understanding of the phenomena that will underpin both global stewardship and future technologies in energy calls for a thoughtful balance between large-scale immediate solutions using existing technology and the fundamental research needed to provide better solutions in the 50-year period. E nimy and limate change are now pre- occupations shared by science. engineer- ing and society. There is a range of views on energy and almost religious levels of advo- cacy for particular technologies. There is also surprisingly broad (although not universal) agree- ment that there is no single solution to the dual problems of meeting future demands for energy and managing the environmental consequences of energy production. Whatever strategy emerges will be a quilt made up of patches representing almost every imaginable technology. The energy problem is often phrasal in terms of developing a strategy that roughly doubles the global production of energy by 2050 (from 13 to about 30 terawatts) (Fig. I) (1-9). The problem of climate change includes two especially important components: (i) understanding the relationship between the climate and the chem- ist*, of the atmosphere and oceans and (ii) predicting the impact on climate of different strategies for energy production. Because at- mospheric C'0, is the dominant greenhouse gas. and because coal is the carbon-rich fossil fuel whose use can most readily be expanded (especially in the rapidly growing economy of China). understanding the linkage between coal and climate is particularly important 16). There is a pervasive sense that "We must do something soon." This urgency mny be justified. but we must also remember that the problems of providing energy and maintaining the environ- ment arc not about to go away, no matter how hard we try using current technologies. In the rush to do something to find technological solutions to global-scale problems—we should not forget that we must ultimately understand than if we are to find the most effective, a:s- tainable solutions. Fundamental research in sci- ence and engineering is important. Understanding phenomena relevant to energy and the environ- ment leads to new technologies and to the ability 'Department of Chemistry and Chemical Biology, Harvard University. Cambridge, IAA 02138. USA, IMaterlals Science Division. Argonne National Laboratory. Argonne, IL 60439, USA. 'Co %horn correspondence should be addressed. E-mail: [email protected] to control the avnomic and environmental out- comes of their applications (7). The cost of large technology demonstra- tion projects is enormous and the time to de- velop them decades, and it is easy to overlook the fundamental research that nourishes them. Today, we have a growing thicket of energy and environmental problems and great enthu- siasm for solving them quickly. In fact. 50 years from now, most of these problems (and more) will still remain unsolved. Developing the best patches we can for the immediate problems is one approach. Understanding the underlying problems is another, and one that is at least as important, much less expensive, and perhaps ultimately time-saving. Energy and climate are problems that will extend over decades or centuries, and the unimaginable technologies of 100 years from now will reit on fundamental research that must start now. What follows is a sketch of nine represent- ative long-tem problems in research that are vital to the development of future technology for energy. We emphasize that this list is per- sonal and idiosyncratic; it tends to emphasize materials. Others might select differently, al- though most lists would probably have areas of broad overlap. The Oxygen Electrode Problem A hydrogen fuel cell operates by extracting eke- trons from i12 and transferring them through an external circuit to 0 2 (to generate I-120). If the H2 is generated electrochemically, the re- verse reactions take place. In either event, the transfer of electrons from H2O to one elec- trode and to 0 2 from another arc slow reac- tions and lower the efficiency of practical fuel cells (considering the free energy of the reac- tions involved). The slow rates of interconversion of 4 e + 02 + 4 W and 2 H,0 exemplify a broader clam of reactions in which a single process requires the transfer of multiple elec- trons. Understanding these reactions and find- ing strategies for circumventing their limitations arc important in developing new, more prac- tical procedures for reactions ranging from the electrochemical production of H2 and the use of 0 2 in fuel cells to the reduction of nitrogen to ammonia. Catalysis by Design Many of the reactions that occur in the pro- duction of energy involve catalysis: the full set used in the processing of crude oil to fucks; all of the biological reactions involved in photo- synthesis, in fixing CO,, and in biodegrada- tion; the hydration of CO, to carbonate ion: the movement of electrons in batteries: the opera- tion of fuel cells: the cleanup of exhaust gas from internal combustion engines: and many others. Given the enormous importance of ca- talysis in the production and storage of energy. in the production of petrochemicals and the ma- tenals derived from them, and in all biological and most geochemical processes it is astonish- ing (and a little disheartening) how little is known of the fundamentals of catalysis: how catalysts operate, how to control them, and es- pecially how to generate new ones. Catalysis by design has periodically been embraced as a grand challenge, and periodically abandoned as too difficult, but nanoseicncc and surface sci- ence offer new approaches to this problem. The fundamental study of catalysis roust be re- animates! across the full spectrum of processes involved in energy and the environment. Transport of Charge and Excitation Pholoexcitation of the semiconductor or dye component of a solar cell creates an exciton: a separated but associated hole and electron (4, 5). To generate current the electron must move to one electrode, and the hole to the otha, before they combine. These processes arc inefficient in materials that might make inexpensive photo- cells: defective. polyclystalline, disordered, or quantum-dot semiconductors (whether inorganic or organic). Understanding them and circum- venting their deficiencies is one key to cost- effective solar cells. Chemistry of CO2 CO2 is a key molecule in global warming (6). in chemical and biological fuel production, and in fuel use. We must know everything possible about its physical and chemical interactions. Important topics include new uses of CO, in large-scale chemistry (where it has the attractive feature that it has negative cast), new chemical reactions of CO2. the movement and reactions of CO2 in the earth, the role of CO2 in determining the behavior of the atmosphere and oceans. and the chemistry and properties of CO2 at high pressures. For decades, there has been little msearch, whether fundamental or exploratory, in this area: it was considered a solved problem. Improving on Photosynthesis The process of uptake and fixation of CO2 in biological photosynthesis is not an optimized 796 9 FEBRUARY 2007 VOL 315 SCIENCE www.sciencemag.org EFTA_R1_01521504 EFTA02444589 SPECIALSECTION IFS iry (144.tPGA Oninthowe Orsects l•Pct, PC at we Omar obt.off•• wnes Ite.3.n.a.1 *.ittl Cr ) .a Commeetsal Ng. 1. The complex system of energy Rows in the United States in 2005 (V. More than half of the energy produced is wasted. Units are in quads; 1 quad = 1015 British thermal units = 1.055 exajoules. [Figure prepared by Lawrence Livermore National Laboratory, University of California, and the U.S. Department of Energy] marvel: It is fairly inefficient thermodynamical- ly. and several key reactions [for example, the key step involving the reaction of CO2 with ribulose diphosphate (with its competing reac- tion with th)) have a surprisingly poor yield. These inefficiencies are an opportunity. Photo- synthesis has immense appeal for the closed. cycle capture of energy from the Sun in forms that arc useful as fuels (4). Thc prospects of re- engineering biological photosynthesis for greater efficiency, maximizing metabolic flux through specific biosynthetic pathways. growing plants in regimes of temperature or salinity where they normally do not flourish, and directly produc- ing fuels such as 112, 0114, or alcohols arc all alluring ones, but ones that will require decades of imaginative research to realize. The prospect of non-biological photosynthesis (where 'pho- tosynthesis" might include No-inspired physical and chemical reactions or more straightfonvard photochemical or photothennal processes that generate fuels or store energy) also warrants new research. Complex Systems Understanding energy and the environment analytically poses a series of problems that we presently have neither the mathematical tools nor the data to solve. Most global systems are "complex" in physicists' definition of the word: They comprise many components, with many degrees of freedom, usually interacting non- linearly. These systems are the natural home of big surprises—often referred to as emergent behavior. Our difficulty in understanding and modeling these systems leads to uncertainties that cloud most discussions of energy and the environment and of the costs and impacts of almost any technology (5). What really is the cost of a kilowatt produced by silicon solar cells? Now important will the burning of coal be to global warming? What, in &tail, are the global sources and sinks for carbon and how do they interact? What can one say about the impact of technologies for generating nuclear power on the potential for proliferation of nu- clear weapons? Development of the theory of complex systems to the point where it gives re- liable results (or at least results whose reliability can be quantified) remains a key enabling ca- pability, and is probably the best way of min- imizing the potential miseries of the law of unintended consequences. The Efficiency of Energy Use Increasing the efficiency of energy conversion and storage is a major opportunity. Many of our standard energy conversion routes are far from their Camot efficiency limits: Electric- ity production with the present mix of fuels is only 37% efficient on average, the typical automobile engine is perhaps 25% efficient, and an incandescent bulb is only 5% efficient for producing visible light. Increasing eflici- ency requires understanding the fundamental phenome- na of existing and alternative energy conversions. Solid- state lighting, for example. can achieve efficiencies of 50% or inure, provided that we understand the mecha- nisms controlling the conva- sion of electronic energy to photons. New understand- ing of mechanisms of fric- tion. wear, and corrosion also provides new strategies for re- ducing lasses. The Chemistry of Small Molecules Thc chemistry of small mol- ecules dominates many as- pects of energy and climate: 112O, 112, O2. CO2. CO (for Fischer-Tropsch chemistry). NO,. O3. NH,.. SO2. O14. ClItOll, NCI, and others arc all vitally important corn- Fitments of these dens:ions. T ere remains a wide range of inlimnation about these molecules and their combi- nations that is needed to un- derstand the complex systems of which they are a part. New Ideas: Separating Wheat from Chaff The spectum of ideas for dealing with prob- lems of energy and global stewardship is not complete. based just on what we now know. We need new ideas, and we need to know which of the current smorgasbord of unex- plored and unproved ideas will work (9). Developing affordable technologies for remov- ing carbon from the atmosphere (for example. by growing biomass and converting it to a stable form of carbon) must be explored now, if they are to be options in the future. Changing the albedo of Earth, stimulating photosynthesis in the oceans by the addition of essential trace ek- malts such as iron, developing new nuclear power cycles, a hydrogen economy, new meth- orb for separating gases (such as CO2 from air) and liquids, mom-temperature superconductivity to carry electric power without loss biological biological II, production, new concepts in batteries, and nuclear fusion all must be explored fundamen- tally and realistically. These problems all require long-tens, patient investment in fundamental research to yield new and validated ideas. These problems arc also. in some cases, sufficiently technical that their importance is most obvious to specialists. The oxygen electrode (as one example) might seem an exotic problem in science, but it is hard to www.sciencemag.org SCIENCE VOL 315 9 FEBRUARY 2007 797 EFTA_R1_01521505 EFTA02444590 Sustainability and Energy believe that a hydrogen economy that used electrolysis to generate H2 and O2 from water. and a fuel cell to convert H2 and Q back to water and electrom, could make a substantial contribution to global energy without a much- improved oxygen electrode. The identification of this problem is not in any sense new: The redox chemistry of oxygen has been a subject of active interest (but limited success) for decades. We simply need new ideas. Another mason to work on these big prob- lems is that they will attract the most talented young people. Over the past 30 years, the Na- tional Institutes of Health has used stable and generous support to recruit and build a very effective community of biomedical scientists. Solving the problems of energy and global stewardship will require the same patient, flexible, and broadly based investment, if society believes that the problems in these area arc sufficiently important to provide a life's work for its most talenteel young people. References and Notes I. President's Council of Adviseas on Science and Technology MASI). the Energy Imperative reehnolo9Y and the Role of Emerging Companies 12006% mwrostp. gowPCASI/pcast.lumt. 2. World Energy Outfooi 1004 (International Energy Agency. Paris. 2004). vnrwriondenergroutIookorgt. 3. Bask Research Needs to Assure o Secure Energy Future. 1 Stringer, L. Nato. Gins loorkihap report. IJS Department of Energy (001) Office of Bask Energy Sciences. 20031. woctsc.doe.godbegrepenstabstracts.htnal6EC. 4. Bask Research Needs for Solar Energy Utilkorion, N. S. Lerns. G. W. Crabtree, Chairs bookshop report, DOE Office of Bask Energy Selerkes, 2005). moise doe. goctecreponstabstracts.hlmlOSEU. 5. Systems and tile-cycle energy Technology Analyses (Nelanal Renewable Energy laboratory!, onwrirel god anahuvton_analysts.hunl. 6. See discussions of global climate science from the National Center for Atmospheric Research, vnew.uut.edur researchAtimate. 7. ). M. Omit R. K. Lester. Meting Technology Wart: Applkohons In Energy and the Environment (Cambridge unw. Press. New Yak, 2004/ 8. N. S. Lewis. D. G. Noses. Pre. Holt Arad. Sci. USA 103. 15729 (2006). 9. M. S. orenethair. I. L. Thomas. Nature 414. 332 (2001). 10.112Edscience.1140362 PERSPECTIVE Toward Cost-Effective Solar Energy Use Nathan 5. Lewis At present, solar energy conversion technologies face cost and scalability hurdles in the technologies required for a complete energy system. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in a cost-effective fashion. New developments in nanotechnology, biotechnology, and the materials and physical sciences may enable step-change approaches to cost-effective, globally scalable systems for solar energy use. m ore energy from sunlight strikes Earth in I hour than all of the energy con- sumed by humans in an entire year. In f ct, the solar energy resource dwarfs all other renewable and fossil-based energy resources combined (I). With increasing attention to- ward carbon-neutral energy production, solar electricity—or photovoltaic (PV) technology—is receiving heightened attention as a potentially widespread approach to sustainable energy pro- duction. The global solar electricity market is currently more than S I 0 billion/year. and the in- dustry is growing at more than 30% per annum (2). However, low-cost, base-loadable. fossil- based electricity has always served as a for- midable cost competitor for electrical power generation. To provide a truly widespread primary energy source, solar energy must be captured, convened, and stored in a cast-effective fashion. Even a solar electricity device that operated at near the theoretical limit of 70% efficiency would not provide the needed technology if it were expensive and if there were no cost-effective mechanism to store and dispatch the converted Beckman Institute and Ko4i Nanosoence Institute, 210 Noyes Laboratory, 127-72, California Institute of Tedinal. ogy, Pasadena, CA 91125. USA E-mail: nslovisapits. caltech.edu solar energy upon demand (3). Hence, a com- plete solar-basal energy system will not only require cost reduction in existing IN manufac- turing methods, but will also requite science and technology breakthroughs to enable, in a conve- nient, scalably manufacturablc tom . the ultralow- cost capture, onwasion, and storage of sunlight. One key step is the capture and conversion of the energy contained in solar photons. Figure I shows the fully amortized cost of elec- tricity as a function of the efficiency and cost of an installed IN module (2, 4). Because the total energy provided by the Sun is fixed over the 30- year lifetime of a PV module, once the energy conversion efficiency of a PV module is estab- lished, the total amount of "product" electricity produced by the module al a representative mid- latitude location is known for the lifetime of the system. The theoretical efficiency limit for even an optimal single band gap solar conversion device is 31%, because photons having energies lower than the absorption threshold of the active PV material are not absorbed, whereas photons having energies much higher than the band gap rapidly release heat to the lattice of the solid and therefore ultimately contain only a useful in- ternal energy equal to that of the band gap (2). Small test cells have demonstrated efficiencies of >20%. with the remaining losses almost en- tirely due to small reflection losses, grid shading losses• and other losses at the 5 to 10% level that any practical system will have to some extent. Shipped PV modules now have efficiencies of IS to 20% in many cases. At such an efficiency, if the cost of a module is —53001m2 (2), and if we take into account the accompanying fixed costs in the so-called "balance of system" (such as the inverter, grid connection, etc., which add a factor of -2 to the total installed system cost), then the sale price of grid-connected PV elec- tricity must be 50.25 to 50.30 per kilowatt-hour (kWh) to recover the initial capital investment and cost of money over the lifetime of the PV installation (2. 4). Qurently, however, utility- scale electrical power generation costs BM much less, with current and new installations costing -S0.03 to 50.05 per kWh (1). Hence. for solar electricity to be cost-competitive with fossil- based electricity at utility scale, improvements in efficiency are helpful, but manufacturing costs must be substantially reduced. In current manufasiuring schemes for Si- based solar cells, the cost of the processed and purified Si is only about 10% of the final cost of the PV module. Some of the Si is lost in cutting up boules into wafers, and other costs arc incurred in polishing the wafers, making the diffused junction in the Si into a photovoltaic device, fabricating the conducting transparent glass, masking and making the electrical con- tacts. sealing the cells, connecting the cells together reliably into a module, and sealing the module for shipment. I knee, in such systems, the energy conversion efficiency is at a premium so as to bate amortize these other fixed costs involved with snaking the final PV module. Improvements in efficiency above the 31% theoretical limit are possible if the constraints that are incorporated into the so-called Shockley- Qucisser theoretical efficiency limit are relaxed (2). For example. if photons having energies greater than the band gap of the absorbing material did not dissipate their excess energy as heat. but instead produced more voltage or 798 9 FEBRUARY 2O07 VOL 315 SCIENCE www.sciencemag.org EFTA_R1_01521506 EFTA02444591