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EFTA02444589 | EFTA02444591 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 muchimproved 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 problems is that they will attract the most talented young people. Over the past 30 years, the National 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. 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. ore energy from sunlight strikes Earth in I hour than all of the energy consumed 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 toward carbon-neutral energy production, solar electricity—or photovoltaic (PV) technology—is receiving heightened attention as a potentially widespread approach to sustainable energy production. The global solar electricity market is currently more than S I 0 billion/year. and the industry is growing at more than 30% per annum (2). However, low-cost, base-loadable. fossilbased electricity has always served as a formidable 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 of70% 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 m 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 798 solar energy upon demand (3). Hence, a complete solar-basal energy system will not only require cost reduction in existing IN manufacturing methods, but will also requite science and technology breakthroughs to enable, in a convenient, scalably manufacturablc tom . the ultralowcost 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 electricity 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 30year lifetime of a PV module, once the energy conversion efficiency of a PV module is established, the total amount of "product" electricity produced by the module al a representative midlatitude 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 internal energy equal to that of the band gap (2). Small test cells have demonstrated efficiencies of >20%. with the remaining losses almost en- 9 FEBRUARY 2O07 VOL 315 SCIENCE 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 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 ofa 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 electricity 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, utilityscale 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 fossilbased electricity at utility scale, improvements in efficiency are helpful, but manufacturing costs must be substantially reduced. In current manufasiuring schemes for Sibased 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 contacts. 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 ShockleyQucisser 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 www.sciencemag.org EFTA_R1_01521506 EFTA02444591
April 9, 2026 at 7:22 PM