Basic Research Needs for Solar Energy Utilization - Office of ...

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and quantum dots. Quantum dots are especially exciting for their tunable absorption wavelength, their quantum conversion efficiency above 100% through multiple-exciton generation, and their easy fabrication through self-assembly. The diversity of new materials in accessible nanoscale architectures promises to spawn breakthroughs in photovoltaic concepts and performance that could break the 50% efficiency level. Solar thermal conversion is one of the most promising directions of solar energy research. Almost all our existing energy networks rely on heat and heat engines as essential links, from industrial processing of chemicals to electricity production to space heating itself. Most of the required heat is produced by combustion of fossil fuels, but it could be supplied as effectively by solar thermal conversion. Solar concentrators produce temperatures up to 3,000°C, far hotter than conventional furnaces and capable of driving reactions like splitting water or decomposing metal oxides that produce chemical fuel without expensive catalysts. Lower-temperature heat can be used to drive steam engines that produce electricity from sunlight with 30% efficiency, higher than the present generation of commercial photocells. Reduced cost, higher efficiency and longer lifetime for solar space-heating systems would create enormous energy impact. Promising breakthrough directions for solar thermal research are materials that can withstand the high temperatures and corrosive chemical environments in solar furnaces, and materials for highcapacity, slow-release thermal storage. The ability to accumulate heat during the day, possibly storing it in an embedded phase transition, and release it in a controlled manner at night would enable solar thermal applications to operate 24 hours per day. Research in solar energy conversion sits squarely at the intersection of physics, chemistry, biology and nanoscience. Photosynthesis is the most successful solar energy converter on Earth, converting 90 TW of solar energy to sugars and carbohydrates. Through advances in structural biology, genome sequencing, and proteomics, we are learning the molecular pathways of photosynthesis, and beginning to reproduce some of its simpler functions in artificial selfassembled structures in the laboratory. The insights generated by these fascinating studies will enable us to produce faster-growing energy crops; adapt the cheap, efficient catalysts of enzymes to synthetic solar conversion schemes; and create artificial molecular machines that turn sunlight, water, and CO2 into chemical fuel. Photovoltaic systems are turning to organic semiconductors and metal oxide-organic dye composites for cheaper high performance materials. Organic molecules display a host of novel photo-excitation and electron transfer phenomena that open promising and still-uncharted new horizons for photovoltaic conversion. Biology provides energy conversion paradigms that are qualitatively different from those of hard materials. The biological concepts of self-repair and defect-tolerant networks give plants lifetimes of 30 years or more. Their application to artificial photovoltaic and solar thermal conversion systems promises to dramatically extend their useful lives. The physical, chemical, and biological pathways of solar energy research meet in nanoscience. Top-down and bottom-up fabrication of complex nanoscale architectures enables assembling and interconnecting functional molecular units for solar energy capture, conversion, and storage with precision and reliability never before attainable. Scanning probe microscopies on the benchtop, electron microscopy with atomic resolution, and in situ experiments at high-intensity X-ray and neutron sources can now reveal the structure and dynamics of solar conversion at the molecular level. Sophisticated quantum chemistry simulations using density functional theory can now predict the electronic and structural behavior of multithousand atom clusters like those that 182
mediate solar conversion. These emerging tools of nanoscale fabrication, characterization, and simulation bring fundamental understanding and control of the molecular mechanisms of solar energy conversion within reach. The cross-fertilization of nanoscale biology, chemistry, and physics is a new feature that creates fascinating interdisciplinary routes to solar conversion. The strong need for doubling our energy output in the next half-century, the singular capacity of the sun as a source of clean, abundant energy, and rapid advances in the physics, chemistry and biology of solar conversion at the nanoscale are compelling motivation for a coordinated basic research program on solar energy utilization. 183
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On the Cover: One route to harvesti
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Revisions, September 2005 p ix, par
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CONTENTS (CONT.) Appendix 4: Additi
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OEC oxygen-evolving complex PCET pr
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viii
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thin films, organic semiconductors,
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genetic sequencing, protein product
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INTRODUCTION The supply and demand
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showing promise to overcome them. T
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GLOBAL ENERGY RESOURCES 7
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energy can be exploited on the need
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BASIC RESEARCH CHALLENGES FOR SOLAR
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CONVERSION OF SUNLIGHT INTO ELECTRI
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PHYSICS OF PHOTOVOLTAIC CELLS Inorg
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Figure 4 Learning curve for PV prod
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Needs of Direct-gap, Thin-film Phot
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Figure 6 Current record efficiencie
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devices. The molecules and material
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Figure 8 Structure for high-efficie
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One critical property of the photoa
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PHOTOELECTROCHEMICAL STORAGE CELLS
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BASIC RESEARCH CHALLENGES FOR SOLAR
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information about the proteins that
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convert it into H2 and liquid fuels
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charge separation ensures highly ef
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Efficient Photo-initiated Charge Se
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artificial RCs, have proven to be i
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CATALYSTS FOR CO2 REDUCTION Photo-d
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Upon photoexcitation, the electron
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(5) determining the physical and ch
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following challenges must be met: (
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A.F. Heyduk and D.G. Nocera, “Hyd
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J. Seth, V. Palaniappan, R.W. Wagne
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THREE TYPES OF CONCENTRATED SOLAR
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Solar Thermal to Electric Energy Co
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Tm = mean temperature Z = measure o
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control and further diode developme
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An overview of solar thermochemical
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for electron and phonon band struct
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coefficient. The thermal conductivi
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J.J. Greffet, R. Carminati, K. Joul
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CROSS-CUTTING RESEARCH CHALLENGES B
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ange of time and length scales span
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Research Issues Advances in the syn
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Advances across several science fro
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Figure 19 Transparent conductive el
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Carrier generation, relaxation, and
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E. Hutter and J.H. Fendler, “Expl
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PRIORITY RESEARCH DIRECTIONS Revolu
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RESEARCH DIRECTIONS Several paths e
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Multiple Energy Level Solar Cells I
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Strain Relaxation. Growth of layers
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the relevant elastic and inelastic
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A.J. Nozik, “Quantum Dot Solar Ce
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these new organic structures, and t
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Figure 30 Schematic diagram (right)
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carrier injection between the indiv
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108
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Figure 32 One important example of
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interface for charge separation. Fo
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• Light harvesting, • Advanced
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116
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Multicomponent structures are often
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REFERENCES A.J. Bard and M.A. Fox,
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PLANT PRODUCTIVITY AND BIOFUEL PROD
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een made in identifying and charact
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126
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SUMMARY OF RESEARCH DIRECTION The k
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developing multi-electron catalytic
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Page 150 and 151: low toxicity, and processibility. T
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Page 154 and 155: largely unknown are the fundamental
Page 156 and 157: nanostructure design, and developme
Page 158 and 159: ealize an efficient artificial phot
Page 160 and 161: Figure 46 Defect-tolerant solar cel
Page 162 and 163: equired 20-30 years of operation to
Page 164 and 165: H2SO4 at 1,130K, and the University
Page 166 and 167: the sequestration step, these solar
Page 168 and 169: P. von Zedtwitz and A. Steinfeld,
Page 170 and 171: The continued advances in energy- a
Page 172 and 173: for the detailed understanding and
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Page 176 and 177: A shortcoming of this approach is t
Page 178 and 179: of ~2.4 at 300K and quantum-dot PbT
Page 180 and 181: High-throughput Experimental Screen
Page 182 and 183: Solar Concentrators and Hot Water H
Page 184 and 185: G. Chen, M.S. Dresselhaus, J.-P. Fl
Page 186 and 187: Materials and Processing Methods to
Page 188 and 189: materials and fabrication approache
Page 190 and 191: multi-electron H2O and CO2 activati
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Page 202 and 203: Flux (mA/eV.cm2 Flux (mA/eV.cm ) 2
Page 204 and 205: Table 1 Worldwide PV Module Product
Page 206 and 207: Amorphous Silicon. From its discove
Page 208 and 209: Table 2 Are There Enough Materials
Page 210 and 211: passivating window and back-surface
Page 212 and 213: R.R. King, C.M. Fetzer, K.M. Edmond
Page 214 and 215: currently accounting for only a sma
Page 216 and 217: Anaerobic Digestion One gasificatio
Page 218 and 219: Electrolysis is the process for bre
Page 220 and 221: alkaline electrolyzers require a mi
Page 222 and 223: maintenance costs of the small prod
Page 224 and 225: P. Courty, P. Chaumette, and C. Rai
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Page 228 and 229: periods and extend the system opera
Page 230 and 231: When ηopt < 1, Equation (2) become
Page 232 and 233: Receiver & Power Conversion Unit So
Page 234 and 235: Figure 77 General concept of solar-
Page 236 and 237: Other areas whose development could
Page 238 and 239: THERMOPHOTOVOLTAICS Thermophotovolt
Page 240 and 241: N. Benz and Th. Kuckelkorn, “A Ne
Page 242 and 243: R.A. Sherif, H.L. Cotal, R.R. King,
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Sub-panelists Edmond Amouyal, Centr
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Terry Tritt, Clemson University Joh
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Plenary Closing Session — Wednesd
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Panel 1C (Photoelectrochemistry)
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Agenda for Solar Fuels Breakout Ses
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Tuesday, April 19, 2005, 8:00 a.m.
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Wednesday, April 20, 2005, 8:00 a.m
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Wednesday, April 20, 2005, 8:00 a.m
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250
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R. Corkish, S. Kettemann, and J. Ne
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T. Polivka and V. Sundstrom, “Ult
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256
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258
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microscopy, 81, 85, 101, 155-157, 1
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