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

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een made in identifying and characterizing the various enzymes involved in microbial digestion of lignocellulose. However, in many cases, the enzymes or enzyme complexes found in nature are not well-suited to industrial-scale processes for conversion of lignocellulose to fermentable sugars and other useful chemicals. Progress in enzyme chemistry, structural biology, and computational chemistry have created exciting new opportunities to greatly improve the properties of enzymes for lignocellulose conversion. Additional investments in understanding the structure and function of polysaccharide and lignin hydrolyases will create significant improvements in the overall efficiency of lignocellulose conversion to liquid fuels. Additional research is also required to improve the efficiency with which sugars other than glucose are bioconverted to useful chemicals. Downstream processes for fermentation of cellulose degradation products are extremely important for biofuel production, and more work on maximizing these microbial processes is extremely important. MICROBES AND SOLAR BIOFUELS Globally, biological processes produce more than 250 metric tonnes of hydrogen per year. However, because other organisms in the biosphere rapidly use most of the metabolically produced hydrogen, this gas is not released into the atmosphere and the phenomenon of biological hydrogen evolution is not widely recognized. Algae and cyanobacteria employ the same basic photosynthetic processes found in green plants. They capture sunlight and use the energy to split water, release oxygen, and fix atmospheric carbon dioxide. All these microbes can adapt their normal photosynthetic processes to produce hydrogen directly from water using sunlight and the Ech hydrogenase H2 F420-non-reducing hydrogenase H2 2 H + H + H2 F420H2 dehydrogenase H + Na + Mphen Mphen Heterodisulfide reductase Fdred CO2 Formyl-methanofuran F420H2 HS-CoB + HS-CoM Formyl-H4MPT Methenyl-H4MPT Methylene-H4MPT F420H2 F420H2 CoM-S-S-CoB Methyl-H4MPT Methanogenesis Methyl-S-CoM CH4 F420H2 HS-CoB Acetyl- CoA Acetate Acetylphosphate Methanol Trimethylamine Dimethylamine Monomethylamine Figure 38 Mechanism of methanogenesis as currently formulated 124 Figure 37 Anaerobic phototrophs also produce hydrogen and are very active nitrogen fixers. enzymes hydrogenase or nitrogenase. In anaerobic photosynthetic bacteria (see Figure 37), there are several enzymes that catalyze hydrogen metabolism and evolution, including reversible hydrogenases and the nitrogenase complex. In addition, some of these organisms can even couple the degradation of toxic halogenated compounds and lignin monomers to hydrogen production. While these capabilities have been known as laboratory curiosities for many years, only recently as the result of a number of advances in basic physiology, enzymology, protein structure, and molecular biology has the prospect of using these unique metabolisms as the basis for new energyproduction technology become a possibility. The emerging tools and modern plant biology hold promise that significant amounts of global energy will be supplied by algal farms that access desert and coastal areas. In addition to hydrogen production, biological methane production (see Figure 38), is a well-
established biotechnology used worldwide, both to reduce waste biomass and to generate biogas fuel. Under anaerobic conditions, complex and largely undefined consortia of microorganisms depolymerize biopolymers and transfer the solar energy trapped by photosynthesis in these molecules into intermediate fatty acids, alcohols and methylamines. In the terminal reactions, methanogenic Archaea use hydrogen from fermentations to reduce carbon dioxide and methyl groups from the intermediates to generate methane. An integrated effort is needed to expand the application of anaerobes to a wider range of biomass and to construct microbial consortia that most efficiently convert the biomass to natural gas. RELEVANCE AND POTENTIAL IMPACT Modification of the biochemistry of plants and bacteria, either genetically or through breeding, along with an understanding of the mechanisms by which natural systems produce fuel, are needed to improve the efficiency of such systems by a factor of 5–10 and to provide a convenient fuel for end-use. The research directions identified here build upon advances in modern biology by the broader biological research community that can be directed toward substantial improvements in solar biofuels production. The new capabilities in computational chemistry, structural biology, molecular machines, and nanotechnology that have become available only recently will allow these ambitious goals to be reached. REFERENCE C. Somerville, S. Bauer, G. Brininstool, M. Facette, T. Hamann, J. Milne, E. Osborne, A. Paredez, S. Persson, T. Raab, S. Vorwerk, and H. Youngs, “Toward a Systems Approach to Understanding Plant Cell Walls,” Science 306, 2206–2211 (2004). 125
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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
Page 89 and 90: CROSS-CUTTING RESEARCH CHALLENGES B
Page 91 and 92: ange of time and length scales span
Page 93 and 94: Research Issues Advances in the syn
Page 95 and 96: Advances across several science fro
Page 97 and 98: Figure 19 Transparent conductive el
Page 99 and 100: Carrier generation, relaxation, and
Page 101 and 102: E. Hutter and J.H. Fendler, “Expl
Page 103: PRIORITY RESEARCH DIRECTIONS Revolu
Page 106 and 107: RESEARCH DIRECTIONS Several paths e
Page 108 and 109: Multiple Energy Level Solar Cells I
Page 110 and 111: Strain Relaxation. Growth of layers
Page 112 and 113: the relevant elastic and inelastic
Page 114 and 115: A.J. Nozik, “Quantum Dot Solar Ce
Page 116 and 117: these new organic structures, and t
Page 118 and 119: Figure 30 Schematic diagram (right)
Page 120 and 121: carrier injection between the indiv
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Page 124 and 125: Figure 32 One important example of
Page 126 and 127: interface for charge separation. Fo
Page 128 and 129: • Light harvesting, • Advanced
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Page 132 and 133: Multicomponent structures are often
Page 134 and 135: REFERENCES A.J. Bard and M.A. Fox,
Page 136 and 137: PLANT PRODUCTIVITY AND BIOFUEL PROD
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Page 142 and 143: SUMMARY OF RESEARCH DIRECTION The k
Page 144 and 145: developing multi-electron catalytic
Page 146 and 147: coupling in multi-cofactor-containi
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Page 150 and 151: low toxicity, and processibility. T
Page 152 and 153: 8 electrons) are extremely limited.
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
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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
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materials and fabrication approache
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multi-electron H2O and CO2 activati
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178
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180
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and quantum dots. Quantum dots are
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184
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Flux (mA/eV.cm2 Flux (mA/eV.cm ) 2
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Table 1 Worldwide PV Module Product
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Amorphous Silicon. From its discove
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Table 2 Are There Enough Materials
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passivating window and back-surface
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R.R. King, C.M. Fetzer, K.M. Edmond
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currently accounting for only a sma
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Anaerobic Digestion One gasificatio
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Electrolysis is the process for bre
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alkaline electrolyzers require a mi
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maintenance costs of the small prod
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P. Courty, P. Chaumette, and C. Rai
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212
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periods and extend the system opera
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When ηopt < 1, Equation (2) become
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Receiver & Power Conversion Unit So
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Figure 77 General concept of solar-
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Other areas whose development could
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THERMOPHOTOVOLTAICS Thermophotovolt
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N. Benz and Th. Kuckelkorn, “A Ne
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R.A. Sherif, H.L. Cotal, R.R. King,
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230
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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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microscopy, 81, 85, 101, 155-157, 1
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