EVALUATION OF SOLAR CELL MATERIALS FOR A SOLAR POWER SATELLITE1 Dr. Peter E. Glaser - Dr. David W. Almgren - Ms. Katinka I. Csigi Arthur D. Little, Inc. - Cambridge, Massachusetts As originally conceived, a solar power satellite (SPS) can utilize current approaches to solar energy conversion, e.g., photovoltaic, thermal electric, and others, which may be developed in the future. Among these conversion processes, photovoltaic conversion represents a useful starting point because solar cells already are in wide use in satellites. An added incentive is the substantial progress being made in the development of low-cost reliable photovoltaic systems for terrestrial applications, the increasing confidence in the capabilities to achieve the required production volumes to sustain expanded markets. Figure 1 projects the cost reduction of silicon solar cells and the necessary transition to thin-film solar cells which will have to be developed to meet projected terrestrial photovoltaic system production and cost goals. Several photovoltaic energy conversion systems applicable to the SPS concept have been evaluated. Two alternative photovoltaic systems—one employing single-crystal silicon, and the other, gallium arsenide—have been selected for the SPS reference system. The silicon photovoltaic system utilized 50 pm thick, single-crystal, silicon solar cells sandwiched between layers of 75 pm and 50 pm thick borosilicate glass, while the alternative design utilizes 5 pm thick, single-crystal heteroface gallium arsenide solar cells formed by a chemical vapor-deposition process on a 20 pm layer of sapphire as the substrate/cover glass for the solar cell. The array is then encapsulated with 13 pm of Teflon bonding the solar cell to a 25 pm Kapton cover. The gallium arsenide solar cells are integrated with thin-film Kapton solar reflectors with a concentration ratio of 2 to reduce the required solar cell area. The mass of the photovoltaic systems represents 40% of the mass of the SPS reference system. The beginning of life (BOL) and end of life (EOL) conversion efficiencies of the SPS solar cells has a significant impact on the total size of an SPS and the total mass that has to be carried to geosynchronous orbit. The SPS system studies have been primarily concerned with such photovoltaic factors as: • Beginning of life solar cell efficiency;
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