Solar Photovoltaics - Since the SPS concept was first postulated, the passive system advantages of photovoltaics has provided a standard of comparison for other solar col 1ection/conversion systems. The MSFC in-house study (ref. 3) looked at numerous photovoltaic systems to select a solar cell technology suitable for the SPS. The design utilized silicon solar cells although their studies indicated some other compositions showed promise. The MSFC-Boeing (ref. 5) study modeled a silicon photovoltaic and gallium arsenide photovoltaic SPS concept to determine total power generation costs. Although most of the component definitions were extremely preliminary, cost estimates allowed reasonable trade studies. The JSC in-house study (ref. 2) emphasis was on the silicon photovoltaic SPS. The JSC-Boeing study (ref. 7,8,9) included evaluations of several photovoltaic options including single crystal silicon, single crystal gallium arsenide and thin film options and some other less developed thin film approaches such as copper indium selenide. The MSFC-Rockwel1 study (ref. 11) included silicon and gallium-aluminum-arsenide solar cell evaluations and several concentration ratios. The data base at the beginning of the study was evaluated to determine which SPS approaches should be seriously considered as candidates for further analysis. In evaluating the various photovoltaics options, a number of factors have been considered including performance (efficiency), mass, materials availability, susceptibility to radiation damage (performance degradation), development status and cost. In addition to the system definition efforts, surveys have been made (ref. 18) to assess materials availability, manufacturing processes requirements, and energy payback of several candidate solar cell designs. This work included an assessment of SPS solar cell requirements with respect to DOE's U.S. Photovoltaic Conversion Program. In comparing the various photovoltaic options, the single crystal silicon cell and the gallium-aluminum-arsenide cells have emerged as the most promising for SPS application. Other solar cells that have been considered include amorphous silicon, polycrystalline silicon, cadmium sulfide, copper indium selenide and polycrystalline gallium arsenide. These cell types generally have the potential advantage of lower costs; however, at present, the performance (efficiency) is low and mass production methods have not been devised.
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