CuInSe2, which has none8), but it is believed that this can be improved upon. Because of its favorable characteristics, and the fact that it is the only thin-film photovoltaic material available on the Moon, amorphous silicon will serve as the basis for this study. If SPS designers wish to substitute a different material for the SPS built of terrestrial materials, the results of this study are still applicable, since the efficiencies are similar. Literature on photovoltaics often contains present-day, near-term future, and long-term future figures on efficiencies. The near term-future figures will be used in this study. For a-Si, the efficiency used is 11.5%718, with a degradation of 4% (Ref. 9, page 164), so that the overall cell efficiency is 0.115 x 0.96 x 100% or 11.04%. The materials for the substrate are Kapton polyimide for an SPS built of terrestrial materials (25|1 thick in the near-term; 7 g thick in the long term) and steel foil for an SPS built of lunar materials (25|1 thick in the near-term; 7.5g thick in the long term)10. The long-term figures were used in this study. For array mass calculations (cells plus substrate), specific powers (watts per kilogram) given in the literature7’10 were based on cell efficiencies of 5, 10, or 15%, so they were rescaled for an efficiency of 11.5%. Support structures for a bicycle wheel-type SPS should consist of lightweight materials, in order to take advantage of the mass savings available from thin-film solar cells and substrates. The materials to be considered are silicon carbide (SiC), for an SPS built of terrestrial materials) and glasses such as basalt fiber, glass ceramic, and fiberglass, for an SPS built of lunar materials)11’16. Densities and tensile strengths of these materials are given in Table 3 (Section 8 of this report). 5. Bicycle Wheel Versus Inflatable Sphere SPS Two different SPS designs were considered, based on Reference 7: bicycle wheel and inflatable sphere. The inflatable sphere has the advantage of needing no support structure, other than a low-pressure gas to inflate it. In addition, it always has an entire hemisphere facing both the sun and the Earth. However, its effective area for both the solar cells and the transmitting antenna array is equal to its cross-sectional area, not its total surface area. Thus, the array is four times more massive than if it were a flat disk. Power from the sun-facing part of the array must be redistributed to the Earth-facing part. The power must be redistributed still further, since the limb of the Earth-facing side will have more transmitting elements per cross-sectional area than will the center, due to the fact that the limb is seen edge on. This may not greatly increase the mass or complicate the design;
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