Table 3 presents a design summary for a silicon planar power conversion system which delivers 9 GW to the spacecraft bus. The mass summary includes the catenary tensioning system but not the primary supporting structure. Table III. Mass analysis of 9 GW silicon planar system. The silicon cells used by SRA were assumed to operate with the same base efficiency as those used in the Earth baseline design. The loss factors were also assumed to be the same, except for the summer solstice cosine loss. Gallium Arsenide Concentrator The gallium arsenide (GaAs) concentrator concept uses GaAs cells to produce electricity from concentrated sunlight. Neither gallium nor arsenic is abundant on the Moon, but GaAs cells can be used with highly concentrated sunlight. Thus, the mass of the cells - and hence the mass of non-lunar material - need be only a small fraction of the SPS mass. Like silicon, GaAs cells must be protected from radiation or must be periodically annealed. GaAs is more likely to be annealable than silicon, but questions remain as to whether the process is workable [5], An advantage of the GaAs concentrator concept is that its mass is not greatly affected by whether GaAs cells can be repeatedly annealed. Two factors account for this: the high radiation resistance of GaAs (roughly three times that of silicon) and the relatively small size of the cells. Thus, very thick cover glass could be added without significantly affecting the mass (total or non-lunar) of the SPS. It was assumed by SRA that GaAs could be annealed. In the SSI/SRA design, each GaAs cell has a Cassegrain optical system with a concentration ratio of 260. Aluminium primary and secondary mirrors focus sunlight onto the cell, which is fixed to the centre of the primary mirror. The thin layer of insulator/adhesive between the cell and the primary reflector was assumed to be non- lunar. A small compound parabolic concentrator (CPC) around the photovoltaic cell serves as a tertiary reflector to compensate for scattering losses (see Fig. 3). Four aluminium legs connect the secondary to the primary reflector. Heat from the cell is radiated from the anodized back surface of the primary, which
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