2.2 GALLIUM ARSENIDE CONCENTRATOR SYSTEM 2.2.1 Introduction The gallium arsenide (GaAs) concentrator option is based on replacing the bulk of the solar cells used in a planar configuration with an aluminum concentrator. Such a system would typically have a concentration ratio in the 100 - 1000 range and is therefore suitable to a strategy of minimizing the use of non-lunar materials. Although GaAs is not available on the Moon, its ability to operate efficiently at high concentration ratios and high temperatures allows a system to be designed which employs minimal amounts of non-lunar mass (less than one percent). 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.(18) 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 here that GaAs could be annealed. Three major areas were considered in this analysis: the concentrator, the cooling system and the optimization of concentrator dimensions. These areas are addressed in the technical discussion (section 2.2.4), along with supporting comments on the cell itself, the structure and manufacturing. 2.2.2 Design Description The concentrator system used in this study consists of a parabolic dish for the primary reflector and a hyperbolic mirror for the secondary reflector. This makes up a typical Cassegrainian concentrator. In addition a small Compound Parabolic Concentrator (CPC) has been added around the photovoltaic cell as a tertiary reflector. This, along with an oversized secondary reflector, is designed to compensate for imperfections in the mirrors which cause dispersion in the reflected light. (See Fig. 2.2-1) The secondary reflector is located at a point where the radius of the image striking it is the same as that of the photovoltaic cell. This point is very near the focal point of the primary reflector. The two reflectors are connected by four aluminum legs. The Ga’s photovoltaic cell is attached directly to the primary reflector which serves as a radiator. The Design Summary allows for a thin layer of possibly non-lunar material between the cell and the primary reflector to serve as an insulator and/or adhesive. All three reflectors are made of aluminum and have a coating of vapor deposited aluminum to enhance reflectivity. In addition the back surface of the primary reflector has been anodized to increase the emissivity. This
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