1.1.1.2 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. GaAs cells are more resistant to radiation damage than silicon, but still require periodic annealing or thick cover glass. The design used here has a Cassegrain optical system with a concentration ratio of 260. Al uni num primary and secondary mirrors focus sunlight onto a small GaAs cell which is fixed to the center of the primary mirror. Excess heat from the cell is radiated from the back surface of the primary, which constrains the design to use many small units. The cell operates at 200 C, producing 4.04 watts per unit. As shown in Figures 1.1-3 and 1.1-4, this design is second only to silicon planar in minimizing non-lunar materials. Further optimization of the design could probably reduce the GaAs system's total mass by a factor of at least two. 1.1.1.3 Thermophotovoltaic (TPV) A TPV converter moves the spectral peak of concentrated sunlight from the visible region to the infrared, which is more efficiently converted to electricity by photovoltaic cells. The cells require active cooling. A parametric model of a TPV converter using silicon photovoltaic cells was developed. The result of optimizing this model for minimal non-lunar mass is shown in Figures 1.1-3 and 1.1-4. The high mass of the TPV system is due to active cooling of the silicon cells. The cooling system also accounts for more than 90% of the non-lunar mass. An advantage of TPV conversion over other photovoltaic systems is that the cells are enclosed within a substantial structure which protects them from harmful radiation, so no annealing is required. TPV technology is advancing rapidly, so it should be considered in future SPS studies. TPV would be especially appealing if a cooling system with very little non-lunar mass were developed. 1.1.1.4 Brayton Cycle High temperatures in the Brayton design yield high efficiency and low total mass but make lunar material substitution difficult due to the need for advanced materials. The degree of lunar material substitution possible was conservatively pstimated for two different temperature cvcles. The hot cvcle has a turbine inlet temperature of 1617 K; the cold cycle, 1100 K. Both use helium as the working fluid. Estimates of total specific mass and non-lunar specific mass for the two temperature ranges are shown in Figures 1.1-3 and 1.1-4. The low temperature cycle is found to use less non-lunar material, but the increase in total mass is forty times larger than the decrease in non-lunar mass. Since neither this figure nor the fifty-to—one cost advantage of
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