lunar materials in the small subsystems which had not been considered in the General Dynamics study. The total mass of the system is about 8% greater than for the Earth baseline. The best design considered uses silicon photovoltaic cells for power conversion. Its structure is primarily aluminium. Thermal radiators are more than 99% composed of lunar material. A flywheel system is used for energy storage during eclipses. Fig. 1 compares total mass and non-lunar mass of this design, the General Dynamics design, and the Earth baseline design. Fig. 1 compares the estimated transport cost per SPS for each design, based on the assumption that non-lunar material is 50 times more costly than lunar material. The cost units are equivalent to thousands of metric tons of lunar material. An alternative design used gallium arsenide cells with solar concentrators. This roughly triples total SPS mass, but non-lunar mass remains less than 2% of that for the Earth baseline. Four other power conversion systems were investigated: thermophoto- voltaic (TPV), Brayton, Rankine, and Stirling. These were found to use significantly more non-lunar material than either the silicon system or the gallium arsenide system, and will not be discussed here. Solar Power Conversion Systems Both the power conversion designs were sized to produce 9 GW of electricity at the SPS bus. This was asssumed to correspond to 5 GW of usable electricity on the ground, i.e., overall electrical efficiency is assumed to be 55.6%. This assumption is conservative, since earlier studies have achieved higher efficiency. The non-lunar material requirements of both systems are shown in Table 2. Table II. Comparison of power conversion systems.
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