2. SOLAR POWER CONVERSION SYSTEMS Solar power conversion systems include transducers which convert sunlight to electrical Energy, cooling systems to dissipate waste heat from the conversion process, and optical systems which filter or concentrate sunlight. Several power conversion concepts considered in this study require large concentrating reflectors and large active cooling systems. These subsystems are discussed in sections 2.7 and 2.8. Primary structure is discussed in chapter 3; it is not included in the mass estimates for any of the conversion systems. Most of the power conversion designs discussed in this chapter are sized to produce 9 GW of electricity at the SPS bus. This is assumed to correspond to 5 GW of usable electricity on the ground, i.e., overall electrical efficiency is assumed to be 55.62. This assumption is quite conservative, since earlier studies have achieved higher efficiency. Two categories of solar power converters were considered in this study: photovoltaic (PV) systems and heat engines. A third category, thermionic conversion, was not considered because of its low efficiency, high mass, and large non-lunar material requirements. Photovoltaic systems convert light energy directly to electrical energy. In all photovoltaic systems considered in this study, electricity is produced when photons of light form electron-hole pairs in a semiconductor material. The photovoltaic systems considered here are silicon planar, gallium arsenide concentrator, and thermophotovoltaic (TPV). Silicon planar was found to require least non-lunar material of all conversion systems considered, as shown in Table 2-1. TABLE 2-1 PERFORMANCE OF POWER CONVERSION SYSTEMS System Max. Temp. Z Eff. Total kg/kW Non-lunar kg/kW Si Planar N/A 14.34 2.22 0.0000001 GaAs Cone. N/A 9.5 6.93 0.0104 TPV N/A 20.5 13.0 0.08 Brayton 1617 X 43 3.01 0.43 Brayton 1100 X 35 10.77 0.234 Rankine, steam 1644 X 42 6.80 1.23 Rankine, potass. 1600 X 31 3.14 1.47 Stirling 720 X 25 34.0 0.66 Photovoltaic systems require less non-lunar material than heat engine systems and are simple to design and build. The semiconductor cells must be protected from radiation or periodically annealed to repair radiation damage- The study team assumed that repeated annealing is effective, but was unable to determine whether this is the case. This matter is discussed further in sections 2.1 and 2.2.
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