2.3 THERMOPHOTOVOLTAIC (TPV) CONVERSION Photovoltaic cells do not efficiently use the solar spectrum, which resembles the spectrum of a 6000K blackbody. Thermo-photovoltaic energy conversion refers to an energy conversion system which uses concentrated sunlight to maintain a radiator at about 2200K. Silicon solar cells can convert the lower temperature spectrum to electricity at a theoretical efficiency of up to 75%. Experimental efficiencies of 40% have been achieved.(4) A schematic diagram of the TPV process is shown in Fig. 2.3-1< FIGURE 2.3-1, THERMOPHOTOVOLTAIC CONVERSION PROCESS 2.3.1 TPV Design Summary Table 2.3—1 summarizes a design for a 5 GW SPS which requires 9 GW at the spacecraft bus. The design was developed usi~g a computer model whose parameters were optimized for minimal non—lunar mass. Structural mass for the radiator cavity is not included, but is assumed to be small compared to the concentrator mass. The maximum radiator temperature is limited to 2300K by the evaporation rate of tungsten.(5) The minimum non-lunar mass occurs at a cell temperature of 405 K, for an overall conversion efficiency of 20.5% from intercepted sunlight to electrical power.
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