Fig. 5-24. Compound Parabolic Concentrators Figure 5-25 shows the GaAs SPS configuration; it begins geosynchronous operation with 16 modules present. Two additional modules are added (after 12 years) to bring the system to its total of 18. The ground output is 10 GW or more throughout the 30 year life. The GaAs SPS end of life area is 113.2 km^ (43.69 miles^) compared to 201.2 km^) for the silicon SPS. 5.8 NUCLEAR THERMIONIC (CONCEPT 7) Nuclear reactors are discussed in Section 4- . The nuclear thermionic SPS configuration investigated was the molten salt breeder reactor (MSBR). Thermionics are discussed in Section 4-6. Probable maximum effective power densities for thermionic diodes of 10 w/cm^ cause a 17.5 GW orbit busbar SPS (10 GW ground output) to require 175,000 m^ (1.88x10° ft^) of active diode area. The baselined reactor quantity for a 10 GW SPS was 16; their total thermal power would be approximately 40 GW. The surface area of all of these reactors (diameter of 4 meters each) is only 804 (8654 ft^). Thus the reactor walls do not provide sufficient area for mounting of the diodes. Therefore a sensible heat sodium loop is provided to heat the diode emitters, which located separately from the reactors. The molten salt flow from the reactors of course contains sufficient thermal energy but has insufficient volume to heat the required 175,000 m^. Thus a molten salt to sodium heat exchanger is required, as shown in Figure 5-26. Fig. 5-26. MSBR Nuclear Thermionic System Fig. 5-25. GaAs SPS Configuration As with the silicon SPS, minimum concentrator thickness was assumed to be 75 micrometers (0.003 inch); the concentrators are the most massive element of the GaAs SPS, but permit the required power to be produced by the SPS program while procuring only approximately one- -fifth of the cells that would otherwise be required. With the 80% production rate factor (learning curve) assumed for cell costs, this reduction in cell area is extremely significant. Table 5-6 gives the total GaAs mass statement, including the beginning of life (BOL) condition masses and the mass of the two add-on modules. Table 5-6. GaAs SPS Mass Statement
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