2.4.3 Brayton Cycle Technical Discussion 2.4.3.1 Radiator In the high temperature Brayton cycle, 12.2 GW of waste heat must be rejected into space to produce 9 GW of electricity. The inlet and outlet temperatures are 644 K and 377 K respectively. The low temperature cycle must reject 16.9 GW of heat, with high and low temperatures of 550 K and 322 K. .One of the 16 modules of the 10 GW Brayton SPS configuration is shown in Figure 2.4-3. The cavity absorber with the engines is in the center. The concentrator is below with its concentrated sunlight entering the aperture. Radiator panels surround the absorber. The basic element of the radiator panels is the heat pipe. For the range of temperatures used here, a water heat pipe is a reasonable choice. This heat pipe can be made with thin stainless steel and an internal stainless steel wick. Figures 2.4-4 and 2.4-5 show more details of the HPR configuration and its panels. Stainless steel is the primary material used in this radiator system. Water is used as the working fluid, which is estimated to have about 0.2Z of the total panel mass. Hydrogen in the water and carbon in the steel (0.3Z) are assumed to be the only non-lunar materials. The mass breakdown for this radiator for the 9 GW high temperature Brayton cycle is shown in Table 2.4-3. TABLE 2*4-3 RADIATOR MASS FOR 9 GW BRATTON CYCLE 2*4.3.2 Concentrator An all-lunar aluminum concentrator like that described in section 2.7 is assumed. The mass of the concentrator for both temperature cycles is shown in Table.2.4-4, TABLE 2.4-4 CONCENTRATOR MASS FOR 9 GW BRAYTON CYCLE
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