Solar Brayton Cycle - Figure A-6 shows a schematic diagram of a typical solar Brayton cycle system. Solar energy is collected by a concentrating reflector and is focused into a cavity absorber. The cycle working fluid, usually an inert gas such as helium or argon, passes through the absorber where it is heated to turbine inlet temperature conditions. The hot gas then expands through a turbine which drives a compressor and generator. The generator produces useful electric power. After passing through the turbine, the gas is further cooled in a recuperator heat exchanger where residual heat then preheats the gas passing into the absorber. The working fluid receives final cooling in a cooler heat exchanger where cycle waste heat is transferred to a coolant fluid for rejection to space via a radiator system. The cooled gas then passes through the compressor where its pressure is raised to the turbine inlet pressure level. Typically, the compressor uses about two-thirds of the turbine output work. The conversion efficiency of a Brayton cycle system ranges from 20 to 35 percent at turbine inlet temperatures in the 1700°F to 2200°F range to greater than 40 percent with turbine inlet temperature in the 2500 to 3000°F range. The higher temperatures require use of more advanced technology ceramic components whereas refractory metal alloys may be used at the lower temperature level. There are several variations of the basic Brayton cycle including gas (working fluid) radiator systems, alternative working fluids, single versus multiple shaft systems, and dual cycle concepts using thermionics concept at the high temperature end of the cycle or a Rankine cycle at the heat rejection end of the cycle. The MSFC in-house study (ref. 3) configured a 10 GW satellite with a concentration ratio of 2000:1, a helium working fluid, and a high temperature thermionic generating loop in combination with a Brayton cycle conversion system. The MSFC-Boeing study (ref. 5) investigated a thermionic/Brayton combined cycle system and a closed cycle Brayton system. The JSC in-house study included examination of the closed cycle Brayton concept and investigated several subsystem alternatives. Following the earlier studies, the JSC-Boeing study (ref. 7) probed deeper into many of the apparent problem areas of the closed system
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