Brayton cycle design. Reflector degradation was reevaluated resulting in size modifications. The configuration was altered to conform to new construction concepts and geometric improvements. These refinements and more detailed investigation of the construction techniques and materials requirements resulted in determining better cost and mass estimates in the study. This resulted in the determination that the system not only was significantly heavier than photovoltaic and Rankine cycle systems (due primarily to large radiator requirements) but was also more complex to construct. The MSFC-Rockwel1 concept definition study comparatively analyzed several solar thermal concepts including the closed Brayton cycle and found that, although the technology for the Brayton system was much further advanced than the others, the weight penalties made it less competitive than less developed, but higher performance cascaded Rankine systems. Solar Rankine Cycle - Like the Brayton cycle, the Rankine cycle utilizes a solar concentrator to collect and focus energy into cavity absorbers (figure A-7); however, instead of an inert gas, a liquid working fluid such as water or potassium is passed into the absorber where it is vaporized (boiled). The hot vapor is then expanded through a turbine which drives a liquid pump (not a compressor as in the Brayton cycle) and a generator which produces electricity. After passing through the turbine the vapor is cooled (condensed to a liquid) either directly in a space radiator or indirectly by an intermediate coolant loop. The liquid then passes through the pump which raises the liquid pressure to boiler inlet conditions. As with the Brayton system, there are several variations of the basic cycle. Typical Rankine cycle conversion efficiencies are 15 to 40 percent depending upon cycle arrangement. The JSC in-house study (ref. 2) reported on the potential of the solar Rankine cycle to SPS applications. Although the technology was not as advanced as some other thermal systems, the Rankine cycle utilizes higher heat rejection temperatures which results in lower radiator mass. Several working fluids were investigated in the JSC study, but the overall analysis was only a preliminary investigation and no cost and mass estimates were reported. Later, the JSC-Boeing study (ref. 7) developed sufficient detail for comparative analysis
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