15 minutes of an orbit. It must be capable of automated startup and shutdown operations and accommodate all the electric power system functions on Freedom's manned base. These include initiating service, maintenance operations, corrective actions following failures or accidents, contingency operations, and safe haven operations. Throughout the life of Freedom, insolation will vary over a range of 1.33 kW per square metre to 1.42 kW per square metre. The SD module must be capable of accepting this varying insolation and delivering the required power at all times. In addition to the operating performance requirements outlined above, the SD hardware must be designed so that two complete modules can be delivered to orbit in one Shuttle launch. Finally, the SD modules must provide power during the total expected 30-year life of Space Station Freedom. SD Principles of Operation The operation of the solar dynamic electric power system for Freedom is shown in simplified form in Fig. 2. A reflecting concentrator focusses incident solar energy into a cavity-type heat receiver. The receiver includes heat exchanging tubes through which the gaseous working fluid for the closed loop Brayton cycle (CBC) heat engine passes. Also in the receiver is a quantity of a eutectic mixture of LiF-CaF2 salts in capsules around the tubes for storage of thermal energy by the heat of fusion of that mixture. The phase change (freeze-melt) temperature of the salt mixture is near 1040°K (1420°F). During the sunlit portion of Freedom's orbit, sufficient thermal energy is stored so that the temperature of the gaseous working fluid at the receiver outlet remains within a range of about 990°K (1330°F) to 1030°K (1400°F) throughout the orbit. The Brayton thermodynamic cycle is a single phase gas cycle. The SD power module uses the closed loop form of this cycle. A compressor raises the pressure of the gas working fluid after which it flows to a recuperator in which its temperature
RkJQdWJsaXNoZXIy MTU5NjU0Mg==