Heat Rejection Assembly To reject any waste heat left over from the PCU, a radiator is required. The amount of energy radiated into space is proportional to the temperature to the fourth power, so the higher the temperature the more energy is radiated into space, i.e., the higher the Watts/m2. However the efficiency of the SDS is dependent on the ratio of the turbine inlet temperature and the radiator outlet temperature, with the higher the ratio the more efficient the system will be. Thus although it is possible to reduce the radiator size by increasing the temperature within the radiator, it is at the expense of the thermodynamic efficiency. At 293 K the heat rejection capability of a radiator in orbit is -200-250 W/m2. The waste heat can be transferred from the PCU and other electrical equipment by a pumped liquid loop to the radiator which radiates the heat to space. The choice of method depends on thermal cycle and system trades-offs. Interface Structure An interface structure is required to mount the SDS to the satellite. The exact structure will depend on the satellite but it will need to maintain the accurate alignment between the concentrator and the receiver as well as the alignment between the sun and the concentrator. Because pointing of better than 0.1° is required a 2-degrees of freedom pointing mechanism will be required. Electrical Equipment The electrical equipment contains all of the controls for the SDS module. This controls the pointing controllers, PCU, and the pump motors. Ground Demonstrations Past and Present Systems Although there is no experience of operating a solar dynamic system in space there is an extensive database of operating these systems on the ground. This helps build confidence in the development of Space Heat engines. Brayton Cycle Systems Closed Brayton systems represent a mature technology .benefiting from a long history of closed cycle system development and an extensive technology base provided by the open cycle gas turbine industry. Brayton cycles use gas as the working fluid and therefore are unaffected by the micro-gravity environment in space. NASA have a long history of developing their heat engines for demonstration the necessary technologies, which are summarised below. [Harper, 1989] 1962 3kW demonstrator 1963 lOkW demonstrator (engine A) 1965 lOkW demonstrator (engine B) 1972 lOOkW nuclear powered 1976 1.3kW radioisotope-powered satellite Stirling Engine A Space Power Demonstrator engine (SPDE) has been developed by NASA which generated 25 kW using a dynmically balanced opposed-piston Stirling engine at a temperature ratio of 2. This gave an efficiency of - 22% and was operated at a temperature of 650 K.
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