7. 2 THERMAL CONVERSION SYSTEM The SPS solar thermal concept configuration has a smaller area and a higher efficiency than the photovoltaic concept. Of the possible thermal conversion systems available, the cascaded thermionic-Brayton system is the most efficient and requires the least concentrator/radiator area and, hence, the smallest satellite planform area. 7. 2.1 REQUIREMENTS AND ANALYSIS The power requirements for the microwave antenna of the solar thermal concept are the same as for the photovoltaic concept. However, because of lower power distribution losses, the bus power required at the output of the thermal conversion equipment is less than that required at the output of the solar arrays. This was not considered in this study, however, and the bus power required of the thermal conversion system was set at 17. 6 GWg for a ground power output of 10 GW . This margin, which could be up to 10 percent, can be c considered as a contingency to compensate for increased pointing requirements of the reflectors or the total satellite (perpendicular to sunline instead of perpendicular to orbit plane) in the solar thermal concept. One approach to the study of the solar thermal concept has resulted in four extremely large power modules to produce the 17.6 GW required bus o power. Component sizes are attendantly very large. Liquid metal radiators are required that must be assembled and welded on-site. It is expected that the on-orbit assembly and maintenance equipment could be smaller and have a larger utilization factor if a large number of smaller power modules were used. A small power module will likely be required for the early phases of the SPS development program, and the impact of stopping the development cycle at a small size was assessed. Advantages of a small module include: 1. Absorber/receiver/con version system (excluding radiator) can be ground assembled and remain a high density launch package. 2. Surface and pointing accuracy of reflector surface (due to the shorter focal length) are reduced. 3. Redundancy is provided (i. e., several modules may fail before a significant SPS power loss is experienced).
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