generation techniques. In addition to these overall areas, a number of specific implementation approaches are also of interest. Some of these are common to several approaches while others are unique to one approach, but they are all associated with some specific energy conversion requirement and include the following: • Cooling provisions/disposal of waste heat • Reflecting/collecting surfaces for solar energy • Solar energy concentration provisions • Power collection implementation • Protection/susceptibility to environmental factors In addition, parameters of interest include the conversion efficiency and, since it influences other considerations, satellite mass in-orbit. a. Overall Conversion Approach Considerations For the principal solar-photovoltaic SPS concept, energy is converted directly by the solar cells, thus the prime considerations involve solar cell technology. The status and requirements associated with this are discussed in Section 2 above, but for consistency can be summarized here as: (1) improvements and new techniques in manufacturing are required to produce the necessary solar blankets, (2) improvements in cell efficiency from the value expected in the mid-80's of about 10% (at a concentration ratio of 2) to a nominal value of 14% for an operational SPS are required, (3) cells with a thickness of 2 mils (50 microns) are required, and (4) improvements in the resistance of the cells to radiation are required if excessive array degradation is to be prevented. Meeting these requirements may require the introduction of a solar cell based on a material other than silicon, which has thus far been baselined. For the solar-thermal SPS concepts utilizing thermionic converters, the energy conversion device is the thermionic diode. This device has been considered in the past mostly with a nuclear system for a heat source. In the SPS, it would receive its heat input via a solar concentrator/cavity absorber arrangement with waste heat dissipated into space to produce the temperature differential required by the diode for energy conversion. The
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