vehicles for use during the development of the other three vehicle types, there is essentially complete agreement that Shuttle systems would be used. 2. Space Solar Power The proposed concepts to utilize space solar power fall into two categories, photovoltaic and thermal. In the solar thermal approach, mirrors reflect heat into cavity absorbers, which in turn supply the heat for energy conversion. In the photovoltaic approach, solar cells perform the energy conversion directly. In the documents examined, requirements for technological advances are more thoroughly presented for the photovoltaic approach. Photovoltaic power systems currently utilize silicon solar cells approximately 8 mils (200 mm) thick. These cells have typically exhibited operational efficiencies of 10-12% (Ref. A4), and beginning-of-life efficiency for current solar cells is 12-14% (Ref. All). To meet the energy conversion and mass requirements of a photovoltaic SPS, cells approximately 2 mils (50 mm) thick with a beginning-of-life efficiency of 18-20% will be needed. In addition, the 30 year SPS life and the Van-Allen belt transit impose requirements on the solar cells for greater resistance to radiation damage than cells currently exhibit. Ref. Al 6 indicates that over a period of 3-6 years, degradation due to radiation (particularly, ultraviolet) is now typically 25%, and for the SPS must be reduced to 6%. Implicit in all these requirements, as well as additional cost requirements, is the need for new cell manufacturing techniques to produce improved cells and/or the development of alternative types of cells. Various aspects of the silicon solar cell and its alternatives are discussed in subsections a through e, below. With respect to the solar thermal SPS concept, the materials required for the expanse of reflecting surfaces and the thermal properties of the cavity absorbers present new requirements on technology. These requirements are described in more detail in subsection f. a. Solar Cell Efficiency As indicated above, beginning-of-life cell efficiencies of
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