Fig. 4. Shuttle power extension package and polycrystalline silicon cells. One intriguing possibility is a solar cell “sandwich,” which exploits the selective spectral absorption of both gallium and silicon to produce a composite cell that may achieve a conversion efficiency between 30 and 40%. In the last several years, research and development programs (12) to improve solar cell performance and to devise low-cost manufacturing techniques have been greatly expanded with gratifying results. The programs emphasizing low-cost technology are oriented toward the production of solar cells to be used on Earth; consequently, these specific cells and techniques may not be completely applicable to space requirements. These programs, however, do provide an expanded technological and industrial base on which to build the space capability and consequently are of critical interest to the Solar Power Satellite program. Although these ongoing solar cell programs are important, there is a need to specifically assess the applicability of present research to Solar Power Satellite requirements and possibly to augment it in areas such as automated fabrication techniques, radiation degradation, annealing techniques, stowage and deployment techniques, concentrator material evaluation, and assessment of large-scale gallium recovery from bauxite. Concurrent with the laboratory research and development activities, new solar arrays are being developed for near-term space use. Figure 4 illustrates one such array which is now being considered to increase the Shuttle capability to support its payloads. Although these arrays are small in comparison with Solar Power Satellite requirements, they represent a continuing development of technology and space experience in this critical area. Other energy conversion systems have also been considered for the Solar Power Satellite, including thermal cycle heat engines which use large solar reflectors to concentrate the sunlight on a boiler surface integral to the spacecraft. These engines are coupled to turbine-driven generators and space radiators.
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