On the other side, our production methods have to be drastically improved in order to meet the technical specifications discussed above, and to meet the cost goals, which have been figured out in the system studies. According to NASA estimates the solar cell blankets will represent about 20% of the recurring cost of one SPS launch at a cost level of $35/m2 or $0.15/W (1,6). This cost goal is identical with the DOE cost target for terrestrial arrays ($0.1-$0.3/W) at an annual production rate of 10 GW. Due to intensive governmental support for research and development, comparatively large annual production quantities, standardized hardware, automated manufacturing equipment, and terrestrial solar generators have already arrived at cost levels which are by a factor of 10-20 lower than for space generators. Therefore it has often been suggested to adapt terrestrial technology to space application. This is possible as far as the terrestrial technology is compatible with space requirements, which is still the case (7). Several space qualified manufacturing processes like wafer preparation, diffusion, contact evaporation, AR coating, and welding have been taken from the space production line so that terrestrial production facilities are able to manufacture large quantities of solar cell modules for space solar arrays either with terrestrial cells against relaxed specifications or with original space solar cells. But the primary optimization criterion for terrestrial solar generators is a minimum cost/power ratio while several space related requirements such as ultrathin solar cells, extremely high conversion efficiency, radiation resistance, etc., are of secondary importance or are not applicable. Although terrestrial technology is governed by quite different requirements, it is being developed for low-cost mass production, which is also required for future large space arrays. The experience gained in terrestrial mass production should be utilized for the production of large space arrays by combining terrestrial mass production methods with advanced space technology. From these arguments it seems mandatory to increase the R&D effort for large space arrays in order to meet the discussed solar array requirements for the SPS. Increased effort would even be justified if it turns out that a nonphotovoltaic SPS would be the optimum concept since any improvement in solar array technology would also be for the benefit of numerous other array-powered space missions. SUMMARY AND CONCLUSION Some major requirements for the SPS solar array, as identified in U.S. system studies, have been discussed and assessed from a state-of-the-art point of view. It has been shown that several aspects, such as large solar cells, ultra-thin solar cells, and elevated array operation voltage are already addressed in actual R&D programs. However, it is necessary to intensify the research effort in order to meet the extreme SPS requirements within the given time frame. Points of major future effort should be seen in those areas which will also help to improve the array performance for missions in the near future. These areas are • solar cell efficiency • radiation resistance of solar cells • technology of elevated voltage solar arrays. Any improvement in these technological areas would have a direct impact on future solar arrays and would be a further step for the realization of solar power satellites.
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