such as improvement in material purification, the cell structure, the cell fabrication processes and the introduction of protective materials against space radiation in order to obtain higher output, reliability and a light weight. Cost Reduction of SPS Cells The scale of SPS is thought to be such that its output covers from 10 MW to 10 GW per system. One example shows that the fabrication capability of SPS is two systems per year, and at maximum 60 systems in total, according to feasibility studies in the USA. As the production numbers of SPS directly influence SPS costs, they must be assumed at first. But in this paper it is assumed that terrestrial-use solar cells are manufactured through a mass production process against the background of large demand, and that SPS cells with special requirements are produced according to a terrestrial process. From the above, the cost efficiency of mass production for SPS cells is ignored in some measure. In the case of a single crystal solar cell in Fig. 5, the SPS cell cost is expected to be ¥400 per watt, twice as much as ¥200 per watt for a terrestrial-use solar cell, and in the case of amorphous Si solar cell, the SPS cell cost is expected to be about ¥250 per watt by comparison to ¥80 per watt for terrestrial use. Conclusion It is necessary that the SPS solar cell costs as little as a terrestrial cell in large-scale manufacturing, has high power and high reliability nearly the same as that of a current space-use solar cell. Although we cannot expect a low cost for SPS cells, in so far as
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