LL Conclusions The results shown above indicate that the reference designs for a thin-film SPS should be based on a power beaming frequency of approximately 10 GHz. Lower frequencies may be desirable if larger SPS's are desirable, although communications needs may render these frequencies unavailable. Higher frequencies may make possible a "mini- SPS," which can serve as a demonstration project, or be used to supply power to small, remote villages in areas with little rainfall. A non-tracking bicycle wheel design may be the simplest in the short run, and it has the highest array specific power of the three designs considered. However, its variation in delivered power during the course of its orbit may make it more difficult to integrate into existing power grids. Thus, the bicycle wheel with mirror SPS will probably be more feasible in the long run, although future developments may tip tire balance in favor of the inflatable sphere. A major issue in the deployment of thin-film SPS’s is the use of lunar versus terrestrial materials. It is likely that terrestrial designs will prove useful in the short run, allowing thin-film SPS’s to be deployed before a lunar infrastructure is built. Due to the large number of SPS’s that will eventually be needed, combined with the need to replace aging SPS’s, a lunar infrastructure may eventually become necessary. The ultimate decision about SPS deployment strategies will depend upon future global energy needs, launch costs, and the cost of building and operating a lunar infrastructure. To date, thin-film solar cells have been produced in relatively small modules at manufacturing volumes far below that required for SPS construction. The substrates commonly used are not lightweight. Research in depositing thin film solar cells on lightweight substrates is only just beginning. However, the promise of thin-film technology, combined with future world energy needs, suggests that it is worthwhile to develop manufacturing technologies which would allow thin-film solar cells and solid state microwave transmitters to be deposited on lightweight substrates and produced in large quantities. 12. Acknowledgments I would like to thank Geoffrey Landis and Ronald Cull of NASA Lewis Research Center and Brandt Goldsworthy of Goldsworthy and Associates for their discussions and the information they provided, Professor Martin Hoffert of New York University for his
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