The support and exploitation of silicon-alternate technologies may produce more cost effective solutions to the photovoltaic conversion problem. Several candidates should receive further study and support. Development of silicon-alternate technologies should not, however, ignore the availability of raw materials. Silicon, for example, is approximately 45 000 times more abundant than gallium, and the energy economics of resource isolation must not be ignored (see subsection 9.1. 3). A further study of resource and energy economics should be conducted at an early stage to assure that the selected photovoltaic system can supply enough energy to pay for its extraction from the raw material. Some simplified calculations indicate the revenue will offset the monetary investment in a silicon photovoltaic SPS after a few months of operation, but similar work for other systems remains to be done and will require access to information that has not yet been made available because of proprietary and other interests. While some studies indicate great potential savings in the energy used to refine and process silicon, these savings have not been demonstrated. Many advanced cell concepts require special processing steps such as selective etching to texture silicon surfaces, fine geometry control for vertical multijunction cells, and complex contact metallurgy. These aspects of cell manufacture must all be examined to identify and exploit advantages in performance. Solar cells have been used with thick, heavy glass or silica covers for protection from radiation and other damage. The SPS baseline has assumed that a very thin, light plastic can be used for this function in the GEO environment. This assumption has little evidence to support it, and much effort will be needed to verify and test light plastic covers. There is evidence that some of the silicon alternates will not require such protection; if this is proven, such characteristics may override the desirable attributes of the silicon. Similar requirements hold for the flexible substrate, but the problem is simplified by the fact that the substrate need not be transparent. Structural and life properties of the substrate need investigation. Test and engineering data for detailed design of the supporting structures must be derived and verified. Finally, the environmental aspects of the solar array technology selection will need study. These include material toxicity, handling, waste products, and impacts of manufacturing large quantities of solar cells and related components.
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