3. Silicon is the second most abundant element on Earth but is never found in the free state. The problem of manufacturing low cost solar cells is recognized as a major development effort that is not associated with material availability. 4. The production of graphite and Kapton was not assessed for this analysis. 5. If galium arsenide is chosen as an alternate to silicon for solar cells, the availability of both galium and arsenic must be considered. 9.1. 2 ENERGY BALANCE ANALYSIS An analysis was conducted to assess the energy required to produce a power satellite and the time required for an SPS to generate an equivalent amount of energy. A concentrator concept with a mass of 185 x 10 kg was used in this study since it is heavier and, hence, more energy intensive. This analysis included all energy necessary for processing of SPS materials and replacement parts and for ground transportation including both rail and truck transportation. Orbital transportation was included for both the ground to LEO and LEO to GEO phases. Rectenna and microwave transmission corridor (approximately 100 km2) was assumed to be unavailable for agricultural use, specifically com production. The production of one SPS requires 4. 5 x 1011 kWh of thermal energy as summarized in Table 9-2. If instead of producing an SPS this thermal energy was converted directly to electrical energy with a conversion efficiency of 30 percent, the electrical energy produced would be 1. 35 x 1011 kWh. The SPS produces 8. 0 x 1010 kWh electrical energy per year, consequently the SPS would return the energy in 1. 7 years. This compares favorably with estimates for conventional ground systems. 9. 1. 3 MATERIAL RESOURCES In general, none of the required resources are expected to be in short supply. The expected United States consumption of some materials until the year 2000 is shown in Figure 9-1 compared to United States resources. The term ''Reserves" applies to resources that are known to exist and that can be mined with current technology and economics. ''Identified Marginal Resources" apply to thos^ that are known to exist and could conceivably be produced economically in the future at higher prices or with proper advances in technology. ''Hypothetical Resources" attempt to quantify deposits that have so far eluded discovery in known areas of favorable ground. ''Speculative Reserves" refer to new regions that have potential for new discoveries.
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