and stem from the relative lack of definition in the constituent materials, uncertainties regarding their energy intensities and the variety of models available for deriving the energy ratio or payback period. 9 2 9 4. Materials Requirements Materials assessment studies resulted in both a methodology for performing an assessment and actual assessment results. The heart of the methodology is a computerized materials screening process using a data base containing information on raw and bulk materials—including energy consumption—from which new systems, as well as their components and subsystems, are manufactured. The data base currently contains about 2,000 entries covering more than 260 materials as well as estimates of present and future U.S. and world consumption, prices, U.S. imports, and dominant non-U.S. suppliers. The screening program tells planners how much expansion in capacity will be needed to produce the projected quantities of each material, how much of the material comes from abroad, and its cost per unit of electricity produced. Materials that exceed critical threshold values are flagged to assure that they will be studied more closely. Thresholds can be changed and the analysis rapidly rerun to determine sensitivities. Quantities of basic materials required for the SPS were estimated and compared against projected supplies, production capabilities and sources. Assessment of these SPS material requirements indicated a number of potential supply problems. The more serious of these were solar cell materials (gallium, gallium arsenide, sapphire, and solar grade silicon), and the graphite fiber required for the satellite structure and space construction facilities. Two options for solar cell material (silicon and gallium arsenide) are part of the Reference System. In general, the gallium arsenide SPS option exhibits more serious problems than the silicon option, possibly because gallium arsenide technology is not as well developed as that for silicon. The only problems of serious concern involving a material that appears in both SPS reference concepts are those associated with graphite fiber production. The annual production growth rate to meet the combined requirements of the SPS and the automobile industry could be in the 20-30 percent range sustained for a decade or more. Also, depending on the type of fiber selected,
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