Fig. 4 Alternative Futures Analysis of Land Requirements The need for large contigious areas, as for SPS rectennas, is a further complicating factor. Water use in m^ x 10^/GW/year, is 22 for coal, 60 for LWR, 22 for LMFBR, 12 for fusion and negligible for TPV and SPS. Total water requirements for the three scenarios, with and without SPS, are shown in Fig. 5. Results indicate that deployment of SPS can save large volumes of water; in scenario CI, SPS saves an amount equal to 40% of the total used in 1980 for baseload electrical generation by coal and nuclear; in scenario UH, the saving is 170% of today's total. Due to large uncertainties in determining the resource/reserve levels for both the United States and the world, the analysis of materials problems was less quantitative than the land and water analyses. A screening methodology included a reliance on imports as a criterion as well as availability and total demand considerations. These screening factors identified gallium as being a material of serious concern. Gallium is used extensively in the GaAlAs solar cell option for SPS. Also of serious concern is tungsten, which is used both in SPS and coal technologies. Net energy analysis shows that the payback period for most of the technologies studied is small (less than 1.5 years). The payback periods for the SPS GaAlAs option, coal, and the nuclear options are about one year, and those for the SPS Si option and TPV (silicon cells) are about 6 and 20 years, respectively. Thus, the GaAlAs design affords SPS with an option that compares favorably with conventional technologies on a net energy basis.
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