A Planning Research Corporation study[174] used I/O analysis of a GaAlAs SPS. The study assumed higher energy intensiveness and resulted in a 2.5 year payback (vs. 1.3 years calculated here). Herendeen[172] used I/O analysis and uncertainty to set a range for the lifetime ratio from 0.7 to 6.6, with a mean of 2.1, for a 10-GW SPS. this is surprisingly close to our result of 4.0, considering major differences in the assumption about silicon cell life. Herendeen assumed an exponentially decreasing cell power output over a 30-year lifetime. This analysis assumes maintenance to obtain constant power output and a 30-year life. A Jet Propulsion Laboratory (JPL) study^^^ computed a payback period in the range of 1.2 to 1.6 years for a system of 48 10-GW silicon satellites (vs. 6.4 years here). Three major factors account for the difference in the estimates. JPL assumes a reduction in the requirements for silicon cell production to about one-fourth of the energy requirements assumed here, and only half as much silicon. Most important, JPL converts electrical energy to a thermal equivalent by multiplying by a factor of three. Removing only this last difference results in a payback of 3.6 to 4.8 years for the JPL study. Summary. The current state of knowledge indicates that energy balance is not a significant factor precluding the development of certain coal, nuclear, and satellite power systems. There are indications that reductions in energy requirements for silicon cell production will be needed, since those requirements constitute a large factor in net energy calculation for silicon systems. In addition, although current data indicate that GaAlAs cells provide a viable alternative, further studies are warranted because information on the energy requirements of these cells is extremely limited. In conclusion, the net energy analysis has shown that the SPS system with silicon cells is a viable alternative, having a payback period substantially smaller than its lifetime; a TPV system using silicon cells does not look as attractive. However, if the most optimistic projections were realized for silicon cell production, the TPV option would become a viable alternative from an energy-balance perspective. If development of a gallium photocell proceeds as currently projected, an SPS system with GaAlAs cells will result in a payback period comparable to CG/CC and nuclear. In any event, the SPS is a viable alternative using state-of-the-art technologies, with promise of even better technology in the foreseeable future. 4.5.4 Water Overview. As for the land assessment, Digitized by the Space SStudies Institute both a side-by-side analysis and an alternative futures analysis were completed for water use. The side-by- side comparison is based on the technology characterization data. Scenario data are coupled with the side-by-side data to provide water use estimates over the 1980-2030 time frame.
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