Fuel supplies, particularly uranium, could be a problem after 2000 if increasing use of electricity continues in the next 20 years. If the demand for electricity continues to fall off as it has in the last several years, then the uranium supplies for use in LWRs could go on for many years beyond the time horizon of study (2000-2030). The introduction of the LMFBR would eliminate uranium resource concerns. Terrestrial photovoltaic systems, particulary those using silicon cells, have a very poor energy payback because of the energy intensity of silicon cell production. If the energy requirements of silicon cell production are diminished, then energy payback does not appear to be a problem. Economic/Societal Issues. The SPS, TPV, and fusion are different technologies than those currently producing electricity and will entail many new industries that will require large investments in infrastructure. These large investments, coupled.with the capital intensiveness of the technology, could have the largest impact on the GNP compared to the other technologies. The infrastructure and R&D costs for TPV and fusion have not been estimated; however, because the SPS would require many different types of systems (space transportation, space construction), many new types of jobs and training would be created to support it. Nuclear and fusion technologies would not be as regionally biased as would the other technologies. Terrestrial photovoltaics would probably be the most regionally applied technology because its implementation is determined by climatological factors, and the proper climate is only available in certain areas of the country. Institutional Issues. The environmental regulatory climate for coal and nuclear technology has increased substantially in the last 10 to 15 years. If this trend continues, the advanced technologies may also feel these institutional restraints at early stages of their development. SPS raises additional institutional questions because of its international nature. Institutional restraints concerning orbital ownership and microwave exposure standards pose the most serious institutional questions for the implementation of an SPS technology. As serious as the regulatory difficulties are within the United States, the international issues will most likely be even more difficult. 5.3 ALTERNATIVE FUTURES CONCLUSIONS OF THE COMPARATIVE ASSESSMENT The energy supply/demand analyses that were part of this assessment produced several energy-demand and supply trajectories for the future. The two extremes of these scenarios are discussed here to form conclusions about the comparative viability of the SPS in an alternative futures framework. The other scenarios not discussed in this concluding section are intermediate cases and offer no more perspective on SPS and the alternative technologies. The two scenarios that were chosen were the unconstrained high energy demand scenario and the constrained intermediate demand scenario. These
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