designed to assemble the solar energy conversion system and the microwave transmission antenna. Construction costs, including transportation of the required construction crew of about 550 people and amortization of the bases, are projected to account for about 8% of the total SPS capital cost. The construction crew's primary activity would be monitoring, servicing, and repairing, with little need for extra-vehicular activities. The SPS hardware throughput in the construction facility is projected to be 15 t/h, for a construction rate of one SPS per year (10). The repetitive automated production process of space construction activities is projected to result in a productivity per crew member of 10 man-hours per ton of materials handled (the experience with terrestrial steel construction projects). To reduce the cost of space construction the production process will have to be equipment-intensive rather than labor-intensive. Thus the significant capital investments will be amortized over a number of SPSs. 7. SPS/UTILITY POWER POOL INTERFACE The large power output potential of the SPS will require careful design of the utility power pool interface to reduce the impact on the stability of a total utility system. Electrical power grids are designed to provide this stability of power supply to the user by incorporating redundant installations of reliable equipment. In addition to mechanical reliability, the reliability of the SPS will depend on generic system effects, and small variations (2-4%) caused by atmospheric absorption at the receiving antenna. Although the eclipse periods, occurring during the periods of minimum demand, are predictable outages, they are not planned outages since they are not deferrable. Thus, since they may affect the total system operation, they have to be included when calculating the forced outage availability of the SPS. The stability of the SPS will have a substantial effect on the stability of the power pool which it serves. Low-frequency fluctuations could cause the power level delivered by the SPS to the receiving antenna to vary; high-frequency fluctuations could cause line surges which might disturb the transient stability of other generators in the power pool. The magnitude of these fluctuations will have to be investigated to establish the required degree of surge protection which would be supplied by shortterm power storage (of the order of minutes) acting as a buffer. The resolution of issues inherent in the SPS utility interface will significantly influence specific design approaches and selection of technology options. A study of utility interface requirements indicated that one or more 5-GW SPSs could be installed in the utility systems of the southern states in the year 2000. The 345-kV and 500-kV transmission systems which will probably exist at that time could be readily extended to accommodate the SPSs (20). 8. SPS IMPACT CONSIDERATIONS 8.1. Economic The economic justification for proceeding with an SPS development program is based on a classical risk/decision analysis which acknowledges that it is not possible to know the cost of a technology which will not be fully developed for at least 10 years — and commercialized, i.e. produced, operated and maintained, in not less than 20 years. Justification, of course, is equally difficult to provide for other ad-
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