4. TERRESTRIAL vs LUNAR TRADEOFFS There are many scenario choices for manufacturing SPS. Examples are: (a) Assembly in LEO from terrestrial materials and transportation of the completed SPS to GSO (Geosynchronous Earth Orbit) (b) Same as (a) but assembly in GSO (c) Assembly in GSO from lunar materials. Variations of (c) would be: (cl) materials refined and parts manufactured on the moon (c2) materials refined on the moon but parts manufactured at GSO (c3) lunar materials refined and parts manufactured in GSO (c4) lunar material refined and parts manufactured in low lunar orbit (LLO) or L5 (c5) lunar materials refined on the moon with parts manufactured in LLO or L5. Transportation options in all these cases involve: (Tl) Chemical propulsion throughout using oxidizer and fuel from earth (T2) Chemical propulsion using hydrogen from earth but lunar oxygen for other than earth launch (T3) Chemical propulsion for earth launch only and electromagnetic propulsion from lunar to orbit and orbit to orbit. In all cases transportation of the assembled SPS, if applicable, is assumed to be by low thrust electric propulsion using power from the unit itself. No attempt will be made to examine all combinations of these and other possible scenarios. A few examples will suffice to illustrate the tradeoffs and potentials of the system. Figure 6 shows the cumulative cost as a function of time, undiscounted for 20 SPS manufactured at the rate of one per year. A comparison is presented between option (a) and option (c4) using propulsion system (T3). Evidently a crossover exists at a production of around five units for the baseline cases. Also shown in Fig. 6 is the effect of varying the initial R and D costs. Since the major costs in producing power with the SPS will probably be the capital investment involved, lines of $/kW are a good indication of the eventual bus bar costs of the SPS produced electricity. The terrestrial receiving system, essentially the rectenna, has not been included and will add an additional cost depending on land values. The capital investment costs may be corrected to 0/kWh by assuming a return on investment of 10% and dividing by the yearly usage. For base load this would be of the order of 8000 hrs/year. A capital investment of 1000 $/kW (about that of a nuclear system) would therefore result in a cost Figure 7 shows the effect of increasing the production rate to five units/year. Total usage of electricity in the U.S. in 1990 is estimated to be 4 x 1012 kWh/year. Consequently, about 50 SPS would be able to provide all the demand at that time. Figure 8 shows a comparison between options (a) and (c4) with propulsion option
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