Eventually photovoltaic solar cells located in the American Southwest may be competitive with one type of intermediate-load service: the supply of energy for air conditioning. Base-load plants (mainly coalfired and nuclear) supply power at IS to 17 mills. Nuclear plants in particular are best suited to base-load service: they must run nearly all the time to amortize the heavy capital investment required for their construction. Once started, a nuclear plant is kept running for another reason also: each time it is turned off there is a risk of component failure due to temperature changes. If electricity could be obtained from an inexhaustible source at 4 to 8 mills, lower even than base-load rates, it could have a profound impact on economic security and independence: residential and industrial heating could then be shifted to electricity, relieving demands on natural gas and oil supplies, and the production of synthetic fuel alternatives to gasoline could become practical. Like a nuclear plant, an SSPS would have to operate nearly all the time to amortize its construction cost. Economic viability of an SSPS would require, therefore, that it operate in base-load service, at rates not over IS to 17 mills. If SSPS power is to have major impact on the problems of energy resources and dependence, a way must be found to build and locate large numbers ofSSPS plants (up to 20 to 40 per year of S-Gw size) and the electricity rates at which they operate must be low enough so that they will achieve market penetration, being chosen for new construction in preference to alternative (coal or nuclear) plants. If those two conditions are not met, SSPS power can be no more than an exotic rarity, classed with hydroelectric and geothermal power among fringe sources (I to S percent) of energy. My purpose in stating these necessary economic conditions is not to discourage the development of a prototype SSPS. Clearly, though, it will be difficult to meet these conditions with SSPS plants built on, and launched from, the earth. In support of the viewpoint that SSPS development is justified nevertheless, I will outline what may be a way to meet the conditions of SSPS mass production and low electrical rates. The Space Manufacturing Alternative The effectiveness of an SMF program for the achievement of economical solar power on the earth would depend on two key elements: the use of lunar materials and the "bootstrap process" - the construction by the first SMF not only of SSPS units but of additional SMF's. The 5 DECEMBER 1975 Table I. Critical factors in satellite power station economics. The numbers assumed in several studies for the factors specific power plant mass. component lift cost from the earth, transmission loss factor (,-1 ) , and interest rate are summarized; in each case a higher number corresponds to a more conservative assumption. Earth-launched SSPS values are from (13) for those with turbogenerators and fro m (3) fo r those with photovoltaic cells. Data in the last column are from th is article. The li ft cost from the earth to geosynchronous orbit is approximately equal to the cost for lift to Lagrange point LS. For base-load service. busbar power costs are now typically 15 to 17 mill/ kwh. Specific Lift SSPS mass cost (kg/ kw) ($/ kgsy) Ea rth -laun ched Turbogenera tor 5 75 Photovoltaic 0.8 220 Built in space from lunar material 10 950 use of lunar materials would circumvent the problem of lift cost ($/ kgsy) and therefore of power plant mass (kg/kw). The bootstrap process would replace linear growth in the number of SSPS units by exponential growth. The establishment of the first SMF would require the transport of 3,000 to I0,000 tons to the lunar surface, and 10,000 to 40,000 tons to LS (2). The structural mass of the SMF has been estimated as IS0,000 tons (18), and the total mass including cosmic-ray shielding could be 2S to 6S times larger. The SM F would be built almost entirely of lunar surface materials. The lunar soil (regolith) as found, unselected, contains 20 to 30 percent metals, 20 percent silicon, and 40 percent oxygen by weight (19). Depending on whether the first SMF were provided at the outset with a massive cosmic-ray shield, or acquired such a shield over a period of years by the accretion of industrial wastes (slag) from the manufacturing operations at LS, the transport machine (mass-driver) for lunar surface materials would be required to lift 80,000 to 700,000 tons per year from the moon to LS. With full-time operation at a cycling rate of 30 kg/sec, the mass-driver previously described (2) would transport 940,000 tons per year. After completion of the SMF, the lunar mass-driver would continue to export raw materials to the SMF site. There, the processing plant already used for SMF construction would continue to produce metals, glass, ceramics, and other materials. In zero or low-gravity construction bays adjacent to the SM F habitat, those materials would be formed into SSPS components. An SSPS built at a space colony would be considerably simpler than one launched from the earth, because the colony-built SSPS could be designed without launch vehicle constraints. Turbogenerators could be fewer and of the most efficient size rather than kept within vehicle limits. Solar reflectors and waste-heat radiators could be built in large sizes and would never have to Interest Initial busbar ,. , rate power cost (%) (mill / kwh) 1.43 8 25 1.54 1.6 10 15 withstand launch accelerations. That is a significant advantage because an SSPS would be mechanically fragile: the specific mass figures of Table I imply an overall average thickness for the SSPS, including solar energy converters, radiators, conductors, mirrors, supports, and transmitting equipment, of only 0.08 to 0.6 mm of aluminum. The linear dimension of the SSPS would be several kilometers, about ten times larger than those of the SM F. On completion, the SSPS would be tested in space close to the construction site. It would then be moved to geosynchronous orbit through the small velocity interval (2.1 km/sec) which separates that orbit from LS. A second mass-driver, similar to the one which by then would have been in operation on the moon for several years, could be used for this task. It would be assembled outside the SM F and attached to the completed power station, to serve as a reaction engine. It would use as reaction mass industrial wastes, possibly liquid oxygen, left over from the processing of materials for the SSPS. As a reaction engine, the massdriver would have an exhaust velocity of 2.4 to 3.7 km/sec and a thrust controllable from zero up to a maximum of several tons. It would be powered by the SSPS during the orbital transfer time of I to 4 months. The economics of SSPS construction at LS requires a fresh viewpoint: in that construction almost no materials or energy from the earth would be required. The colony itself, once established, would be selfsustaining, and its residents would be paid mainly in goods and services produced by the colony. The economic input to the combined colony-SSPS program (Fig. I) is the sum of development and construction costs for the first colony, the cost of lifting the materials needed from the earth for subsequent colonies and for noncolony-built SSPS components, a payment on the earth of $10,000 annually to every colonist, repre945
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