that 60 satellites would be constructed between 2000 and 2030, was not a plan or a prediction of what will or should happen, but a basis for studies pertaining to space transportation, material resources, and manufacturing requirements. The report comments correctly that unless the cost of an SPS is competitive with alternative energy conversion technologies, it “simply would not be built." Therefore, the statement that the price tag for the SPS reference scenario will be in the range of 3 trillion dollars lacks economic justification. There is no basis for projecting the costs of 60 SPSs to be constructed 30 to 50 years hence without accounting for the evolution of advanced technologies, including low-cost, efficient, space-qualified solar cells; low-cost space transportation systems from Earth to low Earth orbit, and from low Earth orbit to geosynchronous orbit; space construction techniques; and technologies associated with the transmission of power from space to Earth. Alternative energy conversion technologies, whether based on development of coal, fast breeder, or fusion power plants, must also achieve a competitive cost range, or such technologies would not be commercialized on the scale projected. The finding that single crystal silicon cells will be too expensive for use in the SPS is moot because more advanced photovoltaic materials for thin film solar cells are projected to be available by the end of this decade.* Gallium .arsenide solar cells are used in an advanced SPS concept, evolved by Rockwell International, at a projected capital cost of $1500 per kW, about half that of the SPS reference system. The report states that “If constraints are placed on the use of coal or uranium (in conventional and breeder reactors) and practical fusion reactors do not achieve their goal, an SPS could become an attractive option for development in the next century," and further, "... there is a possibility that some future combination of high demand and constrained supply could make a more advanced SPS an important option in the more distant future. For the most part, building an SPS requires advances in materials and techniques in system development on a large scale rather than the discovery of new science.” Unless research is performed, how are these advances to be accomplished? Whether the most effective way to advance the SPS concept will be to perform generic research, as the NAS report recommends “where areas relevant to SPS technologies may be investigated in pursuit of goals of other programs, research should be vigorously conducted and the results evaluated for the implications on the SPS concept,” or whether a specific SPS research program to define relevant technologies and indicate ways to lower the cost of the SPS should be pursued is a matter for debate. Consideration should be given to planning the most effective SPS research program, rather than focusing on a specific design, or costs of technologies which will not be commercialized until the 21st century. The SPS option should be kept open because, as the NAS report acknowledges, “there are uncertainties concerning the availability of large-scale sources of energy for electricity beyond the early part of the next century. The use of coal is, of course, technically feasible and may set the standard for economic competition; but concerns about carbon dioxide emissions and other environmental effects could constrain coal development. The nuclear breeder reactor is also technically feasible, but it faces problems of political acceptability because of concerns about reactor safety, waste management, and the potential for proliferation of nuclear weapons. Even though advances are being made in terrestrial photovoltaic cells, the technol- *Aerospace Power Systems, R.R. Barthelemy and D.J. Curtin, Astronautics and Aeronautics, July/ August 1981, p. 71.
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