TECHNOLOGY ALTERNATIVES FOR THE SPS P. E. Glaser and P. K. Chapman Arthur D. Little, Inc. - Cambridge, Massachusetts 02140 The NASA/DOE reference design adopted for the SPS is based on current technology, with realistic projections for improvements in areas such as the cost and specific mass of photovoltaic cells. It provides a common benchmark for use in assessments of the implications of the SPS in societal, economic, industrial, military, environmental and other areas. However, it is recognized that new technologies are emerging which may offer advantages over those selected for the reference system. It is important to maintain a continuing evaluation of the technological alternatives, so as to exhibit potential improvements in the SPS, permit estimates of the technical and cost risk involved, and develop guidelines for future research. It is clearly not possible to make an exhaustive list of all conceivable technical innovations which might affect the SPS, but it is nevertheless feasible to develop a systematic methodology for the assessment of technological alternatives, which may be of value both in evaluating new technologies as they are proposed and in identifying high-priority areas for research. Such a methodology involves several components: 1) Variation of Guidelines. There are a number of guidelines underlying the reference design (a build-up rate of 10 GW per year, a design life of 30 years, a microwave power beam with an ionospheric flux limit of 23 mW/cm^, etc.), which were originally adopted as reasonable but somewhat arbitrary assumptions. These assumptions need to be clearly identified, possible changes in them should be documented, and consideration should be given to the effect of such changes on the optimal design of the SPS, the construction scenario, and the overall cost of the system. 2) Analysis of System Functions. The primary functions which must be performed by the SPS are: • Collection of solar energy in space. • Conversion to an intermediate form of energy (thermal and/or electric). • Conversion to a power beam. • Reception and conversion to electricity on Earth. A number of secondary functions are also required, including station-keeping and attitude control, beam control and steering, transportation and construction, etc. Alternative technical approaches exist for most of the sub-systems required to carry out these functions, and some of them may offer advantages over those assumed in the reference design. However, changes in one sub-system often propagate throughout the design, requiring changes in many other sub-systems as well, and may involve major revision of overall system parameters — for example, using laser instead of microwave power transmission leads to much lower optimum power output. Fortunately, a relatively elementary analysis of the system effects of sub-system changes will generally suffice for a preliminary assessment of new technologies — in fact, it appears to be possible to set up a system tree, analogous to a decision tree, in which the branches are different sub-system choices and which explicitly displays the costs and benefits involved. Fig. I shows the first step in the development of such a
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