• Rate of degradation of output on - orbit - solar cel 1/array/system performance: • Photovoltaic efficiency recovery techniques; • Cell/array cost; • Mass; and • Availability of photovoltaic materials. In addition to these parameters, consideration has to be given to the cell/array manufacturing processes to meet SPS production requirements and cost goals. Silicon is presently the only photovoltaic material that is being considered for mass production as part of the U. S. Department of Energy (DOE) Photovoltaic Program.2 Single-crystal silicon solar cells could meet the projected needs of the ground-based exploratory development and space technology verification during the period 1 980 to 1985, and is the most likely photovoltaic material for use in SPS pilot and prototype systems. Gallium arsenide is a promising photovoltaic material, however, gallium arsenide solar cells in suitable form, sufficient quantity, having reproducible characteristics and an acceptable cost are unlikely to be available before 1985 unless there is a near-term significant R&D program commitment. Furthermore, other advanced photovoltaic materials, e.g., amorphous silicon, are being investigated as part of the DOE Photovoltaic Program which may deserve consideration as candidates for SPS solar cell arrays if they could be mass produced at low cost. The industrial capability needed to manufacture the SPS solar cells and arrays will require the development of large-volume production technology which could serve both the SPS and terrestrial PV system requirements. Figure 2 presents a solar cell/array production scenario based on the deployment of one 10 GW on two 5 GW SPS's per year including inventory accumulation. This implies that the selection of solar cell materials and array designs will be completed by 1985, and that a long-term on-orbit solar cell array test program will provide data on the performance of candidate solar cell arrays in the space environment prior to commitment to a pilot-plant program. The scale of commitment of capital material and labor resources to construct large-scale manufacturing facilities will require that the risks and uncertainties in achieving required solar cell/array performance and cost goals have been evaluated and that they are acceptably low. Funding commitments for gallium arsenide, or other promising photovoltaic materials, would have to approach the present level of funding for silicon solar cells in the next several years so that they could be considered for an operational SPS in the 2000 time frame. xArthur D. Little, Inc., "Evaluation of Solar Cells and Arrays for Potential Solar Power Satellite Applications." Final Report to NASA Lyndon B. Johnson Space Center, March 1978. 2U.S. Department of Energy, National Photovoltaic Program, Multi-Year Program Plan, June 6, 1979- D0E/ET-0105-D.
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