A concentration factor of 2, however, will reduce the efficiency of an 18 percent solar cell to about 14 percent. This is due to the increased operating temperature which will also require heat rejecting coatings for the solar array. Increases in solar cell thicknesses and lower efficiencies will be reflected in increased capital cost because more material will have to be transported into orbit. There is a balance between concentrating the solar radiation per unit area of solar cells, and declining efficiencies and rising temperatures. The baseline concentrator structure is a flat-plate channel design with a concentration ratio of N=2. At this concentration ratio, the power (delivered in space after 5 years of life) from the blanket corre- 2 2 2 spends to 245 mW/in (38 mW/cm ) or an area to power ratio of 4.08 in 2 per watt (26.3 cm per watt). The reflector or concentrator surface areas required to achieve this concentration ratio is 2.12 times the solar cell (blanket) area. The material selected is one-half mil Kapton 2 with aluminized coating. It weighs approximately 0.004 Ib/ft (0.002 / 2. gm/cm ). In order to achieve this baseline requires: • Lightweight mirror design concepts • Accurate thermal analysis and control • Technology for coating large area plastic panels with filters and mirrors • Low-cost filter/mirror structures • Low degradation filter/mirror surfaces (6) Summary To make the photovoltaic process viable rather large and significant improvements are required in solar arrays. These are briefly summarized below. Item Current Status Goal Solar Cell Cost $20,000/kilowatt $200/kilowatt Solar Cell Efficiency 10 percent 18-20 percent Solar Cell Thickness (weight) 250 microns 50 microns Solar Cell Lifetime 10 years 30 years Concentration Optimization N/A $310/kW; 3 Ib/kW
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