2.2.4.8 Energy Conversion Process Incoming Light Intensity ■ 1.353 kW/m2 Blocked by Secondary Reflector: 10% (4,pl09) Absorbed by Primary Reflector: 16% (16,pl593) Light to Secondary Reflector Blocked by Tertiary Reflector: "0 Absorbed by Secondary Reflector: 16% (16,pl593) Light to Photovoltaic Cell Absorbed by Tertiary Reflector: 0 Reflected from Cell Face: 25% (3,pl616) Converted to Electricity: 15% (3,pl616) Rejected as Waste Heat: 60% Thus the sunlight projecting onto the concentrator is converted to electricity with an overall efficiency of: (0.90)*(0.84)*(0.84)*(0.15) - .095 or 9.5% The 15% efficiency of the GaAs cell assumes the losses listed in Table 2.1-2. 2.2.4.9 High Reflectivity Coating This study assumes a value of .84 for the reflectivity of vapor deposited aluminum. This is based on the value presented in a report by TRW (16,pl593) for ’advanced aluminum’ technology. The reference does not define the term ’advanced aluminum’ so the relationship between TRW’s approach and vapor deposited aluminum is not clear. Due to similarity in the applications, however, and a general lack of information on this subject, this number was assumed. Other references(ll,p267) and members of the Advisory Committee have suggested that a higher reflectivity, perhaps as high as .92, is probable for aluminum. Since the assumed coating absorbs (or transmits) nearly 30 percent of the incoming light, it is recommended that efforts to design a concentrator based SPS should include efforts to identify the maximum achievable reflectivity.
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