A second potential SPS orientation angle is for the longitudinal axis to lie in the orbital plane. With the correct initial conditions the SPS orientation would vary ±19° and, assuming a ±1° pointing tolerance, the total misorientation angle would be ±20°. The orientation of the SPS longitudinal axis perpendicular to the orbit plane was used in the system efficiency analysis. The development of the GaAlAs cell was projected for the post-1985 period. A 20-percent cell at AMO and 30°C was used as the basis of the cell performance. Taking into account the higher operating temperature of the cell in orbit as a result of the 2-to-l concentration ratio, the cell efficiency was calculated to be 17.6 percent. Additional design factors for sizing the array include: (1) assembly and cell mismatch losses, (2) interconnect failure, (3) fatigue failures, (4) cell packing factor, and (5) blanket packing factor. The combined efficiencies of the array design factors used in the study were 87.6 percent. The efficiency of the reflector, considering reflectivity and degradation, was 90 percent. The efficiency of electrical distribution in the array was 92 percent and is a compromise between lower conductor weights with larger arrays and larger conductors and weight with lower power losses and, therefore, a smaller array. The efficiency of wiring from the arrays to the slip ring, through the slip ring, and in the microwave antenna was calculated to be 96 percent. Based on the overall system efficiency of 7.5 percent, the concentrator solar cell configuation has to be designed to generate 6.6-6.7 GW of electrical power for the SPS at the end of mission life. The GaAlAs solar cells were assumed to be operated at sufficient temperatures to be self-annealing from the effects of ionized radiation degradation for the 30-year mission life. If the solar cells are not self-annealing, then 22 to 30 percent more solar cell area has to be added for a 30-year mission dr the solar cell blankets have to be replaced every 5 to 15 years, depending on weight and cost tradeoffs. The SPS also is sized for the nominal distance from the sun of 1 AU. However, there is a ± 3.3-percent variation in the solar constant as the earth rotates around the sun because of the eccentricity of the earth's orbit. During the winter solstice, the solar constant is 3.3-percent higher than the nominal and during the summer solstice the solar constant is 3.3-percent lower than the nominal of 135.3 mW/cm2, 2.2.2 Solar Cells and Blanket GaAlAs solar cells were selected as the reference cells in the study. A preliminary specification sheet for the SPS solar cell and blankets, indicating the design parameters, operating conditions, and weights, is presented in Table 2.2-1. The solar energy spectrum and the response of the GaAlAs and Si solar cells at AMO are shown in Figure 2.2-2.
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