B. Solar Energy Collection System The SECS includes the necessary elements for the collection and conversion of sunlight to electrical power, the distribution of that power to the antenna interface, structural loads, and attitude and orbit control. A preliminary analysis indicated that the most promising conversion systems from the standpoint of current state of development were the photovoltaic silicon solar cell and the thermodynamic Brayton cycle. It was also recognized that more efficient and advanced systems might be required to establish SPS viability. For the purpose of the immediate study, however, systems effort was concentrated on the photovoltaic silicon solar cell approach to provide a departure point for comparative evaluation with other approaches in future studies. 1. Solar Array Silicon solar cells have been developed and utilized in spacecraft for a number of years. More recently, under the impetus of proposed terrestrial use, an intensive effort has been initiated to improve the efficiency and reduce the cost of silicon cells. Typical characteristics of space operational solar arrays and those projected to result from the present development efforts for Earth use are as follows. For the purposes of the present study, it has been estimated that efficiencies of 15 to 17 percent at 30° C are achievable within the projected SPS time frame. Cost and weight of the total solar array can be reduced by concentrating the sunlight so that the entire area need not be covered with solar cells. Accordingly, a parametric study of performance as a function of concentration ratio for both silicon (Si) and gallium arsenide (GaAs) cells was performed. It was found that GaAs becomes cost competitive only above concentration ratios of 4 to 6. At these ratios, the solar array requires relatively complex structure and must be oriented toward the Sun more accurately to avoid excessive losses. Silicon cells were used, at a concentration ratio of 2, as a reference for the current study. At this ratio, a simple trough can be used (fig. IV-8). Nominal conversion efficiency, including losses within the cell blanket, is estimated at 10.3 percent for the 100° C cell temperature expected with 2:1 concentration. Cell degradation due to radiation damage and thermal cycling is expected to be a total of 6 percent for the first 5 years and 1 percent/yr thereafter.
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