SPS SILICON REFERENCE SYSTEM Gordon R. Woodcock Boeing Aerospace Company P.O. Box 3999, Seattle Wash 98124 The silicon reference SPS is one of two reference designs developed by the NASA Systems Definition Studies, the other being a Gallium Arsenide option. The JSC/Boeing study emphasized silicon and the MSFC/Rockwell study emphasized Gallium Arsenide. These two options provide a balance between a more mature, relatively well-understood photovoltaics technnology and a more advanced one which offers performance advantages and possibly cost advantages. The composite drawing of Figure 1 illustrates the main features of the silicon reference system. The solar array consists of glass-encapsulated 50-micrometer silicon solar cells, interconnected in series-parallel arrangement to provide the necessary voltage and cell failure redundancy. The array blankets are suspended in a space-frame cubic trusswork of 128 bays, each 667.5 meters square and 470 meters deep. A tension catenary system maintains the solar oell blanket in each bay adequately flat, with a "trampoline” natural frequency about twice that of the solar array support structure as a whole. The array area of 49*6 square kilometers generates 8766 megawatts (de) electric power at 44 kV. This electric power is conducted by an arrangement of ten pairs of power busses to the electrical slip ring interface between the power transmitter. The transmitter converts the electric power to 6700 megawatts of radiated RF power at 2450 megahertz. Details of the solar blanket are illustrated in Figure 2. The individual solar cells are about 50 square centimeters. The size was selected to be compatible with the electrical arrangement of the solar array depicted in Figure 3 and the number of cells in series (about 77,000) required to deliver the required voltage. The cells are encapsulated in panels roughly one meter square and the panels are interconnected by welded flexible tabs. Each panel incorporates a pair of shunting diodes to protect it from reverse voltage in the event of shadowing. Panels are mechanically interconnected by glass fiber tape and are folded or rolled in shipping containers for delivery to space. Figure 3 also shows the general scheme for power disrtribution to the array-antenna interface. Sets of solar cell strings are connected to the satellite main busses at the 2000-amp level through sets of switchgear to provide fault protection as well as isolation of strings of cells for annealing. The main busses are 1-mm aluminum. Passively-cooled conductors of this nature become lighter as they are made thinner. The 1-mm figure provides reasonable minimum gauge and conductor width. The conductors are supported below the solar array by secondary structure. Silicon solar cells degrade in the space environment as a result of ionizing radiation. The principal source of damage is solar protons from flare events. Prediction of the amount of degradation is complicated by the statistical nature of flare phenomena as well as by problems in extrapolating available proton spectral data to the 2 to 10-MeV energy range that will cause most of the damage. There is also some uncertainty in the amount of degradation to be expected at any given fluence as well as uncertainty in converting from test results (usually isoenergetic) to the solar proton spectrum. The Boeing studies used a generally pessimistic radiation model (more fluence than the expected value) and measured proton damage data for experimental 50-micrometer cells. The result was an estimate of 30% output loss for the silicon satellite at the end of a 30-year ”book life” period* The reference system therefore includes an in-situ annealing system that would be used every few years to restore array performance. Characteristics of the annealing system were based on extrapolation of results of preliminary experimental laser annealing of proton-damaged 50-micrometer solar cells.
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