5.0 DEFINITION OF SPS CONCEPTS 5.1 SOLAR PHOTOVOLTAIC The initial photovoltaic concept developed for study of the SPS is illustrated in Figure 5-1. Overall dimensions for this configuration, designed to supply 10 GW of power to the utility interface, are a maximum length of 21.05 km, a width of 9. 53 km, and a structural depth of 0. 215 km. The antenna is located at the center of the configuration to minimize distribution losses. With the selected 20 kV de distribution voltage, these losses are a greater mass factor than the nonmetallic carry-through structure at the center of the antenna. The significant mass influence of power distribution at 20 kV can be effectively eliminated by distributing power at 40 kV. The nonmetallic structure could possibly be eliminated by using the power distribution mast as the central load carrying structure. The solar array of Figure 5-1 is sized for a perpendicular attitude with respect to the Sun. This minimizes the solar array area and, in turn, the mass of the array. However, the mass of the attitude control propellant, the complexity of the rotary joint to correct for the rotation (approximately 23°) of the antenna with respect to the rectenna, and the complexity of the attitude control system to maintain solar pointing are increased. Early study results indicated that comparison of the solar perpendicular attitude to a perpendicular to orbit plane attitude results in comparable overall system masses; however, the lower complexity of the perpendicular to orbit plane attitude and later study mass estimates suggest a trend toward this attitude for future configurations. The solar array consists of trapezoidal shaped modules that are 493 m square at the top and 215 m deep. The sides of the trapezoid are typically aluminized film reflectors that provide a concentration ratio of two for the solar cells located at the base of the trapezoid. The solar cells are passively cooled. The planform shape of the photovoltaic configuration is elliptical. This shape is an efficient structural configuration with regard to the distribution of large quantities of current to the geometric center and, when analyzed for attitude control propellant consumption, results in a slightly lower mass system in comparison to an optimized rectangular configuration, because the elliptic shape is widest at the center where the structural bending moment is greatest. Pertinent descriptive data for this configuration are shown in Table 5-1 for a two antenna configuration.
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