The solar array area is 88 km2, the antenna diameter is 1 km, and the system mass is approximately 117 x 106 kg, which includes a contingency of 30 percent. Alternate photovoltaic configuration concepts are shown in Figure 5-2. These concepts feature two antennas, one on each side of the SPS or at each end, and in each option the antennas are nominally 5 GW each. A side location minimizes distribution losses but causes an attitude control propellant increase, because this offsets the advantage of having the major axis of the elliptical shaped SPS perpendicular to the orbit plane for minimum attitude control propellant. Two antennas, when used to transmit 5 GW each of power to the ground, will reduce the power density to one-half as compared to transmission of this power by one antenna of the same size. This is potentially a desirable configuration change, since transmission of 10 GW of power with a 1 km diameter antenna from a geosynchronous Earth orbit at a frequency of 2. 45 GHz results in a peak power density at the center of the rectenna above 20 mW/cm2. For the transmission of 10 GW of power, two complete 5 GW power systems are lower in weight than a single power system with two antennas because of the large distribution losses for the larger solar array when distributing power at 20 kV. This conclusion could change if the distribution voltage was raised to 40 kV. In any configuration, each transmitting antenna has a companion ground rectenna. An end location of the antennas is shown by the configuration at the bottom of Figure 5-2. This antenna location does effect minimum attitude control propellant, but distribution voltage must be high to reduce distribution power loss. 5. 2 SOLAR THERMAL CONCENTRATOR A typical solar thermal concentrator configuration is shown in Figure 5-3. This configuration consists of 544 independent modules sized to minimize demonstration costs by the delivery of individual components to Earth orbit by the shuttle. The number of independent modules for solar thermal concepts which have been studied ranges from 4 to approximately 550. Recent optimization activities indicate the appropriate number of independent modules would be nearer the small end of the spectrum. AC power is distributed from the modules of Figure 5-3 to antenna locations, where the power is converted to de and transmitted to the ground. Each power module consists of a concentrator, with a concentration ratio of approximately 2000; a ''light pipe" absorber
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