7.0 SATELLITE POWER STATION 7.1 PHOTOVOLTAIC POWER CONVERSION SYSTEM 7.1.1 REQUIREMENTS AND ANALYSIS The SPS solar array is required to produce sufficient power output to deliver 10 GW to a ground station designated to receive, condition, and transmit the power to the utility interface. This photovoltaic system design allows for typical solar array degradation and will provide the required output power without maintenance for 5 years. After 5 years, the array output will be maintained by repair and maintenance operations as the need arises. The system shall be capable of satisfactory performance for at least 30 years. The overall efficiency of the photovoltaic concept, excluding solar cells, is currently calculated to be 52 percent; this efficiency chain is shown in Figure 7-1. The solar cell output power of 19.4 GW encounters losses in the electrical connections, the power conductors, the rotary joints, and in the operating power to the SPS subsystems. The microwave system receives 17. 3 GW for conversion and transmission. After reception and dc-RF conversion, 10 GW at low voltage is available from the de power grid at the ground site. The dashed block of Figure 7-1 shows unconfirmed ground system losses estimated as discussed in section 8.1. The low conversion efficiency of photovoltaics makes the solar array size highly sensitive to all system losses. Distribution of large quantities of power across the spacecraft is inherent to the SPS and indicates that high voltage distribution is desirable to minimize losses in conductors. Selection of distribution voltage has little effect on array configuration, although losses cause increases to the solar array area required. Power distribution and control are discussed in subsection 7.1.4. To evolve the appropriate geometric solar array configuration, an early decision must be made on the concentration ratio of the solar energy collection scheme. Integral to the concentration ratio selection process is the consideration of how structural weight, solar blanket weight, and, ultimately, the cost can be minimized. Although the results are not conclusive for all concentration ratio geometries, some trades have been completed (Fig. 7-2) that indicate the concentration ratio should be near two for the geometry considered in the study thus far.
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