SMALLER SPS SYSTEM SIZING TRADEOFFS G. D. Arndt and L. Monford NASA-Johnson Space Center Houston, TX Introduction- The present solar power satellite (SPS) system was optimized to provide 5~GW of electrical power at the ground using a 1 Km diameter antenna and a 10 Km diameter rectenna. This antenna sizing and maximum power transmission were determined by two constraints; a 23 KW/m2 thermal limitation in the transmit antenna and a 23 mW/cm2 maximum RF power intensity in the ionosphere. This paper considers technical and economic tradeoffs of smaller optimized SPS systems configured with larger antennas, reduced output powers, and smaller rectennas. The advantages of smaller systems are two-fold: (1) commercial utility companies prefer to integrate lower power levels into their grids, and (2) smaller rectenna sizes than the 10 Km diameter reference configuration may be preferred from a land utilization and site location viewpoint. The differential costs in electricity for seven antenna/rectenna configurations operating at 2.45 GHz and five satellite systems operating at 5.8 GHz have been determined and are described in detail in a report to be published. Because of space limitations only the results are summarized in this paper. Microwave Systems and Cost Considerations The thermal limitation at the center of the transmit antenna is due to the heat radiated by the DC-to-RF power converters, i.e., klystrons. The present configuration has 72 KW klystron tubes operating at 85% conversion efficiency and cooled by passive heat - pipe radiators. This thermal limitation is a severe constraint on higher frequency (5.8 GHz) systems which have lower efficiency klystrons (80%) and smaller antenna areas. An improved thermal design using graphite composite materials with high emissivity coatings which provide a 33% increase in heat rejection is proposed in the report and used in these calculations. The ionospheric power density limitation, a critical parameter in the 2.45 GHz systems, is to prevent possible nonlinear interactions between the ionosphere and the power beam. These nonlinear heating effects are of concern because of possible disruptions in low frequency communications and navigation systems produced by radio frequency interference (RFI) and multipath effects. Theoretical studies of the ionosphere completed in the early phases of the SPS evaluation program indicated the power density should be limited to 23 milliwatts per square centimeter or less in order to prevent nonlinear heating effects. This theoretical value, 23 mw/cm2, was taken as the SPS design guideline. Subsequent ionospheric heating tests conducted at Plattville, Colorado, and Arecibo, P. R. during the past year (the results of which are reported elsewhere at this conference) have indicated this 23 mw/cm2 threshold may be too low. The 2.45 GHz downlink power beam frequency is in the center of a 100 MHz IMS (Industrial, Medical and Scientific) band which allows users to interfere with other users in that frequency region. This 2400-2500 MHz band is not particularly affected by weather conditions and an SPS system should not suffer weather outages. Another IMS band (5800 + 75 MHz) is also available for possible SPS usage. However an SPS system operating in this frequency region may have to be shut down under very poor weather conditions. The microwave systems were resized with higher gain antennas and considering various ionospheric and thermal power density limitations. A 10 dB gaussian antenna illumination provides maximum rectenna collection efficiency while minimizing sidelobe levels. Other illumination tapers were investigated but the 10 dB gaussian was the most efficient as was true for the reference SPS system.
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