1.1.3 Power Distribution The Power Distribution System (PDS) controls, conditions, and transmits power from the conversion system to the microwave transmission system. It also provides energy storage for eclipse periods and handles fault detection and isolation. Major PDS subsystems are discussed below. 1.1.3.1 Main Buses Four options were considered for the main power buses: sheet aluminum conductors, refrigerated buses, superconducting buses, or microwave transmission. The sheet aluminum option requires essentially no non-lunar material, and resistance losses can be made arbitrarily small by increasing the width or thickness of the bus. It is also the simplest to manufacture. Thus, the sheet aluminum option was selected. 1.1.3.2 DC-DC Converters Previous studies(l,2,5) have used an oscillator-transformer-rectifier system for DC to DC conversion at the microwave antenna. Transformers are about 98* efficient, making the overall conversion process about 96* efficient. Transformers require active cooling, so non-lunar material is needed for pumps and coolant. Modern power electronics provide DC-DC conversion at over 98* efficiency without using transformers.(6) This increased efficiency reduces non-lunar mass needed for cooling and reduces the mass of the power conversion system, so this conversion technology was selected. 1.1.3.3 Energy Storage Stored energy is required during eclipses to maintain essential systems and to keep RF amplifier cathodes hot. Three energy storage technologies were considered: nickel-hydrogen, sodium chloride, and counter-rotating flywheels wrapped with glass fiber. The flywheel system requires much less non-lunar material per stored kilowatt-hour than the other systems. The energy density of a glass-wrapped flywheel was conservatively estimated from the best demonstrated performance of a steel flywheel. 1.1.3.4 Electrical Slipring All major SPS studies to date have used a slipring-and-brush assembly as the electrical rotary joint. Typically these designs have used silver molybdenum sulfide brushes on coin silver to minimize abrasion, drag, and electrical losses. However, this calls for a large mass of non-lunar materials. The silver in the sliprings (10.6 metric tons for a 5 GW SPS) could cause supply problems on Earth. To decrease the mass of non-lunar material in the electrical slipring, a thin plating of coin silver on an aluminum slipring was selected. This design reduces the requirement for a 5 GW SPS from 11.8 metric tons of coin silver to 1.2 metric tons and gives equivalent electrical performance with less mass. It also reduces the total slipring mass.
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