(2) Solar Thermionic, Actively Cooled In this configuration a liquid metal cooling loop is used to remove waste heat from the diode collectors. In effect, the coolant loop couples the diodes to a greater radiating area than is practical for fins directly attached to the diodes, thereby producing a lower collector temperature, a greater temperature differential across the diode and greater electrical output. Thus the diodes are more efficient, so that fewer diodes are required. However, active cooling uses power drawn from the diodes and requires a liquid metal loop with thermal radiators. Rotary converters are again used to step-up the diode output voltage. (3) Solar Brayton Cycle The Brayton cycle turbomachine provides a rotating shaft output which drives the generators. Thermal energy is added to the helium working fluid in heat exchanger tubing located within the cavity absorber. The hot gas is expanded through the turbine, providing power to turn the generator. A recuperator exchanges energy across the loop to increase the system efficiency. Waste heat is rejected through a gas-to-liquid heat exchanger to a liquid metal cooling loop; the liquid metal pumps use power drawn from the generators. The 50,000 volt ac output of the generators is stepped-up to 382,000 volts in transformers since this high voltage facilitates on-board distribution. Step-down to the 20,000 volts required by the transmitter occurs in rotary transformers. (4) Solar Thermionic/Brayton Cycle This "cascaded” system offers potentially high efficiency. All waste heat from the thermionic diodes is available to the Brayton cycle; the diodes are cooled by the helium flow in the Brayton loop. The Brayton loop is cooled by a liquid metal radiator. The de output of the diodes is stepped-up to 50,000 volts ac in the rotary converters; the turbomachine generators produce 50,000 volts ac which is combined with the output of the rotary converters.
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