following table, reproduced from Ref. A17, indicates the status and requirements for thermionic converter efficiency: Efficiency at emitter temperature of 1800K Barrier Index* (2,780°F) and collector Year (electron volts) temperature of 900K (1,160 F) 1960 3.0 0.03 1970 2.4 0.10 1975 2.1 0.15 1985 (projection) 1.6 0.24 1995 (projection) 1.2 0.36 Ref. Al 7 states that this is based on "historic trends in converter efficiency improvement along with a projection based on predictions of authorities in the field." Ref. A4 reports that efficiencies of 14-16% have been achieved. Since the electrodes of a thermionic diode must, of necessity, operate at high temperatures, operating temperature becomes a prime consideration. Ref. A4 indicates that thermionic diodes currently operate in the range of 2750-2900°F (1800-1850°K). Since efficiency is a function of temperature, Ref. A4 indicates that new or improved electrode materials will be required to achieve the emitter operating temperatures of 2050°F (1400 K) or lower necessary for the projected'—40% efficiency requirement. For the solar-thermal SPS concepts utilizing the Brayton cycle, a helium working fluid is heated via a solar concentrator/cavity absorber/ heat exchanger arrangement, and the hot gas is expanded through a turbine, which in turn drives a compressor and generator. In the cascaded solar thermal concept, thermionic diodes are added for turbine electrical drive. Such Brayton-cycle approaches have been under development for several years for both space and terrestrial applications, and Ref. A4 reports that a space-derived 10 KWe engine, using a helium-xenon mixture as the working fluid, has accumulated over 17,000 hours of operating time. This system ♦Barrier index is the difference between the ideal and actual electrode to electrode voltage of the thermionic diode.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==