Performance comparison of potassium and sodium vapor anode, multi-tube AMTEC converters

An analysis is conducted to compare the performance of Mo-41%Re, eight-tube, potassium and sodium alkali metal thermal-to-electric converters of identical design. The results showed that the operation envelope of the potassium converter is severely limited by incipient dryout in the evaporator wick due to the lower surface tension and higher molecular weight of potassium compared to sodium. As a result of this operational limit, at the same load voltage, the potassium converter delivers lower electrical power and has a lower conversion efficiency than the sodium converter. Moving the evaporator wick surface further away from the hot plate (and shortening its total length) expands the operation envelope of the potassium converter by raising the incipient dryout limit. At the same load voltage, the potassium converter with the remote evaporator wick can deliver the same electrical power as the reference sodium converter, while operating at a 50 K lower hot side temperature and a 90 K lower optimum condenser temperature. When operating at a hot side temperature of 1174 K and 2.7 V, the potassium converter with the remote evaporator wick delivers a peak electrical power of 8.5 W-e at an efficiency of 17.4%, compared, respectively, to 6.2 W-e and 16.4% for the reference potassium converter, and 7.3 W-e and 15.5% for the reference sodium converter. (C) 2002 Elsevier Science Ltd. All rights reserved.