Pergamon Energ? Con~w-s. Mgmt Vol. 38. No. 6. pp. 533-549. 1997 8 1997 Elsevier Science Ltd PII: SO196-8904(96)00067-2 Printed in Great Britain. All rights reserved 0196-8904/97 $17.00 + 0.00 A REVIEW OF CESIUM THERMIONIC CONVERTERS WITH DEVELOPED EMITTER SURFACES DMITRY V. PARAMONOV and MOHAMED S. EL-GENK Institute for Space and Nuclear Power Studies, University of New Mexico, Albuquerque, NM 87131, U.S.A. (Received 23 September 1995) Abstract-Studies from U.S. and Russian literature on experimental and theoretical investigations of thermionic converters with developed emitter surfaces are reviewed and analyzed. Experimental data for these converters is compared with that for close-spaced and traditional Cs arc converters. The former is most advantageous at low temperatures from 1300 to 1700 K, with the power gain increasing as the emitter temperature decreases and/or its emission properties deteriorate. This power gain is attainable at a larger load voltage and significantly larger optimum interelectrode gap size than traditional converters. Additional experiments and mathematical modeling are needed to understand the physical processes taking place and assess the performance of thermionic converters with developed emitter surfaces as compared to traditional ones operating under identical conditions. 0 1997 Elsevier Science Ltd. All rights reserved Thermionic converter Developed surface emitter Work function Plasma voltage drop INTRODUCTION In addition to space electric power systems, thermionic energy conversion is being considered for terrestrial applications, such as topping cycles in fossil power plants, electric power systems for remote sites [ 1,2] and domestic heating and electric power supply [3]. One main limitation on using thermionic conversion for terrestrial applications is the high emitter temperature (1900 K or more) in traditional arc mode thermionic converters. Such high temperatures require using a high temperature heat source and refractory materials, which limit the operational lifetime of the converter. If thermionic converters can be made to operate efficiently at lower temperatures, their potential applications will greatly increase. Proposed ways for achieving this goal include using developed surface electrodes, non-traditional electrode materials with superior emission characteristics, closed-spaced converters, or converters with pulsed or auxiliary discharge capability [4-6]. The power density and conversion efficiency of a thermionic converter operating in arc mode can be increased by either reducing the voltage drop in the interelectrode gap associated with volume ionization or increasing the discharge current while keeping the emitter temperature and cesium pressure the same. Early studies of cesium filled converters, which operated in the pulse mode or employed an auxiliary discharge, showed that plasma voltage losses were lower than in well-optimized traditional cesium arc converters. In these operation schemes, external power is needed to induce sufficient ionization and provide space charge compensation. Performance improvement due to internal ion generation, but without an external energy source, is desired. This can be achieved in converters employing developed surface emitters. These converters separate the processes of space charge neutralization and load current generation. The small cavities formed in the emitter surface provide an effective means for generation of ions without additional external power, thus reducing the voltage drop in the plasma and effectively increasing the emission from the emitter surface, Using developed surface emitters appears promising in thermionic converters operating at low emitter temperatures, which are applicable to power systems with low temperature heat sources or in converters used for a topping cycle. 533