IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991 1191 Thermionic Energy Conversion Plasmas Ned S. Rasor, Senior Member, IEEE (Invited Review Paper) 5, T, Abstruct- The history, application options, and ideal basic C performance of the thermionic energy converter are outlined. The basic plasma types associated with various modes of con- EMITTERn verter operation are described, with emphasis on identification and semi-quantitative characterization of the dominant physical processes and utility of each plasma type. The frontier plasma HEAT IN HEAT OUT science issues in thermionic converter applications are briefly qE - -qc summarized. I. INTRODUCTION HE thermionic energy converter is a nonmechanical ELECTRON CURRENT J Tgaseous-electronic device for converting heat directly into electric power by thermionic electron emission. In its LOAD simplest form-i.e., the diode shown schematically in Fig. l-electrons are emitted from a hot electrode and collected OUTPUT POWER = qE - qc by a colder electrode at a higher potential energy (lower electrical potential). Part of the heat removed from the emitter Fig. 1. Basic thermionic energy converter. by the evaporating electrons is rejected to the collector by the condensing electrons, and the remaining part is converted into of scientific issues addressed, the detailed qualification and electric power in the load as the electrons return to emitter documentation required for a comprehensive review is not potential. possible here. Key publications are cited, however, which in- Although thermionic converter operation usually is de- clude comprehensive review of and reference to the supporting scribed in physical electronic terms (i.e., as a plasma- detail. electron tube), the heat-power engineer may prefer to consider Table I summarizes the variety of technological options and thermionic energy conversion as a thermodynamic heat applications associated with thermionic converters. This article engine cycle. The thermionic cycle is similar to a modified is concerned primarily with the physics of the various types of Rankine (steam engine) cycle that uses electrons directly plasma employed to conform to the constraints of particular as the sole working fluid [l]. The emitter is the “electron options and applications. The various plasma types summa- boiler” and the collector is the “electron condenser,” which rized in Table I1 will be described and their characteristic develop an electrical pressure (potential) difference to produce advantages identified with particular applications. electrical work rather than a vapor-pressure difference to produce mechanical work. Although the efficiency of the ideal thermionic converter at practical power densities is about 11. BRIEFHISTORICAL PERSPECTIVE 60% of the Carnot thermodynamic limit, the technology on The history of thermionic energy conversion and its appli- which present applications is based achieves about 25-35% of cations is summarized briefly in Table 111. The possibility of Carnot efficiency. Advanced thermionic converter types have using electron emission for energy conversion was recognized been investigated that can more closely approach the ideal by sev+eral scientists and inventors after the discovery of converter-efficiency limit. thermionic emission by Edison in 1885, the discovery of the The purpose of this article is to provide a perspective electron by Thomson in 1897, and the quantitative physical of the nature and scientific characterization of the unique description of thermionic emission by Richardson in 1902. and remarkable plasmas occurring in the various modes of The confluence of high-temperature materials technology, the thermionic converter operation associated with different appli- development of nuclear heat sources, and the emerging need cation requirements. Because of the large number and variety for efficient and compact electrical power sources in space in Manuscript received September 26, 1989; revised August 21, 1991. This the mid-1950’s led to the first experimental demonstration of work was partially supported by the U.S. Department of Energy, and by the practical levels of thermionic power generation by Marchuk Strategic Defense Initiative Office of the U.S. Department of Defense through in the USSR in 1956, demonstration of the more practical the U.S. Air Force Wright Aeronautical Laboratories. The author is with Rasor Associates, Inc., Sunnyvale, CA 94089. ignited mode of converter operation by Wilson in the U.S. in IEEE Log Number 9104419. 1957, and demonstration of converter operation in the core 0093-3813/91$01.00 0 1991 IEEE 1192 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991 TABLE I SUMMARYOF THERMIONIC CONVERTEROFTONS APPLICATIONS: Envi ronmnt Nuclear In-core/Ex-core/On-core Nuclear Radioisotope reactor Moderated/Fast Fossil fueled Topping Cogeneration Auxi 1 iary power Solar __-___ Power conditioning TABLE I1 MATRIXOF THERMIONICCONVERTER PLASMA TYPES PLASMA ION SOURCE I VOLUME SURFACE INJECTION I " TRIODES SPACINGiMFP RATIO d/X of a nuclear reactor by Grover et al. in 19.59, all apparently objectives and was planning to construct a test reactor. The acting independently. USSR, however, began ground testing its low-power TOPAZ Early consideration was given to the use of thermionic thermionic reactors in 1970, and ground-tested eight versions converters in solar and radioisotope space-power systems. by 1983. By 1965 it was clear that the emerging thermionic technol- In 1Y73 the US. terminated its entire space nuclear-reactor ogy could not displace the well-established photovoltaic and program, including thermionic reactor development. From thermoelectric technologies in these applications. By 1965, 1973 to 1983 U.S. thermionic technology development was however, the basic physics of the ignited mode of converter directed toward application to fossil-fueled terrestrial power operation was understood sufficiently, and the practical tech- systems. Since performance in the ignited mode of operation nology was developed sufficiently, to permit initiation of that had been used in the reactor application was marginal at engineering development of in-core thermionic nuclear reactor the lower temperatures required for fossil-fueled applications, systems in the US, USSR, West Germany, and France. basically new types of converter operation were developed The multiple modular redundancy and high heat rejection during this period, involving advanced electrode and plasma temperature capabilities of in-core thermionic space reactor technology. systems give them inherent and decisive advantages over In 1983 work was resumed in the US. on space nuclear- turboelectric systems in reliability, development cost, and reactor systems in response to the greatly increased electric system weight. power requirements of prospective military space-based sys- The US. and USSR took substantially different approaches tems. Major emphasis has been placed on recovering the 1973 in thermionic reactor development. The former concentrated thermionic fuel element technology and extending it to meet on perfecting the thermionic technology through interative the 7-10 y lifetime requirements projected for a variety of development tests of thermionic fuel element (TFE) modules. military systems [2]. Alternate approaches also were explored The USSR essentially froze the thermionic technology at for applying advanced converter technology to innovative the 196.5 level and proceeded immediately to construct full- thermionic reactor systems [3], and for applying advanced scale thermionic reactors for testing. By 1973 the US. had very-high-temperature nuclear fuels to achieve the high power achieved its thermionic fuel element lifetime and performance densities required in multimegawatt applications [4]. RASOR: THERMIONIC ENERGY CONVERSION PLASMAS 1193 TABLE Ill HISTORICALSUMMARY OF MAJORTHERMIONIC ENERGY CONVERSION PROGRAMS US THERMIONIC PROGRAM BASIC PHYSICS ELEMENTARY CESIUM IGNITED CESIUM DIODE EXPLORATION DIODE CONSOLIDATIO SOLAR,RADIOISOTOPE THERMIONIC NUCLEAR FOSSIL-FUELED APPLICATIONS & NUCLEAR REACTOR FUEL ELEMENT (TFC) SYSTEM EXPLORATION DEVELOPMENT I I II I 1957 1965 1973 1975 1983 IYM5 1990 USSR THERMIONIC PROGRAM (ESTIMATED) c~~~~~~~~~~v~FB~~~~u~~~~~~L TOPAZ REACTOR TOPA7 FLIGHT TEST TOPA7 SOLAR, RADIOISOTOPE TOPAZ EXPERIMENTS SYSTEM DEVFLOPMENT DEPLOYMENT APPLICATIONS e NUCLEAR REACTOR REACTOR ' REACTOR DEW. e TCST EXPLoRAT1oN DEVELoP"ENT TFE DEVELOPMENT HIGH POWER IIEACTOIt EXPLORATION WESTERN EUROPEAN PROGRAMS -1 I IGNllED MODE IGNITED MODE4 BASIC PHYSICS IGNITED CESIUM DIODE CONSOLIUATION' CERMET EMITTER' EXPLORATION 1 41 THERMIONIC NUCLEAR' FOSSIL FUELED APPLICATIONS F02~'~L{~~~~U3 FUEL LLEMENT (TFE) DIODE DEVtLOPMENT5 DEVELOPMENT 1 1 POWER 1 I 1 -73 1965-73 19/75 1482 1985 1990 1- W. Geniiany, FranLe, Eurdtoill 4- Sweden 2- W. Gerniany, Netherlands 5- Netherldndr 3- W. Gennany In 1987 and 1988 the USSR announced operation and testing in earth-orbit of two of its 6 -kW TOPAZ thermionic reactor systems [5]. The USSR identified thermionic reactor systems as the basis for fulfilling its future space electric- power system requirements into the megawatt region [6]. Because of the continuity and central coherence of its scientific support, the basic physical research on thermionic energy conversion in the USSR has been more coherent, more thorough, and much more completely documented in scientific publications than that in the U.S. The U.S.'s basic work, being tied primarily to a variety of transient