Thermionic converter

A thermionic converter consists of a hot electrode which thermionically emits electrons over a potential energy barrier to a cooler electrode, producing a useful electric power output. Cesium vapor is used to optimize the electrode work functions and provide an ion supply (by surface contact ionization or electron impact ionization in a plasma) to neutralize the electron space charge.

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Definition

From a physical electronic viewpoint, thermionic energy conversion is the direct production of electric power from heat by thermionic electron emission. From a thermodynamic viewpoint (1)it is the use of electron vapor as the working fluid in a power-producing cycle. A thermionic converter consists of a hot emitter electrode from which electrons are vaporized by thermionic emission and a colder collector electrode into which they are condensed after conduction through the interelectrode plasma. The resulting current, typically several amperes per square centimeter of emitter surface, delivers electrical power to a load at a typical potential difference of 0.5–1 volt and thermal efficiency of 5–20%, depending on the emitter temperature (1500–2000 K) and mode of operation. Details of the history, science and technology of thermionic energy conversion can be found in books on the subject (2, 3).The summary here is brief but more current.

History

After the first demonstration of the practical arc-mode cesium vapor thermionic converter by V. Wilson in 1957, several applications of it were demonstrated in the following decade, including its use with solar, combustion, radioisotope and nuclear reactor heat sources. The application most seriously pursued, however, was the integration of thermionic nuclear fuel elements directly into the core of nuclear reactors for production of electrical power in space (4, 5). The exceptionally high operating temperature of thermionic converters, which makes their practical use difficult in other applications, gives the thermionic reactor decisive advantages over competing energy conversion technologies in the space power application where radiant heat rejection is required. Substantial thermionic space reactor development programs were conducted in the U.S., France and Germany in the period 1963-1973, and the US resumed a significant thermionic nuclear fuel element development program in the period 1983-1993.

A massive thermionic reactor development program was conducted continuously in the USSR throughout the period 1960-1989, during which a full-scale thermionic reactor system was developed and first tested in 1972. Two thermionic reactor power systems (TOPAZ) were orbited and operated in space in 1988-1989.

Although the priority for thermionic reactor use diminished as the US and Russian space programs were curtailed, research and technology development in thermionic energy conversion have continued. In recent years technology development programs for solar-heated thermionic space power systems were conducted. Prototype combustion-heated thermionic systems for domestic heat and electric power cogeneration, and for rectification, have been developed (6).

Description

The scientific aspects of thermionic energy conversion primarily concern the fields of surface physics and plasma physics. The electrode surface properties determine the magnitude of electron emission current and electric potential at the electrode surfaces, and the plasma properties determine the transport of electron current from the emitter to the collector. All practical thermionic converters to date employ cesium vapor between the electrodes, which determines both the surface and plasma properties. Cesium is employed because it is the most easily ionized of all stable elements.

The surface property of primary interest is the work function, which is the barrier that limits electron emission current from the surface and essentially is the heat of vaporization of electrons from the surface. The work function is determined primarily by a layer of cesium atoms adsorbed on the electrode surfaces (7). The properties of the interelectrode plasma are determined by the mode of operation of the thermionic converter (8). In the ignited (or “arc”) mode the plasma is maintained via ionization internally by hot plasma electrons (~ 3300 K); in the unignited mode the plasma is maintained via injection of externally-produced positive ions into a cold plasma; in the hybrid mode the plasma is maintained by ions from a hot-plasma interelectrode region transferred into a cold-plasma interelectrode region.

Recent work

All the applications cited above have employed technology in which the basic physical understanding and performance of the thermionic converter were essentially the same as those achieved before 1970. During the period 1973-1983, however, significant research on advanced low-temperature thermionic converter technology for fossil-fueled industrial and commercial electric power production was conducted in the US, and continued until 1995 for possible space reactor and naval reactor applications. That research has shown that substantial improvements in converter performance can be obtained now at lower operating temperatures by addition of oxygen to the cesium vapor (9, 10), by suppression of electron reflection at the electrode surfaces (11), and by hybrid mode operation. Similarly, improvements via use of oxygen-containing electrodes have been demonstrated in Russia along with design studies of systems employing the advanced thermionic converter performance (12).

See also

• Atomic battery
• Betavoltaics
• Optoelectric nuclear battery
• Radioisotope piezoelectric generator
• Radioisotopic Thermoelectric Generator

References

1. N. S. Rasor, "Thermionic energy converter," in Fundamentals Handbook of Electrical and Computer Engineering, vol. II, S.S.L. Chang., Ed., New York: Wiley, 1983, p. 668.

2. G. N. Hatsopoulos and E. P. Gyftopoulos, Thermionic Energy Conversion, vol. I, (1973); vol II, (1979); MIT Press, Cambridge, MA.

3. F.G. Baksht, et al., Thermionic Converters and Low-Temperature Plasma, Russian Edition (B. Moyzhes and G. Pikus, Eds), Acad. of Sciences USSR, Moscow, 1973. English Edition (L.K.Hansen, Ed.) available as DOE-tr-1 from NTIS, Springfield, VA.

4. J. Mills and R. Dahlberg, “Thermionic Systems for DOD Missions”, Proc. 8th Symp. on Space Nucl. Power Syst., (Albuquerque, NM), pt.3, p. 1088.

5. G. M. Griaznov, et al., “Thermoemission Reactor-Converters for Nuclear Power Units in Outer Space”, Atomnaya Energiya 66, 371-383 (1989); English translation available from Plenum.

6. E. van Kemenade & W. B. Veltkamp, “Design of a Thermionic Converter for a Domestic Heating System”, Proc. 29th Intersoc. Energy Conv. Eng. Conf., Vol. 2, p1055 (1994). Also see V.I. Yarygin, Ye. A. Meleta, V.V. Klepikov, V.A. Ruzhnikov, & L.R. Wolff, “Test of a TEC-Module”, ibid, p1061.

7. N. S. Rasor and C. Warner, “Correlation of Emission Processes for Adsorbed Alkali Films on Metal Surfaces”, J. Appl. Phys. 35, 2589 (1964).

8. N. S. Rasor, “Thermionic Energy Conversion Plasmas”, IEEE Trans. Plasma Sci., 19, 1191 (1991); invited review.

9. N.S. Rasor, “Physical-Analytical Model for Cesium/Oxygen Coadsorption on Tungsten”, Proc. 27th Intersoc. Energy Conv. Eng. Conf., Vol.3, p3.529 (1992).

10. J-L. Desplat, L.K. Hansen, G.L. Hatch, J.B. McVey and N.S. Rasor, “HET IV Final Report”, Volumes 1 & 2, Rasor Associates Report #NSR-71/95/0842, (Nov. 1995); performed for Westinghouse Bettis Laboratory under Contract # 73-864733; 344 pages. Also available in total as C.B. Geller, C.S. Murray, D.R. Riley, J-L. Desplat, L.K. Hansen, G.L. Hatch, J.B. McVey and N.S. Rasor, “High-Efficiency Thermionics (HET-IV) and Converter Advancement (CAP) programs. Final Reports”, DOE DE96010173; 386 pages (1996).

11. N.S. Rasor, “The Important Effect of Electron Reflection on Thermionic Converter Performance”, Proc. 33rd Intersoc. Energy Conv. Engr. Conf., Colorado Springs, CO, Aug., 1998, paper 98-211.

12. V. Yarygin, et al., “Energy Conversion Options For NASA’s Space Nuclear Power Systems Initiative – Underestimated Capability Of Thermionics”, Proc. 2nd International Energy Conversion Engineering Conference, Providence, RI, Aug. 2004.