Gary O. Fitzpatrick
Daniel T. Allen

Advanced Energy Technology, Inc.
P. O. Box 327
La Jolla, CA 92038 USA
(619) 455-4310

An efficient cascaded thermionic converter is described. The top stage is a barium-cesium converter, and the bottom stage consists of a close-spaced cesium converter.

Experimental results have been reported for barium-cesium converters operated in Germany (1) and USSR (2). These results are notable in that useful power densities at good calculated efficiency are demonstrated for high collector temperatures. For emitter temperatures in the range of 2000-2200 K, output of 5 We/sq cm at near 15% lead efficiency can be calculated for a 1200 K collector temperature. The average work function used in this calculation is 2.2 eV, which is supported by the experimental data (1).

Close-spaced cesium converters in the USA (3, 4 & 5) and USSR (6) have been shown experimentally to give good output at relatively low emitter temperatures. These data support a calculated lead efficiency of approximately 12% at an emitter temperature of 1200 K and a collector temperature of 750 K.

Since the collector temperature of the barium-cesium converter is the same as the emitter temperature of the close-spaced converter, it is possible to propose a cascade combination of these two experimentally verified converter designs would yield an overall lead efficiency of 25%.

Barium-Cesium Converters

The cesium in a conventional high-pressure cesium, ignited mode thermionic converter serves two functions. One function is the neutralization of the space charge in the gap between the emitter and the collector. The interelectrode plasma accomplishes this function. The other function of the cesium in the conventional converter is to cover the surfaces of the refractory metal electrodes in

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order to reduces their for emission of electrons. In its first function (space charge neutralization) the cesium contributes to the voltage drop between the electrodes, and for high performance converters (which would be those having high temperatures and relatively high interelectrode current densities) lowering this voltage drop can provide significant improvement in conversion efficiency. Lowering cesium pressure results in lower voltage drop, but it undermines the other function of the cesium, which is the electrode work function modification that brings about higher voltage output.

A solution put forward early in the development of thermionic technology (7) has been the utilization of a mixed vapor where an "additive" provides the electrode work function coverage and the cesium is needed only for the space charge neutralization and its pressure can be reduced. Experiments (8) have shown that the heat of adsorption of the alkaline earth’s Ba, Sr and Ca are greater that of cesium, and therefore they selectively adsorb on the electrodes and dominate the effective work function. On the other hand, they have a larger ionization potential than cesium and therefore contribute little to the processes of the interelectrode plasma. Thus, the addition of barium allows the operating cesium pressure to be lower, reducing the voltage drop.

Experimental studies in Germany by Henne about fifteen years ago (1) demonstrated two important characteristics of the barium-cesium converter. One is that the lower interelectrode voltage drop (due to the lowering cesium pressure) enables good performance at greater interelectrode spacings, compared to conventional cesium only converters. The second characteristic is that the high collector work function (due to the barium) results in good converter output at collector temperatures much higher that those for which output falls-off in conventional cesium only converters.

Henne’s experiments quantified the sensitivity to spacing with cesium plus barium. Shown in Fig. 1 are the results of the variation of interelectrode spacing from 0.1mm to 1.0mm in the experimental converter. As can be seen, the decline in maximum power is on the order of 12% for the change from 0.1 to 0.5mm.

Experimental work in the USSR, notably that by Kalandarishvili and co-workers at Sukhumi (2), has verified the performance of barium-cesium converters at high collector temperature. As shown in Fig. 2, the identical converter with cesium only has optimum performance at a lower collector temperature as compared to operation with barium-cesium at emitter temperature of 1773, 1923 and 2073 K. These data are from experiments with converters employing oriented tungsten emitters and niobium collectors.

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Close-Spaced Cesium Thermionic Converter

Experiments were sponsored by the U.S. Department of Energy in1982 through 1984, which proved the feasibility of SAVTEC close-spaced thermionic converter. The key to the SAVTEC technology is that in manufacture the emitter and collector are in physical contact, and the differential thermal expansion of the emitter lead upon heating to power opens and maintains the small gap.

Fig. 3 is a J-V plot for experimental cesium close-spaced converter SAVTEC 12A. Tests were done at emitter temperature from 1100 K to 1750 K. A SAVTEC test at 1750 K had an indicated efficiency of 18%.

Recently published experimental work by the Scientific-Industrial Association Luch, located in Podolsk, describes results with a close-spaced converter of a different design and which utilizes single crystal alloys for both emitter and collector. The emitter was a W-Ta-Re crystal and the collector Mo-Nb-Re.

For our hypothetical cascaded converter Stage 1 is a barium-cesium converter. Stage 2 is a cesium close-spaced converter.

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Tables 1 shows the separate performance of the barium-cesium converter and close-spaced cesium converter derived from the experiments of Henne and SAVTEC 12A. Fig. 4 is a plot showing the calculated lead efficiency of the barium-cesium converter and close-spaced converter as calculated from these experimental points. Fig. 5 is a bar graph showing the lead efficiency of the barium-cesium converter the close-spaced converter and the cascaded converters. Tables 2 shows their temperature. Cascaded efficiency is 26%.

It should be especially noted that the "rejected" heat from Stage 2 converter is at a temperature of 750 K. This heat could be further utilized to produce more output power or to do other useful work, and so there is the possibility that the cascaded converter could be applied in systems of even greater overall efficiency.

A difficulty in actually making such cascaded converter would be the disparity in input power density and current densities between the stages, which is approximately a factor of ten. The barium-cesium converter has higher current densities, 5 A/sq cm and the close-spaced cesium converter has current densities of 0.7 A/sq cm. Use of a heat pipe is one way to get around the problem. A niobium heat pipe with lithium working fluid could couple the stages. Electric current flows down the heat pipe. A schematic for a two-stage converter is in Fig. 6.

A cascaded converter with very attractive efficiency is a real possibility. The converter described is based on experimental converters built and tested. For the barium-cesium stage the converters were built and tested in the USSR and Germany. For the close-spaced stage the converters have been built in the USSR and USA.

This cascaded converter has potential beneficial application in addition to space nuclear power, such as space and terrestrial solar power conversion and for topping cycles in fossil or nuclear central stations.

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(1) Henne, R., "Features of a Barium-Cesium Diode with Plane Polycrystalline Molybdenum Electrodes for Thermionic Energy Conversion," Report, Holland, Deutsche Forschungs-und Versuchsanstalt fur Luft-und Raumfahrt (DFVLR) E.V. Institute for Energy Conversion and Electric Drives, Stuttgart, West Germany, 1975

(2) Kalandarishvili A.C., Working Medium Sources for Thermionic Power Converters, Energoatomizdat, Moscow, USSR, 1986, Sec. 5.4

(3) Dick, R.S., J.R. McVey, G.O. Fitzpatrick and E.J. Britt, "High Performance, Close-Spaced Thermionic Converters," Proceeding 18th Intersociety Energy Conversion Engineering Conference, Orlando, Florida, USA, August, 1983, v. 1, p.198, (839030).

(4) Nyren, T., M. Korringa, J. McVey, T. Sahines and G.O. Fitzpatrick, "Design and Testing of a Combustion-Heated Nineteen-Converters SAVTEC Array," Proceeding 19th Intersociety Energy Conversion Engineering Conference, San Francisco, California, USA, August, 1984, v. 4, p. 2300 (849308).

(5) Fitzpatrick, G.O., R.C. Dahlberg, "SAVTEC Thermionic Converter with an SP-100 Heat Source," 6th Symposium on Space Nuclear Power Systems, Albuquerque, New Mexico, USA, January, 1989.

(6) Nikolaev, Yu.V., R.Ya. Kucherov, et al, "Close-Spaced Thermionic Energy Converter," 7th Symposium on Space Nuclear Power Systems, Albuquerque, New Mexico, USA, January, 1990.

(7) Psarouthakis, J., "Thermionic Energy Converter with Barium and Cesium Vapors," pp. 100-109, IEEE 1964 Thermionic Conversion Specialist Conference, Cleveland, Ohio, 1964.

(8) Bondarenko, V.D. and Guskov, Yu.K., "Characteristics of Thermionic Converters Filled with Vapor Mixture," p. 999, EAEC-OECD 2nd International Conference on Thermionic Electrical Power Generation, Stresa, Italy, May, 1968.

(9) Hatsopoulos, G.N., and J. Kaye, "Measured Thermal Efficiencies of a Diode Configuration of a Thermo Electron Engine," J. Appl. Phys., 29(7): 1124-1125, 1958