Thermionic conversion

THERMIONIC CONVERSION. The process whereby electrons released by thermionic emission are collected and utilized as electric current. The simplest example of this is provided by a vacuum tube, in which the electrons released from a heated anode are collected at the cathode or plate. Used as a method of producing electrical power for spacecraft. 

Thermionic conversion is a technology that needs, and can immediately use, research on high temperature properties of alkali metals. Electron transport properties of alkali vapors and characteristics of atomic clusters are particularly Important. Improved understanding in these areas could lead to performance improvements that would more than double the output power density and efficiency of cesium ignited mode thermionic converters. 

The principle of thermionic conversion is that of a diode, in which the cathode (emitter) emits electrons that are collected at the anode (collector). Alloys of W, Re, Mo, Ni or Ta are used as emitters, and the diodes operate at temperatures of about 2200 K. The efficiency varies between about 1 and 10%, depending on the power. In order to take advantage of thermionic conversion, radionuclides of high specific power output (high power per mass unit) are needed, such as Pu, Ac,... 

D. Lieb et al.. Proceedings of the Thermionic Conversion Specialist Meeting, Eindhoven (Netherlands), 11 (1975)... 

The principle of thermionic conversion derives from Edison s discovery in 1885 that current could be made to flow between two electrodes at different temperatures in a vacuum. The analysis and ejqrerimental investigation of thermionic emission from a hot electrode were performed by O. W. Richardson in 1912. W. Schlichter in 1915 recognized this means of converting heat to electricity. A patent was submitted on this topic in 1923.1. Langmuir and his associates characterized the electron and ion emission from cesium-adsorbed films on tungsten in the 1920s. 

There was some interest in thermionic conversion in Russia in the 1940s. In particular, A. F. Ioffe discussed the thermionic converter as a vacuum thermoelement in 1949. Analytical studies of thermionic conversion that neglected the effects of space charge and collector temperature were published in the early 1950s. The first thermodynamic analysis of a thermionic converter was given by G. N. Hatsopoulos in 1956. 

By the eariy 1980s, the wheel had made another cycle, and the U.S. combustion thermionic conversion program was redirected to nuclear space activities. By 1985, only a token terrestrial thermionic effort existed. 

The demise of the U.S. space reactor effort in 1973, along with the accompanying OPEC oil action, provided the motivation for utilizing on the ground the thermionic technology that had been developed for space. There are a number of potential terrestrial applications of thermionic conversion. These include power plant topping, cogeneration with industrial processes, and solar systems. 

Figure 4. Schematic drawing of Cg resonant multiphoton detachment mechanism for excitation at the band origin of the C2ng - Y2I1U transition (IC, internal conversion TE, thermionic emission).

Thermionic energy conversion is a method of converting heat directly to electricity. A metal electrode, the emitter, is heated sufficiently to emit electrons, as shown in Figure 1. The electrons cross a narrow interelectrode gap and are collected by another metal electrode, the collector. Heat is removed from the collector so that its temperature is lower than the emitter, and the electrons striking the collector cannot be returned except by... 

Britt, E. J. and McVey, J., "Advanced Thermionic Energy Conversion, Joint Highlights and Status Report" Rasor Associates., COO-2263-16, NSR 2-16, 1979. 

Smith, M. D., Manda, M. L., and Britt, E. J., "Utilization of Low Temperature Insulators and Seals in Thermionic Con verters", 15th Intersociety Energy Conversion Engineering Conference, Washington, 1980. 

Goodale, D. B., Reagan, P. Miskolczy, G., Lieb, D. and Huffman, F. N. "Characteristics of CVD Silicon Carbide Thermionic Converters", 16th Intersociety Energy Conversion Engineering Conference, Atlanta, GA, 1981. 

Development of collector electrode materials with lower work function ( 1.0 eV) is the most effective methods to improve energy conversion efficiency of a thermionic converter. At present refractory metals such as Mo, W, Nb with cesium adsorption are used as a collector with the work function values of about 1.7eV. It is suggested that metal oxide collectors can adsorb cesium more strongly and show lower values of work function. In the study, refractory metal oxides and AgO x were experimentally examined concerning the work function values and high temperature durability. A research thermionic converter of a W emitter and an AgO x collector was fabricated and power generation tested to examine the effectiveness of the AgO x collector. A new type of a FGM collector which integrates a... 

Thermionic energy conversion is a power generation method which can be used to convert thermal energy into electric energy directly. A thermionic energy converter is a nonmechanical device that has high reliability. Research has been carried out on thermionic energy converters for use in space and ocean environments. 

Hatsopoulos, G.N., Gyftopoulos, E.P. (1973), Thermionic Energy Conversion, Vol. 1 Processes and Devices, MIT Press, Cambridge, MA. 

Brayton conversion systems have advantages and disadvantages. They are more efficient than most static power conversion systems (e g., thermoelectric or thermionic based systems), and they are more durable and simpler than the other dynamic power conversion systems. However, Brayton cycles do require higher temperatures to achieve the same efficiency as other dynamic power conversion systems. The energy density of the working fluid is low compared to the other dynamic power conversion systems. 

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