Thermionic Converter

(or thermionic generator), a device for the direct conversion of thermal energy into electrical energy on the basis of the phenomenon of thermionic emission.

The simplest type of thermionic converter consists of two electrodes separated by a vacuum gap (a simple cesium thermionic converter is shown in Figure 1 on page 604). One electrode is the cathode, or emitter, and the other is the anode, or collector. The electrodes are made of refractory metals, usually molybdenum, rhenium, or tungsten. A heat source supplies enough thermal energy for appreciable thermionic emission to occur from the surface of the metal. After passing through the interelectrode gap, which is a few tenths of a millimeter in size, the electrons impinge on the surface of the collector, where they create an excess of negative charge and increase the collector’s negative potential. If heat is continuously supplied to the emitter and if the collector,

Figure 1. Schematic of a thermionic converter: (C) cathode, or emitter; (A) anode, or collector; (R) external load; (Q_c) heat supplied to cathode; (Q_A) heat removed from anode; (1) cesium atoms; (2) cesium ions; (3) electrons

which takes up heat from the electrons that reach it, is correspondingly cooled, then an electric current will be maintained in the external circuit, and work will thus be done. Since a thermionic converter is essentially a heat engine whose working fluid is an “electron gas” (the electrons “evaporate” from the heated emitter and “condense” on the cooled collector), the efficiency of a thermionic converter cannot exceed that of the Carnot cycle.

The thermionic converter produces a voltage of 0.5–1 volt. This voltage is of the order of the contact potential difference but differs from it by the value of the voltage drop across the inter-electrode gap and the voltage losses in the switching lines (Figure 2). The maximum density of the current generated by a thermionic converter is limited by the emission capacity of the emitter and may reach a few tens of amperes from 1 cm² of surface. To obtain optimal values of the work function of the emitter (2.5–2.8 electron volts [eV]) and collector (1.0–1.7 eV) and to compensate for the electron space charge formed near the electrodes, a slightly ionized cesium vapor is usually introduced into the interelectrode gap. Positive cesium ions are formed when cesium atoms collide with electrons (1) at the hot cathode or (2) in the interelectrode space. The first case is known as surface ionization. In the second case, a cesium atom may be ionized by a single electron impact or through stepwise ionization, in which the cesium atom is brought into an excited state by an initial collision with an electron and is ionized by subsequent collisions. In this second case, arcing occurs; this mode of operation is the one most widely used. Present-day thermionic converters operate at cathode temperatures of 1700°–2000°K and anode temperatures of 800°–1100°K. At such temperatures, the power density at the cathode surface reaches tens of watts per cm², and the efficiency of the converters may exceed 20 percent.

According to the nature of the heat source, thermionic converters are classified as nuclear, solar, and flame-heated. The heat for nuclear thermionic converters may come from a nuclear fission reaction (in reactor-powered converters) or the decay of a radioactive isotope (in isotope-powered converters). The first reactor-powered thermionic converter in the world was built in the USSR in 1970. Called Topaz, it has an electric power output of about 10 kilowatts. In solar thermionic converters, the emitter is heated by the thermal energy of solar radiation, which is collected with solar concentrators. Flame-heated thermionic converters operate on the heat released in the combustion of organic fuel.

Figure 2. Distribution of potential energy of electrons in the interelectrode gap: (1) when there is an insufficient concentration of cesium Ions, (2) when the space charge is compensated, (3) when arcing occurs; (FLC) Fermi level of cathode (emitter), (FLA) Fermi level of anode (collector), (ε) energy, (ε_c) cathode work function, (ε_A) anode work function, (ΔV_g) voltage drop across interelectrode gap, (ΔV_line) voltage drop in switching lines, (V) voltage drop in external circuit, (e) electron charge, (d) interelectrode spacing

Thermionic converters have several advantages over traditional electromechanical converters: the absence of moving parts, compactness, high reliability, and the possibility of operation without regular servicing. As of the mid–1970’s, a continuous operating life of over 40,000 hours had been achieved for an individual thermionic converter. A promising application of thermionic converters is their use as high-temperature units of multistage energy converters—for example, in combination with thermoelectric converters operating at lower temperatures. Intensive work on the development of thermionic converters suitable for large-scale industrial use is being conducted in the USSR, the USA, France, and several other countries.