In this article we will discuss about the construction and working of thermionic generator with the help of a suitable diagram.
i. A thermionic converter/generator comprises a heated cathode (electron emitter) and an anode (electron collector) separated by ‘vacuum’, the electrical output circuit being connected between the two as shown in the Fig. 9.7.
ii. The heat which is supplied to the cathode raises the energy of its electrons to such a level that it enables than, to escape from the surface and flow to anode. At the ‘anode’ the energy of electrons appears partially as heat, removed by cooling and partially as electrical energy delivered to the circuit.
iii. Although the distance between anode and cathode is only about 1mm. The negative space charge with such an arrangement hinders the passage of the electrons and must be reduced, this can be achieved by introducing positive ions into the inter electrode space, ‘cesium vapour’ being valuable source of such ions.
a. In order to materialise a substantial electron emission rate (per unit area of emitter), and hence a significant current output as well as a high efficiency, the emitter temperature in a thermionic converter containing cesium should be at least 1000°C, the efficiency is then 10 per cent.
Efficiency as high as 40 per cent can be obtained by operating at still higher temperatures. Although temperature has little effect on the voltage generated, the increase in current (per unit emitter area) associated with a temperature increase results in increase in power. Electric power (P) is the product of voltage (E) and current (I) i.e. , P = EI.
b. Anode materials should have a low work function e.g. barium and strontium oxides while that of the cathode should be considerably higher, tungsten impregnated with a barium compound being a suitable material.
Even with these materials temperatures upto 2000°C will be required to secure for the generator itself, efficiencies of 30-35 per cent. Electrical outputs of about 6 W/cm2 of anode surface are envisaged with about 13 W/cm2 removed by coolant.
c. A thermionic generator, in principle, can make use of any fuel (may be fossil fuel, a nuclear fuel or solar energy) subject to the condition that sufficiently high temperatures are obtainable. The thermionic conversion can be utilized in several different situations—remote locations on the earth and in space.