Power Plant Engineering

2.18 THERMIONIC SYSTEMS AND THENNIONIC EMISSION
A thermionic converter transforms heat directly into electrical energy by utilizing thermionic
emission. All metals and some oxides have free electrons which are released on heating. These elec-
trons can travel through a space and collected on a cooled metal. These electrons can return to hot
metal through an external load thereby producing electrical power.
A thermionic converter has two elec-trodes enclosed in a tube. The cathode is called an emitter
and is heated enough to release electrons from its surface. The electrons cross a small gap and accumu-
late on a cooled metal anode called the collector. The space between the electrodes is maintained at
high vacuum or filled with a highly conducting plasma like ionised cesium vapour to minimal energy
losses. The external load R is connected through anode to cathode. The electrons return to cathode
through the external load and electrical power is produced.

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NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION
79
Thermionic conversions is a sealed and evacuated device comprising of
1. A heated cathode (electron emitter)
2. An anode (electron collector)
3. Vacuum gap between 1 and 2 (with ionised vapour to neutralise space charge).
The gap is only about a mm. External electrical circuit is connected between anode and cathode.
(Ref. Fig. 2.25). The thermionic converter converts thermal energy directly to electrical energy by
virtue of flow of electrons through the vacuum gap.
Heat is supplied to emitter. Electrons released from emitter flow through small vacuum gap
seeded with ionising substance. Heat is rejected from collector. Electrical energy is tapped from the
terminals.
+
–
DC
Load
Terminals
Emitter
Collector
Exhaust Heat
Input Heat
Casing
Vaccum
Chamber
Electrons