Power Plant Engineering

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

Te r m i n a l s

Emitter

Collector

Exhaust Heat

Input Heat

Casing

Vaccum

Chamber

Electrons

Fig. 2.25. Principle of Thermoionic Converter (Generator).

2.18.1 Thermoionic Conversion

The emission of an electron from a metal surface is opposed by a potential barrier equal to the
difference between the energies of an electron outside and inside the metal. Therefore, a certain amount
of energy has to be spent to release the electron from the surface. This energy is called surface work
function (Φ).

The maximum electron current per unit area emitted from the surface is given by the following
Richardson Dushman equation:

J = A 1 T^2 e kT

−σ



where J = current density, [A/m^2 ]
T = Temperature, [K]
Φ = work function, [eV]
K = 1.38 × 10–23 J/molecule K
= Boltzmann constant
A 1 = 120 A/cm^2 -K^2 ) = Emission constant.
The kinetic energy of the free electrons at absolute zero would occupy discrete energy levels
from zero upto some maximum value defined by the Fermi energy level, εf. Each energy level contains
a limited number of free electrons.