Power Plant Engineering

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
kT
e
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, 
ε
f
. Each energy level contains
a limited number of free electrons.

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80
POWER PLANT ENGINEERING
Above absolute zero temperature, some electrons may have energies higher than the Fermi level.
The energy that must be supplied to overcome the weak attrac-tive force on the outermost orbital
electrons is the work function, 
Φ
, so that the electron leaving the emitter has an energy level 
Φ
+ 
ε
f
.
When emitter is heated, some high energy free electrons at the Fermi level receive energy equal to
emitter work function 
Φ
c
, and escape the emitter surface. They move through the gap and strike the
collector. The K.E. (
ε
fa
) plus the energy equal to collector work function 
Φ
a
is given up and this energy
is rejected as heat from the low temperature collector.
The electron energy is reduced to the Fermi energy level of the anode 
ε
fa
This energy state is
higher than that of the electron at the Fermi energy level of cathode 
ε
fc
. Therefore, the electron is able
to pass through the external load from anode to cathode. The cathode materials are selected with low
Fermi levels as comprised to anode materials which must have higher Fermi level.