Thermionic conversion directly converts heat into electrical energy using thermionic emission. A thermionic generator has two electrodes in a vacuum tube - a heated cathode that emits electrons and a cooled anode that collects them. As electrons travel through an external load, electrical power is produced. Thermal efficiencies of 10-20% have been achieved, with higher efficiencies possible in the future. Thermionic generators can utilize various heat sources and have applications for remote power generation on Earth and in space.

1. Thermionic Conversion
PRESENTED BY
YOGESH C. DHOTE
ASSOCIATE PROFESSOR
MGM COLLEGE OF ENGINEERING
NAVI-MUMBAI

2. Principle of working
• A thermionic generator transforms
heat directly into electrical energy by
utilizing thermionic emission.
• All metals and some oxides have free
electrons which are released on
heating.
• These electrons 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.

3. Thermionic Converter
• A thermionic generator has two electrodes
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
accumulate 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 ionized
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.

4. Contd…
• It is a low voltage, high current device where current densities of
20-50 A/cm2 have been achieved at voltage from 1 to 2V.
• Thermal efficiencies of 10-20% have been realized although higher
values are possible in future.
• Thermionic generators can be used for larger power generation.
• The fuel elements of a nuclear reactor may be very suitable high
temperature heat source for thermionic generator.
• In principle any heat source, fossil or nuclear fuel, a radioactive
material or solar energy can be used in a thermionic generator.
• Many applications have been suggested for remote locations on the
earth and in space as the device is robust and reliable in unattended
operation.

5. Thermionic Emitter (Cathode)
Following properties are required in emitter cathode
– The work function of cathode should be low, so that
small amount of energy is required for the emission of
electrons.
– Cathode should have high melting point, so it cannot
melt at higher temperature.
– Cathode should also have high mechanical strength.

6. Performance of Thermionic Generator
Thermal Efficiency of thermionic generator,
η = {(γc - θ γa)[1 – θ2 e(γc
- γa
)]}/{(γc + 2) – θ2 (γa + 2θ) e(γc
- γa
)}
Where, θ = Ta/Tc
γc = Vc/KTc, γa = Va/KTa,
Where K = Boltzmann’s constant
= 1.38 x 10-23
V = Work function for space charge
If γc = γa, and neglecting the smaller terms
ηmax = (1 – θ) . γ/(γ + 2)

7. Contd…

8. Limitations
• The efficiency of nuclear fueled thermionic converter
achieved so far is about 15% and may improve to
40% in future.
• The operating temperature of cathode is very high, so
the metal should have high melting point and low
work function, which increases the cost.
• Special shields of ceramic are required to protect
cathode from corrosive combustion gases.