A thermionic generator (converter) converts heat energy directly to electrical energy by utilizing thermionic emission effect. All metals and some oxides have free electrons which are released on heating. In a thermionic converter, electrone act as the working fluid in place of a vapour or gas. In this device electrons are emitted from the surface of heated metal
The energy required to extract an electron from the metal is known as work function and expressed in electron volts (V).The work function depends upon the nature of metal and its surface condition.
The principle of thermionic converters is illustrated in Fig. . In a thermionic converter two electrodes are placed in a container containing an ionised gas or cesium vapour to reduce the space charge. The cathode is heated by concentrating the rays on it. On heating of cathode the electrons are emitted from it and travel to anode. The cathode and anode are connected externally through the load circuit. The electrons return back to cathode through the external circuit and the current flows through the external circuit as shown in the figure. The heat energy is converted directly into electrical energy through a process similar to that in a steam plant where water is evaporated in a boiler and the steam is condensed after doing the useful work in an engine. With the discovery of materials that provide adequate electron emission rates without getting melted, the performance of the system has been largely improved.
This device is being designed for space power applications where high temperature operation is advantageous. Such converters or generators are suitable for use with nuclear reactor or radioisotope heat sources
A thermionic generator is like a cyclic heat engine and its maximum efficiency is limited by Carnot's law. It is a low-voltage, high current device where current densities of 20-50 A/cm2' have been achieved at voltage from 1 to 2 V.
Thermal efficiencies of 10-20% have been realized. Higher values are possible in future.
Thermionic generators can be used for large power generation. The fuel elements of a nuclear reactor may be very suitable high temperature heat source for thermionic generator. This can be surrounded by cooled anode and the in between space can be filled with ionized cesium vapour.
The energy of high temperature gases can be partly converted into electricity if the riser tubes of the boiler are provided with cathode and anode of a thermionic generator with the interspace filled with ionized cesium vapour.
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.
Thermionic Emitter (Cathode)
The metal used for emission of electrons is known as an emitter or cathode. The cathode is heated in an evacuated space to emit electrons. If the cathode is heated in open air it would burn because of presence of oxygen in air. Therefore,
cathode should have the following properties:
1. The work function of the cathode should be low, so that small amount of energy is required for the emission of electrons.
2. Cathode should have high melting point, so that it may withstand high temperature without melting.
3. Cathode should have high mechanical strength.
Limitations
Thermionic converter is a direct energy conversion device with no moving parts. It is very robust, and does not require any supervision for operation. It is very suitable for remote and space applications. It suffers from the following limitations:
1. The operating temperature of cathode is very high, so costly materials like tungsten, rhenium are required for cathode.
2. Special shields of ceramic are required for protection of cathode from corrosive combustion gases.
3. The collector may also have to be made of molybdenum coated with cesium.
4. Ionised cesium vapour has to be filled in the interspace to reduce the space charge barrier to promote electron emission from the cathode.
Applications
1. Thermionic Converter in a Nuclear Reactor.
The fuel element containing the fissile material carries the cathode surrounded by the anode. The interspace is filled with ionized cesium gas. The anode is cooled by the coolant from outside. Some of the energy released by nuclear fission is directly converted into electrical energy by thermionic conversion.
The remaining heat is used in conventional bottoming steam plant. The overall efficiency of the plant, therefore, increases.
2. Thermionic Converter in The Riser Tube of a Boiler.
The riser tubes of a boiler receive heat by radiation from combustion gases. The riser tube is provided by a cathode and anode of a thermionic converter. The interspace is filled with ionized cesium vapour. The use of the hot combustion gases to produce extra power before the steam cycle improves the overall plant efficiency.
3. MHD Thermionic-Steam Power Plant.
The waste heat from the MHD generator at about 1,900°C is used to heat the cathode of thermionic converter. The heat from the anode is used in the boiler of a steam power plant. The overall efficiency of the plant will, therefore increase.