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Essay on Thermionic Generator
Essay Contents:
- Essay on the Introduction to Thermionic Generator
- Essay on the Principle of Operation of Thermionic Generator
- Essay on the Performance of Thermionic Generator
- Essay on the Applications of Thermionic Generator
- Essay on the Limitations of Thermionic Generator
Essay # 1. Introduction to Thermionic Generator:
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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.
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 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.
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 2V. Thermal efficiencies of 10 – 20% have been realized. Higher values are possible in future.
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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 the cooled anode and the in between space can be filled with ionized cerium vapour.
The energy of high temperature combustion gases can be partly converted to electricity if the riser tubes of the boiler are provided with cathode and anode of a thermionic generator with the interspace filled with ionized cerium 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 are robust and reliable in unattended operation.
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Essay # 2. Principle of Operation of Thermionic Generator:
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:
The work function varies between 1 and 5 eV and data is given in Table 15.1.
Fermi Energy Level:
The kinetic energy of the free electrons at absolute zero would occupy discrete energy levels from zero up to some maximum value defined by the Fermi energy level, ԑf. Each energy level contains a limited number of free electrons.
Above absolute zero temperature, some electrons may have energies higher than the Fermi level. The energy that must be supplied to overcome the weak attractive force on the outermost orbital electrons is the work function, ф, so that the electron leaving the emitter has an energy level φ + ԑf.
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When emitter is heated, some high-energy free electrons at the Fermi level receive energy equal to emitter work function φc and escape the emitter surface. They move through the gap and strike the collector. The K.E. (ԑfa) plus the energy equal to collector work function φa 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 ԑfa. This energy state is higher than that of the electron at the Fermi energy level of cathode ԑfc. 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.
Essay # 3. Performance of Thermionic Generator:
The positively charged cathode tends to pull the emitted electrons back. The electrons already in the gas exert a retarding force on the electrons trying to cross the gap. This produces a space charge barrier Figure 15.2 shows the characteristic curve of a thermionic generator with an interspace retarding potential equivalent to δ volts above the anode work function φa.
Example 1:
A thermionic generator has the following characteristics:
Cathode work function фc = 2.5 V.
Anode work function фa = 2.0 V.
Temperature of cathode Tc = 20000 K.
Temperature of surrounding Ts = 1000 K.
Plasma potential drop δ = 0.1 V.
Assuming the value of emissivity as 0.2 for the electrode materials used, calculate the efficiency of the generator and also compare with Carnot efficiency.
Solution:
Net output Voltage.
Essay # 4. Applications of Thermionic Generator:
1. Thermionic Generator in a Nuclear Reactor:
The fuel element containing the fissile material carries the cathode which is 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 electricity by thermionic conversion. The remaining heat is used in conventional bottoming steam plant. The overall efficiency of the plant increases.
2. Thermionic Generator 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 generator. 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 MHD generator at about 1900°C is used to heat the cathode of thermionic generator. The heat from anode is used in the boiler of a stream power plant. The overall efficiency of the combined plant will increase.
Essay # 5. Limitations of Thermionic Generator:
Thermionic generator is a direct energy conversion device with no moving parts. It is very robust, no supervision needed for operation. It is very suitable for remote and space applications.
It suffers from the following limitations:
i. The operating temperature of cathode is very high necessitating the use of costly materials such as tungsten or rhenium.
ii. Special shields of ceramic are needed to protect cathode from corrosive combustion gases.
iii. The collector may also have to be made of molybdenum coated with cesium.
iv. Ionised cesium vapour has to be filled in the interspace to reduce the space charge barrier to promote electron emission from the cathode.