1. PH0101 UNIT-5 LECTURE 4 Introduction Thermionic effect Richardson Law
Principle, construction and working of Thermionic power generator
Advantage, disadvantage and applications
PH Unit Lecture-4

2. Thermionic emission is the basis for the working of this system.
1. Introduction
Thermionic power generator (TPG) is a device that converts heat energy into electrical energy.
Thermionic emission is the basis for the working of this system.
The thermionic emission is the emission of electrons from metal surface due to heat.
PH Unit Lecture-4

3. In 1873, the Britain professor Frederic Guthrie invented the Thermionic phenomenon.
In 1883, Thomas A. Edison observed that the electrons are emitted from a metal surface when it was heated. This effect is called Edison effect.
Later in 1904, a British physicist John Ambrose Fleming developed two-element vacuum tube known as diode.
After Fleming, Owen Willans Richardson worked with thermionic emission and received a Nobel Prize in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him
PH Unit Lecture-4

4. A thermionic energy converter (or) thermionic power
A thermionic energy converter (or) thermionic power gnerator is a device consisting of two electrodes placed near one another in a vacuum.  
One electrode is normally called the cathode, or emitter, and the other is called the anode, or plate.   
Ordinarily, electrons in the cathode are prevented from escaping from the surface by a potential-energy barrier.  
When an electron starts to move away from the surface, it induces a corresponding positive charge in the material, which tends to pull it back into the surface.
To escape, the electron must somehow acquire enough energy to overcome this energy barrier. 
PH Unit Lecture-4

5. At ordinary temperatures, almost none of the electrons
At ordinary temperatures, almost none  of  the  electrons can  acquire  enough  energy to escape.  
However, when the cathode is very hot, the electron energies are greatly increased by thermal motion.
  At sufficiently high temperatures, a considerable number of electrons are able to escape.
The liberation of electrons from a hot surface is called thermionic emission
PH Unit Lecture-4

6. The idea of electrons leaving the surface is shown in figure
PH Unit Lecture-4

7. 2. Thermionic effect
Thermionic effect is the ejection of electron from the heated metal surface and forms as electron cloud at the cathode.
The number of electron emitted from the metal surface depends on temperature and work function.
Work function(φ)
For Electrons to leave the surface of the metal, they have to be supplied with enoughEnergy.
We call the minimum energy required to allow an electron to be liberated from a material as its WORK FUNCTION(φ)
Work Function differs from material to material – usually a few eV
PH Unit Lecture-4

8. 3. Richardson Law J = At2 e (-/KT) Ampers/m3
Richardson law states that the emission current density is exponentially depend on work function and inversely depends upon the absolute temperature.
According to Richardson, the emission current density ‘J’ can be expressed as,
J = At2 e (-/KT) Ampers/m3
Where A-Emission constant (A/m3/K2)
 - Work function
T – absolute temperature (Kelvin)
K – Boltzman constant
J - emission current density.
PH Unit Lecture-4

9. 4. Principle, Construction and working of
4. Principle, Construction and working of Thermionic power generator (TPG)
Principle
Thermionic power generator is based on the principles of Thermionic effect that the electrons are emitted from a hot metal surface and responsible for the production of electricity.
Construction
The TPG consist of tungsten metal, which is negatively charged cathode acts as an emitter.
There is positively charged electrode is called collector. It is collecting the ejected electrons. The emitter and collector are kept in a vacuum quartz tube.
PH Unit Lecture-4

10. Thermionic Power generator
Emitter
Quartz tube
Collector
Thermal energy
Electrical energy VL RL
PH Unit Lecture-4

11. Working
The electrons within a metal can be treated as "electron gas" in which individual outer most electrons are capable of moving freely under the influence of a field.
This movement of electrons is responsible for the function of electric circuits.
At the surface of a metal, a potential barrier exists which prevents the electrons from escaping unless certain conditions are met. This concept can be explained as follows.
PH Unit Lecture-4

12. Electrons Promoted from EF
Surface of cathode
Electrons escaped from cathode
Electrons Promoted from EF EF  A B C
It is known from the free electron theory, at 0K, all the energy levels up to EF (fermi energy) are completely filled and all the energy level above the EF are completely empty.
PH Unit Lecture-4

13. The energy level from the surface of metallic cathode to. the
The energy level from the surface of metallic cathode to the level of EF (BC in figure) is the potential energy barrier called work function (.).
If any electron wants to escape from the surface of the metallic cathode, they should cross this potential barrier.
At 0K, all the electrons are bound within fermi energy level and cannot escape from the surface of cathode (emitter).
When the thermal energy is supplied on the emitter side, some of the electrons are promoted to above the fermi level.
PH Unit Lecture-4

14. Good emitters should have low work functions.
These activated electrons can cross the potential energy barrier and escape from the surface of cathode and responsible for the current production.
As long as the temperature increased, the number of electrons escapes from the surface of emitter increases. Collector collects the emitted electrons and there is an external circuit through which the current flows
The thermionic emission current density is determined by the 'work function' of the material, which is basically the magnitude of the potential energy barrier.
Good emitters should have low work functions.
PH Unit Lecture-4

15. The metal can be heated in two different ways.
It is generally accepted that the thoriated tungsten is the best cathode metal because of its lower value of work function
The metal can be heated in two different ways.
One is direct heating and second is the indirect heating.
In the direct heating where the filament itself is the cathode and the indirect heating where the cathode is heated by a separate filament.
Pure tungsten is used as main metal in the case of direct heating method
Nickel (or) Nickel alloys are used as main metal in the case of indirect heating.
PH Unit Lecture-4

16. 5. Advantage, disadvantage and applications
Advantages
Higher efficiency and high power density
Compact to use
Disadvantages
There is a possibility of vaporization of emitter surface
Thermal breaking is possible during operation
The sealing is often gets failure
PH Unit Lecture-4

17. They are used in space power application for spacecraft
Applications
They are used in space power application for spacecraft
They are used to power submarines and boats.
They used in water pump for irrigation,
They used in power plant for industry and domestic purpose
PH Unit Lecture-4