Thermionic Energy Conversion System

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Group 3


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INTRODUCTION

Thermionic Converters

Thermionic Emission

a device that converts heat energy into electrical energythrough the process called thermionic emission.

emission of electrons from metal surface due to heat.


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INTRODUCTION

HISTORY

BritishProfessor

InventedThermionic

Phenomenon.

observed thatthe electronsare emitted

from a metalsurface whenit was heated.(Edison effect)

received aNobel Prize forhis work on the

thermionicphenomenon(Richardson

Law).

British physicist

developed two-element

vacuum tubeknown as diode.

18

73 1883 1904 1928

INTRODUCTION

FrederickGuthrie

Thomas EdisonJohn Ambrose

Fleming

Owen WilliamsRichardson


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OPERATION PRINCIPLE/CONCPEPT

OPERATION PRINCIPLE/CONCEPT

A thermionic converter consists oftwo metals or electrodes placed nearone another in a vacuum.

One electrode is normally called thecathode, or emitter, and the other iscalled the anode, or plate.

At ordinary temperatures, almost

none of the electronscan acquire enough energy toescape.


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However, when heat is applied tothe cathode, the electron energiesare greatly increased by thermalmotion.

The liberation of electrons from ahot surface is calledthermionicemission.

At the anode the energy of electronsappears partially as heat, removedby cooling and partially as electricalenergy delivered to the circuit.


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The idea of electrons leaving the surface


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Surface of cathode

Electrons escaped from cathode

ElectronsPromoted

from EF

EF

A

B

C

It is known from the free electrontheory, at 0K, all the energy levelsup to EF(fermi energy) are

completely filled and all the energylevel above the EFare completelyempty.

The energy level from the surface of

metallic cathode to the level of EF(BC in figure) is the potentialenergy barrier calledworkfunction (.).

HOW DOES IT WORK?


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HOW DOES IT WORK?The number of electron emittedfrom the metal surface dependsupon the temperature and workfunction.

The energy required to extractan electron from the metal.

Measured in eV.

Value of work function varieswith the nature of the metal

and surface condition.

Work function () Surface of cathode


ElectronsPromoted

from EF

EF

A

B

C


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OPERATION PRINCIPLE/CONCPEPT

HOW DOES IT WORK?If any electron wants to escape fromthe surface of the metallic cathode,they should cross this potentialbarrier.

At 0K, all the electrons are boundwithin fermi energy level andcannot escape from the surface ofcathode (emitter).

When the thermal energy issupplied on the emitter side, someof the electrons are promoted toabove the fermi level.

Surface of cathode


ElectronsPromoted

from EF

EF

A

B

C


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Thermionic Converter Materials:

The main criteria for choosing acollector material that it should

have as low a work function aspossible(lower than the workfunction of the emitter).

Collector

Molybdenum (Mo)

A good emitter should have lowwork functions.

Emmiter

Tungsten (W)


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The metal can be heated in two different ways

the filament itself is the cathode.

Pure tungsten is used as mainmetal

Direct Heating

cathode is heated by aseparate filament

Nickel alloys are used asmain metal

Indirect Heating


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ADVANTAGE AND DISADVANTAGE

Liquid-vapor phase is notemployed

Separators for fluids are notrequired

Frictional losses due tobearings are not present

Compact to use

Rotating equipment is notemployed

Advantages

There is a possibility ofvaporization of emitter

surface

Thermal breaking is possibleduring operation

The sealing is often getsfailure

Low voltage, High currentdevices

Disadvantages


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APPLICATION

Used in space power application for spacecraft

Used to power submarines and boats.

Used in water pump for irrigation

Used in power plant for industry and domestic purpose


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REFERENCES

E. F. Reden.Types of Thermionic Power Converter and CurrentMaterials Limiations

R.K. Rajput.(2006. )A Textbook of Power System Engineering.New Delhi. :Laxmi Publications.

www.srmuniv.ac.in/openware_d_loads/u5Lecture-4.ppt

http://fti.neep.wisc.edu/neep602/SPRING00/lecture9.pdf

http://www.eng.fsu.edu/~dommelen/quantum/style_a/cboxte.html

en.wikipedia.org/wiki/Thermionic_emission

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Thermionic Energy Conversion System