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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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OPERATION PRINCIPLE/CONCEPT
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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OPERATION PRINCIPLE/CONCEPT
The idea of electrons leaving the surface
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OPERATION PRINCIPLE/CONCEPT
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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OPERATION PRINCIPLE/CONCEPT
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
Electrons escaped from 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
Electrons escaped from cathode
ElectronsPromoted
from EF
EF
A
B
C
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OPERATION PRINCIPLE/CONCEPT
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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OPERATION PRINCIPLE/CONCEPT
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