This document discusses several direct energy conversion systems, including fuel cells, magnetohydrodynamic (MHD) systems, thermoelectric systems, and thermionic power generation. Fuel cells directly convert the chemical energy of fuels like hydrogen into electricity. MHD systems use magnets and high temperature gases or liquids to directly generate electricity from heat. Thermoelectric systems use the temperature difference between hot and cold junctions of dissimilar metals to create an electric current. Thermionic power generation uses thermionic emission to extract electrons from hot metals and generate electricity.

1. DIRECT ENERGY CONVERSION
SYSTEM
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DINESH

2. FUEL CELL
Fuel cell is electrochemical device which converts chemical energy of cell
is directly converted in to low voltage direct current electrical energy
It is generally describe as a primary battery in which fuel and oxidizer are
stored external to the battery and fed to it as needed.
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3. Fig. shows the diagram of the fuel cell .
# The fuel gas diffuses through the anode and is oxidized and release the electron to
the external circuit.
# The oxidizer diffuses through the cathode and is reduced by the electrons that have
come from the anode by the ways of external circuit.
# From the available fuel cells , hydrogen fuel cells has good results.
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Fuel Cell Volt Reaction
Hydrogen 1.23 Volt
Hydrazine 1.56 Volt
Carbon
(Coal)
1.02 Volt
Methane 1.05 Volt
OHOH 222 22 
22242 2 NOHOHN 
22 COOC 
OHCOOCH 2224 22 

4. Hydrogen fuel Cell :
# It has three chambers separated by two porous electrodes (Anode and Cathode)
# The middle chamber between the electrode is filled with a strong solution of
Potassium Hydroxide
# The surface of the electrode are chemically treated to repel the electrolyte, so
min leakage of potassium hydroxide in to the outer chamber.
# The gases diffuses through the electrodes.
The water formed is drawn off from the side
The electron librated at the anode find their
ways to the cathode through external circuit
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OH
OHeOH
eOHOH
OH
222
22
22
2O2HReactionCell
)(442OCathode
24)(42HAnode
)(44K4KOH









5. These cells when properly designed and operated they have open circuit voltage
1.1 volt. Fuel cell have efficiency up to 60% to 70%
Advantages of Fuel Cell:
1. Conversion efficiency are very high
2. Require little attention and less maintains
3. Can be installed near the use point
4. They does not make noise
5. A little time is needed to go in to the operation
6. Space requirement is very less
Disadvantages:
1. High initial cost
2. Low service life
Applications :
1. Domestic use
2. Automotive vehicle
3. Central power stations
4. Special Applications
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6. Magneto hydrodynamic System(MHD)
MHD Generator is a device which converts the heat energy of the fuel
directly in to the electrical energy without conventional generator.
# The efficiency of MHD Generator is increased by supplying the heat at
highest practical temp.
# In MHD Generator there is a flow of conducting fluid in the presence of
magnetic and electric field
# The fluid may be gas
Principle:
It works on Faraday law of electromagnetic induction which states that
when a conductor and a magnetic field moves respects to each others
then an electric voltage is induced in the conductor
# The conductor need not to be solid it may be liquid or gas
# In MHD Generator high pressure high temp fluid is forced to flow
through a strong magnetic field
MHD System is of two types:
i) Open cycle System
ii) Closed cycle System26 November
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7. OPEN MHD SYSTEM
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8. # In this system fuel such as oil, coal, natural gas is brunt in the combustion chamber
# The hot gases produced in the combustion chamber are then seeded with small
amount of an ionized alkali metal (Cesium or potassium) to increase the electrical
conductivity of the gas
# The ionization of potassium takes place due to gas produced at high temp (2300-
2700 degree)
# This hot pressurized working fluid leaves the combustion chamber and passes
through the convergent divergent nozzle.
# After passing through the nozzle the high speed fluid is enters in to the MHD
Generator with high velocity
# Now the MHD Generator produces the direct current
# By using the inverter this direct current is converted in to the alternating current
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9. CLOSED MHD SYSTEM:
# In this system liquid metal (potassium) is used as a working fluid
# The liquid potassium after being heated in the breeder reactor is
passed through the nozzle where its velocity is increases.
# Then liquid metal with high velocity is passed through the MHD
Generator to produce the D.C. Power
# The vapors formed due to the nozzle action are separated in the
separator and condensed and then pumped back to the reactor.
# The liquid potassium coming out of the MHD Generator is passed
through the heat exchanger to use its remaining heat to run the turbine and
then pumped to the reactor
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11. Advantages of MHD System:
1. More reliable as there is no moving parts
2. Power produced free from Pollution
3. As soon as it is started it can reach to the full power level
4. The size of the plant is smaller
5. Less overall operational cost
6. The capital cost of the MHD Plants is comparable to those of
conventional power plants
7. Its efficiency is about 50%
8. Better utilization of the fuel
9. Suitable for peak power generation and emergency services
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12. Disadvantages of MHD System:
1. There is high friction loss and heat transfer loss Friction loss up to 12% of
the input
2. The MHD System operates at very high temp. To obtain high electrical
conductivity . But the electrodes must be relatively at low temp hence the
gas in the vicinity of the electrode is cooler. This increases the resistivity
of gas near the electrode hence there is large voltage drop. By adding
seed material resistivity can be decreased
3. The MHD Generator needs very large magnets this is the major expense
4. When coal is used as a fuel, poses the problem of molten ash which may
short circuit the electrodes . So natural gas and oil are used as a fuel
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13. Thermo electric system
This system is quite reliable, silent and energy converter with no moving parts
# It works on the thermoelectric effect
Thermoelectric Effect or Seebeck Effect:
If two dissimilar metals are joined to form a loop and two junction of
those loop are maintained at different temp. the an e.m.f. is will be
set up at these junctions
e.m.f.= E = α ∆T Where ∆t is the temp. difference
α is the Seebeck Coefficient
# This effect has been used in thermocouple to measure the temp.
# This method directly produces the electrical energy from heat energy
# This system has very low efficiency of order 1% to 3%
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15. Fig. shows the thermoelectric power generator
# The thermocouple material A and B are joined at hot end, but the other
ends are kept cold
# A electric voltage or e.m.f. is generated between cold ends
# A direct current will flow in the circuit and load is connected in the external
circuit.
# The floe of current will continue as long as the heat is supplied to the hot
junction
# The voltage and the electric power output are increased by increasing
the temp difference between hot and cold end
# In practice several thermocouple are connected in series to increase the
power and the voltage
# To increase the voltage transformer can be used to increase the voltage.26 November 2020 DINESH PANCHAL 15

16. # A typical thermo couple operating with hot and cold junction at v600 degree
and 200 degree gives 0.1 V and 2 A i.e. 0.5 Watt. So 1KW require 5000
couple in series.
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Material Formul
a
Value of Z
Lead Telluride Pb Te 1.5 x 10-3
Bismuth Telluride
Doped with Sb or
Se
Bi2Te3 4 x 10-3
Germanium
Telluride
With Bismuth
Ge Te 1.5 x 10-3
Cesium Sulphate Ce S 1.0 x 10-3
Zinc Antimonide
Doped with silver
Zn Sb 1.5 x 10-3

17. Thermionic Power Generation
# In terms of thermodynamics, it is a heat engine that uses a electron gas
as a working substance
# In terms of electronics , it is a diode that that transform heat to e4lectricity
by the law of thermionic
emission
# In terms of thermoelectricity it is a thermocouple in which an evacuated
space or plasma has been situated for one of the conductor
# Thermionic convertor works because of the phenomena of thermionic
emissions
# Thermionic emission mean emission of electron from the metal when the
metal is heated.
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18. Work Function:
It is defined as the energy require to extract an electron from the metal.
It si measured in electron volt (ev)
# A thermionic converter consist of two metals or electrodes with different
work function sealed in to evacuated vessel.
# The electrode with large work function is maintained at higher temp. then
the metal having smaller work function
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19. # Thermionic converter consist of electron emitter (Cathode) and electron
collector (Anode )
# Anode and Cathode are separated by vacuum
# The electrical output is obtained between the two electrodes which are
joined by external circuit
# The heat energy supplied to the cathode raises the energy of the electron
so that they may escape from the surface of the cathode and floe towards
the anode
# At anode energy of electron partially appears in form of heat which is
removed by cooling
# This system has max. efficiency is 4o% in case of Cesium
# Anode material should have low work function e.g. Barium, Strontium oxide
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20. # Good Emitter have high electron emission capability
# The main criterion for choosing the collector material is that it should have
as low work function as possible. Because the collector is held at any temp
below which it can not emit electrons
# Molybdenum has been widely used as a collector and its work function is
1.7 ev
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