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IntroductionIt is well known that man had always been trying to fined
Something new to achieve a new aims 80% of total electricity
produced in the
world is hydral while remaining 20% is produced from nuclear,
thermal, solar,Geothermal energy and from magneto hydro dynamic power
generation.
MHD power generation is a new system of
electric power
generation which is said to be of high efficiency and low
pollution. In advanced
Countries MHD generator are widely used but in developing
countries like India
it is still under construction. This construction work is in
progress at
Tiruchirapalli in Tamilnadu unders joint efforts of BARC
(Bhabha Atomic
Research Centre), BHEL, Associated cement corporation and
Russiantechnologists.
As its name implies, magneto-hydro-dynamic (MHD) is
concerned with the flow of conducting fluid in presence of
magnetic and
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electric field. This fluid may be gas at elevated temperature or
liquid metal like
sodium or potassium. A MHD generator is a device for
converting heat energyof fuel directly into electric energy without a conventional
electric generator.
The basic difference between conventional generator and MHD
generator is in
the nature of conductor.
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Principle of MHD power generation
The principle of MHD generation is simple, discovered by
faraday when an electric conductor moves across a magnetic field, aemf. Is
induced in it, which produced an electric current. This is the principle of
the
conventional generator also, where the conductors consists of copper
strips. In
MHD generator the solid conductors are replaced by a gaseous
conductor; i.e.
an ionized gas. If such gas is passed at high velocity through a powerfulmagnetic field, i.e. suppose we have a charged particle (having charge q)
moving at a high velocity V towards right and a perpendicular
magnetic field
is applied. A magnetic force F acts on the charged particle. Show in
fig. A
Positively charged particles as forced upward and negatively charged
particle
is forced downward.
The positive ions would be accelerated towards the upperplate P, and negative ions would be accelerated towards the plate P2 . If
the P1
and P2 are externally connected through a resistance, a current would
flow
through the resistance. Thus gas energy directly converted into electrical
energy. This is the principle of MHD generator. A MHD conversion is
known as
direct energy conversion because it produced electricity directly fromheat
source without the necessity of the additional stage of steam generation
as in
a steam power plant.
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Lorentz forceLorentz force law describes the effect of a charge particle moving in a
constant magnetic field.
F=Q.(V x B)
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MHD systems
There are two types of systems.1.open cycle system
2.closed cycle system
In open cycle system the working fluid after generatingelectrical energy is discharged to the atmosphere through a stack. In a
closed
cycle system the working fluid is recycled to the heat sources and thus
used
again and again. The operation of MHD generator directly on
combustion
products in an open cycle system. In open cycle system working fluid is
air. Inclosed cycle system helium or argon is used as the working fluid.
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An elementary open cycle MHD system is shown in fig. The
MHD generator resembles a rocket engine surrounded by a
magnet the coal is burnt to produce hot gas. The hot gas is then
seeded with a small amount of an ionized alkali metal (cesium or
potassium) to increase the electrical conductivities of gas. The
gas expand through the generator surrounded by powerful
magnet. During the motion of gas the positive and negative ions
move to the electrodes and constitute an electric current. The
rejected gas passes through an air heater for preheating the inlet
air. The seed material is recovered for successive use. Thenitrogen and sulphor are removing for pollution controlled and
then gasses are discharged to the atmosphere. The above cycle is
not suitable for commercial use. The exhaust gases of MHD unit
are still at a sufficiently hot temperature it is possible to use for
additional power generation in a steam turbine alternator unit.
This is increase the efficiency of process. Such cycle is known as
hybrid MHD-steam plant cycle. Figure shows hybrid MHD steam
cycle, coal is processed and burnt in the combustor at hightemperature (2750 to 3000 K) and pressure (7 to 15 at
atmosphere) with preheated air to form the plasma. The plasma is
then seeded with small fraction (1%) of an alkali metal
(potassium) introduced usually as a carbonate powder or solution.
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The resulting mixture having an electrically conductivity of about
10 ohm/m is expanded through a nozzle to increase its velocity
and then passed through the high magnetic field (5 to 7 teslas) of
the MHD generator. Electrodes channel provided electric contact
between flow and external load. The power o/p is dc and it is
necessary to use to change it to ac before the power can be fed
to an electric grid. The gas coming out of MHD generator is stillsufficiently hot and is used to raise steam, which generates
additional energy in a steam in a steam turbine alternator unit. A
part of this steam is also used in a steam turbine which driver a
compressor for compressing air for the MHD cycle. The seed
material is recovered from the gas the harmful emissions
(sulphur) are also removed from gas before it is discharged to
atmosphere through a stack. For efficient practical realization a
MHD system must have following features:1) Air superheating arrangement to heat the gas to around 2500
C so that the
electrical conductivity gas is increased.
2) The combustion chamber must have low heat losses
3) A management to add a low ionization potential seed material
to the gas to
increase, its conductivity.
4) A water cooled but electrically insulating expanding dust with
long life electrodes.
5) A magnet capable of producing high magnetic flux density.
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6) Seed recovery apparatus necessary for both environmental and
economics
reasons.
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The closed cycle inert gas MHD system was conceived 1965. The
main disadvantages of the open cycle system is very high MHD
Power Generation temperature requirement and a very
chemically active flow could be removed, by closed cycle MHD
system. As the name suggests the working fluid in closed cycle, is
circulated in a closed loop. The working fluid is helium or argon
with cesium seeding. Figure show a closed cycle MHD system.
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The complete system has three distinct but interlocking loops. On
the left is the external heating loop, coal is gasified and the gas
having a high heat value of about 5.35 MJ/kg and temperature of
about 530C is burnt in a combustor to produce heat. In the heat
exchanger HX, this heat is transferred to argon the working fluidof MHD cycle. The combustion products after passing through the
air preheater (to recover a part of the heat of combustion
product) and purifiers (To remove harmful emissions) and
discharged to atmosphere. The loop in the centre is the MHD loop.
The hot argon gas is seeded with cesium and passed through
MHD generator. The dc power output of MHD generator is
converted to A.C. by the inverter and is then feed into the grid.
The loop shown on the right hand side in fig is the steam loop forfurther recovering the heat of the working fluid and converting
this heat into electrical energy. The fluid passes through the heat
exchanges HX2 where it imparts its heat to water which gets
converted to steam. This steam is used partly for during a turbine
which runs the compressor partly for turbine driver an alternator.
The output of the alternator is also to the grid. The working fluid
goes back to the heat exchanges HX) after passing through
compressor and intercooler. A closed system can provide more
useful power conversion at lower temperatures (around 1900o K
as compared to 2500o K for open cycle system).
11.
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The MHD generator or dynamo transfer thermal energy orkinetic energy directly into electricity.
MHD generators are different from traditional electric
generators in that they can operate at high temperatures
without moving parts
A simple magnetohydrodynamic generator consists of a gas
nozzle. The gas nozzle is a combustion chamber that injects
a pulse of gas into the channel/duct. The walls of the
channel act as an electrode. The induced electric current is
fed to the load by an external circuit that supplies thegeneratedelectricityto the desired destination. The MHD
generators can be constructed in various designs like the
Faraday generator, Hall generator and disc generator.
Faraday generator was the first designed MHD generator. It
was made by Michael Faraday in 1831. The Faraday
generator used copper disks and a horse-shoe magnet to
generate electricity.
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Under high pressure condition, an electrically conducting
gas is produced by burning afossil fuel. Most of the MHD
systems use coal ornatural gasas fossil fuel. However,
inert gases like argon andheliumare also used in some
MHD systems. The gas is passed through a nozzle at a high
speed of 1000 to 2000 meter per second. The
magnetohydrodynamic generators do not create electric
charge, it is inherent in the ionized fluid or gases. Tounderstand it better, think of a water pump that allows the
water to pass through but it is not the source of water.
Conductivity of the fluid can be increased by adopting
various methods. If the fluid is an inert gas, then suitable
amounts of potassium carbonate can be added.
The gas enters the channel or duct where a strong magnetic
field is applied with the help of superconductingmagnets.
The magnetic intensity or magnetic field strength of the
magnetic field inside the channel is usually between 3 to 5
Tesla. As the gas passes through the channel, an
electromotive force is experienced by it. How does this
force arise? According to the Faraday's law ofelectromagnetic induction current/voltage (EMF) is
induced in a coil/wire whenever there is a change in the
magnetic flux linked with the coil. Here, the electromagnets
are stationary but the conductor fluid is moving constantly.
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A simple Faraday generator would consist of a wedge-shaped pipe or tube of some non-conductive material
When an electrically conductive fluid flows through the
tube, in the presence of a significant perpendicular
magnetic field, a charge is induced in the field
This charge can be drawn off as electrical power by placing
the electrodes on the sides at 90 degree angles to themagnetic field
The main practical problem of a Faraday generator is that
differential voltages
Currents in the fluid short through the electrodes on the
sides of the duct
The most powerful waste is from the Hall effect current
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The normal scheme is to place arrays of short, verticalelectrodes on the sides of the duct.
The first and last electrodes in the duct power the load
Each other electrode is shorted to an electrode on the
opposite side of the duct.
These shorts of the Faraday current induce a powerful
magnetic field within the fluid, but in a chord of a circle at
right angles to the Faraday current.
Losses are less than a Faraday generator, and voltages are
higher because there is less shorting of the final induced
current.
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A disc generator has fluid flowing between the center of a
disc, and a duct wrapped around the edge. The magnetic
excitation field is made by a pair of circular Helmholtz
coils above and below the disc
The Faraday currents flow in a perfect dead short around
the periphery of the disk
The Hall effect currents flow between ring electrodes near
the center and ring electrodes near the peripheryAnother significant advantage of this design is that the
magnet is more efficient.
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The commercial use of MHD concept has not been possiblebecause numerous technological advancements are needed prior
to commercialization of MHD systems. Most of these are related
to material problem created by the simultaneous presence of
high
temperature and a highly corrosive and abrasive environment.
The MHD channel operates extreme conditions of temperature,
magnetic and electric fields. Search is on for better insulator and
electrode materials which can with stand the electrical, thermal,mechanical and thermo-chemical stresses and corrosion.
MHD generation offers several advantages as compared to other
methods of electric generation.
1) The conversion efficiency of a MHD system can be 50% as
compared to less than 40 percent for the most efficient steam
plants.
2) Large amount of power is generated.3) It has no moving parts, so more reliable.
4) It has ability to reach the full power level as soon as started.
5) Because of higher efficiency, the overall generation cost of an
MHD plant will be less.
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6) The more efficient heat utilization would efficient heat
utilization would
decreases the amount of heat discharged to environment and thecooling
water requirements would also be lower.
7) The higher efficiency means better fuel utilization. The
reduce
fuel
consumption would offer additional economic and social
benefits.
8) The Closed cycle system produces power free of pollution.Following table show the pollutant emission of an MUD plant
and
a
conventional steam plant.
Pollutant emission m tons per day based on use of coal having-3
percent
sulphur contents.
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1.) Power generation in space craft.
2.) Hypersonic wind tunnel experiments.
3.) Defense application.
4.) Astrophysics
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With the increased industrial and agricultural activities, powerdemand is also highly increased. In the country is sure to fall
short of the energy demand by the first decade of next century.
This means an additional capacity of power is required next 10
year. The answer to this is in non conventional energy. The
MHD
power generation is in advanced stage today and closer to
commercial utilization significant progress has been made in
development of all critical component and sub systemtechnologies coal burning MHD combined steam power plant
promise significant economic and environmental advantages
compared to other coal burning power generate technologies. It
will not be long before the technological problem of MHD
generate are overcome and MHD power generation transform
itself from nonconventional to conventional energy sources.
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Non-Conventional energy source : G.D. Rai
Electrical Power System : Dr. Inamdar
Generation Distribution & Utilization of Electrical Energy :
C.L.Wadwa
www.wikipedia.org