The
principle of MHD generation is simple, discovered by faraday when an electric
conductor moves across a magnetic field, a emf. 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 powerful magnetic
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 upper plate 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 from heat source without
the necessity of the additional stage of steam generation as in a steam power plant.
There
are two types of MHD System
1.) Open cycle System
2.) Closed cycle System
In open
cycle system the working fluid after generating electrical 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. In closed cycle system helium or argon is
used as the working fluid.
OPEN CYCLE
MHD SYSTEM
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 produced 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. The nitrogen
and sulphur are remove 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 high
temperature (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.
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 still sufficiently 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.
6)
Seed recovery
apparatus necessary for both environmental and economics reasons.
CLOSED CYCLE
MHD SYSTEM
The closed
cycle inert gas MHD system was conceived 1965. The main disadvantages of the
open cycle system is very high 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. 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 530°C is burnt in a combustor to produce heat. In the heat
exchanger HX, this heat is transferred to argon the working fluid of MHD cycle.
The combustion products after passing through the airpreheater (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 for further 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
NEED FOR
FURTHER RESEARCH
The
commercial use of MHD concept has not been possible because 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.
ADVANTAGES OF
MHD GENERATION
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.
6) The
more efficient heat utilization would efficient heat utilization would
decreases the amount of heat discharged to environment and the cooling 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
Applications
1.) Power generation in space craft.
2.) Hypersonic wind tunnel experiments.
3.) Defense application.
