Successfully reported this slideshow.
Department of EEE
NEED FOR MHD
ELECTRODE CONFIGURATION FOR VARIOUS MHD
Metric tons of Carbon dioxide per capita
Magneto hydro dynamic (MHD) principle is applied
for generating power from coal fired or reactor power
MHD generator plant does not require any turbine and
does not have any generator shaft. In a MHD generator
the thermal energy in plasma (hot ionized gas) is
directly converted to electrical energy (without
intermediate conversion to mechanical shaft energy).
The field of
, for which he
Nobel Prize in
Physics in 1970 Hannes Alfvén
When an electric conductor moves across a magnetic field, a voltage is
induced in it which produces an electric current.
In MHD generator, the solid conductors are replaced by a gaseous
conductor, an ionized gas. If such a gas is passed at a high velocity
through a powerful magnetic field, a current is generated and can be
extracted by placing electrodes in suitable position in the stream.
The principle can be explained as follows: An electric conductor moving
through a magnetic field experiences a retarding force as well as an
induced electric field and current.
This effect is a result of FARADAYS LAWS OF
ELECTRO MAGNETIC INDUCTION
The induced EMF is given by Eind = u x B
where u = velocity of the
B = magnetic field intensity.
The induced current is given by, Iind = C x Eind
where C = electric
The disturbance of lines of current flow in conducting path is due to the
application of magnetic field leading to an electric potential gradient
transverse to direction of current flow.
The field Ē gets influenced by Hall Effect as follows:-
Electrons moving in field E constitute flow of current. This current flow is
through the strong magnetic field B & hence produces another gradient ε
cause by Hall Effect.
Field ε is in transverse direction to field E. The direction of electron field is
governed by electron angular frequency & electron mean free time and hall
parameters. Due to Hall Effect and resulting, the path of electrons departs
from original field caused by MHD such that the direction becomes inclined
HALL EFFECTHALL EFFECT
HALL EFFECTHALL EFFECT
There are three possible arrangements of providing electrodes in MHD generators to
counter Hall effect.
Segmented electrode configuration: The electrode segments are separated by insulator
segments, so there will be no current flowing in the direction. The electric field vector
has a component both along the channel and across the channel.
Continuous electrode configuration: In this case the electric field is across the channel
only, but the current has components along the channel as well as across it. In this
case the hall angle is minimized and thus losses are reduced.
Hall electrode: In this case the electrodes wrap up the channel all the way in segment.
The electric field becomes parallel to the channel axis. Due to this reason there cannot
be any potential difference across the channel.
The MHD systems are broadly classified into
OPEN CYCLE SYSTEM
CLOSED CYCLE SYSTEM
LIQUID METAL SYSTEM
Fossil fuel (coal) is burnt to very high
temperature in the furnace to very high
temperature to achieve ionized gas. The
electrical conductivity of the plasma is
increased by seeding the gas by addition of
potassium or cesium. High conductivity
plasma is accelerated through the MHD tunnel
convergent-divergent portion of the nozzle
The pressure ratio (P1/P2) from high pressure side to
low pressure side to nozzle throat is so high and angle
of divergence of tunnel is such that very high gas
velocity is achieved in divergent portion. Conductive
gas passes through the magnetic field B of the MHD
generator. The MHD generator delivers DC electric
power through its electrode system. Heat from hot
exhaust gases can be recovered further in the heat
recovery chamber. Heat is supplied to heat exchanger
to produce steam for steam turbine which drives
CLOSED CYCLE SYSTEMCLOSED CYCLE SYSTEM
Two general types of closed cycle MHD generators are being
Electrical conductivity is maintained in the working fluid by ionization of
a seeded material, as in open cycle system.
A liquid metal provides the conductivity.
The carrier is usually a chemical inert gas, all through a liquid carrier is
been used with a liquid metal conductor. The working fluid is circulated
in a closed loop and is heated by the combustion gases using a heat
exchanger. Hence the heat sources and the working fluid are
independent. The working fluid is helium or argon with cesium seeding.
Two working fluids are used, one for liquid metal &
the other for inert gases. Liquid metal is heated by coal
fired plant or nuclear reactor plant. Heating by liquid
metal causes the expansion of inert gases. The mixture
of inert gas & liquid metal is passed through MHD
tunnel. The gas expanding in the tunnel gets
accelerated & increases the velocity of the mixture. The
liquid metal acts as a moving conductor in the tunnel.
After passing through the MHD generator, the two
fluids are separated again. The liquid metal is reheated.
The gas is cooled & recompressed. Both the working
fluids are re-circulated again.
The conversion efficiency of a MHD system can be around 50% much
higher compared to the most efficient steam plants. Still higher
efficiencies are expected in future, around 60 – 65 %, with the
improvements in experience and technology.
Large amount of power is generated.
It has no moving parts, so more reliable.
The closed cycle system produces power, free of pollution.
It has ability to reach the full power level as soon as started.
The size if the plant is considerably smaller than conventional fossil
Although the cost cannot be predicted very accurately, yet it has been
reported that capital costs of MHD plants will be competitive to conventional
It has been estimated that the overall operational costs in a plant would be
about 20% less than conventional steam plants.
Direct conversion of heat into electricity permits to eliminate the turbine
(compared with a gas turbine power plant) or both the boiler and the turbine
(compared with a steam power plant) elimination reduces losses of energy.
These systems permit better fuel utilization. The reduced fuel consumption
would offer additional economic and special benefits and would also lead to
conservation of energy resources.
It is possible to use MHD for peak power generations and emergency service.
It has been estimated that MHD equipment for such duties is simpler, has
capability of generating in large units and has the ability to make rapid start to
In the MHD generator the advantage of having
no moving parts allows to work at higher
temperatures than a conventional energy
conversion. It is possible to work with
temperature around 3000K, and a these
temperature the maximum theoretical efficiency
would be near 90%. In the section of near future
MHD power generation system the plant
efficiency can be increased by increasing the
working temperature. Also, in order to reduce
CO2 emission, use nuclear power with high
ELECTRICAL POWER SYSTEMS- Dr. S.L. UPPAL, Prof. S.
NON-CONVENTIONAL ENERGY SOURCES- G.D. RAI
INTERNATIONAL JOURNAL OF SCIENTIFIC AND
VOLUME 3, ISSUE 6, JUNE 2013