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Transcript of Magneto Hydro Dynamic
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CONTENTSIntroduction 2
Description 3
1.What is MHD? 31.1 Ideal MHD Equations
1.2 Applicability of MHD to plasmas 4
1.3 The importance of resistivity and kineticEffects 4-5
1.4 Structures in MHD Systems 61.5 Applications 6
2.Magneto Hydro Dynamic Generator 7
2.1 Principle 8-9
2.2 Types of MHD Generator Systems 9-14a.Open cycle MHD
b.Closed cycle MHD
c.Liquid Metal MHD Generators
d.Faraday Generator
e.Hall Generator
f.
Disc Generator2.3 MHD Generator Construction 15-172.4 Efficiency and Economics 18
2.5 Toxic byproducts 19
Applications 19Conclusion 20
Reference 20
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INTRODUCTION
The Magneto Hydro Dynamic power generation technology (MHD) is the production
of electrical power utilising a high temperature conducting plasma moving through anintense magnetic field. The conversion process in MHD was initially described by
Michael Faraday in 1893. However the actual utilisation of this concept remained
unthinkable. The first known attempt to develop an MHD generator was made at
Westing house research laboratory (USA) around 1936.
The efficiencies of all modern thermal power generating
system lies between 35-40% as they have to reject large quantities of heat to the
environment. We need to improve this efficiency level as in all other conventional
power plants like nuclear power plant, hydro-electric power plant, for which first the
thermal energy of the gas is directly converted in to electrical energy. Hence it is
known as direct energy conversion system. The MHD power plants are classified in to
Open and Closed cycle based on the nature of processing of the working fluid. With
the present research and development programmes, the MHD power generation may
play an important role in the power industry in future to help the present crisis of
power.
The MHD process can be used not only for commercial
power generation but also for so may other applications. It is economically attractive
from the design point of view and as far as bulk generation of power is concerned.
The MHD process promises a dramatic improvement in the cost of generating
electricity from coal, beneficial to the growth of the national economy. Not only that
the extensive use of MHD can help in saving billions of dollars towards fuel
prospects, lead to much better fuel utilization but the potential of lower capital costs
with increased utilization of invested capital also provides a very important economic
incentive in this case. The beneficial environmental aspects of MHD are probably of
equal or even greater significance. The MHD energy conversion process contributes
greatly to the solution of the serious air and thermal pollution problems faced by all
steam - electric power plants while it simultaneously assures better utilization for our
natural resources. The high temperature MHD process makes it possible to take
advantage of the highest flame temperatures which can be produced by combustion
from fossil fuel. While commercial nuclear reactors able to provide heat for MHD
generators have yet to be developed, the combined use of MHD generators with
nuclear heat source holds great promise for the future. In India, coal is by far the most
abundant fossil fuel and thus the major energy source for fossil fuelled MHD power
generation.
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DESCRIPTION
1.What is Magneto Hydro Dynamics?Magneto Hydro Dynamics (magneto-fluid-dynamics or hydro-magnetics) is theacademic discipline which studies the dynamics of electrically conducting fluids.
Examples of such fluids include plasmas, liquid metals, and salt water. The word
Magneto Hydro Dynamic (MHD) is derived from Magneto- meaning magnetic field,
Hydro- meaning liquid , and Dynamics- meaning movement. The field was initiated
by Hannes Alfven , for which he received the Nobel Prize in physics in 1970. The
idea of MHD is that magnetic fields can induce currents in a moving conductive fluid,
which create forces on the fluid, and also change the magnetic field itself. The set of
equations which describe MHD are a combination of the Navier-Stokes equations of
fluid dynamics and Maxwell's equations of electromagnetism. These differential
equations have to be solved simultaneously, either analytically or numerically.Because MHD is a fluid theory, it cannot treat kinetic phenomena, i.e., those in which
the existence of discrete particles, or of a non-thermal distribution of their velocities is
important.
The simplest form of MHD, Ideal MHD, assumes that the fluid has so little resistivity
that it can be treated as a perfect conductor. (This is the limit of infinite magnetic
Reynolds number.) In ideal MHD, Lenz's law dictates that the fluid is in a sense tied
to the magnetic field lines. To be more precise, in ideal MHD, a small rope-like
volume of fluid surrounding a field line will continue to lie along a magnetic field
line, even as it is twisted and distorted by fluid flows in the system. The connection
between magnetic field lines and fluid in ideal MHD fixes the topology of themagnetic field in the fluid - for example, if a set of magnetic field lines are tied into a
knot, then they will remain so as long as the fluid/plasma has negligible resistivity.
This difficulty in reconnecting magnetic field lines makes it possible to store energy
by moving the fluid or the source of the magnetic field. The energy can then become
available if the conditions for ideal MHD break down, allowing magnetic
reconnection that releases the stored energy from the magnetic field.
1.1 Ideal MHD Equations:The ideal MHD equations consist of the continuity equation (mass), the momentum
equation, Ampere's Law in the limit of no electric field and no electron diffusivity,
and a temperature evolution equation. As with any fluid description to a kinetic
system, a closure approximation must be applied to highest moment of the particle
distribution equation. This is often accomplished with approximations to the heat flux
through a condition of adiabaticity or isothermality.
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1.2 Applicability of MHD to Plasmas:
Ideal MHD is only strictly applicable when:
1. The plasma is strongly collisional , so that the time scale of collisions is
shorter than the other characteristic times in the system, and the particle
distributions are therefore close to Maxwellian.
2. The resistivity due to these collisions is small. In particular, the typical
magnetic diffusion times over any scale length present in the system must be
longer than any time scale of interest.
3. We are interested in length scales much longer than the ion skin depth and
Larmor radius perpendicular to the field, long enough along the field to ignore
Landau damping, and time scales much longer than the ion gyration time
(system is smooth and slowly evolving).
MHD simulation of solar wind
1.3 The importance of resistivity and kinetic effects:In an imperfectly conducting fluid, the magnetic field can generally move through the
fluid, following a diffusion law with the resistivity of the plasma serving as diffusion
constant. This means that solutions to the ideal MHD equations are only applicable
for a limited time for a region of a given size before diffusion becomes too important
to ignore. One can estimate the diffusion time across a solar active region (from
collisional resistivity) to be hundreds to thousands of years, much longer than the
actual lifetime of a sunspot - so it would seem reasonable to ignore the resistivity. By
contrast, a meter-sized volume of seawater has a magnetic diffusion time measured in
milliseconds.
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Even in physical systems which are large and conductive enough, simple estimates
suggest that the resistivity can be ignored, resistivity may still be important: much
instability exists that can increase the effective resistivity of the plasma by factors of
more than a billion. The enhanced resistivity is usually the result of the formation of
small scale structure like current sheets or fine scale magnetic turbulence, introducing
small spatial scales into the system over which ideal MHD is broken and magneticdiffusion can occur quickly. When this happens, Magnetic Reconnection may occur in
the plasma to release stored magnetic energy as waves, bulk mechanical acceleration
of material, particle acceleration, and heat. Magnetic reconnection in highly
conductive systems is important because it concentrates energy in time and space, so
that gentle forces applied to plasma for long periods of time can cause violent
explosions and bursts of radiation.
When the fluid cannot be considered as completely conductive, but the other
conditions for ideal MHD are satisfied, it is possible to use an extended model called
resistive MHD. This includes an extra term in Ampere's Law which models the
collisional resistivity. Generally MHD computer simulations are at least somewhatresistive because their computational grid introduces a numerical resistivity.
Another limitation of MHD (and fluid theories in general) is that they depend on the
assumption that the plasma is strongly collisional (this is the first criterion listed
above), so that the time scale of collisions is shorter than the other characteristic times
in the system, and the particle distributions are Maxwellian. This is usually not the
case in fusion, space and astrophysical plasmas. When this is not the case, or we are
interested in smaller spatial scales, it may be necessary to use a kinetic model which
properly accounts for the non-Maxwellian shape of the distribution function.
However, because MHD is very simple, and captures many of the important
properties of plasma dynamics, it is often qualitatively accurate, and is almost
invariably the first model tried. Effects which are essentially kinetic and not captured
by fluid models include double layers, Landau damping, a wide range of instabilities,
chemical separation in space plasmas and electron runaway.
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1.4 Structures in MHD Systems:
Schematic view of the different current systems which shape the Earths
magnetosphere
In many MHD systems, most of the electric current is compressed into thin, nearly-
two-dimensional ribbons termed current sheets. These can divide the fluid intomagnetic domains, inside of which the currents are relatively weak. Current sheets in
the solar corona are thought to be between a few meters and a few kilometers in
thickness, which is quite thin compared to the magnetic domains (which are thousands
to hundreds of thousands of kilometers across). Another example is in the earth's
magnetosphere, where current sheets separate topologically distinct domains, isolating
most of the earth's ionosphere from the solar wind.
1.4 Applications :MHD as a science has its application in geophysics as well in astrophysics. MHD is
related to engineering problems such as plasma confinement, liquid-metal cooling of
nuclear reactors, and electromagnetic casting, power generation (among others).
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2. Magneto Hydro Dynamic Generator
The MHD (magneto-hydro-dynamic) generator or dynamo transforms thermal
energy or kinetic energy directly into electricity. MHD generators are different from
traditional electric generators in that they can operate at high temperatures without
moving parts. MHD was eagerly developed because the exhaust of a plasma MHD
generator is a flame, still able to heat the boilers of a steam power plant. So high-
temperature MHD was developed as a topping cycle to increase the efficiency of
electric generation, especially when burning coal or natural gas. It has also been
applied to pump liquid metals and for quiet submarine engines. The basic concept
underlying the mechanical and fluid dynamos is the same. The fluid dynamo,
however, uses the motion of fluid or plasma to generate the currents which generate
the electrical energy. The mechanical dynamo, in contrast, uses the motion of
mechanical devices to accomplish this. The functional difference between an MHD
generator and an MHD dynamo is the path the charged particles follow.
MHD generators are now practical for fossil fuels, but have been overtaken by other,
less expensive technologies, such as combined cycles in which a gas turbine's or
molten carbonate fuel cell's exhaust heats steam for steam turbine. The unique value
of MHD is that it permits an older single-cycle fossil-fuel power plant to be upgraded
to high efficiency. Natural MHD dynamos are an active area of research in plasma
physics and are of great interest to the geophysics and astrophysics communities.
From their perspective the earth is a global MHD dynamo and with the aid of the
particles on the solar wind produces the aurora borealis. The differently charged
electromagnetic layers produced by the dynamo effect on the earth's geomagneticfield enable the appearance of the aurora borealis. As power is extracted from the
plasma of the solar wind, the particles slow and are drawn down along the field lines
in a brilliant display over the poles.
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2.1 MHD Power Generation (The Principle):
When an electrical conductor is moved so as to cut lines of magnetic induction, the
charged particles in the conductor experience a force in a direction mutually
perpendicular to the magnetic field (B) and to the velocity of the conductor (v). The
negative charges tend to move in one direction, and the positive charges in the
opposite direction. This induced electric field, or motional emf, provides the basis for
converting mechanical energy into electrical energy
The Lorentz Force Law describes the effects of a charged particle moving in a
constant magnetic field. The simplest form of this law is given by the vector equation.
Where
F is the force acting on the particle (vector),
Q is charge of particle (scalar),
v is velocity of particle (vector),
x is the cross product,
B is magnetic field (vector).
The vectorF is perpendicular to both v and B according to the Right hand rule.
At the present time nearly all electrical power generators utilize a solid conductor
which is caused to rotate between the poles of a magnet. In the case of hydroelectric
generators, the energy required to maintain the rotation is supplied by the gravitationalmotion of river water. Turbo-generators, on the other hand, generally operate using a
high-speed flow of steam or other gas. The heat source required to produce the high-
speed gas flow may be supplied by the combustion of a fossil fuel or by a nuclear
reactor (either fission or possibly fusion). It was recognized by Faraday as early as
1831 that one could employ a fluid conductor as the working substance in a power
generator. To test this concept Faraday immersed electrodes into the Thames River at
either end of the Waterloo Bridge in London and connected the electrodes at mid span
on the bridge through a galvanometer. Faraday reasoned that the electrically
conducting river water moving through the earth's magnetic field should produce a
transverse emf. Small irregular deflections of the galvanometer were in fact observed.
The production of electrical power through the use of a conducting fluid moving
through a magnetic field is referred to as magneto-hydro-dynamic, or MHD, power
generation. One of the earliest serious attempts to construct an experimental MHO
generator was undertaken at the Westinghouse laboratories in the .period 1938-1944,
under the guidance of Karlovitz. This generator (which was of the annular Hall type)
utilized the products of combustion of natural gas, as a working fluid, and electron
beam ionization. The experiments did not produce the expected power levels because
of the low electrical conductivity of the -gas and the lack of existing knowledge of
plasma properties at that time. A later experiment at Westinghouse by Way, OeCorso,
Hundstad, Kemeny, Stewart, and Young (1961), utilizing a liquid fossil fuel-seeded
with a potassium compound, was much more successful and yielded power levels inexcess of 10 kW. Similar power levels were achieved at the Avco Everett laboratories
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by Rosa (1961) using arc-heated argon at 3000K seeded with powdered
potassium carbonate. In these latter experiments- seeding the working gas with
small concentrations of potassium was essential to provide the necessary number of
freeelectrons required for an adequate electrical conductivity. (Other possible seeding
materials having a relatively low ionization potential are the alkali metals Cesium and
Rubidium.)
2.2 Types of MHD generator systems:
During the decade beginning, about 1960 three general types of MHD generator
systems evolved, classified according to the working fluid and the anticipated heat
source. They are as follows:
Open Cycle MHD generators: They operate with the products of combustion
of a fossil fuel and are closest to practical realization. Closed Cycle MHD generators: They are usually envisaged as operating
with nuclear reactor heat sources, although fossil fuel heat sources have also
been considered. The working fluid for a closed cycle system can be either a
seeded noble gas or a liquid metal. Because of temperature limitations
imposed by the nuclear fuel materials used in reactors, closed-cycle MHD
generators utilizing a gas will require that the generator operate in a non-
equilibrium mode.
Liquid Metal MHD generators: They operate basically with liquid metals,
flowing through ducts, while the operating principle remains the same.
Typically for a large scale power station to approach operational efficiency incomputer models, steps must be taken to increase the electrical conductivity of the
conductive substance. The heating of a gas to plasma or the addition of other easily
ionizable substances like the salts of alkali metals accomplishes this increase in
conductivity. In practice a number of issues must be considered in the implementation
of a MHD generator: Generator efficiency, Economics, and Toxic byproducts. These
issues are affected by the choice of one of the three MHD generator designs. These
are the Faraday generator, the Hall generator, and the disc.
Faraday generator: The Faraday generator is named after the man whofirst looked for the effect in the Thames River. 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, which can be drawn off as electrical power by placing the electrodes
on the sides at 90 degree angles to the magnetic field.
There are limitations on the density and type of field used. The amount of power that
can be extracted is proportional to the cross sectional area of the tube and the speed of
the conductive flow. The conductive substance is also cooled and slowed by this
process. MHD generators typically reduce the temperature of the conductive
substance from plasma temperatures to just over 1000 C.
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The main practical problem of a Faraday generator is that differential voltages and
currents in the fluid short through the electrodes on the sides of the duct. The most
powerful waste is from the Hall effect current. This makes the Faraday duct very
inefficient. Most further refinements of MHD generators have tried to solve this
problem. The optimal magnetic field on duct-shaped MHD generators is a sort of
saddle shape. To get this field, a large generator requires an extremely powerfulmagnet. Many research groups have tried to adapt superconducting magnets to this
purpose, with varying success.
Hall generator: The most common answer is to use the Hall effect tocreate a current that flows with the fluid. The normal scheme is to place arrays
of short, vertical electrodes 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. This secondary, induced field makes
current flow in a rainbow shape between the first and last electrodes.
Losses are less than a Faraday generator, and voltages are higher because there is less
shorting of the final induced current. However, this design has problems because the
speed of the material flow requires the middle electrodes to be offset to "catch" the
Faraday currents. As the load varies, the fluid flow speed varies, misaligning the
Faraday current with its intended electrodes, and making the generator's efficiency
very sensitive to its load.
Disc generator: The third, currently most efficient answer is the Hall effect
disc generator. This design currently holds the efficiency and energy densityrecords for MHD generation. 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 disk.
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 periphery.
Another significant advantage of this design is that the magnet is more efficient. First,
it has simple parallel field lines. Second, because the fluid is processed in a disk, the
magnet can be closer to the fluid, and magnetic field strengths increase as the 7th
power of distance. Finally, the generator is compact for its power, so the magnet isalso smaller. The resulting magnet uses a much smaller percentage of the generated
power.
An MHD generator, like a turbo generator, is an energy conversion device and can be
used with any high-temperature heat source-chemical, nuclear, solar, etc. The future
electrical power needs of industrial countries will have to be met for the most part by
thermal systems composed of a heat source and an energy conversion device. In
accordance with thermodynamic considerations, the maximum potential efficiency of
such a system (i.e., the Carnot efficiency) is determined by the temperature of the heat
source. However, the maximum actual efficiency of the system will be limited by the
maximum temperature employed in the energy conversion device. The closer the
temperature of the working fluid in the energy conversion device to the temperature of
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the heat source, the higher the maximum potential efficiency of the overall system. A
spectrum of heat source temperatures is currently available, up to about 3000K.
However, at the present time large central station power production is limited to the
use of a single energy-conversion scheme-the steam turbo-generator-which is capable
of operating economically at a maximum temperature of only 850K. The over-all
efficiencies of present central-station power-producing systems are limited by thisfact to values below about 42 percent, which is a fraction of the potential efficiency. It
is clear that a temperature gap exists in our energy conversion technology. Because
MHD power generators, in contrast to turbines, do not require the use of moving solid
materials in the gas stream, they can operate at much higher temperatures.
Calculations show that fossil-fuelled MHD generators may be capable of operating at
efficiencies between 50 and 60 percent. Higher operating efficiencies would lead to
improved conservation of natural resources, reduced thermal pollution, and lower fuel
costs. Studies currently in progress suggest also the possibility of reduced air
pollution.
The essential elements of a simplified MHD generator are shown below in the figure.
This type of generator is referred to as a continuous electrode Faraday generator. A
field of magnetic induction B is applied transverse to the motion of an electrically
conducting gas flowing in an insulated duct with a velocity u.
A Simplified MHD generator
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The Charged particles moving with the gas will experience an induced electric field
u x B which will tend to drive an electric current in the direction perpendicular to
both u and B. This current is collected by a pair of electrodes on opposite sides of the
duct in contact with the gas and connected externally through a load. Neglecting the
Hall effect, the magnitude of the current density for a weakly ionized gas is given by
the generalized Ohm's law as
J = (E + u x B)(1)
The electric field E, which is added to the induced field, results from the potential
difference between the electrodes. For the purposes of initial discussion it is assumed
that both u and are uniform. In terms of the coordinate system shown in the fig, we
have
Jy= (Ey- uB).(2)At open circuit Jy = 0, and so the open circuit electric field is uB. For thecharacteristic conditions u ~1000 m sec-1 and B ~2 T, the open circuit electric field is
uB ~ 2000 V m-1. At short circuit Ey = 0, and the short circuit current is Jy = - uB.
For general load conditions, it is conventional to introduce the loading parameter
given by the expression
K= Ey / uB......(3)
Where0 K 1 and Jy = -uB(1 - K). The negative sign indicates that theconventional current flows in the negative y-direction.Since the electrons flow in the
opposite direction, the bottom electrode must serve as an electron emitter, or cathode,
and the upper electrode is an anode. The electrical power delivered to the load per unit
volume of a MHD generator gas is given by
P = -JE..(4)
For the generator shown in the fig above
P = u2B
2K(1 - K)....(5)
This power density has a maximum value
Pmax
=u2b
2/4...(6)
for K = 1/2.The rate at which directed energy is extracted from the gas by the
electromagnetic field per unit volume is -u (J x B). Therefore the electrical efficiency
of the MHD generator is defined as
e=(JE) / ( u (J x B) )(7)
For the generator being discussed,e=K.The Faraday generator therefore tends tohigher efficiency near open circuitoperation.
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A Faraday MHD generator
To circumvent the deleterious consequences of the Hall Effect, the electrodes may be
segmented in the manner indicated in the fig given below and separate loads
connected between opposed electrode pairs. The various geometrical constructions of
the MHD generator are shown as follows:
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Electrode connections for linear MHD generators.
2.3 MHD generator construction:
This series of diagrams conclude the structural design of a MHD Generator. Below, is
the combustion chamber. This device is designed to produce a explosive pulse of gas.
This most basic component is designed on the same principals as a pulsed rocket
engine. The fuel injection system is not defined but, would require a high pressure
pump, and the utilization of stepping motors to regulate frequency.
The camber shape is correct to produce a flaming smoke ring. This is managed very
simply by a few structural features that are not visible in this diagram, or any other.
Very simply the hole in the dome is one third the diameter of the extended tube. A
simple test of a similar system would be a plastic 1 gallon milk jug filled with smoke.
Then it is just striked without crushing it. Consistently, it will produce smoke rings.
This diagram is intended to produce a flaming smoke ring. The difference is that hightemperature gases are by law of physics, plasmas, and plasmas conduct electricity as
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well as wire. Basically, some fuels to burn, hot enough to produce plasma, welding
gases, and hydrogen often qualify. Plasmas occur around the same temperatures at
which Thermionic Emission takes place, which, is one of the basics of vacuum tube
theory. This power generator works on the principal of two known factors discovered
present in standard rotary generators. A loop of conductive gas, act like a short
circuited loop of wire. Kick EMF then intensifies an existing magnetic field producedby a permanent magnet. Underneath it is another shorted loop of wire that produces an
even more intense magnetic field. From that point a second receiving coil is used to
accumulate the energy in the exploding gas.
This is the position of the coil/permanent magnet pair.
This diagram shows the position of the receiving coil which is used like any typical
generator's output coil.
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Now, if we choose to build a device for continuous operation, then there's a little trick
to keep all but the radiated heat away from the metal. That's by introducing a neutral
gas, like nitrogen, CO2, or liquid helium. This gas will expand explosively but, not
reach a true temperature of conductivity that the burning fuel will, or can, based on
the mix ratio. The objective of the position of the coolant injectors is to produce a
layer of cold gas around and over the combustion plate. The combustion serves two
purposes, one is to shape the detonation of the fuel in order to produce a vortex, or
flaming smoke ring by the time it escapes the dome and enters the exit tube. In the
exit tube the gas must be burning clear past the receiving coil. The good point of it isthat it can convert hydrogen directly to water and electricity, and the velocity or
horsepower of the escaping gas is almost directly converted to watts. By size and
weight a rocket engine is always small horse per horse.
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2.4 Generator Efficiency and Economics
As of 1994, the 22% efficiency record for closed-cycle disc MHD generators was held
by Tokyo Technical Institute. The peak enthalpy extraction in these experiments
reached 30.2%. Typical open-cycle Hall & duct coal MHD generators are lower, near17%. These efficiencies make MHD unattractive, by itself, for utility power
generation, since conventional Rankine cycle power plants easily reach 40%.
However, the exhaust of an MHD generator burning fossil fuel is almost as hot as the
flame of a conventional steam boiler. By routing its exhaust gases into a boiler to
make steam, MHD and a steam Rankine cycle can convert fossil fuels into electricity
with an estimated efficiency up to 60 percent, compared to the 40 percent of a typical
coal plant.
A magnetohydrodynamic generator might also be heated by a Nuclear reactor (either
fission or fusion). Reactors of this type operate at temperatures as high as 2000 C. Bypumping the reactor coolant into a magnetohydrodynamic generator before a
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traditional heat exchanger an estimated efficiency of 60 percent can be realised. One
possible conductive coolant is the molten salt reactors molten salt, since molten salts
are electrically conductive.
MHD generators have also been proposed for a number of special situations. In
submarines, low speed MHD generators using liquid metals would be nearly silent,eliminating a source of tell-tale mechanism noise. In spacecraft and unattended
locations, low-speed metallic MHD generators have been proposed as highly reliable
generators, linked to solar, nuclear or isotopic heat sources.
MHD generators have not been employed for large scale mass energy conversion
because other techniques with comparable efficiency have a lower investment and
operating cost. Advances in natural gas turbines achieved similar thermal efficiencies
at lower costs, with simpler equipment than an MHD topping cycle. To get more
electricity from coal, it's cheaper to simply add more low-temperature steam-
generating capacity. If high efficiency is needed in a new plant, coal gasification
feeding molten salt or solid oxide fuel cells is expected to have superior efficienciesbecause the fuel cell bypasses the inherent inefficiencies of a heat engine.
However, MHD generators for fossil fuels are inherently expensive. A certain amount
of electricity is required to maintain sustained magnetic field over 1 T. Because of the
high temperatures, the walls of the channel must be constructed from an exceedingly
heat-resistant substance such as yttrium oxide or zirconium dioxide to retard
oxidation. Similarly, the electrodes must be both conductive and heat-resistant at high
temperatures, making tungsten a common choice.
2.5 Toxic byproducts
MHD reduces overall production of hazardous fossil fuel wastes
because it increases plant efficiency. In MHD coal plants, the
patented commercial "Econoseed" process developed by the
U.S., recycles potassium ionization seed from the fly ash
captured by the stack-gas scrubber. However, this equipment is
an additional expense. If molten metal is the armature fluid a
MHD generator, care must be taken with the coolant of the
electromagnetics and channel. The alkali metals commonly used
as MHD fluids react violently with water. Also, the chemical
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byproducts of heated, electrified alkali metals and channelceramics may be poisonous and environmentally persistent.
3. Applications:
MHD was developed as a topping cycle to increase the
efficiency of electric generation, especially when burning
coal or natural gas. It has also been applied to pump liquid
metals and for quiet submarine engines.
It is used in the liquid metal cooling of nuclear reactorsand electromagnetcic casting.
MHD power generation fueled by potassium-seeded coal
combustion gas showed potential for more efficient energy
conversion (the absence of solid moving parts allows
operation at higher temperatures.)
MHD has got application in the field of orbital power
generation platforms and space propulsion. It is coupled
with a pulsed detonation rocket engine (PDRE) to
simultaneously create propulsion.
CONCLUSION
MHD process has got a wide range of applications of which the MHD
generator is a major one.It not only helps in increasing the efficiencyproblem in the thermal power plants but in one way it solves the power
deficit problem as far as bulk power generation is concerned.The
beneficial environmental aspects of MHD generator are far more
significant in todays world.In India , by far the most abundant fossil fuel
and thus the major source of energy for fossil fuelled MHD power
generation. Before large central station power plants with coal as the
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7/29/2019 Magneto Hydro Dynamic
20/20
20
energy source can become commercially viable , further development is
necessary.
REFERENCES
www.en.wikipedia.org/wiki/MHD_generator
www.en.wikipedia.org/wiki/magnetohydrodynamics
www.techsearch.htm
www.answers.com www.britannica.com
www.edufive.com
http://www.en.wikipedia.org/wiki/MHD_generatorhttp://www.en.wikipedia.org/wiki/magnetohydrodynamicshttp://www.techsearch.htm/http://www.answers.com/http://www.britannica.com/http://www.edufive.com/http://www.edufive.com/http://www.britannica.com/http://www.answers.com/http://www.techsearch.htm/http://www.en.wikipedia.org/wiki/magnetohydrodynamicshttp://www.en.wikipedia.org/wiki/MHD_generator