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.
http://en.wikipedia.org/wiki/Image:T3e_troy.jpeg


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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).
http://en.wikipedia.org/wiki/Image:Currents.jpg


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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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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

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