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I. INTRODUCTION

A Magneto hydrodynamic generator (MHD generator) is a magnetohydrodynamic

device that transforms thermal energy and kinetic energy into electricity. MHD generators are different from traditional electric generators in that they operate at high temperatures without moving parts. MHD was developed because the hot exhaust gas of an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency.

Fig 1. Magnetohydrodynamic Generator Block Diagram

Typically, for a large scale power station to approach the operational efficiency of computer models, steps must be taken to increase the electrical conductivity of the conductive substance. The heating of a gas to its plasma state or the addition of other easily ionizable substances like the salts of alkali metals can accomplish this increase. In practice, a number of issues must be considered in the implementation of an MHD generator: generator efficiency, economics, and toxic byproducts. These issues are affected by the choice of one of the three MHD generator designs: the Faraday generator, the Hall generator, and the disc generator.

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Fig 2. Power production chart

� 80 % of total electricity produced in the world is hydal, while remaining 20% is produced from nuclear, thermal, solar, geothermal energy and from magneto hydro dynamic (mhd) generator.

� 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 generators are widely used but in developing countries like INDIA, it is still under construction, this construction work in in progress at TRICHI in TAMIL NADU, under the joint efforts of BARC ( Bhabha atomic research center), associated cement corporation (ACC) and Russian technologists.

� As its name implies, magneto hydro dynamics (MHD) is concerned with the flow of a conducting fluid in the presence of magnetic and electric field. The fluid may be gas at elevated temperatures or liquid metals like sodium or potassium

Thermal Power Plants,

66.44%

Hydro Power

Plant,19.13%

Nuclear Power Plant,

2.32%Rest , 12.09%

Thermal Power Plants Hydro Power Plant Nuclear Power Plant Rest

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II. INITIATOR & RESEARCHER

Magneto hydrodynamics (MHD) (magneto fluid dynamics or hydro magnetics) is

the academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water. The word magneto hydro dynamics (MHD) is derived from magneto- meaning magnetic field, and hydro- meaning liquid, and -dynamics meaning movement. The field of MHD was initiated by Hannes Alfvén , for which he received the Nobel Prize in Physics in 1970

Fig 3. Hannes Alfvén

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III. PRINCIPLE OF WORKING

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.

F = Q . ( v × B )

Where

• F is the force acting on the particle. • Q is the charge of the particle, • v is the velocity of the particle, and • B is the magnetic field.

The vector F is perpendicular to both v and B according to the right hand rule.

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III. I PRINCIPLES OF MHD POWER GENERATION:

Fig 4.Working Fluid flow

� The conducting flow fluid is forced between the plates with a kinetic energy and pressure differential sufficient to over come the magnetic induction force Find.

� The end view drawing illustrates the construction of the flow channel. � An ionized gas is employed as the conducting fluid. � Ionization is produced either by thermal means I.e. by an elevated temperature or by

seeding with substance like cesium or potassium vapors which ionizes at relatively low temperatures.

� The atoms of seed element split off electrons. The presence of the negatively charged

electrons makes the gas an electrical conductor.

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IV TYPES OF MHD GENERRATORS

� The Faraday Generator

� The Hall Generator

� The Disc Generator

IV. I THE FARADAY GENERATOR:

The Faraday generator is named after the man who first looked for the effect in the Thames river (see history). 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 voltage 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.

Fig 5.Arangement of MHD generator

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

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generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C.

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 powerful magnet. Many research groups have tried to adapt superconducting magnets to this purpose, with varying success.

IV.II THE HALL GENERATOR:

The most common solution is to use the Hall Effect to create 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.

Fig 6. MHD Hall Generator

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

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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.

IV.III THE DISC GENERATOR :

The third and, currently, the most efficient design is the Hall effect disc generator. This design currently holds the efficiency and energy density records 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.

Fig 7. MHD Disk Generator

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.[citation needed] Finally, the generator is compact for its power, so the magnet is also smaller. The resulting magnet uses a much smaller percentage of the generated power.

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V. GENERATOR EFFICIENCY:

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, near 17%. 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 magneto hydrodynamic generator might also be the first stage of a gas-cooled nuclear reactor.

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.

V.I WORKING :

The principal of MHD power generation is very simple and is based on Faraday’s law of electromagnetic induction, which states that when a conductor and a magnetic field moves relative to each other, then voltage is induced in the conductor, which results in flow of current across the terminals. As the name implies, the magneto hydro dynamics generator shown in the figure below, is concerned with the flow of a conducting fluid in the presence of magnetic and electric fields.

In conventional generator or alternator, the conductor consists of copper windings or strips while in an MHD generator the hot ionized gas or conducting fluid replaces the solid conductor.

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Fig 8.Magnetohydrodynamic Power Generation

A pressurized, electrically conducting fluid flows through a transverse magnetic field in a channel or duct. Pair of electrodes are located on the channel walls at right angle to the magnetic field and connected through an external circuit to deliver power to a load connected to it. Electrodes in the MHD generator perform the same function as brushes in a conventional DC generator.

Fig 9.Operational Flow of MHD Generator

The MHD generator develops DC power and the conversion to AC is done using an

inverter. The power generated per unit length by MHD generator is approximately given by,

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Where, u is the fluid velocity, B is the magnetic flux density, σ is the electrical conductivity of conducting fluid and P is the density of fluid. It is evident from the equation above that for the higher power density of an MHD generator there must be a strong magnetic

field of 4-5 tesla and high flow velocity of conducting fluid besides adequate conductivity.

Fig 10.Block Diagram of MHD Generator

V.II WORKING FLUIDS:

The working fluids in the MHD generators depends up on the type of cycle used in generation. Commonly, air is used as working fluid in open cycle generation and argon or

helium are used in closed cycle systems.

V.II.I COAL-FIRED MHD SYSTEMS:

The choice of type of MHD generator depends on the fuel to be used and the application. The abundance of coal reserves throughout much of the world has favoured the development of coal-fired MHD systems for electric power production. Coal can be burned at a temperature high enough to provide thermal ionization. However, as the gas expands along the duct or channel, its electrical conductivity drops along with its temperature. Thus, power production with thermal ionization is essentially finished when the temperature falls to about 2,500 K (about 2,200 °C, or 4,000 °F). To be economically competitive, a coal-fired power station would have to combine an MHD generator with a conventional steam plant in what is termed a binary cycle. The hot gas is first passed through the MHD generator (a process known as topping) and then on to the turbogenerator of a conventional steam plant (the bottoming phase). An MHD power plant employing such an arrangement is known as an open-cycle, or once-through, system.

Coal combustion as a source of heat has several advantages. For example, it results in coal slag, which under magnetohydrodynamic conditions is molten and provides a layer that covers all of the insulator and electrode walls. The electrical conductivity of this layer is

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sufficient to provide conduction between the gas and the electrode structure but not so high as to cause significant leakage of electric currents and consequent power loss. The reduction in thermal losses to the walls because of the slag layer more than compensates for any electrical losses arising from its presence. Also, the use of a seed material in conjunction with coal offers environmental benefits. In particular, the recombination chemistry that occurs in the duct of an MHD generator favours the formation of potassium sulfate in the combustion of high-sulfur coals, thereby reducing sulfur dioxide emissions to the atmosphere. The need to recover seed material also ensures that a high level of particulate removal is built into an MHD coal-fired plant. Finally, by careful design of the boiler and the combustion controls, low levels of nitrogen oxide emissions can be achieved.

V.II.II OTHER MHD SYSTEMS:

In addition to natural gas as a fuel source, more-exotic MHD power generation systems have been proposed. Conventional nuclear reactors can employ hydrogen, or a noble gas such as argon or helium, as the working fluid, but they operate at temperatures that are too low to produce the thermal ionization used in MHD generators. Thus, some form of non-equilibrium ionization using seeding material is necessary.

In theory, solar concentrators can provide thermal energy at a temperature high enough to provide thermal ionization. Thus, solar-based MHD systems have potential, provided that solar collectors can be developed that operate reliably for extended periods at high temperatures.

The need to provide large pulses of electrical power at remote sites has stimulated the development of pulsed MHD generators. For this application, the MHD system basically consists of a rocket motor, duct, magnet, and connections to an electrical load. Such generators have been operated as sources for pulse-power electromagnetic sounding apparatuses used in geophysical research. Power levels up to 100 megawatts for a few seconds have been achieved.

A variation of the usual MHD generator employs a liquid metal as its electrically conducting medium. Liquid metal is an attractive option because of its high electrical conductivity, but it cannot serve directly as a thermodynamic working fluid. The liquid has to be combined with a driving gas or vapor to create a two-phase flow in the generator duct, or it has to be accelerated by a thermodynamic pump (often described as an ejector) and then separated from the driving gas or vapor before it passes through the duct. While such liquid metal MHD systems offer attractive features from the viewpoint of electrical machine operation, they are limited in temperature by the properties of liquid metals to about 1,250 K (about 975 °C, or 1,800 °F). Thus, they compete with various existing energy-conversion systems capable of operating in the same temperature range.

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VI TYPES OF CYCLES

The MHD cycles can be of two types, namely

1. Open Cycle MHD. 2. Closed Cycle MHD.

The detailed account of the types of MHD cycles and the working fluids used, are given below.

VI.I OPEN CYCLE MHD SYSTEM:

Fig 11. Open Cycle MHD system

In open cycle MHD system, atmospheric air at very high temperature and pressure is passed through the strong magnetic field. Coal is first processed and burnet in the combustor at a high temperature of about 2700oC and pressure about 12 ATP with pre-heated air from the plasma. Then a seeding material such as potassium carbonate is injected to the plasma to increase the electrical conductivity. The resulting mixture having an electrical conductivity of about 10 Siemens/m is expanded through a nozzle, so as to have a high velocity and then passed through the magnetic field of MHD generator.

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During the expansion of the gas at high temperature, the positive and negative ions move to the electrodes and thus constitute an electric current. The gas is then made to exhaust through the generator. Since the same air cannot be reused again hence it forms an open cycle and thus is named as open cycle MHD.

VI.II CLOSED CYCLE MHD SYSTEM:

As the name suggests the working fluid in a closed cycle MHD is circulated in a closed loop. Hence, in this case inert gas or liquid metal is used as the working fluid to transfer the heat. The liquid metal has typically the advantage of high electrical conductivity, hence the heat provided by the combustion material need not be too high.

Fig 12. Closed cycle MHD system

Contrary to the open loop system there is no inlet and outlet for the atmospheric air. Hence, the process is simplified to a great extent, as the same fluid is circulated time and again for effective heat transfer.

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VII ADVANTAGES & DISADVANTAGES

VII.I ADVANTAGES OF MHD GENERATION:

The advantages of MHD generation over the other conventional methods of generation are given below.

1. Here only working fluid is circulated, and there are no moving mechanical parts. This

reduces the mechanical losses to nil and makes the operation more dependable.

2. The temperature of working fluid is maintained by the walls of MHD.

3. It has the ability to reach full power level almost directly.

4. The price of MHD generators is much lower than conventional generators.

5. MHD has very high efficiency, which is higher than most of the other conventional or

non-conventional method of generation.

6. The conversion efficiency of a MHD system can be 50% as compared to less

than 40 percent for the most efficient steam plants.

7. Large amount of power is generated.

8. It has no moving parts, so more reliable.

9. It has ability to reach the full power level as soon as started.

10. Because of higher efficiency, the overall generation cost of an MHD plant will

be less.

11. 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.

12. The higher efficiency means better fuel utilization. The reduce fuel

consumption would offer additional economic and social benefits.

13. The Closed cycle system produces power free of pollution.

VII.II DISADVANTAGES OF MHD GENERATORS:

1. They need high pure superconductor.

2. Working temperature is very high as about 200°K to 2400°K.

3. The loss of power if very high

4. The components get high corrosion due to high working temperature.

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VIII APPLICATIONS

• The MHD generators are used to power submarines and aircrafts.

• Electrical power production for domestic applications

• They are used in a pulsed detonation rocket engine (PDRE) for space application

• They can be used as power plants in industry and uninterrupted power supply

system

• Power generation in space craft.

• Hypersonic wind tunnel experiments.

• Defense application.

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IX 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.

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X CONCLUSION

With the increased industrial and agricultural activities, power demand 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 system technologies 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 non-conventional to conventional energy sources.

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XI REFERENCES

1. Blair M. Smith et al.: Gas Core Reactor-MHD Power System with Cascading Power Cycle. Conference proceedings, ICAPP'02: 2002 International congress on advances in nuclear power plants, Hollywood, FL, (abstract).

2. The correct quasi-one-dimensional model of the fluid flow in a Faraday segmented MHD generator channel

3. A reliable tool for the design of shape and size of Faraday segmented MHD generator channel

4. Australian Academy of Technological Sciences and Engineering (ATSE) website 5. Donald G. ink, H. Wayne Beatty (ed), Standard Handbook for Electrical Engineers,

11th Edition, Mc Graw Hill, 1978 ISBN 0-07-020974-X page 11-52