The electricity requirements of the world including India are increasing at alarming rate and the power demand has been running ahead of supply. It is also now widely recognized that the fossil fuels and other conventional resources, presently being used for generation of electrical energy, may not be either sufficient or suitable to keep pace with ever increasing demand of the electrical energy of the world. Also generation of electrical power by coal based steam power plants or nuclear power plants causes’ pollution. The recent severe energy crisis has forced the world to develop new and alternative methods of power generation. MAGNETO HYDRO DYNAMIC (MHD) power generation is a new unique method of power generation.
When an electrical conductor is moved so as to cut lines of magnetic induction, charged particles in the conductor experience a force in a direction mutually perpendicular to the B field and to the velocity of the conductor. 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 electricalenergy. In conventional steam power plants, the heat released by the fuel is converted into rotational mechanical energy by means of a thermo cycle and the mechanical energy is then used to drive the electric generator. Thus two stages of energy conversion are involved in which the heat to mechanical energy conversion has inherently very low efficiency. Also, the rotating machine has its associated losses and maintenance problems. In MHD generation, electrical energy is directly generated from hot combustion gases produced by the combustion of fuel without moving parts. The conventional electrical machines are basically electro mechanical converters while an MHD generator is heat engine operating on a turbine cycle and transforming the internal energy of gas directly into electrical energy.
PRINCIPLE OF WORKING OF AN MHD GENERATOR…
The basic principle of MHD is just similar to dynamo except that the solid conductor of high electrical conductivity is replaced by a gas with comparatively low value of electrical conductivity. If the gas is heated to high temperature it gets converted into plasma and plasma is conductor of electricity.
HOW AN MHD GENERATOR WORKS???
In an MHD generator, electrically conducting gas at a very high temperature is passed at high velocity through a strong magnetic field at right angles to the direction of flow, there by generating electrical energy. The electrical energy is then collected from stationary electrodes placed on the opposite sides of the channel. The current so obtained is direct current which can be converted into ac by an inverter. Ionized gas can be produced by heating it to a high temperature. On heating of a gas, the outer electrons escape out from its atoms or molecules. The particles acquire an electric charge and the gases passes into the state of plasma. However to achieve thermal ionization of products of combustion of fossil fuels or inert gases extremely high temperatures are necessary. Air becomes highly ionized at temperatures of 5,0000 to 6,0000C. To have a reasonable value of electrical conductivity of gases at temperatures around 2,000 to 3,000 K by reasonable ionization, the gases are seeded with additives of easily ionizing materials (Alkali metals) such as Cesium or Potassium. The induced current are collected by suitably placed electrodes and simplest system would carry direct current to the external load.
Limitations:
· The metallic vapours are poor electrical conductors.
· High velocities cannot be obtained by expansion in the system while it is much easier to achieve a high fluid velocity employing a gas and a nozzle. This is because the liquids are practically in compressible.
· The overall conversions efficiencies obtainable with liquid metal system are quite below to that of plasma system
ADVANTAGES OF MHD GENERATOR:
1. Since high temperatures are involved, operational efficiency is high.
2. No moving part, so more reliable.
3. As there is no limitation to the size of the duct, so high capacity generators are possible.
4. Ability of reaching the full power level instantly.
5. Conceptually these generators are very simpler.
6. The walls can be cooled below temperature of working gas.
7. The more efficient heat utilization reduces the amount of heat discharged to environment and thus the cooling water requirements are reduced.
8. The capital costs of the MHD plants are estimated to be competitive with those of coal fired steam power plants.
9. The overall costs of MHD power generation are estimated to be lower (20%) than those of conventional power plants. This is because of higher efficiency of MHD power generation.
10. The generator has high efficiency, construction is not limited by the working temperature of the gas inside the duct but is controlled by the nature of thermal source upstream of the generator.
11. Direct conversion of heat into electrical energy results in the elimination of gas turbine and both the boiler and turbine and thus in the reduction of energy losses.
12. The closed cycle system produces power, free of pollution.
13. The size of the plant is considerably smaller than conventional fossil fuel plants.
14. It can be started and put on the line within few seconds. It provides almost instantaneous standby power.
15. It can be used most economically as peak load plant.
ACHIEVEMENTS:
v USSR has constructed a pilot plant of 75MW installed capacity, 25MW is provided by the MHD generator. The fuel used is natural gas. The plant is designated as “U-25”.
v A 5-15 MW thermal input pilot plant is being set up in India at Tiruchirapalli. This plant uses fluidized bed combustion.
v Besides the use of MHD system for commercial electrical power generation it has got other special uses. A major effort was made in U.S.A use MHD as the conversion system in a nuclear electrical system for space craft.
v In India also considerable studies have been carried out in this field under the National Council of Science and Technology (NCST) .
v The Department of Science and Technology of government of India has sponsored research and development programmes on coal based MHD power generation.
v MHD conversion has also been considered for ship propulsion, airborne applications, hypersonic wind tunnel experiments and for many other defence applications.
PROBLEMS ENCOUNTERED IN DESIGN OF MHD:
1. Sufficient high temperature for thermal ionization can be sustained by refractory materials. A more practical method of reducing the required temperature is by seeding.
2. Seed material potassium attacks insulating materials and make them conducting.
3. Electrode materials are chemically eroded by combustion of gases.
4. The major problem forced by this generator is the economics. Although the overall thermal efficiency is 60% against 40% for conventional thermal plant, additional investment in the magnet, generator, duct, compressors, scrubbers, seed recovery plant and DC to AC converters may increase the plant cost and it may be much higher than conventional plant.
5. Most of the problems are related to material problems caused by high temperature and highly corrosive and abrasive environment.
CONCLUSION:
The threat of disappearing of the fossil fuels within few decades compel the human beings to search for new energy sources will last for a longer time. Extensive research is going on in different countries through out the world to find out the new resources to replace the conventional fossil one. With the present research and development programmes, the non-conventional power resources play an important role in the power industry . The magneto hydro dynamic power generation is one of the examples of a new unique method of generation of electricity. This power resource play a minor role presently and its use on a vast scale is yet to be confirmed as it is in its childhood stage. Man being optimistic should not leave the hopes on the success of this MHD generator as the technology took the man to moon which was a dream long time back, the same technology will help the human being to survive from the present crisis of power shortage.
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