29-01-2009, 11:30 PM
The Magnetohydrodynamic power generation technology (MHD ) is the production of electrical power utilising a high temperature conducting plasma moving through an intense 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. In all other conventional power plant, 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. The economic attractiveness of MHD for bulk generation of power from fossil fuel has been indicated in many design studies and cost estimates of conceptual plants. MHD promises a dramatic improvement in the cost of generating electricity from coal, beneficial to the growth of the national economy. The extensive use of MHD can help in saving billions of dollars towards fuel prospects of much better fuel utilization are most important, but the potential of lower capital costs with increased utilization of invested capital provides also a very important economic incentive. The beneficial environmental aspects of MHD are probably of equal or even greater significance. The MHD energy conversion process cab contribute 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. It can therefore be claimed that the development of MHD for electric utility power generation is an objective of national significance. 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 have yet to be developed, the combined use to MHD 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 fueled MHD power generation. Before large central station power plants with coal as the energy source can be become commercially viable, further development is necessary.
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Magneto Hydro Dynamic System
Prepared by: Debasish Mishra
Department of Electrical & Electronics
Its future prospectus
Eighty percent of electricity produced in the world is hydal while remaining twenty percent is produced from nuclear, thermal, solar, geothermal energy & from magneto hydro dynamic(mhd) generators.
MHD power generation is new system of power genneration which is said to be of high efficiency & low pollution.in advanced countries it is widely used but in devloping countries like india it is still being in under construction.this construction work is in progress at trichi in tamilnadu under joints effort of BARC(Bhava Atomic Reasearch centre)BHIE, Associated cement corporation (ACC) & russian technologists.
As its name implies,magneto hydro dynamics(MHD) is concerned with the flow of a conducting fluid in the presence of magnetic & electric field.the fluid may be gas at elevated temperature or liquid metals like sodium and pottasium.
As MHD generator is a device for converting heat energy of a fuel directly into electrical energy without conventional electric generator.
In this system,an mhd converter system is a heat engine in which heat taken up in at a higher temperature is partly converted into useful work & the remainder is rejected at low temperature. like all heat engines, the thermal efficiency of a MHD converter is increased by supplying the heat at highest practical temperature and rejecting at lowest practical temperature.
PRINCIPLE OF MHD POWER GENERATION
When an electric conductor moves across a magnetic field a voltage is induced in it which produces a electric current.
This is the principle of CONVENTIONAL GENERATOR where the conductor consist of a copper strip.
In MHD generator solid conductor are replaced by gaseous conductor or ionized gas. if such a gas is passed at high velocity through a powerful magnetic field a current is generated & can be extracted by placing electrodes in suitable position in the stream.
The principle can be explained as follows. An electric conductor moving through a magnetic field experiences a retarding force as well as an induced electric field & current.
The electromagnetic induction principle is not limited to solid conductors. The movement of conducting fluid can also generate electrical energy.
When a fluid is used for energy conversion technique, it is called the magneto hydrodynamic(MHD) energy conversion.
If the flow direction is right angle to magnetic field direction an electromotive force is induced in the direction right angle to both flow and field direction.
The conducting flow fluid is forced between the plates with a kinetic energy and pressure differential sufficient to overcome the magnetic induction force F.
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 vapours which ionze at relatively low temperature.
The atoms of seed element split of electrons. The presence of negatively charged electrons makes the carrier gas an electric conductor.
MHD system can be classified as follows
1>open cycle system
2>closed cycle system
a) Seeded inert gas system
b)Liquid metal system
OPEN CYCLE SYSTEM
Fuel used may be oil through an oil tank or gasified coal through a coal gasification plant.
The fuel(coal,oil or natural gas) is burnt in the combustor or combustion chamber.
The hot gas from the combustor is then seeded with a small amount of an ionized alkali metal (cesium or potasium)to increase the electrical conductivity of the gas.
The seed material potassium carbonate is injected into the combustion chamber, the potassium is then ionized by the hot combustion gases at temperature roughly(2300-2700deg)
To attain such high temperatue,the compressed air used to burn the coal in the combustion chamber must be adequate to at least 1100â„¢ c.A lower preheat temperature would be adequate if the air where enriched is oxygen. An alternative is to use compressed oxygen alone for combustion of fuel, little or no preheating is required. The additional cost of oxygen might be balanced by saving the preheater.
The hot pressurized working fluid living the combustor flows through a convergent divergent nozzle.In passing through the nozzle the random motion energy of the molecules in the hot gas is largely converted into thus directed mass of energy. the gas emerges from the nozzle & enters the MHD generator unit at a high velocity.
The MHD generator is divergent channel made of a heat resistant alloy with external water cooling. the hot gas expands through the rocket like generator surrounded by powerful magnet. during motion of the gas the +ve & - ve ions move to the electrodes & constitute electric current.
The arrangement of the electrode connections is determined by the need to reduce losses arising from the Hall effect. By this effect magnetic field acts on the MHD generated current & produces a voltage in flow direction of the working fluid rather than at right angle to it.
Closed cycle system
Two general types of closed cycle system are being investigated.
1 > electrical conductivity is maintained in the working fluid by ionization of a seed material as in open cycle system.
2>A liquid metal provides the conductivity.
the carrier is usually a chemically inert gas,although a liquid carrier is been used with a liquid metal conductor.The working fluid is circulated in closed loop & is heated by the combustion gas using a heat exchanger.Hence the heat sources & working fluid are independent. The working fluid is helium or argon with cesium seeding.
Seeded inert gas system
In a closed cycle system the carrier gas operates in the form of Brayton cycle.in a closed system the gas is compressed & heat is supplied by the source at essentially constant pressure, the compressed gas then expand in the MHD generator & its pressure and temperature falls. After leaving the generator, heat is removed from the gas by a cooler,this is the heat rejection stage of he cycle. finally the gas is recompressed & returned for reheating.
The complete system has three distinct but interlocking loops. On the left it is external heating loop.coal is gasified and the gas burn in a combustor to provide heat.In the primary heat exchanger the heat uis transferred to a carrier gas argon or helium of the MHD cycle. the combustion product after passing through the air preheated & purified are discharged to atmosphere.
Because the combustion system is separate from the working fluid so also are ash & flue gases.Hence the problem of extracting the seed material from fly ash doesnâ„¢t arise. The fuel gases are used to preheat the incoming combustion air & then treated for fly ash & sulfur dioxide removal, if necessary prior to discharge through a stack to the atmosphere.
The loop in the centre is the MHD loop. the hot argon gas is seeding with cesium and resulting working fluid is passed through the MHD generator at high speed. The dc power out of MHD generator is converted in ac by the inverter and is then fed into the grid.
Liquid metal system
When the liquid metal provides the electrical conductivity, an inert gas is a convenient carrier.
The carrier gas is pressurized and heated by passage through a heat exchanger within combustion chamber. the hot gas is then incorporated into the liquid metal usually hot sodium to form the working fluid. the latter the consist of gas bubble uniformly dispersed in an approximately equal volume of liquid sodium.
The working fluid is introduced into the MHD generator through a nozzle in the usual ways, the carrier gas then provides the required high direct velocity of the electrical conductor.
After passage through the generator, the liquid metal is separated form the carrier gas. Part of the heat remaining in the gas is transferred to water in a heat exchanger to produce steam for operating a turbine generator. Finally the carrier gas is cooled, compressed and returned to the combustion chamber for reheating and mixing with the recovered liquid metal. The working fluid temp is usually around 800 c as the boiling point of the sodium; even under moderate pressure is below 900c.
The lower operating temp then in other (l/IHD conversion systems may be advantageous from the material standpoint, but the maximum thermal efficiency is lower. A possible compromise might be to use liquid lithium, with a boiling point near 1300 c as the electrical conductor Lithium is much more expensive then sodium, but losses in a closed system be small.
ADVANTAGES OF MHD SYSTEMS
1> The conversion efficiency of an MHD system can be around 50 per cent as compared to less then 40 per cent for the most | efficient steam plants. Still higher thermal efficiencies(60-65%) are expected in future, with the improvements in experience and technology.
z) Large amount of power is generated. 3) It has no moving parts, so more reliable.
4.) The dosed cycle system produces power free of pollution. 5) It has ability to reach the full power level as soon as started.
6> The size of the plant (m2/kW) is considerably smaller then conventional fossil fuel plants.
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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.
It is well known that man had always been trying to fined something new to achieve a new aims 80% of total electricity produced in the world is hydral while remaining 20% is produced from nuclear, thermal, solar, Geothermal energy and from magneto hydro dynamic power generation.
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 generator are widely used but in developing countries like India it is still under construction. This construction work is in progress at Tiruchirapalli in Tamilnadu unders joint efforts of BARC (Bhabha Atomic Research Centre), BHEL, Associated cement corporation and Russian technologists.
As its name implies, magneto-hydro-dynamic (MHD) is concerned with the flow of conducting fluid in presence of magnetic and electric field. This fluid may be gas at elevated temperature or liquid metal like sodium or potassium. A MHD generator is a device for converting heat energy of fuel directly into electric energy without a conventional electric generator. The basic difference between conventional generator and MHD generator is in the nature of conductor.
PRINCIPLE OF MHD POWER GENERATION
The principle of MHD generation is simple, discovered by faraday when an electric conductor moves across a magnetic field, a emf. Is induced in it, which produced an electric current. This is the principle of the conventional generator also, where the conductors consists of copper strips. In MHD generator the solid conductors are replaced by a gaseous conductor; i.e. an ionized gas. If such gas is passed at high velocity through a powerful magnetic field, i.e. suppose we have a charged particle (having charge q) moving at a high velocity ‘V’ towards right and a perpendicular magnetic field is applied. A magnetic force ‘F’ acts on the charged particle. Show in fig. A Positively charged particles as forced upward and negatively charged particle is forced downward.
The positive ions would be accelerated towards the upper plate P, and negative ions would be accelerated towards the plate P2 . If the P1 and P2 are externally connected through a resistance, a current would flow through the resistance. Thus gas energy directly converted into electrical energy. This is the principle of MHD generator. A MHD conversion is known as direct energy conversion because it produced electricity directly from heat source without the necessity of the additional stage of steam generation as in a steam power plant.
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