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Presented By:
Magneto Hydro Dynamic System
Prepared by: Debasish Mishra
Regd no.0601259231
Department of Electrical & Electronics

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Introduction
Principle
Various systems
Advantages
Its future prospectus



INTRODUCTION
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.