MHD Generator or Magneto Hydro Dynamics Generator is basically a device which converts heat energy of a fuel into electrical energy directly. In the MHD generator, Ionized fluid which is at high temperature is passes through magnetic field. The ionized fluid has a temperature of 2000K-3000K. This high temperature converts the fluid into plasma state. Plasma state is basically a gaseous state where hot gas contains positive charge ions and negative charged free electrons. Thus the electrically conductive gas plasma flow through a magnetic field and produce direct current in the system due to electromagnetic induction. In the MHD generator unlike conventional generators, No turbine is used to generate the power. Thus the MHD generator has no moving part and hence requires low maintenance as compared to conventional electrical generator. In MHD generator, Thermal energy is converted into electrical energy. Any heat source can be used as a fuel but coal is preferred because of its high availability.
The working principle of MHD generator is based on the Faraday’s law of electromagnetic induction. According to it, When the conductor cut across a magnetic field, EMF induce in it. In the conventional electrical generators, The armature which carrying winding moves inside stationary magnetic field and emf is induced. In MHD generator, conductive gas plasma is used in place of solid conductors. The electrically conductive plasma passes through magnetic field at very high velocity. The magnetic field is placed at 90° to the direction of plasma flow. Thus the high velocity conductive plasma which is working as a moving conductor cuts across stationary magnetic filed and thus emf is induced in it. This induced emf produces a direct current which is drawn through the external circuit by placing the electrode at suitable location.
The direct conversion of kinetic energy into electrical energy by the flow of an electrically conducting fluid, which is usually a gas or gas liquid combination through the magnetic field. If the flow of direction is at right angles to the magnetic field direction, an emf is induced in the direction at right angles to both flow and field directions.
The basic MHD generator arrangement consists of combustion chamber, Nozzle and Electrode plates. The combustion chamber is a part of generator where fuel is burnt to produce hot plasma gas. The Convergent/Divergent nozzle is used in MHD generator to increase the velocity of hot gas. When the high velocity gas passes through the magnetic field, the emf which is induced due to electromagnetic induction act in opposite direction and thus tries to slow down the motion of gas. The divergent duct is made to accommodate some of these slow moving gases. Electrodes in the MHD generator are connected with external circuit.
The MHD generator are divided into following two types:
- Open cycle MHD Generator
- Closed cycle MHD generator
1. OPEN CYCLE MHD GENERATOR
In the open cycle MHD generators, Coal is used as a fuel because of more carbon atoms as compared to hydrogen atoms. The hydrogen atoms presence in the MHD generator affects its performance. The working temperature of these type of MHD generators is above 2300°C, Below this temperature the electrical conductivity of gas become zero. Basically the gas is heated at 2500°C to increase its electrical conductivity.
The open cycle MHD generator has following parts:
1. Combustion Chamber
Combustion chamber or also known as Combustor is a part of MHD generator in which fuel is burnt. The fuel which is basically Coal is burnt in presence of hot air. The combustion chamber required to have very low heat loss so that the heat is not get wasted.
The compressor are used in MHD generator to compress the air. The compression process increase the pressure of air. This high pressurized air is sent to combustion chamber for the efficient burning of coal.
3. Pre Heater
The pre heater is used to heat up the compressed air before it is sent to combustion chamber. The pre heater used the heat energy of exhaust gases, thus improves the efficiency of MHD generator. The compressed air is heated at 1100°C and then sent to combustion chamber.
The nozzle is used in MHD generator to increase the velocity of gas. In the MHD generator, Convergent/ Divergent nozzle is used. The nozzle is installed after the combustion chamber where it increase the velocity of high temperature gas up to 100m/s.
5. MHD Duct
The divergent duct is the most important part of MHD generator because it is the section of generator where magnetic field is placed. The divergent side of nozzle is connected with divergent duct. The stationary magnetic field inside duct is perpendicular to the direction of flow of conductive gas. When conductive gas cuts across magnetic field, emf is produce in this section. The divergent duct is made of high resisting and insulating material and has water cooling facility because the conductive gas has very high temperature.
Inverter is a device which converts the DC supply into AC supply. The MHD generator has a DC output, which is converted into AC supply using inverter.
7. Seed Recovery Unit
Seed recovery unit as the name suggests, used to recover the seed material. Seed material is costly and can be reused after some process. Thus it is recovered and supplied back to combustion chamber.
8. Pollution Control Unit
Pollution control unit is basically a gas cleaner, which is used to clean the exhaust gases. In this unit, The harmful content like Sulphur and nitrogen oxides are removed from gases before exhausting in the atmosphere.
2. CLOSED CYCLE MHD GENERATOR
In the closed cycle MHD generators, Argon or Helium is used as fluid. Due to closed cycle, These type of MHD generators has high thermal efficiency. The working temperature of closed cycle MHD generator is low as compared to open cycle MHD generators. Closed cycle MHD generator can provide power at 1600°C. In these type of MHD generator, Fluid is heated up in heater exchange. To provide the electrical conductivity to fluid, alkali metal is mixed in it.
The closed cycle MHD generator is divided into two types:
- Seeded Inert Gas System
- Liquid Metal System
In the seeded inert gas system, The fluid which is basically Helium or Argon, is seeded with cesium and then this fluid is circulates in a close loop. The fluid is heat up in the heat exchanger. Basically heat exchanger is connected with combustion chamber. In the combustion chamber, Gas is burned and supplied to heat exchanger. In the heat exchanger, the heat is transferred to fluid. Then this ionized fluid is passes through the magnetic field inside the MHD duct. Due to electromagnetic induction, ionized fluid cut the magnetic field and emf is induced.
In the liquid metal system, Metallic vapour are used as fluid. These metallic vapour which are at very high temperature are sent to MHD duct through nozzle. Apart from that, It has similar operation.
- MHD Generator has no moving parts, Thus require very less maintenance.
- The conversion efficiency is high. The efficiency is between 60-65%.
- MHD generator are compact in size and has low weight.
- The operating cost is low.
- MHD generators has low cost as compare to others gas or coal powered plants.
- These generator produce less pollution as compare to other conventional plants.
- The life of MHD generator is less because they work at very high temperature.
- The heat energy is wasted due to thermal losses.
- The manufacturing of MHD generator parts is difficult because they have to withstand very high temperature.
- The large amount of power is wasted to creating a magnetic flux.