Working principle of MHD:
It is also known as Magneto Hydrodynamic power generation is a direct energy conversion system that converts the heat energy directly into electrical energy, without any intermediate mechanical energy conversion.
Principle of MHD Generation:
- The principal of MHD power generation 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 the voltage is induced in the conductor, which results in the flow of current across the terminals.
- It is concerned with the flow of a conducting fluid in the presence of magnetic and electric fields.
- In a 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.
- 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 the 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 a brush.
- The MHD generator develops DC power and the conversion to AC is done using an inverter.
Advantage of MHD:
- Only working fluid is circulated, and there are no moving mechanical parts hence no mechanical loss occurs.
- The temperature of the working fluid is maintained by the walls of MHD.
- It has the ability to reach full power level almost directly.
- The price of MHD generators is much lower than conventional generators.
- MHD has very high efficiency.
Disadvantages of MHD:
- The efficiencies attained so far have been relatively low.
- The power output of the MHD generator is proportional to the square of the magnetic field density. The electromagnets need very large power for creating strong magnetic fields. The MHD technology is waiting for the development of superconducting materials which will need very little power even at ambient temperatures.
- The combustor, MHD duct, electrodes, and air preheaters are exposed to very corrosive combustion gases at very high temperatures. So, the life of these equipment has been reduced.
- The ash (or slag) residue from the burning coal is carried over with the combustion gases and tends to cause erosion of exposed surfaces. However, deposition of the slag on such surfaces may also provide some protection.
- There is a serious problem of separation of seed material from the fly ash and reconversion of potassium sulphate to potassium carbonate.
- Special fuel gas and preheating of air are required to provide adequate working fluid temperatures hence it becomes difficult to use.
Advantages of MHD:
- Since high temperatures are involved, operation efficiency is high. MHD system is normally designed to be a topping power system to a conventional steam power plant. At present, the conversion efficiency of an MHD system is around 50% which can be increased to 60% with improvements in experience and technology.
- No moving part, so more reliable.
- Conceptually such generators are much simpler.
- As there is no limitation to the size of the duct, so high-capacity generators are possible.
- The walls can be cooled below the temperature of working gas.
- Direct conversion of heat into electrical energy results in the elimination of the gas turbine (compared with a gas turbine power plant) and both the boiler and the turbine (compared with a conventional steam power plant) and thus in reduction of energy losses.
