How Magneto Hydro Dynamic (MHD) Generator Works?
Traditional electric generators (Dynamo) are working on the principle of Faraday's laws of electromagnetic induction. A dynamo consists of a moving metal conductor in a strong magnetic field to generate electricity.
There are AC generators and DC generators. AC generators are usually known as alternators. It is possible to convert an alternator to DC dynamo by installing a Commutator (a splitted ring arrangement), which will convert the alternating current to direct current.
Here we are discussing about Magneto hydro dynamic generators. Magneto Hydro-Dynamic Conversion of energy is a direct conversion of thermal energy and kinetic energy into electrical power. A fuel cell is also a direct converter of energy. Read more about fuel cells.
A M.H.D generator works on the same principle as that of a Dynamo. In other words, process of MHD power generation is an application of Faraday's laws of electro-magnetic induction to fluid conductors. MHD generator lacks moving parts but they operate at high temperatures. MHD was developed because of the hot exhaust gas of an MHD generator can heat the boilers of steam power plant, increasing overall efficiency.
In conventional generators magnetic flux is cut by metallic conductors and e.m.f produced is proportional to rate of cutting of flux. In MHD generator, fluid conductors are used. The fluid may be gas at elevated temperatures or liquid like sodium or potassium.
Making gas a conductor of electricity is a difficult process. It is to be heated to a very high temperature to dislodge the electrons of gas atoms which are tied up with electrically neutral particles. This process is called ionization. Atoms as such are electrically neutral. When they are ionised they become electricaly charged particles and gas becomes a conductor.
Unlike the conventional electrical machine which is essentially an electromechanical converter, MHD Generator is a heat engine operating on turbine cycle transformig internal energy of gas into electrical energy.
A plasma of hot gas is passed through a duct of varying rectangular cross section. One pair of walls (top and bottom) are insulated. They form the poles of the magnet. Magnetic field is set up between them.
Motion of the plasma in the magnetic field induces an emf which is at right angles to the magnetic field as well as to the direction of gas flow. Electrical energy is extracted from the plasma through the electrode pair which are externally connected to the load.
Working Principle of MHD generator- The Lorentz Force Law
The Lorentz force law describes the affect of magnetic field on moving charges. A simple explanation for of the law is given by the following formula.
Where F is the force acting on the particle. Q is the charge of the particle, v is the velocity of the particle, and B is the magnetic field. The vector F is perpendicular to both v and B according to the Fleming's right hand rule.
About the Construction of MHD Generators
When gas flows across a magnetic field and current is induced, force tending to slow down the motion of the gas is experienced. Therefore ducts of MHD generators are made diverging to accommodate the same mass of gas at smaller velocity.
Both the insulators and electrods have to withstand as high as 3000 degree celsius of temperature. They must have good resistance to thermal shock, abrasion and chemical attack.
Energy is needed to magnetize the electromagnet used in MHD Generators. To reduce the energy loss due to magnetization
Superconductive materials are used.
The working medium after leaving the generator, still possesses considerable amount of heat which can be utilised for preheating the gas or for generation of steam.
Inorder to achieve a reasonable ionization and high electrical conductivity at high temperatures like 2000-3000 degree celsius, the gases are seeded with additives like alkali metals.
Types of MHD Generators
1. Faraday generator
2. Hall generator
3. Disc generator
A coal plant installed with MHD generator can give up to 60%
efficiency, where a typical coal plant gives 40% only.