How Do MHD Generators Work?

How Do MHD Generators Work?

Magnetohydrodynamic (MHD) generators are used for the generation of electrical energy. In these generators, electricity generation is carried out using a conducting fluid as an electrical conductor. Here's more...
MHD is the acronym for magnetohydrodynamic generator. MHD generators are similar to the conventional electric generators. The only difference is that they use electrically conducting fluid instead of solid conductors to generate electric power. Basically, magnetohydrodynamics is a research area that involves the study of motion of conducting fluids such as plasma and salt water.

A simple magnetohydrodynamic generator consists of a gas nozzle. The gas nozzle is a combustion chamber that injects a pulse of gas into the channel/duct. The walls of the channel act as an electrode. The induced electric current is fed to the load by an external circuit that supplies the generated electricity to the desired destination. The MHD generators can be constructed in various designs like the Faraday generator, Hall generator, and disc generator. Faraday generator was the first designed MHD generator. It was made by Michael Faraday in 1831. The Faraday generator used copper disks and a horse-shoe magnet to generate electricity.

Under high pressure condition, an electrically conducting gas is produced by burning a fossil fuel. Most of the MHD systems use coal or natural gas as fossil fuel. However, inert gases like argon and helium are also used in some systems. The gas is passed through a nozzle at a high speed of 1000 to 2000 meter per second. The generators do not create electric charge, it is inherent in the ionized fluid or gases. To understand it better, think of a water pump that allows the water to pass through but it is not the source of water. Conductivity of the fluid can be increased by adopting various methods. If the fluid is an inert gas, then suitable amounts of potassium carbonate can be added.

The gas enters the channel or duct where a strong magnetic field is applied with the help of superconducting magnets. The magnetic intensity or magnetic field strength of the magnetic field inside the channel is usually between 3 to 5 Tesla. As the gas passes through the channel, an electromotive force is experienced by it. How does this force arise? According to the Faraday's law of electromagnetic induction, current/voltage (EMF) is induced in a coil/wire whenever there is a change in the magnetic flux linked with the coil. Here, the electromagnets are stationary but the conductor fluid is moving constantly. This causes the generation of electric field.

As mentioned earlier the MHD systems consist of channel/duct that are a bridge to the external circuit which will finally let the electricity to flow to the load. The question that arises here is; what is an electrode? Electrodes are the plates, rods, or wires that act as a conductor to the flow of electricity. They act as a connector to the external circuit. Here the channel acts as an electrode. The external circuit is connected to the electrode, and electric power supply is transferred to the desired path.

The basic mathematical equation governing the working of a MHD generator, is the Lorentz force law. Suppose, a particle is projected with certain velocity (v) in an area whose magnetic field intensity is B, then the force acted on the charged particle is given by Lorentz force law. The direction of motion of the charged particle is dependent on the charge of the particle (positive, negative or neutral), and also on the direction of the magnetic field. Here is the vector form of the Lorentz force law.

F = Q × (v × B)
F = Force acting on the particle,
Q = Charge on the particle,
v = Velocity of the particle,
B = Magnetic field

The important thing to remember is that the direction of force vector is perpendicular to the plane of velocity and magnetic field.

However, in depth analysis requires the study of Navier-Stokes equations (included in fluid dynamics) and Maxwell's law of electromagnetism. The Navier-Stokes equations are differential equations that determine the velocity of the fluid at any particular instant of time. The Maxwell's law of electromagnetism are four partial equations that combine together to form complex equations involving either magnetic or electric field or both. Coupled with Navier-Stokes equations, they are very useful in studying the working of a MHD generator.

Magnetohydrodynamic generators were initially developed to heat the boilers of steam power plants, as they require very high temperatures to function. This was not possible with conventional electric generators. MHD generators have high thermal efficiency that is required for power plants. They do not cause any significant harm to the environment. With more research and innovation, MHD systems will lead to development in the work of thermonuclear fusion reactors.