Magnetohydrodynamic (MHD) Generator…?

A Magnetohydrodynamic (MHD) generator produces electrical power. As in a conventional generator, it produces power by moving a conductor through a magnetic field. The moving conductor in a standard generator is a coil of copper wire. Unlike a standard electrical generator, the MHD contains no moving parts. In the MHD, the conductor is a fast moving hot plasma gas.

MHD Basics

The high temperature electrically conductive gas flows past a transverse magnetic field. An electric field is generated perpendicular to the direction of gas flow and the magnetic field. The electric field generated is directly proportional to the speed of the gas, its electrical conductivity and the magnetic flux density. Electricity can be siphoned off with electrodes placed in contact with the flowing plasma gas.

The MHD generators require a strong magnetic field. In order to make MHD generators a practical energy supply, superconductive magnets must be used.


Making Plasma

The Plasma in the MHD is created by a process called thermal ionization, where the temperature of the gas is raised to the point so that the electrons are no longer bound to the atoms of gas. These free electrons make the plasma gas electrically conductive.

To create such a plasma though thermal ionization alone requires extraordinary high temperatures. The gas temperature can be lowered significantly by seeding the gas with an alkali metal, such as potassium nitrate. The alkali metal ionizes easily at lower temperatures.

The gas is continuously seeded with potassium nitrate, making the gas electrically conductive at lower temperatures.

Advantages of MHD Generators

Conventional coal-fired generators achieve a maximum efficiency of about 35%. MHD generators have the potential to reach 50% – 60% efficiency. The higher efficiency is due to recycling the energy from the hot plasma gas to standard steam turbines. After the plasma gas passes through the MHD generator, it is still hot enough to boil water to drive steam turbines that produce additional power.

MHD generators are also ecologically sound. Coal with high sulfur content can be used in the MHD without polluting the atmosphere.

Liquid Metal MHD

A gaseous conductor is used flowing past a magnetic field to induce voltage. A conductive liquid metal could also be used to achieve the same effect. A liquid metal such as mercury has been used in an MHD configuration to produce power.

MHD Propulsion in Space

MHD propulsion systemsseem to be ideally suited for interplanetary space travel. MHD rockets could not replace chemical rockets for the raw power needed to escape Earth’s gravitational field because of the MHD’s low specific thrust. However, once lifted in space the MHD propulsion system can easily surpass and speed past chemical rockets in travelling to other planets.

The reasons are that despite their low thrust, the MHDs use very little fuel, and the exhaust velocity of the propellant is very high. Combine these two properties and you have a rocket engine that canrun contonuously for a long perodof time that will allow the vessel to slowly gather to high speed. Remember Newton’s law of motion, for every action there is an equal and opposite reaction. With chemical rockets, regardless of how powerful the thrust is, the rocket can go no faster than the exhaust velocity of its propellant.

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