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MHD dynamo)
The MHD (magnetohydrodynamic) generator or dynamo, is for the direct transformation of thermal energy or kinetic energy into electricity. An advantage of MHD generators over traditional electrical generators is they operate with few moving parts. This technology is applicable to power generation and engine applications.
The basic concept underlying the mechanical and fluid dynamos is the same. The fluid dynamo, however, uses the motion of fluid or plasma to generate the currents which generate the electrical energy. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. The functional difference between an MHD generator and an MHD dynamo is the path the charged particles follow.
The MHD dynamo is an active area of research in plasma physics and is of great interest to the geophysics and astrophysics communities. From their perspective the earth is a global MHD dynamo and with the aid of the particles on the solar wind produces the aurora borealis.
Principle
The Lorentz Force Law describes the effects of a charged particle moving in a constant magnetic field. The simplest form of this law is given by the scalar equation.
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- F = QvB
where
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- F is the force acting on the particle (vector)
- v is velocity of particle (vector)
- Q is charge of particle (scalar)
- B is magnetic field (vector)
where F, V and B are all perpendicular to each other according to the Right hand rule.
Power generation
An example implementation would consist of a pipe or tube of some non-conductive material. When an electrically conductive fluid flows through the tube, in the presence of a significant perpendicular magnetic field, a charge is induced in the field, which can be drawn off as electrical power by placing the electrodes on the sides at 90 degree angles to the magnetic field. There are some limitations that must be met on the density and type of field used. The amount of power that can be extracted in this manner is proportional to the cross sectional area of the tube and the speed at which the conductive substance flows. The conductive substance is also cooled and slowed in this process. MHD generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C.
A second example, which applies to the earth, the differently charged electromagnetic layers produced by the dynamo effect on the earth's geomagnetic field enable the appearance of the aurora borealis. As power is extracted from the plasma of the solar wind, the particles slow and are drawn down along the field lines in a brilliant display over the poles.
Typically for a large scale power station to approach operational efficiency in computer models, steps must be taken to increase the electrical conductivity of the conductive substance. The heating of a gas to plasma or the addition of other easily ionizable substances like the salts of alkali metals accomplishes this increase in conductivity.
In practice a number of issues must be considered in the implementation of a MHD generator; Generator efficiency, Technical problems and Toxic byproducts.
Generator efficiency
The efficiency of the magnetohydrodynamic generator in a single stage is estimated to be no greater than 10 to 20 percent. This makes it unattractive, by itself, for power generation. However it has a number of places that it would be an ideal fit in series with other forms of power generation.
In series with a fossil fuel power plant a MHD generator could provide an efficiency boost. By routing the exhaust gases of such a plant through a magnetohydrodynamic generator before traditional thermal to electrical conversion plants, it is estimated that one can convert fossil fuels into electricity with an estimated efficiency of up to 65 percent.
Similarly, the employment of a magnetohydrodynamic generator is conceivable in series with a Nuclear reactor (either fission or fusion). Reactors of this type tend to operate with fuel rod temperatures at approximately 2000 °C. By pumping the reactor coolant through a magnetohydrodynamic generator before a traditional heat exchanger is reached an estimated efficiency of 60 percent can be realised.
Technical problems
The employment of the MHD generator for large scale mass energy conversion failed so far because of the economics and chemistry. A certain amount of electricity is required to maintain sustained magnetic flux density over 1.0 tesla (T). Because of the high temperatures, the walls of the channel must be constructed from an exceedingly heat-resistant substance such as yttrium oxide or zirconium dioxide to retard oxidation. Similarly, the electrodes must be both conductive and heat-resistant at high temperatures, making tungsten a common choice.
Toxic byproducts
If some form of liquefied metal is used in the operation of a MHD generator, severe care must be taken with the form of cooling used on the electomagnetics and in the channel. Aside from the chemical byproducts of heated electrified alkali metals and channel material. The alkali metals themselves are highly, even violently reactive with water.
Measures must also be taken to separate any ionizing substance used, from the exhaust gasses if the MHD generator is run on plasma.
See also
External links
Research
Geophysics
