An MHD generator, like a conventional generator, relies on moving a conductor through a magnetic field to generate electric current. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities, utility near billings montana the magnetic fields of the earth and sun are produced by these natural dynamos.
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 vector equation. F is the force acting on the particle. Typically, for a large-scale power station to approach the operational efficiency of computer models, steps must be taken to increase the electrical conductivity of the conductive substance. The heating of a gas to its plasma state or the addition of other easily ionizable substances like the salts of alkali metals can accomplish this increase. A simple Faraday generator would consist of a wedge-shaped pipe or tube of some non-conductive material. There are limitations on the density and type of field used.
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The amount of power that can be extracted is proportional to the cross sectional area of the tube and the speed of the conductive flow. The conductive substance is also cooled and slowed by this process. The main practical problem of a Faraday generator is that differential voltages and currents in the fluid short through the electrodes on the sides of the duct. The most powerful waste is from the Hall effect current. This makes the Faraday duct very inefficient. Most further refinements of MHD generators have tried to solve this problem. The optimal magnetic field on duct-shaped MHD generators is a sort of saddle shape.
The most common solution is to use the Hall effect to create a current that flows with the fluid. The normal scheme is to place arrays of short, vertical electrodes on the sides of the duct. The first and last electrodes in the duct power the load. Each other electrode is shorted to an electrode on the opposite side of the duct. Losses are less than a Faraday generator, and voltages are higher because there is less shorting of the final induced current. However, this design has problems because the speed of the material flow requires the middle electrodes to be offset to “catch” the Faraday currents.
As the load varies, the fluid flow speed varies, misaligning the Faraday current with its intended electrodes, and making the generator’s efficiency very sensitive to its load. The third and, currently, the most efficient design is the Hall effect disc generator. This design currently holds the efficiency and energy density records for MHD generation. A disc generator has fluid flowing between the center of a disc, and a duct wrapped around the edge.