MHD generator

A magnetohydrodynamic generator based upon the effects of the Lorentz force. Thus, moving charge carriers can be deflected in a magnetic field, when this relative to such a move (in the direction of the Lorentz force see left -hand rule or three-finger rule). The MHD generator, an electrically conductive fluid flows through the magnetic field. The consistency of the fluid allows the Lorentz force to separate ungleichnamige charges which then accumulate in the dedicated collectors. So that there is direct conversion of kinetic energy of fluid into electric energy.

Fundamentals of Electrical Engineering

Is generated by a voltage between the electrodes, an electric field then results in an induced current density:

By interaction with the magnetic field of the induced current generates a site-specific power:

With this force, the pressure gradient in the flow channel is in force equilibrium.

From this you can see that it is not possible in an MHD generator to convert heat directly into electrical energy, but you have to add in a thermodynamic cycle, for example, after Joule- Brayton or Rankine, integrate it.

For the case that in the flow direction, no electric current flows, the efficiency can be written to:

In which case, the outer and the inner resistance of the plasma.

Technical Description

On the wall of a flow-through of the combustion gases duct electrodes are mounted in one plane. Perpendicular to these electrodes, the assembly is passed through by a magnetic field. Flows through such an arrangement, an electrically conductive substance ( the ionized combustion gas), is produced on the electrodes, an electrical voltage. This is the volume flow rate proportionally, which makes this configuration can also be used as a flow meter with no moving parts.

The application for power generation in a big way, both in the context of conventional thermal power plants as well as nuclear power plants, requires that the channel flows through the approximately 3000 ° C hot combustion gases. Such a high temperature is necessary to make the gas electrically conductive enough. However, the addition of readily ionizable substances, such as salts of alkali metals is necessary to increase the electrical conductivity further. Due to the high gas temperature, the walls of the duct shall be made of highly heat- resistant materials. The material used for this purpose, inter alia, yttria or zirconia come into question.

The electrodes must be made of very resistant material such as tungsten, graphite or silicon carbide. After the channel, a device may be necessary, in which the alkali salts are separated from the exhaust gas.

The efficiency of a magneto- hydrodynamic generator is from 10 to 20 percent. Since, however, still have the exhaust gases of magnetohydrodynamic generator, a temperature of over 1000 degrees Celsius, they can still be used as a heat source for a conventional steam power plant ( efficiency up to 50%).

With such a combined arrangement can fuel with an efficiency of up to 65 percent in electrical energy to implement, since the MHD process extends the crucial for the overall thermal efficiency temperature difference up have ( gas and steam turbines due to the thermal stress of the turbine blades a upper limit temperature at about 600 ... 1000 ° C).

In March 1971, the first MHD generator ( called " U- 25") was completed in the Soviet Union, which was used about 25 megawatts of electricity for the Moscow power grid generated and also for scientific research. Even when nuclear power plants with high coolant temperature, the use of the magneto hydrodynamic generator is conceivable. Due to the necessary electrical conductivity of the cooling medium of the reactor the liquid metal would have to be cooled and to be operated at high fuel temperatures (about 2,000 degrees Celsius). The hot liquid metal flowing through the reactor located directly behind the magneto- hydrodynamic generator, before it reaches the heat exchanger where it gives up its heat to a cooling circuit in which a normal steam power plant is located. In this way the efficiency of the nuclear power generation can be increased to about 60%.

Due to the enormous difficulties of the operation of reactors with liquid metal cooling, such arrangements have not yet been realized. For liquid metal cooling only the highly reactive with water, alkali metals (eg, sodium ) come into question, so there are high security problems.

Possibly, the method is subsequently used in fusion reactors.

MHD generators are used in the present, to investigate such as the Earth's crust with an electromagnetic geophysical methods. To this end, strong electrical pulses are produced that penetrate 20 to 30 miles deep into the earth. This gives information about the geological structure and mineral resources. For this purpose, the pulsed MHD generator PAMIR was built with 15 megawatts of power in the USSR and Russia; it produces 500 to 600 volts and a current 25000-30000 amps.

Reversal of the magneto hydrodynamic generator

The magneto hydrodynamic generator can also be operated as a motor, in which you can through the electrode current to flow. Applications this can be found in medical technology, but also in the magneto- hydrodynamic propulsion of ships.

Practical application of the reversal of the MHD principle

It can also be used to increase the discharge speed of the exhaust gases of rocket engines, so as to make rocket engines performance, but this is not due to the high weight of the magnet practicable ( magnetoplasma dynamic drive ).

Another application of the magnetohydrodynamic generator as a motor is located in the propulsion of ships (magnetic Hydrodynamic drive ). Since this, the water must have the best possible electrical conductivity, this form of propulsion for ships sailing in waters where fresh water is unsuitable. In Japan related studies have been carried out. Mid-1990s were some of Mitsubishi prototype of a MHA - powered ship built, but reached the vehicles, in addition to numerous other difficulties arise, only a speed of about 15 km / h

Technical Problems

The use of the magneto hydrodynamic generator for electricity has failed so far to the following problems:

  • Complex generation of the necessary high magnetic fields ( field strengths of about 1 Tesla are to produce in such large volumes only with superconducting coils )
  • Short lifetime of the thermally highly stressed materials of the channel and the electrode

Model test

We derive, for example, the exhaust gases of a fixed model rocket propellant through the pole pieces of a magnet. At right angles to this are located behind two electrodes between which the voltage generated can be tapped. The generated voltage can be read on an erected at a safe distance meter. In this experiment, care must be taken to a secure attachment of the rocket motor as well as the necessary security clearances!

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