Magnetohydrodynamic generator

A magnetohydrodynamic generator is based on the effects of the Lorentz force . According to this, moving charge carriers can be deflected in a magnetic field if they move relative to such a field (for the direction of the Lorentz force, see left-hand rule or three-finger rule ). In the MHD generator, an electrically conductive fluid flows through the magnetic field. The consistency of the fluid enables the Lorentz force to separate unlike charges, which then collect on collectors provided for this purpose. This leads to the direct conversion of mechanical energy (as displacement work or volume work ) into electrical energy.

Principle of magnetohydrodynamic generator

Electrotechnical basics

Field distribution in the magnetohydrodynamic generator

If an electric field is generated between the electrodes by a voltage , an induced current density results: ${\ displaystyle U _ {\ mathrm {A}}}$ ${\ displaystyle {\ vec {E _ {\ mathrm {A}}}}}$

{\ displaystyle {\ vec {i}} _ {\ mathrm {ind}} = {\ frac {1} {\ varrho}} \ left ({\ vec {v}} \ times {\ vec {B}} + {\ vec {E}} _ {\ mathrm {A}} \ right)}

Through interaction with the magnetic field, this induced current generates an area-specific force:

{\ displaystyle {\ vec {F}} _ {\ mathrm {ind}} = {\ vec {i}} _ {\ mathrm {ind}} \ times {\ vec {B}}}

.

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

{\ displaystyle \ nabla p = - {\ vec {F}} _ {\ mathrm {ind}} = - {\ vec {i}} _ {\ mathrm {ind}} \ times {\ vec {B}}}

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

In the event that no electrical current flows in the direction of flow, the efficiency can be written as:

{\ displaystyle \ eta _ {e} = {\ frac {U _ {\ mathrm {A}}} {U _ {\ mathrm {A}} + U _ {\ mathrm {i}}}} = {\ frac {R_ { \ mathrm {A}}} {R _ {\ mathrm {A}} + R _ {\ mathrm {i}}}}}

Where here is the external and internal resistance of the plasma. ${\ displaystyle R _ {\ mathrm {A}}}$ ${\ displaystyle R _ {\ mathrm {i}}}$

technical description

On the wall of a channel through which the combustion gases flow, electrodes are attached in one plane. The arrangement is penetrated by a magnetic field perpendicular to these electrodes . If an electrically conductive substance (the ionized combustion gases) flows through such an arrangement, an electrical voltage is generated at the electrodes . This is proportional to the volume throughput, which is why this arrangement can also be used as a flow measuring device without moving parts.

Use in power plants requires that the approx. 3000 ° C hot combustion gases flow through the sewer. Such a high temperature is necessary to make the gas sufficiently electrically conductive. Nevertheless, an addition of easily ionizable substances, such as salts of alkali metals, is necessary in order to further increase the electrical conductivity. Due to the high gas temperature, the walls of the channel must be made of very heat-resistant materials. As a material for this come u. a. Yttrium oxide or zirconium dioxide in question. The electrodes must also be made of very heat-resistant material such as tungsten, graphite or silicon carbide. After the channel, a device may be required in which the alkali salts are separated from the exhaust gas.

The efficiency of a magnetohydrodynamic generator is 10 to 20 percent. However, since the exhaust gases from the magnetohydrodynamic generator still have 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, fuels with an efficiency of up to 65 percent can be converted into electrical energy, since the MHD process expands the temperature difference that is decisive for the overall thermal efficiency (gas and steam turbines have a upper temperature limit at approx. 600… 1,000 ° C). The use of a magnetohydrodynamic generator as the first stage is also conceivable in gas-cooled nuclear power plants.

In March 1971 the first MHD generator (designation "U-25") was completed in the Soviet Union, which generated around 25 megawatts of electrical power for the Moscow power grid and was also used for scientific research.

Reversal of the magnetohydrodynamic generator

The magnetohydrodynamic generator can also be operated as a motor by allowing a current to flow through the electrodes. Applications for this can be found in medical technology, but also in magnetohydrodynamic propulsion of watercraft.

Practical application of the reverse of the MHD principle

It can also be used to increase the ejection speed of exhaust gases from rocket engines in order to make rocket engines more efficient, but this is not practicable due to the heavy weight of magnets ( magnetoplasmic dynamic drive ).

Another application of the magnetohydrodynamic generator as a motor is in the propulsion of ships (magnetohydrodynamic propulsion ). Since the water must have the best possible electrical conductivity for this purpose, this type of drive is unsuitable for ships that sail in waters with fresh water. Studies in this regard have already been carried out in Japan. In the mid-1990s, Mitsubishi built some prototypes of an MHA-powered ship, but the vehicles only reached a speed of about 15 km / h, along with numerous other difficulties that occurred.

Technical problems

The use of the magnetohydrodynamic generator for power generation has so far failed due to the following problems:

costly generation of the necessary high magnetic fields (flux densities of over 1 Tesla can only be generated in such large volumes with superconducting coils)
short service life of the thermally highly stressed materials of the duct and the electrodes

Model test

One leads z. B. the exhaust gases of a fixed model rocket propellant through the pole pieces of a magnet. At right angles to this are two electrodes behind it, between which the generated voltage can be tapped. The voltage generated can be read on a measuring device set up at a safe distance. In this attempt, you must ensure that the rocket motor is securely fastened and that the necessary safety distances are maintained!

literature

Karl Strauss: Power plant technology . 7th edition. Springer-Verlag Berlin Heidelberg 2016, ISBN 978-3-662-53029-0 , 15.2 Magnetohydrodynamic energy converters.
Hugo K. Messerle: Magnetohydrodynamic electrical power generation. Wiley, Chichester 1995, ISBN 0-471-94252-9
Rolf Bünde, Jürgen Raeder: MHD power generation - selected problems of combustion MHD generators. Springer, Berlin et al. 1975, ISBN 3-540-07296-9

Individual evidence

^ Blair M. Smith et al .: Gas Core Reactor-MHD Power System with Cascading Power Cycle . Conference proceedings, ICAPP'02: 2002 International congress on advances in nuclear power plants, Hollywood, FL, ( abstract ).
^ Committee on the Strategic Assessment of the US Department of Energy's Coal Program: Coal - Energy for the Future . National Academy of Sciences, 1995.

Web links

Commons : Magnetohydrodynamic generators - collection of images, videos and audio files

Drawings and further explanations

[1] Blair M. Smith et al .: Gas Core Reactor-MHD Power System with Cascading Power Cycle . Conference proceedings, ICAPP'02: 2002 International congress on advances in nuclear power plants, Hollywood, FL, ( abstract ).

[2] Committee on the Strategic Assessment of the US Department of Energy's Coal Program: Coal - Energy for the Future . National Academy of Sciences, 1995.