MAGNETOHYDRODYNAMICS

Magnetohydrodynamics (MHD) is a promising technology for electric bulk power generation. MHD is accomplished by forcing an electrically conducting fluid or a plasma through a channel with a magnetic field applied across it and electrodes placed at right angles to flow and field (Figure 1). An MHD plant can be directly fired with coal and there are no moving parts. To achieve extra high efficiencies, MHD is combined in a binary thermodynamic cycle with a conventional steam plant to add an extra 40 percent to the total power output and to boost the overall combined efficiency into the 60 percent range. The high temperature MHD process extracts part of the heat energy in the plasma at the high temperature end. The gas leaving the MHD generator, still at relatively high temperatures, is then used in a conventional bottoming steam plant.

A schematic coalfired MHD generator in Figure 2 is shown using a combustion plasma at up to 3,000 K, seeded with alkali salts for high conductivity, fed directly into the generator with a superconducting magnet providing up to 7 tesla. The combustion products, still at about 2,000 K, then pass through an air preheater, where the hot gas exhausted from the MHD generator preheats the air for the combustor leading to the high temperatures required to create the plasma.

To use lower gas temperatures (under 1,500 K), noble gases like argon and helium offer very high electrical conductivities. Recycling the noble gas leads to the "closed cycle" MHD process.

Nuclear reactors have been used for small generators providing the high temperature and pressure plasma to drive an MHD process, and a nuclear plant could offer a considerable reduction in pollution because of high overall efficiency without CO₂ emission.

EXPERIMENTAL DEVELOPMENTS

In the early part of the nineteenth century Michael Faraday (1832) conducted MHD experiments using the brackish water of the river Thames flowing through the Earth's magnetic field. The first successful power generation experiment, developed by Richard Rosa in 1959, generated 10 kW with a timber walled channel on the AVCO "Mark 1" facility in Boston, Massachusetts. This success and the possibility of cheap MHD power led in the 1960s to national programs in Britain, the Soviet Union, The Netherlands, France, Germany, Poland, Italy, India, Australia and Israel. In 1965 the AVCO "Mark 5" generator successfully generated 32 MW over a one minute run using alcohol at 45 kg/sec fired with oxygen. AVCO later developed a sophisticated coal fired MHD channel for a 2,000 hour test program and demonstrated technical feasibility under the most stringent conditions.

In 1972 in Moscow, a large experimental facility, the "U-25," used a 250 MW natural gas combustor and generated 20 MW. The Soviets have been using very successfully mobile, pulsed MHD generators throughout the Soviet Union, for seismic studies.

MHD programs in the United States are concentrated in two major facilities. A "Component Development and Integration Facility" is located in Butte, Montana, and a "Coal Fired Flow Facility" at the University of Tennessee to studies coal fired MHD, slag processing, seed handling and downstream systems.

A circular disk MHD generator geometry requires a lower cost circular ring magnet structure as shown in Figure 3. Work in the United States using shock tunnels with large discs proved the feasibility of very high enthalpy extraction. Continuous operation on a fossil fuel fired disk facility was demonstrated at the University of Sydney, Australia. The potential advantages offered by closed cycle disc systems was made evident in the 1980s and 1990s by the Japanese MHD research effort and by the Dutch team at the University of Eindhoven.

A great deal of work has been done in the United States, Russia, Israel, and France showing that low temperature operation is possible using liquid metal as the MHD driver. A liquid metal MHD-generator can be very much smaller because of the much higher conductivity of liquid metal.

POWER SYSTEM REQUIREMENTS

Studies carried out in the United States, Russia, and Japan indicate that a combined cycle MHD-steam plant should be able to achieve an overall power station efficiency of at least 60 percent which is about 20 percent more than offered by a conventional steam plant. This should be possible at capital costs comparable with existing steam plants.

As shown in Figure 4, the operating temperature range for MHD is beyond that of any other generating technology. MHD could still add up to 15 percent at the top end of other combined cycles. MHD is potentially a natural choice for conversion of high temperature energy output from future nuclear power plants whether fission or fusion driven. A combined Nuclear-MHD system design with high efficiency does offer two advantages: (1) a reduction in thermal pollution and (2) no CO₂ emission as with fossil fuel-driven plants.

MHD efforts in many countries, including the United States, have declined substantially, in Russia because of lack of funds and in general because the high costs envisioned in setting up a full-scale power station. If funds become available to set up full-scale power stations and with the advent of high temperature nuclear heat sources, the many advantages provided by MHD may be realised.

Hugh Karl Messerle

See also: Combustion; Electric Power, Generation of Faraday, Michael; Hydroelectric Energy; Magnetism and Magnets; Nuclear Energy; Thermodynamics.

BIBLIOGRAPHY

Faraday, M. (1832). "Experimental Researches in Electricity." First Series, Philosophical Transactions of the Royal Society, pp. 125-162.

Messerle, H. K. (1995). Magneto-Hydro-Dynamic Electrical Power Generation. Chichester, England: John Wiley & Sons Ltd.

Petrick, M., and Shumyatsky, B. Y. (1978). Open-Cycle Magnetohydrodynamic Electrical Power Generation. Argonne, IL: Argonne National Laboratory.

Rosa, R. J. (1960). "Experimental Magnetohydrodynamic Power Generation." Applied Physics 31:735-36.

Rosa, R. J. (1987). Magnetohydrodynamic Energy Conversion. Washington, DC: Hemisphere Publishing Corp.

Shioda, S. (1991). "Results of Feasibility Studies on Closed Cycle MHD Power Plants." Proceedings Plasma Technology Conference, Sydney, Australia, pp. 189-200.

Simpson, S. W.; Marty, S. M.; and Messerle, H. K. (1989). "Open-Cycle Disk Generators: Laboratory Experiments and Predictions for Base-Load Operation." MHD An International Journal 2(1):57-63.

Veilkov, E. P.; Zhukov, B. P.; Scheindlin, A. E.; Vengerskii, V. V.; Shelkov, E. M.; Babakov, Y. P.; Volkov, Y. M.; Zeigarnik, V. A.; Mtveenko, O. G.; and Kolyadin, N. M. (1983). "Status and Prospects of Geophysical MHD Power." Proceedings of the 8th International Conference on MHD Electric Power Generation, Moscow.

Most of the work carried out in MHD power generation is published in the Proceedings of the eleven "International Conferences on MHD Electrical Power Generation" organized by the International Liaison Group on MHD Electrical Power Generation (ILG-MHD) sponsored by UNESCO and originally by IAEA and ENEA, in the annual series of the "Symposia on Engineering Aspects of Magnetohydrodynamics" (SEAM) in the United States and in the Japanese series of CIS MHD Symposia.

magnetohydrodynamics The science of relating magnetic fields, mainly mathematically, within a moving conducting medium. It is mainly applicable to the Earth's core, where the geomagnetic field is generated by the motion of magnetic lines of force that are ‘frozen’ within a moving, electrically conducting medium, but it is also applicable to all systems involving the fluid motion of electrically conducting materials within a magnetic field.