Major types of MHD systems
Coal-fired MHD systems
The choice of type of MHD generator depends on the fuel to be used and the application. The abundance of coal reserves throughout much of the world has favoured the development of coal-fired MHD systems for electric power production. Coal can be burned at a temperature high enough to provide thermal ionization. However, as the gas expands along the duct or channel, its electrical conductivity drops along with its temperature. Thus, power production with thermal ionization is essentially finished when the temperature falls to about 2,500 K (about 2,200 °C, or 4,000 °F). To be economically competitive, a coal-fired power station would have to combine an MHD generator with a conventional steam plant in what is termed a binary cycle. The hot gas is first passed through the MHD generator (a process known as topping) and then on to the turbogenerator of a conventional steam plant (the bottoming phase). An MHD power plant employing such an arrangement is known as an open-cycle, or once-through, system.
Coal combustion as a source of heat has several advantages. For example, it results in coal slag, which under magnetohydrodynamic conditions is molten and provides a layer that covers all of the insulator and electrode walls. The electrical conductivity of this layer is sufficient to provide conduction between the gas and the electrode structure but not so high as to cause significant leakage of electric currents and consequent power loss. The reduction in thermal losses to the walls because of the slag layer more than compensates for any electrical losses arising from its presence. Also, the use of a seed material in conjunction with coal offers environmental benefits. In particular, the recombination chemistry that occurs in the duct of an MHD generator favours the formation of potassium sulfate in the combustion of high-sulfur coals, thereby reducing sulfur dioxide emissions to the atmosphere. The need to recover seed material also ensures that a high level of particulate removal is built into an MHD coal-fired plant. Finally, by careful design of the boiler and the combustion controls, low levels of nitrogen oxide emissions can be achieved.
Other MHD systems
In addition to natural gas as a fuel source, more-exotic MHD power generation systems have been proposed. Conventional nuclear reactors can employ hydrogen, or a noble gas such as argon or helium, as the working fluid, but they operate at temperatures that are too low to produce the thermal ionization used in MHD generators. Thus, some form of nonequilibrium ionization using seeding material is necessary.
In theory, solar concentrators can provide thermal energy at a temperature high enough to provide thermal ionization. Thus, solar-based MHD systems have potential, provided that solar collectors can be developed that operate reliably for extended periods at high temperatures.
The need to provide large pulses of electrical power at remote sites has stimulated the development of pulsed MHD generators. For this application, the MHD system basically consists of a rocket motor, duct, magnet, and connections to an electrical load. Such generators have been operated as sources for pulse-power electromagnetic sounding apparatuses used in geophysical research. Power levels up to 100 megawatts for a few seconds have been achieved.
A variation of the usual MHD generator employs a liquid metal as its electrically conducting medium. Liquid metal is an attractive option because of its high electrical conductivity, but it cannot serve directly as a thermodynamic working fluid. The liquid has to be combined with a driving gas or vapour to create a two-phase flow in the generator duct, or it has to be accelerated by a thermodynamic pump (often described as an ejector) and then separated from the driving gas or vapour before it passes through the duct. While such liquid metal MHD systems offer attractive features from the viewpoint of electrical machine operation, they are limited in temperature by the properties of liquid metals to about 1,250 K (about 975 °C, or 1,800 °F). Thus, they compete with various existing energy-conversion systems capable of operating in the same temperature range.
The use of MHD generators to provide power for spacecraft for both burst and continuous operations has also been considered. While both chemical and nuclear heat sources have been investigated, the latter has been the preferred choice for applications such as supplying electric propulsion power for deep-space probes.
Development of MHD power generators
The first recorded MHD investigation was conducted in 1821 by the English chemist Humphry Davy when he showed that an arc could be deflected by a magnetic field. More than a decade later, Michael Faraday sought to demonstrate motional electromagnetic induction in a conductor moving through Earth’s geomagnetic field. To this end, he set up in January 1832 a rudimentary open-circuit MHD generator, or flow meter, on the Waterloo Bridge across the River Thames in London. His experiment was unsuccessful owing to the electrodes being electrochemically polarized, an effect not understood at that time.
Faraday soon turned his attention to other aspects of electromagnetic induction, and MHD power generation received little attention until the 1920s and ’30s, when Bela Karlovitz, a Hungarian-born engineer, first proposed a gaseous MHD system. In 1938 he and Hungarian engineer D. Halász set up an experimental MHD facility at the Westinghouse Electric Corporation research laboratories and by 1946 had shown that, through seeding the working gas, small amounts of electric power could be extracted. The project was abandoned, however, largely because of a lack of understanding of the conditions required to make the working gas an effective conductor.
Interest in magnetohydrodynamics grew rapidly during the late 1950s as a result of extensive studies of ionized gases for a number of applications. In 1959 the American engineer Richard J. Rosa operated the first truly successful MHD generator, producing about 10 kilowatts of electric power. By 1963 the Avco Research Laboratory, under the direction of the American physicist Arthur R. Kantrowitz, had constructed and operated a 33-megawatt MHD generator, and for many years this remained a record power output. The assumption in the late 1960s that nuclear power would dominate commercial power generation, and the failure to find applications for space missions, led to a sharp curtailment of MHD research. The energy crisis of the 1970s, however, brought about a revival of interest, with the focus centred on coal-fueled systems. By the late 1980s, development had reached the point where construction of a complete demonstration system was feasible. However, the performance and economic risks have deterred electric power utilities from making deep investments in such systems. This situation may change if energy prices or environmental considerations shift significantly.
William D. Jackson G. Ralph Strohl