MHD generator 1502835 214774856 2008-05-25T05:39:33Z 66.81.24.143 /* Economics */ {{Portal|Energy}} {{TOCright}} The '''MHD ''' ([[Magnetohydrodynamics|magnetohydrodynamic]]) '''generator''' or '''dynamo''' transforms [[thermal energy]] or [[kinetic energy]] directly into [[electricity]]. MHD generators are different from traditional [[electric generator]]s in that they can operate at high temperatures without moving parts. MHD was eagerly developed because the exhaust of a plasma MHD generator is a flame, still able to heat the boilers of a [[Rankine cycle|steam]] [[power plant]]. So high-temperature MHD was developed as a [[topping cycle]] to increase the efficiency of [[electric generation]], especially when burning [[coal]] or [[natural gas]]. It has also been applied to pump liquid metals and for [[Magnetohydrodynamic drive|quiet submarine engines]]. The basic concept underlying the mechanical and fluid dynamos is the same. The fluid dynamo, however, uses the motion of fluid or [[Plasma (physics)|plasma]] to generate the currents which generate the electrical energy. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. The functional difference between an MHD generator and an MHD dynamo is the path the charged particles follow. MHD generators are now practical for fossil fuels, but have been overtaken by other, less expensive technologies, such as [[combined cycle]]s in which a [[gas turbine]]'s or [[molten carbonate fuel cell]]'s exhaust heats [[steam]] for [[steam turbine]]. The unique value of MHD is that it permits an older single-cycle fossil-fuel power plant to be upgraded to high efficiency. Natural MHD dynamos are an active area of research in [[plasma physics]] and are of great interest to the [[geophysics]] and [[astrophysics]] communities. From their perspective the earth is a [[global]] MHD dynamo and with the aid of the particles on the [[solar wind]] produces the [[Polar aurora|aurora borealis]]. The differently charged electromagnetic layers produced by the dynamo effect on the earth's [[geomagnetic field]] enable the appearance of the aurora borealis. As power is extracted from the plasma of the solar wind, the particles slow and are drawn down along the field lines in a brilliant display over the poles. ==Principle== 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. ::<math>\mathbf{F} = Q \cdot (\mathbf{v}\times\mathbf{B} )</math> where *'''F''' is the force acting on the particle (vector), *Q is charge of particle (scalar), *'''v''' is velocity of particle (vector), *x is the cross product, *'''B''' is magnetic field (vector). The vector '''F''' is perpendicular to both ''' v''' and '''B''' according to the [[Right hand rule]]. == Power generation == Typically for a large scale power station to approach operational efficiency in [[computer model]]s, steps must be taken to increase the electrical conductivity of the conductive substance. The heating of a gas to plasma or the addition of other easily ionizable substances like the salts of alkali metals accomplishes this increase in conductivity. In practice a number of issues must be considered in the implementation of a '''MHD generator''': Generator efficiency, Economics, and Toxic byproducts. These issues are affected by the choice of one of the three MHD generator designs. These are the Faraday generator, the Hall generator, and the disc. === Faraday generator === The Faraday generator is named after the man who first looked for the effect in the Thames river (see History, below). A simple Faraday generator would consist of a wedge-shaped pipe or tube of some non-[[conductive]] material. When an electrically conductive fluid flows through the tube, in the presence of a significant perpendicular magnetic field, a charge is induced in the field, which can be drawn off as electrical power by placing the electrodes on the sides at 90 degree angles to the magnetic field. There are limitations on the density and type of field used. 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. MHD generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C. 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. To get this field, a large generator requires an extremely powerful magnet. Many research groups have tried to adapt superconducting magnets to this purpose, with varying success. === Hall generator === The most common answer 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. These shorts of the Faraday current induce a powerful magnetic field within the fluid, but in a chord of a circle at right angles to the Faraday current. This secondary, induced field makes current flow in a rainbow shape between the first and last electrodes. 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. === Disc generator === The third, currently most efficient answer 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. The magnetic excitation field is made by a pair of circular [[Helmholtz coils]] above and below the disk. The Faraday currents flow in a perfect dead short around the periphery of the disk. The Hall effect currents flow between ring electrodes near the center and ring electrodes near the periphery. Another significant advantage of this design is that the magnet is more efficient. First, it has simple parallel field lines. Second, because the fluid is processed in a disk, the magnet can be closer to the fluid, and magnetic field strengths increase as the 7th power of distance. Finally, the generator is compact for its power, so the magnet is also smaller. The resulting magnet uses a much smaller percentage of the generated power. === Generator efficiency === As of 1994, the 22% efficiency record for closed-cycle disc MHD generators was held by Tokyo Technical Institute. The peak enthalpy extraction in these experiments reached 30.2%. Typical open-cycle Hall & duct coal MHD generators are lower, near 17%. These efficiencies make MHD unattractive, by itself, for utility power generation, since conventional [[Rankine cycle]] power plants easily reach 40%. However, the exhaust of an MHD generator burning [[fossil fuel]] is almost as hot as the flame of a conventional steam boiler. By routing its exhaust gases into a boiler to make steam, MHD and a [[steam]] [[Rankine cycle]] can convert [[fossil fuel]]s into electricity with an estimated efficiency up to 60 percent, compared to the 40 percent of a typical coal plant. A magnetohydrodynamic generator might also be heated by a [[Nuclear reactor]] (either [[Nuclear fission|fission]] or [[Nuclear fusion|fusion]]). Reactors of this type operate at temperatures as high as 2000 °C. By pumping the reactor coolant into a magnetohydrodynamic generator before a traditional heat exchanger an estimated efficiency of 60 percent can be realised. One possible conductive coolant is the [[molten salt reactor]]'s molten salt, since molten salts are electrically conductive. MHD generators have also been proposed for a number of special situations. In submarines, low speed MHD generators using liquid metals would be nearly silent, eliminating a source of tell-tale mechanism noise. In spacecraft and unattended locations, low-speed metallic MHD generators have been proposed as highly reliable generators, linked to solar, nuclear or isotopic heat sources. === Economics === MHD generators have not been employed for large scale mass energy conversion because other techniques with comparable efficiency have a lower investment and operating cost. Advances in natural gas turbines achieved similar thermal efficiencies at lower costs, by having the turbine's exhaust drive a [[Rankine cycle]] steam plant. To get more electricity from coal, it's cheaper to simply add more low-temperature steam-generating capacity. If high efficiency is needed in a new plant, coal [[gasification]] feeding [[molten carbonate fuel cell|molten salt]] or [[solid-oxide fuel cell|solid oxide]] fuel cells should have superior efficiencies because the fuel cell bypasses the inherent inefficiencies of a [[heat engine]]. Also, MHD generators for fossil fuels are inherently expensive. A certain amount of electricity is required to maintain sustained [[magnetic field]] over 1 [[Tesla (unit)|T]]. Because of the high temperatures, the walls of the channel must be constructed from an exceedingly heat-resistant substance such as [[yttrium]] oxide or [[zirconium]] dioxide to retard oxidation. Similarly, the electrodes must be both conductive and heat-resistant at high temperatures, making [[tungsten]] a common choice. === Toxic byproducts === MHD reduces overall production of hazardous fossil fuel wastes because it increases plant efficiency. In MHD coal plants, the patented commercial "Econoseed" process developed by the U.S. (see below) recycles potassium ionization seed from the fly ash captured by the stack-gas scrubber. However, this equipment is an additional expense. If molten metal is the armature fluid of an MHD generator, care must be taken with the coolant of the electromagnetics and channel. The alkali metals commonly used as MHD fluids react violently with water. Also, the chemical byproducts of heated, electrified alkali metals and channel ceramics may be poisonous and environmentally persistent. == History == [[Michael Faraday]] first proposed the idea in his "Bakerian lecture for 1832" to the [[Royal Society]]. He carried out experiments at Waterloo Bridge, measuring current from the flow of the Thames in the Earth's magnetic field. The first practical MHD power research was funded in the 1938 in the U.S. by [[Westinghouse Electric Corporation (1886)|Westinghouse]] in its [[Pittsburgh, Pennsylvania]] laboratories, headed by [[Bela Karlovitz]]. The initial patent on MHD is by B. Karlovitz, U.S. Patent No. 2,210,918, "Process for the Conversion of Energy", August 13, 1940. World war II interrupted development. In 1962, the First International Conference on MHD Power was held in Newcastle on Tyne, UK by Dr. Brian C. Lindley of the International Research and Development Company Ltd. The group set up a steering committee to set up further conferences and disseminate ideas. In 1964, the group set up a second conference in Paris, France, in consultation with the [[European Nuclear Energy Agency]]. Since membership in the [[ENEA]] was limited, the group persuaded the [[International Atomic Energy Agency]] to sponsor a third conference, in Salzburg, Austria, July 1966. Negotiations at this meeting converted the steering committee into a periodic reporting group, the ILG-MHD (international liaison group, MHD), under the ENEA, and later in 1967, also under the International Atomic Energy Agency. Further research in the 1960s by R. Rosa established the practicality of MHD for fossil-fueled systems. In the 1960s, AVCO Everett Aeronautical Research began a series of experiments, ending with the Mk. V generator of 1965. This generated 35 MW, but used about 8MW to drive its magnet. In 1966, the ILG-MHD had its first formal meeting in Paris, France. It began issuing a periodic status report in 1967. This pattern persisted, in this institutional form, up until 1976. Toward the end of the 1960s, interest in MHD declined because nuclear power was becoming more widely available. In the late 1970s, as interest in nuclear power declined, interest in MHD increased. In 1975, [[UNESCO]] became persuaded the MHD might be the most efficient way to utilise world coal reserves, and in 1976, sponsored the ILG-MHD. In 1976, it became clear that no nuclear reactor in the next 25 years would use MHD, so the [[International Atomic Energy Agency]] and [[ENEA]] (both nuclear agencies) withdrew support from the ILG-MHD, leaving [[UNESCO]] as the primary sponsor of the ILG-MHD. === U.S. development === In the 1980s, the [[U.S. Department of Energy]] began a vigorous multiyear program, culminating in a 1992 50MW demonstration coal combustor at the Component Development and Integration Facility (CDIF) in [[Butte, Montana]]. This program also had significant work at the Coal-Fired-In-Flow-Facility (CFIFF) at [[Tullahoma University]]'s [[Space Research Science Institute]]. This program combined four parts: *1. An integrated MHD topping cycle, with channel, electrodes and current control units developed by AVCO, later known as Textron Defence of Boston. This system was a Hall effect duct generator heated by pulverized coal, with a potassium ionisation seed. AVCO had developed the famous Mk. V generator, and had significant experience. *2. An integrated bottoming cycle, developed at the CDIF. *3. A facility to regenerate the ionization seed was developed by TRW. Potassium carbonate is separated from the sulphate in the fly ash from the scrubbers. The carbonate is removed, to regain the potassium. *4. A method to integrate MHD into preexisting coal plants. The Department of Energy commissioned two studies. Westinghouse Electric performed a study based on the Scholtz Plant of Gulf Power in [[Snead, Florida]]. The MHD Development Corporation also produced a study based on the J.E. Corrette Plant of the Montana Power Company of [[Billings, Montana]]. Initial prototypes at the CDIF were operated for short durations, with various coals: Montana Rosebud, and a high-sulphur corrosive coal, Illinois No. 6. A great deal of engineering, chemistry and material science was completed. After final components were developed, operational testing completed with 4,000 hours of continuous operation, 2,000 on Montana Rosebud, 2,000 on Illinois No. 6. The testing ended in 1993. === Japanese development === The Japanese program in the late 1980s concentrated on closed-cycle MHD. The belief was that it would have higher efficiencies, and smaller equipment, especially in the clean, small, economical plant capacities near 100 megawatts (electrical) which are suited to Japanese conditions. Open-cycle coal-powered plants are generally thought to become economical above 200 megawatts. The first major series of experiments was FUJI-1, a blow-down system powered from a shock tube at the [[Tokyo Institute of Technology]]. These experiments extracted up to 30.2% of enthalpy, and achieved power densities near 100 megawatts per cubic meter. This facility was funded by Tokyo Electric Power, other Japanese utilities, and the Department of Education. Some authorities believe this system was a disc generator with a helium and argon carrier gas and potassium ionization seed. In 1994, there were detailed plans for FUJI-2, a 5MW (electrical) continuous closed-cycle facility, powered by natural gas, to be built using the experience of FUJI-1. The basic MHD design was to be a system with inert gases using a disk generator. The aim was an enthalpy extraction of 30% and an MHD thermal efficiency of 60%. FUJI-2 was to be followed by a retrofit to a 300 MWe natural gas plant. === Australian development === In 1986, Professor Hugo K Messerle at The University of Sydney researched coal-fueled MHD. This resulted in a 28 MWe topping facility that was operated outside Sydney. Messerle also wrote one of the most recent reference works (see below), as part of a UNESCO education program. A detailed obituary for Hugo is located here [http://www.ewh.ieee.org/r10/nsw/] === Italian development === The Italian program began in 1989 with a budget of about 20 million $US, and had three main development areas: *1. MHD Modelling. *2. Superconducting magnet development. The goal in 1994 was a prototype 2 m long, storing 66 [[megajoule|MJ]], for an MHD demonstration 8 m long. The field was to be 5 [[tesla (unit)|teslas]], with a taper of 0.15 T/m. The geometry was to resemble a saddle shape, with cylindrical and rectangular windings of niobium-titanium copper. *3. Retrofits to natural gas powerplants. One was to be at the Enichem-Anic factor in Ravenna. In this plant, the combustion gases from the MHD would pass to the boiler. The other was a 230 MW (thermal) installation for a power station in Brindisi, that would pass steam to the main power plant. === Chinese development === A joint U.S.-China national programme ended in 1992 by retrofitting the coal-fired No. 3 plant in Asbach. A further eleven-year program was approved in March 1994. This established centres of research in: *1. The Institute of Electrical Engineering in the [[Academica Sinica]], Beijing, concerned with MHD generator design. *2. The [[Shanghai Power Research Institute]], concerned with overall system and superconducting magnet research. *3. The Thermoenergy Research Engineering Institute at the Nanjing's [[Southeast University]], concerned with later developments. The 1994 study proposed a 10 MW (electrical, 108 MW thermal) generator with the MHD and bottoming cycle plants connected by steam piping, so either could operate independently. === Russian developments === By 1994, Russia had developed and operated the coal-operated facility U-25, at the High-Temperature Institute of the [[Russian Academy of Sciences]] in Moscow. U-25's bottoming plant was actually operated under contract with the Moscow utility, and fed power into Moscow's grid. There was substantial interest in Russia in developing a coal-powered disc generator. == See also == *[[Magnetohydrodynamics]] *[[Magnetohydrodynamic drive]] *[[Plasma physics]] *[[Electromagnetic radiation]] == External links == ===Research=== * [http://www.physics.ubc.ca/~outreach/phys420/p420_96/reg/main.htm Magnetohydrodynamics and the Lorentz Force Law] * [http://www.utsi.edu/news/release12-1-04mhdgenerator.html MHD generatorResearch at the University of Tennessee Space Institute] - 2004 * [http://icm.krasn.ru/section.php?id=5&page=lab23devices&lang=eng Model of an MHD-generator at the Institute of Computational Modelling, Akademgorodok, Russia] - 2003 * [http://www.die.ing.unibo.it/Labs/MHD/MHlabE.htm The Magnetohydrodynamic Engineering Laboratory Of The University Of Bologna, Italy] - 2003 ===References=== <references/> *Hugo K. Messerle, "Magnetohydrodynamic Power Generation", 1994, John Wiley, Chichester, Part of the UNESCO Energy Engineering Series (This is the source of the historical and generator design information) *Shioda, S. "Results of Feasibility Studies on Closed-Cycle MHD Power Plants", Proc. Plasma Tech. Conf., 1991, Sydney, Australia, pp189-200. *R.J. Rosa, "Magnetohydrodynamic Energy Conversion", 1987, Hemisphere Publishing, Washington D.C. *G.J. Womac, "MHD Power Generation", 1969, Chapman and Hall, London ===Geophysics=== * [http://liftoff.msfc.nasa.gov/shuttle/sts-75/tss-1r/brochure/page_7.html NASA on how the Earth's atmosphere acts like a MHD generator] [[Category:Plasma physics|Magnetohydrodynamic generator]] [[Category:Electrical generators]] [[Category:Chemical engineering]] [[Category:Power station technology]] [[Category:Energy conversion]] [[de:Magnetohydrodynamischer Generator]] [[fr:Générateur MHD]] [[ja:MHD発電]] [[pl:Generator magnetohydrodynamiczny]] [[ru:Магнитогидродинамический генератор]]