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WEEK 03: GENERATION: ALTERNATIVE SOURCES: MAGNETOHYDRODYNAMICS Sections: Definition | Sytems | History | Generation | Future
Magnetohydrodynamics (MHD) is the study of moving magnetic force on a gaseous or liquid conductor for use in power generation. The Guiness Encyclopedia1 defines it as the study of the motion of electrically conducting liquids and gases in the presence of a magnetic field. The subject is also known as hydromagnetics, magnetodynamics, and (when a gas is involved) magneto-gas dynamics. The field of MHD represents a joining of two branches of physics: the study of electromagnetic radiation and the study of the behavior of fluids. One of the most interesting aspects of MHD is that moving fluids and magnetic fields mutually interact with one another. That is, electric currents induced in a magnetic field by the motion of a fluid modify the field. At the same time the motion of the fluid in the field produces mechanical forces that modify the motion. There are three areas in MHD: Magnetoplasmadynamics uses gaseous plasma; Magnetofluiddynamics uses liquid metals; and Magnetogasdynamics uses gaseous fuel. MHDs are "topping" units on a conventional turbine-generator station. Exhaust heat generated are as high as 2,200°K. When used to create stem for turbine-generator tandem, the net thermal overall efficiency is given as h = h1+ h2 (1 - h1), where h1 is the efficiency of MHD generator, usually 0.25 and h2 is the efficiency of the "bottom" steam plant, usually 0.40. The net thermal overall efficiency, h is usually about 0.55. Sections: Definition | Sytems | History | Generation | Future
Sections: Definition | Sytems | History | Generation | Future Development of MHD.3,4 The basic principles of MHD were recognized by the British physicist Michael Faraday and his contemporaries as early as the 1830s. In 1831, Faraday experimented with the flow of mercury in a glass tube between the poles of a magnet in order to observe the MHD effects that might result. The magnetic fields then available, however, were not strong enough for the effects to be measurable. In the early 1930s, the British physicist Sydney Chapman and V. C. A. Ferraro developed a theory accounting for the nature of the magnetic storms that occur in the Earth's atmosphere as a result of changes in solar activity. This theory drew attention to the importance of the mutual interactions of mechanical and magnetic forces in conducting fluids, and studies on such interactions followed in the late 1930s. In 1940, Swedish astrophysicist Hannes Alfven further elaborated the behavior of moving particles in a magnetic field. Two years later, by combining the equations of James Clerk Maxwell--which govern electromagnetic theory--with the equations of fluid mechanics. Alfven was able to predict the existence of a type of wave motion now called the MHD, or Alfven, waves. Alfven Waves2 In 1951 another Swedish scientist, S. O. Lundquist, again used mercury in attempting to demonstrate the existence of MHD waves in the laboratory, as Faraday had a century before. This time, however, magnets of sufficient strength were available. Lundquist placed the mercury in a stainless-steel cylinder with a vertical magnetic field parallel to the axis of the cylinder. He used a rotating circular disk with projecting vanes attached at the bottom of the cylinder to produce wave action. This work, with later experiments, sufficed to demonstrate the existence of MHD waves. When a perfectly conducting fluid moves transverse to magnetic lines of force, it carries the lines of force with it. Therefore, the tube of magnetic lines of force behaves like an elastic string. This situation is comparable to the propagation of waves along a stretched elastic string. These Alfven waves are unimportant in ordinary liquids, where they are strongly damped by the inertia of the liquids. The waves are common, however, in plasmas (see plasma physics). They occur in the Earth's Van Allen Belts and in the solar wind. They are believed to be responsible for transporting energy from the Sun's interior to sunspots and to be a principle mechanism for heating the Sun's corona. On Earth, Alfven waves have been used experimentally to heat plasmas. When using compressible fluids, however, the situation is made much more complicated by the occurrence of other types of MHD waves as well. When an MHD wave is propagating at an arbitrary angle with respect to the magnetic field, the complexity further increases. In contrast to the waves that occur in ordinary fluid dynamics, three wave velocities exist that are classified according to their propagation velocities as fast, slow, and intermediate. Sections: Definition | Sytems | History | Generation | Future
Plasma Propulsion2 The basic principle of electromagnetic pumps and electromagnetic plasma accelerators is the same. A conducting fluid in a pipe is forced to move by the Lorentz force created when mutually perpendicular magnetic fields and electric currents are applied perpendicular to the pipe. So-called electromagnetic pumps are useful for circulating corrosive substances such as liquid sodium carrying heat from the core of nuclear reactors to the heat exchangers outside. An electromagnetic flowmeter consists of an insulating pipe under a uniform transverse magnetic field. The voltage induced by the fluid motion between electrodes placed at the ends of a diameter of the pipe perpendicular to the field is used to indicate the flow rate. MHD plasma accelerators are used to simulate and study reentry conditions for hypersonic vehicles into the atmosphere, as well as to study more basic aerodynamic problems. Accelerators are also undergoing research and development as propulsion devices in which plasmas are accelerated and then expelled to produce a thrust. In the 1980s the United States, the Soviet Union, and Japan all had such MHD projects under way, with Japan planning to launch a small prototype MHD-powered ship in the 1990s. The basic principle of such a ship is to pass seawater, which conducts electricity well, through a pair of large electrodes. At right angles to the current, a large magnetic field generated by a superconducting magnet produces the thrust. Parts of the Generator The following are the basic parts of the MHD Generator: 1) Ducts are of two types:
a) Cold-Wall are composed of water-cooled metals such as copper, stainless-steel, super-alloys or platinum and are maintained at a surface temperature from 500 to 1000°K. Insulation between metals are usually alumina (A2O3), boron nitride or mica. 2) Electrodes usually made of metal (particularly copper) or refractory ceramics. The following factors are considered:
a) The ability to withstand corrosive and erosive actions at high temperature. 3) Magnets could be a) superconducting magnets, b) water-cooled, or c) cryogenic-cooled magnets, usually made of niobium-titanium (NbTi). Types of Generator The MHD Generators are classified into two: 1) Liquid Metal MHD are of two types:
a) Single-Component System uses mercury and mercury-potassium mix; or potassium, cesium, sodium and sodium-potassium mix. 1) Gas MHD are of three types:
a) Air uses either preheated or oxygen-enriched air. This system generates approximately up to 14V. Sections: Definition | Sytems | History | Generation | Future
Distributed SMES Unit6 Portable distributed SMES or D-SMES are placed on key substation location in a loop, on a distributed basis, to provide an instantaneous voltage-control capability at a lower cost compared to traditional network stabilization solutions. The micro-SMES technology unites two critical capabilities that include real energy storage through the use of superconductors and instantaneous response through the use of power electronics. At the heart of the micro-SMES system is a superconducting coil made from niobium-titanium wire that carries large currents at practically zero electrical resistance. Cooled with liquid helium, the magnet is contained within a vacuum-cooled cryostat that is kept at 4.2°K., see above diagram. Integrated within this system, the power electronics module can detect voltage sags and inject precise quantities of real and reactive power to boost voltage on the transmission network within half a millisecond. AN advanced data acquisition system provides continuous local monitoring of SMES units in the field and allows convenient interpretation and analysis of data. Each D-SMES system will approximately provide 2.8 MVA of continuous VAR support with an added capability of 100% over-current capability for 1 sec. The superconducting magnet storage system can instantaneously supply up to 2 MW. Compared to other options, D-SMES provides the following advantages:
1. Performance. The stored power available to the D-SMES system made possible a more rapid response to voltage sags. 1. Maung Hla Pe. The 1998 Grolier Multimedia Encyclopedia, Grolier Interactive Inc.: 1997
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