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MAGNETO HYDRODYNAMIC GENERATOR



Introduction

Eighty per cent of total electricity produced in the world ,is hydal,while remaining 20% is produced from nuclear,thermal,solar,geothermal energy & from magnato hydrodynamic (mhd) generators. MHD power generation is a new system of electric power generation which is said to be of high efficiency and low pollution. In advanced countries MHD generators are widely used but in developing countries like India it is still under construction. This construction work is in progress at trichi in tamilnadu under joint efforts of BARC(Bhabha Atomic Research Centre) BHEl, Associated Cement Corporation (ACC) and Russian technologists. As its name implies, magneto hydro dynamics(MHD) is concerned with the flow of a conducting fluid in the presence of magnetic and electric field. The fluid may be gas at elevated temperatures or liquid metals like sodium or potassium.

The MHD (magnetohydrodynamic) generator or dynamo, is for the direct transformation of thermal energy or kinetic energy into electricity. An advantage of MHD generators over traditional electrical generators is they operate with few moving parts. This technology is applicable to power generation and engine applications. The basic concept underlying the mechanical and fluid dynamos is the same. The fluid dynamo, however, uses the motion of fluid or 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 a MHD generator and a MHD dynamo is the path the charged particles follow. The MHD dynamo is an active area of research in plasma physics and is 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 . aurora borealis

Any of a class of devices that generate electric power by means of the interaction of a moving fluid (usually an ionized gas or plasma) and a magnetic field. Magnetohydrodynamic (MHD) power plants offer the potential for large-scale electrical power generation with reduced impact on the environment. Since 1970, several countries have undertaken MHD research programs with a particular emphasis on the use of coal as a fuel. MHD generators are also attractive for the production of large electrical power pulses.

Magneto Hydrodynamic Generator(MHD)?

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The MHD (magnetohydrodynamic) generator or dynamo transforms thermal energy or kinetic energy directly into electricity. MHD generators are different from traditional electric generators in that they can operate at high temperatures without moving parts. MHD was developed because the exhaust of a plasma MHD generator is a flame, still able to heat the boilers of a 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 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 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 cycles 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 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.

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 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

Principle Of MHD Power Generation:

The underlying principle of MHD power generation is elegantly simple. Typically, an electrically conducting gas is produced at high pressure by combustion of a fossil fuel. The gas is then directed through a magnetic field, resulting in an electromotive force within it in accordance with Faraday’s law of induction (named for the 19th-century English physicist and chemist Michael Faraday). The MHD system constitutes a heat engine, involving an expansion of the gas from high to low pressure in a manner similar to that employed in a conventional gas turbogenerator (see ). In the turbogenerator, the gas interacts with blade surfaces to drive the turbine and the attached electric generator. In the MHD system, the kinetic energy of the gas is converted directly to electric energy as it is allowed to expand.

Interest in MHD power generation was originally stimulated by the observation that the interaction of a plasma with a magnetic field could occur at much higher temperatures than were possible in a rotating mechanical turbine. The limiting performance from the point of view of efficiency in heat engines was established early in the 19th century by the French engineer Sadi Carnot. The Carnot cycle, which establishes the maximum theoretical efficiency of a heat engine, is obtained from the difference between the hot source temperature and the cold sink temperature, divided by the source temperature. For example, if the source temperature is 3,000 K (about 2,700 °C, or 4,900 °F) and the sink temperature 300 K (about 30 °C, or 85 °F), the maximum theoretical efficiency would be 90 percent. Allowing for the inefficiencies introduced by finite heat transfer rates and component inefficiencies in real heat engines, a system employing an MHD generator offers the potential of an ultimate efficiency in the range of 60 to 65 percent. This is much better than the 35 to 40 percent efficiency that can be achieved in a modern conventional plant. In addition, MHD generators produce fewer pollutants than conventional plants. However, the higher construction costs of MHD systems have limited their adoption.

In an MHD generator the hot gas is accelerated by a nozzle and injected into a channel. A powerful magnetic field is set up across the channel. In accordance with Faraday’s law of induction, an electric field is established that acts in a direction perpendicular to both the gas flow and the magnetic field. The walls of the channel parallel to the magnetic field serve as electrodes and enable the generator to provide an electric current to an external circuit

The power output of an MHD generator for each cubic metre of its channel volume is proportional to the product of the gas conductivity, the square of the gas velocity, and the square of the strength of the magnetic field through which the gas passes. For MHD generators to operate competitively with good performance and reasonable physical dimensions, the electrical conductivity of the plasma must be in a temperature range above about 1,800 K (about 1,500 °C, or 2,800 °F). The turbine blades of a gas-turbine power system are unable to operate at such temperatures. An adequate value of electrical conductivity—10 to 50 siemens per metre—can be achieved if an additive, typically about 1 percent by mass, is injected into the hot gas. This additive is a readily ionizable alkali material, such as cesium, potassium carbonate, or sodium, and is referred to as the “seed.” While cesium has the lowest ionizing potential (3.894 electron volts), potassium (4.341 electron volts) is less costly. Even though the amount of seed material is small, economic operation requires that a system be provided to recover as much of it as possible.

The hot gas with its seed is at a pressure of several million pascals. It is accelerated by a nozzle to a speed that may be in the range of 1,000 to 2,000 metres (about 3,300 to 6,600 feet) per second. The gas then enters the channel or duct, across which the magnetic field is applied. To produce a competitive MHD system, this magnetic field must have high intensity. Typically, a superconducting magnet is employed to provide a magnetic field in the range of three to five teslas across the channel. An electromotive force acting in a direction perpendicular to both the flow and the field is set up, and the walls parallel to the magnetic field serve as electrodes to provide current to an external electric circuit. The remaining two walls of the channel are electric insulators. Theoretically, an MHD system with a gas conductivity of 25 siemens per metre, an average magnetic field of three teslas, and an average gas velocity of 1,000 metres per second is capable of generating electric power with a density of about 250 million watts per cubic metre of channel volume.

A complicating feature of a plasma MHD generator is the occurrence of a pronounced Hall effect. This results from the behaviour of electrons in the presence of both magnetic and electric fields. Electrons in the plasma have a much higher mobility than ions. When electric load current flows across the channel, the electrons in this current experience a force directed along the channel. This is the Hall effect—named for its discoverer, the American physicist Edwin H. Hall. As a result of this effect, the electric current flows at an angle across the channel. An additional electric field, called the Hall field, is established along the axis of the channel. This in turn requires that either the electrode walls in a typical generator configuration (see ) be constructed to support this Hall field or that the Hall field itself be used as the output to drive current through the electric circuit external to the MHD system.

A number of generator configurations have been devised to accommodate the Hall effect. In a Faraday generator, as shown in part A of the , the electrode walls are segmented and insulated from each other to support the axial electric field and the electric power is taken out in a series of loads. In the alternate configuration known as a Hall generator, the Faraday field across each sector of the channel is short-circuited and the sectors are connected in series. This allows the connection of a single electric load between the ends of the channel. Consideration of the electric potentials at different points in the channel leads to the observation that an equipotential runs diagonally across the insulator walls and that electrodes may be appropriately staggered to match the equipotentials. The series connection of these electrodes in this diagonal generator permits a single electric load to be used.

Types Of  MHD’s

Faraday generator:

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The Faraday generator is named after the man who first looked for the effect in the Thames river. 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.

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 fuels 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 fission or 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.

The efficiency of the magnetohydrodynamic generator in a single stage is estimated to be no greater than 10 to 20 percent. This makes it unattractive, by itself, for power generation. However it has a number of places that it would be an ideal fit in series with other forms of power generation. In series with a fossil fuel power plant a MHD generator could provice and efficency boost. By routing the exhaust gases of such a plant through a magnetohydrodynamic generator before traditional thermal to electrical conversion plants, it is estimated that one can convert fossil fuels into electricity with an estimated efficiency of up to 65 percent. Similarly, the employment of a magnetohydrodynamic generator is conceivable in series with a Nuclear reactor (either fission or fusion). Reactors of this type tend to opperate with fuel rod temperatures at approximately 2000 C. By pumping the reactor coolant through a magnetohydrodynamic generator before a traditional heat exchanger is reached an estimated efficency of 60 percent can be realised.

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.

ADVANTAGES OF MHD SYSTEMS:

The conversion efficiency of an MHD system can be around 50 per cent as compared to less then 40 per cent for the most efficient steam plants. Still higher thermal efficiencies(60-65%) are expected in future, with the improvements in experience and technology. Large amount of power is generated. It has no moving parts, so more reliable. The closed cycle system produces power free of pollution. It has ability to reach the full power level as soon as started. The size of the plant (m2/kW) is considerably smaller then conventional fossil fuel plants.

MHD-generator (magneto-hydrodynamic generator) is a complex physical installation based on a solid fuel booster of a missile engine. MHD-generator creates a short high-power electromagnetic impulse used for depth sounding of Earth. The natural electromagnetic field of the Earth (NEMF) is a source of interference and noise in these experiments. Therefore, the starting time of the MHD-generator must be selected with the minimal level of interference. Besides that, it is necessary to provide a real-time control over numerous measuring stations located on a geophysical range of several dozens of miles.

Conventional coal-fired generators achieve a maximum efficiency of about 35%. MHD generators have the potential to reach 50% – 60% efficiency. The higher efficiency is due to recycling the energy from the hot plasma gas to standard steam turbines. After the plasma gas passes through the MHD generator, it is still hot enough to boil water to drive steam turbines that produce additional power.

MHD generators are also ecologically sound. Coal with high sulfur content can be used in the MHD without polluting the atmosphere.

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