Magneto hydrodynamics (MHD) (magneto fluid dynamics or hydro magnetics) is the academic discipline which studies the dynamics of electrically conducting fluids . Examples of such fluids include plasmas , liquid metals, and salt water . The word magneto hydro dynamics (MHD) is derived from magneto- meaning magnetic field , and hydro- meaning liquid , and -dynamics meaning movement. The field of MHD was initiated by Hannes Alfvén , for which he received the Nobel Prize in Physics in 1970
80 % of total electricity produced in the world is hydal , while remaining 20% is produced from nuclear, thermal, solar, geothermal energy and from magneto hydro dynamic (mhd) generator.
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 in in progress at TRICHI in TAMIL NADU , under the joint efforts of BARC (Bhabha atomic research center), Associated cement corporation (ACC) and Russian technologists.
Several MHD projects were initiated in the 1960s but overcoming the technical challenges of making a practical system proved very expensive. Interest consequently waned in favour of nuclear power which since that time has seemed a more attractive option .
Serbian engineers in Bosnia had built the first experimental Magneto-Hydrodynamic facility power generator in 1992. It was here it was first patented
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.
An MHD generator is a device for converting heat energy of a fuel directly into electrical energy without conventional electric generator.
In this system. An MHD converter system is a heat engine in which heat taken up at a higher temperature is partly converted into useful work and the remainder is rejected at a temperature. Like all heat engines, the thermal efficiency of an MHD converter is increased by supplying the heat at the highest practical temperature and rejecting it at the lowest practical temperature.
Created By Michael Faraday in 1831.
Homopolar generator used a copper disc that rotated between the two poles of a horseshoe magnet.
It created a lot of current, but only a tiny amount of DC voltage.
The first MHD generator to actually be used industrially was the Dynamo.
In this case, the generator is made up of a stationary structure that has a magnetic field that is constant, and which contains rotating wingdings that turn inside the magnetic field.
On a large dynamo such as would be used in industry, the constant magnetic field is created by electromagnets called field coils.
Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities.
When an electric conductor moves across a magnetic field, a voltage is induced in it which produces an electric current.
This is the principle of the conventional generator where the conductors consist of copper strips.
In MHD generator, the solid conductors are replaced by a gaseous conductor, an ionized gas. If such a gas is passed at a high velocity through a powerful magnetic field, a current is generated and can be extracted by placing electrodes in suitable position in the stream.
The principle can be explained as follows. An electric conductor moving through a magnetic field experiences a retarding force as well as an induced electric field and current.
This effect is a result of FARADAYS LAWS OF ELECTRO MAGNETIC INDUCTION .
The induced EMF is given by,
E ind = u x B where u = velocity of the conductor. B = magnetic field intensity.
The induced current is given by,
J ind = σ x E ind where σ = electric conductivity
The retarding force on the conductor is the Lorentz force given by F ind = J i nd X B
The flow direction is right angles to the magnetic fields direction. An electromotive force (or electric voltage) is induced in the direction at right angles to both flow and field directions, as shown in the next slide.
There are several ways to achieve electrical conductivity with an MHD generator.
The conducting fluids that are usually considered are all gases that are made from alkali metal vapors, noble gases and combustion.
When combustion gases are chosen as the conducting fluid, then potassium carbonate is added to the flow in tiny amounts.
It is thermally ionized and makes up the electron density necessary for conductivity.
Cesium is used in the case of monatomic gases, and the electron temperature is raised above the gas, which makes electrical conductivity possible at a lower temperature than would be the case with thermal ionization.
Finally, in the case of liquid metal, electrical conductivity happens when the liquid metal is injected directly into the vapor or gas stream. This makes a continuous liquid phase possible.
Various MHD Generator:
Named after the man who first looked for the effect in the Thames river.
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.
Power generation depends on the density and type of field used.
Power generation proportional to
1. The cross sectional area of the tube
2. 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.
Faraday Generator Contd.
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.
Various MHD Generator Contd.:
The most common solution is to use the Hall effect to create a current that flows with the fluid.
Arrays of short, vertical electrodes on the sides of the duct are placed.
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.
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.
Various MHD Generator Contd.:
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 disc.
The Faraday currents flow in a perfect dead short around the periphery of the disc.
The Hall effect currents flow between ring electrodes near the center and ring electrodes near the periphery.
Advantage of this design
1. It has simple parallel field lines.
2. 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.
3. The resulting magnet uses a much smaller percentage of the generated power.