08-07-2014, 02:17 PM
MAGNETO HYDRO DYNAMIC POWER GENERATION (MHD )
MAGNETO HYDRO DYNAMIC POWER GENERATION.ppt (Size: 974.5 KB / Downloads: 160)
INTRODUCTION
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.
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
PRINCIPLES OF MHD POWER GENERATION
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.
PRINCIPLES OF MHD POWER GENERATION
This effect is a result of FARADAYS LAWS OF ELECTRO MAGNETIC INDUCTION.
The induced EMF is given by Eind = u x B where u = velocity of the conductor. B = magnetic field intensity.
The induced current is given by, Jind = C x Eind where C = electric conductivity
The retarding force on the conductor is the Lorentz force given by Find = Jind X B
PRINCIPLES OF MHD POWER GENERATION
The electro magnetic induction principle is not limited to solid conductors. The movement of a conducting fluid through a magnetic field can also generate electrical energy.
When a fluid is used for the energy conversion technique, it is called MAGNETO HYDRO DYNAMIC (MHD), energy conversion.
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.
PRINCIPLES OF MHD POWER GENERATION
The conducting flow fluid is forced between the plates with a kinetic energy and pressure differential sufficient to over come the magnetic induction force Find.
The end view drawing illustrates the construction of the flow channel.
An ionized gas is employed as the conducting fluid.
Ionization is produced either by thermal means I.e. by an elevated temperature or by seeding with substance like cesium or potassium vapors which ionizes at relatively low temperatures.
The atoms of seed element split off electrons. The presence of the negatively charged electrons makes the gas an electrical conductor.
OPEN CYCLE SYSTEM
The fuel used maybe oil through an oil tank or gasified coal through a coal gasification plant
The fuel (coal, oil or natural gas) is burnt in the combustor or combustion chamber.
The hot gases from combustor is then seeded with a small amount of ionized alkali metal (cesium or potassium) to increase the electrical conductivity of the gas.
The seed material, generally potassium carbonate is injected into the combustion chamber, the potassium is then ionized by the hot combustion gases at temperature of roughly 2300’ c to 2700’c
LIQUID METAL SYSTEM
After passage through the generator, the liquid metal is separated from the carrier gas. Part of the heat exchanger to produce steam for operating a turbine generator. Finally the carrier gas is cooled, compressed and returned to the combustion chamber for reheating and mixing with the recovered liquid metal. The working fluid temperature is usually around 800’c as the boiling point of sodium even under moderate pressure is below 900’c.
At lower operating temp, the other MHD conversion systems may be advantageous from the material standpoint, but the maximum thermal efficiency is lower. A possible compromise might be to use liquid lithium, with a boiling point near 1300’c as the electrical conductor lithium is much more expensive than sodium, but losses in a closed system are less.