Magneto hydrodynamics

INTRODUCTION• 80 % of total electricity produced in the world is

hydal, while remaining 20% is produced from nuclear, thermal, solar, geothermal energy and from magneto hydrodynamics(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 Tamil Nadu under joint efforts of BARC, BHEL, ACC and Russian technologists.

What is Magnetohydrodynamics?

• Magnetofluiddynamics

• Dynamics of electrically-conducting fluids and their interactions with magnetic fluids.

• An MHD generator is a device for converting heat energy of a fuel directly into electrical energy without a conventional electric generator.

• It is concerned with flow of conducting fluid in the presence of magnetic and electric field.

BASIC OPERATION OF MHD GENERATOR

MHD covers phenomena in electrically conducting fluids, where the velocity field V, and the magnetic field B are coupled.

Any movement of a conducting material in a magnetic field generates electric currents j, which in turn induces a magnetic field.

Each unit volume of liquid having j and B experiences MHD force j x B, known as the “Lorentz force”.

In MHD flows in blanket channels, interaction of the induced electric currents with the applied plasma-confinement magnetic field results in the flow opposing Lorentz force that may lead to high MHD pressure drop, turbulence modifications, changes in heat and mass transfer and other important MHD phenomena.

A few facts about MHD Alfvén was the first to

introduce the term “MAGNETOHYDRODYNAMICS”. He described astrophysical phenomena as an independent scientific discipline.

The official birth of incompressible fluid Magneto hydrodynamics is 1936-1937. Hartmann and Lazarus performed theoretical and experimental studies of MHD flows in ducts.

The most appropriate name for the phenomena would be “MagnetoFluidMechanics,” but the original name “Magneto hydrodynamics” is still generally used.

Hannes Alfvén (1908-1995), winning the Nobel Prize for his work on Magneto hydrodynamics.

Magnetic fields• Earth – 0.5 10-4 T• Sun – 10-4 T, up to 0.4 T

at sunspots• Jupiter – 10-2 T

(strongest planetary magnetic field in the solar system)

• Permanent laboratory magnets with ~0.1 m gap – about 1-2 T

• Electromagnets – 25-50 T

• Fusion Reactor (ARIES RS) – 12 T

• Experimental Fusion Reactor (NSTX) – 1.5 T

Supplying power to the world's strongest long-pulse magnet at Los Alamos' National High Magnetic Field Laboratory is a 1.4 billion-watt generator, itself the largest among magnetic power sources. It can produce enough energy to power the entire state of New Mexico.

Important aspects of MHD:1. Accelerate fluids through Lorentz force J x B which refers

to MHD accelerators.

2. Convert the kinetic energy of a fluid and its enthalpy into

electric energy. This refers to MHD generators.

Magnetohydrodynamic Waves

• Alfvén waves: vibrations in

the magnetic field lines.

• Fast magnetosonic waves

• Slow magnetosonic waves

• Properties of MHD waves

depend upon the angle

between the wave vector

and the magnetic field

Non-homogeneities in the field results

• Guided propagation of MHD waves

• Linear coupling of different MHD waves

• Phase mixing of Alfvén waves

• Resonant absorption

• Appearance of wave dispersion

Magnetohydrodynamic (MHD) sensors • Linear accelerometers • Automotive safety research instrumentation • Line of-sight (LOS) stabilization for handheld and platform

mounted imaging systems• Sensor controlled LOS stabilization• Defense related space based pointing and tracking experiments

Magnetohydrodynamic Power Generation

• Le Chatelier-Brown Principle

• MHD Generators

• MHD Turbines

• MHD Power Plants

Plasma-driven MHD generators

• Open cycle MHD generators

– Involves the use of combustion products seeded by alkaline

compounds such as potassium carbonate

• A closed cycle MHD generator

– Involves an inert gas which is heated in a high temperature heat

exchanger of the ceramics pebble or cored-brick type and seeded with

an alkaline metal such as cesium vapor.

MHD Turbines

• Mobile power sources

• powered by hydrogen produced by veneered algae on desert

hydrogen farms.

MHD Developmental Status

Coal fired MHDs Natural Gas Fired MHDs

Nations involved: • USA• Former USSR• Japan• India• China• Australia• Italy• Poland• Yugoslavia

Working Principle •The MHD generator can be considered to be a fluid dynamo. This is similar to a mechanical dynamo in which the motion of a metal conductor through a magnetic field creates a current in the conductor except that in the MHD generator the metal conductor is replaced by a conducting gas plasma.•When a conductor moves through a magnetic field it creates an electrical field perpendicular to the magnetic field and the direction of movement of the conductor. This is the principle, discovered by Michael Faraday, behind the conventional rotary electricity generator. Dutch physicist Antoon Lorentz provided the mathematical theory to quantify its effects.

The flow (motion) of the conducting plasma through a magnetic field causes a voltage to be generated (and an associated current to flow) across the plasma , perpendicular to both the plasma flow and the magnetic field according to Fleming's Right Hand Rule Lorentz Law describing the effects of a charged particle moving in a constant magnetic field can be stated asF = QvBwhereF is the force acting on the charged particlev is velocity of particleB is magnetic field 

Structure of MHD

Structure of MHD Generator 

•A simple magneto hydrodynamic generator consists of a gas nozzle. •The gas nozzle is a combustion chamber that injects a pulse of gas into the channel/duct. •The walls of the channel act as an electrode.• The induced electric current is fed to the load by an external circuit that supplies the generated electricity to the desired

destination.

The MHD System

The MHD generator needs a high temperature gas source, which could be the coolant from a nuclear reactor or more likely high temperature combustion gases generated by burning fossil fuels, including coal, in a combustion chamber. The diagram below shows possible system components.

• The expansion nozzle reduces the gas pressure and consequently increases the plasma speed through the generator duct to increase the power output). Unfortunately, at the same time, the pressure drop causes the plasma temperature to fall which also increases the plasma resistance,.

• The exhaust heat from the working fluid is used to drive a compressor to increase the fuel combustion rate but much of the heat will be wasted unless it can be used in another process.

•  

Types of MHD•There are two types of MHD System1.) Open cycle System2.) Closed cycle System

•In open cycle system the working fluid after generating electrical energy is discharged to the atmosphere through a stack. In a closed cycle system the working fluid is recycled to the heat sources and thus used again and again. The operation of MHD generator directly on combustion products in an open cycle system. In open cycle system working fluid is air. In closed cycle system helium or argon is used as the working fluid

OPEN CYCLE MHD SYSTEMAn elementary open cycle MHD system, is shown in fig.The MHD generator resembles a rocket engine surrounded by a magnet the coal is burnt to produced hot gas. The hot gas is then seeded with a small amount of an ionized alkali metal (cesium or potassium) to increase the electrical conductivities of gas. The gas expand through the generator surrounded by powerful magnet. During the motion of gas the positive and negative ions move to the electrodes and constitute an electric current. The rejected gas passes through an air heater for preheating the inlet air. The seed material is recovered for successive use. The nitrogen and sulphur are remove for pollution controlled and then gasses are discharged to the atmosphere.

The above cycle is not suitable for commercial use. The exhaust gases of MHD unit are still at a sufficiently hot temperature it is possible to use for additional power generation in a steam turbine alternator unit. This is increase the efficiency of process. Such cycle is known as hybrid MHD-steam plant cycle.• Figure shows hybrid MHD steam cycle, coal is processed and burnt in the combustor at high temperature (2750 to 3000 °K) and pressure (7 to 15 at atmosphere). With preheated air to form the plasma. The plasma is then seeded with small fraction (1%) of an alkali metal (potassium)introduced usually as a carbonate powder or solution.

Closed Cycle SystemThe closed cycle inert gas MHD system was conceived temperature requirement and a very chemically active flow could be removed, by closed cycle MHD system. As the name suggests the working fluid in closed cycle, is circulated in a closed loop. The working fluid is helium or argon with cesium seeding.Figure show a closed cycle MHD system. The complete system has three distinct but interlocking loops. On the left is the external heating loop, coal is gasified and the gas having a high heat value of about5.35 MJ/kg and temperature of about 530°C is burnt in a combustor to pro heat.

In the heat exchanger HX, this heat is transferred to argon the working fluid of MHD cycle. The combustion products after passing through the air pre-heater (to recover a part of the heat of combustion product) and purifiers(To remove harmful emissions) and discharged to atmosphere.The loop in the centre is the MHD loop. The hot argon gases seeded with cesium and passed through MHD generator. The dc power output of MHD generator is converted to A.C. by the inverter and is then feed into the grid .The loop shown on the right hand side in fig is the steam loop for further recovering the heat of the working fluid and converting this heat into electrical energy. The fluid passes through the heat exchanges HX2where it imparts its heat to water which gets converted to steam. This steam is used partly for during a turbine which runs the compressor partly for turbine.

The output of the alternator is also to the grid. The working fluid goes back to the heat exchanges HX) after passing through compressor and intercooler. A closed system can provide more useful power conversion at lower temperatures.

MHD Generators

Faraday Magnetohydrodynamic Generator

Hall Magnetohydrodynamic Generator

Faraday Generator

• 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.

Hall GeneratorThe 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.

MHD Power Plants• Can generate electricity without using the conventional

turbo-alternator

• Offer the prospect of more efficient use of coal for power

generation

Thank you!