(1) Explain with neat sketch a closed cycle MHD System. (OCT/Nov. 2009, May/June 2009, Nov/Dec. 2004, Nov/Dec. 2007) (2) Discuss Principle of MHD System. (OCT/Nov. 2009, Nov/Dec. 2008, Nov/Dec.2001, Nov/Dec. 2000) (3) Explain Open Cycle MHD System.( Nov/Dec. 2008, May/June 2007, Nov/Dec. 2005, May/June 2008, Nov/Dec. 2005) (4) Explain Seeding in MHD Power generation(Nov/Dec. 2004) (5) Write merits and demerits of MHD Closed System. (Nov/Dec. 2003, Nov/Dec. 2002, Nov/Dec. 2001, Nov/Dec. 2000)

(1) Discuss Principle of MHD System. (OCT/Nov. 2009, Nov/Dec. 2008, Nov/Dec.2001, Nov/Dec. 2000) Magneto hydrodynamic is a device which converts the heat energy of a fuel directly into electrical energy without a conventional electric generator. Basic principle is a based on Faradays Law. It states that when a conductor is moved in a magnetic field an emf is induced in the conductor. This conductor may be solid, liquid or gas. The directions of the magnetic field, the direction and the emf induced will be mutually perpendicular to each other. This is given by right hand rule as shown in fig.1.

Fig.1. principle of Magneto-hydrodynamic Working principle is explained below. An MHD generator is like a turbo generator is an energy conversion device. It can be used with any high temperature heat source like chemical, nuclear, solar energy etc. in an MHD generator system, hot gases acts as a conductor.

When hot gases are heated to sufficient temperature, one or more of the valence electrons are expelled from their orbit which they are revolving. This ionized the gas. These ions are the electrical conductors. The gases are moved in a duct called MHD Duct at a very high velocity. The movement of these ions in the MHD duct in which a strong magnetic field is applied leads to the electromagnetic induction of an emf in a pair of electrons which are placed in a direction perpendicular to both the direction of the flow of the ionized gases and that of the magnetic field. The electrodes are connected to the external circuit which drives the current in the external load. (2) Explain Open Cycle MHD System.( Nov/Dec. 2008, May/June 2007, Nov/Dec. 2005, May/June 2008, Nov/Dec. 2005) Fig. shows the open cycle MHD power plant. This system uses an MHD generator in conjunction with conventional steam power plant. The fuel used may be coal, oil or natural gas. The surrounding air is compressed to high pressure in the air compressor. The gases are then heated in the low temperature and high temperature air preheaters before supplying to the combustion chamber upto temperature of about 1100 C.

Fig.2. Open Cycle MHD Power Plant The fuel is burnt in presence of hot air in the combustor. The hot gases so produced are seeded with 1 % alkaline potassium to increase the electrical conductivity of the gases. These hot gases expand in the nozzle to high velocity and enter to MHD duct at about 2300 C to 2700 C temperature. Some part of the

internal energy of the gas or plasma is directly converted into DC power. The DC AC inverter converts the DC power into AC power. Hot gases leaving the MHD duct is used in high temperature air preheater to heat the air. Then these gases are used in heat exchanger to generate steam from feed water. The steam so generated is used in a conventional steam power plant to generate power as shown in fig.2. the remainder of heat of hot gases is utilized for heating the compressed air in low temperature air preheater. Hot gases then pass through a seed recovery unit or ESP (electrostatic Precipitator). The clean hot gases are either exhausted to the surrounding through chimney as shown in fig or again recycled by compressing again, it is decided by type of MHD Plant.

(3) Explain Closed Cycle MHD System.( Nov/Dec. 2008, May/June 2007, Nov/Dec. 2005, May/June 2008, Nov/Dec. 2005) Closed cycle MHD generators can either operate on seeded inert gas (helium or argon) system or liquid metal system. These systems are discussed below: Seeded inert gas MHD generator: Fig.3. shows the closed cycle scheme using helium or argon gas seeded with Cesium MHD generator. Inert gas helium seeded with cesium is heated to a very high temperature in a nuclear reactor. The hot helium gas is passed into nozzles to increase its velocity before supplying to MHD duct to produce DC Power which is converted into AC Power by inverter. The gas is passed through a heat exchanger called steam generator to convert feed water into steam to be used in the conventional steam power plant as shown in fig.

Fig.3. Closed cycle Seeded inert gas, MHD Generator

The helium gas leaving the steam generator is cooled in the cooler and after compressing in the compressor is supplied back to the nuclear reactor. In the above system instead of using the conventional steam power plant as bottoming unit, it can be also employ a gas turbine power plant. The seeded inert gas MHD System is most promising amongst all system.

Liquid metal MHD System: A liquid metal MHD Sytem is as shown in fig.4. It is similar to inert gas system except that is used liquid metal (potassium) instead of helium gas as the working fluid.

Fig.4. Liquid metal MHD System Liquid potassium is heated in the nuclear reactor is passed through the nozzles to increase its velocity. Small amount of vapour formed during the nozzle passage is separated in the separator and it is pumped back into the reactor. High velocity liquid metal is passed through MHD duct to produce DC Power. Comparatively lower temperature liquid potassium is then passed through a heat exchanger before pumped to the reactor. It transfers heat to feed the water which is converted into steam. This steam is used in a conventional steam power plant.

(4) Write merits and demerits of MHD Closed System. (Nov/Dec. 2003, Nov/Dec. 2002, Nov/Dec. 2001, Nov/Dec. 2000),May/June 2008, Nov/Dec. 2005) Closed cycle MHD generators can either operate on seeded inert gas (helium or argon) system or liquid metal system. These systems are discussed below: Seeded inert gas MHD generator: Fig.3. shows the closed cycle scheme using helium or argon gas seeded with Cesium MHD generator. Inert gas helium seeded with cesium is heated to a very high temperature in a nuclear reactor. The hot helium gas is passed into nozzles to increase its velocity before supplying to MHD duct to produce DC Power which is converted into AC Power by inverter. The gas is passed through a heat exchanger called steam generator to convert feed water into steam to be used in the conventional steam power plant as shown in fig.

Fig.3. Closed cycle Seeded inert gas, MHD Generator The helium gas leaving the steam generator is cooled in the cooler and after compressing in the compressor is supplied back to the nuclear reactor. In the above system instead of using the conventional steam power plant as bottoming unit, it can be also employ a gas turbine power plant. The seeded inert gas MHD System is most promising amongst all system.

Liquid metal MHD System:

A liquid metal MHD Sytem is as shown in fig.4. It is similar to inert gas system except that is used liquid metal (potassium) instead of helium gas as the working fluid.

Fig.4. Liquid metal MHD System Liquid potassium is heated in the nuclear reactor is passed through the nozzles to increase its velocity. Small amount of vapour formed during the nozzle passage is separated in the separator and it is pumped back into the reactor. High velocity liquid metal is passed through MHD duct to produce DC Power. Comparatively lower temperature liquid potassium is then passed through a heat exchanger before pumped to the reactor. It transfers heat to feed the water which is converted into steam. This steam is used in a conventional steam power plant.

There are following advantages and limitations of liquid metal system: This system can use energy as high temperature is not the requirement of this system as in case of the plasma converter. It can be easily provide ac power supply directly, which it is almost impossible to do so in a plasma system. The size of the system including that of the magnets, while it is almost comparatively smaller. This is because of high power density. The metallic vapors are poor electrical conductors.

The overall conversion efficiencies obtainable with liquid metal system are quite below to that of the plasma System.

(5) Explain Seeding in MHD Power generation(Nov/Dec. 2004) One way of increasing the conductivity o gas, without the need of exceedingly high temperatures, is to introduce into gas a material whose ionization potential lower than that of the gas atoms themselves. This means that in addition; or seeding agent, will ionize more readily than the gas itself, and will thus increase the electrical conductivity. Main materials are alkali metals like cesium, potassium etc. the addition of the seeding material allows the attainment of suitable conductivities at very much lower temperatures. It is seen that conductivities between 10 and 100 mhos/m can be achieved in the temperature in the range of 2000 2400 C according to the percentage of seed material added and the gas pressure. These temperatures are still very high, but materials technology has now advanced sufficiently to make working at such temperatures at least feasible.