TITLE OF PAPER

M H D GENERATOR

(Non-conventional energy sources)

LAKIREDDY BALI REDDY COLLEGE OF ENGINEERING

ABSTRACT:-Energy generation, conservation and management are some of critical issues facing

society today. However, Energy resources are finite. Increasing demand is being made for

diminishing supplies. The cost of energy is enormous and the cost is rising. Proper usage of

renewable energy sources is the only solutions to the problem of energy insufficiency. Non-

conventional energy sources are any energy source that can be either replenished

continuously or within a moderate timeframe. The use of Non-conventional energy by the

industries and the public will considerably reduce the dependence on fast depleting

conventional energy sources such as coal, oil etc and prevent the damage to environment.

Non-conventional power sources include solar power, biomass power, wind power,

hydropower, geothermal power and MHD generator. MHD generator is also one of the non-

conventional energy sources which will use natural gas of air to generate power.

Power generated by using MHD generator is in the range of 1000MW. This plant is

also used in combined power plants not only used for generating bulk amount of power but

also for increasing efficiency of the plant which is working in combined with MHD

generator. It will produce pollution free power.

This paper deals with working principle & construction of MHD generator, different

types of MHD systems, advantages & disadvantages of MHD generator & international status

of MHD systems.

INTRODUCTION:- Eighty percent 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. In advanced countries MHD generators are widely used but in developing

countries like India it is still under construction. This construction is in progress at Trichi in

Tamil Nadu.

The word magnetohydrodynamics (MHD) is derived as magneto- means magnetic

field, and hydro- means fluid and dynamics- means movement. The field of MHD was

initiated by Hannes Alfven, for which he received the Nobel prize in Physics in 1970.

An MHD generator is a device used for converting heat energy of a fuel directly into

electrical energy without a conventional electric generator. Magneto-hydrodynamic (MHD)

electric generator works on the principle that any conductor that is moved through a magnetic

field will generate electricity. This applies not only to copper wires (as in conventional

generators), but also to gases, which become conductors when they are made so hot that some

of their atoms separate (ionize) into electrically charged particles. A small amount of potassium

chloride fed into it increases its ionization and makes it a better electrical conductor. If this gas

is forced through a magnetic field, a stream of ionized gas causes an electrical current to flow

across it. The gas must be so hot at least 2000 – 30000k .To heat the gas, the fuels like coal, oil,

natural gas are mainly used.

The thermal efficiency of an MHD converter is increased by supplying the heat at

highest practical temperature and rejecting it at the lowest practical temperature. It is observed

that economic and physical factors will lead to design outputs of the order of 1000MW.Infact

MHD is really of interest only for central power generation. MHD generators are big enough to

supply commercial power. A major advantage of MHD generator has no primary moving parts.

MHD power generation is a new system of electrical power generation which has no turbine to

rotate any rotor and there is no usage of copper winding.

PRINCIPLE:-

The principle of MHD generator is based on Faraday’s laws of electro magnetic

induction which states that a changing magnetic field induces an electric field in any conductor

located in it. This electric field while acting on the free charges in the conductor causes a current

to flow. As in case of conventional electric generator conductor crosses the line of magnetic

field and a voltage is induced. Similarly in MHD generator when an ionized gas which acts like

a conductor at high practical temperature flows across a magnetic field and voltage is induced. If

the circuit is closed then current is generated and can extracted by placing electrodes in a

suitable position in the stream. This arrangement is illustrated in below figure and it provides

d.c. power directly.

The principle of MHD power generation is “an electric conductor moving through a

magnetic field will induce emf and the electric flux is produced. Due to the interaction of both

electric and magnetic fields a retarding force is developed on the gas. From energy point of

view, “the retarding force is converted to electric work by means of

electromagnetic induction”.

If the fluid flow is at right angles to the magnetic field direction an emf is

induced in the direction at right angles to the both flow and field directions as shown in the

above figure.

CONSTRUCTION:-

SECTIONAL VIEW OF MHD GENERATOR

PARTS OF MHD GENERATORS:-

The main parts of the MHD generator are listed &explained below:

1. DUCT :-It is a divergent channel made up of heat resistant alloy with external water

cooling. The hot gas expands through this duct which is surrounded by powerful magnet.

Materials used for the duct should withstand upto the temperature of nearly 30000C. The

material used should also withstand thermal&electrical stresses. Al2O3reacts with

potassium seed at high temperature to give potassium beta alumina (KAl11O17), on the

other hand, MgO, stands seed and slag corrosion. It also gives good thermal shock

resistance. Hence use of MgO as duct wall is favourable.

2. MAGNETS :-The output power delivered by an MHD generator is proportional to B2,

hence the magnet should be as large as possible. The magnet being one of the most

expensive items of an MHD generator. There are three types of magnets listed below:

Permanent magnets ,

Water cooled electro magnets,

Super conduction magnets.

Among those super conduction magnets are most preferable because Super conductivity

is exhibited by some metal compounds & alloys when they are in magnetic field, and below a

certain temperature usually a few degree Kelvin, their resistance becomes zero. The transition

temperature and critical field for some superconducting materials are given in the table below:

Properties of superconductivity materials

Material Critical field wb/m2 Transition temp.OK

Tin

Niobium

Niobium 33zirconium

Niobium-titanium

Niobium 3 tin

0.08

0.25

8.00

12.00

21.10

7.18

8.0

10.50

9.80

18.00

3.ELECTRODES:-Electrodes are mainly used to collect the power generated. A

number of oppositely located electrode pairs are inserted in the channel to conduct the electric

current generated to an external load. If the electrodes are connected in improper way then a

halleffect will arises. By this effect, the magnetic field acts on MHD generated current and

produces a voltage in the flow direction of the working fluid rather than at right angles to it. The

resulting current in an external load is then called the hall current. The hall current minimizes

the generated current. The electrode pairs may be connected in various ways to minimize hall

current. A better, but more complicated, alternative is to connect each electrode pair across a

separate load, as in fig.1. Another possibility is to utilize the hall current only; each electrode

pair is short circuited outside the generator, and the load is connected between the electrodes at

the two ends of the MHD generator is shown in fig.2.

fig.1 fig.2

SEEDING PROCESS:-

One way of increasing the conductivity of gas, without the need of exceedingly high

temperatures, is to introduce into the gas a material whose ionization potential is lower than that

of the gas atoms themselves. This means that the addition, or seeding agent, will ionize more

readily than the gas itself, and will thus enhance the electrical conductivity.

The main seeding materials used are alkali metals like cesium, potassium etc. The

addition of the seeding material allows the attainment of suitable conductivities at very much

low temperature.

SIMPLE MHD GENERATOR:-

A schematic diagram of MHD generator is shown in the fig. below. The conducting fluid

is forced between the plates with a kinetic energy and pressure differential sufficient to

overcome the magnetic induction force. An ionized gas is employed as conducting fluid.

Ionization is produced either by thermal means i.e., by an elevated temperatures or by

seeding with substance like cesium or potassium. The atoms of seed element split off electrons.

The presence of negatively charged electrons makes the carrier gas as an electrical conductor.

Cycle of Simple MHD power plant

The MHD power plant works on the Rankine cycle. The cycle consists of generator, heater,

compressor and cooler. Still, a Compressor must be used to elevate the pressure, heat is added at

high pressure and the flow is accelerated before entering the converter. A schematic of MHD

power cycle is shown in above figure.

MHD SYSTEMS:-

A combined cycle is characteristic of a power producing engine or plant that employs

more than one thermodynamic cycle. Combining two or more "cycles" such as the Brayton

cycle and Rankine cycle results in improved overall efficiency. If MHD generator is used in

combined power plant then the system is said to be as MHD system. Thus the MHD systems can

be broadly classified as follows:

1. Open cycle systems.

2. Closed cycle systems.

These are explained in the following sections:

1.OPEN CYCLE SYSTEMS:-

The arrangement of the system is shown schematically in the below fig. In this system,

fuel used may be oil or coal or natural gas. The fuel is burnt in the combustion chamber. The hot

gases from combuster are then seeded with a small amount of an ionized alkali metal, to

increase the electrical conductivity of gas. In order to attain high temperatures (2300 -27000C),

the compressed air used to burn the fuel (taking fuel as coal) in the combustion chamber. The

compressed air is preheated to atleast 11000C.

Schematic of Open cycle MHD generators

The hot, pressurized working fluid leaving the combustor flows through a convergent –

divergent nozzle. In passing through the nozzle, the random motion energy of the molecules in

the hot gas is largely converted into directed, mass motion energy. Thus, the gas emerges from

the nozzle and enters the MHD generator at high velocity and it passes through the duct. During

the motion of the gas the positive and negative ions move to the electrodes and constitute an

electric current. This current is drawn by the electrodes which are in D.C. and it is fed to the

load by converting it into a.c using inverter. The hot gases from the MHD generator are send to

the steam generator through air-preheater. The steam generator generates the steam and it

impinges on the turbine. It converts steam energy into mechanical energy and this energy is

converted into electrical energy by using alternator.

2. CLOSED CYCLE SYSTEM:-

There are two types of closed cycle systems. In one type, electrical conductivity is

maintained in the working fluid by ionization of a seed material as in open cycle system; and in

the other, a liquid metal provides the conductivity. The carrier is usually a chemical inert gas,

although a liquid carrier has been used with a liquid metal conductor. The working fluid is

circulated in a closed loop and is heated by the combustion gases using the heat exchanger.

Hence the heat source and the working fluid are independent. The working fluid is Helium or

Argon with Cesium seeding.

1.Seeded inert gas system:- In the closed cycle system the carrier gas (argon/helium) operates

in a form of Brayton cycle. The gas is compressed and heat is supplied by the source, at

essentially constant pressure; the compressed gas then expands in the MHD generator and its

pressure and temperature fall. After leaving the generator heat is removed from the gas by a

coolant, this is the heat rejection stage of the cycle. Finally the gas is recompressed and returned

for reheating.

Schematic of Seeded inert gas system

Initially coal is gasified and the gas is burned at a temperature of about 5250C in a

combuster to provide heat. In the primary heat exchanger, this heat is transferred to a carrier

gas (working fluid) of the MHD cycle. The combustion products after passing through the air-

preheater and purifier then they are discharged to atmosphere. Hence the problem of extracting

the seed material from fly ash does not arise. The hot argon gas is seeded with cesium and

resulting working fluid is passed through the MHD generator at high speeds. The d.c power out

of MHD generator is converted to a.c by the inverter and is fed into the grid. The heat of

working fluid is converted into electrical energy in the diffuser the working fluid is slowed

down to a low subsonic speed. Then hot fluid enters the secondary heat exchanger which serves

as a waste heat boiler to generate steam and to drive the turbine which runs the argon

compressor. The output of this generator can also fed to main grid.

2. Liquid metal system:-When a liquid metal provides electrical conductivity, an inert gas is a

convenient carrier. The carrier gas is pressurized and heated by passing through the heat

exchanger with in combustion chamber. The hot gas is then incorporate into the liquid metal,

usually hot sodium to warm the working fluid.

Schematic of Liquid metal system

The working fluid is introduced into the MHD generator through a nozzle in the usual ways ;

the carrier gas then provide the required high directed velocity of the electrical conductor(liquid

metal). After passing through a generator, the liquid metal is separated from the carrier gas. Part of

the heat remaining in the gas is transferred to the water in a secondary 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 around 8000C. The major advantage is high electrical conductivity.

Advantages of MHD generation:-

The following are advantages of MHD generation over other power plants:

The efficiency of a combined power plant which runs along with a MHD system can be around

50% as compared to less than 40% for the most efficient steam plants.

Bulk amount of power is generated.

It has no moving parts, so more reliable.

It has ability to reach the full power level as soon as started.

The size of the plant is considerably small as compared to conventional power plants.

The closed cycle system produces power which is free from pollution.

Direct conversion of heat into electricity permits to eliminate the gas turbine(compared with a

gas turbine power plant) or both the boiler and the turbine(as compared with steam power

plant).This elimination reduces losses of energy and maintainance cost.

It has been estimated that the overall operation costs in an MHD plant would be about 20%less

than in conventional steam plants.

Disadvantages:

It is difficulty to handle the fluid which operates at high temperatures.

The initial cost of setting of an MHD power plant is slightly higher than that of conventional

thermal power plants.

It is difficult to produce the strong magnetic field with permanent magnets without proper

cooling. To overcome this either we will provide a necessary cooling to the field coils or by

using super conduction magnets, which are expensive.

The main disadvantage is to convert d.c to a.c, we need inverting equipment. This equipment is

much expensive.

Life time of plant is small in order to with stand those high temperatures.

Conversion of seed material from the fly ash and its reconversion into its original form is

difficult.

INTERNATIONAL STATUS OF MHD POWER GENERATION :-

International co-operation has been prominent in the development of MHD power

generation over the past 3 decades; research and development in MHD electric power generation

have been accompanied by a considerable upswing in International Corporation. The first

demonstration of a feasibility of MHD was made in USA in 1959. Beginning in 1960, related

scientific research& engineering development was undertaken by USA, USSR, JAPAN, UK,

POLAND and other countries.

A 25 MW MHD pilot plant was commissioned in USSR in 1970, which has successfully

supplied electricity to the Moscow grid. In Japan the work on a 20MW demonstration plant near

Tokyo is in progress. Poland &China are also believed to be developing MHD systems.

The Indian R&D programme on MHD started in 1977. It concentrates on developing

necessary competence in this area and associated technology which will form the basis of

creating commercially viable equipment for coal based MHD generators. The department of

non-conventional sources of energy (DNES) as sponsored research and development

programme on coal based MHD power generation. These power plants run at very poor

efficiency of 15-30% depending upon the season of the year. The over all efficiency of each

Unit of Harduaganj thermal power plants near Aligarh India is given for different years and

found to be quite low around 18% as an average. Efficiency of a proposed unit of Magneto

Hydro Dynamics (MHD) channel of 20 MW (BHEL) is calculated and this channel is

retrofitted as topper unit with Harduaganj thermal power plant which is used as trailer unit. It

is seen that the efficiency increases by more than 50% of its existing value. This can be

further enhanced if larger size of MHD generator is used. This scheme enhances the

efficiency, conserve coal and the pollution caused by thermal power plant is also reduced to a

great extent.

A 5-15MW pilot MHD plant has been prepared and the construction work is in

progress at Tiruchirapalli in Tamil nadu. In march 1985 an experimental 5MW (Thermal

input) plant was commissioned at Tiruchirapalli which is an open cycle MHD pilot plant

using blue water gas as fuel.

CONCLUSION:-

Really MHD is a boon to the growing world. Hence we conclude that MHD is one of

the special& eco-friendly technique that cannot be depleted.

Mainly it is useful when we use it as a combined plant with the steam power plants.

Thermal power plants are the major source of electrical power. Due to improper utilization of

the heat they run at very poor efficiency. The efficiency of thermal power plants can be

improved to a great extent if they are used in retrofit with MHD power plants. This will cause

a great reduction in coal consumption and also in pollution level.

BIBILIOGRAPHY:-

1. “Non-conventional Energy Sources” by G.D.RAI

2. “Power Plant Engineering” by G.R.NAGPAL

3. “Electrical power” by Dr.S.L.UPPAL

REFERENCES:-

1. www.wikipedia.com

2. www.google.com