Here is a compilation of essays on ‘MHD Power Generation’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘MHD Power Generation’ especially written for school and college students.
Essay on MHD Power Generation
Essay Contents:
- Essay on the Introduction to MHD Power Generation
- Essay on the Arrangement of MHD Power Generator
- Essay on the Design Problems of MHD Power Generator
- Essay on the Thermodynamic Performance Analysis of MHD Power Generation
- Essay on the Electrical Analysis of MHD Power Generation
- Essay on the MHD Generator Efficiency
- Essay on the Open Cycle MHD Power Generator System
- Essay on the Closed Cycle MHD Power Generation System
- Essay on the Hybridisation of MHD Power Generator
- Essay on the Advantages of MHD Power Generation
- Essay on Indian Experience in the Development of MHD Technology
- Essay on the Limitations of MHD Power Generation
Essay # 1. Introduction to MHD Power Generation:
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Magnetohydrodynamic (MHD) is a direct heat-to-electricity conversion technique based on Faraday Law that when an electric conductor moves across a magnetic field, a voltage is induced in it which produces an electric current. Here the conductor is an ionized gas which is passed at high velocity through a powerful magnetic field; a current is generated and can be extracted by placing electrodes in a suitable position in the stream.
It produces d.c. power directly. MHD power generation is the most promising of direct energy conversion techniques where the mechanical link can be avoided. It can overcome some of the limitations of conventional power generation by improving the efficiency from 40% to 55%, thus better utilising the fuel resources and reducing the environmental pollution.
MHD power generation has great potential for power production in excess of 1000MW. It can be used as topper for a coal-fired thermal power plant. This will increase the thermal plant efficiency by affecting direct conversion of heat to electricity. In addition to fuel economy and reduced environmental pollution, the capital cost of the power plant will also be reduced.
Essay # 2. Arrangement of MHD Power Generator:
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The arrangement of MHD power generator is shown in Fig. 13.1. The ionized gas working as electrical conductor experiences a braking force due to electromagnetic interaction. The degree of ionization required in practice is very small of the order of 0.1%. The gas is still composed of neutral particles which carry nearly all the kinetic energy of stream and are unaffected by magnetic force.
The retarding force is a complex function of collisions, cross-sections and magnetic flux density. The applied magnetic field manifests itself through the force that it exerts on the electrons in the gas. This force is then coupled to the neutral particles by the electron-ion Coulomb force and ion-neutron collisions.
In order to achieve a large power output, the gas velocity should be high (103m/s) and applied magnetics flux density must be as large as possible. There should be adequate gas conductivity (more than 10 mhos/m). To achieve equilibrium conditions in the pure gas by thermal ionisation, temperatures of tens of thousands of degrees are required.
By seeding the gas with elements which have low ionisation potential such as alkali metals Cesium and Potassium, It is possible to achieve reasonable conductivity at temperature in the region of 2000°C. This temperature is within the limits of material technology of MHD.
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The electrical power is proportional to magnetic flux density (B) and gas velocity (U) and gas conductivity (σ).
If a particle with positive charge q is moving in a duct with plate walls P1 and P2 with a velocity v’, the magnetic flux of density B pointing into the paper will apply a magnetic force F on the particle. The force, magnetic field and velocity are vector quantities.
The positive charge particle moves upwards and a negatively charged particle moves downwards. If P1 and P2 are externally connected through a resistance, a current will flow and mechanical energy (kinetic energy) of gas is converted into electrical energy. If an electrical field of strength E is also applied, the total force on the particle.
Is gas flows in the x direction and magnetic field B is applied in the + Z direction.
These are the scalar equations and can be solved for vx and vy. There will be no force acting in y direction.
These are the basic equations for analysis of MHD generator.
Both jx and jz can be extracted. In a Faraday MHD generator the current jZ is extracted and in a Hall MHD generator jX is extracted. The flow of current in the x direction in called Hall Effect.
Essay # 3. Design Problems of MHD Power Generator:
1. Gas Velocity:
The power is proportional to square of gas velocity; the latter should be as high as possible. This can be achieved by converting thermal energy of gas emerging from the furnace into directed kinetic energy in a nozzle. This kinetic energy and thermal energy are then converted into electrical energy by the MHD process.
The conversion of thermal energy into directed kinetic energy can be brought about by conventional nozzle-expansion techniques and velocities of the order of 103 m/s can be obtained. For seeded inlet gas, the duct is designed to give a constant optimum Mack Number of 0.5 which ensures that maximum density of power is extracted at each section of the duct.
The temperature, pressure and velocity will fall in such a way as to keep the optimum Mack Number constant. However, as the mass flow rate through the duct must remain constant, the area of the duct will vary with velocity. The area of the duct will increase and velocity will decrease with the gas flow.
2. Magnetic Flux Density:
The power is proportional to magnetic flux density (B) which should be as strong as possible. This can be achieved by passing heavy currents though the coils. The energy dissipated in the magnet is proportional to the length of the duct but is independent of its cross-sectional area. It is also proportional to the resistivity of the winding material. The capital cost of the magnet is proportional to the bulk cost of the winding material.
Magnets constructed with copper windings on a soft core can carry flux density up to 3 Wb/m2. Water cooling must be used to remove the heat generated in the windings by Joule effect. For a 100 MW MHD plant, heat dissipated is about 12 MW.
The resistivity of certain metals falls very fast with the decrease of temperature. The decrease in the power needed to generate magnetic field is much greater than additional power needed for the refrigeration equipment. This decrease in running cost is produced at the expense of capital cost of the refrigeration equipment. Magnets made of superconducting materials (NbZr, Nb3Sb) can reduce power dissipation in the windings to zero but a refrigeration plant would be required to keep the winding at liquid-helium temperature.
3. Gas Conductivity:
It is a difficult task to attain reasonable (10 to 100 mhos/m) conductivity in a gas. The gas must be made a conductor by ionising some fraction of it. Thus the gas in the MHD generator can be called plasma, i.e., an ionized gas in which positive ions and electrons are present in equal number. In order to attain reasonably high temperature plasma, it is necessary to have fuel gas with a calorific value of at least 8500 kJ/Nm3.
At 3000°C, a small degree of ionisation equal to 0.1% is observed which gives a conductivity of 50%. By increasing the degree of ionisation to 1%, the conductivity can be raised to 80%. Hence, high degree of ionisation is not needed for practical purposes. However, it is difficult to find suitable structural materials for combustion chamber, ducts and nozzles to withstand flow of ionised gases at 3000°C.
One way of increasing the gas conductivity without the need of exceedingly high temperatures is to introduce seeding agent which will ionise more readily than the gas itself and will enhance the electrical conductivity. The best seeding material is metal cesium which has ionisation potential of 3.89 eV. Cesium is very expensive and the next seeding material being used is potassium with ionisation potential of 4.34 eV.
In this case conductivity of 10 to 100 mhos/m can be achieved at temperatures of 2000°C to 2400°C, which are within the range of material technology. However, material selection has to take care of enhanced corrosiveness of plasma due to presence of seeding material.
4. MHD Duct:
The main difficulty encountered in the design of MHD power generator is finding suitable material of construction for the duct to handle highly corrosive and high temperature gases and seeding materials. The development of a long- lived duct is one of the prime requirements for MHD generator. Ceramics were initially used. If uncooled walls are cooled, severe ablation and erosion takes place and extremely high thermal stresses are set up which leads to rapid disintegration of material. Temperature gradients of 10,000°C/cm may be obtained.
A more promising mode of construction is the use of water-cooled metal walls. A ceramic material, such as zirconia or alumina is sand-witched between water-cooled metal segments. This makes the wall electrically non-conducting. This results in large pressure drops due to rough surfaces on gas flow side.
Water cooled peg wall or modular construction have been used successfully. But smooth walls of ceramic construction gives better flow conditions and greater efficiency. The most important insulating materials are- magnesia, alumina, calcium zirconate and strontium zirconate. For electrodes, graphite and water- cooled copper electrodes have been used.
Essay # 4. Thermodynamic Performance Analysis of MHD Power Generation:
A schematic diagram of MHD power cycle along with its T-s diagram are shown n Fig. 13.2 and Fig. 13.3 respectively.
The following factors can adversely affect the thermal efficiency:
i. Dissipation of energy in the internal resistance of ionised gas.
ii. A space charge barrier at the electrode surface.
iii. Heat loss through the electrode and insulator walls.
iv. Losses associated with fluid friction
v. Hall effect losses due to current induction in the direction of flow of gas.
Essay # 5. Electrical Analysis of MHD Power Generation:
When a current I flows across load resistance RL with voltage V across the load, the electrical intensity across the electrode plates.
Example:
Calculate the open circuit voltage and maximum power output for the following MHD generator:
Plate area = 0.25 m2
Distance between plates = 0.50 m
Magnetic flux density = 2 Wb/m2
Average gas velocity = 103 m/s
Gas conductivity = 10 mhos/m
Solution:
Essay # 6. MHD Generator Efficiency:
Possible arrangements in MHD generator are shown in Fig. 13.5.
The current generated in a Faraday generator at the maximum power output is given by:
The efficiency of energy conversion in MHD generator is defined as the ratio of useful electrical power output and the power required transporting the plasma through the MHD duct.
The variation of ƞmax with L/h is shown in Fig. 13.6, where L is the length of duct and h is the height of duct. Ƞmax of 0.5 can be achieved with optimum design.
Essay # 7. Open Cycle MHD Power Generator System:
An Open Cycle MHD Power generation system is shown in Fig. 13.7.
The system consists of the following units:
1. Combustion Chamber:
The fuel (coal, oil or natural gas) is burnt with preheated oxygen (or air) at 1100◦ C. The hot, pressurised working fluid at 2300◦ C to 2700◦ C is selected with potassium carbonate (or cesium) to ionize the gas.
2. Nozzle:
A convergent-divergent nozzle is used to increase the velocity to 103 m/s to get directed mass motion energy.
3. MHD Duct:
It is a divergent channel made of heat resistant material externally water cooled. The magnetic field acts perpendicular to the direction of gas motion. The electrode pair may by connected in different ways as per Fig. 13.5 to reduce losses. The d.c power produced is converted into ac power with the help of an inverter.
4. Preheater:
The gas at 1900°C enters the gas preheater where oxygen or oxygen enriched air or compressed air is heated to a temperature of 1100°C. The preheated gas helps to produce working fluid at 2300°C to 2700°C.
5. Seed Recovery Unit:
The seed material is recovered for successive use for seeding of hot working fluid in the combustion chamber. The original potassium carbonate seed is converted into potassium sulphate due to presence of sulphur in the fuel. The potassium sulphate is converted back to potassium carbonate chemically in the seed recovery unit.
6. Hot Gas:
The hot gases are passed through pollution control device to remove sulphur and nitrogen oxides before exhausting through a chimney.
Essay # 8. Closed Cycle MHD Power Generation System:
In a closed system, helium or argon is used as working fluid which is heated in a heat exchanger. Higher temperature and better thermal efficiency are possible. However, seeding is required to attain reasonable gas conductivity at temperatures workable with available structural materials. Instead of seeding with cesium or potassium carbonate a liquid metal is mixed with an inert gas to form the working fluid. The liquid metal provides the conductivity.
1. Seeded Inert Gas System:
The main components of a closed cycle seeded inert gas MHD system are shown in Fig. 13.8:
i. Combustor and Heater:
The carrier gas (argon or helium) is heated by the combustion of fuel gas to 1900°C and seeded by cesium injection.
ii. MHD Generator:
The seeded hot working fluid is passed through the MHD generator at high speeds. The dc power from MHD generator is converted to a.c. power by the inverter. The working fluid is slowed down in the diffuser and precooled.
iii. Compressor:
The precooled gas is compressed for heating.
2. Liquid Metal System:
The carrier gas (argon or helium) is pressurised and heated in a heat exchanger within the combustion chamber. The hot gas is incorporated into the liquid metal (hot sodium) to form the working fluid.
i. MHD Generator:
The working fluid consisting of gas bubbles uniformly dispersed in an equal volume of liquid sodium is passed through the MHD generator with high directed velocity.
ii. Breeder Reactor:
The exhaust from MHD duct is passed through a condenser where potassium liquid is formed and pumped to breeder reactor. The liquid potassium is heated in vapour form and accelerated through a nozzle.
iii. Separator:
The vapours are separated, condensed and pumped to breeder reactor. The schematic diagram is shown in Fig. 13.9.
Essay # 9. Hybridisation of MHD Power Generator:
The overall energy utilization can be improved by employing combined-cycle power plant consisting of MHD generator as a topping plant and a gas or steam turbine as a bottoming plant. The overall efficiency of about 60% can be achieved in the combined cycle. The schematic diagram is shown in Fig. 13.10.
If the gas entering the MHD duct at about 3000°C could be expanded to the ambient temperature of 30°C, the Carnot efficiently would have reached 90%. Unfortunately, the MHD power output is restricted because by the time the gas temperature falls to 2000°C, the electrical conductivity becomes very low with the electrons combining with ions to form neutral atoms, and the generator then ceases to operate satisfactorily.
Therefore, the MHD generator is used as a topping plant and the MHD exhaust at about 2000°C is utilized in raising steam to drive turbine and generate electricity in a conventional steam power plant. If the fraction Z of the fuel energy is directly converted to electricity in the MHD generator, the remainder (1 – Z) is converted with an efficiency Ƞ in the bottoming steam plant so that overall efficiency.
Ƞ = Z + Ƞ’ (1 – Z)
If Z = 0.3 and Ƞ’= 0.4, then Ƞ = 0.58 which is a good power plant efficiency. MHD-topped steam plants can operate either in an open cycle or in a closed cycle. A gas turbine plant can also be used as a bottoming unit. Since the combined cycle plant operates over a larger temperature difference, the efficiency will obviously be higher.
Essay # 10. Advantages of MHD Power Generation:
i. The conversion efficiency of MHD generator in combined cycle plants can be 60-65%.
ii. Large amount of power can be generated.
iii. It is very reliable and robust without moving parts.
iv. Pollution-free power can be generated in a closed cycle system.
v. The plant can be operated on full capacity in a very short time.
vi. The plant is very compact.
vii. Capital costs can be lower than conventional power plants.
viii. The operating costs will be lower.
ix. It is a direct conversion device eliminating very large plants which reduces loss of energy and enhances reliability of operation.
x. The higher energy utilization helps savings in fuel and pollution.
Essay # 11. Indian Experience in the Development of MHD Technology:
Many countries world over are in the development of MHD technology.
The specifications of a Japanese pilot plant (Tokyo) are given below:
Russia has been the pioneer of this new technology. The world’s first U-O2 MHD unit was designed by the Russian scientists and power engineers in 1964.
Under intergovernmental agreement, Indian scientists from BARC and power engineers from BHEL worked on a pilot plant of MHD of 5 MW capacity with close interaction with the scientists of the High Temperature Science Institute of Moscow.
The pilot plant was set up at BHEL, Trichy. BARC had developed special magnet, MHD duct and power tap-off system. BHEL was responsible for the development and installation of fuel gas generator, oxygen plant, water treatment plant, high temperature regeneration air heaters and valves, the main combustion chamber, nozzles as well as seed injection and seed recovery system.
The main specifications of the pilot plant are:
Essay # 12. Limitations of MHD Power Generation:
The main limitations hindering the commercial application of MHD technology are:
i. The efficiencies attained so far have been relatively low.
ii. The power output of MHD generator is proportional to the square of the magnetic field density. The electro-magnets require very large power to create strong magnetic fields. The MHD technology is waiting for development of superconductor materials which will require very little power even at ambient temperatures.
iii. The combustor, MHD duct, electrodes and air preheaters are exposed to very corrosive combustion gases at very high temperatures. The life of these equipments has been very short.
iv. The ash (or slag) residue from the burning coal is carried over with the combustion gases and tends to cause erosion of exposed surfaces. However, deposition of the slag on such surfaces may provide some protection.
v. There is a serious problem of separation of seed material from the fly ash and reconversion of potassium sulphate to potassium carbonate.
vi. Special fuel gas and preheating of air are needed to provide adequate working fluid temperatures.
vii. There are serious problems associated with the fabrication of MHD duct, high temperature and high pressure heat exchangers and reactors.