MHD Generator Working Principle

Hi friends,
 
In this article, I am going to discuss the MHD generator working principle and mhd power generation. Also I will provide you some basic information about it.
 
A Magneto Hydrodynamic Generator is a device which converts heat energy of a fuel directly into electrical energy without a conventional electric generator. This system eliminates all the intermediate linking conversion processes thereby improving the efficiency. It can be used with any high-temperature heat source like nuclear, solar energy, chemical etc.
 

MHD Generator Working Principle

 
The working principle of MHD generator is based on Faraday’s Law. It states that when a conductor is moved in a magnetic field an EMF is induced in the conductor. In an MHD system, hot gases act as the conductor.
 
When gases are heated to sufficient temperature by burning of fuel, one or more of the valence electrons are displaced from their orbit in which they are spinning. Thus, the neutral atoms are split into the positive and negative ion. These ions are the electrical conductors.
 
After that, ionized gases are moved in a duct known as the MHD duct at very high velocity. This movement of gas ions in the MHD duct in which strong magnetic field is applied leads to the electromagnetic induction and an EMF is induced in the pair of electrodes. The electrodes are connected to the external circuit and they deliver current to the external load. The three major components of an MHD generator are shown in Figure. These are:

  • MHD duct
  • magnetic field and
  • electrodes.

The directions of the magnetic field, the flow of gases and the electrodes are mutually perpendicular to each other.

mhd generator working principle

Power Generated by MHD Generator

 
P = LƱVB2/ρ
 
Where,
L = length of duct
Ʊ = electrical conductivity of gas
V = velocity of gas
B = Magnetic field density
ρ = density of gas.
 
The power produced by an MHD generator is DC. For transmission purposes, DC is converted in AC.
 
Practically, ionization temperature of gases in pure form is very high. Handling of these high temperatures is very difficult. To overcome this problem, the gas is seeded with a small quantity of alkaline metals like sodium and potassium. This drops the ionization temperature of gases considerably.
 
To increase the overall efficiency of the system MHD system is used in conjunction with steam power plants. Initially, when temperatures of gases are high, the energy of gases is used in the MHD system for direct conversion into electricity.
 
After this, when the temperature of gases falls to a low value, the hot gases are used for steam generation. This steam is used to generate electrical power in a conventional steam power plant.
 

Fuels and Materials Used in MHD Plants

 
All conventional fuels like oil, coal, natural gas or any other fuel which can produce high temperature can be used in MHD plants. It can also use nuclear energy.
 
The material used in the construction of MHD plants must have temperature resistance. Also, medium inside the MHD duct is very corrosive, therefore, highly corrosion resistant materials are used in its construction. The insulating materials used for duct walls are magnesium oxide, strontium zirconate. Electrodes are made of tungsten of carbon.
 

MHD Plants Working Fluids

 
MHD system may be an open cycle system or a closed cycle system. In an open cycle system, the working fluid is used on a once-through basis. 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 source and thus used again and again.
 
The operation of MHD generators directly on combustion products is an open cycle system. The working fluid is air.
 
In closed-cycle systems, helium or argon is used as the working fluid.
 
The use of a nuclear reactor using solid fuel elements for providing the heat energy for an MHD process requires that the working fluid should have the following three properties:
(i) it should be capable of providing heat transfer under reactor operating conditions,
(ii) it should not require excessive compressor work,
(iii) it should not be rendered active within the reactor. Helium, most nearly, fulfills all these three requirements.
 

MHD Generator Advantage and Disadvantage

 
Advantages

  • It converts heat energy directly into electrical energy and hence conversion efficiency is high.
  • Although the costs cannot be predicted very accurately, yet it has been reported that the capital costs of MHD plants will be competitive with those of conventional steam plants.
  • Because of higher efficiency, the overall generation cost of an MHD plant will be less. It has been estimated that the overall generation cost in an MHD plant would be about 20% less than in conventional steam plants.
  • The higher efficiency means better fuel utilization. The reduced fuel consumption would offer additional economic and social benefits and would also lead to the conservation of energy resources.
  • The more efficient heat utilization would decrease the amount of heat discharged to the environment and the cooling water requirements would also be lower.
  • The research studies have shown that the use of precipitator not only recovers the seed material but also effectively traps most other pollution agents.
  • It has no moving parts.
  • On and OFF time is very low.
  • It is a pollution free system.
  • The size of MHD power plants is small as compared to conventional power plants for same power output.
  • The cost of power generation per unit is less.
  • The operational and maintenance costs are low.
  • The efficiency of MHD plants is 50% whereas the efficiency of most efficient conventional power plants is about 40%.
  • It can work on any type of fuel.

Disadvantages

  • It has high fluid friction losses and heat transfer losses.
  • A large voltage drop takes place across the gas film.
  • The requirement of large-sized magnets increases the cost of an MHD system.
  • Working temperature ranges from 200°K to 2400°K. This causes fast corrosion of components.

 

Applications of MHD Generation

 
MHD power generation can be used not only for commercial electrical power generation but also for some other special applications.
 
A major effort was made in the USA to use MHD as the conversion system in a nuclear electric system for space crafts.
 
MHD generation has also been considered for ship propulsion, airborne applications, hypersonic wind tunnel experiments, and many other defense applications.
 
Thanks for reading about MHD generator working principle and mhd power generation.

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