Open Cycle MHD System
The block diagram of an open cycle MHD system in conjunction with a conventional power plant is shown in Figure. The step by step working of an open cycle MHD generator is as under:
The atmospheric air is compressed to a high pressure in an air compressor. Thereafter, compressed air is heated in the low temperature and high-temperature air preheaters up to a temperature of about 1100oC and fed to the combustion chamber.
The fuel is burnt in the 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.
To increase the velocity of hot gases, they are passed through a nozzle. These high velocity and high temperature (about 2300oC to 2700oC) gases are fed to the MHD duct. Here, some part of the internal energy of the gas (plasma) is directly converted into DC power.
The exhausted gases from MHD duct are used in air preheater to heat the air. Then these gases are used to produce steam from feed water in a heat exchanger (steam generator).
The surplus heat of the gas is used for heating the compressed air in the low-temperature preheater.
Hot gases then pass through a seed recovering unit (electrostatic precipitator). This unit removes seeded alkaline potassium from gases.
The clean hot gases are expelled to the atmosphere through the chimney. In some systems, they can be recycled by compressing again.
Closed Cycle MHD System
A closed cycle MHD system can either operate on seeded inert gas (helium or argon) or liquid metal.
Seeded inert gas closed cycle system: A closed cycle MHD system using helium or argon gas seeded with Cesium is shown in Figure. Step by step working of the system is as under:
In this system, helium gas seeded with Cesium is heated to a very high temperature in a nuclear reactor.
The hot helium gas is passed through a nozzle to increase its velocity and then supplied to MHD duct to produce DC power. Here, some part of the internal energy of the gas is directly converted into DC power.
In next step, the gas is passed through the heat exchanger (i.e. steam generator) to convert feed water into steam. Now, this steam is used in a conventional steam power plant to generate electricity.
The exhausted helium gas from steam generator is cooled in the cooler and compressed in the compressor. Thereafter, it is supplied back to the nuclear reactor and complete cycle is repeated again.
Liquid metal Closed Cycle System: It is similar to the inert gas system except that it uses liquid metal (potassium) in place of helium gas as the working fluid.
Liquid potassium is heated in the nuclear reactor and is passed through the nozzle to increase its velocity. Thereafter, the high-velocity liquid metal is passed through the MHD duct to produce DC power.
The liquid potassium leaving the MHD duct is fed to the heat exchanger where it transfers heat to feed water and feed water is converted into steam. This steam is used in a conventional steam power plant.
The liquid potassium, leaving the heat exchanger is supplied back to the nuclear reactor and complete cycle is repeated again.
A closed-cycle system can provide more useful power conversion at lower temperatures (around 1900 K as compared to 2500 K for open cycle system). However, its use is still a distant dream.
The heat exchanger design is one of the difficulties because the heat exchanger works up to the highest temperature of the gas. Moreover, the working fluid must be kept pure.
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