The maximum steam temperature and pressure being fixed by metallurgical considerations, the minimum temperature by the ambient conditions, and with the optimum degree of regeneration and number of reheats, the ceiling for the conversion efficiency of a conventional thermal power station is somewhere near
45%.
There is a great deal of world-wide interest to achieve a higher conversion efficiency and hence, fuel economy, by converting “heat” directly to electricity eliminating the link process of producing mechanical energy via steam.
The magneto hydrodynamic (MHD) power generation seems to be the most promising for a utility system.
The maximum limiting temperature for turbine blades being 750 —800 ° C, the
MHD generator is capable of tapping the vast potential offered by modern furnaces, which can reach temperatures of more than 2500 K, and up to 3000 K with preheating of air.
Principle of MHD Power Generation
Faraday’ s law of electromagnetic induction states that when a conductor and a magnetic field move relative to each other, an electric voltage is induced in the conductor.
The conductor may be a solid, liquid or gas. In an MHD generator, the hot ionized gas replaces the copper windings of an alternator.
When a gas is heated to high temperatures, the valence electrons of the excited atoms move on to higher quantized orbits and ultimately, at certain energy levels they fly off and become free electrons.
For a gas to be conducting, a certain number of free electrons must be present along with an equal number of ions and the main body of neutral atoms.
Since a very high temperature is required to ionize a gas (thermal ionization) which cannot be endured by the materials available, the hot gas is seeded with an alkali metal, such as cesium or potassium (K or KOH) having a low ionization potential (energy needed to ionize one g mol of atoms) before the gas enters the MHD duct.
An adequate electrical conductivity of the order of 10 mho/m can thus be realized at somewhat lower temperatures in the range 2200—2 700 ° C.
A simple view of the MHD generator is shown inFig The duct through which the electrically conducting ionized gas flows has two sides supporting a strong transverse magnetic field of 4 —5 tesla (1 tesla = iü gauss) at right angles to the flow and the other sides forming the faces of electrodes which are joined through an electrical circuit.
As the hot ionized gas or plasma enters the MHD duct, due to the effect of the strong magnetic field and the consequent Lorentz force, there is a decrease in the kinetic energy of the plasma, and the electrons and ions get deposited on the opposite electrodes.
The power generated per unit length is approximately proportional to cru B where c-is the electrical conductivity, u is the velocity of the gas, B is the magnetic field strength and p is the density.
The power produced being dc, the conversion to ac is done by an inverter. Figure
3.31 shows the principal components of a typical MHD plant and its cycle of operations on T —s diagram.
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