The overall efficiency of steam power station is much lower and that is about 29%,it is mainly due to two reasons.Firstly a large amount of heat is wasted in condensers and secondly the heat wastage in other parts of the plant.The heat loss in the condenser is unavoidable since heat energy cannot be converted into mechanical energy without temperature difference.The greater the temperature difference greater is the heat energy converted into mechanical energy.This necessitates to keep the temperature of condenser to a minimum value.But greater the temperature difference,greater the amount of heat lost.This explains the low efficiency of steam plants.

1) Thermal Efficiency

The ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion of coal is known as the thermal efficiency of steam power station.

Thermal efficiency ,η thermal = Heat equivalent of mech.energy transmitted to turbine shaft/Heat of coal combustion

The thermal efficiency of a modern steam power station is about 30% ,ie if 100 calories is produced only 30 calories of mechanical equivalent is available at the turbine shaft and it may be also noted that 50% of combustion heat is lost in condenser.

2) Overall efficiency

The ratio of heat equivalent of electrical output to the heat of combustion of coal is known as overall efficiency of steam power station ie

Overall efficiency , η overall = Heat equivalent of electrical output/Heat of combustion of coal

The overall efficiency of steam power station is about 29%.It is seen that overall efficiency is less than thermal efficiency.A loss of about 1% occurs in the alternator.

There is relation between various efficiency as shown below

Overall efficiency = Thermal efficiency × Electrical efficiency

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