Thermal efficiency of Steam Power Plant:
In steam power plant overall efficiency will be in the order of about 30%. In case of modern thermal power plats where super critical parameters (pressure and temperature) are employed and by employing number heat savig devices the power plant efficiency will be raised to around 40%. Quality of the coal also plays significant role in improving the efficiency of the Thermal Plant.
Some the losses occurring i the steam power plants are:
Loss at Boiler - 15%
Loss at the Turbo-Generator
Heat Rejection to the condenser - 55%
Alternator loss - 1%
From above mentioned figures we understand that more than 50% of total heat combustion is lost as heat rejected to the condenser. This loss cannot be avoided as heat energy cannot be converted into mechanical energy without (some heat rejected) drop in temperature (From the second law of thermodynamics-A fraction of heat can only be converted to mechanical work and remaining heat must be rejected). Steam in the condenser is at lowest temperature.
Thermal efficiency of the power plant mainly depends on three factors.
a) Pressure of the steam entering the turbine
b) Temperature of the steam entering the turbine
c) Pressure in the condenser
Thermal efficiency of the plant can be improved by increasing the temperature and pressure of the steam. For this reason high temperature and pressure is used and also thermal efficiency is improved by decreasing the pressure in the condenser
Also efficiency can be improved by reheating the steam between the turbine stages
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