Steam power plant:

Steam power plant converts heat energy Q from combustion of fuel into mechanical work W of shaft rotation which is used to generate electricity. This type plant operates on thermodynamic cycle in closed loop of processes following one another such that working fluid of steam and water repeats the cycles continuously. If first law of thermodynamics is applied to thermodynamic cycle in which working fluid returns to the initial condition of it, the energy flowing into fluid at the time of cycle should be equal to that flowing out of cycle.

Q_in + W_in = Q_out + W_out Or,    Q_in  - Q_out = W_out - W_in Where;

Q = Rate of Heat transfer

W = Rate of work transfer, that is power

Heat and work are mutually convertible. However, quantity of work energy can be converted into heat energy, the converse is not true. A quantity of heat cannot be converted into work.

Heat flows because of temperature difference, and this means that in order to flow, two heat reservoirs should be present; a hot source and a cold sink. During heat flow from the hot source to cold sink, a fraction of flow can be converted into work energy, and function of power plant is to generate this conversion. However, some heat should flow into the cold sink because of its presence. Thus rate of heat transfer Qout of the cycle should always be positive and the efficiency of conversion of heat energy into work energy can never be 100 percent at all. Thermodynamic efficiency for the cycle, nth is the measure of how well cycle converts heat into work.