Rankine Cycle

The Rankine cycle is the most commonly used cycle in thermal power plants—steam engine and steam turbines. The Rankine cycle is sometimes referred to a practical Carnot cycle because when an efficient turbine is used, the T–s diagram begins to resemble the Carnot cycle. The main difference is that heat addition and rejection are at constant pressure in the Rankine cycle and isothermal in the theoretical Carnot cycle. A pump is used to pressurize the water received from the condenser. To pump the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it. Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station.

The working fluid in a Rankine cycle follows a closed loop and is reused continuously. Work is required to drive the pump, the working fluid being in its liquid phase at this point. By condensing the fluid, the work required by the pump consumes only 1–3% of the turbine power and contributes to a much higher efficiency for a real cycle.