The Rankine cycle is based on the simple Carnot cycle. Invented by William John Macquom Rankine (1820 - 1872), the Rankine cycle models a simple steam powerplant. The Rankine cycle is important since it forms a basis of understanding for other cycles, such as the refrigeration cycle. It is recommended that the student reviews the concepts of the Carnot cycle before attempting to learn the Rankine cycle.
In brief, there are three limitations of the Carnot cycle that renders the model unpractical:
1) It is difficult to compress a two phase mixture through a pump.
2) It is difficult to control the quality of the working fluid throughout the cycle.
3) The cycle efficiency is limited by the upper operating temperature since the cycle has to remain inside the vapour dome.
The Rankine cycle rectifies all three limitations:
1) The turbine exhaust steam is condensed to a saturated liquid.
2) By always condensing to saturated liquid, the quality of the working fluid can be accurately controlled.
3) Modifications of the basic Rankine cycle model allows for the working fluid to go into the sub-cooled and superheated regions.
Another difference between the Carnot cycle and the Rankine cycle is the state of the working fluid at the exit of the boiler. In a Carnot cycle, heat is added to the system at constant temperature. For isothermal heating, pressure of the superheated working fluid must drop as the heat is added. This process is extremely difficult to achieve. More realistically and easier to implement, the Rankine cycle heats the working fluid at constant pressure.
Even though the aforementioned differences lowers the cycle efficiency, the Rankine cycle remains the most efficient practical model of a steam powerplant system.