Comparison between Rankine & Reverse Rankine Cycle

This page will give you the basic differences and thermodynamical comparison between Rankine cycle and reverse Rankine cycle. The comparison of the two is easy as one is the reverse of the other. Both are identical cycles but their process diagrams are in opposite direction. The basic idea of Temperature-Entropy (TS) diagram of Rankine and reverse Rankine cycle is a prerequisite for reading this article. The following are the basic comparison between both the cycles.

Rankine Cycle

• It is a work producing cycle and is the basic vapor standard thermodynamic cycle for the steam power plant.

• This concept is centered around Kelvin-Plank statement of the 2nd law of Thermodynamics. This statement gives rise to the idea of work producing devices.

• Steam is the working fluid in this cycle.

• The direction of the various thermodynamic processes in Rankine cycle is in the clockwise direction.

• In Rankine cycle, expansion process in the turbine is considered as isentropic.

• In Rankine cycle, we are interested in finding out the cycle efficiency and net work output and it is desirable to increase both.

• An increase in boiler pressure in Rankine cycle results in the reduction of heat rejection or increment in mean temperature of heat addition which ultimately results in an increase in efficiency. However, dryness fraction decreases and requires a higher capacity feed water pump.

• A decrease in condenser pressure reduces the heat rejection and increases the turbine work output and thus increasing the efficiency. But here also dryness fraction decreases.

• Superheating in Rankine cycle improves the efficiency as well as the dryness fraction of steam.

• Regeneration in Rankine cycle also improves the efficiency.

Reverse Rankine Cycle or Vapour Compression Refrigeration

• It is a work consuming cycle and is the basic vapor standard thermodynamic cycle for the vapour compression refrigeration system.

• This concept is centered around Clausius statement of the 2nd law of Thermodynamics. This statement gives rise to the idea of work consuming devices.

• Working fluid is usually referred to as a refrigerant.

• The direction of the various thermodynamic processes in reverse Rankine cycle is in the counterclockwise direction.

• In reverse Rankine cycle, expansion process in the throttling device is considered as isenthalpic.

• In reverse Rankine cycle, we are interested in finding out the coefficient of performance (COP) and it is desirable to increase it.

• COP decreases with increase in condenser pressure and a decrease in evaporator pressure.

• The variation of COP is unpredicted in the case of superheating. This is because of the increment in refrigeration effect with the same work consumed by the compressor.

• Subcooling increases COP of reverse Rankine cycle.