Rankine cycle

Rankine cycle layout

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Rankine cycle Ts

Rankine cycle with superheat

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Rankine cycle with reheat

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Regenerative rankine cycle

Rankine cycle is a thermodynamic cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. The Rankine cycle is the fundamental operating cycle of all thermal power plants where an operating fluid is continuously evaporated and condensed. It is named after William John Macquorn Rankine, a Scottish polymath.

Description

The Rankine cycle operates in four basic steps:

1. Pumping: Liquid water is pumped from low to high pressure.
2. Heating: Water at high pressure is heated in a boiler until it becomes high-pressure steam.
3. Expansion: The high-pressure steam expands in a turbine, generating power. This expansion cools the steam into a partially condensed state.
4. Condensation: The partially condensed steam is then condensed in a condenser to become liquid water again, completing the cycle.

The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid and the inability to operate at a high temperature. The efficiency can be improved by superheating the steam at the boiler and by increasing the pressure of the working fluid.

Variations

Several variations of the Rankine cycle exist, including:

• Reheat Rankine Cycle: Steam is expanded in a turbine, then reheated before expanding in a second turbine.
• Regenerative Rankine Cycle: Heat is extracted from certain points in the turbine to preheat the boiler feedwater.
• Organic Rankine Cycle (ORC): Uses organic fluids with a low boiling point instead of water, suitable for capturing low-grade heat.

Applications

The Rankine cycle is widely used in power generation systems, including:

• Fossil fuel power plants
• Nuclear power plants
• Geothermal power plants
• Solar thermal power plants

Efficiency and Improvements

The efficiency of the Rankine cycle is often enhanced by increasing the temperature and pressure of the steam. However, there are practical limits due to material strength and thermal stresses. Other methods to improve efficiency include using multiple reheat and regenerative feedwater heating stages.

Environmental Impact

While the Rankine cycle itself is a clean process, the environmental impact depends on the source of heat. Fossil fuel-based plants emit greenhouse gases and pollutants, whereas nuclear, geothermal, and solar thermal plants have lower emissions.

See Also

• Thermodynamics
• Carnot cycle
• Brayton cycle
• Stirling engine

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