What is the Rankine cycle?

Short Answer:

The Rankine cycle is a thermodynamic cycle that describes the working of steam power plants, where heat energy is converted into mechanical work using water or steam as the working fluid. It is widely used in thermal power stations to generate electricity.

The cycle consists of four key processes: heat addition in a boiler, expansion in a turbine, heat rejection in a condenser, and compression in a pump. The Rankine cycle is more practical than the Carnot cycle and is the standard model for evaluating the performance of steam engines and turbines.

Detailed Explanation:

Rankine cycle

The Rankine cycle is one of the most important thermodynamic cycles used in power generation systems. It forms the basis of almost all steam-based power plants around the world. The cycle uses water or steam as the working fluid and operates between two pressure and temperature levels to generate mechanical work, which is later converted into electricity.

The Rankine cycle is more practical and closer to real-world applications compared to the ideal Carnot cycle because it considers phase changes and can be implemented using common components like boilers, turbines, condensers, and pumps.

Processes in the Rankine Cycle

The basic Rankine cycle consists of four major processes:

  1. Process 1–2: Isentropic Compression (Pump)
    • Water at low pressure enters the pump.
    • The pump compresses the liquid isentropically (without entropy change), raising its pressure.
    • This process requires a small amount of work input.
  2. Process 2–3: Constant Pressure Heat Addition (Boiler)
    • High-pressure water enters the boiler.
    • Heat is added at constant pressure, converting water into high-temperature steam.
    • No work is done, but energy is added to the system.
  3. Process 3–4: Isentropic Expansion (Turbine)
    • The high-pressure steam enters the turbine.
    • It expands isentropically, producing mechanical work.
    • The pressure and temperature of the steam drop.
  4. Process 4–1: Constant Pressure Heat Rejection (Condenser)
    • Low-pressure steam enters the condenser.
    • Heat is rejected to the surroundings at constant pressure.
    • The steam condenses into saturated liquid water, completing the cycle.

After this, the water is again pumped to the boiler, and the cycle repeats.

Rankine Cycle on T-s and P-v Diagrams

  • On a Temperature-Entropy (T-s) diagram, the Rankine cycle is represented by a loop that shows phase change during heating and cooling.
  • On a Pressure-Volume (P-v) diagram, it shows compression and expansion phases along with phase changes.

These diagrams help visualize how temperature, pressure, and entropy change during the cycle.

Improvements to the Rankine Cycle

To increase efficiency, engineers often modify the basic Rankine cycle by adding:

  • Reheating: Steam is partially expanded, reheated, and then expanded again.
  • Regeneration: Feedwater is preheated using steam bled from the turbine.
  • Superheating: Steam is heated beyond the saturation temperature to increase work output.

These changes help in reducing fuel consumption and improving thermal efficiency.

Importance of the Rankine Cycle

  1. Widely Used in Power Plants
    – Coal, nuclear, and solar thermal power stations all use Rankine cycle principles.
  2. Suitable for Phase Change Fluids
    – Handles water–steam transitions efficiently.
  3. Realistic and Practical
    – Accounts for real system limitations better than the Carnot cycle.
  4. Supports Multiple Enhancements
    – Can be improved using superheating, reheating, and regeneration.
  5. Foundation of Steam Engineering
    – Helps analyze and design steam turbines, condensers, and pumps.

Efficiency of the Rankine Cycle

The thermal efficiency of the Rankine cycle is given by:

η = (W_turbine – W_pump) / Q_boiler

Efficiency increases with:

  • Higher boiler pressure and temperature
  • Lower condenser pressure
  • Better turbine and pump performance

Although not as efficient as the Carnot cycle, the Rankine cycle is practical and implementable.

Conclusion

The Rankine cycle is a practical thermodynamic cycle used in steam power plants to convert heat energy into work. It includes processes of pumping, heating, expansion, and condensation. It is more applicable to real-world systems than the Carnot cycle and forms the basis for designing efficient thermal power stations. Enhancements like reheating and regeneration help in increasing the performance of the Rankine cycle in modern power engineering.