Short Answer:

A regenerative Rankine cycle is a modified form of the simple Rankine cycle used to improve the thermal efficiency of power plants. In this cycle, a portion of steam is extracted from the turbine at intermediate stages to preheat the feedwater before it enters the boiler. This process reduces the fuel required to heat water in the boiler.

In simple words, the regenerative Rankine cycle increases efficiency by using part of the steam’s energy to heat the feedwater instead of wasting it. This raises the average temperature of heat addition, improving the overall performance and fuel economy of the power plant.

Detailed Explanation :

Regenerative Rankine Cycle

The regenerative Rankine cycle is an advanced version of the simple Rankine cycle that improves thermal efficiency by using steam extraction for feedwater heating. In a simple Rankine cycle, all the heat for converting water into steam is supplied by the boiler. However, in the regenerative cycle, part of this heat is supplied by the extracted steam from the turbine, which preheats the feedwater before it enters the boiler.

By preheating the feedwater, the average temperature of heat addition in the boiler increases. This reduces the fuel consumption and enhances the efficiency of the cycle. Regeneration also helps in maintaining the boiler and turbine components at a more uniform temperature, improving the life and performance of the system.

1. Principle of Regenerative Rankine Cycle

The regenerative Rankine cycle works on the principle of heat regeneration or feedwater heating. The idea is to recover some of the heat from the expanding steam in the turbine and use it to heat the feedwater before it enters the boiler.

This process increases the average temperature of heat addition, which improves the thermal efficiency according to the basic thermodynamic principle that:

“The efficiency of a heat engine increases when the average temperature of heat addition increases.”

Thus, regeneration is a method of internal heat recovery within the system to make better use of available energy.

2. Processes in Regenerative Rankine Cycle

The regenerative Rankine cycle consists of the following major processes:

Process 1–2: Isentropic Compression (in Pump)

• The condensate (water) from the condenser is pumped to the boiler pressure by the feedwater pump.
• This process occurs nearly isentropically, meaning with no heat transfer.
• The water’s pressure increases, preparing it for heating in the boiler.

Process 2–3: Constant Pressure Heat Addition (in Boiler)

• The high-pressure water enters the boiler and absorbs heat from fuel combustion.
• The water is converted into high-pressure, high-temperature steam.
• This steam is then sent to the turbine for expansion.

Process 3–4: Isentropic Expansion (in Turbine with Steam Extraction)

• The superheated steam expands in the turbine, doing mechanical work and losing pressure and temperature.
• During expansion, a portion of the steam is extracted at an intermediate stage and diverted to the feedwater heater.
• The remaining steam continues to expand until it reaches condenser pressure.

Process 4–5: Constant Pressure Heat Rejection (in Condenser)

• The low-pressure steam leaving the turbine enters the condenser, where it is cooled and condensed into water at constant pressure.
• This heat rejection occurs to the cooling water in the condenser.

Process 5–6: Feedwater Heating (Regeneration)

• The condensate from the condenser is pumped to an intermediate pressure and passed through the feedwater heater.
• The extracted steam transfers its heat to the feedwater and condenses.
• The feedwater, now at a higher temperature, is returned to the boiler.

This completes the regenerative Rankine cycle, where part of the steam energy is reused for feedwater heating instead of being wasted.

3. Components of Regenerative Rankine Cycle

The major components used in the regenerative Rankine cycle are:

1. Boiler: Generates high-pressure steam from water.
2. Turbine: Expands steam in multiple stages, extracting part of it for regeneration.
3. Condenser: Converts exhaust steam into water by removing heat.
4. Feedwater Pump: Increases pressure of condensate water before entering the boiler.
5. Feedwater Heater (Regenerator): Heats the feedwater using extracted steam from the turbine.

Depending on the design, there can be one or more feedwater heaters in the cycle.

4. Types of Regenerative Feedwater Heaters

There are two main types of feedwater heaters used in regenerative Rankine cycles:

1. Open (Direct Contact) Feedwater Heater

• In this type, extracted steam directly mixes with the feedwater.
• The heat transfer occurs through direct contact.
• The mixture reaches a uniform temperature and pressure.
• Commonly used because of its simplicity and high efficiency.

1. Closed Feedwater Heater

• In this type, the extracted steam and feedwater do not mix.
• The steam passes through one side of a heat exchanger while feedwater flows on the other side.
• The steam condenses after transferring its heat and is drained separately.
• Suitable for systems where purity of feedwater must be maintained.

Large power plants often use a combination of open and closed heaters for maximum efficiency.

5. Advantages of Regenerative Rankine Cycle

1. Higher Thermal Efficiency:
   • Increases the mean temperature of heat addition, resulting in higher efficiency.
2. Reduced Fuel Consumption:
   • Less fuel is required in the boiler due to preheated feedwater.
3. Improved Turbine and Boiler Life:
   • Reduces thermal stresses by maintaining uniform temperature differences.
4. Lower Irreversibilities:
   • Makes the heat transfer process more reversible, improving cycle performance.
5. Water Conservation:
   • Reuses condensed steam, reducing fresh water requirements.

6. Disadvantages of Regenerative Rankine Cycle

1. Complex Construction:
   • Requires additional piping, valves, and feedwater heaters, increasing design complexity.
2. Higher Cost:
   • The installation and maintenance of extra equipment increase the overall cost.
3. Difficult Operation:
   • Requires precise control of steam extraction and pressure balance.
4. Slight Decrease in Turbine Work:
   • Since some steam is extracted before full expansion, the turbine produces slightly less work.

Despite these drawbacks, the efficiency gain justifies its use in most large-scale thermal power plants.

7. Efficiency of Regenerative Rankine Cycle

The thermal efficiency (η) of the regenerative Rankine cycle is given by:

However, because feedwater heating increases the average temperature of heat addition, the efficiency of the regenerative cycle is always higher than that of the simple Rankine cycle.

Typical efficiency improvements range from 3% to 8% depending on the number of heaters used.

8. Applications

The regenerative Rankine cycle is widely used in:

• Thermal power plants for large-scale electricity generation.
• Nuclear power stations for efficiency improvement.
• Industrial steam systems where high efficiency is required.

Most modern power plants operate on a reheat-regenerative Rankine cycle, which combines the benefits of both reheating and regeneration.

Conclusion

The regenerative Rankine cycle is a modified version of the simple Rankine cycle designed to increase thermal efficiency and reduce fuel consumption. By using extracted steam to preheat feedwater, the cycle increases the average temperature of heat addition, leading to better performance. Though it adds complexity and cost, its efficiency advantages make it an essential system for modern thermal and nuclear power plants. Therefore, the regenerative Rankine cycle is one of the most practical and efficient methods for sustainable power generation.