What is regeneration in gas turbines?

Short Answer:

Regeneration in gas turbines is a process used to improve the efficiency of the plant by using the heat from the turbine exhaust gases to preheat the compressed air before it enters the combustion chamber. This reduces the amount of fuel required for combustion and increases overall thermal efficiency.

In simple words, regeneration means recovering waste heat from the exhaust gases and using it to heat the compressed air. By doing so, less fuel is needed to reach the desired temperature in the combustion chamber, saving energy and improving the performance of the gas turbine plant.

Detailed Explanation :

Regeneration in Gas Turbines

Regeneration in gas turbines is a method of waste heat recovery that improves the efficiency of the gas turbine cycle by utilizing the heat of exhaust gases. The basic idea is to transfer heat from the hot gases leaving the turbine to the compressed air coming out of the compressor before it enters the combustion chamber.

This process reduces the fuel consumption required to raise the temperature of the air to the combustion level. The device used for this purpose is called a regenerator or heat exchanger. It allows heat transfer between two fluids — the hot exhaust gas and the cool compressed air — without allowing them to mix.

By recovering part of the exhaust heat, regeneration increases the thermal efficiency of the gas turbine and reduces overall fuel cost.

Working Principle of Regeneration

The working principle of regeneration is based on the concept of heat exchange between the exhaust gases and the compressed air.

  1. Compression Process:
    • Atmospheric air is drawn into the compressor where it is compressed to a high pressure.
    • During compression, the temperature of air increases due to the work done on it.
  2. Regeneration Process:
    • The compressed air, still at a high pressure but moderate temperature, passes through the regenerator.
    • Simultaneously, the exhaust gases leaving the turbine are passed through the same regenerator on the opposite side.
    • Heat flows from the hot exhaust gases to the cooler compressed air through the regenerator’s metal walls.
    • The compressed air temperature increases before it enters the combustion chamber.
  3. Combustion Process:
    • The preheated air now enters the combustion chamber, where fuel is injected and burned.
    • Since the air is already hot due to regeneration, less fuel is needed to reach the required turbine inlet temperature.
  4. Expansion Process:
    • The high-temperature gases from the combustion chamber expand through the turbine, producing mechanical power.
    • The exhaust gases leaving the turbine still contain significant heat, which is reused in the regenerator.

Thus, the regenerator continuously exchanges heat between the turbine exhaust and the compressor outlet air to improve efficiency.

Construction of Regenerator

A regenerator is typically a heat exchanger made of metal tubes or plates that separate the two gas streams.

Main parts include:

  1. Hot Gas Side: Carries exhaust gases from the turbine.
  2. Cold Air Side: Carries compressed air from the compressor.
  3. Heat Transfer Surface: Metal walls or fins that transfer heat between the gases.

Depending on design, regenerators are of two main types — recuperative and regenerative (rotary) types.

Types of Regeneration

  1. Recuperative Regenerator
  • The recuperative regenerator uses stationary heat exchange surfaces such as tubes or plates.
  • Hot exhaust gases and cold compressed air flow continuously on opposite sides of the metal surface.
  • Heat is transferred from exhaust to air without direct contact.
  • Common in industrial and stationary gas turbines.
  1. Rotary (Regenerative) Regenerator
  • In this type, a rotating matrix or wheel made of metal or ceramic material alternately passes through the hot and cold gas streams.
  • The matrix absorbs heat from exhaust gases and then transfers it to the compressed air as it rotates.
  • Used in large gas turbine plants due to compact design and high heat recovery efficiency.

Effect of Regeneration on Efficiency

The efficiency of a gas turbine plant can be expressed as:

Where  is the net work output, and  is the heat supplied by the fuel.

In a simple Brayton cycle, the hot exhaust gases are discharged directly into the atmosphere, and their heat is wasted. By introducing regeneration, some of this wasted heat is reused, reducing the heat input .

Hence, the overall efficiency increases because the same work output is achieved with less fuel.

The thermal efficiency with regeneration is given as:

Where:

  •  = inlet air temperature
  •  = temperature after compression
  •  = temperature after combustion
  •  = exhaust gas temperature

The regenerator is most effective when the temperature of exhaust gases is significantly higher than the compressor outlet temperature.

Advantages of Regeneration

  1. Improved Thermal Efficiency:
    • Reduces fuel requirement by reusing waste heat from exhaust gases.
    • Efficiency can increase by 10–15%.
  2. Reduced Fuel Consumption:
    • Since the compressed air is preheated, less fuel is needed for combustion.
  3. Lower Operating Cost:
    • Reduces fuel expenses, leading to long-term economic operation.
  4. Environmental Benefits:
    • Less fuel burning means lower CO₂ emissions and reduced pollution.
  5. Increased Component Life:
    • Lower fuel flow and moderate combustion temperature reduce thermal stress on turbine components.

Limitations of Regeneration

  1. High Initial Cost:
    • Regenerators are expensive to design and install.
  2. Increased Maintenance:
    • Heat exchanger surfaces may get fouled with dust and require regular cleaning.
  3. Pressure Loss:
    • There is a small pressure drop on both air and gas sides, reducing efficiency slightly.
  4. Limited Effectiveness:
    • At very high pressure ratios, the temperature difference between exhaust and compressed air decreases, making regeneration less useful.
  5. Space Requirement:
    • The addition of the regenerator increases the system size and weight.

Despite these drawbacks, regeneration remains a practical method to improve performance, especially in medium and small gas turbine plants.

Applications of Regeneration

  • Industrial gas turbines for power generation.
  • Marine propulsion systems where fuel economy is important.
  • Aviation auxiliary power units (APUs).
  • Combined-cycle plants for waste heat recovery.
  • Cogeneration plants for both power and heat production.
Conclusion

In conclusion, regeneration in gas turbines is a technique used to improve efficiency by utilizing the exhaust gas heat to preheat the compressed air before combustion. The process reduces fuel consumption, operating costs, and environmental impact. A regenerator or heat exchanger is used to transfer heat between the hot exhaust and compressed air. Though it increases system cost and complexity, regeneration significantly enhances thermal efficiency and is widely used in modern gas turbine systems for economical and sustainable energy production.