A combined cycle power plant (CCPP) is a power generation facility that combines a gas turbine cycle with a steam turbine cycle to generate electricity more efficiently. Instead of releasing the hot exhaust gases produced by the gas turbine, a combined cycle plant recovers this thermal energy to produce steam and generate additional electricity.
This combination allows the plant to extract more useful energy from the same fuel than a simple-cycle gas turbine plant.
In this article, we explain how a combined cycle power plant works, its main components, how the gas and steam cycles interact, and the factors that influence its performance.
What is a combined cycle power plant?
A combined cycle power plant combines two thermodynamic cycles to produce electricity: a gas turbine cycle and a steam turbine cycle.
In the first cycle, fuel — typically natural gas — is burned in a gas turbine. The turbine drives a generator to produce electricity, but the process also generates high-temperature exhaust gases containing significant thermal energy.
Instead of releasing this heat directly into the atmosphere, a Heat Recovery Steam Generator (HRSG) captures it and uses it to produce steam.
The steam then drives a steam turbine, which generates additional electricity. This ability to recover energy that would otherwise be wasted is the fundamental principle behind combined cycle power generation.
How does a combined cycle power plant work?
The operation of a combined cycle power plant can be divided into four main stages:
1. Gas turbine cycle
Ambient air enters the gas turbine through the inlet system and is compressed. Fuel is then mixed with the compressed air and burned in the combustion chamber. The resulting high-temperature gases expand through the gas turbine, producing mechanical power that drives an electrical generator.
2. Heat recovery
After leaving the gas turbine, the exhaust gases still contain a considerable amount of thermal energy. Instead of rejecting this heat, the exhaust gases enter the Heat Recovery Steam Generator (HRSG).
The HRSG transfers heat from the exhaust gases to water, producing high-pressure steam without requiring a separate primary combustion process.
3. Steam turbine cycle
The steam produced by the HRSG expands through a steam turbine. The steam turbine drives another generator, producing additional electricity from energy recovered from the gas turbine exhaust.
4. Steam condensation and recirculation
After passing through the steam turbine, the steam is cooled in a condenser and converted back into water. The water can then be returned to the HRSG, completing the steam cycle.
In this way, the gas and steam cycles work together, allowing the plant to generate more electricity from the fuel input than would be possible with the gas turbine alone.
Combined cycle power plant diagram
Figure 1. Schematic representation of a combined cycle power plant.
A combined cycle power plant diagram illustrates the relationship between the gas turbine, HRSG and steam turbine.
The key element connecting both cycles is the HRSG. By recovering thermal energy from the gas turbine exhaust, it enables the second power generation cycle without requiring the same level of additional fuel input.
Figure 2. Conceptual diagram of a combined cycle power plant
Main components of a combined cycle power plant
Although plant configurations vary according to capacity, operating conditions and project requirements, a typical CCPP includes several key components.
Gas turbine
The gas turbine represents the first power generation cycle. Its main sections include the compressor, combustion chamber and turbine. Air is compressed and mixed with fuel before combustion, and the resulting gases expand through the turbine to produce mechanical power.
Heat Recovery Steam Generator (HRSG)
The HRSG connects the gas and steam cycles. It captures thermal energy from the gas turbine exhaust and transfers it to water to produce steam.
This heat recovery process is one of the fundamental reasons why combined cycle power plants can achieve significantly higher overall efficiency than simple-cycle configurations.
Steam turbine
Steam produced in the HRSG expands through the steam turbine, converting thermal energy into mechanical power and subsequently electricity through a generator.
The steam cycle therefore allows the plant to obtain additional electrical output from heat already produced during gas turbine operation.
Condenser and cooling system
After leaving the steam turbine, the steam must be condensed before returning to the water-steam cycle. The cooling system removes heat from the exhaust steam, while the condenser converts it back into liquid water.
Cooling performance can therefore influence the overall operation and efficiency of the steam cycle.
Electrical generators
Both the gas turbine and steam turbine can be connected to electrical generators, depending on the plant configuration. The electricity generated through both cycles represents the plant's total combined output.
Combined cycle vs. simple cycle power plants
The main difference between a combined cycle power plant and a simple cycle power plant is what happens to the gas turbine exhaust heat.
In a simple-cycle configuration, the gas turbine generates electricity and the exhaust gases are discharged without being used to operate a second power generation cycle. In a combined-cycle configuration, this heat is recovered through the HRSG and used to produce steam for the steam turbine.
| Simple cycle | Combined cycle | |
|---|---|---|
| Main turbine | Gas turbine | Gas + steam turbine |
| Exhaust heat recovery | No | Yes |
| HRSG | No | Yes |
| Configuration | Simpler | More complex |
| Overall efficiency | Lower | Higher |
| Start-up | Generally faster | Generally slower |
The appropriate configuration ultimately depends on factors such as operating profile, required flexibility, investment considerations and project objectives.
Combined cycle power plant efficiency
One of the main characteristics of a combined cycle power plant is its high thermal efficiency. The reason is straightforward: the system generates electricity twice from the energy available in the fuel.
First, the gas turbine generates electricity directly. Then, instead of wasting the thermal energy contained in its exhaust gases, the HRSG recovers part of that energy and uses it to generate steam and additional power.
Modern combined-cycle plants can achieve efficiencies above 60% under suitable conditions, although actual plant performance depends on turbine technology, configuration, ambient conditions and operating load. ARANER's existing technical content also identifies heat recovery as the fundamental reason CCGT efficiency exceeds that of standalone gas-turbine configurations.
What affects the performance of a combined cycle power plant?
CCPP performance is influenced by both plant design and operating conditions. One particularly important variable is ambient temperature.
Gas turbines draw large quantities of ambient air into their compressors. As air temperature increases, its density decreases. Under high-temperature conditions, this can reduce the mass flow entering the gas turbine and consequently reduce its power output.
The effect does not necessarily stop at the gas turbine. Changes in gas turbine exhaust flow and temperature can also affect the HRSG and the amount of steam available to the steam turbine. For this reason, ambient conditions should be considered when assessing the overall performance of a combined cycle plant.
How can Turbine Inlet Air Cooling improve CCPP performance?
Turbine Inlet Air Cooling (TIAC) is designed to cool the air entering the gas turbine compressor.
Maintaining lower and more stable inlet-air temperatures can help reduce the loss of gas turbine output associated with high ambient temperatures.
This is particularly relevant in hot climates, where periods of high ambient temperature can coincide with periods of high electricity demand.
In a combined-cycle configuration, the impact of TIAC should be evaluated across the entire plant rather than only at the gas turbine. Changes in gas turbine output and exhaust conditions can also influence HRSG operation and steam turbine output.
ARANER designs TIAC systems according to the specific operating conditions and requirements of each power plant rather than applying a standard configuration.
Advantages and limitations of combined cycle power plants
Combined cycle technology offers several operational characteristics compared with conventional single-cycle power generation.
Recovering exhaust heat allows higher fuel utilization and greater overall efficiency, while the combination of gas and steam cycles can provide substantial power output from the same fuel input.
However, CCPPs also involve a more complex plant configuration, additional equipment and different operational considerations compared with open-cycle gas turbines. Rather than developing those considerations here, they deserve a separate analysis.
Optimizing combined cycle power plant performance
A combined cycle power plant improves fuel utilization by integrating gas turbine generation, waste heat recovery and steam turbine generation into a single system.
However, plant performance depends not only on its basic configuration but also on equipment design, operating profile and environmental conditions.
In regions where high ambient temperatures cause significant gas turbine output losses, technologies such as Turbine Inlet Air Cooling can help maintain power production when electricity demand is high.
At ARANER, we engineer customized TIAC solutions for new and existing power plants based on each project's climate, operating profile and performance requirements. Contact us for further information.
Frequently asked questions about combined cycle power plants
What is a combined cycle power plant?
A combined cycle power plant generates electricity using both a gas turbine and a steam turbine. Heat from the gas turbine exhaust is recovered through an HRSG to produce steam and generate additional electricity.
What does CCPP stand for?
CCPP stands for Combined Cycle Power Plant. The term CCGT, or Combined Cycle Gas Turbine, is also commonly used when referring to gas-fired combined-cycle plants.
What are the main components of a combined cycle power plant?
The main components include the gas turbine, Heat Recovery Steam Generator (HRSG), steam turbine, electrical generators, condenser and associated cooling systems.
What is an HRSG in a combined cycle power plant?
A Heat Recovery Steam Generator captures heat from gas turbine exhaust gases and uses it to generate steam. This steam drives the steam turbine to produce additional electricity.
Why are combined cycle power plants more efficient?
They recover thermal energy from gas turbine exhaust that would otherwise be wasted and use it to generate additional electricity through a steam turbine.
How does ambient temperature affect a combined cycle power plant?
Higher ambient temperatures can reduce air density and gas turbine mass flow, decreasing gas turbine output. They can also influence exhaust conditions and therefore the performance of the steam cycle.
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