Combined Cycle Gas Turbine (CCGT) power plants are widely used for electricity generation because they combine high efficiency with operational flexibility.
Unlike an Open Cycle Gas Turbine (OCGT), a CCGT power plant generates electricity through two interconnected thermodynamic cycles. First, a gas turbine produces electricity. Then, instead of releasing its hot exhaust gases directly into the atmosphere, their thermal energy is recovered to produce steam and generate additional electricity in a steam turbine.
Figure 1 below shows the basic difference between open cycle and combined cycle configurations, illustrating how a CCGT power plant recovers heat from the gas turbine exhaust and uses it to generate additional power.
As shown in Figure 1, the fundamental difference lies in the use of the exhaust gases. In an OCGT power plant, hot exhaust gases are released after passing through the gas turbine. In a Combined Cycle Gas Turbine plant, this thermal energy is instead recovered through a Heat Recovery Steam Generator (HRSG) and used to produce steam for a steam turbine.
This ability to generate additional electricity from heat that would otherwise be wasted is one of the main reasons why combined cycle power plants can achieve significantly higher efficiencies than open cycle configurations.
What is a Combined Cycle Gas Turbine (CCGT) power plant?
A Combined Cycle Gas Turbine, or CCGT, is a power generation system that combines a gas turbine cycle with a steam turbine cycle.
In the first cycle, air is compressed and mixed with fuel in the combustion chamber. The resulting high-temperature gases expand through the gas turbine, driving a generator and producing electricity.
However, the exhaust gases leaving the gas turbine still contain a considerable amount of thermal energy.
In a CCGT power plant, this heat is recovered in a Heat Recovery Steam Generator (HRSG). The HRSG uses the exhaust heat to produce steam, which then drives a steam turbine and generates additional electricity.
Therefore, the plant obtains more useful energy from the same fuel input.
This is the fundamental difference between a combined cycle and an open or simple cycle configuration.
How does a Combined Cycle Gas Turbine work?
The operation of a combined cycle power plant can be divided into five main stages.
1. Air compression and combustion
Ambient air enters the gas turbine through the air intake system and passes through the compressor, where its pressure increases significantly.
The compressed air then enters the combustion chamber and mixes with fuel, typically natural gas. Combustion produces high-temperature, high-pressure gases that are directed toward the gas turbine.
2. Power generation in the gas turbine
The hot combustion gases expand through the turbine stages. As they expand, they rotate the turbine shaft, which drives both the compressor and the electrical generator.
This process represents the first power generation cycle of the plant. In an Open Cycle Gas Turbine (OCGT), the exhaust gases would normally be discharged after this stage. In a CCGT configuration, however, their remaining thermal energy is recovered.
3. Waste heat recovery in the HRSG
After leaving the gas turbine, the exhaust gases enter the Heat Recovery Steam Generator. The HRSG acts as the connection between the gas turbine cycle and the steam cycle.
Instead of allowing the thermal energy contained in the exhaust gases to go unused, the HRSG transfers that energy to water, producing high-pressure steam.
This heat recovery process is one of the main reasons why combined cycle power plants can achieve considerably higher efficiencies than open cycle plants.
4. Additional power generation in the steam turbine
The steam produced in the HRSG is directed toward a steam turbine. As the steam expands through the turbine, it drives another generator, producing additional electricity without requiring a proportional increase in fuel consumption.
A CCGT power plant therefore generates electricity twice: first through the gas turbine and then through the steam turbine by recovering heat that would otherwise be wasted.
5. Steam condensation and cycle completion
After leaving the steam turbine, the steam must be condensed back into water before it can return to the HRSG. A condenser removes heat from the exhaust steam using a cooling system. The condensed water is then pumped back through the steam cycle.
The type and performance of the cooling system can affect the overall operation and efficiency of the power plant, particularly under demanding ambient conditions.
Main components of a CCGT power plant
Although CCGT configurations vary depending on plant capacity and design requirements, the main components typically include:
- Gas turbine: converts the energy from combustion gases into mechanical energy that drives the generator.
- Air compressor: compresses the ambient air required for combustion.
- Combustion chamber: mixes compressed air with fuel and produces high-temperature gases.
- Electrical generator: converts mechanical energy from the turbine into electricity.
- Heat Recovery Steam Generator (HRSG): recovers thermal energy from the gas turbine exhaust to generate steam.
- Steam turbine: uses the steam produced in the HRSG to generate additional power.
- Condenser: converts exhaust steam back into water.
- Cooling system: rejects the heat removed during the condensation process.
Together, these systems allow a combined cycle power plant to extract significantly more useful energy from its fuel than a simple-cycle gas turbine.
Combined Cycle Gas Turbine efficiency
Efficiency is one of the main advantages of Combined Cycle Gas Turbine power plants.
In an open cycle configuration, a significant amount of energy leaves the system with the hot exhaust gases. Combined cycle technology captures part of this otherwise wasted thermal energy and uses it to generate additional electricity.
Modern combined cycle power plants can achieve efficiencies above 60% under suitable design and operating conditions, although actual plant efficiency depends on multiple factors, including gas turbine technology, plant configuration, ambient conditions, operating load and equipment performance.
This makes CCGT technology significantly more efficient than conventional open cycle gas turbine configurations.
For a deeper look at the factors behind this performance, read What makes combined cycle power plants so efficient?
CCGT vs. OCGT: what is the difference?
The main difference between Combined Cycle Gas Turbine (CCGT) and Open Cycle Gas Turbine (OCGT) power plants lies in what happens to the gas turbine exhaust heat.
In an OCGT plant, the gas turbine generates electricity and the exhaust gases are discharged without being used in a secondary steam cycle.
In a CCGT plant, the exhaust heat is recovered through an HRSG to produce steam and generate additional electricity.
| CCGT | OCGT | |
|---|---|---|
| Power cycles | Gas + steam | Gas |
| Heat recovery | Yes | No |
| HRSG | Required | Not required |
| Efficiency | Higher | Lower |
| Plant complexity | Higher | Lower |
| Start-up | Generally slower | Faster |
| Typical application | Sustained and efficient power generation | Peak demand and fast-response generation |
The choice between CCGT and OCGT therefore depends on the requirements of each project.
OCGT plants have historically been attractive where rapid start-up, lower capital investment or peak-load operation are priorities.
CCGT power plants require additional equipment and greater initial investment, but their higher efficiency can provide substantial benefits when plants operate for longer periods or fuel efficiency becomes a priority.
For this reason, plant operating profile, expected running hours, fuel prices, electricity market conditions, flexibility requirements and investment costs should all be considered when evaluating both configurations.
What factors affect CCGT power plant performance?
Although plant design and equipment selection determine much of the potential performance of a combined cycle power plant, actual output and efficiency are also affected by operating and environmental conditions.
Relevant factors include:
- Ambient temperature and humidity
- Gas turbine inlet air conditions
- Part-load operation
- Gas turbine degradation
- HRSG performance
- Steam cycle conditions
- Condenser and cooling system performance
- Maintenance and equipment availability
Among these variables, ambient conditions are particularly important for gas turbine performance.
Gas turbines are air-breathing machines. As ambient temperature increases, air density decreases. This can reduce the mass flow entering the compressor and, consequently, decrease gas turbine power output.
The effect can become particularly relevant in hot climates or during periods when high electricity demand coincides with high ambient temperatures.
How Turbine Inlet Air Cooling can improve CCGT performance
Because gas turbine performance is directly affected by inlet air conditions, cooling the air before it enters the compressor can help recover power output lost at high ambient temperatures.
Turbine Inlet Air Cooling (TIAC) systems are designed to reduce and control gas turbine inlet air temperature, increasing air density and mass flow through the turbine.
This allows the gas turbine to operate closer to its design conditions even when ambient temperatures rise.
In a Combined Cycle Gas Turbine plant, the benefits are not limited to the gas turbine itself. Increasing gas turbine output can also influence the energy available to the bottoming cycle and therefore the overall performance of the combined cycle plant.
The actual improvement depends on the gas turbine model, ambient conditions, cooling technology, plant configuration and operating strategy.
ARANER: efficiency in combined cycle plants
Selecting an open cycle gas turbine technology is normally a compromise between fixed cost and marginal cost. This compromise is based on plant utilization. If a facility is to be used regularly, then efficiency is expected to be high. Consequently, the owner must be prepared to pay a higher fixed cost (investment). This means that a CCGT would be the most suitable solution. If the plant operates only for short periods of time, it might be more economically feasible to avoid high fixed cost even if the efficiency is low. That way, recovery of investment over the turbine’s lifetime is almost a guarantee.
Obviously, it is not easy to decide for either OCGT or CCGT considering that you also have to consider fuel availability, demand profile, logistics and so on. To sum up, it is worth stating that with the increasing energy demand, power producers must make technological choices with more keenness. ARANER performs thorough engineering studies to factor in all the constraints of a project. This includes considering TESTIAC technologies for efficiency improvement, which is the trend nowadays. If you like this article, then you must read this about the efficiency of combined cycle power plants
Frequently asked questions about Combined Cycle Gas Turbines
What does CCGT stand for?
CCGT stands for Combined Cycle Gas Turbine. It refers to a power plant configuration that combines a gas turbine with a steam cycle to generate additional electricity from the heat contained in the gas turbine exhaust.
How does a CCGT power plant work?
A CCGT power plant first generates electricity with a gas turbine. The hot exhaust gases then enter a Heat Recovery Steam Generator, which produces steam to drive a steam turbine and generate additional electricity.
What is the difference between CCGT and OCGT?
A CCGT recovers heat from the gas turbine exhaust and uses it in a secondary steam cycle. An OCGT generates electricity only through the gas turbine and does not include this heat recovery steam cycle.
Why are combined cycle power plants more efficient?
Combined cycle power plants are more efficient because they recover thermal energy from gas turbine exhaust gases that would otherwise be wasted and use it to generate additional electricity through a steam turbine.
What is the role of the HRSG in a CCGT plant?
The Heat Recovery Steam Generator recovers thermal energy from the gas turbine exhaust gases and uses it to produce steam. This steam powers the steam turbine, enabling the second power generation cycle.
Can Turbine Inlet Air Cooling increase CCGT power output?
Yes. Turbine Inlet Air Cooling can reduce gas turbine inlet air temperature, increasing air density and helping recover power output during hot ambient conditions. The achievable improvement depends on plant design, turbine characteristics and site conditions.
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