A combined cycle power plant generates electricity in two stages, using the same fuel. A gas turbine burns natural gas to produce power, then the hot exhaust from that turbine feeds a second system that makes steam to drive another turbine. This two-for-one approach pushes efficiency well beyond what either system achieves alone, reaching 60% or higher in modern plants.
How the Two Cycles Work Together
The first cycle is straightforward: natural gas is burned in a combustion chamber, and the expanding hot gases spin a turbine connected to a generator. This is the same basic principle behind a jet engine. On its own, this gas turbine cycle converts roughly 40% of the fuel’s energy into electricity. The rest leaves the turbine as extremely hot exhaust, typically around 500 to 600°C (930 to 1,100°F).
In a simple cycle plant, that heat escapes into the atmosphere. A combined cycle plant captures it instead. The exhaust flows into a heat recovery steam generator (HRSG), essentially a large, high-efficiency boiler. The HRSG transfers heat from the exhaust gases to water, producing steam at multiple pressure levels. That steam then drives a separate steam turbine, generating additional electricity without burning any more fuel.
The steam cycle alone operates at about 30% efficiency. But because it’s running on energy that would otherwise be wasted, the two cycles together achieve a combined efficiency far greater than either one individually. MIT’s thermodynamics calculations show that a 40%-efficient gas turbine paired with a 30%-efficient steam cycle yields a combined efficiency of roughly 58%. The current world record, set at SSE’s Keadby-2 power station in the UK in May 2024, reached 64.2% net thermal efficiency using a Siemens Energy gas turbine. That same plant also holds the record as the most powerful combined cycle facility, producing nearly 850 megawatts.
The Heat Recovery Steam Generator
The HRSG is the bridge between the two cycles and the component that makes the whole concept work. It functions as an energy recovery heat exchanger: hot gases flow across tubes filled with water, transferring their heat without the two streams ever mixing. Modern HRSGs typically operate at three pressure levels (high, intermediate, and low), squeezing as much usable energy from the exhaust as possible. An economizer section preheats the water before it enters the main evaporator, further improving steam generation efficiency.
HRSGs aren’t exclusive to power plants. The same technology recovers waste heat in manufacturing and industrial settings, where the steam can be used for process heating or additional electricity generation. But the combined cycle power plant is their most common and highest-profile application.
Emissions Compared to Coal and Simple Gas
Combined cycle plants produce significantly less pollution per unit of electricity than coal plants or even standard natural gas plants. NOAA data covering 1997 to 2012 found that coal plants emitted an average of 915 grams of CO2 per kilowatt-hour, while standard natural gas plants emitted 549 grams. Combined cycle natural gas plants came in at 436 grams, just 44% of coal’s carbon output per kilowatt-hour.
The difference extends beyond CO2. Combined cycle plants also produce less nitrogen oxide and far less sulfur dioxide per unit of energy than coal. Because they burn natural gas rather than coal, there’s no ash, no mercury, and no coal dust to manage. This emissions profile is a major reason combined cycle plants have replaced coal generation across much of the United States and Europe over the past two decades.
Grid Flexibility and Start-Up Speed
One of the practical advantages of combined cycle plants is their ability to ramp output up and down to match electricity demand. Modern plants typically increase their output at a rate of about 2 to 3 megawatts per minute during start-up. That’s considerably faster than coal or nuclear plants, which can take hours to reach full power.
This flexibility makes combined cycle plants useful as partners for wind and solar generation. When the wind drops or clouds roll in, a combined cycle plant can increase its output to fill the gap. Research into optimal ramp rates has found that pushing start-up speeds beyond about 0.05 MW per second doesn’t improve grid voltage quality any further, meaning there’s a practical ceiling to how fast is useful. The sweet spot balances speed with stable power delivery, making modern combined cycle plants one of the more responsive large-scale generation options available.
Water Use and Cooling
Like all thermal power plants, combined cycle facilities need water to cool the steam back into liquid after it passes through the turbine. Plants draw water from nearby rivers, lakes, or oceans, pass it through a condenser to cool the steam, and return most of it to the source.
That said, combined cycle plants use far less water than coal. In 2021, U.S. combined cycle plants averaged 2,803 gallons of water withdrawn per megawatt-hour of electricity produced. Coal plants averaged 19,185 gallons per megawatt-hour, nearly seven times more. The efficiency advantage explains much of this gap: when you get more electricity from less fuel, you also produce less waste heat that needs cooling. Some plants use dry cooling systems (essentially large air-cooled radiators) to reduce water use further, though these are more expensive and slightly less efficient.
Hydrogen Blending for Lower Carbon Output
Natural gas is a fossil fuel, and even at 436 grams of CO2 per kilowatt-hour, combined cycle plants still produce greenhouse gas emissions. One path toward reducing that footprint is blending hydrogen into the fuel supply. When hydrogen burns, it produces water vapor instead of carbon dioxide.
Several U.S. plants have already tested this approach. The Long Ridge Energy Generation Project in Ohio, a 485-megawatt combined cycle facility, burned a blend containing 5% hydrogen by volume in March 2022. Georgia Power’s Jack McDonough plant tested blends with up to 20% hydrogen in one of its turbines a few months later. Across the industry, operators have successfully tested blends ranging from 5% to 44% hydrogen. The higher the hydrogen fraction, the greater the reduction in CO2, though turbine modifications are needed at higher percentages. Running on 100% hydrogen remains a development goal for several turbine manufacturers.