How a combined-cycle gas power plant works
A combined-cycle plant recovers more energy from its fuel: a gas turbine generates electricity, then its hot exhaust makes steam for a second turbine.
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Combined-cycle plant with two generator shafts. The illustrated heat-recovery circuit uses a single steam pressure; many real plants use multiple pressure levels. Motion is slowed for inspection. Coloured overlays reveal flow through enclosed equipment; gold pulses show net electrical energy transfer, not individual electrons.
How it works
- Fuel energy
- Combustion heat
- Gas-turbine + steam-cycle work
- Generators
- Grid electricity
A gas-fired station converts fuel energy into shaft rotation and then electrical power. In a gas turbine, air is compressed before fuel burns in it. Expanding hot gas turns turbine blades, supplying both the compressor and the generator. This continuous-flow arrangement follows the Brayton-cycle principle. The compressor consumes a substantial part of the turbine’s mechanical output, so the generator receives the remaining shaft power rather than everything the turbine develops.
The illustrated combined-cycle plant adds a second engine: a steam turbine. Exhaust heats water through the walls of a heat-recovery steam generator, without mixing combustion gas with steam. The steam circuit follows the Rankine-cycle principle; cooling and pumping return its water for reuse. Both generators supply the station’s electrical system, while pumps, fans and other auxiliaries consume some output. Recovering exhaust heat increases electricity obtained from the fuel; it does not eliminate combustion emissions or the need to reject unused heat.
Inside the engineering
Follow the machinery, the working fluids and the electrical connection.
Mechanical
An axial compressor, combustors and expansion turbine form the gas-turbine train. The turbine drives both its compressor and generator. A separate steam turbine uses recovered exhaust heat, with an air-cooled condenser returning water to the feedwater circuit.
Thermal & chemical
Fuel burns in compressed air. The gas turbine follows the Brayton-cycle principle; the heat-recovery steam circuit follows the Rankine-cycle principle. Exhaust gas and steam exchange heat across tube walls without mixing. Combustion produces CO₂ and can form nitrogen oxides.
Electrical
Both generator trains require electrical export paths. Their output passes through switchgear and step-up transformation to a common network. Electrical energy delivered to the grid is less than gross generation because the station consumes power itself.
Main types and variations
The model shows a combined-cycle plant with separate gas- and steam-turbine generators. These alternatives describe machinery and heat use, not different names for the fuel. A gas engine, a gas turbine and a CHP installation can all consume natural gas; approved alternative fuels depend on the equipment.
Open-cycle gas turbine
Also called simple cycle, this arrangement generates electricity from the gas-turbine shaft without a bottoming steam turbine. Air enters, fuel burns and the exhaust leaves after expansion. The turbine still drives its compressor as well as its generator, and much of the unused energy leaves as exhaust heat.
What changes in practice
Removing the steam plant simplifies the installation. Compared with adding heat recovery for power generation, more fuel energy remains unused in the exhaust.
Combined-cycle gas turbine
A heat-recovery steam generator captures gas-turbine exhaust energy for a steam turbine. The two cycles share an energy source while keeping their working fluids separate. Plants may put the turbines on a common generator shaft or use separate generators; the model illustrates the latter arrangement.
What changes in practice
It obtains additional electricity from heat that would otherwise be discarded. More equipment, water treatment and steam-system controls accompany that gain in electrical efficiency.
Reciprocating gas engine
Fuel burns inside cylinders, pushing pistons connected to a crankshaft and generator. Many natural-gas engines use spark ignition; other designs use a small pilot fuel charge. A station can operate several engine-generator sets, starting or stopping units as the required electrical output changes.
What changes in practice
Modular units can follow changing demand while keeping operating engines usefully loaded. Exhaust and engine-cooling heat can also serve a nearby heat customer.
Gas combined heat and power
A turbine or engine produces electricity while heat exchangers recover energy for steam, hot water or an industrial process. CHP describes this useful heat supply, rather than one particular engine. The recovered heat must reach a real consumer instead of simply being rejected to the surroundings.
What changes in practice
The useful output includes both heat and electricity. Performance depends on matching the plant to the customer’s heat demand as well as its electrical needs.
Worth knowing
- Combined-cycle plants are among the most efficient fossil-fuel plants.
- Open-cycle gas turbines skip the steam stage: less efficient, but they start quickly for peak demand.
- Burning gas emits carbon dioxide; methane leaks upstream add to its climate impact.
See real examples
The atlas includes 4,090 gas records worldwide.
Show on the mapRelated answers
Textbook references
The science on this page follows these standard engineering textbooks. See the full bibliography.
- Combined-Cycle Gas & Steam Turbine Power PlantsGas turbines, heat-recovery steam generators and combined-cycle performance.
- Thermodynamics: An Engineering ApproachBrayton and combined Brayton–Rankine cycle thermodynamics.
Sources
- EIA: Use of natural gas ↗
- US DOE: Gas turbines and combined-cycle heat recovery ↗
- EIA: How electricity is generated ↗
Cutaways reveal enclosed equipment. Layouts, dimensions, materials and animation speeds are illustrative; this is an educational model, not a plant design or a thermodynamic simulation.