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Gas turbines burn fuel in compressed air and expand the resulting hot gas through turbine blades. Steam turbines do not burn fuel inside the turbine; they expand high-pressure steam made by a boiler, heat-recovery steam generator (HRSG), or another external heat source. In a combined-cycle power plant, both machines operate in sequence: the gas turbine generates electricity first, its exhaust makes steam in an HRSG, and that steam drives a steam turbine for additional power.
The choice is therefore not simply “gas or steam.” It depends on the available fuel and heat, whether the site needs electricity alone or useful process heat, and whether exhaust energy can be recovered economically.
The essential difference at a glance
| Characteristic | Gas turbine | Steam turbine |
|---|---|---|
| Working fluid | Hot combustion gas produced inside the turbine system | Steam supplied from an external boiler, HRSG, or other heat source |
| How heat enters the cycle | Air is compressed, fuel is added and burned, and the hot gas expands through the turbine | Water is heated elsewhere to make high-pressure steam, which then expands through the turbine |
| What the turbine shaft drives | The compressor and an electrical generator | Usually an electrical generator; in some industrial arrangements, mechanical equipment |
| Typical plant arrangement | Simple cycle or the gas-turbine stage of a combined cycle | Standalone steam cycle, the steam stage of a combined cycle, or an industrial CHP system |
| Heat available after expansion | Very hot exhaust that can be released, used for process heat, or recovered in an HRSG | Exhaust steam must be condensed, returned, or sent to a process load according to the plant design |
| Fuel requirement at the turbine | Direct fuel combustion; equipment may be designed for more than one fuel | No fuel is burned in the turbine itself; the upstream steam generator determines the heat source |
A gas turbine describes a machine and its working fluid. A combined cycle describes a plant arrangement. A combined-cycle plant can therefore contain both a gas turbine and a steam turbine rather than choosing one type exclusively.
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The U.S. Department of Energy describes three principal sections in a gas-turbine power system: the compressor, the combustion system, and the turbine. The same rotating shaft normally connects the turbine to the compressor and to an electrical generator.
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1. Compressing the air
Air enters the compressor, where rotating and stationary blade rows raise its pressure. Compressing the air prepares it for efficient combustion and provides the high-pressure flow needed for expansion through the turbine.
2. Adding fuel and burning the mixture
Fuel is injected into the compressed air in the combustion system. The mixture burns continuously, creating a stream of high-temperature, high-pressure gas. The combustion chamber is part of the gas-turbine system; the plant does not need a separate steam boiler to make the gas that turns the turbine.
3. Expanding the hot gas
The combustion gas expands through turbine blade stages. As it loses pressure and temperature, it gives up energy to the rotating shaft. Part of that shaft power runs the compressor; the rest is available to drive a generator or other equipment.
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4. Managing the exhaust
In a simple-cycle plant, the exhaust leaves after passing through the turbine, carrying substantial heat away. In a combined-cycle plant, that exhaust is routed to an HRSG instead of being treated only as waste heat. Gas-turbine exhaust can also serve a site that needs steam or process heat directly.
Heavy-frame and aeroderivative designs
Land-based gas turbines include heavy-frame machines and aeroderivative machines. They are different design families with different size and operating characteristics. The appropriate family depends on the required output, site conditions, operating profile, and integration with the rest of the plant; the label alone does not establish which design is better.
How a steam turbine produces power
A steam turbine is an expander, not a boiler. It receives steam made outside the turbine and converts the steam’s pressure and temperature into shaft work as the steam flows through blade stages.
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1. Making and conditioning the steam
An upstream heat source raises water into high-pressure steam. That source may be a conventional boiler that burns fuel, an HRSG heated by gas-turbine exhaust, or another process that supplies suitable steam conditions. The steam generator, fuel system, feedwater equipment, and water-treatment equipment are separate plant systems from the turbine itself.
2. Expanding through the turbine
High-pressure steam enters the first turbine stages and expands as it passes through stationary nozzles and rotating blades. The pressure drop produces torque on the shaft, which normally turns an electrical generator. Additional stages can extract energy as the steam reaches lower pressures.
3. Using or condensing the outlet steam
The outlet arrangement reflects the plant’s purpose. In an electricity-focused plant, the exhaust may go to a condenser so water can be returned to the steam cycle. In combined heat and power (CHP), steam can instead be extracted or discharged to a process load, with the remaining steam continuing through the turbine when the configuration allows it.
Where steam turbines are used
DOE identifies steam turbines in central-station generation, industrial facilities, commercial facilities, and CHP systems. Their long-established technology makes them suitable where a facility already has a dependable steam source or has a large, continuous need for process steam.
Gas turbine versus steam turbine: the plant-cycle distinction
Confusion often comes from treating a machine name as though it were a complete cycle. A gas turbine includes compression, combustion, and expansion in one integrated train. A steam turbine is one part of a steam cycle whose heat addition occurs elsewhere.
- “Gas turbine” identifies the turbine that expands combustion gas.
- “Steam turbine” identifies the turbine that expands externally produced steam.
- “Simple cycle” usually means a gas turbine produces power while its exhaust heat is not used to make a second power-producing cycle.
- “Combined cycle” means a gas-turbine cycle and a steam cycle are thermally linked so the gas-turbine exhaust makes steam for a steam turbine.
- “CHP” or cogeneration means the plant supplies electricity plus useful heat, such as process steam; it may use a gas turbine, a steam turbine, or both.
Thus, comparing “a gas turbine” with “a steam turbine” without naming the surrounding plant can produce a misleading result. A standalone gas turbine and a combined-cycle plant are not equivalent boundaries, and a steam turbine supplied by a fuel-fired boiler is not equivalent to one supplied by recovered exhaust heat.
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How a combined-cycle power plant uses both turbines
Combined cycle captures energy that would otherwise leave in the gas-turbine exhaust. The gas turbine is the first stage and the steam cycle is the second.
- Compress air: The gas turbine draws in air and raises its pressure in the compressor.
- Burn fuel: Fuel mixes with the compressed air and burns in the combustion system.
- Generate first-stage power: Expanding combustion gas turns the gas-turbine shaft and generator.
- Recover exhaust heat: Instead of sending the hot exhaust directly away, the plant passes it through an HRSG.
- Make steam: Heat transferred in the HRSG converts feedwater into steam at conditions selected for the steam cycle.
- Generate second-stage power: The steam expands through a steam turbine connected to another generator or the same generating train, depending on the design.
- Return or use the steam: Exhaust steam is condensed and recycled, extracted for a process load, or directed according to the plant’s CHP configuration.
The two turbines therefore perform different jobs in sequence. The gas turbine converts fuel energy directly to shaft work and leaves high-temperature exhaust. The steam turbine converts part of that remaining heat into additional shaft work.
Efficiency: why the percentage needs a boundary
The DOE explanatory page states that a simple-cycle gas turbine can achieve energy-conversion efficiencies of 20% to 35%. This is DOE’s stated range for simple-cycle gas turbines on an undated page crawled in 2026, not a guarantee for a particular machine, fuel, ambient condition, or operating point.
That range should not be treated as a universal ranking against every steam-turbine plant. The comparison changes when an HRSG and steam turbine are added, because the plant now produces additional electricity from exhaust heat. It also changes when useful process heat is counted.
Electricity-only efficiency
An electricity-only figure counts electrical output against the energy supplied to the plant. For a simple-cycle gas turbine, the boundary generally includes the fuel entering the gas-turbine system and the electricity leaving the generator. For a combined cycle, the boundary includes both the gas-turbine and steam-turbine outputs and the systems needed to recover heat.
Total useful-energy efficiency in CHP
CHP calculations can count electricity plus useful heat delivered to a process. The DOE-hosted CIBO Energy Efficiency Handbook presents cogeneration figures in this kind of boundary. Such overall thermal-efficiency values cannot be compared directly with an electricity-only conversion percentage: they count useful heat as an output and describe a different service from the plant.
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Questions to ask before comparing two percentages
- Is the figure for a simple cycle, a combined cycle, or a CHP plant?
- Does the output include only electricity, or electricity plus useful steam or heat?
- Are the numbers for the turbine alone or for the complete plant, including auxiliaries?
- What fuel, ambient conditions, load level, and operating date apply?
- Is the number a design value, a guaranteed value, or an observed operating result?
Without those qualifications, a larger percentage may simply reflect a broader output definition rather than a more efficient turbine in the same service.
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CHP: when the exhaust or steam has another job
Gas turbines are useful in CHP because their exhaust remains hot after electricity is generated. A facility can recover that heat to make steam, heat water, or support a process. DOE’s gas-turbine CHP fact sheet also notes that gas turbines can be designed for multiple fuels, although the exact fuel options and performance depend on the equipment and site.
Steam turbines are also common in industrial and commercial CHP. A facility that already produces high-pressure steam may expand that steam through a turbine before sending lower-pressure steam to manufacturing, heating, or another process load.
A combined-cycle cogeneration plant can divide steam among several destinations: some can expand through the steam turbine for more electricity, while some can go directly to a process. The best split depends on the site’s heat demand, electric demand, steam pressure requirements, and operating schedule. Maximizing electricity is not always the same as maximizing the value of the plant’s useful energy.
Comparison axes that matter in a real project
| Decision axis | Gas-turbine emphasis | Steam-turbine emphasis | What to verify |
|---|---|---|---|
| Primary energy input | Fuel is burned in the gas-turbine combustion system | Steam must be supplied by a boiler, HRSG, or another heat source | Fuel availability, steam availability, and delivered energy cost |
| Heat recovery | Hot exhaust can feed an HRSG or process load | Steam can be expanded for power and then used or condensed | Whether the site can use recovered heat throughout the year |
| Plant configuration | Simple cycle or first stage of a combined cycle | Standalone steam cycle or second stage of a combined cycle | Required electrical output and the value of a second cycle |
| Fuel flexibility | Multiple-fuel capability may be available on selected equipment | Fuel choice is primarily a property of the upstream boiler or heat source | Approved fuels, emissions limits, storage, and switching requirements |
| Site role | Electricity generation, exhaust-heat recovery, or CHP | Central-station generation, industrial power, commercial power, or CHP | Whether the site is electricity-led, steam-led, or balanced |
| Efficiency metric | Simple-cycle electricity efficiency or combined-cycle output | Electricity from steam expansion, or total CHP useful energy | Identical system boundaries and operating assumptions |
Choosing among simple cycle, steam cycle, and combined cycle
A simple-cycle gas turbine fits when
- The project needs a gas-turbine generator without the added equipment of an HRSG and steam turbine.
- Exhaust heat has no dependable process use and the additional recovery system would not justify its complexity.
- The site can provide an appropriate fuel and wants a gas-turbine arrangement rather than an externally generated steam supply.
A steam-turbine plant fits when
- A boiler, industrial process, or other heat source already produces the required steam.
- The facility has a substantial and predictable steam demand as well as an electricity requirement.
- A central-station, industrial, commercial, or CHP application can use a mature steam-cycle technology.
A combined-cycle plant fits when
- The project can use gas-turbine exhaust heat to make steam.
- Additional electricity from the steam turbine is valuable enough to warrant the HRSG and steam-cycle equipment.
- The plant can manage the linked operation of two cycles and, where relevant, a process-steam load.
This framework does not produce a universal winner. A site with no use for steam may favor a simpler arrangement, while a site with sustained heat demand may gain more value from recovery or CHP than from electricity-only operation.
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Common misconceptions and the precise correction
“A steam turbine includes the boiler.”
No. The turbine converts steam energy to shaft work. The boiler, HRSG, or other heat source makes the steam and is a separate part of the plant.
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“Combined cycle is another kind of turbine.”
No. Combined cycle is a plant configuration that links a gas-turbine cycle to a steam cycle. The configuration contains both turbine types.
“The highest efficiency number always identifies the better plant.”
Only if the numbers use the same boundary and serve the same purpose. Electricity-only and CHP useful-energy efficiencies answer different questions.
“Gas versus steam means a plant must select one machine.”
No. Combined-cycle and CHP arrangements deliberately use both machines or route steam to a process load as well as a turbine.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match“All gas turbines have the same fuel and performance options.”
No. Multiple-fuel capability and operating performance depend on the selected equipment and site conditions.
A practical checklist for evaluating a proposed plant
- Define the output: State the required electrical output and whether process steam, hot water, or other useful heat is also required.
- Identify the heat source: Determine whether the plant will burn fuel in a gas turbine, make steam in a boiler, recover exhaust heat in an HRSG, or combine these sources.
- Set the comparison boundary: Decide whether the key metric is electricity-only efficiency or total useful energy including recovered heat.
- Map the operating profile: Describe expected load, seasonal heat demand, and the consequences of running one cycle when the other is unavailable.
- Check fuel and steam constraints: Verify available fuels, steam pressure and temperature, water systems, and process requirements before selecting turbine equipment.
- Compare complete plant configurations: Include the compressor, combustion system, boiler or HRSG, generators, condenser or process connection, and auxiliary systems rather than comparing turbine nameplates alone.
- Document assumptions: Record the equipment class, site conditions, date of the efficiency claim, and whether the figure is a design, guarantee, or operating value.
Gas turbines and steam turbines are complementary technologies with different working fluids and different places in a power plant. The gas turbine creates hot expanding gas inside its own combustion system; the steam turbine waits for steam made elsewhere. Once that distinction is clear, the role of a combined-cycle plant follows naturally: use the gas turbine first, recover its exhaust heat in an HRSG, and use the resulting steam in a second turbine or in a process load.
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