Energy

Combined-Cycle Power Plant

Stacking two engines to hit 60% efficiency

A combined-cycle power plant is a thermal power station that runs a gas turbine (Brayton cycle) and a steam turbine (Rankine cycle) in series, reusing the gas turbine's hot exhaust to raise steam instead of dumping it to atmosphere. That single move — recovering waste heat through a heat recovery steam generator — lifts overall thermal efficiency from the ~40% of a standalone gas turbine to roughly 60–64%, the highest of any fuel-burning power plant in commercial service. The bottoming steam cycle produces about a third of the total power from heat that would otherwise be lost, all without burning a single extra unit of fuel.

Net efficiency (best in service)~64% (LHV)
Topping cycleBrayton (gas turbine)
Bottoming cycleRankine (steam turbine)
Turbine inlet temp~1,500–1,600 °C
Exhaust into HRSG~550–650 °C
Gas:steam power split≈ 2 : 1

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Two cycles, one fuel

Every heat engine is bounded by the same hard limit: it can only turn heat into work by moving energy from a hot reservoir to a cold one, and the most it can extract is set by the temperature span. The Carnot efficiency is the ceiling:

η_Carnot = 1 − T_cold / T_hot      (temperatures in kelvin)

A gas turbine alone:
  T_hot  ≈ 1,773 K  (1,500 °C inlet)
  T_cold ≈   873 K  (  600 °C exhaust, dumped to air)
  η_Carnot ≈ 1 − 873/1773 ≈ 0.51

The problem: the 600 °C exhaust is still blisteringly hot.
A simple-cycle plant throws that heat away.

A standalone gas turbine running the Brayton cycle — compress air, burn fuel, expand through a turbine — converts about 35–42% of the fuel's energy to shaft work and vents the rest as 550–650 °C exhaust. A standalone steam plant running the Rankine cycle — boil water, expand steam through a turbine, condense, pump back — manages a similar 35–42%, but its top temperature is limited by metallurgy and water chemistry to roughly 600 °C.

The combined-cycle insight is to bolt them together so they cover different parts of the same temperature ladder. The Brayton cycle takes the top rungs (1,500 °C down to ~600 °C); its exhaust becomes the heat source for a Rankine cycle that takes the lower rungs (~600 °C down to ~30 °C in the condenser). Stacking the two efficiencies is multiplicative on what each leaves behind:

η_combined = η_Brayton + η_Rankine · (1 − η_Brayton)

With η_Brayton = 0.40, η_Rankine = 0.33:
  η_combined = 0.40 + 0.33 × (1 − 0.40)
             = 0.40 + 0.33 × 0.60
             = 0.40 + 0.198
             ≈ 0.60   →  60%

The steam turbine is, in effect, harvesting the gas turbine's trash. Because it burns no fuel of its own, every kilowatt it produces is pure efficiency gain.

The heat recovery steam generator

The component that glues the two cycles together is the heat recovery steam generator (HRSG) — a tall duct full of finned-tube banks sitting in the gas turbine exhaust. It is a boiler with no flame: the only heat input is the turbine's exhaust gas flowing across the tubes. Feedwater enters cold, picks up heat as it climbs through the tube banks, and leaves as superheated high-pressure steam.

A single boiling pressure wastes exergy, because water boils at one fixed temperature while the gas cools continuously. The mismatch is largest at the pinch point, the spot where the gas temperature comes closest to the boiling temperature. Modern HRSGs use two or three pressure levels — typically high-pressure (~120 bar), intermediate-pressure (~25 bar) and low-pressure (~4 bar) circuits — so steam is raised at several temperatures, hugging the gas cooling curve and recovering more heat. High-end designs add a reheat stage, sending partly-expanded steam back through the HRSG to be reheated before the next turbine stage.

Heat recovered in the HRSG:
  Q_HRSG = m_gas · c_p · (T_exhaust − T_stack)

  m_gas    = exhaust mass flow (kg/s)
  c_p      ≈ 1.1 kJ/(kg·K) for hot combustion gas
  T_exhaust ≈ 600 °C  (gas turbine outlet)
  T_stack   ≈ 90–110 °C  (limited by acid dew point)

The lower the stack temperature, the more heat is recovered —
but stack gas must stay above the sulfuric/acid dew point
or the cold end of the HRSG corrodes.

How the cycles stack up

	Simple-cycle gas turbine	Coal steam plant	Combined cycle (CCGT)
Cycle(s)	Brayton only	Rankine only	Brayton + Rankine
Net efficiency (LHV)	35–42%	33–40%	58–64%
Top temperature	~1,500 °C (turbine inlet)	~600 °C (superheater)	~1,500–1,600 °C
Heat rejected	Hot exhaust to air (~600 °C)	Condenser to cooling water (~30 °C)	Stack ~100 °C + condenser ~30 °C
Cold start to full load	~10–15 min	4–8 hours	30–60 min (steam side limits)
Ramp rate	Very fast	Slow	Fast (gas), slow (steam)
Capital cost ($/kW)	Low (~$700)	High (~$3,000+)	Medium (~$1,000–1,300)
Typical role	Peaking / backup	Baseload (declining)	Baseload + load following
CO₂ per MWh	~550 kg	~900 kg	~350 kg

Worked example: a 400 MW block

Take a representative single-shaft CCGT block firing natural gas. The gas turbine and steam turbine sit on one shaft driving a shared generator.

Fuel input (LHV)        : 625 MW of natural gas
Gas turbine output      : 270 MW   (η_GT = 270/625 = 43%)
Gas turbine exhaust     : ~340 MW of heat at 600 °C
HRSG recovers           : ~290 MW into steam (~85% of exhaust heat)
Steam turbine output    : 130 MW   (η_ST on recovered heat ≈ 45%)

Combined electrical output = 270 + 130 = 400 MW
Combined efficiency        = 400 / 625 = 64%

Gas : steam power split    = 270 : 130 ≈ 2.1 : 1

The steam turbine adds 130 MW — about a third of the block — from heat that a simple-cycle plant would have sent up the stack. That is the entire economic case for combined cycle: a third more power, same fuel bill.

Why turbine inlet temperature is everything

Every point of combined-cycle efficiency is bought with hotter combustion. Carnot rewards a higher T_hot, so manufacturers push turbine inlet temperatures relentlessly upward — but the gas is now hotter than the melting point of the very blades it flows over. Three technologies make this possible:

Single-crystal superalloy blades. Nickel-based alloys grown as a single crystal eliminate grain boundaries, which are the first thing to creep and crack at temperature. A first-stage blade may cost as much as a small car.
Thermal barrier coatings (TBCs). A ceramic layer (yttria-stabilised zirconia, ~0.3 mm) on the blade surface drops the metal temperature by 100–150 °C for the same gas temperature.
Film cooling. Cooler compressor air is bled through hundreds of laser-drilled holes in each blade, forming a thin protective film of cool air over the surface. This lets blades survive in gas hundreds of degrees above their melting point.

The payoff: raising turbine inlet temperature from 1,300 °C (F-class) to 1,600 °C (J-class) lifted combined-cycle efficiency from ~57% to ~64% over two decades.

Trade-offs and failure modes

Slow steam-side start-up. Thick-walled HRSG drums and the steam turbine rotor develop dangerous thermal stress if heated too fast. A hot restart may take 30 minutes; a cold start an hour. Bypass stacks and steam-bypass valves let the gas turbine run while the steam side warms gradually.
Cold-end corrosion in the HRSG. Drive the stack temperature too low to grab the last bit of heat and the exhaust drops below the acid dew point, condensing sulfuric and nitric acid onto the coldest tubes. Economizer tubes then pit and fail.
Thermal-fatigue cracking from cycling. Plants that follow load start and stop daily. Each cycle thermally shocks HRSG headers and turbine casings; low-cycle fatigue cracks are the dominant maintenance issue for flexible CCGTs.
Efficiency falls off at part load. Best efficiency is at full load. Below ~50% load, the gas turbine's compressor inlet guide vanes close down, exhaust temperature and efficiency both sag, and the plant heat rate worsens.
Hot-section creep and oxidation. The blades that enable high efficiency live close to their limit. Creep, oxidation and TBC spallation set inspection intervals measured in fired hours and start counts, not calendar years.
Water and steam chemistry. The Rankine side demands ultra-pure feedwater. Even trace dissolved oxygen or silica causes pitting in the HRSG and deposits on the last steam-turbine blades, where the steam is wet and erosion-prone.

Configurations you will meet

Single-shaft. Gas turbine, steam turbine and one generator on a common shaft. Compact, high efficiency, but the whole block trips together.
Multi-shaft (2-on-1, 3-on-1). Two or three gas turbines each feed their own HRSG, and the combined steam drives one large steam turbine. Flexible — you can run one gas turbine in simple cycle while the steam side is offline.
Supplementary (duct) firing. Burners added in the HRSG inlet duct boost steam output on hot days or peak demand, using leftover oxygen in the exhaust. It adds capacity at the cost of marginal efficiency.
Combined heat and power (CHP / cogeneration). Bleed steam from the bottoming cycle for district heating or process steam. Electrical efficiency drops, but total fuel utilisation can exceed 80%.

Frequently asked questions

What is a combined-cycle power plant?

A combined-cycle power plant generates electricity from two thermodynamic cycles in series. A gas turbine runs the Brayton cycle, burning natural gas to spin a generator. Its hot exhaust — typically 550 to 650 °C — is not vented but routed through a heat recovery steam generator (HRSG) that boils water into high-pressure steam. That steam drives a second, steam turbine running the Rankine cycle. Recovering the gas turbine's waste heat is what lets the combined plant reach about 60% efficiency versus roughly 40% for the gas turbine alone.

Why is a combined cycle more efficient than a single cycle?

A single gas turbine rejects 55 to 60% of its fuel energy as hot exhaust straight to the atmosphere. Carnot's theorem says you can only extract work between a hot source and a cold sink, and a single cycle uses just one such temperature span. A combined cycle uses the gas turbine's exhaust as the heat source for a second cycle, so the total system spans from the turbine inlet (~1,500 °C) down to the condenser (~30 °C). Capturing two temperature drops rather than one is why combined efficiency (~60%) exceeds either cycle alone (~40% Brayton, ~40% Rankine).

What does the heat recovery steam generator (HRSG) do?

The HRSG is a large finned-tube heat exchanger that sits in the gas turbine exhaust duct. It has no burner of its own — the gas turbine exhaust is the only heat source. As exhaust flows over the tube banks it boils feedwater into steam, usually at two or three pressure levels (e.g. high-pressure ~120 bar, intermediate ~25 bar, low-pressure ~4 bar) to extract heat across a wide temperature range. Multiple pressure levels reduce the temperature mismatch between gas and water, cutting exergy loss and adding a few points of plant efficiency.

How efficient can a combined-cycle plant get?

Modern H-class and J-class combined-cycle gas turbine (CCGT) plants reach 62 to 64% net lower-heating-value efficiency. The current verified record is just above 64%. Efficiency rises with turbine inlet temperature (now ~1,600 °C in the latest machines, enabled by single-crystal blades, thermal barrier coatings and film cooling) and with three-pressure reheat HRSG designs. By comparison, a typical simple-cycle gas turbine is 35 to 42% and a coal steam plant is 33 to 40%.

What is the ratio of gas turbine to steam turbine power?

In a typical CCGT, the gas turbine produces about two-thirds of the total output and the steam turbine produces about one-third. A common rule of thumb is a 2:1 power split. For example, a 400 MW block might come from a ~270 MW gas turbine and a ~130 MW steam turbine fed by the gas turbine's exhaust. Because the steam side is powered entirely by recovered waste heat, that one-third of output is produced with essentially no additional fuel.

Why are combined-cycle plants used for load following?

Gas turbines start fast — a few minutes to part load — and can ramp at tens of MW per minute, which makes CCGT plants well suited to balancing variable wind and solar. The trade-off is the steam bottoming cycle: thick-walled HRSG drums and steam turbine rotors must be heated slowly to avoid thermal-stress cracking, so a cold start of the full combined cycle can still take 30 to 60 minutes. Plants use bypass stacks and steam-bypass systems so the gas turbine can run while the steam side warms up gradually.