Combined-Cycle Power Plants — Stacking Cycles for 60%+ Efficiency
Combined-Cycle Power Plants — Stacking Cycles for 60%+ Efficiency
A gas turbine exhausts at 500–600°C — far too hot to waste. A combined-cycle gas turbine (CCGT) plant captures this exhaust heat in a heat recovery steam generator (HRSG) and uses it to drive a steam turbine. Two cycles, one fuel stream, extraordinary efficiency.
The Math of Stacking Cycles
If the gas turbine has thermal efficiency $\eta_g$ and the steam bottoming cycle recovers a fraction $\varepsilon$ of the exhaust heat with efficiency $\eta_s$, the combined overall efficiency is:
$$\eta_{CC} = \eta_g + (1 - \eta_g) \cdot \varepsilon \cdot \eta_s$$
The first term is gas turbine output. The second term is steam cycle output from otherwise-wasted exhaust heat. With $\eta_g = 38\%$, $\varepsilon = 85\%$, $\eta_s = 35\%$:
$$\eta_{CC} = 0.38 + (1 - 0.38) \times 0.85 \times 0.35 = 0.38 + 0.184 = 56.4\%$$
The world record combined-cycle efficiency stands at 63.08% (GE HA-class turbine, 2016) — verified by independent measurement.
Why This Matters — The Economics of Efficiency
A 500 MW power plant running at 40% vs 60% efficiency burns:
- At 40%: $P_{fuel} = 500/0.40 = 1{,}250$ MW of fuel
- At 60%: $P_{fuel} = 500/0.60 = 833$ MW of fuel
Difference: 417 MW of fuel saved, continuously. At $\$6$/GJ, that's $\$22$ million per year per plant — and a corresponding reduction in CO$_2$ emissions of roughly 750,000 tonnes per year.
Heat Recovery Steam Generator (HRSG)
The HRSG is a sophisticated heat exchanger that extracts energy from the 550°C exhaust gas as it cools toward the stack temperature ($\sim 80$–$120$°C). The heat recovery fraction is:
$$\varepsilon = 1 - \frac{T_{stack} - T_{cond}}{T_{exhaust} - T_{cond}}$$
Minimising stack temperature maximises heat recovery — but risks acid condensation (SO$_3$/H$_2$O) on heat transfer surfaces, requiring careful temperature management.
Gas turbine: $\eta_g = 42\%$, exhaust at 625°C. Steam cycle: condenser at 30°C, $\eta_s = 35\%$, heat recovery $\varepsilon = 87\%$. Find $\eta_{CC}$.
- $\eta_{CC} = 0.42 + (1 - 0.42) \times 0.87 \times 0.35 = 0.42 + 0.177 = 59.7\%$
- Nearly 60%! In practice the GE 7HA achieves 61.4% in combined-cycle configuration — the difference comes from multi-pressure steam cycles and advanced turbine cooling not captured in this simple model.
Combined-cycle plants now generate more than 25% of global electricity. As the world adds renewable energy, fast-starting gas turbines and CCGT plants provide the flexible backup power needed to balance solar and wind variability. The thermodynamics haven't changed since Brayton and Rankine — but the engineering keeps pushing efficiency higher.