Combined-Cycle Plant Overall Efficiency
Combined-Cycle Plant Overall Efficiency
A combined-cycle power plant pairs a gas turbine (topping cycle) with a steam turbine (bottoming cycle) that runs on the gas turbine's otherwise-wasted exhaust heat, and this combination is why combined-cycle plants are the most efficient large-scale fossil-fuel power generation technology in commercial use today. Because the bottoming cycle only recovers a fraction of the heat the topping cycle rejects (rather than adding independently), the combined efficiency is not simply the sum of the two individual efficiencies — it follows a specific formula that reflects heat cascading from one cycle into the next.
The combined efficiency is eta_cc = eta_top + eta_bot·(1 - eta_top), where the bottoming cycle recovers a fraction of the heat the topping cycle rejects. where eta_top is the topping (gas turbine) cycle efficiency and eta_bot is the bottoming (steam) cycle efficiency applied to the topping cycle's waste heat.
Inputs
eta_top := 0.38=0.38
eta_bot := 0.30=0.3
The topping cycle converts its share of the fuel energy directly to work, and the bottoming cycle then converts part of what the topping cycle rejected, so the two contributions add together to give the overall plant efficiency.
eta_cc := eta_top + eta_bot·(1-eta_top)=0.57
Results
A combined efficiency around 57% is consistent with real-world modern combined-cycle plants, which commonly achieve 55–62% versus roughly 35–40% for a simple-cycle gas turbine or steam plant alone — a substantial fuel savings that is the main reason combined-cycle plants dominate new gas-fired generation. Because the formula shows diminishing returns (the bottoming cycle only acts on the leftover 1 - eta_top), pushing the topping cycle efficiency higher actually shrinks the pool of waste heat available to the bottoming cycle, so plant designers optimize the combination rather than either cycle in isolation. Achieving efficiencies meaningfully above 60% generally requires advances in turbine inlet temperature and materials, not just cycle arrangement.
Assumptions & limitations
- Steady-state operation at full load design conditions.
- Perfect heat recovery boundary between the two cycles is idealized.
- No auxiliary power consumption (pumps, fans) subtracted from the result.
- Both cycle efficiencies assumed known/measured independently beforehand.