Gas turbines and combined cycle: Science and limits

How hotter blades and recycled exhaust push efficiency to 64%

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

A gas turbine sucks in air, squeezes it, burns fuel in it and lets the hot gas expand through a turbine that drives both the compressor and a generator (or, in a jet engine, pushes the aircraft). A combined-cycle plant catches the still-hot exhaust to raise steam for a second turbine, reaching about 64% efficiency, the best of any heat engine in service.

The conversion, step by step

Combustion
Chemical (natural gas or jet fuel) → Heat in compressed air · better than 99% of the fuel burns⁸Gas turbine (Brayton cycle)
Heat → Mechanical (shaft) · about 42-44% of fuel energy in the best simple-cycle units; the exhaust leaves at about 650 C¹²⁷Heat recovery steam generator and steam turbine
Exhaust heat → Mechanical · adds about one third of the combined plant's output¹Generator
Mechanical → Electrical · up to 98.9% (air-cooled)¹⁵Whole combined-cycle plant
Chemical (gas) → Electrical · about 64% rated (net, LHV); about 57% best field performance; about 44% for all US gas-fired plants in 2025 (calculated from EIA's 7,781 Btu/kWh, HHV)¹³Jet engine (turbofan)
Chemical (jet fuel) → Kinetic energy of the aircraft · about 40% overall: up to 55% thermal times 70-80% propulsive⁸

The science

The gas turbine runs the Brayton cycle: compress, heat at roughly constant pressure, expand. Because the compressor absorbs a large share of the turbine's work, efficiency depends on the pressure ratio and above all on how hot the gas entering the turbine is. Modern H- and J-class machines run turbine inlet gas at about 1,600-1,650 C, hotter than the melting point of the nickel superalloy blades, which survive only because cooling air flows through internal passages and out through tiny holes, and a ceramic thermal barrier coating shields the metal. A simple-cycle machine throws away exhaust at about 650 C. Combined cycle uses this as the heat source for a Rankine steam cycle, so the plant uses heat across a far wider temperature span. A jet engine is the same core with the turbine power used to drive a large fan; propulsive efficiency rises when a big fan moves a lot of air slowly rather than a little air fast.

Limits set by science

Limit	Value	Why it exists	Best today
Carnot limit for combined cycle	about 85% (calculated: 1 - 288 K / 1,923 K, for 1,650 C turbine inlet and 15 C ambient air)¹²¹⁶	Heat is not all added at the peak temperature, so the practical ceiling is lower than this.	About 64% rated net efficiency (LHV), roughly three quarters of the Carnot figure (calculated). Measured records: 62.22% at Bouchain, France (2016) and 63.08% gross at Nishi-Nagoya, Japan (2018, Guinness-recognised). Mitsubishi reports more than 64% (LHV) at its T-Point 2 validation plant (company claim).
Ideal Brayton cycle	about 59% at a pressure ratio of 22 (calculated: 1 - 22^(-0.286))¹	The ideal gas-turbine efficiency depends only on pressure ratio; real compressors, turbines and cooling flows cut it.	A 1,600 C, 22:1 machine gives about 41.4%; best ratings are about 42-44%.
Turbine inlet temperature (materials)	about 1,650 C today; 1,710 C quoted for one 60 Hz model¹⁸⁹	Superalloys, single-crystal casting, thermal barrier coatings and ceramic matrix composites set how hot the gas can be. Rhenium is added to some nickel superalloys for strength above 1,000 C.	Ceramic matrix composites run at about 1,200-1,300 C (2,200-2,400 F) and the next combined-cycle goal is 70% net.
Jet engine overall efficiency	ideal thermal limit somewhat above 80%; practical 65-70% thermal; propulsive up to 90-95%⁸	Safety, weight and cost cap what can be built.	About 40% overall, up from about 10% in the late 1940s; a further gain of roughly 7% per decade is judged possible with enough investment.

Sources

Gas Turbines: Past, Present and Future (Efficiency Focus), S.C. Gulen, UTSR Project Review Meeting US DOE National Energy Technology Laboratory, 2021
GE HA Turbine Snags Another World Record for CCGT Efficiency POWER Magazine, 2018
Electric Power Annual, Table 8.1: Average Operating Heat Rate for Selected Energy Sources, 2015 through 2025 US Energy Information Administration, 2026
Commercial Aircraft Propulsion and Energy Systems Research, Chapter 3: Aircraft Gas Turbine Engines National Academies of Sciences, Engineering, and Medicine, 2016
Mineral Commodity Summaries 2026: Rhenium US Geological Survey, 2026
Air-cooled generators (GEN-A) GE Vernova, 2025
GTCC business summary 2025 (investor presentation; market data from McCoy Power Reports) Mitsubishi Heavy Industries, 2025
M501J series gas turbine specifications Mitsubishi Power, 2026