Model a power plant that pairs a gas turbine (Brayton cycle) with a steam turbine (Rankine cycle). Adjust each turbine's efficiency, the fuel input and the heat-recovery rate to see the overall thermal efficiency, the output split and the stack loss update in real time — and feel why this is the most efficient thermal power station.
Parameters
Gas turbine efficiency η_gas
Stand-alone efficiency of the topping (Brayton) cycle
Steam turbine efficiency η_steam
Stand-alone efficiency of the bottoming (Rankine) cycle
Fuel heat input
MW
Rate of chemical energy supplied by the fuel
HRSG heat-recovery rate η_HRSG
Fraction of gas turbine waste heat captured by the HRSG
Results
—
Combined efficiency η_cc (%)
—
Total electrical output (MW)
—
Gas turbine output (MW)
—
Steam turbine output (MW)
—
Stack loss (MW)
—
Efficiency gain vs gas turbine alone (points)
—
Plant layout — energy flow
Fuel enters the gas turbine, its hot exhaust raises steam in the HRSG, and that steam drives the steam turbine. Arrow widths are proportional to the energy (MW) carried, and particles show the flowing energy.
Overall thermal efficiency of the combined cycle. The first term is the gas turbine (topping cycle); the second term is the bottoming-cycle contribution — the waste heat (1−η_gas) the gas turbine dumps, captured by the HRSG at rate η_HRSG and turned into electricity by the steam turbine at efficiency η_steam.
$$P_{total}=P_{gas}+P_{steam}$$
Total electrical output is the sum of the gas turbine and steam turbine outputs. The steam (bottoming) cycle harvests the gas turbine's exhaust heat, so it adds power without burning any extra fuel.
What is a Combined Cycle?
🙋
I keep hearing the term "combined-cycle power generation". How is it different from an ordinary thermal power plant?
🎓
Roughly speaking, it is a "two-stage turbine" scheme. First you burn fuel and spin a gas turbine — that is the Brayton cycle. The exhaust gas that comes out is still very hot, around 500-650 °C. An ordinary gas turbine simply dumps it up the stack. A combined cycle instead uses that exhaust to boil water once more and spin a steam turbine — the Rankine cycle. The key idea is "generate power once again from the heat you were about to throw away".
🙋
Wow, the exhaust is that hot? Throwing it away really is a waste, isn't it?
🎓
Exactly — that is the whole point. Set the "gas turbine efficiency" slider on the left to 0.38. Even with 200 MW of fuel going in, the gas turbine only converts 76 MW into electricity. The remaining 124 MW leaves as exhaust gas. Run the turbine alone and that 124 MW is completely wasted. A combined cycle routes that 124 MW through the heat-recovery steam generator, the HRSG for short.
🙋
What exactly does the HRSG do?
🎓
Heat Recovery Steam Generator — a heat-recovery boiler. It is a huge heat exchanger that passes the hot exhaust gas through and uses that heat to turn feedwater into steam. With the "HRSG recovery rate" slider at 0.85, 105.4 MW of the 124 MW becomes steam; the remaining 18.6 MW escapes up the stack. When the steam turbine runs on that 105.4 MW of heat at an efficiency of 0.33, you get about 34.8 MW of extra generation. Look at the plant layout — the arrow widths show how big each energy stream is.
🙋
Adding it up — 76 MW of gas plus 34.8 MW of steam is 110.8 MW... the efficiency is over 55%! Impressive.
🎓
Right. η_cc = 110.8 ÷ 200 ≈ 55.4%. The gas turbine alone was 38%, so it climbed 17 points — and that gain came for free, from heat that was destined for the stack. The latest combined cycles raise the turbine inlet temperature and reach nearly 64%. Conventional steam plants are 40% and even nuclear is around 33%, so as a thermal plant a combined cycle is dramatically efficient. That is why almost every new gas-fired plant built today is a combined cycle.
🙋
So if I want to push the efficiency even higher, what should I change?
🎓
There are three directions. The first is to raise the gas turbine efficiency η_gas — the classic move is a higher inlet gas temperature, but turbine-blade heat resistance is the wall. The second is to raise the HRSG recovery rate η_HRSG and cut the stack loss. The third is to raise the steam turbine efficiency η_steam. Look at the "combined efficiency vs gas turbine efficiency" chart below: raising η_gas lifts the overall efficiency surprisingly gently. From the formula η_cc = η_gas + (1−η_gas)·η_HRSG·η_steam, raising η_gas shrinks the (1−η_gas) in the second term, so the gas side and the steam side are, in a sense, competing for the same heat. Balancing them well is where the skill lies.
Frequently Asked Questions
Overall efficiency is η_cc = η_gas + (1−η_gas)·η_HRSG·η_steam. The first term η_gas is the gas turbine's stand-alone efficiency; the second term is the contribution of the steam side — the waste heat the gas turbine throws away (1−η_gas), the fraction the HRSG captures (η_HRSG), converted to electricity by the steam turbine at efficiency η_steam. For η_gas=0.38, η_HRSG=0.85 and η_steam=0.33, η_cc = 0.38 + 0.62·0.85·0.33 ≈ 0.554, that is about 55%.
A simple gas turbine dumps roughly 60% of the fuel energy to the atmosphere as hot exhaust gas. A combined cycle routes that exhaust through a heat-recovery steam generator (HRSG), boils water to raise steam and drives a second turbine — the steam turbine. Because it generates more power from heat that would otherwise be wasted, it adds about half the gas turbine's output again, lifting the overall efficiency to 55-64%. This is the highest efficiency of any thermal power station today.
The HRSG (Heat Recovery Steam Generator) is a heat exchanger that takes the 500-650 °C exhaust gas leaving the gas turbine and uses that heat to preheat, evaporate and superheat feedwater into high-pressure steam. It is the bridge of the combined cycle, supplying the heat source for the bottoming (steam) cycle itself. The higher the HRSG heat-recovery rate, the less heat escapes up the stack and the more heat reaches the steam turbine, raising the overall efficiency.
A conventional steam plant (boiler plus steam turbine only) reaches about 40% efficiency, and a stand-alone gas turbine 30-40%. A combined cycle connects a gas turbine and a steam turbine in series and reuses the gas-side exhaust heat on the steam side, achieving 55-64%. Because it starts quickly and follows load well, almost all newly built gas-fired plants now use the combined-cycle configuration.
Real-World Applications
Large gas-fired power stations: Large LNG-fired plants near cities are, almost without exception, combined cycles. They come in "single-shaft" arrangements pairing one gas turbine with one HRSG and steam turbine, and "multi-shaft" arrangements feeding the exhaust of several gas turbines into one steam turbine. Each unit delivers several hundred MW up to the 1 GW class at an overall efficiency of 55-64%. Setting the fuel heat input in this tool to 1000 MW gives you a feel for the scale of such large machines.
Balancing power and load following: Combined cycles start quickly and ramp output up and down nimbly, so they are prized as the "filler" for variable renewables such as solar and wind. When solar output rises at midday they throttle down; when demand climbs in the evening they ramp up fast. As the dispatchable balancing power the grid relies on, they form the core of the modern electricity mix.
Cogeneration (combined heat and power): In factories and district heating, the extraction or exhaust steam of the steam turbine is used as process steam or hot water for heating. By using the heat as well as generating electricity, the overall fuel-utilisation rate can be lifted above 80%. A combined cycle paired with this heat use is a classic example of the "cascade utilisation" of energy.
Repowering of thermal plants: Adding a new gas turbine and HRSG to an old steam plant (boiler plus steam turbine) and reusing the existing steam turbine as the bottoming cycle is called "repowering". Because it lifts efficiency from the low 40s into the 50s without a full plant replacement, it is adopted as a low-cost route to better efficiency and lower CO2.
Common Misconceptions and Pitfalls
The most common error is assuming "the overall efficiency is just the gas efficiency plus the steam efficiency". The correct formula is η_cc = η_gas + (1−η_gas)·η_HRSG·η_steam — not a simple sum. The steam turbine can only use "the waste heat the gas turbine threw away", so the second term always carries the factor (1−η_gas). Raising η_gas increases the first term but shrinks the (1−η_gas) in the second, so the gas side and steam side compete for the same heat. A plain sum of the two (for example 0.38 + 0.33 = 0.71) is physically impossible.
Next, the belief that "the HRSG recovery rate can be 100%". The HRSG is a heat exchanger, so cooling the exhaust gas all the way down to feedwater temperature is impossible in principle. If the exhaust temperature drops below the dew point (around 100 °C), the sulphur in the fuel forms sulphuric acid and corrodes the ducting, so real plants deliberately keep the stack outlet temperature at about 90-110 °C. In other words, some exhaust heat is always designed to leave through the stack — which is why this tool caps η_HRSG at 0.95.
Finally, the oversimplification that "raising the gas turbine inlet temperature always raises efficiency unconditionally". It is true that a higher inlet gas temperature raises the Brayton-cycle efficiency, but at temperatures above 1500 °C the turbine blades face a brutal environment, approaching the melting point of metal. Real machines survive with air cooling that flows air inside the blades, ceramic thermal-barrier coatings and advanced materials such as single-crystal alloys — yet there are still trade-offs against blade life, the bleed loss of cooling air and NOx formation. Remember that efficiency is set not by thermodynamics alone but by an integrated design that includes materials, cooling and emissions regulations.
How to Use
Set gas turbine efficiency (typical range 35–42% for modern GE 7FA units) and steam turbine efficiency (78–88% depending on condensing pressure)
Input fuel energy input in MW (coal: 500–1200 MW; natural gas: 150–600 MW) and Heat Recovery Steam Generator (HRSG) effectiveness (85–92%)
Observe combined cycle efficiency, individual turbine outputs, stack losses, and efficiency gain relative to gas turbine alone operation
Worked Example
A 450 MW natural gas facility: fuel input 580 MW, gas turbine η=39%, steam turbine η=85%, HRSG efficiency 89%. Gas turbine extracts 226 MW (580×0.39). Exhaust heat (354 MW) enters HRSG; steam cycle recovers 298 MW (354×0.89), yielding 253 MW after turbine losses (298×0.85). Combined output: 479 MW, combined efficiency 82.6%, stack loss 101 MW, efficiency gain +43.6 percentage points versus 39% baseline.
Practical Notes
HRSG pinch point (minimum temperature difference between exhaust gas and steam): 15–20°C typical; lower pinch increases steam recovery but raises capital cost—verify economizer design matches your heat duty curve
Gas turbine exhaust temperature (500–650°C) directly controls HRSG steam generation rate; off-design operation (part-load, ambient temperature variations) reduces both cycles simultaneously
Condenser pressure elevation from cooling water scarcity increases steam turbine back-pressure, cutting steam turbine output more than proportionally; check auxiliary cooling demand at 12–15% of steam turbine output