Gas Turbines, Combined Cycles & Cogeneration

The most efficient fossil power plants and how to use waste heat. Learn gas-turbine (Brayton) power plants, the heat-recovery steam generator (HRSG) and combined-cycle efficiency, pinch point and approach temperatures, open-cycle peakers, combined heat and power (CHP/cogeneration) and trigeneration, primary energy savings, waste-heat recovery in industry, and hydrogen-ready turbines — with Python for combined-cycle and CHP efficiencies.

A gas turbine alone converts only 35–42% of fuel energy to electricity; its exhaust leaves at 550–650 °C, hot enough to raise steam. Using that exhaust in a steam cycle creates a combined-cycle plant exceeding 60% efficiency — the best of any thermal plant.

Cogeneration goes further by using leftover heat for industrial processes or district heating, reaching 75–90% total fuel utilisation.

An analogy

A combined cycle is like cooking pasta and then using the hot pasta water to make soup: the same fuel does two jobs, so far less energy is wasted.

Gas-turbine power plants

Air is compressed (pressure ratio 15–25), fuel (natural gas, diesel, hydrogen blends) burns, and hot gas expands through the turbine driving both compressor and generator. Open-cycle gas turbines start in minutes — ideal peaking plants to balance solar and wind — but are less efficient than combined cycles.

Combined cycle

The HRSG (economiser, evaporator, superheater, often at two or three pressure levels with reheat) raises steam from the GT exhaust; a steam turbine adds ~50% more power without extra fuel. Design parameters: pinch point (8–15 K), approach temperature, stack temperature (~90 °C). Configurations: 1-on-1, 2-on-1, single-shaft.

Cogeneration

Useful heat configurations:

Flexibility and decarbonisation

Modern turbines ramp fast and start quickly to firm up renewables. Hydrogen-ready combustors burn up to 30–100% H₂ (managing flame speed and NOₓ). Carbon capture on combined cycles and sequential combustion are further options. Economics depend strongly on gas price and utilisation.

Formulas

Worked example: Combined-cycle

Given: η_GT 0.40, ε 0.9, η_ST 0.36

Find: η_CC.

Answer: 59.4%

Worked example: Exhaust heat

Given: 500 kg/s exhaust, c_p 1.1 kJ/kg·K, 600 → 90 °C

Find: Recoverable heat.

Answer: 280 MW

Worked example: Power-to-heat

Given: 3.5 MW electricity, 4.5 MW heat

Find: σ.

Answer: 0.78

Combined-cycle and CHP efficiency

# Combined-cycle efficiency and a CHP energy balance
eta_gt, eta_st_recovery = 0.38, 0.33       # GT efficiency, steam cycle efficiency on GT exhaust heat
eta_cc = eta_gt + (1 - eta_gt) * 0.85 * eta_st_recovery   # 85% of exhaust heat captured in HRSG
print(f"combined-cycle efficiency ~ {eta_cc*100:.1f}%")
# CHP: 10 MW fuel -> 3.5 MW electricity + 4.5 MW useful heat
fuel, el, heat = 10.0, 3.5, 4.5
print(f"CHP total efficiency {(el+heat)/fuel*100:.0f}%")
sep = el / 0.45 + heat / 0.90                  # separate grid (45%) + boiler (90%)
print(f"separate production needs {sep:.2f} MW fuel -> primary energy saving {(1-fuel/sep)*100:.0f}%")
# combined-cycle efficiency ~ 55.4%
# CHP total efficiency 80%
# separate production needs 12.78 MW fuel -> primary energy saving 22%

Adding a steam bottoming cycle raises efficiency from 38% to ~55% (the best H-class plants exceed 63%). CHP saves 22% of fuel compared with buying grid electricity and running a separate boiler — the case for cogeneration in sugar mills, refineries, hospitals and district heating.

Common mistakes

Applications

Key takeaways

Going further