Operational characteristics of combined cycle plant

A combined cycle gas turbine (CCGT) plant combines a gas turbine (Brayton cycle) with a steam turbine (Rankine cycle). The hot exhaust of the gas turbine (about 450–600 °C) produces steam in a heat recovery steam generator (HRSG), which drives a steam turbine.

 Air -> Compressor -> Combustor -> Gas turbine -> G1
                        ^ fuel         |
                                    exhaust
                                       |
                 Steam turbine <-- HRSG --> stack
                   |      |
                  G2   Condenser

Operational characteristics

1. High efficiency – about 50–60% (gas turbine alone 30–40%), the highest of all thermal plants.
2. Quick start – gas turbine starts in 10–30 minutes; full combined cycle in about 1–3 hours. Suitable for intermediate and peak load.
3. Good load following – can ramp fast; supports variable renewables.
4. Part-load behaviour – efficiency falls at part load; usually operated above about 40–50% load; several smaller units improve flexibility.
5. Fuel – natural gas (cleanest) or light oil; fuel cost is the main running cost.
6. Ambient sensitivity – output and efficiency drop at high air temperature and high altitude (lower air density).
7. Low emissions – less CO₂ per kWh than coal; low SO₂ and particulates.
8. Low capital cost and short construction time (2–3 years); compact; little cooling water (only steam part).
9. Phased operation – gas turbine can run in simple cycle while the steam part is built or maintained.