Advantages of combined power plant

A combined power plant uses the waste heat of a gas turbine exhaust to raise steam for a steam turbine. Its main advantages:

• High overall efficiency (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so lower fuel cost per kWh.
• Lower capital cost per kW than coal or nuclear plants; the gas turbine part is compact and factory built.
• Short construction time (2-3 years); can be built in phases (gas turbine runs first in open cycle).
• Quick start-up and good load-following, so useful for intermediate and peak loads as well as base load.
• Less cooling water needed, since only the steam part (about one-third of output) has a condenser.
• Lower emissions of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling.
• Less space and smaller staff; high reliability and availability.
• Flexible: supplementary firing, cogeneration (process steam), repowering of old steam plants.

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$

For example, $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$ give $\eta_{cc} = 0.545$ (54.5%), far higher than either plant alone.