What is a combined cycle power plant?

A combined cycle power plant is a modern electrical generating plant that captures the energy from burning natural gas in two ways.

First - the gas turbine burns fuel and generates electricity:

• The gas turbine compresses air and mixes it with fuel.
• The mixture is ignited, creating an explosion that propels the very hot gas through the turbine.
• The hot gas spins the gas turbine blades which rotates the turbine shaft.
• The fast-spinning turbine shaft drives a generator that converts the spinning energy into electricity.

Second - the steam turbine utilizes the waste heat from the gas turbine exhaust that would otherwise escape through the exhaust stack to create additional electricity:

• A heat exchanger captures exhaust heat from the gas turbine and boils water to create steam.
• The steam spins the steam turbine blades which rotates the turbine shaft.
• The steam turbine shaft drives a generator that delivers additional electricity.

This is the most efficient type of fossil fuel power plant. By combining these two systems, the overall net efficiency of the combustion process can be increased by 50 - 60 percent. Thus, from an overall efficiency of about 35% in a single cycle system one can achieve to an overall efficiency of 50-60% in a combined cycle system.

Either a single shart or multiple shaft configuration can be used for the combined cycle plant. In a single shaft system, the gas and steam turbines turn a common shaft with a single generator. This is the most efficient configuration. However, in larger plants it is more economical to have multiple gas turbines and a single steam turbine.

For large-scale power generation, a typical gas/steam turbine set would be a 270 MW gas turbine coupled to a 130 MW steam turbine giving a total of 400 MW. A typical power plant might consist of between 1 and 6 such sets. GE currently manufactures the largest gas turbine available at just over 500 MW.

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