Combined Cycle

Combined cycle is an electric generating technology that creates additional electricity from waste heat released by gas turbines. The exhaust heat from a gas turbine is routed to a conventional boiler or to a heat recovery steam generator. Subsequently, the exhaust heat is converted into electricity by a steam turbine.¹

A combined-cycle power plant uses both a gas and a steam turbine together to produce up to 50% more electricity than a traditional simple-cycle plant. The waste heat from the gas turbine is routed to the nearby steam turbine, which generates additional power².

Below is a summary of process within a combined cycle power plant³:

1. Gas turbine burns fuel.
   • The gas turbine compresses air and mixes it with fuel that is heated to a very high temperature. The hot air-fuel mixture moves through the gas turbine blades, making them spin.
   • The fast-spinning turbine drives a generator that converts a portion of the spinning energy into electricity.
2. Heat recovery system captures exhaust.
   • A Heat Recovery Steam Generator (HRSG) captures exhaust heat from the gas turbine that would otherwise escape through the exhaust stack.
   • The HRSG creates steam from the gas turbine exhaust heat and delivers it to the steam turbine.
3. Steam turbine delivers additional electricity.
   • The steam turbine sends its energy to the generator drive shaft, where it is converted into additional electricity.

As with single cycle thermal units, combined cycle units may also deliver low temperature heat energy for industrial processes, district heating and other uses.  This is called cogeneration and such power plants are often referred to as a Combined Heat and Power (CHP) plant.

Energy developers are now opting for combined cycle power generation to decrease fuel consumption and increase energy efficiency. While there is an increase in cost, much of this initial increase can be offset by fuel savings.