- Power Cycles
Combined Cycle Power Plants
In the last 30 years the revolution in the industrial base of the power utilities industry in the United States and around the world resulted in combining Rankine cycle engines with Brayton cycle engines. The combined cycle plant uses a Brayton Cycle (or jet engine) as the topping cycle and a Rankine or steam turbine as the bottoming cycle. The exhaust gas from the jet engine is about 700°F (370° C). That is hot enough to make steam. So in the combined cycle the exhaust the gas from the jet engine goes to a boiler. The boiler produces steam. The steam goes to a steam turbine generator. So a combined cycle plant is actually two power plants combined. Electricity is made from an electric generator on the jet engine turbine (also known as a gas turbine) and from a generator on the steam turbine.
Brayton Cycle engines have changed how most of our electricity is generated today. Brayton Cycle refers to the jet engine. Another term commonly used for the Brayton Cycle is “gas turbine.” Jet engines used for power plants normally run on liquid fuels or natural gas. Utilities used to buy these plants as "peakers," meaning that the plant is used for peak load. Historically the gas turbines have used more fuel and more expensive fuels so the power companies only used them when necessary. But combining gas turbines with steam turbines has resulted in the most efficient type of power plant – the combined cycle plant.
In recent years engineers are trying to find ways to run combined cycle plants on solid fuels such as coal or on fuels that are derived from coal. And now with natural gas prices low many new combined cycle plants are being built.
Rankine Cycle Power is uses a boiler and a steam turbine. Rankine Cycle Power is the older more conventional steam turbine power cycle. Almost all coal plants use the Rankine Cycle. All commercial nuclear power plants use the Rankine cycle.