Simple cycle
Simple cycle generators run an engine of some sort (reciprocating, like a car, or a turbine, like a jet plane) that’s fueled by a hydrocarbon (like natural gas) to spin an alternator to generate electricity.
Combined cycle
Combined cycle power plants run two different turbines to capture more electricity from the combustion of a hydrocarbon, typically natural gas:
- Gas turbine: Natural gas is burned, and the combustion products (gases) are used to spin a turbine to generate electricity. This is the same as the simple cycle.
- Steam turbine: The hot gases that exhaust from the gas turbine are then used to boil water and generate steam. That steam passes through a second turbine, generating more electricity.
These power plants typically have higher efficiencies (60% or higher) than simple-cycle generators such as are commonly used with coal. They still emit CO2, but generate more power per unit of greenhouse gas produced.