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A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.
The principle is that after completing its cycle in the first engine, the working fluid is still hot enough that a second subsequent heat engine can extract energy from the heat in the exhaust. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids.
By generating power from multiple streams of work, the overall efficiency of the system can be increased by 50–60%. That is, from an overall efficiency of say 34% , to as much as 64%.This is more than 84% of the theoretical efficiency of a Carnot cycle. Heat engines can only use part of the energy from their fuel, so in a non-combined cycle heat engine, the remaining heat from combustion is wasted.