Historically successful combined cycles have used mercury vapour turbines, magnetohydrodynamic generators and molten carbonate fuel cells, with steam plants for the low temperature "bottoming" cycle. Heat engines can only use part of the energy from their fuel, so in a non-combined cycle heat engine, the remaining heat (i.e., hot exhaust gas) from combustion is wasted. This is more than 84% of the theoretical efficiency of a Carnot cycle. That is, from an overall efficiency of the system of say 34% for a simple cycle, to as much as 64% net for the turbine alone in specified conditions for a combined cycle. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids.īy generating power from multiple streams of work, the overall efficiency can be increased by 50–60%. The principle is that after completing its cycle in the first engine, the working fluid (the exhaust) is still hot enough that a second subsequent heat engine can extract energy from the heat in the exhaust. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. On land, when used to make electricity the most common type is called a combined cycle gas turbine ( CCGT) plant. Gateway Generating Station, a 530-megawatt combined cycle natural gas-fired power station in Contra Costa County, California.Ī 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.
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