Combined Cycle Gas and Steam Turbine Power Plants
The literature has often suggested combining two or more thermal cycles within a single power plant. In all cases, the intention was to increase efficiency over that of single cycles. Thermal
processes can be combined in this way whether they operate with the same or with differing working media. However, a combination of cycles with different working media is more interesting
because their advantages can complement one another. Normally the cycles can be classed as a ''topping'' and a ''bottoming" cycle. The first cycle, to which most of the heat is supplied, is called the "topping cycle." The waste heat it produces is then utilized in a second process which operates at a lower temperature level and is therefore referred to as a "bottoming cycle."
Careful selection of the working media makes it possible to create an overall process that makes optimum thermodynamic use of the heat in the upper range of temperatures and returns
waste heat to the environment at as low a temperature level as possible. Normally the ''topping'' and ''bottoming'' cycles are coupled in a heat exchanger. Up to the present time, only one combined cycle has found wide acceptance: the combination gas turbine/steam turbine power plant. So far, plants of this type have burned generally fossil fuels (principally-liquid fuels or gases.)
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