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.
he 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.) Fig. 1 is a simplified flow diagram for an installation of this type, in which an open-cycle gas turbine is followed by a steam process. The heat given off by the gas turbine is used to generate steam. Other combinations are also possible, e.g., a mercury vapor process or replacing the water with organic fluids or ammonia.
The mercury vapor process is no longer of interest today since even conventional steam power plants achieve higher efficiencies. Organic fluids or ammonia have certain advantages over water in the low temperature range, such as reduced volume flows, no wetness. However, the disadvantages, i.e., development costs, environmental impact, etc., appear great enough to prevent their ever replacing the steam process in a combined-cycle power plant. The discussion that follows deals mainly with the combination of an open-cycle gas turbine with a water/steam cycle.
Certain special applications using closed-cycle gas turbines will also be dealt with briefly. It therefore is quite reasonable to use the steam process for the "bottoming cycle.'' That such combination gas turbine/steam turbine power plants were not more widely used even earlier has clearly been due to the historical development of the gas turbine. Only in recent years have gas turbines attained inlet temperatures that make it possible to design a very highefficiency cycle. Today, however, the installed power capacity of combined-cycle gas turbine/steam turbine power plants worldwide world totals more than 30,000 MW.
810K (980°F), while the combustion temperature in the boiler is approx. 2000 K. Then, too, the temperature of the waste heat from the process is higher than the ambient temperature. Both heat exchange processes cause losses. The best way to improve the process efficiency is to reduce these losses, which can be accomplished by raising the maximum temperature in the cycle, or by releasing the waste heat at as low a temperature as possible. The interest in combined-cycles arises particularly from these two considerations. By its nature, no single cycle can make both improvements to an equal extent.
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