Environmental Engineering Reference
In-Depth Information
Figure 3.7 A model of a gas turbine for use in a combined cycle power plant, the compressor on the right
and the turbine on the left. The combustion occurs in two stages, the first between the compressor exit and
the first turbine stage and the second immediately after the first turbine stage.
high-temperature strength of the turbine blades. It was not until the development of efficient aero-
dynamic blade designs and high-temperature turbine materials that the gas turbine plant became
practical and economically feasible in the twentieth century.
For the simple Brayton cycle, the best thermodynamic efficiencies are about 33%. By use
of heat exchange between the hot exhaust gas and the compressed gas entering the combustion
chamber, this efficiency may be increased by about four percentage points. Economic factors
may require that the plant operate at maximum power output, for which the efficiency would be
somewhat lower than these values.
The compressor and turbine of a gas turbine power plant are usually built into a single rotor,
as shown in Figure 3.7, with the combustion chamber sandwiched between the compressor and
turbine. This is the arrangement for aircraft gas turbines, which must be as light as possible. The
rotor shaft delivers the net power difference between the turbine and compressor powers to the
electric generator.
Combined Brayton and Rankine Cycles
The combustion products gas stream leaving the gas turbine carries with it that portion of the fuel
heating value that was not converted to work. This hot stream of gas may be used to generate steam
in a boiler and produce additional work without requiring the burning of more fuel. The use of a
gas turbine and steam plant to produce more work from a given amount of fuel than either alone
could produce is called a combined cycle.
The thermodynamic efficiency
η cc of a combined cycle power plant may be determined as a
function of the component efficiencies,
η g and
η s , of the gas turbine and steam cycles. For the gas
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