Combined Cycle Systems

Figures 12-1a and b show a simple combined-cycle system featuring a gas turbine-generator with a heat recovery steam generator (HRSG) and a steam turbine-generator, condenser, and auxiliary system. In this case, the HRSG is the heat exchanger that links the two cycles together by transferring the exhaust energy from the gasturbine topping cycle to the steam-turbine bottoming cycle.

The benefit of combining these cycles is that combustion engine cycles are well suited for high-temperature operation, and the rejected heat of those cycles, still at a relatively elevated temperature, is well suited for the steam-turbine cycle. A fundamental limitation on thermal efficiency of combustion engine cycles is the high exit temperature, typically in the range of 900 to 1,200°F (482 to 649°C). The exhaust stream has a high energy level that is wasted if heat recovery applications are not available.

Generally, the basic gas turbine design (i.e., no after-coolers or recuperators) provides a good exhaust temperature for steam turbine power generating efficiency. Increases in exhaust gas temperatures of a few hundred degrees, via supplementary firing, may further optimize HRSG and steam turbine performance. These factors allow the use of basic gas turbines in high thermal-efficiency combined-cycle plants.

Thermal efficiency of a combined-cycle plant, assuming no firing in the HRSG, is the ratio of total power output (combustion engine and steam turbine) to the combustion engine energy input. This can be expressed as:

Where: W

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