Steam injection in gas turbines (steam raised from the energy of the exhaust and injected into one or more of the turbine stages) is an attractive option for cogeneration applications. From a thermodynamic point of view, however, there is little information available about methods for optimizing the use of the steam for injection into a gas turbine.

A computer model for an aeroderivative gas turbine is used to analyze the effect of steam injection on net power output and overall efficiency. The effects of varying the quantity of steam injected, the stations at which the steam is injected, and the temperature of the steam that is injected are assessed on a normalized basis, with the turbine-inlet temperature maintained from the simple-cycle design point.

The energy balance between the exhaust of the gas turbine and the flow of steam to be injected is the final constraint in selecting a steam-injected design point to maximize performance. For the engine in this study, increases of over 64% in net power output and 23% in overall efficiency can be achieved with roughly 16% steam/inlet air by mass, which represents all of the steam that can be produced by the exhaust stream for the given conditions.

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