Steam injection into the combustion chambers of gas turbines (GT) increases their power output. Additionally, the thermal efficiency can be raised, if steam is generated by exhaust heat.

The types of steam injected gas turbines (STIG) are distinguished according to the kind of limit to the amount of steam that can be injected. A gas turbine is called partial STIG, if it cannot utilize the total amount of steam that could be generated by the gas turbine exhaust heat. The limit is given by the flow capacity of the turbine. If, on the other hand, the gas turbine is sized such that the entire amount of steam producible can be utilized, it is called full STIG.

Three different partial STIG cooling models were selected to analyze the power output, the efficiency and the impact on two important components. Since the differences in the results for the three cycles are marginal, the following conclusion can be briefly summarized: Compressor surge turned out to be the strongest limit for overloading the gas turbine. At the point of maximum overload — where safe operation is still guaranteed — the steam mass flow amounts to one tenth of the nominal compressor air mass flow. At this operating point, the power output can be raised by more than 30% with a simultaneous increase in efficiency.

Based on the gas turbine configurations used for the partial STIGs, the preliminary designs of two full STIG cycles have been developed. However, for full STIG operation by injection of the total amount of steam producible, either the compressor or the turbines of the original gas turbine have to be modified. In this case, the steam flow exceeding that required for cooling has to be injected into the compressed air in front of the combustor. Depending on whether the compressor is scaled down or the turbines are scaled up, the power output of full STIGs is 30 to 135% higher than that of the original gas turbine. The gross thermal efficiency is about 50.5.%.

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