In order to study the impact of using a gas turbine coolant other than compressed air, a comparison is made among various gas-turbine based power cycles: the simple-cycle gas turbine, the steam injected gas turbine, the combined cycle and the Humid Air Turbine (HAT) cycle. Depending on the cycle configuration, the coolant, which is compared to the air extracted from the compressor, is either steam or humidified air. The study is based on a modern, medium-sized industrial gas turbine (net power output in base configuration: 40 MW), in order to evaluate the possibilities of different redesign options of the cooling system.

It is found that steam cooling is a very efficient method of cooling; whereas, the possible benefits of humid air cooling are mainly obtained through the temperature decrease that is a result of the humidification process. Hence, for the intercooled HAT cycle, the benefit of cooling with humid air is smaller (∼0.2 percentage points increase in thermal efficiency) than for the HAT cycle without intercooler (∼0.5 percentage points increase in thermal efficiency). For the simple cycle, there is an increase in thermal efficiency of 0.8 percentage points when it is cooled with humid air.

A parameter variation shows that with more advanced cooling technologies and heat resistant materials, the benefit of cooling with steam or humid air, instead of with compressed air, is reduced for the HAT and combined cycles.

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