Evaporative Gas Turbine (EvGT)/Humid Air Turbine (HAT) Cycles

In a combined cycle, a physical separation exists between the working fluids of the topping gas turbine cycle and of the bottoming steam turbine cycle. It is possible to combine the working fluids of these two cycles to result in a single “mixed cycle.” There are a number of alternate approaches to the manner in which these two working fluids may be combined, that is, the manner in which the liquid water is evaporated/combined with the other working fluid resulting in different cycle arrangements with very different thermal performance. This article is devoted to a discussion of the various evaporative gas turbine cycles incorporating this mixed cycle approach such as the steam injected, recuperative water injected, and humid air turbine (HAT) cycles, and will mainly focus on the systems where H2O addition can attain a significant fraction (10–20%) of the airflow. Thermodynamic merits and technological drawbacks will be illustrated and their thermal performance compared on a consistent basis. Major emphasis of this article is the HAT cycle while the other mixed cycles will provide the necessary background on this topic. Development of the HAT cycle is aimed at having the advantages of the combined cycle in terms of its high thermal efficiency but having a low specific cost of an open simple cycle by eliminating the need for the expensive components such as the steam turbine, condenser, and cooling medium circulation circuit of a combined cycle. Challenges related to operating commercial gas turbine engines in the HAT cycle mode are also discussed including the variations to the cycle arrangement to minimize the development of specialized turbomachinery. Keywords: evaporative gas turbine; EvGT; humid air turbine cycle; HAT cycle; steam-injected cycle; recuperative water-injected cycle; compressor inlet air fogging; combustion with humid air; exhaust water recovery; CO2 recovery in wet gas turbine cycles