Humid Air Turbine (HAT) cycle: state of the art and perpesctives

In the last fifteen years several different energy conversion systems configurations were proposed to increase efficiency of power plants. In particular, those based on the increase of gas turbine performance seem to be the most interesting solutions for electrical power production in the next century. The highest efficiency values are actually achieved by the combined cycles that utilize the heat rejected from a gas turbine to generate steam which is then used in a steam turbine to produce additional power. This, however, complicates plant configuration and operation. Another solution to increase gas turbine performance, with a simpler system, is steam injection, which increases the flow rate in the turbine and reduces NOx emissions. Among these cycles, starting from the General Electric STIG (Steam Injection Gas Turbine, Brown and Cohn, 1981, Tuzson, 1992, Rice, 1993a-c) and the Cheng cycle (Saad and Cheng, 1992), the one that seems to provide the best performance in terms of power and efficiency is the Humid Air Turbine (HAT) cycle (Rao and Joiner, 1990). Since the first HAT cycle configuration was presented, some thermodynamic and economic analyses have been done on the cycle behavior. The optimization of the basic parameters of the HAT cycle (Bettagli et al., 1995, Chiesa, et al., 1994, Stecco, et al., 1993a-b, Xiao, et al., 1994, Rosén, et al., 1994, Hansen and Nielsen, 1996, Klara, et al., 1996, Lindgren, et al., 1992), the study of the effect of an external heat exchanger for the refrigerators circulating water (Stecco, et al, 1993a-b), the analysis of heat and water recovery from the exhaust gasses (Bettagli and Facchini, 1994; Bidini et al., 1996; Desideri and Di Maria, 1996; Bombarda, 1995, De Paepe and Dick, 1997, Qun, et al., 1997, Xueyou, et al., 1996), yan, et al. 1996), the effect of blade cooling bleeding point (Gallo et al., 1996), are some of the principal contributions to the knowledge of the HAT cycle. An important contribution has also been given by Nakhamkin, et al. (1995, 1997) with the introduction of the CHAT (Cascaded Humidified Air Turbine). This paper reviews some of the most significant results presented in the above papers, obtained by the research groups at the Universities of Perugia and Florence in Italy and the University of Campinas in Brasil. An economic analysis has been added to understand the influence of the main economic parameters on the cost of generated electricity. This is important when the cycle is studied in different economic scenarios such as the Europe and Latin America.