# Design Point Performance and Optimization of Humid Air Turbine Power Plants
## 2 Methodology  
Figure 1 shows the layout of the HAT system studied herein. It comprises a dual-shaft gas generator with a free power turbine. Water absorption into the compressed air takes place in the saturator. The dry air entering the saturator is previously cooled down in the aftercooler, which increases the performance of the saturator tower as it permits to recover larger amounts of low-quality--i.e., low temperature--heat [10]. Once inside the tower, both heat and mass are  
transferred from the stream of hot water circulating counter-currently into the air, increasing its temperature and mass flow. Due to the relatively low temperatures of the humid air exiting the saturator, the HAT cycle benefits from a recuperator that recovers the high-quality--i.e., high-temperature waste heat from the exhaust gases before streaming the humid air into the combustion chamber. An additional bypass is also present between the high-pressure compressor exit and the inlet of the recuperator, as suggested by [PERSON] and [PERSON] [8; 9], so that a certain fraction of dry air can bypass the humidification process and is mixed with the rest of the humid air between the saturator and the recuperator inlet. The amount of air that bypasses the humidifier is represented by the parameter \(\beta\), which represents the ratio of air flowing through the aftercooler and saturator to the inlet mass flow. Therefore, \(\beta=1\) when all the air goes through the aftercooler and saturator, whereas, for \(\beta=0\), all of the air bypasses them. In this study, an open loop is used to feed water in the heat exchangers, on the assumption that a water source is available. The evaporation process in the saturator and an efficient droplet eliminator at the gas outlet of the saturator acts as a water-treatment device per se, allowing the use of non-demineralized water without undermining the life of the components. The thermal performance of the system is resolved using Turbomatch(r)[22; 23], an in-house 0D performance software developed at Cranfield University, with external modules for the heat exchangers and saturator performance, and blade cooling mass flow requirements calculations. For the design point performance prediction, the power output is set constant at 40 MW with no mechanical losses considered. In terms of fuel, Marine Diesel chemical properties were used for combustion calculations [24]. With regards to turbomachinery performance, a 90% polytropic efficiency is assumed for the compressors and the core turbines, whereas 92% is assumed for the power turbine, to reflect the current technology levels of these components. The turbine inlet temperature (TIT) at the outlet of the main combustion chamber as well as at the outlet of the reheater was set constant at 1600 K. Turbine cooling flows are calculated using the methodology proposed by [PERSON] and [PERSON] [25]. This method accounts for the cooling requirement of the disks, and the stationary and rotating blades when cooled with internal convection and film cooling technology. Typical values for maximum metal temperature (\(T_{m,max}=1300\) K), film cooling effectiveness (\(\varepsilon_{f}=40\%\)) and internal flow cooling efficiency (\(\eta_{c}=70\%\)) are used as discussed by [PERSON] et al. in [26]. Turbine cooling flows are extracted downstream of the high pressure compressor and upstream of the aftercooler. The outlet conditions from all gas-water heat exchangers, namely the intercooler, the aftercooler and economizer, are resolved by imposing the effectiveness, which is viewed as the heat exchanger's technology level, (\(\varepsilon=Q/Q_{max}\)) and the heat capacity ratio (\(C^{*}=C_{min}/C_{max}\)) between the two streams, assuming that \(C_{w}=C_{max}\). Figure 1: Schematic layout of the Humid Air Turbine. Water condensation in the air stream is possible within the intercooler and aftercooler due to the high pressure and low temperature of the gas at the outlet of the heat exchangers (\(T_{g,\ out}<T_{g,\ dew}\)).