# Design Point Performance and Optimization of Humid Air Turbine Power Plants
## 1 Introduction  
Clean and cost effective power generation is a key factor to respond to the challenges arising nowadays in the power generation market. Thermal efficiency enhancements by advanced cycle power plants significantly impacts pollutant emissions and consequently cost of electricity. Therefore, it can be argued that advanced cycle plants with low environmental footprint increase their competiveness in a market dominated by increasingly stringent emission regulations. Techno-economic assessments of such technologies should be, therefore, undertaken to identify the economic viability of such systems. Humid Air Turbine (HAT) or Evaporative Gas Turbine (EvGT) cycles were previously found to offer lower specific investment costs than combined-cycle gas turbines (CCGTs) and notably higher thermal efficiencies, especially in the range of small to medium-sized power plants [1; 2]. Moreover, HAT cycles can also offer greater specific power (higher even than the specific power of the 'dry' gas turbines), faster start-up times (comparable to the typical start-up times of 'dry' gas turbine packages), potential to reduce even further the NOx emissions due to the presence of high levels of humidity within the combustion chamber of the gas turbine, better part-load performances, and low susceptibility to ambient conditions compared to the CCGTs [3]. Since the introduction of HAT cycles in the late 80s [4], the potential of these systems has been studied extensively. [PERSON] et al. [5] performed a comparison of three humid cycles showing that the HAT cycle achieved the highest thermodynamic performance, enhancing the thermal efficiency morethan 10 percentage points (pp) compared to a simple cycle. The enhancement was mainly attributed to the reduction in the discharge losses and the recuperation of the heat rejected in the intercooler within the saturator. The authors also pointed that the thermal efficiency could be further increased if a reheater was placed. [PERSON] and [PERSON] [6; 7] performed a thermodynamic optimization of a HAT cycle where they considered the heat exchanger network as a \"black box\" to be later optimized. The study established that the intercooling temperature for optimum thermal efficiency was the one minimizing the compression work, and that the pressure ratio does not affect the thermal efficiency but only the specific power. When optimizing the heat exchanger network independently, they discover that the best configuration only included one intercooler, one aftercooler, and one economizer. [PERSON] and [PERSON] analysed the potential of part-flow humidification whereby a percentage of the air flow bypasses the humidifier, showing that, in some cases, it can imply a lower initial investment and even higher cycle thermal efficiencies [8; 9]. Moreover, [PERSON] and [PERSON] [1] presented a detailed economic analysis proving the advantages of part-flow humidification, as lower specific investment cost for a similar cost of electricity compared to the combined cycle or the fully humidified version. They also proved, that for larger overall pressure ratios, the benefits of the humidification process increase. In addition, [PERSON] et al. [10] tested the idea of including an aftercooler downstream the compressor, proving that the addition of this component boosts the performance of the whole plant by enhancing the amount of heat recovered in the humidifier. Further studies have been carried out on the part load performance of HAT cycles. [PERSON] et al. [11] and [PERSON] et al. [12] concluded that HAT systems are less susceptible to ambient conditions than simple cycle gas turbines. [PERSON] et al. [13] showed that humid turbines may demonstrate a better part-load performance than combined cycles. Additionally, optimizations performed by [PERSON] et al. [14] and [PERSON] et al. [15] showed that the variables that drive the cycle's thermal efficiency are the overall pressure ratio (OPR), the relative pressure ratio, and the recuperator effectiveness.