# A New Procedure to Design an Open Circuit Blowing Subsonic Moist-Air Wind Tunnel
## 1 Introduction
The experimental and simulated results show an increase in the velocity ratio whenever the air density is reduced. A posterior work [17] showed that, when their models were employed to the changed climatic conditions by the year 2030, it was concluded that the VAWT would be less affected by climate change than the HAWT. From these studies, it was concluded that there is a need to evaluate and optimise wind concentrator design and test with more complex wind tunnels. Now, new concepts, like moist air phase change, are being analysed to improve energy conversion. Therefore, designing an adequate wind tunnel that takes into account the relative humidity of moist air becomes particularly interesting. Previous works analysed the effect of ice or humidity in wind tunnels [18; 19], but none of them showed the way to empty this with a nozzle that causes the moist air phase change and its associated energy release. In this sense, relating the relative humidity of moist air to dry air becomes significant because different densities and energy are noted with each different relative humidity value. For example, to provide a reference for the power output of wind turbines, it is compared with the power of the free-air stream, which flows through the same cross-sectional area (\(A\)), according to Equation (1). \[P_{0}=\frac{1}{2}\cdot\rho\cdot v_{1}^{3}\cdot A \tag{1}\]  
where \(\rho\) is the moist air density (kg/m\({}^{3}\)), \(v_{1}\) is the wind velocity (m/s), and \(A\) is the cross-sectional area (m\({}^{2}\)). From this equation, wind turbine power conversion is observed to mainly depend on the cross-sectional area, moist air density and wind velocity. Hence, the properties of moist air will influence this energy conversion; however, this parameter was hardly ever taken into account in these processes. To consider this, wind velocity, as well as new parameters in weather conditions, like moist air properties, need to be analysed. Thus, if the total monthly wind energy is compared, the next figures can be arrived at. Figure 6 shows outdoor moist air temperature, relative humidity and its corresponding enthalpy. From this figure, it can be concluded that the outdoor moist air presents maximum enthalpy during the months between April and September as a clear function of outdoor air temperature  
Figure 5: Relationship between relative humidity, enthalpy and density of moist air. during the spring and summer seasons. However, Figure 7 shows the outdoor air velocity, density and corresponding kinetic energy. This kinetic energy, as seen, depends on the outdoor air velocity as it presents a cubic power value with respect to the density of moist air. Figure 7 thus shows that the kinetic energy is obtained from February to May. Figures 6 and 7 show that the energy of moist air depends not only on the wind speed, as typically employed in wind turbines, but also on the design and operation of the wind farms. Fluid enthalpy particularly needs to be considered due to the total energy of the moist air, which has been summarised in these two points. From these Figures, outdoor air  
Figure 6: Outdoor air temperature, relative humidity and enthalpy. Figure 7: Outdoor air density, velocity and kinetic energy. enthalpy has been seen to depend on the temperature of the moist air. Additionally, the relative humidity of outdoor air exerts a lower influence, although it does not exert a phase change. Therefore, because of all these parameters, the kinetic energy is transformed under different moist-air conditions into an irregular monthly real wind farm power, as shown in Figure 8. Actually, Figure 8 corresponds to the same region whose weather conditions were shown earlier. Thus, it becomes evident that wind power exerts a maximum value during the period between January and March. On studying the climatic conditions and the wind power conversion during the past years, it became interesting to set their relationship in real wind farms.