## 2 Data and Methodology
### Interaction between Wind Direction and Building Orientation  
#### 2.2.1 The Importance of the Skew Angle and a Small-Building Experiment  
The skew angle of any Vertical Axis Wind Turbine (VAWT) is the angle between the plane perpendicular to the rotation axis and the incoming wind direction, which means that, for a regular VAWT, it has a value of zero when the wind only has horizontal components, as well as a value of 90\({}^{\circ}\) when it has only vertical ones. If the VAWT is mounted horizontally on the edge between the roof and the facade, this is the same angle as the angle between the plane perpendicular to both the facade and the roof and the wind direction. When working with wind turbines in this position, it is very important to take this angle into account when estimating the wind resource because they only work when the wind is facing the facade. To determine the maximum skew angle with which the turbine can generate the potency given by the power curve for a particular wind speed, a small wind tunnel experiment was run. It involved using a mock-up of a building with a miniature Savonious turbine and Power Augmenting Guiding Vanes (PAGVs). The turbine was attached to a dynamo, from which voltage readings were taken. The voltage was later used to calculate the rotation speed for each wind speed and skew angle. Figure 3 shows the design, dimensions, and photos of the small-scale model with a 6-bladed small turbine within the PAGVs and the building. Figure 3a,d shows the first design and the corresponding real construction of the small turbine along the building (plant view). The profile of the design and real methacrylate PAGVs are shown in Figure 3b,e. Figure 3c's profile view shows the design and exact \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Anemometer Data** & **Reanalysis Data** & **Merging of Both Anemometer and Reanalysis Data** \\ \hline Spatial (number of sites) & 1 & 9 & 10 \\ Temporal (years) & 1 & 10 & 10 \\ Height & Rooftop & 10 m and 100 m & Rooftop and 10 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the scope of each data set. dimensions and angles of the 6 blades, while Figure 3f is a photo of the small building with the PAGVs in a previous experiment. The electrical motor attached to the small turbine is a maxon DCX06M EB KL 6V of 0.529 W (speed constant of 3060 min\({}^{-1}\) V\({}^{-1}\), speed-torque relation of 36,600 min\({}^{-1}\) m Nm\({}^{-1}\)). This description and the characteristics of the wind tunnel in which the skew angle experiment is performed are already described in our previous publication [7]. Although the blockage ratio of the small scale experiment (the transverse area of the small building with respect to the wind tunnel circular area) should be below 10% [22], a blockage ratio of around 20% is obtained even with our precise scale model of the turbine. In the future, a bigger wind tunnel in necessary with a wider range of wind speeds. Figure 3: Design and dimensions of the small Savonius model and the PAGVs (Power Augmenting Guiding Vanes) (**a**)–(**c**) and the corresponding photos of the small-scale building with the real Savonius and the PAVs (**d**)–(**f**). Although the experiment is preliminary and should be validated by CFD or in a bigger wind tunnel, it shows very relevant results for our purpose and the configuration of our Savonius turbine. It should be noted that the horizontal plane skew angle with respect to this horizontally mounted Savonius is analogous to the typical vertical plane skew angle with respect to a VAWT (see Figure 4). The experiments were developed for wind speeds around 6 m/s, 7 m/s, and 8 m/s. There are fluctuations in the measured wind speed and the rotational speed (\(rpm\)) of the small Savonius, and that is why boxplots are used for the representation of the graph.