## 6 Experimental Results and Discussion
### Variation of Height Above the Rooftop
(deflected) wind is experienced by the horizontal blades. Hence, the reason for the poor performance of CAWT at higher positions above the rooftop. On the other hand, at lower positions, due to the speed up effect at the rooftop, more vertical (deflected) wind interacts with the horizontal blades therefore improving the performance of the CAWT. The presence of the horizontal blades of the CAWT is very important in case of the skewed flow, which could actually increase the lift production of the CAWT. To verify the experimental result on the variation of height of CAWT above the rooftop (Figures 14 and 15), a simple CFD simulation was carried out to calculate the skewed angle (Figure 16) of the wind at the rooftop shown in Section 4 (Figure 8a). For each height of the CAWT above the rooftop, i.e., \(Y=100\) mm, \(150\) mm, \(200\) mm, and \(250\) mm, the skewed angle is calculated. The result obtained for the calculated skewed angle agrees well with [PERSON] et al. [33] where the skewed angle is largest close to the windward side of the roof edge. Figure 16 presents the results for the skewed angle for each of the turbine height above the rooftop. From the figure; it can be observed that the highest skewed angle is obtained at the \(100\) mm height of the CAWT. This confirms the results obtained in Figure 14 where the CAWT mounted at \(100\) mm height performs better than other heights. The figure shows that the skewed angle decreases with increase in height. Therefore, the performance of the CAWT increases with increase in skewed angle. Similarly, improvement in the performance of VAWT turbine due to skewed flow is reported in [6; 33; 56]. The higher skewed angle experience by the CAWT at \(100\) mm height also contributed to the improved performance of the CAWT at that position (Figure 14). At higher skewed angle the swept area of the turbine is increased due to the contribution of the downwind zone which counterparts the decrease of the projected frontal area with an overall increase of the available surface area to intercept the wind as reported by [PERSON] et al. [35]. Furthermore, Skewed flow increases the performance of H-VAWT by modifying the region of interaction of the downwind blade passage with the upwind generated wake. Increasing the skewed angle will increase the area of the downwind blades passage which is operating outside the upwind generated wake thus experiencing an incoming flow with larger energy content [34]. While skewed angle is believed to improve the performance of wind turbine in urban buildings as reported by [PERSON] et al. [33], [PERSON] et al. [35], and [PERSON] et al. [57]. [PERSON] and [PERSON] [56] reported that for the case of cowling system the wind turbine in a skewed \begin{table}
\begin{tabular}{c|c c c c} \hline \multirow{2}{*}{**Parameter**} & \multicolumn{4}{c}{**Height (mm)**} \\ \cline{2-5}  & **100** & **150** & **200** & **250** \\ \hline RPM & 554 & 512 & 474 & 449 \\ \(\text{C}_{\text{P,max}}\) & 0.1263 & 0.1035 & 0.0859 & 0.0765 \\ TSR for \(\text{C}_{\text{P,max}}\) & 1.1 & 1.16 & 1.08 & 1.12 \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of experimental results on variation of height. Figure 15: Rotational speed against time for CAWT at various heights above the rooftop. flow can lead to power loss of 11.6%. The skewed wind conditions in the cowling system generates excessive blockage thereby becoming an obstacle to the oncoming flow.