## 6 Experimental Results and Discussion
### Variation of Height Above the Rooftop
In addition, the vertical wind guided by the gable roof shape has successfully reduced the self-starting time and increased the maximum rotational speed of the CAWT resulting in a better efficiency. For the case of the straight-bladed VAWT the connecting struts affect the performance by adding parasitic drag to the turbine. Further study was conducted on the effect of the increase in turbine height on the performance of the building integrated CAWT. The height of the CAWT above the rooftop was varied as shown  
Figure 13: Rotational speed against time for building integrated CAWT and VAWT for height: (a) \(Y=100\) mm; (b) \(Y=150\) mm; (c) \(Y=200\) mm; and (d) \(Y=250\) mm. in Figure 14, and comparisons were made between the performances of the building integrated CAWT at various heights above the building model. Figure 14 presents the coefficient of power of the CAWT against tip speed ratio at various heights above the building rooftop. The figure shows that the low performance of the CAWT is observed as it is mounted at higher positions above the rooftop of the building. This implies that the closer the CAWT is mounted above the rooftop, and utilizing the vertical wind deflected by the rooftop, a better performance can be expected from the CAWT. Although the results showed a better performance closer to the roof, the air flow closer to the roof is turbulent and the machine experiences more turbulence and as a result more fatigue which may shorten the lifetime of the turbine. Research has shown that VAWT can operate efficiently in a turbulence environment [12; 51; 52]. [PERSON] [53] reported that Turbulence in the approaching wind can have a significant influence on the wind turbine power output, this is mainly significant for the smaller wind turbines which in practice are often situated near buildings, trees and other Impediments. According to [PERSON] [54] the turbulence intensity has a significant influence on the performance of a wind turbine, due to the cubic variation of power with wind speed, more turbulent winds have greater power than less turbulent winds with the same mean wind speed. Evidence has shown that the turbulence level in the incoming flow affects the rate at which the velocity deficit reduces with downwind distance from the turbine (i.e., wake recovery rate), since the turbulence in the wake is an efficient mixer, it mixes the low velocity fluid in the wake with the high velocity fluid outside it. In this way momentum is transferred into the wake, the wake expands but the velocity deficit is reduced [55]. However, \(Y=100\) mm is the minimum distance in which the CAWT can be mounted above the rooftop, at \(Y<100\) mm, the distance will be too close that the vertical blade almost touch the surface of the rooftop. Moreover, the presence of the horizontal blades interacting with the deflected vertical wind enhances the self-starting capability of the CAWT and improves the overall performance of the CAWT. When the height of the CAWT above the rooftop of the building was increased from 100 to 150 mm and from 100 to 200 mm, the maximum \(C_{\text{p}}\) of the CAWT drops by 18% and 32%, respectively. The summary of the experimental results for the variation of height is presented in Table 4. Since there is non-uniform wind profile, the turbine at each position is expected to experience a different wind velocity and hence different amounts of available wind energy to be extracted. Figure 15 shows the rotational speed of the building integrated CAWT at various heights above the rooftop. The overall results and observations showed that as the height of the CAWT above the rooftop is increased, the performance of the CAWT reduces. This is because the horizontal blades of the CAWT which utilizes the vertical airflow that is deflected from the building is been positioned further away from the deflected wind flow. Therefore, at higher position above the rooftop, very small vertical  
Figure 14: Coefficient of power of CAWT at various heights above the rooftop.