## 4 Results and Discussion
In addition, according to Figure 3a,c, an increase in the height difference of 1 m leads to an increase in the air velocity \(v_{\rm o}\) by 38.43-39.84% and 23.01-23.46% for the cases of \"Roof\" and \"Facade\", respectively. Figure 4 illustrates the dependence of the average temperature of the PV section \(T_{\rm PV}\) on the solar irradiance \(q_{\rm E,S}\) for the PV sections that consisted of 1 and 6 PV modules and the solar chimney heights of (a) \(L_{\rm TS}+L_{\rm AP}\approx\) 0.5 m; (b) \(L_{\rm TS}+L_{\rm AP}=1\) m; and (c) \(L_{\rm TS}+L_{\rm AP}=1.5\) m. The PV sections with one and six PV modules have the lengths of \(L_{\rm PV}=0.52\) m and \(L_{\rm PV}=3.12\) m, respectively. The labels \"Roof\" and \"Facade\", as well as the marks of the curves, have the same meanings as in the case of \[T_{\text{PV}}=\frac{(T_{\text{PVb}}+T_{\text{PVu}})}{2}, \tag{23}\]0  
According to Figure 4, the average temperature of the PV section \(T_{\rm PV}\) increases with the increase in the solar irradiance \(q_{\rm E,S}\) and decreases with the increase in the solar chimney height. The effect of the variation in the height of the solar chimney \(L_{\rm TS}+L_{\rm AP}\) on the average temperature of the PV section \(T_{\rm PV}\) is almost negligible for the cases of the PV sections composed of six PV modules (the curves represented by solid red lines and red circles). In the cases of the PV sections composed of one PV module, the effect is more pronounced. Furthermore, this decreasing effect is more pronounced in the case of \"Roof\" than in the case of \"Facade\". This is a consequence of the increase in the air velocity \(v_{\rm o}\) with the increase in the solar chimney height from \[T_{\text{PV}}=\frac{(T_{\text{PVb}}+T_{\text{PVu}})}{2}, \tag{23}\]1 Therefore, the greater the amount of heat transferred between the PV section and the air in the ventilation channel, the smaller the effect of the air velocity \(v_{\rm o}\) on the average temperature of the PV section \(T_{\rm PV}\). For the curves represented by dashed blue lines and blue squares in Figure 4, in the range of the considered values for \(q_{\rm E,S}\), an increase in the solar chimney height of 0.5 m leads to a decrease in the temperature \(T_{\rm PV}\) by 1.31-4.71% and 0.37-1.75% for the cases of \"Roof\" and \"Facade\", respectively. In addition to this, an increase in the solar chimney height of 1 m leads to a decrease in the temperature \(T_{\rm PV}\) by 1.95-7.21% and 0.65-2.92% for the cases of \"Roof\" and \"Facade\", respectively. \[T_{\text{PV}}=\frac{(T_{\text{PVb}}+T_{\text{PVu}})}{2}, \tag{23}\]2 The average temperature of the PV section depending on the solar irradiance for two different numbers of PV modules per section and (a) \(L_{\rm TS}+L_{\rm AP}\approx\) 0.5 m; (b) \(L_{\rm TS}+L_{\rm AP}=1\) m; and (c) \(L_{\rm TS}+L_{\rm AP}=1.5\) m.  
In connection with the results shown in Figures 3 and 4, it is interesting to establish the flow types of free convection from the upper surfaces of the PV section and the acrylic plate. This can best be established based on the value of the fundamental dimensionless number for free convection. Accordingly, Figure 5 presents the dependence of the fundamental dimensionless number for free convection on the solar irradiance \(q_{\rm E,S}\) for the PV sections with the lengths \(L_{\rm PV}=0.52\) m (consisting of one PV module) and \(L_{\rm PV}=3.12\) m (consisting of six PV modules) and the solar chimney heights \(L_{\rm TS}+L_{\rm AP}\approx 0.5\) m, \(L_{\rm TS}+L_{\rm AP}=1\) m, and \(L_{\rm TS}+L_{\rm AP}=1.5\) m. In Figure 5, the solid blue line, dash-dot red line, and black circles correspond with the PV section consisted of 1 PV module, and the solid green line, dashed red line and black triangles correspond with the PV section that consisted of six PV modules. According to Figure 5, the fundamental dimensionless number for free convection is lower than \(10^{9}\) for the PV section that consisted of one PV module, which corresponds to laminar flow.