## 5 Results and Discussion  
Figure 7 shows the results of experiments under the following conditions: covering the solar panel with Grafplast PDA roofing material, using TOPTERM MULTILAYER PIPE PEX/AL/PEX tubes, and an intensity of solar radiation \(I=500\) W/m\({}^{2}\).  
Figure 8 presents the results of experiments under the following conditions: covering the solar panel with roofing material, TOPTERM MULTILAYER PIPE PEX/AL/PEX tubes, and intensity of solar radiation \(I=1000\) W/m\({}^{2}\).  
Figure 7: Dependence of solar panel efficiency \(\eta\) covered with Grafplast PDA roofing material on the distance \(h\) (m) between the PEX/AL/PEX, the heat-carrying medium mass flow rate \(G\) (kg/h), and tube diameter \(d\) (mm).  
Figure 10 presents the results of experiments under the following conditions: covering the solar panel with roofing material, PRANDELLI/TUBORAMA 02 STOP tubes, and intensity of solar radiation \(I=1000\) W/m\({}^{2}\).  
Figure 8: Dependence of solar panel efficiency \(\eta\) covered with Graftplast PDA roofing material on the distance \(h\) (m) between the PEX/AL/PEX tubes, the heat-carrying medium mass flow rate \(G\) (kg/h), and tube diameter \(d\) (mm).  
Figure 10: Dependence of solar panel efficiency \(\eta\) covered with Graftplast PDA roofing material on the distance \(h\) (m) between the PRANDELLI/TUBORAMA 02 STOP tubes, the heat-carrying medium mass flow rate \(G\) (kg/h), and pipe diameter \(d\) (mm).  
Figure 9: Dependence of solar panel efficiency \(\eta\) covered with Graftplast PDA roofing material on the distance \(h\) (m) between the PRANDELLI/TUBORAMA 02 STOP tubes, the heat-carrying medium mass flow rate \(G\) (kg/h), and tube diameter \(d\) (mm).  
The graphs in Figures 7-10 are approximated by Formulas (1)-(4), respectively.  
\[\eta=44.0+(4.72-0.14d)(0.4G-5.2+18.5h-1.5 Glh), \tag{1}\]  
\[\eta=32.55-0.22d+(0.056d+1.24)(10h+0.35G-Gl-4.35), \tag{2}\]  
\[\eta=36.71+0.3d+0.09G-5.4h-0.18 Glh+0.027 Gd-1.6 hd-0.05 Gld, \tag{3}\]  
\[\eta=27.5+(0.02d+0.95)(0.334G-20h-0.67 Gl+1.0). \tag{4}\]  
The research results under these conditions (Figures 7-10) proved that the efficiency of the solar panel increases with a decrease in the pitch between the tubes, as well as with a decrease in their diameter, as well as with an increase in the flow of the heat carrier with an increase in the intensity of solar radiation. However, increasing the mass flow rate and reducing the diameter of the tubes results in an increase in hydraulic resistance, which is an undesirable phenomenon. Under the selected conditions of the experiments, the optimal values are the following: diameter of the tubes \(d=16\) mm, step between the tubes \(h=0.1\) m, mass flow rate of heat-carrying medium \(G=45\) kg/h. As for the material of the tubes, PRANDELLI/TUBORAMA 02 STOP and PEX/AL/PEX are equivalent since the advantage of 67% PRANDELLI/TUBORAMA 02 STOP over 65% PEX/AL/PEX may be due to experimental error.  
By way of discussion, it should be noted that Figures 7-10 and Formulas (1)-(4) are summarized as follows: to increase the efficiency of the panel, values \(d\) and \(h\) should be reduced, and \(G\) should be increased. However, they should be optimized in terms of hydraulic resistance.  
The direction of future research will involve the selection of other tube materials and coatings in order to compare the obtained results and carry out optimization in technical and economic aspects. In addition, it is advisable to conduct research on other values of the intensity of solar radiation, for example, 700-800 W/m\({}^{2}\) and 1200-1300 W/m\({}^{2}\).