## 3 Experimental Setup
### Test Conditions
#### 3.6.5 Duration of Each Test  
Each test used a longitudinal tilt angle, dissipated thermal power, and five wind speeds. The duration of each test was five hours. A total of seventy hours were spent on the research. Each test was carried out on a different day.  
Figure 11: Environmental conditions.  
3.6.6. Thermal Power  
Depending on the \(PV\) cell mode of operation, a significant part of the absorbed solar irradiance may not be converted into electricity and is, therefore, completely dissipated as heat. This internal heat source can be expressed according to the following equation [55]:  
\[Q_{th}=I_{t}\cdot A_{PV}\cdot(1-\eta_{e}) \tag{5}\]  
where \(Q_{th}\) is the internal heat source (W), \(I_{t}\) is the total absorbed solar irradiance for the \(PV\) system (W/m\({}^{2}\)), \(A_{PV}\) is the total area of the \(PV\) system (m\({}^{2}\)), and \(\eta_{e}\) is the electrical efficiency of the \(PV\) system.  
Part of the heat \(Q_{th}\) is transferred by conduction to the wall of the flat tube, according to [PERSON]'s law of conduction [56]:  
\[Q_{cond}=\frac{k\cdot A_{AC}\cdot(T_{PV}-T_{AC})}{\delta_{AC}} \tag{6}\]  
where \(k\) is the thermal conductivity of the flat tube (W/m\({}^{\circ}\)K), \(A_{AC}\) is the area of the flat tube (m\({}^{2}\)), \(T_{PV}\) is the temperature of the \(PV\) cells (\({}^{\circ}\)K), \(T_{AC}\) is the temperature of the flat tube (\({}^{\circ}\)K), and \(\delta_{AC}\) is the wall thickness of the flat tube (m). In this study, \(k=50\) (W/m\({}^{\circ}\)K).  
The heat is transferred by convection to the working fluid through the wall of the flat tube, according to Newton's Law of Cooling [56]:  
\[Q_{conv}=h_{i}\cdot A_{AC}\cdot(T_{AC}-T_{w}) \tag{7}\]  
where \(h_{i}\) is the heat transfer coefficient of the wall of the flat tube (W/m\({}^{2\circ}\)K), \(A_{AC}\) is the area of the flat tube (m\({}^{2}\)), \(T_{AC}\) is the surface temperature of the wall of the of the flat tube, and \(T_{w}\) is the working fluid temperature. \(h_{i}\) can be determined using the following equation [57]:  
\[h_{i}=\frac{k_{w}}{D_{h}}\cdot 0.23\cdot R_{ew}^{0.8}\cdot P_{rw}^{0.4} \tag{8}\]  
where \(k_{w}\) is the thermal conductivity of the working fluid (W/m\({}^{\circ}\)K), \(D_{h}\) is the hydraulic diameter of the flat tube (m), \(R_{ew}\) is the Reynolds number of the working fluid, and \(P_{rw}\) is the Prandtl number of the working fluid.  
Part of the heat \(Q_{th}\) is transferred by convection (\(Q_{convPV}\)) and radiation (\(Q_{radPV}\)) to the environment. According to the Stefan-Boltzmann law [56],  
\[Q_{radPV}=\sigma\cdot\varepsilon_{PV}\cdot A_{PV}\cdot\left(T_{PV}^{4}-T_{amb }^{4}\right) \tag{9}\]  
where \(\sigma\) is the Stefan-Boltzmann constant which is equal to \(5.67\cdot 10^{-8}\) (W/m\({}^{2}\)K\({}^{4}\)), \(\varepsilon_{PV}\) is the emittance of the \(PV\) system, \(A_{PV}\) is the total area of the \(PV\) system (m\({}^{2}\)), \(T_{PV}\) is the temperature of the \(PV\) cells (\({}^{\circ}\)K), and \(T_{amb}\) is the ambient temperature (\({}^{\circ}\)K). In this study, \(\varepsilon_{PV}\)= 0.9 [57]. According to [PERSON]'s Law of Cooling [56],  
\[Q_{convPV}=h_{PV}\cdot A_{PV}\cdot(T_{PV}-T_{amb}) \tag{10}\]  
where \(h_{PV}\) is the heat transfer coefficient of the \(PV\) system (W/m\({}^{2\circ}\)K), \(A_{PV}\) is the total area of the \(PV\) system (m\({}^{2}\)), \(T_{PV}\) is the temperature of the \(PV\) cells (\({}^{\circ}\)K), and \(T_{amb}\) is the ambient temperature (\({}^{\circ}\)K). The heat losses of the flat tube are considered negligible due to the insulation consisting of a glass wool filling 100 (mm) thick. Figure 12 shows a simplified heat transfer scheme.