## 3 Results and Discussion  
In the present section, the results of this study are provided with detailed analysis. Firstly, Figure 4 depicts the optical efficiency of the collector for the mean day of June. An hourly analysis was performed to determine the optimum value for the collector slope, as described in Section 2.3.1. The presented findings exclusively pertain to the morning \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Outer air temperature (\({}^{\circ}\)C) & 26.5 \\ Outer air speed (m/s) & 3.3 \\ Heat convective coefficient with ambient (W/m\({}^{2}\)/K) & 12.59 \\ Heat convective coefficient with air in the gap (W/m\({}^{2}\)/K) & 5 \\ Solar irradiance—direct (W/m\({}^{2}\)) & 725 \\ Solar irradiance—diffusive (W/m\({}^{2}\)) & 218 \\ Flow rate—volumetric (It/min) & 2.2 \\ Inlet water temperature (\({}^{\circ}\)C) & 20–80 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermal simulation operating conditions. period, with the understanding that the same outcomes apply to hours beyond 12:00. To illustrate, the optimal inclination angle for 13:00 corresponds to that of 11:00. It is noteworthy that the optical efficiency of the solar collector is low in the early mornings and in the late afternoon period. Furthermore, sudden shifts in the performance of the collector are observed, especially for higher inclination angles. This phenomenon arises from the fact that, during these circumstances, the reflector does not contribute to solar utilization, leading to a drastic and abrupt reduction in the collector's useful reflective area. In addition, Figure 4 shows that a time period exists around the solar noon in which the optimal angle of inclination remains 10\({}^{\circ}\). The maximum value of the calculated optical efficiency is 75%. It is also remarkable to notice that the optical efficiency curves seem to be parallel to certain slope ranges which are different in each time moment, and that happens because in these ranges, the concentrator of the collector does not contribute to the solar energy utilization. Hence, in these cases, only the upper part of the receiver remains active for the solar energy absorption. After establishing the optimal inclination angle based on Figure 4, the investigated solar unit was subjected to an analysis involving different inlet temperatures. This analysis aimed to evaluate the unit's performance under various operational conditions, specifically for the determined tilt angle. Figure 4: Optical efficiency of the solar unit for the mean day of June. Firstly, so as to verify the results from the simulation model, the Darcy friction factor was calculated. The Darcy friction factor was determined by comparing values obtained from the simulation and the theoretical calculations. The theoretical values were derived using Equation (16), whereas the numerical analysis utilized the pressure drop generated during the simulation by Equation (15). The values from the prementioned analysis are presented in Figure 5. A maximum deviation of 2.5% is observed. Therefore, the findings from the numerical analysis demonstrate close agreement with the theoretical values, affirming the reliability of the developed simulation model. Figure 6 illustrates the thermal solar collector performance, determined by applying Equation (8) and utilizing data from the two simulation tools employed. The obtained results demonstrate a strong correspondence between the two types of software, with a maximum deviation of 5.3% to be observed. These findings confirm the reliability and validity of the simulation method employed regarding the thermal modeling. The thermal performance line shows a downward trend with an increase in the water temperature at the inlet. This is a logical outcome because a higher temperature at the inlet results in greater losses, ultimately leading to a decrease in thermal efficiency. Figure 7 showcases the temperature profiles of both the upper and the lower photovoltaic components, calculated using the simulation tools.