## 4 Experimental Results and Discussion  
The aim of this section is to estimate the effect of a longitudinal tilt angle for the secondary system of a low-concentration \(PV\) system based on a small-scale linear Fresnel reflector and the wind speed on the proposed evaluation indicators: useful heat gain, thermal efficiency, solar irradiation gain incident on the \(PV\) cells, and total useful energy gain. Two configurations were analysed for this purpose: a base configuration and a longitudinal tilt configuration. The base configuration (\(C_{B}\)) is the classical configuration of large-scale \(LFRs\) and is characterised by \(\beta_{M}=0\) (\({}^{\circ}\)) and \(\beta_{S}=0\) (\({}^{\circ}\)). The longitudinal tilt configuration (\(C_{S}\)) is characterised by the optimal longitudinal tilt angles [24]\(\beta_{M}=\lambda/2\) (\({}^{\circ}\)) and \(\beta_{S}=\lambda\) (\({}^{\circ}\)). Experimental tests were carried out for various geographical locations and compared with the longitudinal tilt angle used in large-scale \(LFRs\).  
The available flat roof surface of the building was considered to be the same for all studies carried out. Specifically, the dimensions of the flat roof were a width of 78.68 (mm) and a length of 658 (mm). The electrical efficiency of the \(PV\) cells was also assumed to be the same for both configurations. In fact, 0.2 was assumed for the two. Moreover, the operating temperature of the \(PV\) cells was assumed to be the same in both configurations for the same wind speed.  
The useful heat of the secondary system was determined for various longitudinal tilt angles, various powers that the \(PV\) cells did not convert into electricity, and various wind speeds. Section 3 shows the dimensions and materials of the prototype under testing. The conditions of the experimental tests conducted were as follows: (i) a range of longitudinal tilt angles from 0 (\({}^{\circ}\)) to 36.86 (\({}^{\circ}\)) at intervals of approximately 6 (\({}^{\circ}\)); (ii) a wind speed range of 0 (m/s) to 10.03 (m/s); (iii) an initial ambient temperature range of 19.00 (\({}^{\circ}\)C) to 20.70 (\({}^{\circ}\)C); (iv) two situations of unconverted electricity power from the \(PV\) cells of 300 (W) and 400 (W); and (v) a water flow rate through the cooling system of 0.5 (L/min). The thermal performance of the prototype was examined by monitoring the thermal behaviour while varying the wind speed and the longitudinal tilt angle of the secondary system.  
Mathematica(tm) Computer Algebra System software was used to determine the irradiance incident on the \(PV\) system. This software has been used extensively in similar studies [14]. In order to be able to analyse how the experimental results affect the \(SSLFR\) configuration, the beam solar irradiance incident on the \(PV\) system must be determined. Thus, the beam solar irradiance on the horizontal surface of the site under study and, therefore, the specific meteorological conditions must be taken into account. The method proposed by [60] can be used to determine the particular meteorological conditions at agiven location. This method uses the \(PVGIS\)[54] database to obtain the monthly average beam solar irradiance over horizontal surfaces. The equations used for the calculation of the incident solar irradiation on the \(PV\) system are presented in [14]. Table 5 shows the beam solar irradiation incident (\(H_{b}\)) on the \(PV\) cells for each configuration and city studied. This table also shows the beam solar irradiation incident gain (\(H_{b}G\)) with respect to the base configuration (\(H_{b_{C_{B}}}\)).  
As can be seen in Table 5, the effect of the longitudinal tilt of the secondary system is positive with respect to the incident solar irradiation on the \(PV\) cells.