## 5 Conclusions  
This paper experimentally studied the influence of a longitudinal tilt angle for the secondary system of a small-scale linear Fresnel reflector used in a low-concentrating photovoltaic system based on several evaluation indicators (useful heat gain, thermal efficiency) under various wind speed conditions and in different locations throughout the Northern Hemisphere. Other evaluation indicators were also analysed, such as solar irradiation gain incident on the photovoltaic cells and total useful energy gain. A comparative study between nonconcentrating and low-concentrating photovoltaic systems was also carried out. The experiments to obtain the evaluation indicators were carried out with two configurations: a base configuration and a longitudinal tilt configuration. The base configuration is the classical configuration of large-scale linear Fresnel reflectors and is characterised by \(\beta_{M}=0\) (\({}^{\circ}\)) and \(\beta_{S}=0\) (\({}^{\circ}\)). The basis of the study is the power not converted into electricity by the photovoltaic cells, i.e., the thermal power wasted in a nonconcentrated photovoltaic system. Depending on the dimensions of the prototype used, a resistor simulating the thermal power of 300 and 400 (W) was used. Therefore, this power takes into account the operating temperature of the photovoltaic cells. The longitudinal tilt configuration is characterised by the optimal longitudinal tilt angles [24]\(\beta_{M}=\lambda/2\) (\({}^{\circ}\)) and \(\beta_{S}=\lambda\) (\({}^{\circ}\)). The experiments were carried out with five wind speeds (around 0 (m/s), 1.05 (m/s), 2.55 (m/s), 4.99 (m/s), and 10.03 (m/s)), and six cities in the Northern Hemisphere (Medellin, Bangkok, Morelia, Karachi, Cairo, and Almeria). To carry out the research, an experimental platform was designed and built, consisting of the secondary system of a small-scale linear Fresnel reflector, an open-loop wind tunnel, an electrical subsystem, a thermal subsystem, a longitudinal tilt system of the secondary system, and measuring equipment. The following conclusions were drawn:  
1. As concerns useful heat production, the longitudinal tilt configuration performs worse as the longitudinal tilt angle and wind speed increase. For example, the following is  
Figure 24: Total useful energy gain. true for the power not converted into electricity by the photovoltaic cells of 400 (W): (a) When the wind speed is 10.03 (m/s), the useful heat gain shows the worst results, ranging from \(-21.30\%\) (Medellin) to \(-33.91\%\) (Almeria). (b) If the wind speed drops below this value, e.g., 4.99 (m/s), the useful heat gain also decreases, resulting in useful heat gains between \(-17.27\%\) (Medellin) and \(-18.77\%\) (Almeria). (c) For 2.55 (m/s), the useful heat gain is approximately \(-8\%\). (d) When the wind speed is 0 (m/s), the useful heat in the longitudinal tilt configuration is similar to that of the base configuration. As wind speed increases, the difference in useful heat gain between lower and higher latitude locations increases. (ii) The thermal efficiency decreased with increasing wind speed and a longitudinal tilt angle. The thermal efficiency was always below 0.6. As wind speed increases, this value decreases. This effect is more pronounced as the longitudinal tilt angle increases. For example, the following is true for the power not converted into electricity by the photovoltaic cells of 400 (W): (a) When the wind speed is 1.05 (m/s), the thermal efficiency ranges between 0.5 and 0.4. (b) For wind speeds of 2.55 (m/s), it ranges between 0.4 and 0.3. (c) For wind speeds of 4.99 (m/s) and 10.03 (m/s), the thermal efficiency is between 0.3 and 0.2. The lowest thermal efficiency is obtained in Almeria (location with the highest latitude). (iii) The total useful energy in a low-concentration photovoltaic system is the sum of the electrical energy generated and the useful thermal energy produced. The longitudinal tilt configuration shows the best increase in total useful energy in the absence of wind.