## 3 Experimental Setup
### The Prototype  
\(LCPV\) systems require uniform illumination of the \(PV\) cells, otherwise their fill factor and overall electrical efficiency decrease [29] and may even damage the cells [30]. In [14], \(SSLFR\) parameters were identified which reflect a bad design that can cause nonuniform illumination on \(PV\) cells. These parameters are as follows: (i) the contribution of shading and blocking phenomena between adjacent mirrors (small \(d\)), (ii) high mirror spacing (large \(d\)), (iii) the number of mirrors, and (iv) the width of the \(PV\) system. Figure 4: Experimental platform installed. Figure 3: Experimental platform scheme. Therefore, an \(SSLFR\) for \(LCPV\) system applications can be considered to be optimally designed when the so-called \"optimal design conditions\" are met [14]: (i) the flux density over the \(PV\) cells is homogeneous; (ii) there is no shading phenomenon between adjacent mirrors; and (iii) there is no blocking phenomenon between adjacent mirrors. Before defining the measurements of the secondary system being tested, the parameters of the \(SSLFR\) need to be calculated. The method proposed in [14] can be used for this purpose. The following parameters were used to apply the methodology proposed in [14]:  
1. The width and length of the prototype are limited by the dimensions of the available wind tunnel. Therefore, a commercial \(PV\) cell with a width of \(60\) (mm)  and a length of \(30\) (mm)  was chosen. A total of \(21\)\(PV\) cells were arranged in the \(PV\) system. Therefore, the length of the \(PV\) system was \(658\) (mm) (\(21\cdot 30\) (mm)). 2. Thermal interface material plays a key role in devices operating at high temperatures, such as photovoltaic concentrator devices or integrated microelectronic devices [31]. Silicone grease [32] was used as the thermal interface material between the \(PV\) cells and the cooling system. 3. Normally, the height to receiver (\(f\)) is taken as \(1500\) (mm) [14; 33; 34]. 4. The choice of the number of mirrors is directly related to the cost of the \(SSLFR\)[35]. Nine mirrors were chosen so the cost would not be too high. Therefore, \(N=4\). 5. Based on the length of the \(PV\) system, a mirror length of \(658\) (mm) was chosen. 6. The optical properties of the materials used in the calculations were already used in previous studies [14; 36]: mirror reflectivity \(\rho=0.94\)[37], mirror cleanliness \(CI_{m}=0.96\)[38], glass cleanliness \(CI_{g}=0.96\)[38], glass transmissivity \(\tau_{g}=0.92\)[39], encapsulant transmissivity \(\tau_{e}=0.90\)[40], and \(PV\) cell absorptivity \(\alpha=0.90\)[40]. 7. The optimal design conditions are impossible to meet from sunrise to sunset. Therefore, Ref. [14] defines the so-called \"optimum operation interval\" (\(\theta_{t_{0}}\)). This \(\theta_{t_{0}}\) guarantees the homogeneous distribution of solar irradiance without shadows or blocking. Several aspects have to be taken into account when choosing \(\theta_{t_{0}}\), such as the length of the \(SSLFR\), the optimal hours of operation, and the total annual solar irradiation effectively reaching the \(PV\) cells. As an example, Figure 5 shows three curves related to the choice of \(\theta_{t_{0}}\) for the city of Almeria, Spain (latitude \(36^{\circ}50^{\prime}07^{\prime\prime}\) N, longitude \(02^{\circ}24^{\prime}08^{\prime\prime}\) W, and elevation \(22\) (m)), as well as the parameters chosen in this section. Figure 5a shows the relationship between the number of hours with a guaranteed homogeneous distribution of solar irradiance without shading or blocking and the optimum operation interval. If \(\theta_{t_{0}}\) is high, the number of guaranteed hours will also be high. Figure 5b shows the variation of the % of annual irradiation that actually reaches the \(PV\) system as a function of \(\theta_{t_{0}}\). If \(\theta_{t_{0}}\) is high, the % of annual irradiation will also be high. Figure 5c shows the optimal width of the \(SSLFR\) for each value of \(\theta_{t_{0}}\).