## 4 Experimental Investigation  
Experiments were carried out at the Applied Energy Research Centre (CREA) at the University of Lleida (Spain), which is located in Lleida (latitude 41.36\({}^{\circ}\) N and longitude 0.37\({}^{\circ}\) E). The experimental phase comprised different stages, ranging from individual characterisation of the concentrator elements to system characterisation. Three photographs of the characterisation of one of the concentrators are included in Figure 11. Figure 11a illustrates the concentrator that was mounted on a two-axis solar tracker so that solar irradiance was always perpendicular to the PV-cell plane. The tracker was equipped with a pyranometer (Kipp & Zonen CMP6, Delft, the Netherlands) and a pyrheliometer (Kipp & Zonen CHP1, Delft, the Netherlands) to measure global and direct irradiance, respectively. Figure 11b shows details of the hydraulic connection and the concentrated spot (indicated by an arrow). Also, in Figure 11c, the concentration effect and the optimised module are shown. The 5 mm cell optically increased to 60 mm. After checking the performance of the 8 concentrator units to ensure their correct operation, these components were mounted on the structure described in the previous section. The system, with a gross aperture area of approximately 1 \(\times\) 1 m\({}^{2}\) and 8 concentrator modules (4 modules with DIW circulating in series through all of them and 4 modules with IPA in a similar way), was installed on a specially built testing unit. The system was monitored and the thermal/electrical characteristics were obtained. Its effect on the temperature/illumination in the interior space of the testing unit was examined. In order to investigate the behaviour of the proposed system, taking into account building integration and the effect of the system on the interior temperature and illumination, two testing units were built. These testing units had the same dimensions and materials--1 \(\times\) 1 \(\times\) 1 m\({}^{3}\) cubes with all the walls made of prefabricated sandwich panels of 10 cm thickness--except the one oriented to the south (in this case, a sheet of methacrylate (8 mm thickness) was used). Both testing units (Figure 12) were oriented to the south. Moreover, both testing units were  
Figure 11: Characterisation of the modules: (**a**) A PV module mounted on a two-axis tracker together with a pyrheliometer and a pyranometer for solar radiation measurements, (**b**) details of the hydraulic connection and the concentrated light, (**c**) details of the module concentration effect. placed 50 cm above the ground to minimise the albedo effect. It should also be mentioned that the differential thermal and illumination effects in the interior space were evaluated. As can be seen in Figure 13, a pyranometer (Kipp&Zonen CMP6) was placed in the vertical plane of the modules in order to register the proper global irradiance received. Furthermore, each module had T-type thermocouples at the inlet/outlet and on the rear surface of the PV PERC solar cells [42] located at the half-length of the module. Both units were equipped with flux meters (Hukseflux HFP01, Delft, the Netherlands) on each wall, except for the one where the element to be tested was located. A luxmeter (PCE-174, Tobarra, Spain) was placed in the centre of each testing unit. In addition, a T-type thermocouple was suspended from the ceiling (in the centre of the air volume). The thermocouple had a reflective shielding element to avoid the influence of incident irradiance and monitor the interior air temperature. The IV curves were measured with a source meter (Keithley 2460, Cleveland, OH, USA), and all the data were stored in a datalogger (Campbell Sci. CR3000, Leicestershire, United Kingdom). Finally, there were two liquid-holding tanks: one was filled with IPA, and the other was filled with DIW. The circulation of the two liquids was maintained by two precision peristaltic pumps ([PERSON], Schwabach, Germany) at 0.0018 kg/s.