## 5 Results  
Both units (TU1 and TU2) were tested prior to the installation of the concentrator system in one of the units. Heat flux through all the walls (except the south-facing walls) and the interior temperature were monitored. It was found that both configurations exhibited satisfactory behaviour, resulting in almost identical heat fluxes and temperature values. Figure 14 shows, as an example, the values that were recorded over three days. Different weather conditions (sunny and cloudy days) were examined. Therefore, as a first step, the testing units were characterised. As a second step, the concentrators were installed. As described in Section 4, one unit included the concentrator and glazing (8 mm PMMA) and the other unit had PMMA glazing only and is considered the reference case. Both glazing elements were south-facing. Figure 15 clearly depicts the benefits of the unit with the concentrating system. The results show that, in this case, the temperatures registered are less subjected to daily-temperature and irradiance variations, achieving temperature and flux values quantitatively lower than those of the reference unit. In addition to the shading effect, the modules located on the outer face of the transparent construction may improve the isolation of the interior space when the exterior temperature is lower. Furthermore, numerical and experimental stationary thermal characteristic curves were determined for both concentrator configurations (DIW and IPA). As can be seen in Figure 16, the heat-loss coefficients are high: the values range from around 14 to 20 W\({}^{\circ}\)C\({}^{-1}\)m\({}^{-2}\) (experimental and simulated wind velocity is 2 m/s). This is attributed to the fact that the solar collector morphology (glazing) was optimised for concentrating the incident irradiance but not for minimising heat losses. These losses could be reduced by embedding vacuum layers into the system with minor design changes, but the fabrication of such systems could not be conducted at the available facilities. Good agreement  
Figure 14: Comparison of heat flux (q) and temperature (T) of the twin testing units (TU1 and TU2): (a) north wall, (b) west wall, (c) east wall, (d) floor, (e) ceiling and (f) interior temperatures. between experimental and numerical values was obtained. Minor discrepancies are mainly attributed to wind speed fluctuations around the value of 2 m/s (steady-state experiments). Figure 16: Thermal characteristic curves for (**a**) the DIW configuration and (**b**) the IPA configuration. Wind velocity = 2 m/s. T\({}_{\rm m}\) is the mean fluid temperature. T\({}_{\rm a}\) is the ambient air temperature, and G stands for solar irradiation. Figure 15: Comparison of heat flux (q) and temperature (T) of the twin testing units with concentrator (TU1-Conc and TU2-Ref): (**a**) north wall, (**b**) west wall, (**c**) east wall, (**d**) floor, (**e**) ceiling, (**f**) interior temperatures and (**g**) weather information. In Figure 17, the average IV (electric current-voltage) curves of DIW and IPA modules can be seen. As shown in Figure 17, these curves were obtained under similar direct-irradiance conditions (around 965 Wm\({}^{-2}\)). Furthermore, the temperatures were controlled. More specifically, the temperatures were maintained close to 25 \({}^{\circ}\)C (26.4 \({}^{\circ}\)C for the DIW option; 26.8 \({}^{\circ}\)C for the IPA option). The FF was found to be 62.8% in the case of the IPA configuration and 61.7% in the case of the DIW one. The values of DIW were lower (comparing to those of IPA) but relatively close, as expected. It should be noted that negative impacts on V\({}_{\text{oc}}\) and especially on the FF, compared to the original cell, were observed. These effects are attributed to the following facts: (1) a reduction in the parallel resistance due to edge shunts and higher recombination rate, (2) an increased series resistance due to the six cells connected in series and (3) the limitation of maximum current by the most damaged cell in the series connection [43].