## 3 Results
gains due to spectral effects reported in the literature, being those gains for CIGS technology more remarkable at high latitudes in the north Hemisphere, as in the case of Reference [31]. Furthermore, CdTe modules, because of the lower power temperature coefficient and characteristic for this technology in high latitudes spectral gain in summer, have a better response than pc-Si, as shown in Figure 6[31; 33]. Nevertheless, the efficiency of CdTe modules is lower, and, finally, their yield is not as good as CIGS technology but better than pc-Si. Experimental efficiency of each studied PV technology calculated as the average of daily efficiencies (Equation (6)) measured over the period February 2015-November 2015 can be seen in Figure 7. Efficiencies of pc-Si, CIGS and CdTe technologies were found to be 14.5%, 11.0%, and 8.7%, respectively. The results of calculations based on experimental data indicate lower efficiency values for all considered PV technologies in comparison to those obtained under STC presented in manufacturer datasheet (Table 1). The highest difference (17.9%) was noted for CdTe technology. In the case of CIGS and pc-Si technologies, these differences were found to be 12.7% and 5.8% respectively. Analysis of daily energy efficiency calculated for chosen sunny days of 2015 with relatively high solar irradiation level (winter sunny days are characterized by lower values of solar irradiation in comparison with summer sunny days), presented in Figure 8, shows the linear decrease of daily  
Figure 6: Daily energy yield vs. daily average temperature of the different technology modules calculated for sunny summer days of 2015 with similar solar irradiation levels. Figure 7: Comparison of the photovoltaic (PV) modules’ efficiencies presented in manufacturer datasheet with the efficiencies obtained experimentally over the period February 2015–November 2015.  
efficiency with the increase of the daily average modules temperature. Temperature coefficients of efficiency for studied types of PV technology were derived from experimental data according to the following equation [34]:  
\[\eta=\eta_{r}-\mu(T_{c}-T_{r}), \tag{7}\]  
in which: \(\eta_{r}\)--reference module efficiency at reference temperature \(T_{r}\) = 25 \({}^{\circ}\)C, \(\mu\)--temperature coefficient, \(T_{c}\)--measured temperature of the module. The temperature coefficient value is a slope of a line that represents the efficiency changes for each module type in Figure 3:0 The values of temperature coefficient, both obtained from experimental data and calculated based on manufacturer information (Table 1) as \(\mu=\beta\eta_{r}\), are presented in Figure 3:1  
The comparison of the data obtained by two methods shows that the temperature coefficient resulting from experimental data exhibits a higher value for pc-Si modules than the value calculated according to the datasheet. The experimentally estimated value of pc-Si modules temperature coefficient is the highest compared to other technologies, which is directly connected to the strong influence of temperature on pc-Si energy production (Figure 6). The temperature coefficients of both thin film technologies are lower than these for pc-Si. In realistic operational conditions, their temperature coefficients achieve even lower values than the values determined from the datasheet, which is favorable and confirms the observations of high energy production by these technologies presented in Figure 3:2  
Figure 9 shows the dependency of DC output power on the PV modules temperature under real outdoor conditions for selected sunny days of 2015. The effect was determined by measuring DC output power and instantaneous module temperature at incident solar irradiance equal to 1000 W/m\({}^{2}\) (+/\(-\)0.6%). The lowest module temperature at irradiance of 1000 W/m\({}^{2}\) was found to be 28 \({}^{\circ}\)C (registered at 11.18 on 23 rd of March 2015). The highest value was equal to 49 \({}^{\circ}\)C (registered at 12.09 on 2 nd of July 2015).