## 3 Results and Discussion
### Measurements  
#### 3.2.1 Thermographic Validity Criterion  
The new modules used for the power plant renovation were subjected to analysis of the thermal pattern in the short-circuit state, which can be seen in Figure 10. At the time of recording, the solar irradiance was 677 W/m\({}^{2}\) and the contact measured module temperature was 43.8 \({}^{\circ}\)C.  
When performing the thermographic analysis, it was necessary to continuously consider the recording angle because the emissivity parameter takes on different values. Different sources give different information on the emissivity and the viewing angle perpendicular to the module, where shooting at an angle of 5-60\({}^{\circ}\) is acceptable [19]. FLIR in particular points out that the emissivity of glass is a great challenge. \"Even though glass has an emissivity of 0.85-0.90 in the 8-14 \(\upmu\)m waveband, thermal measurements on glass surfaces are not easy to do\". One of the first papers to provide information on temperature differences taken from a helicopter [20] showed a graphical representation of the emissivity, given in Figure 11.  
Figure 10: Thermography analysis of the new module. The analysis of all previous findings, presented in [10], from the emissivity spectrum can be seen in Figure 12.  
The work in [21] states that emissivity should be selected as 0.85 for glass and 0.95 for a polymer backsheet, if the view angle is within 90-60\({}^{\circ}\) (glass) and 90-45\({}^{\circ}\) (polymer). FLIR's recommendation is visible in the form of the green space in Figure 12. Information on emissivity changes is especially important when conducting an analysis of a larger area composed of several different thermographic records is necessary because, sometimes, this is the only way we can get complete information about the temperature distribution of the surface of a PV module. Table 5 provides information on the temperature distribution on the module shown in Figure 10.  
Figure 11: Emissivity of glass as a function of reflection angle. Figure 12: Dependence of module emissivity on the shooting angle. Analyzing the data, we can conclude that the maximum temperature difference of the individual parts of the module is 27.9 \({}^{\circ}\)C. Table 6 gives the statistical data of the apparent temperature of the new module. Considering the pilot project results, where the apparent temperature only increased by 17%, the new module in the short-circuit state shows a temperature increase of 67% in some parts. Considering that the short-circuit current of the new module is up to 8 times higher, it is clear that the pattern of determining the \(\Delta\)T criterion depends on the short-circuit current and will be unique for each module. The limiting criterion will be the maximum temperature that individual parts of the module can withstand under normal operation. From the above, it is clear why the IEC TS 62446-3:2017 standard does not specify temperature values, and only the three basic classes of abnormality are specified, as listed in Table 7. \begin{table}
\begin{tabular}{c c} \hline \hline Module temperature & 41.0 \({}^{\circ}\)C \\ Average cell temperature & 48.6 \({}^{\circ}\)C \\ Maximum cell temperature & 68.9 \({}^{\circ}\)C \\ Minimum cell temperature & 41.4 \({}^{\circ}\)C \\ Cell temperature difference & 27.5 \({}^{\circ}\)C \\ Module maximum temperature difference & 27.9 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Statistical data of the apparent temperature of the new module.