## 3 Results and Discussion
### Measurements
#### 3.2.4 Infrared Thermographic Analysis of Undamaged Modules
Considering the observed extremes of the hotspots (maximum temperature values of the modules are shown in Figure 27), we can conclude that module 4 is defective, i.e., not suitable for further use because the temperature difference is 65.3 \({}^{\circ}\)C. In addition, module 8 should be monitored, considering the lower value of solar irradiance with a temperature difference of 33.3 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Module Number** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **Average** \\ \hline Minimum temperature [\({}^{\circ}\)C] & 40.8 & 40.9 & 43.4 & 48.9 & 45.0 & 48.0 & 48.5 & 47.0 & 45.3 \\ Maximum temperature [\({}^{\circ}\)C] & 55.7 & 63.7 & 60.8 & 69.8 & 69.5 & 64.9 & 75.7 & 66.0 & 65.8 \\ Difference [\({}^{\circ}\)C] & 14.9 & 22.8 & 17.4 & 20.9 & 24.5 & 16.9 & 27.2 & 19.0 & 20.5 \\ Percentage difference [\%] & 36.52 & 55.75 & 40.09 & 42.74 & 54.44 & 35.21 & 56.08 & 40.43 & 45.16 \\ Measurement time [h] & 10:27 & 10:48 & 11:11 & 11:38 & 11:52 & 12:49 & 13:11 & 13:38 & \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Numerical data comparison of the thermographic analysis. Figure 25: Temperature correction values for equalization of the results due to the influence of different amounts of solar irradiance. Figure 26: Minimum temperature of modules during measurements. The analysis performed on 62% of the total studied modules indicates that another faulty module may occur. Another conclusion is that, if reusing entire modules, it is necessary to monitor at least two modules for further development of potential hotspots. This is due to damage that can develop as the module ages during operation. The results of the visual inspection described in [13] are shown in Figure 28, which illustrate the different forms of damage found on the modules. A detailed overview of all the different module damage types can be found in the NREL report [23]. Interesting examples of damage and appearance of a PV module after 27 years of operation can be found in [22]. A further analysis of the studied modules was done by classifying the visible damage into five levels. Level 1 represents a cell without any visible damage, level 2 represents barely visible damage, level 3 represents visible damage present in the right photograph of Figure 28, level 4 represents damage presented in the centre photograph in Figure 28, and level 5 is visible in the left photograph of Figure 28. The visual inspection and damage classification of module number 3 for each cell is given in Figure 29, along with the thermogram of the same module. The thermogram of module number 3, given in Figure 29 (right), is captured right after the thermographic analysis described in Section 3.2.4 and is given in Figure 20 (right) before disposal of the PV plant at the microlocation. The purpose of the thermogram given in Figure 29 (right) was not to conduct a thermographic analysis but rather to determine the thermal pattern for comparison with the damage classification via visual inspection. Comparison of the thermogram for module number 3 with the visual inspection damage classification results in Figure 29 shows that there is a significant difference. The thermal pattern does not correspond to the results of the visual inspection, i.e., conclusions that could be made on the basis of the visual inspection, whereby evaluation of the individual cell state via visual inspection emphasizes the complexity of the studied problem. Figure 27: Maximum temperature values of the module surface. Figure 28: Observed damage determined by visual inspection of the module. In a review of PV module failure [21], in a section on documenting visual failures in the field, the observed patterns were classified as snail tracks. Snail tracks are discolorations of the silver paste used for the gridlines of cells. This discoloration appears along cell cracks.