Hot spots are areas where an uncontrolled change in resistance has occurred causing a significant amount of Joule-Lenz heat to be released and the cell temperature to rise in these areas. The presence of hot spots can be due to the presence of current path defects such as the presence of undersoldings and changes in cross-section, as well as cell microdamage. In the defective area, the cell dissipates energy as heat instead of generating it. The problem of hot spots becomes very serious when part of the cell is shaded. In such a situation, the shaded cell does not produce electricity, but the reverse current, generated by unshaded cells, flows through it, causing an increase in temperature [1]. In order to prevent the occurrence of hot spots in modules, by-pass diodes are used. The purpose of the bypass diodes is to redirect the generated current in such a way that no current flows through the damaged and shaded cell [2].
The temperature of the hot spot depends on the location and conductivity of the material used to produce the photovoltaic cells [3]. The hot spot near the edge of the silicon module achieves much higher temperatures than the hot spot occurring in the center of the module. This is because the thermal conductivity in the silicon cell is higher than in the laminate. For this reason, heat is distributed over a larger area if the hot spot occurs in the center of the panel. Hot spots can be detected by a thermal imaging survey of the photovoltaic system. An image taken with a thermographic camera is shown in Fig. 1. It shows the temperature distribution on the surface of the photovoltaic module. The surface temperature of a properly working module should be identical over its entire area. The image shows an area with an increased temperature in comparison with the rest of the module, which indicates the presence of hot spots.

Figure 1: Photographs of modules taken with a thermal imaging camera showing areas of overheating of cells (so-called hot spots). Photo by Stanisław Bajorski – PV thermovision measurements – used with permission of Solar Building Polska.

Photovoltaic cells in hot-spot areas can heat up to over 250 \( _{}^{o}\textrm C{} \) [1]. Heating the cell to temperatures exceeding the strength of the EVA film can lead to permanent damage, resulting in a dark discolouration on the cell surface. The occurrence of "hot-spots" accelerates the ageing of the cell, which can result in shortened lifetime of the module and decrease in energy efficiency by over \( 15\% \) [4], and in extreme situations can lead to a system fire.