**2.6. Snail tracks**

Snail tracks are discolorations of the silver fingers on solar cells. A significant example can be seen in **Figure 3**. The effect looks like a snail has passed across the front glass of the PV module. The discoloration takes place on cell cracks that are not visible at the edge of the solar cell. This typically happens between 3 months and 1 year after installation of the PV modules [18]. Discoloration speed is initially dependent on seasonal and environmental conditions, such that snail tracks seem to spread faster during summer months and in hot climates [5]. PV modules affected by snail tracks have been compared with reference modules under laboratory conditions [19] and results showed a 40% reduction in maximum power under standard conditions with a 25% lower yield than expected when measured over a 30-day period.

#### **2.7. Hot spots and burn marks**

A typical and very common failure in silicon PV modules is burn marks. This failure occurs due to part of the module becoming very hot and can be because of ribbon breakage, solder bond failure, or localized heating from reverse current flow or other hot spots [5]. Burn marks can produce power losses and serious safety problems. They are usually located on or closed to the metal contacts of the PV solar cells, such as it can be seen in **Figure 4**.

Hot spots are areas in a photovoltaic module that have very high operating temperatures when compared to surrounding areas. This may be due to interconnection failures, defects in the cell, dispersion of characteristics between modules of a generator and between cells of the same module connected in series, potential-induced polarization in modules manufactured with novel techniques, or when a cell generates less current than other cells connected in series as a consequence of intermittent cloud cover or partial shading [20]. As a consequence, the cell becomes polarized (the voltage between the terminals becomes negative) and starts to dissipate the power generated by the other serial cells in the form of heat.

**Figure 3.** Example of a snail track over a PV cell.

Recently, a new maximum power point tracking (MPPT) method was proposed to avoid the consequences of hot spots. It is based, firstly, on a bidirectional buck converter to control the operating point of each module and uses a boost converter to control the terminal voltage of each branch. Secondly, MPPT is modeled as an large-scale global optimization (LGSO), and a novel, multicontext, cooperatively coevolving particle swarm optimization (PSO) algorithm (CCPSO-m) is proposed to solve this large-scale problem [21].
