First we need to mention something, “Snail Tracks” also known as “Worm Marks” are not as the name implies traces of slime left on the modules by snails or other animals. They are only looking like this, but are a phenomenon which is inside of the module.
You can see them mostly on mono- and polycrystalline modules. We have observed the development of them on a few pv-plants, on some they appeared quite fast, after 5 months, on others it took one or two winters for them to appear.
Snail Tracks are discolorations of the conductor fingers, mostly visible on the front of the module. The function of the conductor fingers is to transport the current from the silicon to the wire. The conductor fingers are normally printed with a silver-metal paste onto the silicon.
What do Snail Trails tell us?
Short: It is a microscopic tear, visible because of chemical reactions. This means, every module with Snail Tracks definitely has tears – but no Snail Trails is no sign of a module not having tears.
The Institut für Solarenergieforschung (institute of solar energy research) found out about the forming of Snail Trails around microscopic tears. The scientists think the discolorations are because of a chemical reaction of hydrogen (moisture is transformed in the tears through electrolysis) with silver-metal paste and halogens. It is similar to the discoloration of silverware when using the wrong cleaning agent. The reaction is on top of the silver-metal paste so the contact of the paste to the silicon is not directly disturbed.
There is a connection supposed with the EVA-lamination of the module: In the EVA lamination, the individual cells are embedded; EVA stands for “ethylene-vinyl acetate”. The layer is located between the glass and the cells and, therefore, must be transparent. At the same time, EVA is in its raw form not known to be very resistant to ultra violet light. Therefore, this EVA films are often provided with additives providing fire resistance and other desirable effects. Those additives are often from the chemical group of halogens and so they do provide one of the reaction partners.
Settlement through warranty, product and performance guarantees
The assessment of worm tracks in different modules is not always the same. Relevant for the revenue are primarily the micro cracks. However, they can be found in each module that has already been mounted. If the worm traces occur only sporadically, so they are rather irrelevant for the yield. Unless the warranty and product warranty periods have expired, a fallback to the performance guarantee will be possible only in the case that the appearance of worm tracks is massively. For instance, if every third cell in a module has one or more of these cracks.
If your photovoltaic plant is still in warranty, you should persistently criticize this. It is important to ensure there are many cells affected; at least 5 in a 60 cell module would be an example. In this case, you should speak in the complaint clearly a series of errors in order to possibly get replaced even after the expiry of the periods other modules.
The warranty differs very much from country to country and where you did buy, so look carefully what kind of warranties apply in your case.
Text & Picture: Andreas Witte; Translation: Ralph Hunderlach