There exist some studies on the pollution of PV plants in southern regions with very little rainfall. But there are many causes of pollution and often complete self-cleaning by rain, even in regions with higher rainfall, cannot be assumed. The self-cleaning effect depends significantly on the inclination of the solar modules. The smaller the angle of inclination, the lesser is the effect. Module structure also plays a role. Water can flow more easily on frameless modules and thus fewer deposits are left on the modules. Whether the modules are mounted horizontally or vertically plays a role as the water travels a smaller distance on horizontal modules and the edge on which the water collects is significantly larger.
The influencing factors and the most common causes of pollution are shown below.
• Material used for the module (Texture of the glass surface)
• Mounting method
• Edge distance between cell and frame
Common causes of pollution
• Foliage and tree blossoms
• Pollen and grass seed
• Soot from fireplaces
• Plants such as lichens, algae and mosses on the modules’ seals and glass
• Bird droppings
• Dust (especially in industrial and agricultural areas)
• Stable venting (ridge venting)
If one or more of the aforementioned causes for a PV system are present, it should be checked for any contamination. This means both a visual check of the modules as well as continuous monitoring of the energy yields with the help of monitoring software. Only after they have carried out an inspection should the operator decide whether cleaning the PV installation is necessary.
Consequences of pollution
The chief consequences of pollution are reductions in yield. In special cases, however, irreversible damage to the modules can occur, which cannot be remedied by module cleaning.
Types of pollution
There are many causes and types of contamination on PV modules. If the technical effects are considered, however, a distinction must be made only between uniform and selective types of pollution.
For pollution from e.g. dust, generally speaking all the cells in a module are equally affected by the pollution. This can be equated with a reduction in solar radiation. Due to the fact that the change in voltage due to small changes in radiation is negligible, the degree of pollution can be determined directly via the short-circuit current. However, the following problem also becomes apparent: the difference between a reduction in radiation and pollution cannot be seen by the I-U characteristic curve alone. Therefore, a reference is always necessary in order to recognize and evaluate uniform pollution.
Selective pollution, e.g. bird droppings have the same effects as partial shading. These have no effect on the short-circuit current, but the I-U characteristic curve will have a kink. The background is that in the case of crystalline modules bypass diodes are installed (usually 3 diodes per 60 cells) in order to protect the solar cells.
The experience in operational monitoring and maintenance in the field has shown that, in most cases, the individual strings of a PV system are exposed to different degrees of pollution. While some sections of the entire module array are very clean, others are heavily polluted. A good example is installations on cattle sheds with ridge ventilation. The top row of modules is by far the most exposed to pollution. Accordingly, the strings near the ventilation will also experience the strongest reduction in yield caused by pollution. The advantage of such an uneven pollution distribution on the module array is that this pollution can be detected much more easily through system monitoring than if the pollution is uniformly distributed over all the modules. If deviations in the energy yield of individual strings are too great, this can be detected automatically by error detection.
We will describe this in detail in the third part of this series “Cleaning a solar installation – loss of earnings due to pollution“.
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