Dirt on solar modules endangers their durability, is a safety risk and leads to yield losses. What degree of soiling requires a proper solar cleaning? Find it out here as well as which cleaning techniques are the most economical.

It is obvious that less light can be converted to electricity in the case of contaminated solar cells. It is less known that dirt can also damage photovoltaic modules and even lead to fires. That is why  as a PV operator, you should conduct regular checks to see whether the modules are at risk and if they are suffering a permanent yield loss, that can easily be prevented with a thourough cleaning.

Solar cleaning can help you maintain the value of your investment in the long term.

Contamination

Here are five common types of dirt:

• Bird droppings: they are everywhere but even more common in coastal areas.
• Ammoniacal exhaust air from stables: cattle farms in particular are highly polluted with such exhaust.
• Moss/lichen on the frames: standing water on modules can collect on the frames. This can damage silicone layers and frames, and dissolve laminations.
• Pollen layers: depending on the location, pollen layers may appear within flowering periods.
• Dust layers: the amount of dust will on the location of the plant, e.g. it will of course be heavier in industrial or agricultural areas. Equally, the abrasion of metal on railroad tracks can affect modules.

Rain cleans solar modules. The steeper the inclination of the modules, the better the water and dirt flow off. In contrast, the natural cleaning effect does not apply to modules with a flatter incline or in areas with low precipitation.

Even dirt spots can become dangerous, because individual solar cells can heat up strongly at these points: these “hot spots” will then damage the modules. In some cases, even flammable material underneath ignites. More often, however, hot spots cause unnoticed long-term damage.

Since modules are connected in series (string), even a single dirty module reduces the output of the entire string. In other words: each module can deliver at most the power of the weakest module in the chain.

When is solar cleaning necessary?

At the latest when you notice a drop in performance, it is worthwhile to take a closer look at the modules. You can use an expert’s view – their secret is not only their experience, but also the right angle: the flatter your view of the module surface, the more clearly you can detect any soiling. And if you gently wipe the modules’ surface with your finger, you can see any dirt on your fingertip.

You can use the following degrees of soiling as indicator to assess whether cleaning is necessary:

1 – very slight: a slight film of dust hardly visible to the naked eye.

2 – slight: small deposits are visible.

3 – medium: stronger deposits, traces of bird droppings or the first signs of moss and lichens on the module edges are visible.

4 – strong: strong dirt film up to encrustations, deposits caused by ammoniacal exhaust air from stables, bird droppings, high proportion of mosses and lichens at the edges.

5 – very strong: strong and persistent encrustations, strong moss and lichen growth at the edges, possibly the module surface is no longer recognizable as such, possibly irreversible soiling on the modules.

You should start to think about cleaning your photovoltaic system even if it is only slightly soiled. Moss and lichen formation should be avoided, as these are irreversible soiling. It is recommended to consult an expert if you are unsure about your assessment.

Solar cleaning technologies at a glance

Manual cleaning: heavy dirt can be removed particularly well with rotating hand brushes on telescopic rods. Cost-intensive manual cleaning is irreplaceable where machines or robots are not operational, or when the plant is too small.

Large brush attachment systems: cleaning attachments for vehicles (e.g. tractors) and mechanical large brush systems clean faster and more cost-effectively, but they require enough space on site.

Robots: cleaning robots are cost-effective and controlled by means of sensor technology or remote control via the modules. They can be used with a module inclination of up to 20 degrees.

Demineralized water should always be used while cleaning agents should absolutely be avoided, as these often pollute the environment and attack the module surface. The cleaning instructions of the module manufacturers should in any case always be observed.

When does cleaning pay off?

You can use the profitability calculators of cleaning companies for the calculation. They compare cleaning costs with additional yields that result from solar cleaning. It is recommended to always compare the results of several calculators. They should be comprehensible and plausible.

An annual additional yield of 1 to 4 percent is a realistic empirical value. On the roof of a barn up to 10 percent more yield per year is possible, because the vapors produced by livestock are deposited on the modules like a film of grease and the ammonia in the deposits attacks the modules and places a heavy burden on them.

Not only the additional yield after cleaning is decisive, but also the protection of your modules from damage caused by soiling and thus the value retention of your investment.

Putting cleaning into practice

Ask for offers from different cleaning companies. When a company has several cleaning techniques, you can be sure that you will be offered the appropriate technique for your installation. Have references shown to you and contact previous customers of the supplier if necessary.

You should take care of the cleaning early enough, because good cleaning companies usually have their agenda from spring to summer during the cleaning season. However, professional cleaning is also profitable into the autumn.

You should only clean by yourself if you are familiar with occupational safety, master the right cleaning technique, and have access to appropriate equipment.

This article is a free translation from the article created with the kind support of Pascal Liebold, owner, Photovoltaik Liebold GbR. Image source: Photovoltaik Liebold GbR.

To read the original version (in German): here.