There is no single answer to that question, as it depends on the exact localization of the PV structure to be cleaned. Even in desertic areas without industrial or agricultural pollution, water-based cleaning may be necessary (occasionally), for instance to clean sand that has been cemented over the PV surface by dew overnight making dry cleaning ineffective or at least less relevant than in “normal” sandy conditions.

• climatic factors (rain, humidity, dew, wind direction and temperature)
• the type and composition of the dirt (dust and sand, bird droppings, pollen, industrial and/or agricultural emissions, lichen growth, moss, etc.)
• the architecture of the PV system itself: panel layout, tilt and module type (framed or unframed)

• it would already require a long study over at least 1 year to calibrate the loss induced by the absence of cleaning and for a specific site;
• it also supposes that no modification of the environment of the installation (road construction, agricultural activity, etc…) is undertaken during the duration of the project;
• another requirement would be to take into account from day one the exceptional weather events (such as sandstorms, long periods without rain, etc.) that may occur more or less frequently;
• finally, cleaning is not only a question of performance, but also of maintenance. Solar panel manufacturers do not guarantee the integrity of the modules during the whole lifetime of the PV project: therefore, more and more developers/operators dedicate, for example, a cleaning session after the summer period in order to avoid the formation of lichens and mosses that could penetrate between the glass and damage the PV cells.

Studies have demonstrated that flat PV modules are much more prone to the effects of soiling when compared directly to high angle structures. As mentioned above, other key dimensions such as the type of fouling material, wind direction, frequency of rain events, etc., must be taken into consideration when defining the cleaning procedure for a specific PV installation.

Let’s take the example of your car windshield: in the absence of rain, without the mechanical contribution of water and the simultaneous action of the windshield wipers, it can quickly become clogged. More seriously, all studies done on the effects of clogging on PV production take into account “one-off” weather events such as rain, which have a positive (but limited) impact on cleaning, since it lacks the mechanical action of the brush.

Solar farms are built with an average lifetime of 20-30 years; not only will production losses be greater over time without a cleaning procedure in place, but the risk of hot spots will also be multiplied, rapidly damaging the panels, sometimes to the point of requiring complete panel replacement.