The most important parameter of a PV plant is the soiling rate. The soiling can be induced by the presence of dust, air pollution, sand, salt, bird droppings, snow etc. Therefore, the rate needs to be as accurate as possible.

The impact on the production is commonly assessed during the initial Energy Yield Assessment (EYA). It is usually assumed that the soiling losses are estimated to be at least 1% per year (linear) for a typical plant design (tilt angle of the panels between 15° to 25°) and a “standard” location (country or city).

For specific location, it is highly recommended to evaluate the potential losses with more precision in terms of seasonality (non-linear losses), location and environmental specificities (sandstorms in deserts, bird droppings close to the sea..). The losses are heightened if the tilt angle of the panels are low (below 10°).

This impact on losses must be followed during Operation as well:
(i) to ensure the performance of the plant
(ii) to adjust the soiling losses value
(iii) thus, to evaluate if a module cleaning is necessary.
It will allow the adjustment of OPEX in the Business plan by adding or removing cleaning modules interventions.

Several methodologies can be used to assess the degradation, due to soiling on panels.

For greenfield projects, if the location and the design are standard, it is recommended to consider a soiling rate of the PV plant between 1% and 2% per year.
It is then highly recommended to re-assess this rate after at least 1 year of operation, to check the soiling seasonality (heavy rain, snow, sandstorm, pollen etc). Several research papers [2][3] discuss the methodologies to calculate this rate and to evaluate the losses. For example, the bird dropping’s impact on the Yield can be three times higher than dust [3]).

For the first approach, an evaluation of the historical PR can be carried out by identifying periods with sudden drops. As part of the O&M contract scope, monthly I-V curve measures can be undertaken on a module’s sample (which is more adapted for ground-mounted plants). This rate can also be assessed using a specific sensor [4], which can monitor in real time the soiling on modules.

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By David Roisse, Asset Manager

[2] M. Deceglie, L. Micheli, and M. Muller, “Quantifying soiling loss directly from PV yield,” IEEE Journal of Photovoltaics” ; January 2018.
[3] Dounia Dahlioui, Bouchra Laarabi, Moulay Abdelmjid Sebbar and Abdelfettah Barhdadi (Semiconductors Physics and Solar Energy Research Team High Education School, Mohammed V University in Rabat Morocco) ; Gauthier Dambrine, Etienne Menard, Jay Boardman (HELIOSLITE, Savoie Technolac) ; “Soiling Effect on Photovoltaic Modules Performance” ; October 2017.