Easily causing more than 1% power loss per day, soiling is a site-specific phenomenon, strongly influenced by local climatic conditions. The different types of soiling materials include: mineral dust deposits (Figure 1A), bird droppings (Figure 1B), biofilms of bacteria, algae, lichen, mosses, or fungi (Figure 1C), plant debris or pollen (Figure 1D), engine exhausts or industry emissions (Figure 1E), and agricultural emissions such as feed dusts (Figure 1F).

Figure 1. Soling Types.

The physics of dust deposition and adhesion are complex due to the many influencing factors. Airborne dust concentration is considered the major determinant factor, together with rain frequency. Other factors include: wind speed, tilt angle, relative humidity and dew.

Consequences

Apart from the evident optical losses, soiling can cause permanent degradation of PV modules. In cases of omitted cleaning, cemented dust layers, lichens, and fungi can become practically irremovable, whereas harsh cleaning can lead to the scratching or abrasion of anti-reflective coatings or glass corrosion.

In addition, mechanical loads during cleaning or thermal shocks when a hot element is cleaned with cold water may lead to breakage of solar cells and glasses or expansion of micro cracks. Furthermore, potential induced degradation (PID) in PV can be enhanced by soiling and partial shading due to non-uniform soiling can lead to the formation of hot spots.

All of these effects damage the equipment, ageing PV modules faster and reducing power production.

Soiling is estimated to have reduced global solar power production by at least 3%–4% in 2018, causing global revenue losses of at least 3–5 billion €. Global soiling losses could rise significantly to 4%–7% of annual power production, causing more than 4–7 billion € economic losses by 2023. This increase in costs is mainly driven by an increased deployment of PV in high insolation and also in highly soiling-affected regions, as well as the low predicted electricity price, which reduces the incentive for cleaning. Additional factors that increase the impact of soiling are: rising PV module efficiencies, climate change (increase in temperature, rise in the global soil aridity, higher risk of droughts and wildfires and irregular precipitation patterns) and anthropogenic activities (agriculture and deforestations).

Solutions and Mitigation Technologies

Site Selection, Adaption, and Monitoring

As it is not yet possible to accurately predict soiling only from climate information; daily loss rate, rain frequency, and dust characteristics should be analyzed at each potential site during resource assessment measurement campaigns using soiling measurement devices at their intended tilt or tracking pattern and orientation. Industrial dust sources, agriculture and livestock farms, and dirt roads or high traffic roads should be avoided by site selection.

Many times, site selection is not a choice and such sources are unavoidable. Then, preventive measures can reduce the impact of fugitive local dust sources (e.g., by water spray, vegetation, paved roads, dust barriers, or increased height of installation).

Monitoring is an essential soiling mitigation tool, as it helps to detect extreme soiling conditions and to adapt the cleaning schedule depending on the interannual variability of the climatic conditions. For large PV systems, ideally also soiling non-uniformity is mapped to identify sections that are economically worth cleaning.

Tilt Angle and Solar Tracker

Field studies consistently show that soiling rates significantly decrease at steeper surface tilt angles suggesting soiling mitigation by vertical (90°) overnight stowing. This may not always be an option due to wind loads.

Heating of Surfaces Preventing Water Condensation

Dew has been identified as a crucial factor in soiling in many places by increasing cementation, decreasing particle rebound and causing distinct soiling patterns. Condensation typically peaks before dawn, when the relative humidity is high, and PV modules are colder than the ambient air temperature because of their infrared radiative emission (radiative cooling).

Accordingly, new approaches were proposed for soiling mitigation by preventing condensation through active and passive surface heating. This includes heat generation by controlled current supply to solar cells, adapted application of photovoltaic thermal hybrid solar collectors or using latent heat from phase-change materials (PCM). In addition, low-emissive coatings could significantly reduce the radiative cooling and therefore the occurrence of dew. Active heating with relatively high power indicated up to 65% soiling reduction, but so far no results, models, or practical conclusion on the economic feasibility of heating approaches to reduce soiling exist. Still, heating modules at night might offer potential for soiling mitigation in situations with high cleaning and maintenance costs (e.g., remote locations, street lighting, and building-integrated PV) in arid environments.

Electrodynamic Screens (EDSs)

Transparent electrodynamic screens (EDSs), repel dust particles by creating a time-varying electric field over a surface. EDSs have been successfully demonstrated in the lab. However, they have proved difficult to translate to the field, where harsh conditions interfere with the electronic systems.

A recently launched commercial device reported 32% soiling rate reduction in Saudi Arabia, but large-scale implementation has not occurred yet due to its relatively high cost.

Anti-soiling coatings (ASCs)

ASCs applied to the front glass of PV modules aim to reduce soiling and the demand for cleaning. They are considered as a holy grail by the soiling community. Originally ASCs had important issues with dew as particles typically become tightly adhered to the surface. Consequently, small particles remain stuck to the PV and, over time, comprise an increasing fraction of the soiling layer and optical losses. This is being solved by the replacement of hydrophilic ASCs with hydrophobic ASCs.

Soiling rate reductions of more than 80% have been reported from outdoor exposure studies; however, over longer periods, average anti-soiling performances are typically much lower as they tend to degrade over time due to abrasion, particle settlement, UV irradiation and other external factors.

Nevertheless, the attraction of a passive anti-soiling solution is great, so that development continues, with many promising approaches. Learn about the vendors developing ASCs on this framework.

Cleaning

In general, cleaning methods can be categorized into manual, semi-automatic, and fully automatic. A further distinction can be made between dry and wet cleaning technologies, the ability for these to translate from row to row and the use of machine learning and/or AI to manage cleaning schedules and get data from PV modules. Learn more about the different features and vendors involved through our PV module cleaning framework.

The fully autonomous cleaning market, which represented only 0.13 % by 2018 of the global solar capacity, is growing exponentially thanks to the recent developments of dry, fully automated robots, which can be already integrated into the plant design.

Conclusions

Currently, there is no one-solution-fits-all to the problem of soiling due to its site-specific and seasonal variability, differences in local energy costs, and the availability and costs of resources required for cleaning, such as water or labor.

Optimized cleaning plans, automated cleaning robots, anti-soiling coatings, tracking system modifications, improved soiling monitoring and site adaption can be economically feasible and effective solutions to reduce the negative impact of soiling at utility scale. In contrast, electrodynamic screens and heating solutions appear too expensive, or the technology is not mature enough.

There are several ways to reduce the effect of soiling, but cleaning is the most widely used technique and so far no passive anti-soiling technology eliminates the need for cleaning. Darcy has identified Ecoppia as the leader provider of autonomous cleaning robots and we would like to invite you to join our forum next week to learn more about soiling losses and the solution Ecoppia provides.

References

Joule. (2019). Techno-Economic Assessment of Soiling Losses and Mitigation Strategies for Solar Power Generation. (2018). Elsevier Inc. Link.