Reduction in PV solar output due the accumulation of sand,
dust, and particulate matter on solar panels is known as soiling losses.
Soiling losses are both site-specific and region specific. In areas with adequate
and frequent rainfall soiling losses are lower since rain washes away the dust
and particles. Soiling losses on fixed tilt panels are higher than on panels that
track the sun since gravity may help remove some particles as the panels change
to a higher pitch angle. Time-limited regional events like wildfires can drastically
increase airborne particulate matter and subsequent soiling losses. Places
where precipitation is very low like desert areas and events like sandstorms
are frequent, also desert areas, are likely to have very high soiling losses.
The National
Renewable Energy Laboratory (NREL) noted in the section about soiling losses in
their 2016 PV Module Reliability Workshop:
“Soiling is a complex problem that increases
uncertainty and drives up LCOE through lost energy production, increased
O&M costs, and higher finance rates. – Overall, soiling may be adding
¢1/kWh to LCOE in the United States (may be worse in some other parts of the
world) depending on the site.”
Companies and
academia have developed modeling based on continuous monitoring of particulate matter
and weather conditions. SolarAnywhere has developed a global soiling loss map
based on their own data and that of Humboldt State University (HSU) data. Parameters
used by HSU include rainfall, cleaning threshold (amount of rain needed to
clean panels), tilt of panels, PM2.5 (particulate matter less than 2.5 microns),
PM10 (particulate matter less than 10 microns), depositional or settling velocity
of particulates, and rain accumulation period. SolarAnywhere’s modeling is
based on fixed-tilt panels
Soiling loss
stations can be set up on solar farms to gather site-specific data on soiling
losses. These typically involve a panel section being regularly cleaned compared
to one that is left to accumulate dust.
In urban environments the composition of particulates is more
likely to come from air pollution and may be much different than in deserts or
other dry environments. A study using sensors in Los Angeles found that fine
and coarse particulate matter accumulating on panels for several years without
cleaning can be very difficult, even impossible to remove so that those soiling
losses are permanent. LA is known to have high smog rates due to weather
inversions.
A study
published in 2019 by HSU researchers showed that simple modeling can accurately
predict soiling losses after being compared with actual data for the local
sites evaluated. They do not know if their model is applicable to other sites
or all sites, but it seems reasonable to suspect that it would be close. They
also noted: “The model demonstrated strong dependency on the frequency and
magnitude of rain the PV system experiences.” Thus, rain is likely the
biggest factor. Manually cleaning solar panels is an option but for large solar
farms this would result in increased maintenance costs which could offset the
gains of cleaning the panels.
Soiling losses
in the US are estimated between 0 and 7%. California and parts of the Southwest
have the highest soiling losses in the US. In some parts of the world soiling
losses can be as much as 50%! A 2020 study in Nature Sustainability based on a
combination of “solar PV performance modelling with long-term
satellite-observation-constrained surface irradiance, aerosol deposition and
precipitation rates” that in Saharan and Middle East regions, which have
some of the best solar resources in the world, soiling losses led to capacity
factor losses greater than 30% and in some cases 50%. Of course, in those regions
it would be worth it to manually clean the panels. Morning dew can make the
problem worse by cementing particles together. This can make the particles
stick better to the panels, requiring more energy to remove them.
Other
remediation methods include applying coatings to make particulates less likely
to deposit on the panels. In the Middle East autonomous robots are deployed to
clean solar panels with rotating brushes. This keeps the panels clean and
prevents the cementation problems caused by morning dew. However, this solution
does require frequent cleaning, which as I have mentioned would likely be
worthwhile in places where soiling losses are high. An issue in very humid
areas that leads to high soiling losses is fungus which may grow on the panels
that requires both chemicals and hard scrubbing to remove.
References
Estimate PV soiling
losses to reduce solar risk with SolarAnywhere. Akansha Bhat and Marc Perez,
January 31. 2022. SolarAnywhere. Estimate
PV soiling losses to reduce solar risk with SolarAnywhere - SolarAnywhere
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