Pollen coating can reduce the performance of solar photovoltaic (PV) modules, and rains are not enough to wash them away, according to a report by National Renewable Energy Laboratory (NREL).

In a new paper published in the IEEE Journal of Photovoltaics, scientists found that while rainfall could wash away some of the pollen, it was not enough to return PV performance levels to what they were before.

Dust, pollen, and other elements hinder the light from reaching solar cells. This process of ‘soiling’ has emerged as a major problem to PV performance and realized revenue for solar plant operators.

The team assessed the effects of the utility-scale solar plants located in four counties in North Carolina and found that the performance of solar panels was reduced by 15% during peak pollen season. Without planned cleaning, annual production losses can stand at 10%.

The solar projects had all been operational for more than seven years at the time of the analysis, without any manual cleaning performed. Solar Unsoiled, working with the system owner, measured performance increases from 5% to 11%, following mechanical wet brush cleaning.

Even after the end of the pollen season, the performance of solar panels did not return to their previous “cleaned” levels despite frequent rains. Soiling losses have been estimated in arid and semi-arid locations but not rainier locations; hence, the magnitude of the issue depends on local conditions and climate. As per the researchers, this warrants a careful investigation across regions with frequent rainfall to check if planned cleaning can avoid large financial losses and system underperformance.

“There is still much work to do to fully understand the risks and implications of pollen and bio-soiling in the southeast U.S., but this work has made clear that regular rainfall is not sufficient to assume that solar plants are fully cleaned in certain environments,” said Matthew Muller, a research engineer within the PV Performance and Reliability group at NREL and co-author of a new paper describing the problem with pollen.

Solar accounts for 48% of new power generation capacity added in the first nine months of 2023; in the U.S., the cumulative installed solar capacity stood at 161 GWdc.