Pollen Persistence: Rain Doesn’t Erase Pollen’s Mark on Solar Power
Research Collaboration with National Renewable Energy Lab and University Partners Reveals Challenges of Soiling Losses in the Southeast

Pollen Soiling on Solar Panel

Pollen accumulation blocks light from reaching solar cells.

Energy losses from dirty solar panels – known as soiling – has traditionally been considered an issue unique to dry, desert regions. Rainfall was thought to be an all-natural, cost-free way to keep panels clean and site performance high. However, while rain does (mostly) clean dust off of solar panels, other sources of soiling are not readily removed. In a joint collaboration between Solar Unsoiled, the National Renewable Energy Laboratory (NREL), the University of Jaén, and Duke University, an understudied and misunderstood source of soiling was investigated – pollen. This study also looked into the benefit of panel cleanings performed by partner Cypress Creek Solutions (CCS).

For those of us in the Southeast US, March brings uncontrollable sneezing and a yellow coating on just about every outdoor surface. Pollen on a solar panel surface hinders sunlight from reaching the energy generating cells, but traditional wisdom for operators of solar farms says these losses are short term, with the first major rainfall washing away any layer of pollen.

Pollen Season Drops Performance of Solar

Pollen season coincides with energy losses of 15% at solar farms in the Southeast.

The recently published study, An Investigation on the Pollen-Induced Soiling Losses in Utility-Scale PV Plants, challenges this assertion. The research analyzed five of Cypress Creek’s solar farms in North Carolina and confirmed that pollen causes short-term losses. During the peak of the pollen season, losses at all sites exceeded 15%.

Surprisingly, the recovery of these losses was slow – over the course of months – with no correlation to rain events. Additionally, after each spring, the recovery was never 100%, meaning that soiling losses continued to remain present on the panels. Removing these persistent soiling losses required cleaning methods and expertise from Cypress Creek Solutions. The five sites, ranging in size from 4 to 9 Megawatts, were all cleaned with CCS’s efficient robotic washing method and the 5 to 11% soiling losses were finally recovered.

The already-thin margins of solar, coupled with the fact that soiling was widely ignored at the development stage of many critical assets, reveal the major risk that soiling losses pose to the long-term financial viability of these assets.

Accurate methods of assessing soiling losses and rates are the first step to soiling mitigation. Open-source and widely-accepted data analytics techniques developed by NREL estimate soiling by focusing on performance trends during dry periods and observing performance jumps after rain events. However, due to the rain-independent nature of soiling in the Southeast US, these methods are unable to quantify soiling losses.

Combating soiling requires an owner/operator to perform site cleanings, with the caveat that these expensive cleanings need to be done at the economically-optimal time. Accurate assessments of current and future soiling losses need to be combined with site-specific design and economics to ensure that the tradeoff between lost revenue and the operational expense of site cleanings is balanced in order to maximize profit.

Solar Unsoiled is dedicated to providing optimal cleaning schedules for owners and operators across the US and further understanding soiling losses in the Southeast. An expanded study with Solar Unsoiled and NREL is planned for 2024 – this time focusing on the chemical, physical, and biological mechanisms by which pollen contributes to persistent soiling losses.