Solar performance reduced by up to 30% during peak wildfire seasons, report finds


Smoke from California wildfires in September 2020 darkened the skies so much that it slashed the state’s solar power production during peak hours by 10% to 30%, according to the study Smoke from 2020 United States wildfires responsible for substantial solar energy forecast errors, conducted by the National Center for Atmospheric Research (NCAR) and published in Environmental Research Letters.

It also means solar energy forecasts for several days had overestimated the amount of power that can be generated by solar modules for multiple days during wildfire season.

The study used advanced computer models of wildfire smoke patterns and meteorological conditions, as well as records of solar irradiance and energy production. Although previous research looked at the impact of 2020 wildfires on solar energy production, the new study is notable for using a specialized weather forecasting model, pointing the way to developing better systems to predict smoke impacts.

“The key takeaway from this research is that wildfire smoke can have a substantial and negative impact on solar production in areas near major wildfires,” said Timothy Juliano of NCAR, lead author of the report. “This is something that utilities should keep in mind when wildfires occur.”

The study was funded by the U.S. National Science Foundation, NCAR’s sponsor, as well as by the U.S. Department of Energy’s Solar Energy Technologies Office.

The 2020 California wildfire season featured half of the 10 largest wildfires in the state’s history, including the August Complex, which set the state record by burning more than 1 million acres. In September, dry and windy conditions in California and much of the West were particularly conducive to wildfires, with satellites showing clear images of smoke plumes.

Juliano and his colleagues focused their study on smoke conditions from Sept. 7 to 16, 2020, when fires were peaking. To determine the amount of solar energy generated at various sites across the state, they drew on California Independent System Operator (CAISO) data.

The group compared energy output to the same period during the two previous years, when solar arrays produced far more energy. They also analyzed data from ground stations in California and elsewhere in the United States that measure solar radiation, or irradiance, minute by minute.

During days of heavy smoke, irradiance was sharply reduced and energy production plummeted 10% to 30% during otherwise daily peak solar generation time of noon to 4 p.m., compared with the same days in previous years.

On days when winds blew much of the smoke out of the region, such as occurred on Sept. 9 and between Sept. 12 and 16, solar production declined by just less than 7%.  Although scientists focused on areas in California near the fires, Juliano said other regions of the country where the smoke dispersed likely experienced a small decline in solar energy.

PV forecast improvement

The smoke significantly affected the accuracy of energy predictions in California. During the smokiest days, solar production averaged 27% less than forecast. It also fell to as much as 50% less than forecast during late afternoon and evening hours that coincide with the largest ramp-up in energy demand.

To determine if the forecasts can be improved, the scientists next turned to an NCAR-based model known as Weather Research and Forecasting-Solar, which enhances solar irradiance predictions. By inputting data about the smoke, including airborne particles such as aerosols, they were able to better approximate the magnitude and timing of solar irradiance.

The weather forecast model simulations were run on the Cheyenne supercomputer at the NCAR-Wyoming Supercomputing Center.

Juliano concluded that wildfire activity seldom affects solar energy to the extent that it did in California in 2020.  But with wildfires becoming more common and smoke plumes sometimes extending over large swaths of the nation, the study concluded that scientists need to continue working to better predict smoke impacts — an effort that will require collaborations among experts in different disciplines, including fire behavior, weather, and renewable energy.

“Wildfire smoke can be quite an important factor in energy production,” Juliano said. “Given the increase in large wildfires and society’s greater reliance on solar energy, even regions far downwind of fires may need to consider the potential impacts of smoke.”

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