PNNL warns wind & solar ‘energy droughts’ may last a week, but storage can help

(Photo by Giorgio Trovato on Unsplash)

Researchers at Pacific Northwest National Laboratory (PNNL) warn that in some parts of the country, “energy droughts” can last nearly a week, according to a newly published paper.

When renewable energy generation halts- when the sun isn’t shining and the wind isn’t blowing- it is referred to as a “compound energy drought.” The paper, Standardized benchmark of historical compound wind and solar energy droughts across the Continental United States warns that these occurrences could have a heavy impact on an unprepared grid.

“When we have a completely decarbonized grid and depend heavily on solar and wind, energy droughts could have huge amounts of impact on the grid,” comments Cameron Bracken, an Earth scientist at PNNL and lead author of the paper.

Grid operators need to know when energy droughts will occur so they can prepare to pull energy from different sources, the authors recommend. Understanding where, when, and for how long energy droughts occur can help inform grid-level battery planning so enough electricity is stored to mitigate the loss.

Researchers have studied compound droughts in the past, PNNL notes, but only on the state and regional scale. PNNL’s researchers used weather data and historical energy demand data to understand how often an energy drought occurs when that energy is needed the most.

Overall, researchers found that the longest potential compound energy drought on an hourly timescale was 37 hours (in Texas), while the longest energy drought on a daily timescale was six days (in California).

The team examined four decades of hourly weather data for the continental U.S. and honed in on geographical areas where actual solar and wind energy plants operate today. That data included wind speeds at the height of wind turbines as well as the intensity of solar energy falling on solar panels. Times when the weather data showed stagnant air and cloudy skies translated into lower energy generation from the wind and solar plants- a compound energy drought.

“We essentially took a snapshot of the infrastructure as of 2020 and ran it through the 40 years of weather data, starting in 1980,” Bracken said. “We are basically saying: here is how the current infrastructure would have performed under historical weather conditions.”

The researchers found that energy droughts can occur in any season across the continental United States, though they vary widely in frequency and duration. In California, for instance, cloudy and windless conditions might last several days, whereas the same conditions might last for only a few hours in Texas. Utah, Colorado, and Kansas experience frequent energy droughts both over several-hour timescales as well as several-day timescales. The Pacific Northwest and Northeast, meanwhile, seem to experience energy droughts that last several hours more frequently than several days.

Researchers noted that just because wind and solar power is not generating in some areas, it doesn’t necessarily mean that power generation completely stops. Other forms of generation like hydropower or fossil fuels could be used, or energy could be transmitted from another region. But as the energy transition continues, PNNL stressed the need to understand whether energy droughts will occur during times when the electricity demand might exceed supply.

“Wind and solar droughts happen during peak demand events more than you would expect due to chance,” Bracken said, meaning that more often than not, windless and cloudless periods occurred during times when power demand was high. For now, Bracken isn’t certain that the correlation means causation.

“This could be due to well-understood meteorological phenomenon such as inversions suppressing wind and increasing temperatures, but further study is needed,” Bracken added.

Going forward, the PNNL researchers will examine future weather and demand data to see how climate change will affect the frequency and duration of energy droughts. The team plans to model energy droughts through the end of the century.