Wind and solar droughts can hit during peak grid demand—and last nearly a week

December 13, 2023, 12:05PMNuclear News

Cloud cover cuts solar generation, and on calm days wind turbines won’t spin. When cloudy and windless conditions coincide for hours or days, the result is called a compound energy drought. Pacific Northwest National Laboratory recently found that these energy droughts can last nearly a week in some parts of the country and that they overlap with periods of peak grid demand more often than would be expected by chance.

If you want to keep critical infrastructure running during inevitable bad weather, the findings indicate a need to deploy grid-scale batteries along with new renewable energy installations—or a need to increase investment in reliable, dispatchable nuclear energy. As the news article released by PNNL on December 11 puts it, “It’s one thing to acknowledge the risks that come with renewable energy: the sun doesn’t always shine and the wind doesn’t always blow, but what happens when the grid loses both of these energy sources at the same time?”

The investigation: Compound energy droughts have typically been studied on a state or regional scale. To find out more about the risk of energy droughts over the entire continental United States, researchers dug into four decades of hourly weather data and compared it to historical energy demand data to understand how often energy droughts happen when energy is most needed.

The team focused on historical data for geographical areas that host solar and wind energy installations today, targeting data including wind speeds at wind turbine heights, as well as solar intensity. According to PNNL, the research team published “Standardized benchmark of historical compound wind and solar energy droughts across the Continental United States” on October 31 in the journal Renewable Energy and will be presenting at this week’s annual meeting of the American Geophysical Union. The research was funded by PNNL through its internal GODEEEP (Grid Operations, Decarbonization, Environmental and Energy Equity Platform) initiative.

“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,” said Cameron Bracken, an earth scientist at PNNL and lead author on the paper. “We essentially took a snapshot of the infrastructure as of 2020 and ran it through the 40 years of weather data, starting in 1980. We are basically saying ‘here is how the current infrastructure would have performed under historical weather conditions.’”

This figure displays magnitude, duration, and frequency of energy droughts, with combined wind and solar droughts shown in the left panel and combined load, wind, and solar droughts in the right panel. The points indicate the mean drought duration for the 15 grid balancing authorities (for example, MISO in the upper Midwest or ERCOT in Texas) covered by the study at a given time scale, and the vertical lines indicate the range of drought durations from the minimum to maximum observed duration over the 40-year study period. (Source: Bracken et al, “Standardized benchmark of historical compound wind and solar energy droughts across the Continental United States,” Renewable Energy, fig. 6;

Detecting droughts: The researchers found that energy droughts can occur in any season across the continental United States, though they vary widely in frequency and duration. Droughts may be measured in hours or days, depending on the location. In Texas, for instance, cloudy and windless conditions might last a few hours, while in California they can last several days. The researchers found that other states—including Utah, Colorado, and Kansas—experience frequent energy droughts over both several-hour and several-day timescales. Overall, the 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).

Demand side: A wind and solar energy production drought might not have any grid impacts if load is low. To understand the possible connection between energy droughts and energy demand, the team mapped their hypothetical historical generation data onto 40 years of historical energy supply and demand data across the continent.

The data showed that “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 demand for power was high. “This could be due to well-understood meteorological phenomenon such as inversions suppressing wind and increasing temperatures, but further study is needed,” Bracken said.

Climate change is bringing hotter summers and more intense winter storms, increasing demand for electricity in the service of public safety. Bracken and the team next plan to extrapolate weather and demand data to see how climate change could affect the frequency and duration of energy droughts. According to PNNL, the team plans to model energy droughts through 2100 and analyze it with a companion model of energy infrastructure changes over time.

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