Researchers at Pacific Northwest National Laboratory (PNNL) have found that in some parts of the country, compound energy droughts—i.e., when the sun doesn’t shine and the wind doesn’t blow, both at the sametime—can last nearly a week. The team published the findings in an open-access paper in the journal Renewable Energy and will be presenting at this week’s annual meeting of the American Geophysical Union.

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.

—Cameron Bracken, an Earth scientist at PNNL and lead author

Grid operators need to know when energy droughts will occur so they can prepare to pull energy from different sources. On top of that, understanding where, when, and for how long energy droughts occur will help experts manage grid-level battery systems that can store enough electricity to deploy during times when energy is needed most.

In the past, researchers studied compound energy droughts on a state or regional scale. But not much has been studied on a nationwide scale. To find out more about the risk of energy droughts over the entire continental US, the researchers dug into weather data and then used historical energy demand data to understand how often an energy drought occurs when that energy is needed the most.

The team examined 4 decades of hourly weather data for the continental US and homed in on geographical areas where actual solar and wind energy plants operate today. Weather 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.

The researchers found that energy droughts can occur in any season across the continental US, 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. The different timescales (hourly versus daily) will help inform the energy drought’s impact on the grid.

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).

Energy drought at peak demand. Simply knowing the where and how of energy droughts is just one piece of the puzzle, Bracken said. He also stressed that a drought of solar and wind power won’t necessarily cause an energy shortage. Grid operators can turn to other sources of energy such as hydropower, fossil fuels, or energy transmitted from other regions in the US.

However, as the nation aims to move away from fossil fuels and rely more on solar and wind power, grid operators must understand whether energy droughts will occur during times when the demand for electricity might exceed supply.

To understand the possible connection between energy droughts and energy demand, the team mapped their historical, hypothetical generation data onto 40 years of historical energy demand data that also covered real power plants across the continent.

Magnitude, duration and frequency of energy droughts for all Balancing Authorities (BAs) (and aggregation periods. Wind and Solar (WS) droughts are shown in the left panel and Load, Wind, and Solar (LWS) droughts in the right panel. The points indicate the mean drought duration for a BA at a given time scale, the vertical lines indicate the range of drought durations from the min to the max observed duration in the 40 year period. The curved line is an exponential curve meant to illustrate a rough upper bounding region for the data. Bracken et al.

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. For now, Bracken isn’t certain that the correlation means causation.

Energy storage for energy droughts. Studying patterns in the frequency and duration of energy droughts will also help inform the deployment of long-duration energy storage projects, said Nathalie Voisin, an Earth scientist at PNNL and coauthor on the paper. The paper is the first to provide a uniform standard of what a compound energy drought is and how long it can last in different parts of the country.

Next, Bracken and the team will extrapolate weather and demand data into the future to see how climate change will affect the frequency and duration of energy droughts. The team plans to model energy droughts all the way to the end of the century combined with evolving infrastructure.

This research was funded by PNNL through its internal GODEEEP initiative.

Resources

• Bracken C.W., N. Voisin, C.D. Burleyson, A.M. Campbell, Z. Hou, and D.P. Broman. (2024) “Standardized Benchmark of Historical Compound Wind and Solar Energy Droughts Across the Continental United States.” Renewable Energy 220. PNNL-SA-186419. doi: 10.1016/j.renene.2023.119550