On a power grid partly supported by wind turbines, solar panels and hydropower turbines, weather and climate patterns can significantly affect how much power is produced. And as more wind and solar are added to the grid, utilities will need to understand how seasonal patterns in climate and weather might cause swings in energy generation.

In new research, a team from the Department of Energy's Pacific Northwest National Laboratory shows that compound energy droughts or periods of low energy generation from solar, wind and hydropower simultaneously can last up to five months and occur most often in the fall. 

Importantly, "compound energy droughts don't necessarily cause blackouts, and consumers may not notice one was occurring," said Cameron Bracken, Earth scientist at PNNL and lead author on the paper. And in fact, the team found that in the western United States, the sheer amount of non-renewable generation means that even in the worst-case energy drought scenario, there's still enough energy available to meet demand. 

"The question then becomes about the cost of energy and how to effectively deploy energy storage," Bracken said. "During a compound energy drought, utilities would have to use more energy from burning fossil fuels to meet demand or purchase energy on the market." 

And burning fossil fuels costs more and emits more carbon dioxide. Knowing when a compound energy drought might occur, and how long it might last, allows utility operators to not only plan for how to cost-effectively provide energy to consumers, but also how to invest in energy storage.

Bracken presented the research on December 10 at the annual meeting of the American Geophysical Union in Washington, DC. The team also submitted a paper to the journal Applied Energy.

How the seasons affect energy generation

Bracken and his colleagues previously investigated compound energy droughts in a paper published last fall, where they found that energy droughts in solar and wind power can last nearly a week. Because solar and wind can shift within minutes due to a cloud passing over a field of solar panels or the wind dying down these energy droughts affect a utility's minute-by-minute operations.

But wind and solar power aren't the only renewable energy sources dependent on natural rhythms.

Hydropower generation responds to longer-term climate patterns of dry and wet seasons, Bracken said. In the western United States, melting snowpack in the summer drives an increase in hydropower generation during warmer months, for instance. In the eastern part of the country, hydropower doesn't depend as much on mountain snowpack, but on seasonal rains. 

"Hydrological cycles last months, not days or hours. We wanted to find out how much energy droughts might affect the grid on a seasonal scale, which means we need to investigate how climate phenomena affect hydropower generation," Bracken said. Understanding a seasonal pattern of compound energy droughts would allow utilities to plan ahead on a longer timescale. 

Similarly to the previous paper, Bracken and his coauthors looked at a period of historical climate data between 1982 and 2019 to find cloudy times when solar power might fall, stagnant days when the wind might not blow and dry periods that might decrease hydropower generation. They also investigated climate patterns like El Niño and La Niña to see whether a correlation existed with energy droughts.

The team then applied that data to today's energy infrastructure. Meaning, if today's extent of wind turbines, solar panels and hydropower facilities existed for those 40 years, how often and for how long would compound energy droughts have occurred?

The team found that compound energy droughts would have occurred most often in the fall and could have lasted up to five months. This reflects a period where days begin to shorten (leading to less sunlight) and the summer's snowmelt dwindles. 

During the worst of these compound energy droughts, the researchers found that carbon dioxide emissions could rise up to 20 percent as utilities shifted to fossil fuels to replace lost generation from wind, solar and hydropower. They also found that energy prices rose significantly in the Northwest United States, which is more reliant on hydropower than other parts of the country. 

The good news is, however, that in a model of the Western Interconnect energy production never fell so much that demand could not be met, Bracken said. This is partly because the nation's power grid hosts enough different kinds of energy sources that not every single one is affected at the same time. Another reason for this resilience is that if a compound energy drought occurs in one part of the country, it's unlikely that a neighboring region would experience the same dip in generation. With regional transmission, neighboring regions can send energy where it's needed.

Implications for energy storage, emissions

The researchers also showed how utilities could use energy drought information to inform their operations. The team picked the worst five energy drought months over the study period, when climate patterns caused a simultaneous drop in solar, wind and hydropower, and ran that data through a model of the Western Interconnection's annual operations. 

"This case study can help give utilities an idea of when the generation from all their intermittent resources are all the lowest we have observed historically," Bracken said.

With the modeled scenario, utilities can consider how much energy storage they could install to buffer some effects of a simultaneous drop in wind, solar and hydropower.

The new work offers a foundation for utilities to start thinking in a new way about how to manage and plan for a decarbonized power grid. And in future research, the team plans to investigate how climate change will affect the frequency and duration of compound seasonal energy droughts.