Renewable energy production from wind and solar is often faulted for its gaps: Winds are inconsistent and skies aren’t always sunny. Unfortunately, battery storage has lagged in making up the difference.
But a deceptively simple-sounding adjustment could increase their combined capacity during hours of peak demand by as much as 40%, even without the technologies being located at the same site, researchers at North Carolina State University and North Carolina Central University have detailed in a recently released report.
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“Our work here suggests that solar power can offer greater benefits to reliability than sustainable energy skeptics suggest,” Jeremiah Johnson, an associate professor of civil, construction and environmental engineering at NC State and corresponding author of the paper, said in a June 17 press release. “Investing in both solar power and energy storage systems can unlock reliability value that neither technology would provide on its own.”
When used in tandem rather than deployed separately, the researchers found solar energy and battery storage interact in a symbiotic manner.
“So, what really matters from the system perspective is that you have solar and storage somewhere on the grid and that you’re able to operate them in a way where they sort of mutually benefit each other,” Joseph DeCarolis, a professor of civil, construction and environmental engineering at NC State, said in an interview.
The paper, “The Symbiotic Relationship of Solar Power and Energy Storage in Providing Capacity Value, is one of the first analysis that has looked at the advantage of having the technologies working simultaneously, he said.
“I think a big, big challenge here is utilities system operators, they have to make sure that demand is always met,” DeCarolis said. “The thing that keeps them up at night is what happens during these peak demand periods when everybody wants to stay cool or everybody’s trying to warm up and demand goes really high.”
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In the decades before renewables entered the picture, he said, the power grid was supplied by a variety of dispatchable capacity, including nuclear, coal and natural gas. All were fairly easy to control. With wind and solar, it’s more challenging.
“So, trying to characterize the capacity credit of renewable technologies and storage, taking into account these peak periods is really critical because it helps us understand to what degree we can rely on renewables and storage to help meet those peak demands,” DeCarolis said. “And what this study finds is that there’s actually a lot of mutual benefit to operating these technologies together, in unison.”
Researchers studied reliability of renewable energy in North and South Carolina power systems, working with data on power demand and generation sources, according to the press release. Digital models were built to assess power production from the sources during peak demand, with researchers testing varied amounts of solar energy and storage capacity.
DeCarolis, a co-author of the report, explained that utilities such as Charlotte-based Duke Energy typically create a “demand profile” for their customers’ energy daily usage, and then determine how to match that demand in real time with their energy supply. While there is a lot of predictability in demand — when people wake up they turn everything on — and seasonal variabilities that matches output of renewables, the question has been its reliability in meeting spikes during peak demand.
Independent of each other, solar energy and battery storage have their own value in the system, DeCarolis said. Each is assigned a “capacity credit,” which is essentially the fraction of installed power capacity that can be relied on during peak demand. What the researchers found is that the combined capacity credit for solar and storage on the system together is greater than the sum of their parts.
2GW/8GWh energy storage system. (a) Jan. 22 with 5 GW solar; (b) July 16 with 5 GW solar; (c) Jan. 22 with 10 GW solar; (d) July 16 with 10 GW solar. Source: From the study
“The good thing about solar is that it matches, particularly the summer peaks, pretty well,” he said. “But it’s not a perfect match. For example, those winter morning peaks, sometimes the electricity is peaking just as the sun is rising.”
It’s a given that no one can control when the wind blows and the sun shines, but what the new research shows is that the utilities can control the efficiency of the technology by synchronizing the output.
Typically, DeCarolis said, system operators will look at the hourly demand for electricity, and then look at the hourly output of solar, and then subtract the output from the demand, resulting in a net demand profile.
“It’s basically factoring in the solar production, and then they’re saying, ‘Okay, now this is the net demand that we need to meet with all the other generating assets that we have on the system,’” he said.
If the demand peak is envisioned as an inverted “U,” he explained, energy production from solar photovoltaics, or PVs, generally corresponds with the peak, in the process making the “U” lower and narrower. But under that scenario, known informally as the “duck curve,” DeCarolis said, adding excess megawatts of solar would be firing at the same place in the curve.
“It doesn’t matter if you have 5 terawatts of solar, you will never affect the demand curve when the sun isn’t shining and all you have is solar,” he said.
A terawatt is equal to 1 trillion watts.
Dispatching energy from battery storage during peak periods, however, is controllable, he said. The downside is it is limited: Its use cannot exceed its capacity. Comparing it to the way people keep track of the power on their cellphones, DeCarolis said that operators take care to make sure that batteries have enough charge available to meet the demand.
And here is where tweaking the same ingredients of the power system recipe can make a greater capacity cake. As the report explained, “the presence of solar PV decreases the duration of daily peak demands, thereby allowing energy-limited storage capacity to dispatch electricity during peak demand hours … storage can be dispatched during hours when solar exhibits diminished output, and solar helps to shorten the durations of peak load that must be shaved by energy-limited storage systems.” That is, by deploying solar and storage at the same time, more dispatched stored energy can be accommodated when needed.
“Remember, solar is going to reduce the peak but also narrow the peak,” DeCarolis said. “And when it narrows the peak, it actually makes it easier for storage to basically meet the rest of that peak demand.”
Although the research, which was done with support from the North Carolina Policy Collaboratory, focused on solar energy, power grids function similarly with wind energy, which also is not dispatchable.
The report concluded that the research demonstrated the important distinction between winter and summer peaking systems, showing “significantly different” seasonal capacity values for solar energy.
“These findings are timely as utilities replace their aging peaking plants and are taking energy storage into consideration as part of a low carbon pathway,” the report said.
But will the study change the way the companies use renewable energy on the grid?
“I hope so,” DeCarolis said. “I think the key here is that there should be a coordinated strategy among utilities and policy makers to think about the deployment of these technologies … that could help them maximize their mutual benefit, instead of just treating them independently.
“It’s helpful to think about the advantages they offer when you operate them together.”
