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New Grid Storage Technology Helps Integrate Renewables

Duke Energy installs batteries, ultracapacitors to cover power shortages and store energy for peak demand.

Developing cheap energy storage is a critical step in moving to renewables and away from fossil fuels as the primary source of electricity: it’s the only way such intermittent sources can supply power to the grid when the wind’s not blowing and the sun’s not shining.

Duke Energy, the nation’s largest utility, is trying a novel strategy to tackle this challenge. At its Rankin substation, in North Carolina, it has installed a system of hybrid energy storage that combines a battery from Aquion Energy and ultracapacitors from Maxwell Technologies.

The two technologies are complementary. The ultracapacitors, which are good for large, short-duration power surges, help supply power when the solar power on the grid fluctuates because of cloud cover. The Aquion batteries, which are better for storing energy over longer periods, can supply power later in the day, when the sun goes down and electricity demand rises. Combining the two using smart electronics enables the system to efficiently handle power demand over periods ranging from seconds to several hours. The system is the first of its kind at utility scale, says Thomas Golden, technology development manager for Duke.

Combining ultracapacitors and batteries offers a low-cost, highly flexible grid storage system.

“A hybrid system that uses ultracapacitors for the higher-power needs and lower-cost battery options, such as Aquion’s technology, has the potential to provide a more economical solution when compared to each of the technologies put to use separately—or any other solution, for that matter,” says Apurba Sakti, a research scientist with the MIT Energy Initiative who works on advanced energy storage technologies.

North Carolina ranks fourth among the states for installed solar capacity, and Duke is investing half a billion dollars to expand its solar generation there. The area served by the Rankin substation includes a 1.2-megawatt solar installation just one mile away. The hybrid energy storage system, which began operation in February, includes a battery from Aquion that is well suited for this kind of grid solution. Aquion’s batteries use a saltwater chemistry that can provide long-term storage at a very low cost (see “Storing the Sun”). Cost is important because adding ultracapacitors could make the system more complex, and more expensive, than a battery-only solution.

Lithium-ion batteries can discharge bigger bursts of power than Aquion’s batteries, which would reduce the need for ultracapacitors, but they are more expensive (around $450 per kilowatt-hour of capacity, versus less than $350). Golden says the low cost of the Aquion technology and the ability to use a smaller battery, thanks to the ultracapacitors, lowered the cost of the Rankin installation by 10 to 15 percent compared with a battery-only system.

“There’s no such thing as a perfect battery; each one has its own technical fit,” says Matt Maroon, vice president of product management at Aquion. 

Cheap and practical energy storage could not only address the challenges involved in integrating renewables but also help utilities avoid overbuilding power plants and transmission lines to meet demand spikes. “If you look at the grid today, we build these enormous systems to meet the needs of the worst-case scenario in July when everybody runs their air conditioners,” says Golden. “That’s a lot of infrastructure for just a few days of peak demand we have to cover.”


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