Duke Energy Tries First-of-Its-Kind Hybrid Energy Storage System

Duke Energy is testing a hybrid ultracapacitor-battery energy storage system (HESS) at its Rankin Substation in Gaston County, N.C., in an effort to find a more effective way to integrate renewable energy into its system.

The project became operational in February, but there is still testing in progress to demonstrate the system's multiple service applications. The benefits of the HESS are said to be extended operational life, rapid response, real-time solar smoothing, and load shifting.

According to Win Inertia-one of Duke Energy's partners on the project-the use of a stand-alone battery system is suitable for time-shifting applications, but when batteries simultaneously provide high-power short-term response it can lead to excessive degradation, requiring oversizing. Instead, the HESS uses ultracapacitors (UCAPs), which are capable of storing and discharging energy quickly and effectively, to help manage solar smoothing events in real-time.

Win Inertia says that the combined ultracapacitor-battery solution offered in a HESS reduces the degradation by using UCAPs to compensate for most of the solar-smoothing events. By using a hybrid approach, the total energy throughput of the battery is reduced and the thermal stress caused by high-C-rate responses is mitigated. This is particularly useful when solar power generation fluctuates due to cloud cover or other weather circumstances. UCAPs have already been proven to reduce heat stress on batteries and minimize degradation in other applications.

Installed solar power in North Carolina ranks fourth in the nation, so Duke Energy sees managing and maintaining grid-connected renewable installations as an important task, both now and in the future. The Rankin Substation (Figure 1) has a 1.2-MW solar installation connected just a mile away.

1. The Rankin Substation hybrid ultracapacitor-battery energy storage system. Courtesy: Duke Energy

"With so many solar installations in North Carolina, we must look for innovative ways to better incorporate renewable energy into our system-and still provide reliable service at a competitive price for our customers," said Thomas Golden, technology development manager for Duke Energy. "This approach will allow our energy storage systems to do a variety of tasks," he added.

The 100-kW/300-kWh battery installed at Rankin uses aqueous hybrid ion chemistry, which includes a saltwater electrolyte and synthetic cotton separator. Duke Energy expects the materials to save money, while the water-based chemistry provides a nontoxic and noncombustible product that is both safe to handle and environmentally friendly.

In addition to Win Inertia, Duke Energy partnered with Aquion Energy for the battery technology and Maxwell Technologies for the fast-response UCAPs. Win Inertia supplied its patented SHAD solution, including enhanced power electronics, and the energy management and control systems.

"Energy storage is changing the paradigm on how we generate, distribute and use energy. The demonstration of new technologies will help facilitate wider adoption across the nation," said Matt Roberts, executive director of the Washington, D.C.-based Energy Storage Association.

-Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)

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