Hybrid Energy Storage System Combines Ultracapacitors and Batteries

Batteries hold more energy, but ultracapacitors have quicker response times. Duke Energy uses a hybrid to get the best of both worlds.

Investment in grid-level energy storage is expected to triple over the next five years, with a multitude of players delivering a wide variety of technologies, each with its own benefits and drawbacks. Duke Energy recently installed a hybrid energy storage system (HESS) that combines high-capacity batteries with fast-responding ultracapacitors. The pilot project will provide peak demand response, load shifting, and support for a utility-owned 1.2 MW photovoltaic array.

Image courtesy of Maxwell Technologies

Image courtesy of Maxwell Technologies

Hybrid Energy Storage System (HESS)

Other than fuels, batteries offer the highest energy density of any energy storage system. They’re also quite versatile since they’re easily scalable and, unlike pumped hydro or compressed air energy storage, they don’t depend on local geography. On the other hand, batteries take a while to charge, they have limited lifespans, and they can’t deliver or absorb large power surges.  

Ultracapacitors (or supercapacitors) can charge quickly and survive ten times the number of cycles that most batteries can handle. Ultracapacitors’ quick response times allow them to take in or dish out substantial bursts of power, but for a given size/weight, ultracapacitors can’t pack nearly as much energy as batteries can.

While batteries are the marathon runners of energy storage, ultracapacitors are the sprinters. Duke Energy decided to employ both in its HESS, pairing a 100kW/300kWh Aquion Aqueous Hybrid Ion “saltwater” battery bank with a 277kW/8kWh Maxwell Technologies ultracapacitor module.


The Aqueous Hybrid Ion (AHI) battery is maintenance-free like a Li-ion battery, but according to Aquion, the AHI is more robust than Li-ion. For example, the AHI can operate over a wider temperature range (-5°C to 40°C), it’s not susceptible to thermal runaway (even when overcharged), and it is made from recyclable and sustainable materials. AHI batteries don’t have quite as much energy density or power delivery capabilities as their Li-ion competitors, however. The 100kW/300kWh battery bank can power about 80 homes for three hours.

Image courtesy of Aquion Energy

Image courtesy of Aquion Energy


While the battery bank provides long-duration power, the ultracapacitors handle short, intermittent duties. For example, when a cloud passes in front of the PV array, the ultracapacitors can quickly stabilize the power. When supply exceeds demand briefly, the ultracapacitor will absorb the additional energy. In Duke Energy’s hybrid system, the ultracapacitors can hold 8 kWh of energy and can deliver 277 kW of power for up to 104 seconds.

Image courtesy of Maxwell Technologies

Image courtesy of Maxwell Technologies

Other Applications

In addition to the aforementioned applications, grid-level energy storage can be used for frequency regulation, spinning reserve, and even wind turbine blade pitch control.

In the energy storage world, there’s no such thing as “one size fits all.” Likewise, there’s no single storage medium that meets the needs of every application. Smart designers are mixing various capacities and technologies to build the right tool for the job.



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