Ultracapacitor maker Ioxus today said it has raised $21 million from a handful of large industrial companies, a vote of confidence in this relatively young energy-storage technology.

The lead investor was Energy Technology Ventures, a joint venture of General Electric, utility NRG Energy, and fuel company ConocoPhillips. Another investment group involved was Aster Capital, which represents transportation and power companies Alstom, Schneider Electric, and Rhodia.

Ultracapacitors are being evaluated for more energy storage applications, says Ioxus.

The series B round will be used for product development, for sales, and to ramp up the company's manufacturing facility in Oneonta, N.Y., Ioxus CEO Mark McGough said today. These "strategic partner" investors could become high-volume customers, he added.

Like batteries, ultracapacitors store energy but they have different properties that make them well suited for certain applications, such as powering motors for electric windows in a car or to adjust the pitch of wind turbine blades.

Ultracapacitors can discharge a lot of power in quick bursts, but can't store as much overall energy as batteries. They also can be charged very quickly and last for many charge cycles. Improvements in performance and more familiarity among industrial designers are nudging the technology into different areas, said McGough.

"In the last 10 years or so, it's gone from a technology that hardly anybody knows, to people understanding it, and finally to adopting it in design," he said. "It fills an increasingly important place in the designer's toolkit. We're seeing a tipping point of adopting this technology in mainstream design."

None of the new investors in Ioxus is now using its products, which look like cylindrical batteries, but the companies are evaluating ultracapacitors. GE, for example, is looking at using them in its locomotives, medical devices, wind turbines, and for hybrid electric cars, McGough said.

One application that could become a boost to ultracapacitors is microhybrids, which use start-stop technology. When a hybrid car is idle, its engine will turn off and then rely on ultracapacitors to accelerate the car and to store braking energy.

Ioxus, a 2008 spin-off from a capacitor company, plans next quarter to release its second-generation product, which will have more power than its current line, making it viable for transportation, McGough said.