Ultracapacitor Delivers a Steady Jolt

Friday, July 20, 2012 @ 04:07 PM gHale

Chemical batteries power different mobile electronic devices, but repeated charging and discharging cycles will wear them out.

On the other hand there is an alternative energy storage device called an ultracapacitor that can recharge hundreds of thousands of times without degrading, but the problem is one disadvantage is voltage output drops as the device discharges.

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That may soon change as there is now an ultracapacitor in design that can maintain a near steady voltage. The novel constant-voltage design may help ultracapacitors find new uses in low-voltage electric vehicle circuits and handheld electronics, according to a researcher from the University of West Florida.

Standard capacitors store energy in an electric field created when opposite electrical charges collect on two plates separated by a thin insulating material. In ultracapacitors, the surface area of the plates increases with a coating of porous activated carbon, packed with tiny holes and cracks that can capture charged particles. The space between the plates then fills with an electrolyte solution containing positive and negative ions. As charge accumulates on the plates, they attract ions, creating a double-layer of stored energy.

In standard capacitors and ultracapacitors, the voltage drops as the stored charge releases. Most electronic devices, however, require constant voltage to operate. An electronic circuit called a DC-DC converter can change the dropping voltage of the capacitor into a constant voltage output, but the converters experience problems below one volt.

“A significant portion of the energy of the ultracapacitor is held below one volt,” said Ezzat Bakhoum, a professor of electrical engineering at the University of West Florida. “Operation in that region is very difficult because the DC-DC converter cannot function at such low voltage. Applications where the use of an ultracapacitor is precluded because of this problem include low-voltage systems in electric vehicles, hand-held power tools, toys, and cameras, just to name a few.”

Bakhoum decided to go out and design an ultracapacitor that maintains a near-constant voltage without a DC-DC converter. The ultracapacitor has an electromechanical system that can slowly lift the core of the device out of the electrolyte solution as the stored charged releases. As the electrolyte drains away, the device can hold less charge, thus lowering, its capacitance. Since the voltage of the capacitor relates to the ratio of the stored charge to the capacitance, the system maintains a steady voltage as the charge siphons off.

Bakhoum built and tested a prototype of the new ultracapacitor. After attaching a 35-watt load to the device, he found he could successfully program the voltage to stay within a 4.9 to 4.6 volt range. Testing also showed the constant-voltage mechanism operates with a 99 percent efficiency or higher. The lifetime of the electromechanical motor should be about the same as the lifetime of the ultracapacitor’s core, Bakhoum said.

“The ultracapacitor is a wonderful new energy storage device that has many advantages by comparison with batteries,” Bakhoum said. In addition to their near limitless ability to recharge, ultracapacitors can release a jolt of energy much more quickly than batteries.

One current disadvantage of commercially available ultracapacitors is they store only a fraction of the energy per unit mass that batteries store. That is another challenge researchers are working on.

Future research includes modifying the design of the constant-voltage ultracapacitor system so users can install it at any angle, Bakhoum said. He may also explore whether the same type of constant-voltage approach is suitable for new, high-energy-density ultracapacitors.

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