Engineers have designed an ultracapacitor that maintains a near-constant voltage and could therefore find use in electric vehicle circuits and handheld electronics.
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 is increased with a coating of porous activated carbon, which is packed with tiny holes and cracks that can capture charged particles. The space between the plates is filled 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 both standard capacitors and ultracapacitors, the voltage drops as the stored charge is released. 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 1V.
‘Applications where the use of an ultracapacitor is precluded because of this problem include low-voltage systems in electric vehicles, handheld power tools, toys and cameras, just to name a few,’ said Ezzat Bakhoum, the West Florida University electrical engineer that built and tested the new prototype ultracapacitor.
The ultracapacitor is fitted with an electromechanical system that can slowly lift the core of the device out of the electrolyte solution as the stored charged is released.
As the electrolyte drains away, the device can hold less charge, thus lowering its capacitance. Since the voltage of the capacitor is related to the ratio of the stored charge to the capacitance, the system maintains a steady voltage as charge is siphoned off.
After attaching a 35W load to the device, Bakhoum found he could successfully program the voltage to stay within a 4.9V to 4.6V range.
Testing also showed that the constant-voltage mechanism operates with a 99 per cent efficiency or higher.
The lifetime of the electromechanical motor is expected to be about the same as the lifetime of the ultracapacitor’s core, according to Bakhoum.