Chemical
batteries power many different mobile electronic devices, but repeated
charging and discharging cycles can wear them out. An alternative energy
storage device called an ultracapacitor can be recharged hundreds of
thousands of times without degrading, but ultracapacitors have their own
disadvantages, including a voltage output that drops precipitously as
the device is discharged. Now a researcher from the University of West
Florida has designed an ultracapacitor that maintains a near steady
voltage. The novel constant-voltage design, which may one day help
ultracapacitors find new uses in low-voltage electric vehicle circuits
and handheld electronics, is described in the American Institute of
Physics’ Journal of Renewable and Sustainable Energy.
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
one volt.
“A
significant portion of the energy of the ultracapacitor is held below
one volt,” notes 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.”
So
Bakhoum has designed an ultracapacitor that maintains a near-constant
voltage without a DC-DC converter. 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.
Bakhoum
built and tested a prototype of the new ultracapacitor. After attaching
a 35-W load to the device, he found he could successfully program the
voltage to stay within a 4.9 to 4.6 V range. Testing also showed that the
constant-voltage mechanism operates with a 99% efficiency or higher.
The lifetime of the electromechanical motor is expected to be about the
same as the lifetime of the ultracapacitor’s core, Bakhoum writes.
“The
ultracapacitor is a wonderful new energy storage device that has many
advantages by comparison with batteries,” says Bakhoum. In addition to
their near limitless ability to be recharged, ultracapacitors can
release a jolt of energy much more quickly than batteries. One current
disadvantage of commercially available ultracapacitors, that they store
only a fraction of the energy per unit mass that batteries store, is a
challenge that is still being researched. Some groups have experimented,
for example, with changing the structure of the electrode to increase
surface area, and thus the amount of charge that can be stored.
For
Bakhoum, future research steps include modifying the design of the
constant-voltage ultracapacitor system so that it can be installed at
any angle. He may also explore whether the same type of constant-voltage
approach is suitable for new, high-energy-density ultracapacitors.
Constant voltage supercapacitor
Source: American Institute of Physics
