Super
Capacitors also called Ultra Capacitors and electric double layer capacitors
(EDLC) are capacitors with capacitance values greater than any other capacitor
type available today. Capacitance values reaching up to 400 Farads in a single
standard case size are available. Super Capacitors have the highest capacitive
density available today with densities so high that these capacitors can be
used to applications normally reserved for batteries. Super Capacitors are not
as volumetrically efficient and are more expensive than batteries but they do
have other advantages over batteries making the preferred choice in
applications requiring a large amount of energy storage to be stored and
delivered in bursts repeatedly.
The
most significant advantage Super Capacitors have over batteries is their
ability to be charged and discharged continuously without degrading like
batteries do. This is why batteries and Super Capacitors are used in
conjunction with each other. The Super Capacitors will supply power to the
system when there are surges or energy bursts, since Super Capacitors can be
charged and discharged quickly while the batteries can supply the bulk energy
since they can store and deliver larger amount energy over a longer slower
period of time.
In
Super Capacitor construction, what makes Super Capacitors different from other
capacitors types are the electrodes used in these capacitors. Super Capacitors
are based on a carbon (nanotube) technology. The carbon technology used in
these capacitors creates a very large surface area with an extremely small
separation distance. Capacitors consist of 2 metal electrodes separated by a
dielectric material. The dielectric not only separates the electrodes but also
has electrical properties that affect the performance of a capacitor. Super Capacitors
do not have a traditional dielectric material like ceramic, polymer films or aluminium
oxide to separate the electrodes but instead have a physical barrier made from
activated carbon that when an electrical charge is applied to the material, a
double electric field is generated which acts like a dielectric. The thickness
of the electric double layer is as thin as a molecule. The surface area of the activated
carbon layer is extremely large, yielding several thousands of square meters
per gram. This large surface area allows for the absorption of a large amount
of ions.
The charging/discharging occurs in
an ion absorption layer formed on the electrodes of activated carbon. The
activated carbon fibre electrodes are impregnated with an electrolyte where positive
and negative charges are formed between the electrodes and the impregnate. The
electric double layer formed becomes an insulator until a large enough voltage
is applied and current begins to flow. The magnitude of voltage where charges
begin to flow is where the electrolyte begins to break down. This is called the
decomposition voltage. The double layers formed on the activated carbon surfaces
can be illustrated as a series of parallel RC circuits.
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