|ultra capacitor or EDLC|
An electric double-layer capacitor, also known by the names super capacitor, super condenser, pseudo capacitor, electrochemical double layer capacitor(EDLC) or ultra capacitor is an electrochemical capacitor that has an unusually high energy density when compared to common capacitors, typically of the order of thousands of times greater than a high capacity electrolytic capacitor. For instance, a typical D-cell sized electrolytic capacitor will have a capacitance in the range of milli farads, but the same size super capacitor would have a capacitance of several farads, an improvement of about two or three orders of magnitude in capacitance, usually at a lower working voltage.
In a conventional capacitor, energy is stored by the removal of charge carriers, typically electrons, from one metal plate and depositing them on another. This charge separation creates a potential difference between two plates, which can be harnessed in an external circuit. The total energy stored will be proportional to both the amount of charge stored and the potential between these plates. Different materials sandwiched between the plates to separate them results in different voltages to be stored.
Super capacitors do not have a conventional dielectric. Rather than two separate plates separated by an intervening substance, these capacitors use “plates” that are in fact two layers of the same substrate, and their electrical properties, so called “electrical double layer”, result in effective separation of charge despite the vanishingly thin physical separation of the layers. The lack of need for a bulky layer of dielectric permits the packing of “plates” with much larger surface area into a given size, resulting in extraordinarily high capacitances in practical-sized packages.
In general, EDLC’s improve storage density through the use of nanoporous material, typically activated charcoal, in place of conventional insulating barrier. Activated charcoal is a powder made up of extremely small and very rough particles which in bulk, form a low density volume of particles with holes between them that resembles a sponge. The overall surface area of even a thin layer of such material is many times greater than a traditional material like aluminium, allowing more charge carriers to be stored in a given volume.
Most recent research in EDLCs has focused on improved materials that offer even higher usable surface areas. Experimental devices developed at MIT replace the charcoal with carbon nanotubes, which can store about the same charge as charcoal but are mechanically arranged in much more regular pattern that exposes a much greater suitable surface area. Other teams are experimenting with custom materials made of activated polypyrrole, and nanotube-impregnated papers.
EDLCs have much higher power density than batteries. Power density combines the energy with the speed that the energy can be delivered to the load. Batteries, which are based on the movement of charge carriers in a liquid electrolyte, have relatively slow charging and discharging time. Capacitors, on the other hand, can be charged or discharged at a rate that is typically limited by current heating of the electrodes. So while existing EDLCs have energy densities that are perhaps 1/10th that of a conventional battery, their power density is generally 10 to 100 times greater.
Activated carbon, graphene, carbon nanotubes and certain conductive polymers or carbon aero goals are practical for super capacitors. Virtually all commercial super capacitors manufactured by Panasonic, Nesscap, Maxwell technologies, Axion power and others use powdered activated carbon made from coconut shells. Some companies also build higher performance devices, at a significant cost increase, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH).
1. Heavy and public transport
New technology in development could potentially make EDLCs with high enough energy density to be an attractive replacement for batteries in all electric cars and plug-in hybrids, as EDLCs charge quickly and are stable with temperature.China is experimenting with a new form of electric bus that runs without power lines using power stored in large onboard EDLCs, which are quickly recharged whenever the bus is at stop and fully charged in the terminus.Other companies from the public transport, manufacturing sector are developing electric double-layer capacitor technology.
2. Consumer electronics
EDLCs can be used in PC cards, flash photography devices in digital cameras, flash lights, portable media players and in automated meter reading, particularly where extremely fast charging is desirable.