Synonyms for ultracapacitors:
Double layer capacitors, or ultracaps, consist of two electrodes which are immersed in an electrolyte system. When a voltage is applied that is smaller than the decomposition potential of the electrolyte, both electrodes attract ions of reversed polarization. They create a zone of immovable current collectors with a thickness of only a few molecular layers. This effect was described by Hermann von Helmholtz in 1879.
The electrodes, with the load-collector layer as a dielectric separator, behave like two capacitors that are connected in series via the two electrically conductive electrolytes. The energy is stored electro-statically, in contrast to batteries which store and supply electrical energy chemically.
The capacitance of a capacitor depends not only on the thickness of the insulation layer (the spacing), but also on the size of the electrode surfaces. The activated carbon commonly used has a surface area of up to 3,000 m2/g. In combination with separator layers with thicknesses of only a few nanometres, a very large capacitance is achieved on a very small area.
The most important parameters of the ultracaps are the capacitance C and the equivalent serial resistance ESR. The idealised equivalent circuit of a capacitor is shown below:
The capacitance of a capacitor is specified in Farad (F) (named after Michael Faraday). A capacitor has the capacistance of 1F when it is charged to 1V in 1s with a current of 1A.
1 F 1 As/V or 1 Farad = 1 ampere second per volt.
The stored energy (work W) of a capacitor results from:
W = 1/2 C U2.
The usable energy is derived from the voltage difference at the capacitor in accordance with the following formula:
Wusable = 1/2 C (U2max - U2min)