How to size ultracapacitors

So how can this be possible? When it comes to electric energy storage the ultracapacitor is a completely different sort of beast—it has a very high capacity to accumulate electronic charge. Unlike the battery that requires reduction-oxidation reactions to chemically store energy, the ultracapacitor is far more adept, it stores energy in the same form it will be used. Let’s explore this difference in more detail.

Ultracapacitor fundamentals
In a broad sense, electrochemical capacitors comprise a wide class of electrical energy storage components in which the symmetrical carbon-carbon cell is the most notable and is now commonly referred to as an ultracapacitor. The terminology of “ultra” comes from the fact that unlike conventional electrostatic field storage components, the ultracapacitor takes the two main contributors to capacity, surface area (S) and charge separation distance (d) to the extreme. The figure below illustrates how an electronic double layer capacitor (EDLC) is formed at each electrode. Ions in the electrolyte remain in charge balance, but when an external electric field is impressed, will diffuse to the oppositely charged electrode. The electrodes being highly porous (3,000 m²/g ) act as very efficient electron and ion accumulators.

Electrons accumulate at the negative electrode in the porous carbon where they are bound to an electrolyte ion. The reverse process occurs at the anode (right side of graphic) where electron vacancies in the carbon become attached to electrolyte anions. The electrolyte remains conductive so that displacement currents during charging or discharging have a conductive path to follow between the double layer capacitor at each electrode.

A model of this behavior is shown below, from which it can be seen that each ultracapacitor cell in effect is the series connection of two EDLC’s. The equivalent series resistance (ESR) of the ultracapacitor consists of the bulk resistance of the aluminum foil current collectors, the adhesion interface between this conductor and the carbon electrode, the spreading resistance of the carbon itself, and the ionic conductivity of the electrolyte that is absorbed into the electrodes and separator.

The ESR of Maxwell ultracapacitors is very low and cell construction is such that parasitic components of ESR are minimized, yielding components that exhibit very high power density. In the application of any ultracapacitor, the user will note two different calculations of its specific power, P_d, the gravimetric attribute (W/kg), and P_v, the volumetric attribute (W/liter). Both P_d and P_v are calculated at the matched load condition (e.g., the point at which external and external power dissipation is equal).