Double Layer Capacitors

Double Layer Capacitors (DLC, Supercaps, Ultracaps, etc.) are efficient energy storage devices, which can store and release electrical energy very quick. They are ideally suited for applications where short time backup power and peak power needs are critical.

The Doble Layer Capacitor is based on a discovery made be Helmholtz in 1856, who observed that a double layer is generated when a voltage is applied to electrodes in a electrically conducting liquid (electrolyte). In discharged condition the positive and negative charged ions in the electrolyte are distributed equally, when a external voltage is applied the positive charged ions move to the negative electrode and vice versa. This is called the electrochemical double layer.

The maximum voltage applied to the electrodes is limited by the dissociation voltage. Above this voltage level electrical loads can move to the direct attached ions on them and chemical reactions are started which cause the evolution of gases and therefore will destroy the capacitor. The use of an aqueos electrolyte limits the voltage to about 1V while electrolytes based on organic solvents allow dissociations voltages up to 2,5V. The high energy content of Double Layer Capacitor originates in the activated carbon electrode material, which has an extremely high specific surface area of about 2000m²/g and the extremely short distance between the opposite charges of the capacitor, which is in the order of a view nanometers (2-5nm). Since the dielectric is very thin, capacitances of a view thopusand Farad can be realized in a device as small as a soda can.

Compared with accumulators Double Layer Capacitors offer a significantly lower internal resistance (Ri) and therefore very high discharge currents (power density). Also the charging of an Double Layer Capacitor can be done in seconds rather than in hours as with standard electrochemical accumulators. The energy density of a Double Layer Capacitor is significantly below that of accumulators. Todays Double Layer Capacitors allow up to 500.000 charge/discharge cycles and a lifetime of up to 20 years and are almost maintenance free.

Power Density and Energy Density (Comparison Battery and Ultracap)

Charge/Discharge curves and internal resistance Ri (Comparison Battery and Ultracap)

Applications

Ultracaps are used for two main types of applications today. One is to backup shorttime power outages or power interruptions (UPS etc.). The other application type is the coverage of peak power requirements where the Double Layer Capacitor has a huge advantage over accumulators . Wherever short power peaks need to be met Ultracaps are an ideal solution. As an example, today car manufacturers intend to use Ultracaps in conjunction with accumulators to provide more power for the starter generator especially at low temperatures.