One of the most recent innovations in the field of passive components is the electrochemical double layer capacitor - in short, the SuperCapacitor. From about ten up to several thousand farad (F) are housed in its case. A 100 F model for example is approximately the size of a match-box, and this capacitance would correspond to the impressive volume of 100 million parallel connected standard polyester film capacitors with an individual capacitance of 1 μF each! With the SuperCapacitor, despite the limitations of its low maximum operating voltage of 2.5 V, several units can be built up to enormous capacitance of the desired rated voltage by means of series or parallel connection (cascade). The typical application is the quick provision of several 100 A in the direct current field. The SuperCapacitor is therefore the connecting link between the conventional capacitor and the battery. It combines the advantage of the capacitor as a fast supplier of electricity with that of the battery as a notable energy reservoir. Its energy density is, at present, still limited to about 1/10th of the battery value - as yet!

The working principle of the electrochemical double layer capacitor was discovered by the well-known physicist Helmholtz [1] as early as 1856. He described the phenomenon of the construction of a double layer made up of charge carriers which is formed on electrodes in a conductive liquid when voltage is applied. It was not until the middle of the 20th century that some scientists took up the Helmholtz model again in order to examine to what extent it could be used for the realization of technical capacitors. At the beginning of the 1980s the Japanese were the first to succeed in the technical realization: - the so-called “Gold Cap”, thumb-sized with a capacitance of 10 F, came onto the market. It was first used for voltage support on computer circuit boards. In the meantime, other manufacturers, including recently WIMA, are active in this field which is becoming increasingly widespread in connection with power electronics too.

The WIMA Double Layer Capacitor

Building on the above-mentioned Helmholtz principle of energy storage in the electrochemical double layer of electrolyte systems, WIMA has developed “SuperCap” capacitors which have about a million times the capacitance of traditional capacitors of comparable size. The WIMA SuperCaps can take up energy which is comparable to the stored energy of smaller batteries, however they deliver considerably higher electric currents and are also maintenance-free.   With the choice of a material system on the basis of an electrode containing carbon, which is attached with low resistance to electron-conductive foil, the WIMA SuperCap has extremely large surfaces within the smallest volume.

For technical enthusiasts, it should be mentioned that the secret of its comparably enormous capacitance is to be found in its material construction and working principle. The capacitance rate of the well-known plate capacitor from the physics lesson was determined by the size of the plate or electrode, the small distance between the plates and the dielectric value of the insulator material in between. In the case of the SuperCap, the inner surface accessible to the charge carriers, its electrode surface, is extremely large as it takes the form of a carbon layered electrode. Casually speaking, the inner surface would cover a whole football stadium. The strength of the dielectric, here fictively resulting from the electrochemical double layer, is, on the other hand, tiny. It corresponds to half the diameter of an ion. The double layer itself consists of ions which, when voltage is applied, attach to the positive or negative electrode corresponding to their opposite poles and create a dielectric of only a few angstrom. According to the formula for the capacitor from the dielectric constant of the double layer of about 40, the extremely thin dielectric as well as the extremely large surface area, the result is a very high capacitance yield.

WIMA double layer capacitors are available in the standard capacitance range of 100 F to 300 F with a rated voltage of 2.5 VDC and the highest discharge current. The prismatic case makes space-saving serial and parallel connections possible. WIMA SuperCaps replace, protect or support batteries e.g. in the context of new traction technologies.

Technical Data

Rated capacitance: CR 100 F 200 F 300 F Capacitance tolerance: - ±20% ±20% ±20% Rated voltage: UR 2.5V 2.5V 2.5V Rated current: IC 30A 45A 50A Internal resistance: RDC 12m 7m 6m Max. stored energy: ±20% Emax. 313J 625J 938J Operating temperature: Top -30°C ... +65°C Storage temperature: Tst -40°C ... +70°C Weight: m 65g 80g 90g Volume: v 0.075l 0.075l 0.075l Additional Data Case: - Al99.5 Al99.5 Al99.5 Lug terminal: - Brass Brass Brass Comparative Data Density of capacitance: gravimetric: Cd 1500F/kg 2500F/kg 3400F/kg volumetric: Dv 1400F/l 2900F/l 4400F/l Energy density: gravimetric: Ed 1.2Wh/kg 2.0Wh/kg 3.0Wh/kg volumetric: Dv 1.3Wh/l 2.5Wh/l 4.0Wh/l

Dims. in mm. Lug terminals for 6.3mm slip-on connection When connected in series cases should be kept isolated   Rights reserved to amend design data without prior notification

Application of Double Layer Capacitors

The areas of use for SuperCapacitors conform to the most up-to-date developments in power electronics and their application [2-5]. In general they are used for voltage support, for the fast supply of electric energy, e.g. to cover the requirements of peak power demand, or for the saving of batteries, which can then be made smaller, as the capacitor lowers the peak current.

A frequent use of the SuperCap is to be found in the consumer field: a bicycle is standing in the dark, for example at a red traffic light. To protect the cyclist, the lights remain active while the bicycle is standing still. A small SuperCap supplies energy to the front and back lights for several minutes.

The SuperCap has just entered the field of uninterruptable power supply (UPS). In the simplest case it supplies the emergency generator, usually a diesel generator or a micro-turbine, with the necessary start-up energy completely maintenance-free. Even alone, in special cases, it can mitigate short-term and shortest voltage dips, when connected to the grid. More than 90% of the dips in our mains are, statistically calculated, shorter than 10 seconds.

In a recently described application in the telecommunications area of a well-known service provider, SuperCaps supply emergency energy to switch-over stations which transmit the calls from mobile phones. The best thing about the systems is their lack of maintenance and their reliability. The numerous stations have to be able to interrupt safely in the case of power failure and to restart automatically. Special demands are made on interruption-free electricity supply in the generation of wind power. High up in the gondola is a special kind of system, the pitch control, which makes sure that the rotation speed of the rotary blades remains constant. Especially in a storm, the installation must not over rotate, the rotor blades are then turned with their slim side into the wind and thus loose their traction. This is done by the pitch control, which can be electric or pneumatic. In all events it must function self-sufficiently on each windmill, in all weathers, at temperatures between -40°C and +70°C, always at the ready for many years to come. The use of SuperCaps in the windmills considerably reduces maintenance work. They are important because they can help increase the profitability of the windmills, especially when, in the future, they are placed on the continental shelf in the sea.

At present there is a lot of discussion about new types of traction systems for motor vehicles. A number of car manufacturers world-wide are working on vehicles with fuel cell drive. At this year's IAA in Frankfurt, new projects were set up even on this topic. In state of the art technology the portable fuel cell works most reliably in a continuous manner. Some manufacturers pin their hopes on load levelling, by means of a power reservoir in parallel, to put it simply: an array of several SuperCapacitors which are resilient due to their lowest internal resistance and their low weight and therefore predestined for this purpose. The SuperCap is very well suited to support the on-board electrical system from decentralized demand, so to speak where needed, by providing electrical energy which is consumed in modern vehicles by the ever-increasing number of electric consumers. The reform of the on-board electrical system up to higher voltage with adapted wiring can thus be postponed for a time. A further advantage of the SuperCapacitor in the car, when electric drive is being used anyway, is the possibility of recuperating brake energy. New projects describe how up to 80% of the kinetic energy which has, up to now, been wasted at traffic lights, with accumulated currents in the range of kilo ampere, can be collected for recuperation in the SuperCaps and, with delay, be made available for driving again. Local public transport could profit from this too, with bus systems which could start emission-free, trolley buses which could, to a certain extent, travel outside their network system and electric airport buses which would not be a further burden on the airport areas which are already polluted.

Even the support of underground networks (subway systems) by SuperCapacitors connected in series on high voltages, which have been built into highly frequented tunnels as a space-saving device, has been described. For the railway too, the use of SuperCaps offers diverse advantages. A reliable start for diesel-electric locomotives is child's play for the SuperCap. Indeed, the majority of today's engines are electrically powered, whether by traction wire or diesel generator. The electric motor is far superior to the combustion engine due to its large torque when starting. At a time when Europe is growing together, but also in countries with traditionally different traffic systems, the relatively light energy reservoir SuperCap offers distinct advantages over battery packs while bridging the systems.

An ideal case for the use of double layer capacitors as an energy buffer is to be found in photovoltaic conversion. In countries which are blessed or plagued by eternal sun, the advantage of a completely independent, permanently available and largely maintenance-free energy converter is obvious: as long as the sun shines, the capacitor is charged via the photocell. If the energy is sufficient, the pump for watering works and the game starts again from the beginning. In our climatic conditions these advantages are more modest, but progress is being made here too.

Well-Known WIMA Quality

The WIMA double-layer capacitor is excellently suited for all these applications. Its low capacitance drift over and beyond its lifetime guarantees that the electric function is maintained for decades. Careful studies show that the WIMA SuperCap always renders its service reliably even under the hardest conditions. Its metallic rectangular case is tightly sealed by laser welding and therefore defies the
most severe temperature fluctuations. The WIMA SuperCap copes with short-term over-heating, which in other system solutions, can cause undesirable problems in application. Its prismatic shape allows for a greater packing density in the assembling of larger units. In this way unused cavities are avoided. Where natural cooling is not sufficient e.g. in the case of fast pulse sequences, permanent over- heating can easily be prevented by a ventilator.

Constant quality assessments of the capacitors is an obligation for us. In tests of the different types, the usual run-in of the capacitance for double layer capacitors is recorded. For this purpose, the WIMA SuperCaps are subjected to ten thousands of charging and discharging cycles, whereby charging takes place about every 8 minutes from an empty state with 50 A and the complete discharge with just under 200 A. The enormous inner surface structure is typically reduced on first use only by about 10%. After that the capacitance remains constant even with higher temperature applications. Therefore the user can be sure of a lifelong stable function. The run-in is compensated by a capacitance tolerance of ±20% depending on type. It is thus guaranteed that the capacitance given on the performance label is really available. A further test is carried out on a series/parallel connection of several double-layer capacitors, for example on 300 F at 7.5 volt. Due to the prismatic structure of the WIMA capacitor, a lot of space can be saved in the construction of the series/parallel connection. For a more demanding load, it is recommended that the SuperCaps be fixed by means of metal plates.

At WIMA the individual capacitors are, of course, tested according to the recommendations of the norm draft DIN IEC 62391-1 to 2 for “Fixed Electric Double Layer Capacitors” in accordance with their type.

Generally speaking, the time is ripe for the Helmholtz capacitor. Its success cannot yet, of course, be chalked up en masse. However, the environmental awareness which has set in increasingly in recent years, the current discussion about energy consumption and the explosion of fuel prices are grist to the mill of the SuperCap. The looming change in the climate, whatever it is caused by, is reinforcing acceptance of new resource-protective technologies.
Here the WIMA SuperCap is to be found in the front line, because it makes a revolutionary contribution to the increase of efficiency in the use of electric energy. Its application in combination with alternative traction may also be seen in this connection. Increasingly rigorous exhaust emission regulations e.g. the “Zero Emmission Vehical Program” in California are driving this develop-ment forward. Thus the boom of these new technologies in the field of passive components corresponds to the prognoses for the coming years.

WIMA has taken up its position. WIMA is committed. WIMA is involved.

Source Material:

[1] Von Helmholtz, H.L.F., Studies of electric boundary layers, Wied. Ann.,7,337-382(1879)

[2] DGES - Fachtagung in Aachen, 8. - 9. Mai 2003, Elektrofahrzeuge, Hybrid-Technologien im Aufschwung

[3] 11-13th International Seminar on Double-Layer-Capacitors, Deerfield Beach, FL, U.S.A., 2001-2003

[4] International Symposium on Power Sources for Stationary and Distributed Systems
16.-18. September 2003, München

[5] Siebentes Kassler Symposium Energie, Systemtechnik, Erneuerbare Energien und Rationelle Energieverwendung, Energiespeicher und Energietransport, 14.-15. November 2002