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Posted on 03 April 2019

Super Capacitors, the Unknown Capacity Giants

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The unit of capacitance farad (F) has been known only in combination with the prefixes micro, nano, and pico for a long time. Nowadays, however, kilofarads have become possible. How do these new super capacitors work, which base on a principle that was discovered 160 years ago?

By Wolf-Dieter Roth, HY-LINE Power Components

Super capacitors have become known by the name of gold caps after their market launch. These were capacitors that were capable of carrying only low voltages but providing sufficient capacity to replace backup batteries of RAM retention or real time chips. They were also used for LED tail lights of bicycles and astonished people seeing a bicycle stopping at a traffic light whose tail light was still shining for minutes and no battery to be seen. Initially, these super capacitors featured only a low peak current but a relatively high equivalent serial resistance.

In the mean time, however, their technology has been very much advanced. Today, even mass produced super capacitors up to 7,000 F are offered (figure 1). With respect to their storage capacity they can compete with smaller accumulators. The physics of super capacitors, however, differs from the one of accumulators. That is why they have a completely different electrical behaviour.

3,000 F super capacitor from SPSCAP compared in size to a 9 volt battery

To begin with, super capacitors are basically capacitors: their capacity is determined by two opposing conductive surfaces. The larger the surface, the smaller the clearance between the surfaces and the higher the dielectric constant – the higher is the capacity. The formula for that is:

C = \epsilon \cdot \frac{A}{d}

                where C = capacitance, A = area, d = distance and ε = dielectric constant.

Increasing capacitance values

So an air or vacuum capacitor has a lower capacitance value, because d is high whereas ε and A are low. But a high dielectric strength is achieved.

A film capacitor, however, has a markedly higher capacity because it has a larger surface and a higher dielectric constant. In addition to that the film allows for reducing clearance without compromising dielectric strength. Depending on the dielectric constant of the materials used, ceramic capacitors feature even higher capacities but accompanied by a possibly reduced voltage persistance and capacitance stability.

E capacitors have an even increased capacity because there is no mechanically manufactured dielectric but a thin chemically generated oxide layer instead. A rough base material results in a larger surface and higher capacity. Dielectric strength is lower and the capacitor requires the user to observe the correct polarity. Improper handling such as reverse polarity, overvoltage, overcurrent, and overtemperature can lead to capacitor failure.

Charging principle of double layer capacitors

Super capacitors are double layer capacitors whose working principle bases on the Helmholtz double layers and has been known for more than 130 years. These layers have a thickness of only a few molecules, that is to say in the nanometer range, which results in an increased capacity compared with E capacitors of up to factor 10,000 and a lower dielectric strength which, in comparison with the stateof-the-art technology, ranges for individual cells under 3 V. For higher voltages the cells may be connected in series as it is the case with batteries. More than two cells which reach a peak operating voltage of 5 up to 5.5 V require that measures for a symmetrical voltage division are to be taken.

The alternative to accumulators

Electrochemical reactions as they occur in batteries and accumulators lead to wear and tear of the electrode material. They only play a minor part with double layer capacitors and contribute to the capacity of state-of-the-art super capacitors in the percent range.

What is of some relevance, however, is ionic shift and the formation of ions in the double layer. That is why super capacitors are also known as electrochemical capacitors and the reason for the exceptionally high field strengths of up to 5,000 kV/mm in the double layer which would lead to electrical breakdown in a normal dielectric.

Charge and discharge currents of double layer capacitors can be very high, whereas deep discharge is no problem. 100,000 charge and discharge cycles and even more are possible which means a life of more than 20 years. The capacities already lie at 1/10 of those of accumulators. Consequently, super capacitors have a far better performance in cyclic operations than accumulators. Even racing cars or means of public transport such as electric buses, which are being recharged during a short stop at the bus stop, can be powered by these capacitors. The Fraunhofer Institute, for instance, had hybrid buses manufactured in Dresden, which after having been recharged for 15 seconds at the bus stop were able to reach the next charging station 2 km away with this charge.

The principles of E and super capacitors were discovered almost at the same time: in 1875 by Eugène Adrien Ducrete (E capacitor) and even some time before in 1853 by Hermann von Helmholtz (Supercap) who also detected the double layer effect in 1879. But while the aluminium E capacitor was industrially used from 1892 and from 1931 onwards manufactured in the known technology of today, the super capacitor was ignored for many years. The first patents occurred in 1957 and in 1962 a canoe of Standard Oil powered by a super capacitor, which had the size of a car battery, made a demonstration for ten minutes on a lake in Ohio. Standard Oil, however, decided that there was no market chance for the capacitor and sold the patents to NEC. At the time even the developers of super capacitors did not know the difference from the principle of E capacitors. Thus Standard Oil regarded them as E capacitors. In 1971 NEC launched the first market-ready products and in 1978 they were followed by Panasonic’s 10 F “Goldcap” versions. Since 1992 capacities of 1,000 F have been available.

Temperature characteristics

Epcos went out of the super capacitor business at the end of 2006. Super capacitors have a higher temperature resistance (see figure 3) and a much better performance at low temperatures than accumulators. Certain limit values, however, must not be exceeded lest the electrolyte evaporates. At the end of its life there is a 30% loss of capacity or a doubling of the internal resistance. When used properly total failure of a super capacitor is seldom.

Combination with batteries

Besides the extremely thin insulation layer the high capacity of super capacitors is gained by the fact that super capacitors use carbon electrodes. They are very porous and rough – mostly active coal is used. With only one gram carbon powder a surface of 3,000 square metres can be realised.

Super capacitors are no filters like normal and E capacitors – they are primarily energy storages. The internal resistance at higher frequencies makes them unsuitable for sieving especially with switched-mode power supplies and converters – already at 10 Hz only a fraction of the super capacitor’s capacity is effective. This is due to the fact that the ions of the double layer do not move fast enough and the internal resistance is generally higher than the one of E capacitors. As a consequence the use of super capacitors as filter and smoothing capacitor is far from satisfactory and may even result in overheating and failure of the capacitor. However, in uninterruptible power supplies they are capable of bridging a potential power failure for several seconds without the need for permanent maintenance and inspection as is the case with a battery-powered UPS. They can even be used as starters of cars because in contrast to conventional starter batteries their capacity does not drop at low temperatures. Only the price for their use in these applications is still too high and therefore not yet competitive.

Likewise super capacitors are suitable for use as backup (see figure 4) if the device such as an optical smoke detector, despite the fact that it is supplied by batteries, draws current rather discontinuously. In this case the ESR of the batteries becomes too high, especially during the continuous discharge of the batteries.

With an additional 6 F super capacitor a digital camera supplied by two alkaline mignon (AA) batteries can be operated three times as long

With a super capacitor connected in parallel, however, these batteries can be used much longer before transients are able to cause undervoltages. Alternatively, long term high capacity lithium ion batteries can be used instead of the high current alkaline batteries.

Electrical characteristics

Super capacitors are used exclusively as secondary storage components. Even though they feature high capacity and a low self-discharge, they are not suited for use as an independent power supply of equipment for several months. Their self-discharge, however, is low enough for bridging days and even weeks.

For safety reasons, super capacitors are not delivered charged like accumulators, and are not mounted as plug-in or replaceable components. Peak currents caused by improper handling (short-circuit) would be very high and could do some serious damage. Unlike batteries or accumulators, super capacitors do not supply voltage which is chemically defined and constant for some time, and rapidly drops at discharge end, but like every capacitor supply constantly decreasing voltage at constant current drain. By using voltage regulators the output voltage of a UPS powered by super capacitors can be kept constant. However, when the capacitor voltage drops to half of its initial value, three quarters of the stored energy will be discharged. Consequently it is not worth while using wide range converters for discharging further.

Deep discharge is principally no problem for super capacitors, and you need not be afraid of a sudden failure when the electrical storage component has reached the discharge voltage.

Calculating super capacitor arrays

The end of life of a super capacitor is defined by a reduction of capacity to 70 per cent of the initial capacity and/or an increase of ESR to 200 per cent. Thus a circuit designed to supply a certain voltage and capacity by means of a super capacitor array is to be dimensioned with sufficient reserve capacities. If a discharge within seconds instead of hours is planned, the internal resistance will result in a drop of voltage which has to be compensated by a higher charging voltage and several super capacitors connected in series. The series circuitry, however, will lead to a reduction of capacity. A correct dimensioning of the individual capacitors will be calculated with end-of-life parameters instead of parameters of a brand new super capacitor. Only this way the long term operation of the circuit within its working limits is ensured.

The rule of thumb is that the life of a super capacitor is increased by a factor of 2.2 when:

  • the operating voltage is reduced by 0.2 V
  • the ambient temperature drops by 10 °C

Consequently, if you want the capacitor to have an especially long life, it is best to use it at reduced voltage and not too high temperatures. Likewise, it is necessary to prevent overvoltages and incorrect polarity, since this will cause decomposition of the electrolyte as well as capacitor gassing as it is the case with E capacitors.

When used within a wide temperature range, it is to be taken into consideration that the ESR will increase considerably below 0 °C whereas the capacity will be slightly reduced (see figure 5), however not to a level as it is usual with common accumulators.

Applications for super capacitors

There are mainly four fields of application (see figure 5):

The 4 classes of double layer capacitors

Low – internal resistance – high (verticle axis in figure 5)
Low – capacity – high (horizontal axis in figure 5)

Class 1
Data preservation
Class 2
For energy storage
Class 3
For power applications
Class 4
For instantaneous power

IEC 62576
Automotive applications
DIN EN 61881-3
Railway applications

Class 1: Characterized by low capacities and slow discharge. This type is designed for storage retention and uninterruptible power supply of real-time chips. They feature 0.1 up to 1 F capacity and low leak current values. Typical of this category are the Powerstor B, HB, and K series as well as the SPSCAP-SCV series.

Class 2: Characterized by low to medium capacities and slow discharge. This type is used for energy storage and power supply of torch lights, toys, emergency exit lights, small electrical tools, tail lights of bicycles, solar lamps, and for shutting down machines in case of a blackout. They come with capacities from 5 to 400 F. Typical for this category are the Powerstor XB and XV series as well as the SPSCAP-SCE series.

Class 3: This type of capacitor is marked by high capacities and strong discharge. They are typically used for electric vehicles (energy recovery, starting aid, start-stop systems), renewables (wind turbines and PV installations), x-ray devices and construction machines. They come with capacities from 100 to 5,000 F. Typical of this category is the SPSCAP-SCP series.

Class 4: These capacitors feature only low capacities but short transients. Their capacity ranges from 1 to 22 F and their ESR values are low. Typical of this category are the Powerstor HV and M series.

The first super capacitors introduced to the market belonged to class 1, whereas under class 3 fell all capacitors that were used for all kinds of vehicles. Class 2 capacitors are much cheaper if there is no need for the extremely high current ratings of class 3 capacitors, whereas class 4 capacitors are only poorly represented in the market.

With some applications it is difficult to see the possibilities they open up for capacitors as, for instance, their use in wind turbines: Here they are used among other things to quickly move the blades out of the wind in the event of a mains failure, before they get damaged. Other most obvious applications such as energy recovery during elevator operation have not been realised yet because of the aging of accumulators. With super capacitors, however, this has become possible.

Even a combination of different application modes is possible. One such example is the emergency power supply whose first task is to immediately shut down a high-power installation and after that to ensure its storage retention by way of supplying low current for long periods of time. Contrary to conventional battery-supported backup systems, there is no need for maintenance and the regular replacement of batteries with super capacitors and the temperature dependence of the installation is much lower. This is highly beneficial for traffic lights, data centres, telecommunications systems, or one-armed bandits, where a failure due to grid perturbations does not endanger lives but, nevertheless, can cause a lot of annoyance. A further advantage is that, after returning of the power supply, a super capacitor UPS system can be rapidly recharged and is capable of handling disturbances occurring anew.

More generally, it should be noted that some parameters increase the capacitor’s capacitance density and others its energy density. High capacitance density is required for class 3 and 4 applications, whereas high energy density is important to class 2 and 3 applications. Thus porous active carbon layers, for instance, ensure a higher capacity but the expanded surface area and the resulting longer ways will increase the ESR and thus reduce the capacitor’s energy density.

Choosing the right electrolyte

Today’s customary super capacitors use either propylene carbonate or acetonitrile as electrolytes. The formula of propylene carbonate is C4H6O3. It is a water-soluble carbon acid ester which becomes liquid at -48.8 °C and boils at 242 °C. Although falling under the German Ordinance on Hazardous Substances it is regarded as an unproblematic and environmentally friendly material – as a solvent it has replaced more hazardous substances. Super capacitors using it as an electrolyte can be operated without limitation at temperatures ranging from -25 °C to +70 °C. There are, however, limitations at temperatures up to +85 °C (due to voltage and power derating). At temperatures below -25 °C the capacitors cannot be operated any longer because the electrolyte freezes. Powerstor A, B, HB, P, K and XB capacitor series use propylene carbonate as electrolyte. There are series, however, where you have a choice as for SCV-P (propylene carbonate) or SCVA (acetonitrile).

The formula of acetonitrile is C2H3N. It has a higher conductivity, so capacitors using it as electrolyte have a somewhat lower ESR. It is, however, highly flammable. It becomes liquid at -45 °C and boils at +82 °C. It is deemed more critical in environmental terms and might cause poisoning because it undergoes decomposition and finally forms hydrogen cyanide (HCN) in case of fi re or when swallowed. The small quantity of acetonitrile in the enclosure of the super capacitor, however, does not pose a threat to anybody. The formerly existing dangerous goods regulations related to super capacitors fi lled with acetonitrile no longer apply except for certain limitations or transport regulations with regard to the generally high capacitance density of super capacitors, as is the case with lithium batteries.

The Powerstor HV, PHV and XV series use actonitrile as electrolyte, whereas the M and PM series use a mixture of 50% propylene carbonate and 50% acetonitrile. Super capacitors filled with acetonitrile can already be used at -40 °C but not above +65 °C, because this is not far from boiling point. With 2.7 V instead of 2.5 V the permissible voltage might be slightly higher, which is not due to the electrolyte but to the upper limiting temperature. If it is limited at +65 °C the 2.7 V can also be realized with propylene carbonate. Which of the two electrolytes is more appropriate for a certain application must be assessed case by case. In most cases, however, propylene carbonate is the more cost-effective solution.

Round or square?

In order to exploit a specified volume to the maximum square capacitors seem to be more beneficial at first. No different from fi lm capacitors, round windings have a superior performance and are cheaper to produce in the case of super capacitors. Compared with dense packed square capacitors, arrays are much easier to cool. Although square designs for special applications such as the SPP series from SPSCAP (see figure 6) are available, round designs are principally the better choice.

For further reference
Double layer (interfacial) http://en.wikipedia.org/wiki/Double_layer_%28interfacial%29
Excel spread sheet for calculating super capacitor UPS http://www.hy-line.de/supercapcalc
SPSCAP super capacitors http://www.hy-line.de/spscapPowerstor super capacitors http://www.hy-line.de/powerstor

 

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