Ultracapacitors

[soumyakanta]

Electric double-layer capacitors, also known as supercapacitors, electrochemical double layer capacitors (EDLCs) or ultracapacitors are electrochemical capacitors that have an unusually high energy density when compared to common capacitors, typically on the order of thousands of times greater than a high-capacity electrolytic capacitor. For instance, a typical D-cell sized electrolytic capacitor will have a capacitance in the range of tens of millifarads. The same size electric double-layer capacitor would have a capacitance of several farads, an improvement of about two or three orders of magnitude in capacitance, but usually at a lower working voltage. Larger, commercial electric double-layer capacitors have capacities as high as 5,000 farads. The highest energy density in production is 30 Wh/kg

Supercapacitors aim to fill the gap between capacitors and batteries

Electric double-layer capacitors have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. Some of the earliest uses were motor startup capacitors for large engines in tanks and submarines, and as the cost has fallen they have started to appear on diesel trucks and railroad locomotives.[4] More recently they have become a topic of some interest in the green energy world, where their ability to soak up energy quickly makes them particularly suitable for regenerative braking applications, whereas batteries have difficulty in this application due to slow charging rates. New technology in development could potentially make EDLCs with high enough energy density to be an attractive replacement for batteries in all-electric cars and plug-in hybrids, as EDLCs are quick charging and exhibit temperature stability. They can also be used in PCMCIA+ cards, flash light devices in digital cameras, portable media players and automated meter reading

In a conventional capacitor, energy is stored by the removal of charge carriers, typically electrons, from one metal plate and depositing them on another. This charge separation creates a potential between the two plates, which can be harnessed in an external circuit. The total energy stored in this fashion is proportional to both the number of charges stored and the potential between the plates. The former is essentially a function of size and the material properties of the plates, while the latter is limited by the dielectric breakdown between the plates. Different materials sandwiched between the plates to separate them result in different voltages to be stored. Optimizing the material leads to higher energy densities for any given size of capacitor.

In contrast with traditional capacitors, electric double-layer capacitors do not have a conventional dielectric. Rather than two separate plates separated by an intervening substance, these capacitors use "plates" that are in fact two layers of the same substrate, and their electrical properties, the so-called "electrical double layer", result in the effective separation of charge despite the vanishingly thin (on the order of nanometers) physical separation of the layers. The lack of need for a bulky layer of dielectric permits the packing of "plates" with much larger surface area into a given size, resulting in their extraordinarily high capacitances in practical sized packages.

In an electrical double layer, each layer by itself is quite conductive, but the physics at the interface where the layers are effectively in contact means that no significant current can flow between the layers. However, the double layer can withstand only a low voltage, which means that electric double-layer capacitors rated for higher voltages must be made of matched series-connected individual electric double-layer capacitors, much like series-connected cells in higher-voltage batteries.

In general, electric double-layer capacitors improve storage density through the use of a nanoporous material, typically activated charcoal, in place of the conventional insulating barrier. Activated charcoal is a powder made up of extremely small and very "rough" particles, which in bulk form a low-density volume of particles with holes between them that resembles a sponge. The overall surface area of even a thin layer of such a material is many times greater than a traditional material like aluminum, allowing many more charge carriers (ions or radicals from the electrolyte) to be stored in any given volume. The downside is that the charcoal is taking the place of the improved insulators used in conventional devices, so in general electric double-layer capacitors use low potentials on the order of 2 to 3 V.

Activated charcoal is not the "perfect" material for this application. The charge carriers are actually (in effect) quite large - especially when surrounded by solvent molecules - and are often larger than the holes left in the charcoal, which are too small to accept them, limiting the storage. Recent research in electric double-layer capacitors has generally focused on improved materials that offer even higher usable surface areas. Experimental devices developed at MIT replace the charcoal with carbon nanotubes, which have similar charge storage capability as charcoal (which is almost pure carbon) but are mechanically arranged in a much more regular pattern that exposes a much greater suitable surface area.Other teams are experimenting with custom materials made of activated polypyrrole, and even nanotube-impregnated papers.

Ragone chart showing energy density vs. power density for various energy-storage devices

In terms of energy density, existing commercial electric double-layer capacitors range around 0.5 to 30 W h/kg, with the standardized cells available

For More Read This Links
http://en.wikipediawiki/Supercapacitor
http://fplreflib.findlay.co.uk/articles/...p-fits.pdf
http://ultracapacitorsforum/56.html

[Ahmad]

The electrochemical ultracapacitor is an emerging technology that promises to play an important role in meeting the demands of electronic devices and systems both now and in the future. This newly available technology of ultracapacitors is making it easier for engineers to balance their use of both energy and power. Energy storage devices like ultracapacitors are normally used along with batteries to compensate for the limited battery power capability. Evidently, the proper control of the energy storage systems presents both a challenge and opportunity for the power and energy management system. This paper traces the history of the development of the technology and explores the principles and theory of operation of the ultracapacitors. The use of ultracapacitors in various applications are discussed and their advantages over alternative technologies are considered. To provide examples with which to outline practical implementation issues, systems incorporating ultracapacitors as vital components are also explored. This paper has aimed to provide a brief overview of ultracapacitor technology as it stands today. Previous development efforts have been described to place the current state of the technology within an historical context. Scientific background has also been covered in order to better understand performance characteristics. Possible applications of ultracapacitor technology have also been described to illustrate the wide range of possibilities that exist. Because of the advantages of charging efficiency, long lifetime, fast response, and wide operating temperature range, it is tempting to try and apply ultracapacitors to any application that requires energy storage. The limitations of the current technology must be fully appreciated, however, and it is important to realize that ultracapacitors are only useful within a finite range of energy and power requirements. Outside of these boundaries other alternatives are likely to be the better solution. The most important thing to remember about ultracapacitors technology is that it is a new and different technology in its own right. There may exist some similarities between ultracapacitor operation and the operation of electrostatic capacitors, but there are fundamental differences that result from the different physical processes involved and these must be appreciated. Problems may be encountered if systems are designed based on the assumption that ultracapacitors behave like normal capacitors. Ultracapacitors are, at any rate, a part of the new wave of advanced energy storage devices that will further the push towards greater energy efficiency and more sustainable alternatives. They will be a useful tool with which to engineer highly efficient electrical and electronic systems, and as the state of the technology advances they will become progressively more commonplace.

[simran]

Electrical energy storage is required in many applications ? telecommunication devices,such as cell phones and pagers, stand-by power systems, and electric hybrid vehicles. The specifications for the various energy storage devices are given in terms of energy stored (Who) .and maximum power as well as size and weight, initial cost and life. A storage device to be suitable for a particular application must meet all the requirements. As power requirements for many applications become more demanding, the battery was found wanting. It is often reasonable to consider separating the energy and power requirements by providing for the peak power by using a pulse power device battery. For applications in which significant energy is needed in pulse form, traditional capacitors as used in electronic circuits cannot store enough energy in the volume and weight available. For these applications, the development of high energy density capacitors gained importance and which eventually lead to the development of Ultracapacitors. Ultracapacitors are basically capacitors having capacitance in the order of kilofarads and can store much larger energy as compared to ordinary capacitors, but lesser than batteries.

[hemu]

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