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A supercapacitor or an ultracapacitor is an electrochemical capacitor that has an unusually large amount of energy storage capability relative to its size when compared to common capacitors. These are of particular interest in automotive applications for hybrid vehicles and as supplemental storage for battery electric vehicles.
First trials of using the supercapacitors in industrial applications were carried out for supporting the energy supply to robots *
In 2005 leading producer of aerospace systems and controls Diehl Luftfahrt Elektronik GmbH chose ultracapacitors Boostcap® (of Maxwell Technologies) to power emergency actuation systems for doors and evacuation slides in passenger aircraft, including the new Airbus 380 jumbo jet. *
History
The first supercapacitor based on a double layer mechanism was developed in 1957 by General Electronics in a patent using a porous carbon electrode H.I., “Low voltage electrolytic capacitor”, [http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=2800616 U.S. Patent 2800616, 23 July 1957]. It was believed that the energy was stored in the carbon pores and it exhibited "exceptionally high capacitance", although the mechanism was unknown at that time. It was the Standard Oil Company, Cleveland (SOHIO) in 1966 that patented a device that stored energy in the double layer interface R.A., “Electrical energy storage apparatus”, *.
Current State
As of spring 2006, EEStor Inc. claims to have a supercapacitor with a barium titanate dielectric nearing production. The company claims a unit with 31 farads capacitance and an operating voltage of 3.5 kV, capable of storing up to 340 Wh/kg (1232 kJ/kg)and charging or discharging at up to 3.5 kW/kg (52 kWh = 187 MJ and 520 kW - 6 minute charging time - for the 152 kg unit), lifetime of over 1,000,000 discharge cycles and leakage of less than 0.1% per month *" target="_blank" >US Patent 7,033,406 with a cost of $40-$60 per kWh ($3,200 - $2,100 per unit). BusinessWeek, 3 September 2005" target="_blank" >*. The technology is scheduled for third-party verification during the summer of 2006.
Features
Such energy storage has several advantages relative to batteries:
- Very high rates of charge and discharge.
- Little degradation over hundreds of thousands of cycles.
- Good reversibility
- Low toxicity of materials used.
- High cycle efficiency (95% or more)
Disadvantages:
- The amount of energy stored per unit weight is considerably lower than that of an electrochemical battery (3-5 W.h/kg for a UC compared to 30-40 W.h/kg for a battery).
- The voltage varies with the energy stored. To effectively store and recover energy requires sophisticated electronic control and switching equipment.
- Exhibits the highest dielectric absorption of all types of capacitors.
Technology
Carbon nanotubes and polymers, or carbon aerogels, are practical for supercapacitors. Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. Polymers have a redox (reduction-oxidation) storage mechanism along with a high surface area. MIT's Laboratory of Electromagnetic and Electronic Systems (LEES) is researching using carbon nanotubes.*
Supercapacitors are also being made of carbon aerogel. Carbon aerogel is a unique material providing extremely high surface area of about 400-1000 m2/g. Capacitances of up to 104 F/g and 77 F/cm3 have been achieved. Some corporations, such as Cooper Electronic Technologies, are already producing aerogel-based supercapacitors. Their maximum voltage is 2.5 V (or 2.7 V but they can achieve an energy density of 325 kJ/kg (disputed as 10.6 kJ/kg, see Discussion), which is about 70% of that provided by the state-of-the-art lithium polymer batteries. Power densities achieved are even higher, up to 20 kW/kg, orders of magnitude higher than what Li-poly offers. Small aerogel supercapacitors are being used as backup batteries in microelectronics, but applications for electric vehicles are expected. [http://www.aip.org/tip/INPHFA/vol-10/iss-5/p26.html
The electrodes of aerogel supercapacitors are usually made of non-woven paper made from carbon fibers and coated with organic aerogel, which then undergoes pyrolysis. The paper is a composite material where the carbon fibers provide structural integrity and the aerogel provides the required large surface.
Capacitance of a single cell of ultracapacitor can be up to 2.6 kF (see photo at the beginning).
See also
External links
- http://www.doc.ic.ac.uk/~mpj01/ise2grp/energystorage_report/node9.html
- Ultracapacitor used in Honda's FCX fuel cell car
- Encyclopedia article on Electrochemical Capacitors
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"Supercapacitor"