12-04-2012, 03:30 PM
SUPER CAPACITOR
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Definition
Electric double-layer capacitors, also known as supercapacitors, pseudocapacitors, 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 double-layer capacitors have capacities up to 5,000 farads as of 2010. The highest energy density in production is 30 W•h/kg.
1. INTRODUCTION
The EDLC effect was first noticed in 1957 by General Electric engineers experimenting with devices using porous carbon electrodes.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.
General Electric did not immediately follow up on this work, and the modern version of the devices was eventually developed by researchers at Standard Oil of Ohio in 1966, after they accidentally re-discovered the effect while working on experimental fuel cell designs.Their cell design used two layers of activated charcoal separated by a thin porous insulator, and this basic mechanical design remains the basis of most electric double-layer capacitors to this day.
2. CONCEPT BEHIND SUPER CAPACITOR
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 amount of charge stored and the potential between the plates. The amount of charge stored is essentially a function of size and the material properties of the plates, while the potential between the plates is limited by dielectric breakdown of the substance separating 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.
3. MATERIALS USED IN SUPER CAPACITOR
Activated carbon, graphene, carbon nanotubes and certain conductive polymers, or carbon aerogels, are practical for supercapacitors:
Virtually all commercial supercapacitors manufactured by Panasonic, Nesscap, Maxwell Technologies, Nippon Chemi-Con, Axion Power, and others use powdered activated carbon made from coconut shell Some companies also build higher performance devices, at a significant cost increase, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH)
• Graphene has excellent surface area per unit of gravimetric or volumetric densities, is highly conductive and can now be produced in various labs. It will not be long before large volumes of Graphene is produced for supercapacitors.
• Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. MIT's Laboratory of Electromagnetic and Electronic Systems (LEES) is researching using carbon nanotubes.
6. ADVANTAGES
• Long life, with little degradation over hundreds of thousands of cycles. Due to the capacitor's high number of charge-discharge cycles (millions or more compared to 200 to 1000 for most commercially available rechargeable batteries) it will last for the entire lifetime of most devices, which makes the device environmentally friendly.
• Low cost per cycle
• Good reversibility
• Very high rates of charge and discharge.
• Extremely low internal resistance (ESR) and consequent high cycle efficiency (95% or more) and extremely low heating levels
• High output power
• High specific power. According to ITS (Institute of Transportation Studies, Davis, California) test results, the specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency[12]
• Improved safety, no corrosive electrolyte and low toxicity of materials.
7. DISADVANTAGES
• The amount of energy stored per unit weight is considerably lower than that of an electrochemical battery (3-5 W•h/kg for an ultracapacitor as of 2010compared to 30-40 W•h/kg for a lead acid battery), and about 1/10,000th the volumetric energy density of gasoline.
• As with any capacitor, the voltage varies with the energy stored. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent losses of energy
• Has the highest dielectric absorption of any type of capacitor.
• High self-discharge - the rate is considerably higher than that of an electrochemical battery.
8. APPLICATIONS
Vehicles
Heavy and public transport
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.More recently they have become a topic of some interest in the green energy world, where their ability to store energy much faster than batteries makes them particularly suitable for regenerative braking applications. 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 charge quickly and are stable with temperature.
In 2001 and 2002 VAG, the public transport operator in Nuremberg, Germany tested an hybrid bus which uses a diesel-electric battery drive system with electric double-layer capacitors.
CONCLUSION
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