27-12-2012, 01:53 PM
Supercapacitor (electrochemical double-layer capacitor)
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Brief description of technology
Electrochemical double-layer capacitors (EDLCs), often called ‘Supercapacitors’ or ‘Ultracapacitors’, are storage devices that can store or deliver energy at a high rate, but have limited storage capacity when compared with most batteries. They have a high power density (power per unit volume), but a low energy density (energy per unit volume). Traditionally, EDLCs have been installed complementary to a battery storage system to increase the overall power density – the rate at which the system can charge or discharge – of the facility, handling the peak transients in supply or demand while the battery storage system provides the overall energy storage capacity. Figure 1 compares the energy density with the power density for various energy storage devices, including ‘Ultracapacitors’.
Technical/economic data
2008 figures for EDLCs put the average cycle life at over half a million cycles, with an overall cycle efficiency range from 75% to 95%. Costs are estimated at $10-20/Wh and $25-50/kW, with a specific energy of 5Wh/kg and specific power 5-10kW/kg [2] (See table 1).
Energy efficiency is current dependent as losses occur in the devices proportional to current, and self-discharge of the EDLC is high, at 6.5% loss after 12 hours at room temperature [3].
Application/markets
Until higher energy densities are realised outside of the academic environment, EDLCs can find application in power quality and voltage stabilisation on timescales of seconds, for system operator ancillary services. However, due to their high power handling capability, they can be combined with low power density battery storage systems to improve response times in an otherwise high energy capacity system. This complementary system can have benefits to the battery technology too, where high current can have a detrimental effect on the battery lifetime.
Advantages/disadvantages
EDLCs can handle very many charge-discharge cycles and therefore typically have a low cost per cycle when their long lives are accounted for. However, the cost per kWh of energy storage remains high, as they possess very low energy density and require a large volume to store a large amount of energy. This means they are typically only useful for short endurance applications. Lifetimes are measured in hundreds of thousands, to millions, of cycles, as compared to batteries which have lifetimes limited to hundreds or thousands of cycles at deep discharge. However, unlike practical batteries, the voltage across any capacitor, including EDLCs, drops significantly as it discharges. Effective storage and recovery of energy therefore requires complex electronic control and switching converters, with consequent energy loss in the converter electronics, lowering the overall system efficiency.
Safety, security, environmental and public perception issues
Because of their capacity to deliver very high transient currents, EDLCs are more prone to sparking which can become a safety consideration when used in conjunction with batteries which may produce flammable gasses. However, in general, most EDLCs have a low environmental impact given they do not use heavy metals or toxic electrolytes, although the use and disposal of materials within them will be influenced by legislation [10]. Also, the high cycle life tends towards a storage device which will last the lifetime of the given application.