With characteristics of both batteries and capacitors, supercapacitors (also called electrochemical capacitors or ultracapacitors) could be used by utilities to regulate power quality. A capacitor is a device that stores energy in the electric field created between a pair of conductors on which electric charges of equal magnitude, but opposite sign, have been placed. A supercapacitor 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, as well as power electronics applications such as in wind turbines.
Capacitors are electronic devices that that can provide enormous amounts of power, but only store very small amounts of energy. Alternatively, batteries can store large amounts of energy, but provide relatively low power outputs. Supercapacitors can provide both higher power outputs and store lots of energy.
Supercapacitor Technology
When a supercapacitor is charged, the energy is stored as a charge or concentration of electrons on the surface of a material. This means a supercapacitor is capable of very fast charges and discharges which can achieve a very large number of cycles without degradation, even at 100% depth of discharge (DOD). Capacitors are made from various materials in many ways, from multilayer ceramics, ceramic disc, multilayer polyester film, tubular ceramic, axial and radial polystyrene, to carbon nanotubes (Wikipedia, 2006a).
Supercapacitors found their first application in military projects such as starting the engines of battle tanks and submarines or replacing batteries in missiles. Common applications today include starting diesel trucks and railroad locomotives, actuators, and in electric/hybrid-electric vehicles for transient load levelling and regenerating the energy of braking. NASA has used 30 large supercapacitors in its turbo-electric city bus (EERE, 2006).
Carbon nanotubes and polymers, or carbon aerogels, are practical for supercapacitor designs. 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 (LEES, 2006).
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 104F/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 2.5V, 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 increasing (Wikipedia, 2006b).
Supercapacitor Applications
The newly developed Honda Fuel Cell Stack and ultra-capacitor combine to power the motor, with onboard high-pressure hydrogen tanks for fuel storage for the new Honda FCX hydrogen fuel cell car. The fuel cell vehicle is powered by an electric motor running on electricity generated by a fuel stack which uses hydrogen as its energy source. Considering factors such as energy efficiency during power generation and driving, overall system weight, and packaging efficiency, Honda has equipped the FCX with a system that combines a fuel cell stack and ultra-capacitor (see Figure 1) with onboard high-pressure hydrogen tanks (Honda Worldwide, 2006).
Renewable energy technologies feature in the applications for supercapacitors with Alain Riedo, vice president and general manager of Maxwell’s Swiss subsidiary, Maxwell Technologies SA, said that Enercon currently uses BOOSTCAP® ultracapacitors (see Figure 3) for backup energy storage and power delivery in wind turbine models ranging in output from 300 kW to 6 MW. “In addition to being one of the world’s largest wind turbine producers, Enercon is recognized as a leading innovator in the design and manufacture of megawatt class turbines,” Riedo said. “To optimize energy output and enhance system reliability and longevity, each of Enercon’s turbines’ three blades has an independent braking and pitch adjustment mechanism with backup power to ensure continuous operation in the event of a power failure. Each turbine incorporates from 200 to 700 BOOSTCAP ultracapacitors for backup power.” (Maxwell Technologies, 2006).
Ulrich Neundlinger, Enercon’s managing director of switching units, said that the company is expanding its use of ultracapacitors for blade pitch system backup power after initial deployments confirmed their significant advantages over traditional battery solutions. “Ultracapacitors enabled us to overcome a number of battery-related design challenges, including poor low temperature performance and limited operational life,” Neundlinger said. “Maxwell’s products emerged as the clear choice for this application on the basis of their robust construction, long operating life and cost-effectiveness. Wind turbine operators need low-maintenance systems that operate reliably for many years, and BOOSTCAP products have proven that they can help us to continue to meet our customers’ expectations.” (Maxwell Technologies, 2006).
Source:
http://www.rise.org.au/info/Tech/scap/index.html.
