Super-capacitors take charge in Germany
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Energy Quarterly
Supercapacitors take charge in Germany By Philip Ball Feature Editor Yury Gogotsi
T
he yellow-and-white Stadtbahn trams criss-crossing the streets of Mannheim in southern Germany look unremarkable, but some of them are literally carrying what could be the key to greener public transportation. The electrically powered vehicles use 30% less energy than their equivalents in most other cities because they contain onboard systems for capturing the energy that would otherwise be squandered when the trams brake. This energy is converted into electricity, which is then stored in devices called supercapacitors mounted on the tram roof. Supercapacitors are power-storage devices that can supply onboard electrical power in hybrid vehicles. Unlike batteries, supercapacitors can be charged and discharged in seconds and can withstand many hundreds of thousands of such charging cycles. This makes them ideal for energy-saving applications that capitalize on transient opportunities for recharging, such as energy capture during braking, and that require power to be delivered in short bursts of perhaps half a minute or less. They can, for example,
help with acceleration or can restart engines that cut out to reduce fuel use and pollution in stop-and-go traffic. Whereas batteries store energy in chemical form—in substances that can react to release electrical energy—capacitors store it by simply piling up electrical charge on two electrodes. The larger the electrodes and the closer they are, the more energy that can be stored. An ordinary capacitor consists of two conductive plates separated by an insulating (dielectric) layer. But a supercapacitor (sometimes called an ultracapacitor) holds its charge a little differently. Typically it contains two conductive porous electrodes—usually made of carbon— immersed in a liquid electrolyte and separated by a very thin insulating film, usually made of a porous polymer. The charge is stored by adsorption of ions onto the high-surface-area electrodes. When the electrodes are charged, this produces a layer of oppositely charged ions on their surfaces: a so-called electrical double-layer, which is why this type of supercapacitor is often called a doublelayer capacitor. The potential of supercapacitors to assist in powering vehicles was displayed in dramatic fashion in the 24-hour speed race at Le Mans this June, when Toyota fielded a hybrid TS030 that used them for energy-capture during braking. The devices performed perfectly, but a crash scuppered the vehicle’s bid for glory. The Mannheim trams are rather more sedate, but with their own onboard power, they can keep running across short gaps or disruptions in the electricity line, for example due to ice or where overhead power lines cannot be deployed for aesthetic or technical reasons. The energy source can also be tapped to drive air conditioning, automatic windows, or passenger doors. These trams have been reaping the benefits of supercapacitors since 2003 and are now joined by a host of other publictransportation systems in
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