Super- Capacitors Take Charge in Germany
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help with acceleration or can restart en- French trains and hybrid buses all over gines that cut out to reduce fuel use and the world, on construction equipment pollution in stop-and-go traffi c. such as cranes, and on garbage-collection Whereas batteries store energy in trucks. On buses, it can reduce the effec- chemical form—in substances that can tive carbon-dioxide emissions by around react to release electrical energy—capac- 30%, while the Munich-based heavy-ve- itors store it by simply piling up electri- hicle manufacturer MAN estimated that cal charge on two electrodes. The larger their supercapacitor-fi tted coaches each the electrodes and the closer they are, save around $4,500 a year on fuel costs. the more energy that can be stored. An Reducing fuel consumption and emis- ordinary capacitor consists of two con- sions during the “dead time” of standing ductive plates separated by an insulating at stops, intersections, and traffi c lights is Regional Initiative Regional (dielectric) layer. But a supercapacitor a particularly pressing concern for buses. • (sometimes called an ultracapacitor) Since 2001, MAN has been developing Super- holds its charge a little differently. Typi- hybrid supercapacitor buses called the cally it contains two conductive porous Lion’s City Hybrid. The current com- capacitors electrodes—usually made of carbon— mercial model, available since 2010, immersed in a liquid electrolyte and cuts diesel consumption by up to 30%, take charge separated by a very thin insulating fi lm, and is now being used on a small scale usually made of a porous polymer. The in Paris and some other European cities. in Germany charge is stored by adsorption of ions In principle, buses can recharge their su- By Philip Ball onto the high-surface-area electrodes. percapacitors not just during braking, but Feature Editor Yury Gogotsi When the electrodes are charged, this also at every bus stop by making contact produces a layer of oppositely charged with overhead charging lines for just a he yellow-and-white Stadtbahn ions on their surfaces: a so-called electri- few seconds. Ttrams criss-crossing the streets of cal double-layer, which is why this type So far the take-up of the technology Energy Quarterly Mannheim in southern Germany look of supercapacitor is often called a double- has been relatively modest. But it looks unremarkable, but some of them are lit- layer capacitor. set to expand both as energy-saving and erally carrying what could be the key to The potential of supercapacitors to as- low-emission technologies become more greener public transportation. The elec- sist in powering vehicles was displayed imperative and as the technical capabili- trically powered vehicles use 30% less in dramatic fashion in the 24-hour speed ties of supercapacitors improve. Super- energy than their equivalents in most race at Le Mans this June, when Toyota capacitors are not by any means a pana- other cities because they contain on- fi elded a hybrid TS030 that used them cea for green transportation: they have a board systems for capturing the energy for energy-capture during braking. The lower total energy density than batteries that would otherwise be squandered when devices performed perfectly, but a crash by one or two orders of magnitude, for the trams brake. This energy is converted scuppered the vehicle’s bid for glory. example. But they have a higher power into electricity, which is then stored in The Mannheim trams are rather more density, delivering much more power devices called supercapacitors mounted sedate, but with their own onboard pow- over a short period of time. on the tram roof. er, they can keep running across short “There is no single perfect energy- Supercapacitors are power-storage gaps or disruptions in the electricity line, storage solution, no ‘one size fi ts all,’” devices that can supply onboard elec- for example due to ice or where over- said materials scientist Yury Gogotsi at trical power in hybrid vehicles. Unlike head power lines cannot be deployed for Drexel University in Philadelphia. “A batteries, supercapacitors can be charged aesthetic or technical reasons. The en- ‘battery of the future’ may be a battery- and discharged in seconds and can with- ergy source can also be tapped to drive supercapacitor hybrid having the long stand many hundreds of thousands of air conditioning, automatic windows, or lifetime, fast charging, and high power such charging cycles. This makes them passenger doors. of a supercapacitor combined with a high ideal for energy-saving applications that These trams have been reaping the ben- energy density of a battery.” capitalize on transient opportunities for efi ts of supercapacitors since 2003 and The main producers of supercapacitors recharging, such as energy capture dur- are now joined by a host of other public- worldwide are Nesscap Energy, based in ing braking, and that require power to be transportation systems in Germany and South Korea, and the California-based delivered in short bursts of perhaps half beyond. Supercapacitor technology is company Maxwell Technologies. These a minute or less. They can, for example, deployed, for example, on Spanish and and other manufacturers supply the basic Yury Gogotsi, Drexel University Philip Ball, [email protected] 802DownloadedMRS fromBULLETIN https://www.cambridge.org/core • VOLUME 37 • SEPTEMBER 2012. IP address: • www.mrs.org/bulletin 170.106.33.14, on • 02Energy Oct 2021Quarterly at 03:07:24, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2012.222 cells to companies such as Siemens and government that has been enjoyed by, transport operator RNV, which serves the Belgian fi rm 4Esys, which incorpo- say, photovoltaic energy (where subsi- the Rhein-Neckar region and includes rate them into modules that can be added dies and guarantees of competitive rates Mannheim, Heidelberg, and Ludwig- directly into vehicle designs. In Germany, have boosted production and use) has shafen, ordered 11 more Bombardier the electrical-component manufacturer not been extended to the supercapacitor trams for its 200-km network. WIMA in Mannheim, which special- market. “German authorities are pretty In part, the obstacles to wider use are izes in capacitors and supercapacitors, conservative,” Auer feels. There are also matters of infrastructure and design: manufactures both individual cells and no coherent plans for how and where the engineers are more used to thinking in modules. Siemens has developed two technology might be introduced. “Each terms of hybrid vehicles with batteries energy-storage supercapacitor modules energy or transport company has a hybrid that require charging cycles of several called Sibac and Sitrac, which are incor- solution, whether with supercapacitors or hours, rather than the several seconds porated into the vehicles or the power- batteries,” he said, “but the fi nal custom- needed by supercapacitors. But as their Regional Initiative Regional supply lines respectively to capture ener- ers like cities aren’t ready to buy it or im- use hinges mostly on economics, it is vi- gy during braking. Sitras can also be used plement it into their fl eets. That makes it tal to improve performance while reduc- • to maintain the voltage of the network a very diffi cult product to market.” Auer ing costs. Supercapacitors can have an during peaks of high power demand or contrasts this ruefully with China, where impressive lifetime—15 to 20 years—but temporary outages. efforts to use supercapacitors in transport the initial outlay is still rather high. began only about four A key problem is that the individual years ago but have re- cells develop relatively low voltages— sulted already in tens typically less than 3 V—because of the of thousands of “super- limited electrolyte stability. This means cap” buses on the road, that to obtain the 24 V typically needed especially in Shanghai. for vehicle systems, several cells must be “Right now the biggest connected in series, making for relatively market is in China,” costly and bulky modules. “We need to Herrmann agreed. increase voltage per cell in order to have However, Herrmann higher energy density and fewer cells,” Energy Quarterly thinks that Germany’s said Herrmann. energy policy to use Auer said that some of the raw ma- renewable rather than terials, such as the porous carbon elec- nuclear energy will in- trodes, are also somewhat costly, and so directly help the tech- is the manufacturing process. However, nology. Offshore wind Gogotsi insists that “there is no funda- farms, for example, mental reason for supercapacitors to be need regular mainte- expensive, because they use just carbon, Trams in Germany powered by supercapacitors use 30% less energy than their equivalents in other regions. Credit: RNV nance using ships and polymer fi lm, an inexpensive aluminum helicopters, and super- foil, and an organic electrolyte, with capacitors should help no rare or expensive elements.” Costs Despite the promise of supercapacitors to reduce the running costs of these fl eets. should fall, for example, simply as the in transportation, their uptake has been Germany’s federal system offers scope scale of production increases and manu- rather slow and cautious over the past for regional projects, and individual cit- facturing methods improve. That would decade. “There have been a lot of projects ies such as Mannheim have sometimes not only boost current uses but enable where prototypes are tested over a con- launched their own initiatives. Siemens’ new applications to emerge. siderable time,” said Frank Herrmann, Combino trams, which use the Sibac This all leaves Auer optimistic about an engineer at WIMA. The market has, system, have been deployed in several the prospects. “There’s still a lot of room however, been picking up speed over the German cities, including Augsburg, Düs- for future growth,” he said.