Batteries - are their days numbered?

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Hans vd Groenendaal, EIT 27 August, 2008 02:00:00
Are supercapacitors the answer?

Supercapacitors exhibit vastly greater capacitance than conventional capacitors. In developing them there was no discovery of new physics laws. In fact, the theory behind them goes back to Helmholtz. Like all capacitors, ultracaps are still about storing power in the form of an electrical charge between two "plates."

The capacitance is directly related to the area of the plates and the permittivity of the material between the plates, and it’s inversely related to the distance between them. After that, the story gets interesting writes Don Tuite on the Electronic Design website.

"Before we had ultracaps to provide astonishingly high values of capacitance, we had electrolytics. Ultracapacitors aren’t electrolytics, but understanding the older tech is helpful in understanding the new tech. Electrolytics are so named because one (or both) of the "plates" is a nonmetallic electrolyte on top of a metallic backing. During manufacturing, a voltage drives a current from the anode metal through a conductive bath to the cathode. That produces an insulating metal oxide on the surface of the anode - the dielectric.

"One of the phenomena that happens inside electrolytics is the charge accumulation and charge separation that occurs at the interface when any electrode is immersed in an electrolyte solution. An accumulation of oppositely charged ions in the solution compensates for excess charge on the electrode surface. The interface is called the Helmholtz layer.

"To understand ultracaps, stop thinking about flat plates (or flat plates rolled up into tubes) with a dielectric between them, much like peanut butter in a sandwich. In an ultracap, charging/discharging takes place on the interfaces between porous carbon materials or porous oxides of certain metals in an electrolyte.

"The Helmholtz layers give rise to an effect called double layer capacitance. When a DC voltage is applied across the porous carbon electrodes in an ultracap, compensating accumulations of cations or anions develop in the solution around the charged electrodes. If no electron transfer can occur across the interface, a "double layer" of separated charges (electrons or electron deficiency at the metal side and cations or anions at the solution side of the interface boundary) exists across the interface

"The Helmholtz-region capacitance depends on the area of those porous carbon electrodes and the size of the ions in solution. The capacitance per square centimetre of electrode double layers is on the order of 10 000 times larger than those of ordinary dielectric capacitors. That’s because the separation of charges in double layers is about 0,3 to 0,5 nm, instead of 10 to 100 nm in electrolytics and 1000 nm in mica or polystyrene caps.

"There’s a catch to this "double-layer" characteristic, though. The double-layer configuration reduces the potential capacitance of a practical device because the ultracap consists of a pair of electrodes, each with half the total mass. In addition, the ultracapacitor is effectively two capacitors in series. Taken together, that means the ultracap achieves one quarter of the theoretical capacitance based on electrode area and ion size.

"Batteries and ultracapacitors are often lumped together, obscuring a number of important differences:

Batteries store watt-hours of energy. Capacitors store watts of power.
Batteries depend on chemical reactions with long time constants. They take a relatively long time to charge, and they’re fussy about the profile of the current that charges them. Conversely, capacitors are charged by applying a voltage across their terminals, and their charge rate depends mostly on external resistance.
Batteries deliver power in the form of a more or less constant voltage over long time periods. Capacitors discharge rapidly, and their output voltage decays exponentially.
Batteries are good for only a limited number of charge/discharge cycles, and the number of cycles depends on how deeply they are discharged. Capacitors, especially ultra capacitors, can be charged and discharged repeatedly for tens of millions of cycles.
Batteries are big and heavy. Capacitors are small and light.
Applications

"The most basic applications for ultracaps lie in stabilising DC bus voltages. Ultracaps have become widely used in automobiles to protect the various engine control units and other microcontrollers from voltage dips associated with the application of sudden transient loads

"Elsewhere in transportation, the ultracap’s ability to absorb and discharge energy rapidly makes it far better than batteries for regenerative braking schemes. Most of these applications have been in public transportation. The Bombardier rail cars in the light-rail system in Mannheim, Germany, use packs of 600 2600-F ultra capacitors for braking energy recapture. The stored energy is used to boost acceleration and to bridge non-powered sections and intersections. Operation there represents between 100 000 and 300 000 load cycles/year. This is an all-electric rail system, so recaptured braking energy reduces demand on the grid. In that regard, the prototype has demonstrated a potential for energy savings of up to 30%.

"In hybrid transportation applications in the U.S., ISE Corporation’s buses now run in Elk Grove and Long Beach. The buses accelerate more quickly than standard buses. At gross vehicle weight, the bus can accelerate from zero to 31 mph in 17 seconds and can reach a maximum speed of 100 km per hour. Preliminary data indicates better average fuel efficiency compared to competitive battery-based hybrid electric drive systems."

Don Tuite wrote his first technical article (on circular antenna polarisation) in 1973 for Microwaves magazine, which is a sister publication of Electronic Design. He went on to author four books for electronics hobbyists. Since 1985, he has concentrated on semiconductors, working on his own as well as within chip manufacturers, for public relations agencies, and as a trade-press editor. He holds an MS degree in communications and technical writing from Rensselaer Polytechnic Institute (Troy, NY) and a BS in electrical engineering from the New Jersey Institute of Technology.

Will the supercapacitor replace batteries in the future? Get the lowdown on supercapacitors: http://electronicdesign.com/Articles/ArticleID/17465/17465.html

EngineerIT

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