A couple years ago Tesla CEO Elon Musk said in an offhand remark that he thought capacitors — rather than batteries — might be the energy storage tech to deliver an important breakthrough for electric transportation. Tesla cars, of course, use lithium ion batteries for storing energy and providing power for their vehicles, but Musk is an engineer by nature, and he likes what ultracaps offer for electric cars: short bursts of high energy and very long life cycles.
Capacitors are energy storage device like batteries, but they store energy in an electric field, instead of through a chemical reaction the way a battery does. A basic capacitor consists of two metal plates, or conductors, separated by an insulator, such as air or a film made of plastic or ceramic. During charging, electrons accumulate on one conductor, and depart from the other.
A bus using ultracapacitor tech from Maxwell. Image courtesy of Maxwell.
Supercapacitors also have two metal plates, like capacitors, but they’re usually coated with a spongelike, porous material known as activated carbon. The porous material makes the surface area a lot larger and boosts the energy density. They're also immersed in an electrolyte made of positive and negative ions dissolved in a solvent. Finally, each of the ultracapacitor’s carbon electrodes ends up having two layers of charge coating its surface. Ultracapacitors and supercapacitors are often used interchangeably to mean the same thing — basically, a double-layer capacitor.
According to an article in the Economist this weekend, super and ultracapacitors are already starting to be used in a variety of novel ways in electric vehicles, including in racecars and also in some buses, trams and trains. Ultracapacitors are often used to aid batteries in hybrid and electric cars, something we’ve covered over the years.
One of the more common uses of ultracapacitors is in hybrid buses in China. Many of the buses there are equipped with stop-start engines (which turn off when the vehicle stops) and the ultracapacitor is used to turn the engine back on, thus reducing the load on the battery itself. Maxwell Technologies is a significant supplier of these start-stop engine ultracapacitors. China also has a half-dozen trams that are using ultracapacitors to efficiently charge up between stops.
Paper Battery Company’s ultracapacitor tech demonstrated in an iPhone, courtesy of Paper Battery Company.
One of the more fun uses of ultracapacitors in electric vehicles is as a way to provide a short burst of power for racecars. Toyota has a hybrid racecar called the TS040 (pictured in the thumbnail), which uses a supercapacitor to add an additional 480 horsepower. The TS040 also has regenerative braking, which recharges the supercapacitor. Toyota is using what it’s learned with the TS040 in a concept car called the Yaris Hybrid-R, which uses a supercapacitor.
Beyond electric vehicles, ultracapacitors could play an interesting role in ultra-thin battery-powered devices and gadgets. A team out of Singapore has developed an ultracapacitor based on graphene and carbon nanotubes that can be woven into clothing, opening up applications for wearable devices. A startup called the Paper Battery Company has created a thin ultracapitor sheet that can be wrapped around the battery in a cell phone to squeeze out extra energy and help a cell phone last longer.
The biggest drawback of ultracapacitors for energy storage is that they’re just not as energy-dense as lithium ion batteries and other types of batteries. But the Economist notes that newer ultracapacitors are getting much better energy densities and a new hybrid supercapacitor has recently reached 30 to 40 percent of a lithium-ion battery's energy density.
Related research and analysis from Gigaom Research:
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