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The Dark Horse in the Race to Power Hybrid Cars

Are ultracapacitors the key to making hybrids king of the auto market?



Courtesy of Toyota Motor Sales, U.S.A., Inc.

Many motorists chuckle smugly after giving their cars a little extra gas to leave a Toyota Prius or some other eco-friendly automobile in the dust. But Toyota and its Earth-loving ilk may yet have the last laugh as they cultivate encouraging new advances in ultracapacitor technology that promise to one day put hybrids in the driver's seat.

The greatest victory so far for the cars, fueled by a combo of electricity and gas, came just weeks ago when an ultracapacitor-equipped Toyota Supra HV-R coupe became the first hybrid to win the 24-hour endurance car race held at Japan's Tokachi International Speedway. The hybrid Supra finished 616 laps of the 5.1-kilometer (roughly three¿mile) course—19 more laps than the second-place nonhybrid Nissan Fairlady Z. "The Toyota that won was able to deliver energy more quickly, accelerate faster, and use braking generation more efficiently," says Kevin Mak, an analyst with research and consulting firm Strategy Analytics and author of a recent study that explores the potential for ultracapacitors to complement and possibly even replace batteries in hybrid vehicles. "The days of the large hybrid vehicle battery pack may be numbered," he adds.

The reason, he says: capacitor technology that stores energy in the electric field between a pair of closely spaced conductors. An ultracapacitor, also called a supercapacitor, is an electrochemical capacitor with a higher energy density than normal capacitors, which potentially makes them a better fit for hybrid vehicles.

Ultracapacitors store electricity by physically separating positive and negative charges. Batteries store energy using toxic chemicals and their effectiveness fades over time. In addition, recycling the heavy metals in batteries is a difficult task. Capacitors, on the other hand, are constructed of much smaller fine carbon nanotubes, Mak says.

A major advantage of ultracapacitors is their ability to efficiently capture electricity from regenerative braking systems and provide that electricity to power a car's acceleration. Ultracapacitors not only charge more quickly than batteries, they also release energy more quickly, Mak says.

A drawback to their use is the technology's inability to store as much energy as a battery. But the Tokachi race proved that ultracapacitors could be more widely used in conjunction with smaller batteries to power hybrid cars. "Without the need for chemicals, capacitors can be lighter, thereby enabling the hybrid car maker to improve fuel economy further and reduce costs," Mak says. "The low weight would then make hybrid power trains more readily available to compact car segments as [has been] seen on Honda and Mazda concept cars since 1997."

Most car companies, however, are more interested in advances in lithium ion batteries than in ultracapacitor evolution. Earlier this month General Motors Corp. signed an agreement with A123Systems to develop its nanophosphate lithium ion battery technology for automobiles. A123Systems' batteries today are used primarily in cordless power tools. Hybrids such as the Toyota Prius and Ford Escape use nickel-metal hydride batteries that are larger than lithium ion systems, the latter of which can pack more electric power into a smaller space. The first A123 car batteries are expected to be ready for GM to test by October, and the company plans to have its next generation of electrically powered vehicles on the market by the end of 2010.

Capacitors haven't been competitive with batteries in the past, because they have not offered a higher energy density, says Olgierd Palusinski, a University of Arizona professor of electrical and computer engineering. Palusinski argues that energy density is an even better measure than storage capacity of how effective a power source can be. "You could have a very high storage of charge but at a very low voltage," he notes.

Researchers at the University of Arizona—including Palusinski—have been developing a new capacitor technology designed to give hybrid cars the best of both worlds: the ability to rapidly convert mechanical energy into electricity and store that energy as well as to quickly charge and discharge energy to help cars accelerate or brake.

This new device is called a Digitized Energy Storage Device (DESD), which has a capacitance-to-volume ratio that is more than 10,000 times larger than a conventional parallel-plate capacitor of the same size. The researchers make DESD capacitors by using porous membranes as template platforms. The membranes have a pore diameter ranging from 15 nanometers to one micron and a hole density of 10 million to 100 trillion pores per square centimeter (0.16 square inch). DESDs will be able to provide 130 joules per gram (0.35 ounce), Palusinski says, adding that a chemical battery provides about 100 joules per gram. Lithium ion batteries, however, can provide as much as 600 joules per gram.

Palusinski envisions a day when DESDs will replace batteries in automobiles, although currently no carmakers are working with the technology he and his colleagues developed. It's more likely that near- term deployments of DESD technology would be used to power something much less demanding, such as sensors used to detect motion for alarms and lighting systems.

Despite Toyota's success with ultracapacitors, most automakers in recent years have been focused on new developments in the lighter, more stable lithium ion batteries, says Brett Smith, an alternative-fuel analyst at the Center for Automotive Research in Ann Arbor, Mich.

"Moving from the type of lithium battery used in laptops to a nanophosphate lithium ion battery may have been the invention or the paradigm shift that the industry was requiring," Smith says. "Now the challenge is can they get the cost down." That's the pivotal question that is likely to ultimately determine the fate of ultracapacitors used in hybrid cars.

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