A Battery-Ultracapacitor Hybrid

A device for power tools may also help regenerative braking.
BY PRACHI PATEL

Battercapacitor: A new energy storage device blends the chemistry of an ultracapacitor with that of a lithium-ion battery.
Credit: Ioxus

By combining the chemistries of ultracapacitors and lithium-ion batteries, a company calledIoxus has created a hybrid energy-storage device that could recharge power tools in minutes and might never need to be replaced. The company says future incarnations could perhaps be used to capture energy from braking vehicles.

Ultracapacitors capture and release energy in seconds and can do so millions of times, but they store only about 5 percent as much energy as lithium-ion batteries. The hybrid can store more than twice the energy by volume of standard ultracapacitors. That's still much less than a lithium-ion battery, but the hybrid can be recharged quickly over 20,000 times as against a few hundred cycles for a typical battery.

A power tool using the lithium-ion ultracapacitor would run for only a 15th as long as it would on a battery but would recharge in just a minute. "Our product is for weekend warriors who don't use the power tool much every day" but want very fast charging, says Mark McGough, CEO of Ioxus. The company, which is based in Oneont, New York, already makes conventional ultracapacitors for hybrid-electric buses and for engine start-stop systems that are used to increase fuel economy in cars.

The hybrid energy-storage device consists of an etched aluminum film coated on one side with carbon slurry, which is similar to the electrode found in an ultracapacitor. The other electrode, on the other side of the film, is coated not with carbon but with a lithium-ion material, providing more energy-storage capacity. The film is wound into a cylinder to make the finished device.

Ultracapacitors are being tested in some city buses as a way to capture the energy generated by braking and quickly release it for reacceleration, an approach that promises to improve fuel efficiency. If the hybrid lithium-ion ultracapacitor can be scaled up, it could improve fuel efficiency further by storing more energy. But its cycle life will need to be improved, as vehicle breaking systems need to be recharged hundreds of thousands of times.

The concept of hybrid lithium-ion ultracapacitors has been around for 20 years, but there is more demand for other types of energy-storage devices, says Theodore Bohn, an engineer at Argonne National Laboratory's Advanced Powertrain Research Facility.

Bohn says the hybrid technology could nonetheless be ideal for small, lightweight applications that would benefit from having some of the power advantage of ultracapacitors and some the energy advantage of a battery. "The hybrid is good for the power pulse and OK for the energy," he says.

Only one other company—JSR Micro, in Tokyo—makes hybrid devices of this type, having brought them to market in 2009. The company says its device has three times the energy density of a conventional ultracapacitor and a cycle life of 100,000 recharges. Jeff Myron, a program manager at JSR Micro, says the device is mainly intended as a backup power supply in medical-imaging equipment.

Ultracaps Could Boost Hybrid Efficiency

Recent studies point to the potential of ultracapacitors to augment conventional batteries.
BY KEVIN BULLIS

Energy storage devices called ultracapacitors could lower the cost of the battery packs in plug-in hybrid vehicles by hundreds or even thousands of dollars by cutting the size of the packs in half, according to estimates by researchers at Argonne National Laboratory in Argonne, IL. Ultracapacitors could also dramatically improve the efficiency of another class of hybrid vehicle that uses small electric motors, called microhybrids, according to a recent study from the University of California, Davis.

The use of ultracapacitors in hybrids isn't a new idea. But the falling cost of making these devices and improvements to the electronics needed to regulate their power output and coordinate their interaction with batteries could soon make them more practical, says Theodore Bohn, a researcher at Argonne's Advanced Powertrain Research Facility.

Although batteries have improved significantly in recent years, the cost of making them is the main the reason why hybrids cost thousands of dollars more than conventional vehicles. This is especially true of plug-in hybrids, which rely on large battery packs to supply all or most of the power during short trips. Battery packs are expensive in part because they degrade over time and, to compensate for this, automakers oversize them to ensure that they can provide enough power even after 10 years of use in a vehicle.

Ultracapacitors offer a way to extend the life of a hybrid vehicle's power source, reducing the need to oversize its battery packs. Unlike batteries, ultracapacitors don't rely on chemical reactions to store energy, and they don't degrade significantly over the life of a car, even when they are charged and discharged in very intense bursts that can damage batteries. The drawback is that they store much less energy than batteries--typically, an order of magnitude less. If, however, ultracapacitors were paired with batteries, they couldprotect batteries from intense bursts of power, Bohn says, such as those needed for acceleration, thereby extending the life of the batteries. Ultracapacitors could also ensure that the car can accelerate just as well at the end of its life as at the beginning.

Reducing the size of a vehicle's battery pack by 25 percent could save about $2,500, Bohn estimates. The ultracapacitors and electronics needed to coordinate them with the batteries could cost between $500 and $1,000, resulting in hundreds of dollars of net savings.

Ultracapacitors would also make it possible to redesign batteries to hold more energy. There is typically a tradeoff between how fast batteries can be charged and discharged and how much total energy they can store. That's true in part because designing a battery to discharge quickly requires using very thin electrodes stacked in many layers. Each layer must be separated by supporting materials that take up space in the battery but don't store any energy. The more layers used, the more supporting materials are needed and the less energy can be stored in the battery. Paired with ultracapacitors, batteries wouldn't need to deliver bursts of power and so could be made with just a few layers of very thick electrodes, reducing the amount of supporting material needed. That could make it possible to store twice as much energy in the same space, Bohn says.

Ultracapacitors could also be useful in a very different type of hybrid vehicle called a microhybrid, says Andrew Burke, a research engineer at the Institute of Transportation Studies at UC Davis. As designed today, these vehicles use small electric motors and batteries to augment a gasoline engine, allowing the engine to switch off every time the car comes to a stop and restart when the driver hits the accelerator. A microhybrid's batteries can also capture a small part of the energy that is typically wasted as heat during braking. Because ultracapacitors can quickly charge and discharge without being damaged, it would be possible to design microhybrids to make much greater use of an electric motor, providing short bursts of power whenever needed for acceleration. They could also collect more energy from braking. According to computer simulations performed by Burke, such a system would improve the efficiency of a conventional engine by 40 percent during city driving. Conventional microhybrids only improve efficiency by 10 to 20 percent.

In both plug-in hybrids and microhybrids, ultracapacitors would offer improved cold weather performance, since they do not rely on chemical reactions that slow down in the cold. "In very cold weather, you have to heat the battery, or you can't drive very fast--you'd have very low acceleration," Bohn says. In contrast, ultracapacitors could provide fast acceleration even in cold temperatures.

Mark Verbrugge, director of the materials and processes lab at GM, says that of the two uses for ultracapacitors, it will be easier to use them in microhybrids. In this case, he says, ultracapacitors would simply replace batteries, since they store enough energy to augment the gasoline engine without the help of batteries. In plug-in hybrids, which require much more energy, ultracapacitors would need to be paired with batteries, and this would require complex electronics to coordinate between the two energy storage devices. "By and large, you never want to add parts to a car," he says. "You want the simplest system possible" so that there are fewer things to go wrong.

For ultracapacitors to be practical in microhybrids, Verbrugge says, the cost of making them has to decrease by about half, which may be possible because many parts of the manufacturing process for large ultracapacitors aren't yet automated. But to justify the added complexity in plug-in hybrids, he says, the entire system would have to cost significantly less than using batteries alone.

The researchers at Argonne have already taken steps toward proving that ultracapacitors can provide these savings, having shown that they reduce the heat stress placed on batteries by a third. They are continuing to test ultracapacitors to demonstrate that they can make batteries last longer, which would allow automakers to use smaller batteries and save money.