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Ultracaps Could Boost Hybrid Efficiency

Recent studies point to the potential of ultracapacitors to augment conventional batteries.
August 20, 2009

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.

Rugged power: This ultracapacitor is being used to capture energy from the brakes in hybrid buses. Similar devices could soon be used to reduce the cost of hybrid cars.

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 could protect 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.

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