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.