Photo: Maxwell Technologies
The Best of Both Worlds
Ultracapacitors are better than batteries in some ways, but worse in others. They can be charged and discharged very quickly, they last almost forever, and their performance doesn't degrade appreciably with use. But for an equal size/weight, they can't hold anywhere near as much energy as a chemical battery (about an order of magnitude less). This means that unless there's a breakthrough, ultracap-only electric vehicles won't happen in the short term. But it doesn't mean that ultracaps can't be used to supplement and improve current hybrids and plug-in hybrids.
Photo: Maxwell Technologies
Bringing PHEVs Closer to Reality?
A study conducted by researchers at the Argonne National Laboratory shows that ultracapacitors could be used to make hybrid vehicles more efficient (especially plug-in hybrids, which require bigger and more expensive battery packs), and that even micro-hybrids could benefit significantly.
One of the things that make plug-in hybrids so expensive is that the battery pack needs to be oversized to make sure that it will last long enough (10+ years). Since chemical batteries degrade with age, that's the only way to avoid a marked performance decline over the years. But this also adds to the cost and weight of the vehicle.
That's where ultracapacitors come in: "Ultracapacitors offer a way to extend the life of a hybrid vehicle's power source, reducing the need to oversize its battery packs. [...] If 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."
The researchers estimate that if the size of a PHEV battery could be reduced by about 25%, that would represent a saving of about $2,500. The ultracapacitors and electronics required to make up for the smaller battery size would cost between $500-$1,000. Nothing to sneeze at.
Ultracapacitors and Micro-Hybrids
Ultracaps could also be used in microhybrids because they require much less energy storage than full hybrids or PHEVs. Their main role would be to capture energy bursts from regenerative braking and supply bursts of power for acceleration, two things that ultracaps can do even better than chemical batteries. It is estimated that in city driving, a micro-hybrid with ultracaps could improve the fuel efficiency compared to a regular car by about 40% (compared to 10-20% for a regular micro-hybrid).
But that's not all! From Technology Review:
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.
The photo above is of an ultracap by Maxwell Technology. You can find more information about it on their website.
Via Technology Review
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