While the big focus today is on battery-assisted hybrids, research is going into the use of supercapacitors to offer a regenerative power boost. Able to quickly charge and discharge, supercapacitors could be used in place of batteries in some applications, or as an additional energy source to add power when a vehicle is climbing hills. Check this article for more info. Meanwhile, we're waiting for the bercapacitor, on which Bosch is working with Doctor Emmett Brown.
Supercapacitors to push cars uphill [Deccan Herald, India]
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More on Hydraulic Hybrids [Internal]