Increasing the range of electric vehicles or other hybrids and avoiding running the engine while the vehicle is idling are both reasons which have led to increased use of deceleration energy recovery, particularly during braking.
Most hybrid technologies transform kinetic energy into electric energy, or kinetic energy into a flywheel, or even pneumatic energy into a hydraulic-pneumatic accumulator.
In electric vehicles the battery (or auxiliary in the case of fuel cells) recovers deceleration energy produced by the motor-generator during energy conversion.
Internal combustion engines that are equipped with stop-start systems can also recharge their batteries during deceleration via the starter-alternator. The energy that is recovered can then be used to turn off the combustion engine during stops and start it up again without discharging the battery. This allows for a 15% reduction of polluting emissions in the city.
Turbo diesel engines, however, don't like frequent stops and starts. They run on very high rotational speeds requiring efficient lubrication that can be lacking when the engine stops. It is therefore important to maintain minimum oil pressure for the turbo axles to restart under good conditions. This is achieved by the current devices.
But in all cases recovering deceleration energy, particularly during braking, must be done very quickly. The battery can only do this if its charge level is fairly weak.
Supercapacitors are therefore used to accumulate an electric charge very quickly and to release it instantly when it is needed. Current supercapacitors only store a limited amount of electric energy, around 3 to 4 Wh/kg. But the new graphene types are promising from 60 to 200 Wh/kg! They can deliver the energy instantly, and therefore provide considerable peak power. It then recharges around 10 seconds later. These characteristics (rapid cycles and frequent stops and starts) are particularly well suited to heavy vehicles that need significant instant power, such as urban buses.
Supercapacitors tolerate up to one million charge and discharge cycles. Their capacity still is 98 % after 10,000 cycles. However they are expensive.
Alternatives to supercapacitors are currently being developed, such as auxiliary batteries with short time constants which always carry a low charge and have compatible life spans.