It just occurred to me that the engine block itself could be a capacitor. The waterjacket is perfectly suited if you could figure out a way to use the water as a dielectric.... which is unlikely. Never mind...

Dual use materials still intrigue...

These units are roughly 8.25" by 10.25" by 2.4".  A stack of 10 of them would be roughly 2 feet long and hold about 3/4 kWh over a voltage range of 7.5 V to 15 V.  That's enough to push a 4000 lb vehicle from 0-60 with almost 100 kJ left over.  50 kW is 67 horsepower, or enough to brake that 4000 lb vehicle at 4.5 MPH/sec from a speed of 30 MPH.

These things look like they'd make one hell of a hybrid.

Thanks for the numbers EP. My mind is racing with ideas of downsizing.

"Among the applications for the modules suggested by FDK are use in a stop-start system which can regenerate electric power during deceleration or use to extend the product life of lead-acid or lithium-ion batteries."

Is this the beginning of the end for the lead acid starter battery?
Presumably in an HEV this ultracapacitor could replace the lead acid starter battery.
At 3kg it's heavy but weighs less than a lead acid starter battery.
The ultracapacitor holds just 10.5 Wh but could be recharged from the battery pack if the car fails to start immediately.

The lead acid starter battery seems out of place in a modern car. It is heavy and its performance drops off in cold winter weather when you need it most.
This ultracpacitor sounds like it would outlast the car and work well in cold weather.

I goofed on the energy calculations above.  The stack would hold 105 Wh, or enough to drive the vehicle to about 20 MPH.  But you'd have pretty good braking performance from there on down.

At least 100 KW is required for braking and acceleration.

You need much more than 100 kW peak, which is why friction brakes are likely to remain no matter what.  However, for careful drivers in city driving, 50 kW would allow a great deal of (perhaps most, if speeds are low) braking energy to be recovered.