The hybrid drive system being used by Porsche in its new 911 GT3 R isn't what you find in your average Prius or Fusion. Instead of a battery for energy storage, the 911 will use an electro-mechanical flywheel. The system being used was actually developed by the Williams formula one team and its Williams Hybrid Power subsidiary.
The system is comprised of an electrically driven flywheel and a motor/generator on the gearbox. During braking the gearbox mounted generator drives the flywheel to spin it up to 40,000 rpm. When needed for extra acceleration, the flywheel drives its integrated motor/generator to spin provide power back to the unit on the gearbox.
In the case of the 911, a pair motor generators are actually on the front wheels instead of the single unit on the gearbox as it was on the Williams F1 car last year. This provides more regenerative braking capability as well as all wheel drive.
The flywheel itself is made of a composite material that is infused with magnetic particles when it is being molded. This actually acts as the permanent magnet for the motor, helping to keep the overall weight down. The result is a very efficient system with less mass than a battery system and the ability to absorb energy more rapidly than a battery similar to an ultracapaitor. Like ultracapacitors the downside for automotive applications is limited energy storage capacity.
As a result, electric driving is limited meaning that this really behaves more like a mild hybrid system than a strong hybrid. However, the power output is closer to what is available from most strong hybrids. Williams Hybrid Power is working with a number of automakers on implementing its flywheel system although only Porsche has been publicly identified. Porsche's has setup a microsite for its new Hybrid racecar, and for more details, make the jump for the release and a video explaining the system, along with two promo videos.