Most promising green technologies number seven: flywheels

There have been many cases where racing technology has made its way down the ladder right into the cars that we use to drive to work. Flywheels could be the next such advancement. The pinnacle of racing motorsports, Formula 1, is investigating the possibility of using flywheels for energy storage. The idea is old, but recent advancements in low-mass high-strength materials may finally make flywheels a reality.

What's Number 6?