Flywheels can do a similar thing by acting as a temporary store of energy. But they can do it much more efficiently. In electric hybrids only 35% of the kinetic energy lost during braking is retrievable. With flywheels more than 70% is, according to Dick Elsy, boss of Torotrak, another British firm working on the technology. That is because regenerative braking converts kinetic energy into electrical energy, and then into chemical potential energy in the battery. Flywheels, by contrast, merely turn one sort of kinetic energy (of the wheels) directly into another (of the flywheel), which is far less wasteful.
Moreover, unlike batteries, which need to be replaced every few years, flywheels are designed to last the lifetime of the vehicle, and contain no nasty chemicals which need to be disposed of. Small wonder, then, that carmakers have at last begun to sit up and take notice.
