Flywheel energy storage

> Sounds promising. So could flybrids compete with battery hybrids? Rob Thring of Loughborough University's department of aeronautical and automotive engineering wonders if that's a fair question. "Flywheels are a good way of storing energy. But as for comparing them to batteries – it's like comparing petrol and diesel. Both have merit," he says. "Flywheels' have a lower energy density compared to batteries, but their power density is higher."

> Meaning flywheels can release stored energy far more quickly than batteries. So despite the fact that they can't store as much energy for the same weight, they might actually be great for city-centre driving, characterised by short distances and endless stop-starts.

Also, batteries can come in diverse shapes.