Flywheels

Every vehicle on the road has a flywheel. It smooths out the fuel explosions in each cylinder and makes gear change possible.

An over-sized flywheel can propel a light vehicle but due to the constant speed transitions in regular traffic this application is problematic.

A "mega-sized" flywheel can become a formidable energy storage device that can avert blackouts, facilitate the smart grid and even store energy from daytime production (solar) for nighttime consumption.

This Washington Post article... Reinventing the (Fly)Wheel contains a lot of useful development including the first in the nation 20 MW electricity power plant located in Massachusetts.