Mechanical energy storage

• Energy can be stored in other forms. E.g. the use of a flywheel. We have used flywheels in machines for centuries; they're a heavy spinning mass that continues to spin when the driving force stops

  
• A recent development used in some buses in Belgium and Switzerland is a flywheel which is accelerated to a very high speed. When spinning at speed stores the energy (rotational energy) and slows as it's released
• Electricity is used to accelerate and slow the flywheel, so an energy conversion takes place. High-efficiency bearings are an important part of this technology and frequently magnetic bearings are used to reduce friction, which increases the efficiency of the system. In this system, energy is also stored when the buses are braking
• Clockwork is still used for storing energy in toys and mechanical devices. Normally a key is used to wind up a spiral string by forcing it into a smaller space, where it is stored as potential energy with a system of gears being used to release the energy slowly