Cheat sheet: Five alternative energy stores

(i) Compression: Use renewable energy during off-peak hours to push air into sturdy containers at high density. When released later, the air can drive turbines. 

(ii) Flywheels: Used by NASA in space and children the world over, wind up a flywheel when energy is abundant, then letting it rotate back later to free the power. 

(iii) Gravity: Lift a heavy object high up and secure it there. Then later when energy is required, release it and the pull towards earth generates power. 

(iv) Pumping: Move water uphill in times of abundant energy and store it in a dam or other container. Open sluice gates to drive a turbine when power is needed.

(v) Sand: Heat sand or similar substances to great temperatures (500 degrees C). It will hold the heat for months under the right circumstances, ready for release on demand.