Mechanical energy storage...

Mechanical energy storage ... is getting a lot of attention right now.  All of these are less efficient than electrochemical systems (aka “batteries”), but have other advantages (most especially, cost at scale) that outweigh the efficiency differences.  I know of one other mechanical storage technique being actively pursued: inertial storage.  In an inertial storage system, a large and heavy flywheel is spun up to store energy and down to recover it.  To my knowledge, the largest inertial storage system is in Israel, where a small town uses one to store solar energy during the day and recover it at night...