Grid scale energy storage systems are all such cool hacks.  Here’s a new one which will be popular with all you model train geeks.  In this scheme they build a specialized railroad, based on those used in mining, and haul freight cars full of somehting heavy (gravel?) back and forth between two rail yards.  The video (which is only tollerable if you turn the sound off, also skip the 1st 2 minutes) illustrated the idea.

668MW Facility from Advanced-RES on Vimeo.

Sadly one of my favorite schemes, Iowa Stored Energy Park, didn’t work out (lessons learned (pdf)).