🚀 go-pugleaf

I have seen energy storage as a dc current in a superconducting
inductor (solenoid) demonstrated.  A reasonable demonstration
solenoid a foot or so in diameter and a couple of feet high
stored several MegaJoules.  It was connected to a commercial
3-phase a.c.  line and charged via SCRs.  The same SCRs perform
d.c.  to a.c.  conversion for discharging the solenoid by
changing the phase of the gate triggers.  The "quench" was
spectacular (when the inductor goes resistive), and the solenoid
must be designed to withstand the expansion and contraction
during a quench.  The system I saw used liquid helium and some
sort of copper-lead conductor, but that was before the recent
developments in higher temperature superconductors.

From what I have read, energe storage as heat in salts that
change phase near room temperature would also provide substantial
storage in a reasonable space, though I am not sure what
mechanism you would use to effeciently convert the heat energy
back into electrical energy.  Such "phase-change" salts are used
in some solar energy storage systems.