It’s a question that inventors have been tackling for decades. No one wants the fridge, or the hospital, going on the blink when demand surges or the power plant needs repairs.

It turns out to be a surprisingly tricky question to answer. Today, with the rise of green energy sources like solar and wind, the need for industrial-scale energy storage is becoming ever more vital to make sure there’s power even after the sun sets or the breeze dies down.

It’s usually (but not always) still too impractical to string together enough traditional batteries — those powered by chemical reactions, like the ones in smoke alarms and Teslas — to do the job. Instead, with remarkable ingenuity, technicians have relied on a host of physical forces and states such as temperature, friction, gravity and inertia to keep energy locked up for later release.