Gravitricity, an Edinburgh-based green engineering start-up, is working to make this a reality. In April last year, the group successfully trialled its first gravity battery prototype: a 15m (49ft) steel tower suspending a 50 tonne iron weight. Inch-by-inch, electric motors hoisted the massive metal box skyward before gradually releasing it back to earth, powering a series of electric generators with the downward drag. 

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