Gravitricity
Dutch-British project involving storing/retrieving electricity by lifting/sinking large weights in a mine shaft (59,000 of those in the UK alone). Claim made by startup “Gravitricity”: power up to 20 MW per installation, life span ±50 year and a storage efficiency of 80 à 90%.
[wattisduurzaam.nl] – Nederlands-Brits team bouwt mijnschacht om tot energieopslag
[euanmearns.com] – Short-term energy storage with “Gravitricity” – iron versus ion
[physics.stackexchange.com] – Why don’t we use weights to store energy?
DeepResource 