the idea behind gravity batteries is really simple. During times when energy sources are producing more energy than the demand, the excess energy is used to move weights (in the form of water or sometimes sand) upwards, turning it into potential energy. When the power supply is low, these objects can then be released, powering turbines as our good friend (and deadly enemy) gravity sends them towards the Earth.
Though generally gravity batteries take the form of reservoirs, abandoned mines moving sand or other weights up when excess power is being produced have also been suggested. Scottish company Gravitricity created a system of winches and hoists that can be installed in such disused mineshafts. The company will install the system in the 1,400-meter-deep (4,600 feet) zinc and copper mine in Pyhäjärvi, Finland.
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