One of the deepest metal mines in Europe – the Pyhäsalmi Mine in central Finland – is to be turned into an enormous gravity battery capable of storing 2 megawatts of energy.

As the planet moves towards renewable energy, we are faced with the problem of storage. The problem is that the wind doesn't blow and the sun doesn't shine at precisely the time when people want to consume their power. On particularly windy or sunny days, too much electricity can be generated, leading to a situation where consumers are paid to consume electricity rather than overload the grid. But power that isn't used becomes lost.

It would be better, of course, if we could store that energy away for later use. Gravity batteries are one way of doing that.

Despite the cool name, 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.

“As the world generates more electricity from intermittent renewable energy sources, there is a growing need for technologies which can capture and store energy during periods of low demand and release it rapidly when required,” Gravitricity co-founder Martin Wright said of the technology late last year.

“Our GraviStore underground gravity energy storage uses the force of gravity to offer some of the best characteristics of lithium-ion batteries and pumped hydro storage – at low cost, and without the need for any rare earth metals."