For those who don’t know, a sand battery is a thermal energy storage system, built around a big, insulated steel tank -- around 4 meters (13.1 ft) wide and 7 meters (23 ft) high – and full of sand. When this sand is heated up, using a simple heat exchanger buried in the middle of it, it can storing an impressive 8 megawatt-hours of energy, at a nominal power rating of 100 kW, with the sand heated to somewhere around 500-600 degrees Celsius (932-1112F).

When it's needed, the energy is extracted again as heat in the same way. Vatajankowski is using this stored heat, in conjunction with excess heat from its own data servers, to feed the local district heating system, which uses piped water to transmit heat around the area.

It can then be used to heat buildings, or swimming pools, or in industrial processes, or in any other situation that requires heat.

Polar Night CTO Markku Ylonen said that it was a doddle to convert electricity into heat, but going back from heat to electricity was a bit more tricky because it needs turbines and more complex things. “As long as we're just using the heat as heat, it stays really simple."

The company claims an efficiency factor up to 99 percent, a capability to store heat with minimal loss for months on end, and a lifespan in the decades.

The company says it'll scale up, too, with installations around 20 gigawatt-hours of energy storage making hundreds of megawatts of nominal power, and the sand heated as far as 1,000C (1,832F) in certain designs. It's possible to create bulk underground storage facilities out of disused mine shafts, if they're the right shape. There are no high-pressure vessels needed, and the biggest cost involved is often the pipework.