Inside the world's largest sand battery.

John Koetsier

The world’s largest sand battery doesn’t have a copper top. It doesn’t really look like a battery. But it stores 100 megawatt-hours of energy, delivers 1 megawatt consistently, and has slashed a rural Finnish town’s carbon emissions by 70%, cutting 160 tons of carbon dioxide emissions annually. And it’s about to lose its title as the world’s largest sand battery, because the company that made it is building another one, twice its size.

How does it work?

"You heat the air with electricity, store it in the sand, and then take it out through the heat exchanger," Sauli Onttila, chair of the board of the company, Loviisan Lämpö, that runs the facility, told me. "It's quite simple. The genius part is how you make the warmth concentrate and stay in the sand."

This works very well for Finland because, like some countries in Eastern Europe, winter heating is often generated centrally, then distributed to homes and facilities in the local community. The previous solution in Pornainen, where the battery has been installed, was a wood chip burner. That’s still on-site for backup, and sees occasional use, but the sand battery stores enough energy to heat the community for about a week when full with no further inputs.

That gives the local community the ability to schedule energy purchases – increasingly electricity from wind and solar farms – when it’s cheap, further reducing costs. Sometimes, with renewable systems that are generating more power than a grid can handle, those costs can approach negative territory: you get paid to use and store energy.

And if you happen to need electricity out of the battery instead of heat, you can stick a generator on the output instead of a heat exchanger: out comes electrons instead of hot water or super-heated steam.

Round-trip efficiency (energy in versus energy out) sits at around 83–85%, which holds up well against the woodchip boiler it’s largely displacing. The legacy system runs at 80–90% efficiency when fully loaded during heating season, but drops to 50–60% outside of it, while the sand battery runs at 83–85% year-round.

The switch for this one small community results in 14 hectares of forest saved per year, according to Polar Night, the company that designed and built the battery.

But the world’s largest sand battery won’t hold the title much longer.

Polar Night is doubling its size with a new 250 megawatt-hour facility in Vaaksy, Finland, which will output 2 megawatts of energy consistently. That newer and bigger battery will use only slightly more sand – 2,400 tons – because the sand battery’s design renders it more efficient with size. What will become the new world’s largest sand battery is already under construction and is scheduled for completion in summer 2027.

City and rural use is a major growth opportunity for a thermal storage facility like this, but there’s another that might be just as important.

According to Polar Night, around 36% of global industrial process heat demand falls within the sand battery’s operating output range of 80–400°C. That means you can get hot air, hot water, or super-heated steam out, but a version capable of converting stored heat back into electricity is also possible, and it’s currently in testing. Industrial heat is one of the largest sources of global emissions. If Polar Night Energy can crack that market at scale, it could help decarbonize industrial economies.

For now, Polar Night is driving consumer costs for heat and power much lower, says Polar Night communications manager Miika Peltola. Perhaps tomorrow, industrial production costs too.