The world's biggest sand battery is heating homes in Finland. Can it solve industrial heating too?

Filling up a massive silo with sand may not sound like a particularly modern way of storing renewable energy in this era of high-tech AI solutions.

But for a small Finnish municipality, the world’s largest sand battery is proving to be a profitable and green way of providing district heating thanks to its unique capacity for storing heat for several weeks.

The battery in Pornainen, a municipality of about 5,000 people, uses excess renewable energy to heat the sand inside the silo up to 450 degrees. That energy is then stored until its needed to heat the water for the local district heating network, helping warm up buildings across the municipality.

The ability to trap the heat for so long enables maximum savings on electricity as the battery can be "charged" when prices are at their lowest.

“It's simple, but it's a very clever way of doing it,” says Sauli Antila, investment director at CapMan Infra, which owns local district heating provider Loviisan Lämpö.

“If you use water to store heat, it lasts maybe a day and a half. But electric heating is all about utilizing the lowest prices and avoiding the peaks,” Antila tells Impact Loop in an interview. ”And if you only have a one-day window to select the cheapest rates, it’s very difficult. But if you store the heat in sand, it can be there for months.”

District heating is often used in cold-weather countries like Finland and Sweden. The system generates heat centrally and distributes it through a network of pipes to homes, schools, and businesses, meaning buildings don't need their own boiler.

Developed by Polar Night Energy

For Loviisan Lämpö, the sand battery project started as a practical response to a looming problem. Its aging wood-chip plant had only about five years of life left if used at full capacity. Replacing it with another biomass unit was a tempting choice.

“90% of companies would have done that,” Antila says. “But we thought, we don’t want to be dependent only on wood chips. There needs to be something else”.

So the company turned to a new technology developed by Finnish startup Polar Night Energy: a sand-based thermal storage unit. The facility is 13 meters tall and 15 meters wide and has a storage capacity of 100MWh, with an output of 1MW.

Polar Night Energy had initially planned to use ordinary waste sand from construction sites, but found a better alternative in crushed soapstone from a local factory producing fireplaces.

Any type of sand will do, though, as long as there’s enough of it: the silo uses about 2,000 tonnes.

And even though the technology sounds old-school, it does use AI-powered software to determine the optimal times of heating up the sand and releasing the heat to the network.

The battery was inaugurated in June and the aim is to reduce emissions from Pornainen’s district heating by nearly 70 percent, saving around 160 tonnes of CO₂. It will also help extend the lifetime of the old wood-chip plant by up to 15 years as it won’t be used as much.

“We have been running the battery for close to three months now and everything is working,” Antila says. “Because there are basically no moving parts, so what could break down?”

Bigger batteries in the works

For Polar Night Energy, this first battery is hopefully only the beginning.

It already has plans for launching even bigger sand battery projects, targeting outputs of up to 10MW.

The ultimate goal is even bigger: producing sustainable thermal energy storage solutions that can be deployed worldwide.

“Industrial applications are particularly promising, especially where heat above 100°C is required, something electric boilers and heat pumps cannot provide,” says Liisa Naskali, COO of Polar Night Energy.

The Pornainen battery heats the water used for district heating up to 90 degrees. But the sand can easily store much higher temperatures in order to produce steam, which is needed for industrial applications. “It’s really scalable, there is really not any kind of limit to that,” Antila says.