A commercial-scale sand battery in southern Finland is demonstrating how thermal energy storage could help address one of renewable energy’s biggest challenges: balancing intermittent electricity generation.

Located in the town of Pornainen, the facility stores clean electricity as heat in 2,000 metric tons of crushed soapstone, delivering up to 100 megawatt-hours of thermal energy. Standing 13 meters tall and 15 meters wide, it is the largest sand battery of its kind in the world. For the town’s roughly 5,000 residents, that provides nearly a month of heating in summer and about a week in winter.

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Commissioned by district heating company Loviisan Lämpö and developed by Polar Night Energy, the system began operating last year as part of efforts to introduce more flexible heat production and reduce emissions. According to the company, greenhouse gas emissions from Pornainen’s district heating network have fallen by almost 70%, while wood-chip consumption has declined by about 60%. A conventional wood-chip plant remains available to provide backup and meet peak demand.

How The Technology Works

Sand batteries store surplus electricity generated during periods of strong wind or solar production by converting it into heat. The energy is retained in heavily insulated crushed soapstone for days or weeks before being released through heat exchangers to warm water circulating in the district heating network.

Polar Night Energy Chief Executive Tommi Eronen said the shift from conventional power plants to renewable energy would require significantly more storage capacity.

“We’re changing from a world where big power plants were doing the energy production to where solar and wind are producing the energy, then we need a massive amount of storage.”

He said sand-based storage could complement renewable energy without relying on rare earth materials used in many conventional battery technologies.

Scaling Electrothermal Storage

The Pornainen installation is around 10 times larger than an earlier sand battery launched in Finland in 2022, reflecting the company’s ambition to expand electrothermal storage.

Interest is also growing beyond Finland. Polar Night Energy said it has received enquiries from potential customers on every continent, particularly from communities seeking alternatives to fossil fuel-based heating.

Jan Rosenow, professor of energy and climate policy at the University of Oxford, said electrothermal storage could play a broader role because it relies on widely available materials and can store heat for much longer than conventional batteries.

“You don’t need rare earths, critical raw materials and the beauty is you can also charge up the battery when the electricity is cheap and discharge whenever you need the heat.”

Supporting District Heating

Unlike conventional batteries, sand batteries are designed to store heat rather than electricity, making them particularly suitable for district heating systems.

Loviisan Lämpö Chief Executive Mikko Paajanen said the battery allows the company to separate electricity purchases from heat production by buying power when prices are low and storing the energy for later use. One charge can provide heating for about one week during winter or up to one month during summer.

Paajanen said the company expects the sand battery to supply 55% to 60% of Pornainen’s district heating during the first months of the year, compared with about 30% in 2025.

Finland’s Climate Minister Sari Multala described the project as “very inspiring” and said the technology could support more advanced energy applications in the future.