A small town in Finland is about to ditch fossil fuels in its heating network thanks to a sand-filled energy storage tank the size of a house.

Finnish startup Polar Night Energy recently turned on the so-called sand battery in the municipality of Pornainen, an hour north of Helsinki. 

The machine, which uses dirt to store excess renewable energy as heat, will warm the homes and businesses in the town of 5,000 people. It is expected to replace natural gas and oil in Pornainen’s district heating network entirely, slashing emissions by an estimated 70%.  

“This project is a powerful example that effective solutions for mitigating climate change do exist,” said Liisa Naskali, COO at Polar Night Energy. “Combustion is not a sustainable option for the climate or the environment.” 

The energy-storing structure measures about 13 metres tall and 15 metres wide and is filled with 2,000 tonnes of crushed soapstone, a byproduct of the construction industry. The new battery is 10 times larger than the startup’s first pilot plant in Pornainen, launched in 2022. 

When renewables are abundant, like on a sunny or windy day, clean electricity is wired to the battery. There, it powers a heater that sends hot air through a series of pipes into the giant vat of sand, heating it to a toasty 600°C. 

Thanks to the battery’s insulated wall, this energy can be stored for weeks or even months. When needed, the battery discharges the hot air on demand — warming water in the district heating network. This can provide heat to households, factories, and even swimming pools.

“Of course, we alone cannot solve the whole problem of climate change, but we need different solutions, and our sand battery is one of them,” said Naskali in a video.    

With a power output of 100 MWh, Polar Night estimates the battery will be able to heat the whole town of Pornainen for a week in winter, or an entire month in summer when demand is lower — on just one charge. The town will still maintain a biomass boiler, which burns wood chips, as a backup source of energy during peak demand. 

Charging the sand battery from ambient temperature to 600°C takes about four days. However, in practice, it’s continuously topped up with excess renewable energy whenever it’s available — so it rarely drops back to the temperature of the surrounding air.

Polar Night said it is currently discussing installing new, bigger sand batteries in Finland and internationally. It aims to offer a cleaner alternative to fossil fuels for heating homes. 

According to the International Energy Agency (IEA), heating accounts for around half of total energy consumption. In Europe, the majority of this heat comes from burning natural gas, oil, wood chips, or waste. 

For European towns, especially ones with access to lots of renewable energy, sand batteries could be low-hanging fruit. If scaled, they could become a significant part of the energy storage toolbox, alongside other options such as lithium-ion, gravity, hydropower, and CO2 domes.  

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