Finland’s Polar Night to build 250 MWh sand battery for district heating provider

Polar Night Energy will deploy an industrial-scale thermal energy storage unit for Lahti Energia’s district heating network in Vääksy, Finland. The sand battery will provide 2 MW of thermal output and 250 MWh of storage capacity, positioning it as the world’s largest sand-based thermal energy storage system upon completion.

The installation will supply heat to the Vääksy district heating network and is expected to lower fossil-based emissions by approximately 60% annually, primarily through an estimated 80% reduction in natural gas consumption and reduced reliance on wood chips.

“We want to offer our customers affordable district heating and make use of renewable energy in our heat production. The scale of this Sand Battery also enables us to participate in Fingrid’s reserve and grid balancing markets. As the share of weather-dependent energy grows in the grid, the Sand Battery will contribute to balancing electricity supply and demand”, says Jouni Haikarainen, CEO of Lahti Energia.

A sand battery is a high-temperature thermal energy storage system that uses sand or a similar solid material to store renewable energy. For the Lahti project, the energy storage medium will be approximately 2,400 tons of locally available natural sand – a material which is well suited for this purpose and enables storage temperatures above 500 degrees Celsius.

Polar Night Energy will serve as the main contractor for the project. On-site construction is scheduled to begin in early 2026, with completion expected in summer 2027. Once finished, the Sand Battery will stand approximately 14 meters high and 15 meters wide, accompanied by a separate technical building.

Lahti Energia has received energy aid for the project from Business Finland, the Finnish government agency responsible for promoting innovation, trade, investment, and tourism.

“We are truly proud to collaborate with a major energy company like Lahti Energia. This project is not only an important reference for us but also another step toward cleaner heat production in demanding high-temperature applications,” said Tommi Eronen, CEO of Polar Night Energy.

The company’s previous sand battery, built for Loviisan Lämpö in Pornainen, stands nearly 13 meters tall and 15 meters wide. It delivers 1 MW of thermal power, provides 100 MWh of storage, and contains approximately 2,000 tons of crushed soapstone as the storage medium. According to Polar Night Energy, the system “has been operating for half a year and has demonstrated strong, reliable performance in real-world operation.”

In 2022, Polar Night Energy switched on the world’s first commercial sand-based, high-temperature heat storage system in the Finnish city of Kankaanpää, with 100 kW of power output and 8 MWh of storage capacity. In December 2023, the company announced a partnership with Nordic energy company Ilmatar to develop its sand battery with power-to-heat-to-power capabilities.

In its latest press release, the company stated that it is currently in negotiations for “several additional sand battery projects for district heating as well as industrial applications.”