The world’s largest sand battery has achieved all of its performance targets during its first year of operation, helping cut carbon emissions from the district heating network in Pornainen, Finland, by 70%.
The thermal energy storage system, developed by Finnish company Polar Night Energy and operated by Loviisan Lämpö (website in Finnish and Swedish only), was commissioned in June 2025 and now serves as the primary heat production facility for the municipality’s district heating network.
According to Polar Night Energy, the installation has met all of its design specifications and has demonstrated the ability of high-temperature thermal storage to replace combustion-based heat production.
“We have demonstrated that the sand battery works as designed and can deliver significant emissions reductions in a cost-effective way. This is an excellent practical example of the energy transition, where energy production based on combustion is replaced with renewable energy,” said Liisa Naskali, chief operating officer of Polar Night Energy.
The Sand Battery stores renewable electricity as heat in sand or similar materials and can retain energy for periods ranging from days to weeks. In Pornainen, the installation uses 2,000 tonnes of crushed soapstone as the storage medium.
Loviisan Lämpö said the system had operated reliably throughout its first year.
“The first year went even better than we expected. Commissioning was exceptionally smooth, and reliability has been excellent: there have been zero interruptions in district heating delivery. Our cooperation with Polar Night Energy has been very successful,” said Mikko Paajanen, chief executive officer of Loviisan Lämpö.
The investment was accelerated by the European energy crisis and the approaching end of life of the utility’s biomass heating plant.
Sand battery: providing protection from energy price volatility
“We wanted to reduce our dependence on a single fuel source. Recent developments have shown that volatility in energy prices is no longer an exception but the new normal. The sand battery provides protection against exactly these kinds of situations,” said Sauli Antila, operating partner at CapMan Infra.
The large storage capacity enables the system to charge during periods of low electricity prices and also allows participation in reserve markets operated by Finnish transmission system operator Fingrid.
“One of the most interesting performance indicators has been how much cheaper electricity we have been able to procure compared to average market prices. On average, we have been able to charge the battery at prices approximately 70–80 percent below the spot market average. In some months, the difference has exceeded 90 percent,” said Antila.
CapMan has highlighted the project in its sustainability reporting and believes its successful operation could encourage wider adoption of the technology.
“If we as a private equity investor have been willing to invest in a new technology, conducted thorough profitability analyses, and after one year of operation can confidently say that we are satisfied with the outcome, I believe that helps reduce uncertainty for other organizations as well,” said Antila.
Beyond district heating, the partners see opportunities for thermal storage in industrial applications requiring steam and high-temperature heat.
“Many industrial applications still rely on natural gas or oil to generate high temperatures. Thermal energy storage could play a significant role in these applications in the future,” said Antila.
The municipality of Pornainen, whose largest buildings are connected to the district heating network, said the project had brought benefits beyond emissions reductions.
“The most significant measurable benefits have been improved reliability of supply, stable pricing, and reduced emissions. The positive media attention generated by the project has also increased awareness of Pornainen. Overall, the outcome has been extremely positive,” said Antti Kuusela, mayor of Pornainen.
Kuusela said the installation had also strengthened energy security.
“The Sand Battery has significantly strengthened the security of supply of our district heating system. The network now has greater flexibility and a better ability to cope with various disruptions,” he said.
The increased capacity has enabled the district heating network to be expanded to serve new facilities, including a sports and events hall and a renovated comprehensive school.
“I am more convinced than ever that thermal energy storage technologies can play a very important role in helping municipalities reduce emissions. At the same time, projects like this strengthen Finland’s competitiveness and support climate targets,” said Kuusela.
The Sand Battery is 13m high and 15m wide. Its storage capacity corresponds to approximately one month of summer heat demand in Pornainen or around one week of winter demand.
According to Polar Night Energy, the installation has achieved energy efficiency of more than 85%, reduced biomass consumption by 60% and eliminated the use of oil entirely, while maintaining 100% district heating supply reliability.
The project received support from Business Finland and has attracted international recognition, including selection in Time magazine’s Best Inventions of 2025 in the Green Energy category and Popular Science’s list of the 50 Greatest Innovations of 2025.
