Finnish Startup Polar Night Energy Stores Solar Energy in Sand Heated to 600°C Without Rare Metals

In Finland, the startup Polar Night Energy is turning ordinary sand into a reservoir of thermal energy. The process involves heating the material up to about 600 °C to store surplus solar and wind power. Two commercial installations are already in operation. A third, even larger, is under construction.

SAND HEATED TO 600 °C: THE SIMPLE PHYSICAL PRINCIPLE THAT ALLOWS MEGAWATT-HOURS TO BE STORED FOR MONTHS

Polar Night Energy’s approach relies on the thermal physics of sand. Surplus renewable electricity heats air, which then flows through large silos. The sand is brought to temperatures close to 600 °C. The heat is stored for weeks on end.

When a demand arises, the system returns the energy as hot water or hot air. It is immediately usable. Urban heating thus benefits from a stable supply, independent of weather conditions. No rare metals, no complex chemical reactions are required in this process.

From Kankaanpää to Pornainen: the two commercial batteries already in operation cut CO₂ emissions by 70%

Polar Night Energy operates two commercial installations in Finland. The first, in Kankaanpää, has been running since 2022. It offers a capacity of 8 megawatt-hours and feeds the local district heating network. This first site has allowed the company to refine its thermal management algorithms.

The second battery, inaugurated in Pornainen in 2025, marks a major technological leap. Its 13-meter-high silo contains 2,000 tonnes of crushed steatite. This rock, a by-product of Finland’s chimney industry, is heated to 600 °C. It stores 100 megawatt-hours of thermal energy.

This installation can cover up to a month of the commune’s summer heat demand. In winter, autonomy reaches one week. Thanks to this system, Pornainen reduces its CO₂ emissions by 160 tonnes per year, bringing the municipality closer to carbon neutrality by 2035.

Reversing heat into electricity: the P2H2P project that could raise overall efficiency to 90%

The current installations only deliver heat. However, Polar Night Energy is developing a process named P2H2P. This system reconverts stored heat into electricity. A pilot plant is being developed in southern Finland. This step marks a decisive scale-up.

With this conversion, overall efficiency could reach 90%. Without it, the system operates at 30–35%, a level comparable to nuclear power plants. The storage battery also helps rebalance the electricity grid. It reduces dependence on fossil fuels and lowers large-scale storage costs.

A high cost and commercial challenge: the future Vääksy battery targets 250 MWh to prove economic viability

The main limitation of the sand battery remains its deployment cost. Liisa Naskali, Chief Operating Officer, openly acknowledges this challenge. Selling this innovative solution when the initial investment is very high remains a real obstacle. The biomass-fired boilers used significantly add to the overall bill.

Despite these hurdles, the Vääksy project is moving forward. Lahti Energia and Polar Night Energy have announced the construction of a new industrial unit. This future site will boast a power rating of 2 megawatts and a capacity of 250 megawatt-hours. Construction begins in 2026 with commissioning planned for 2027.

Once operational, this Vääksy site will become the largest sand battery in the world. The system will reduce the local district heating network’s natural gas consumption by 80%. Several European countries, as well as Canada, are already showing interest in this technology and its export potential.