Finland switches on world's largest commercial sand battery, storing 100 MWh of thermal energy

Modern power grids require a constant, real-time balance between supply and consumption. The integration of wind and solar energy, however, makes power grids unstable due to the intermittent nature of renewable energy generation. To prevent localized grid blackouts or harmful voltage spikes, utilities are increasingly pairing renewables with battery storage and/or rapid-response natural gas plants to smooth out fluctuating supply.

Finland has now introduced an unconventional method to store excess renewable energy and release it to the grid on-demand: sand batteries.

How the Sand Battery Works

Finnish technology startup Polar Night Energy has constructed the world's largest commercial sand battery—a high-temperature thermal energy storage design capable of storing 100 MWh of renewable energy.

Located in Pornainen, Southern Finland, and commissioned by district heating company Loviisan Lämpö, the industrial-scale silo stands 13 meters tall and contains 2,000 metric tons of crushed soapstone, a readily available byproduct from local fireplace manufacturing.

The battery draws clean, cheap electricity from the grid during periods of peak production when wind and solar power are abundant. This electrical energy powers a resistive heating element—similar to a large-scale industrial hairdryer—to heat air.

A closed-loop design of embedded pipes circulates this superheated air through the crushed soapstone, raising the internal temperature to 400°C–600°C. The soapstone retains this energy for weeks or even months with minimal losses, thanks to the material's high thermal density and heat capacity.

To discharge energy during periods of high power demand, cool air is blown through the internal pipe network to extract heat from the stones. This heat is then fed into a heat exchanger to produce hot water, reaching temperatures up to 400°C. The water is pumped directly through the local Pornainen district heating network to warm homes, schools, and public buildings, providing enough thermal energy for the 5,000 residents of Pornainen for one week during winter and nearly one month through the summer.

Automated Operations and Environmental Impact

The entire operation is fully automated using software. An AI monitor tracks live spot-market electricity prices to automatically:

• Charge the battery when electricity is virtually free or negatively priced
• Discharge it when grid power spikes or heating demands peak

This ensures high financial viability.

Polar Night Energy estimates that the sand battery has:

• Cut greenhouse gas emissions by nearly 70%
• Reduced wood chip use by roughly 60%

An older wood-chip plant remains active for peak loads and backup.

Advantages Over Conventional Batteries

Unlike conventional lithium-ion batteries, sand batteries:

• Do not degrade over time
• Require no rare-earth materials (which China controls)
• Offer an ultra-sustainable, low-cost solution for seasonal, long-duration energy storage

"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," Tommi Eronen, CEO of Polar Night Energy, told CNBC. "Hopefully a big portion of those storages could be sand battery-type of storages where we don't need rare earth materials, like in many chemical batteries, such as lithium-ion batteries."

Previous and Upcoming Projects

The Pornainen sand battery is not the company's first.

In 2022, Polar Night Energy launched a smaller 0.1 MW / 8 MWh pilot project in Kankaanpää, proving the commercial viability of storing renewable heat for local district networks.

The company is now developing a massive 250 MWh capacity system (2 MW thermal power) in Vääksy, expected to be completed in the summer of 2027. This project will utilize natural sand to cut local fossil emissions by 60% and support national grid balancing.

Broader Industry Context

Long-duration energy storage is emerging as one of the fastest-growing segments of the energy transition as grids absorb larger volumes of intermittent wind and solar generation. While lithium-ion batteries dominate today's market, utilities are increasingly evaluating alternative storage technologies capable of storing energy for days, weeks, or even months at lower cost.

Finland's sand battery is one of the first commercial-scale technologies to demonstrate what long-duration thermal energy storage could look like.

Institutional investors and private equity firms are increasingly backing integrated renewable projects that combine generation with dedicated storage, rather than standalone wind or solar assets. As utilities look beyond lithium-ion for longer-duration storage, technologies such as thermal storage are beginning to attract greater attention.