A small municipality in Finland is switching off fossil fuels and switching on a new era of green heating, thanks to an innovative sand-based energy storage system.
Pornainen Turns to Thermal Storage to Ditch Oil and Gas
The Finnish town of Pornainen, just over an hour from Helsinki, has become the first community to heat its buildings entirely using a sand battery, officially the world’s largest of its kind. Developed by clean-tech startup Polar Night Energy, the sand-filled system went live in early 2025 and now powers the district heating network serving the town’s 5,000 residents.
Heating Bills Slashed and Fossil Fuels Replaced
Crucially, the project slashes heating emissions by an estimated 70 per cent and replaces imported fossil fuels with stored renewable energy. According to Polar Night Energy’s COO, Liisa Naskali, “This project is a powerful example that effective solutions for mitigating climate change do exist. Combustion is not a sustainable option for the climate or the environment.”
How a Sand Battery Actually Works
At the heart of the system is a 13-metre-tall, 15-metre-wide insulated steel silo filled with 2,000 tonnes of crushed soapstone, an industrial by-product similar to sand. The sand battery stores thermal energy, not electricity. During periods of high renewable electricity availability e.g., such as windy or sunny days, clean power is routed to a resistive air heater, which warms air to around 600°C. That hot air is then circulated through pipes embedded in the sand, storing energy as heat.
How It Works
The silo’s insulation is key and once charged, Polar Night Energy says the sand can retain its high temperature for weeks, or even months, with only minor heat loss. When heating is needed, cooler air is pumped through the silo, absorbing heat from the sand and passing it through a heat exchanger. This warms water for the town’s district heating system, which supplies homes, businesses, and even public buildings such as swimming pools.
Charging the system from ambient temperature takes several days, but in reality, the battery is topped up continually from available surplus energy. This means it rarely cools fully, enabling more efficient long-term performance.
Heating for a Week (Or a Month) On One Charge
The Pornainen sand battery has a power output of around 1 MW and a total energy storage capacity of 100 MWh. Polar Night Energy estimates that’s enough to heat the entire town for a week during winter, or up to a month in summer when demand is lower. In a cold Nordic climate heavily reliant on heating, that’s a significant step.
This installation builds on an earlier, smaller 2022 pilot by the company in Kankaanpää. That earlier model had just one-tenth the capacity of the Pornainen system and served as a proof of concept. Now, Polar Night Energy is scaling up, with further deployments under discussion across Finland and other European nations.
Emissions and Efficiency Gains
According to the town’s heating provider, Loviisan Lämpö, the sand battery will reduce the use of oil by 100 per cent and cut consumption of wood chips (the previous main heat source) by 60 per cent. This is expected to save around 160 tonnes of carbon dioxide emissions per year, a major environmental gain for such a small town.
The battery also contributes to energy security by reducing reliance on imported fossil fuels and improving resilience during energy price spikes. With around 50 per cent of Europe’s final energy consumption still used for heating (most of it fossil-fuel-based), thermal energy storage could play a crucial role in decarbonisation strategies.
Thermal Efficiency Higher Than Chemical Batteries
Round-trip thermal efficiency of the sand battery is reported to be around 85–90 per cent, significantly higher than many chemical battery systems. While it cannot return energy to the grid as electricity (yet), a pilot project is underway to develop a Power-to-Heat-to-Power (P2H2P) version by 2026, which would allow stored heat to be converted back into power during peak demand periods.
Who It’s For?
The sand battery is primarily targeted at district heating providers, industrial users, and large buildings or campuses. Applications include heating water for municipal systems, generating hot air for industrial drying or manufacturing, and producing process steam for sectors such as chemicals, food production, or pharmaceuticals.
It’s also suitable for facilities aiming to participate in grid balancing or reserve energy markets. The system can adjust its charging rate to respond to energy price changes or availability, using AI-based optimisation, an approach developed with telecoms provider Elisa.
Sand Means It’s Scalable
Also, because sand is cheap, abundant, and not in demand for construction, the system is also highly scalable and cost-effective. “We aim to provide a viable alternative to fossil fuels without introducing new dependencies,” said co-founder Markku Ylönen.
Competitors and Comparisons
It’s worth noting here that Polar Night Energy isn’t the only company exploring thermal energy storage. For example, German firm Kraftblock uses a proprietary granulate material to store heat at temperatures of up to 1,300°C for industrial processes. Also, in the US, Antora Energy has developed carbon-block-based thermal storage to power industrial operations, while Siemens Gamesa has experimented with volcanic rock as a medium for grid-scale storage in Hamburg.
However, sand, or in this case, crushed soapstone, offers a unique combination of affordability, local availability, and self-insulating properties. It’s also inert, safe, and non-toxic, making it suitable for use near residential areas.
Compared to lithium-ion batteries, which degrade over time and require mining of critical materials, sand batteries have far lower lifecycle impacts and do not face the same safety concerns. That said, they are limited to heat-based applications and cannot directly power electrical appliances or vehicles.
Challenges and Criticisms
One of the main criticisms of thermal storage systems is that they don’t address all aspects of the energy transition, particularly where electricity, rather than heat, is the end use. Converting heat back into power is possible but involves efficiency losses and greater technical complexity.
There are also infrastructure constraints. Not all towns have district heating networks in place and retrofitting them can be costly and disruptive. In the UK, for example, the dominance of individual gas boilers and a lack of widespread district heating limits immediate applicability.
Another concern is scalability. While sand batteries are modular and cost-effective at medium scale, it remains to be seen whether they can fully replace existing heating systems in large urban areas or high-density cities.
That said, advocates argue that sand batteries are not a silver bullet but a strategic piece of the puzzle. “Of course, we alone cannot solve the whole problem of climate change,” said Liisa Naskali. “But we need different solutions, and our sand battery is one of them.”
What Does This Mean For Your Organisation?
For now, the technology remains most viable in towns or industrial zones with established district heating systems, but its potential reach is growing. As more renewable electricity becomes available and the need for long-duration storage intensifies, thermal solutions like sand batteries are likely to gain traction. What makes Pornainen’s example compelling is that it shows how even a small town can take meaningful climate action using infrastructure that is low-cost, low-maintenance, and relatively simple to integrate.
For UK businesses, particularly those involved in manufacturing, utilities, or large-scale building management, sand-based thermal storage could offer a new route to decarbonisation. While domestic adoption faces barriers due to the limited rollout of district heating, commercial and industrial users may find opportunities to cut fuel costs and emissions by incorporating heat storage alongside renewable generation. Energy-intensive sites with processes that rely on steam or hot air could benefit most immediately, especially where peak demand or volatile energy pricing creates operational risks.
There’s also a clear advantage in terms of supply chain resilience. For example, by using abundant, non-toxic materials and sidestepping the rare minerals used in conventional batteries, sand storage avoids many of the geopolitical and environmental concerns linked to lithium and cobalt. Also, for those designing future-ready infrastructure, the option to add electricity recovery later may future-proof investments made today.
While sand batteries won’t replace all forms of energy storage, they do challenge the assumption that high-tech solutions must always rely on complex chemistry or cutting-edge electronics. In an energy landscape that needs diversity and flexibility, simplicity might turn out to be one of the most powerful tools we have.