June 28, 2026 · Tags: renewable energy, thermal storage, engineering, Finland
A Finnish startup built the world's largest sand battery in a village of 5,000 people, heated it to 600 degrees, and ran it through one of Finland's most brutal winters. The system delivered 100% oil reduction, 70% emissions cuts, and didn't skip a beat. By most viral technology standards, that would be enough. But the real story is both more interesting and more nuanced than "electricity stored in sand."
Here's what's actually happening in Pornainen, Finland, and why the details matter more than the headline.
What's Actually in the Silo #
The technology is called a "Sand Battery," and the company — Polar Night Energy — uses that name because the system works with any granular solid. But the Pornainen installation doesn't use ordinary quartz sand. It uses 2,000 tons of crushed soapstone — a byproduct from Tulikivi, a Finnish fireplace manufacturer that produces waste stone fragments in the process of cutting heat-retaining fireplaces.
Soapstone was chosen over ordinary quartz sand for good reason. It has a higher specific heat capacity (about 0.98 kJ per kilogram per degree at room temperature, versus roughly 0.83 for quartz sand), a significantly higher density (about 2,700 kg/m³ versus 1,500), and can store roughly 1.5 to 2 times more energy per unit volume. It's also more thermally stable across thousands of heating and cooling cycles. Earlier Polar Night Energy projects — the Kankaanpää pilot from 2022 and the Tampere prototype — used regular sand. Pornainen is the first to upgrade to soapstone, effectively using someone else's industrial waste as a high-performance storage medium.
The "sand" branding also remains because, geologically, "sand" refers to grain size (0.0625 to 2mm), not mineral composition. Crushed soapstone of the right particle size is technically sand. The underlying technology - packed-bed thermal storage - works with any granular solid material. "Sand" is the universally understood concept; soapstone is the engineering optimization.
How It Actually Works #
The system is fundamentally simple, which is part of its appeal.
Charging: When electricity is cheap and abundant — typically when wind and solar are generating surplus power — the system uses that electricity to run resistive heating elements. Air is heated and circulated through the packed bed of crushed soapstone via forced convection, raising the temperature inside the insulated silo to 500-600 degrees Celsius.
Storage: The hot soapstone retains its thermal energy in the insulated silo. Depending on insulation quality and storage duration, the system can hold usable heat for days to weeks with modest standing losses. Polar Night Energy's lead scientist has stated that storage-time losses are typically no more than 5% for practical discharge cycles, though the silo would lose about 50% of its energy if left completely idle for roughly three months.
Discharging: When heat is needed — which in Finland is most of the year — air is circulated back through the hot soapstone bed, picking up thermal energy. That hot air passes through a heat exchanger to produce hot water for the district heating network. The heated water flows to the municipal school, town hall, library, and residential buildings across Pornainen.
The system has a thermal power rating of 1 megawatt and a storage capacity of 100 megawatt-hours. At full discharge, that's roughly 100 hours of continuous heat output. It covers almost a month of Pornainen's heat demand in summer and about a week in winter.
What It Actually Achieved #
The performance numbers, corroborated by the customer (Loviisan Lämpö) and the investor (CapMan Infra), are real:
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100% oil reduction. The old oil backup boiler hasn't been fired once since the sand battery went online. The sand battery replaced oil as the primary heat source, and the existing biomass (wood chip) boiler now serves as backup instead. This is literally zero oil use, not just a reduction.
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70% CO2 emissions reduction. After one year of operation, Polar Night Energy confirmed that climate emissions from the Pornainen district heating network dropped by 70 percent. The system was commissioned in June 2025, and by mid-2026 the company confirmed it had met and even exceeded its guaranteed performance targets.
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60% reduction in wood chip combustion. The biomass boiler still runs during peak winter demand, but far less than before. This is a reduction, not an elimination.
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Survived winter 2025-2026. The CEO described it as "a very challenging winter... super cold, even compared to our standards." Electricity prices swung from 3 euros per megawatt-hour to 373 euros in a single week. The system operated through all of it without interruption in district heating delivery.
The project won a Certificate of Merit at the Global District Energy Climate Awards 2025 and was listed in TIME magazine's 100 Best Inventions of the Year.
The Critical Distinction: Storing Heat, Not Electricity #
Here's where the popular framing gets misleading.
The headline "store electricity in sand" sounds like a grid-scale battery — something that absorbs excess renewable electricity and gives it back as electricity when needed. That's not what this does.
The sand battery converts electricity into heat, stores that heat, and returns it as heat. The 100 megawatt-hours is thermal energy — not electrical energy. The system cannot power your lights, your computer, your electric vehicle, or your factory's motors. It heats buildings and water.
This distinction matters because heat accounts for roughly 50% of global final energy demand. Decarbonizing heat is arguably a bigger challenge than decarbonizing electricity, and the sand battery addresses exactly that problem. But anyone who hears "battery" and imagines a competitor to lithium-ion grid storage is misunderstanding the technology entirely.
Polar Night Energy is developing a Power-to-Heat-to-Power (P2H2P) pilot at Valkeakoski that would convert stored heat back into electricity. But the thermodynamic penalty is severe — the Carnot efficiency limit means converting 600-degree heat back to electricity would yield perhaps 30-40% efficiency at best, cutting the overall round-trip figure dramatically below the 80-90% thermal-only number.
The Efficiency Question #
The 80-90% round-trip efficiency figure is plausible but comes with important caveats.
First, it's company-claimed. No independent third-party audit has verified the figure across a full operating year. The underlying physics supports it as reasonable — resistive heating is nearly 100% efficient at converting electricity to heat, and the losses (circulation of air through the bed, some heat escape through insulation) are modest at multi-day storage durations. But as one third-party analysis noted, "vendor claims have not been independently verified across full operating years, although the underlying physics supports them as plausible."
Second, the 80-90% applies to electricity-to-heat-to-heat round trips. Comparing this to lithium-ion batteries (85-95% electricity-to-electricity) or pumped hydro (70-80% electricity-to-electricity) is misleading. Only a fraction of the energy value of electricity is captured when it's used to make heat that's then used only as heat. That said, for the specific application — using cheap surplus renewable electricity to displace oil-burning heating — the economics make sense even without the thermodynamic perfection of a true electrical battery.
Third, the efficiency drops significantly for long-duration storage. At regular charge/discharge cycles (days to weeks), the 80-90% figure holds. But if the system were used for true seasonal storage — charging all summer and discharging all winter — standing losses would consume roughly half the stored energy.
Not a Breakthrough — A Brilliant Application #
The underlying technology is not new. Packed-bed thermal storage — heating rocks, sand, or ceramic material with forced air — has been used in industrial processes, blast furnaces, and concentrated solar power plants for decades. Molten salt thermal storage has operated at grid scale in CSP plants since the 2000s. Refractory brick thermal storage dates back centuries.
What Polar Night Energy has done is take an established concept and apply it to a specific problem with elegant packaging:
- The right use case: District heating in a cold climate where heat demand is massive and sustained
- The right storage medium: Soapstone waste from a local fireplace manufacturer — better performance than sand, effectively free raw material
- The right business model: Selling heat to a municipal utility, participating in grid balancing markets for additional revenue
- The right grid integration: Charging when electricity is cheap (surplus renewables), displacing oil and biomass when heat is needed
Multiple companies globally are pursuing similar approaches. Rondo Energy in California uses refractory bricks heated to 1,100-1,500 degrees Celsius and claims 98% efficiency. Antora Energy uses solid carbon blocks. EnergyNest in Norway uses concrete-based modules. The technology category is real and actively scaling — Polar Night Energy's distinction is being first to commercial operation at this scale for district heating specifically.
What the Video Gets Right vs. What It Omits #
Gets right: Nearly everything. The specs, the performance numbers, the basic mechanism, the funding, the project count — all verified against company announcements, press coverage, and on-site journalism. This is one of the more accurate technology videos to go viral.
Omits:
The 100 MWh is thermal energy, not electrical energy. The "store electricity in sand" framing implies grid-scale electrical storage, which this isn't.
The efficiency figure is company-claimed, not independently verified. It's physically plausible but unconfirmed by third-party audit.
All performance numbers originate from Polar Night Energy and are corroborated by the customer and investor — parties with a financial interest in the technology's success.
The technology is not new. Packed-bed thermal storage is decades old. What's new is the application, the branding, and the business model.
The "closed-loop system" terminology isn't used by the company itself. The system is more accurately described as a high-temperature thermal energy storage system with forced-air heat transfer.
The Bigger Picture #
The Pornainen sand battery is genuinely impressive engineering. It replaced oil heating for an entire town using surplus renewable electricity and waste stone from fireplace manufacturing. It survived a brutal Finnish winter without interruption. It cut emissions by 70% on its first year of operation.
But it's a heat solution, not an electricity solution. The distinction matters because the world needs both. Storing renewable energy as heat for district heating is a meaningful piece of the decarbonization puzzle — but it's a piece that addresses heating demand, not the grid storage problem that lithium-ion, pumped hydro, and other electrical storage technologies are tackling.
The sand battery isn't competing with your Tesla Powerwall. It's competing with oil boilers and gas furnaces. In that arena, it's already winning.
Sources #
- Polar Night Energy official press releases: https://polarnightenergy.com/news/
- pv magazine (June 2025): https://www.pv-magazine.com/2025/06/11/worlds-largest-1-mw-100-mwh-sand-battery-commissioned-in-finland/
- Energy-Storage.News (June 2025): https://www.energy-storage.news/100mwh-sand-battery-goes-into-commercial-operation-in-finland/
- Euronews (June 2025): https://www.euronews.com/2025/06/15/sand-batteries-could-be-key-breakthrough-in-storing-solar-and-wind-energy-year-round
- Interesting Engineering (March 2026): https://interestingengineering.com/energy/sand-battery-polar-night-energy
- CapMan Infra: https://capman.com/articles/finnish-innovation-the-worlds-largest-sand-based-thermal-storage-inaugurated-in-pornainen/
- Tulikivi Group: https://tulikivigroup.com/en/tulikivi-will-supply-crushed-soapstone-to-loviisan-lampos-thermal-energy-storage-in-cooperation-with-polar-night-energy/
- PNE efficiency explainer: https://polarnightenergy.com/news/sand-batterys-efficiency-explained/
- datadeep.tech analysis: https://datadeep.tech/sand-batteries/