On the outskirts of Kankaanpää, a small town in southwest Finland with just over 11,000 residents, is a steel silo that holds 100 tonnes of sand. It doesn’t appear to be much. It is 23 feet high and 13 feet wide, and if you didn’t know what was inside, you might think it was just regular industrial storage. However, the events taking place within that silo are anything but typical.
That sand is quietly storing enough energy to heat about a hundred homes while being heated to temperatures as high as 500 degrees Celsius.
| Category | Details |
|---|---|
| Company Name | Polar Night Energy |
| Founded | 2021, Finland |
| Headquarters | Finland (Operations in Kankaanpää & Valkeakoski) |
| Co-Founders / Key People | Tommi Eronen (CEO), Markku Ylonen (CTO), Liisa Naskali (COO), Matti Ulvinen (Product Sales Manager) |
| Core Technology | Sand-based thermal energy storage battery — world’s first commercially operated model |
| Battery Capacity | 8 megawatt-hours (current); new deal: 200 MWh storage with 2 MW discharge |
| Operating Temperature | Up to 500–600°C (current model); new pilot targeting higher ranges |
| Battery Location (Pilot 1) | Vatajankoski Power Plant, Kankaanpää, Southwest Finland |
| Battery Location (Pilot 2) | Valkeakoski, ~150 km north of Helsinki (construction started October 2025) |
| Silo Dimensions | 23 feet high, 13 feet in diameter — holds 100 tonnes of recycled sand |
| Expected System Efficiency | ~90% (heat + electricity combined output); 30–35% electricity-only |
| Primary Use | District heating for residential, commercial, and industrial customers |
| Cost Advantage | Up to 10x cheaper than existing high-temperature storage methods |
| Key Challenge | High upfront investment cost discouraging customers |
| Market Ambition | Negotiating in tens of countries including the United States |
This is the sand battery from Polar Night Energy. Furthermore, it may be one of the more subtly significant energy concepts to emerge from Europe in recent years.
The company was started by a small group of people—six at its core—who were convinced that costly materials and advanced chemistry weren’t always necessary to meet the world’s growing need for large-scale energy storage. They contended that sand would be just fine. It’s a big pile of sand. The sand isn’t particularly noteworthy by itself. The company’s product sales manager, Matti Ulvinen, stated, “It’s just sand,” in a tone that makes you question why no one had done this before. It turns out that the solution is complex.
The reasoning behind sand batteries is fairly simple. Solar and wind energy are examples of renewable energy sources that are sporadic by nature. They don’t always produce power when people need it most; instead, they do so when the sun is shining or the wind is blowing.
That discrepancy between supply and demand is more than just a technical annoyance in Finland, where winter temperatures can drop to minus 30 degrees Celsius and the sun can set as early as 3 PM. It’s a survival issue. In a way that lithium batteries, despite their sophistication, are unable to match at scale, excess renewable electricity can be stored as heat in sand, which retains temperature with surprisingly little loss over extended periods of time.
The physics are beautiful. Resistors that heat the air inside the silo are powered by grid electricity that is drawn during times of excess solar or wind generation. Heat is progressively transferred into the mass by a fan moving that hot air through a system of pipes buried in the sand. The energy doesn’t dissipate quickly because sand conducts heat slowly.
The company’s chief technology officer, Markku Ylonen, has noted that heat loss over days or even weeks is negligible because the sand’s core is sufficiently remote from the silo’s walls. Theoretically, the sand battery can store heat for months, which is a seasonal storage capability that most traditional battery technologies just cannot provide.
It’s difficult to ignore how well-suited this technology is to Finland’s current circumstances. Following Finland’s application to join NATO, Russia cut off gas supplies, exposing the nation in a way that swiftly focused minds. The local systems of insulated pipes that move hot water throughout Finnish towns, known as district heating networks, had been largely dependent on that gas. Unsurprisingly, utility companies have become increasingly interested in sand batteries since they provide a means of supplying those networks with stored renewable energy.
The more recent work by Polar Night Energy goes one step further. A power-to-heat-to-power model is being tested in a pilot project in Valkeakoski, about 90 miles north of Helsinki. This means that the stored heat would eventually be transformed back into electricity rather than being used only to warm buildings. The CEO of the company, Tommi Eronen, has described this new configuration as “basically totally different” from the original design.
Construction started in October 2025. A reconsideration of the system’s integration into the current infrastructure is reflected in the shape, which is now horizontal rather than vertical. The anticipated electricity-only efficiency is between 30 and 35 percent, which is comparable to conventional combustion plants but not particularly impressive by some standards. Total system efficiency increases to about 90% when heat output is taken into account, which makes the economics much more appealing for businesses that require both.
Observing this from the outside gives the impression that the sand battery narrative is being undervalued in the larger discussion of energy transition. Maybe it doesn’t have the slick branding of next-generation lithium technology or the drama of hydrogen fuel cells. Sand is not a glamorous substance. However, Liisa Naskali, the chief operating officer, is open about the true challenges. The technology is functional. The cost of the investment is the problem.
“Almost nobody has had the courage to invest in our product,” she stated, pointing out that even when the long-term economics are favorable, it is difficult to convince customers to pay more up front because wood-chip boilers are still inexpensive and well-known. Other clean energy technologies are familiar with this tension.
However, according to Ylonen, the business is negotiating in dozens of nations, including the US. The Finns are quick to point out that their battery works with low-grade sand, which is available in practically infinite quantities worldwide, despite the fact that the lack of high-quality construction sand is sometimes cited as a potential barrier.
The managing director of the Vatajankoski power plant, Pekka Passi, has observed that the material cost is actually quite low. Minimal maintenance is anticipated. It’s still unclear if those benefits will be sufficient to overcome risk-averse utility buyers’ reluctance.
Sand batteries may continue to be a specialized solution, helpful in cold climates with well-established district heating infrastructure but slow to proliferate elsewhere. The economics may also change as long-term storage becomes more urgent and renewable energy becomes more affordable.
The Kankaanpää silo continues to operate as of right now. The sand maintains its heat. Additionally, a small Finnish startup continues to argue that sometimes the most basic material—when handled creatively—is precisely what the situation calls for.
