Finnish Town Powers Winters with Heated Sand

The small Finnish town of Kankaanpää made an unusual bet in 2022: they buried 100 tons of sand in a steel silo and heated it to 600°C. The goal wasn't to set a record or conduct an experiment. It was to keep people warm through the winter using electricity generated months earlier, when solar panels across Europe were producing more power than anyone needed.

The Physics of Patience

Thermal batteries work on a principle so simple it sounds like it shouldn't qualify as innovation: heat something up when energy is cheap and abundant, then extract that heat when you need it. Yet the engineering challenges are considerable. The storage medium must retain heat for months without significant loss, withstand extreme temperatures without degrading, and release energy efficiently when called upon.

Sand excels at this task because it's both abundant and stable. Unlike water, which boils away, or most metals, which expand and contract destructively at high temperatures, sand can sit at 600°C indefinitely. The Kankaanpää system uses resistive heating—essentially giant heating elements—to warm the sand with excess wind and solar power. When winter arrives and heating demand spikes, air blown through the sand emerges hot enough to feed the local district heating network.

The round-trip efficiency hovers around 85-90%, meaning most of the energy put in comes back out as useful heat. This compares favorably to lithium-ion batteries, which lose about 10-15% of electricity to conversion losses but can't store energy for months without degradation. Sand doesn't care if you store heat for a day or a season.

What Works Underground

While sand batteries make intuitive sense—hot sand stays hot—the idea of storing heat in frozen ground seems paradoxical. Yet researchers at the National Renewable Energy Laboratory demonstrated in 2025 that borehole thermal energy storage works even in Fairbanks, Alaska, where permafrost locks 50-90% of the ground in permanent ice.

The system drills narrow vertical holes 91 meters deep, creating a subsurface heat exchange network. During summer, waste heat from buildings—normally vented to the atmosphere—gets pumped underground. The surrounding earth slowly warms, creating a thermal reservoir. In winter, when heating demand in Alaska runs 5.6 times higher than cooling needs, geothermal heat pumps extract that stored warmth.

The counterintuitive part: the permafrost helps. Frozen ground acts as insulation, preventing heat from dissipating horizontally. Wells at the center of the borehole field produced one-third more thermal energy than outer wells, since the frozen perimeter trapped heat in the middle. The NREL study found that preheating the system for five years before regular operation dramatically improved performance, suggesting these installations improve with age rather than degrade.

The Industrial Heat Problem

Residential heating gets attention because everyone understands cold houses, but industry consumes far more thermal energy. Manufacturing cement, steel, chemicals, and food products requires sustained high temperatures—often provided by burning natural gas or oil. About 36% of industrial process heat falls between 100°C and 400°C, precisely the range where sand batteries operate.

Electrified Thermal Solutions pushed this concept to its extreme in early 2026, unveiling a thermal battery that reaches 1,800°C. At that temperature, the system can replace fossil fuels in processes like steel annealing or glass manufacturing. The economics work because industrial facilities can charge these batteries overnight when electricity is cheap, then discharge heat during production hours without maintaining a constant fuel supply.

The catch is location. Unlike electricity, which travels efficiently across power lines, heat dissipates quickly over distance. Thermal batteries need to sit next to their users, making them infrastructure investments rather than grid-scale solutions. This explains why early adoption clusters around district heating networks in Nordic countries, where pipes already connect heat producers to consumers.

When Storage Becomes Strategy

The Solana Generating Station in Arizona illustrates how thermal storage reshapes energy economics. This concentrated solar plant uses mirrors to heat molten salt to 550°C. The salt stores enough energy to run turbines for six hours after sunset, pushing the plant's capacity factor to 38%—nearly double what solar photovoltaic achieves without storage.

Molten salt costs more than sand and requires careful temperature management to prevent solidification, but it offers something sand can't: the ability to generate electricity on demand. Heat converts to mechanical work through steam turbines, the same technology that's powered grids for a century. Polar Night Energy expects to commercialize this "power-to-heat-to-power" approach by late 2026, turning their sand batteries into electricity sources rather than just heating systems.

This matters because it changes how utilities plan capacity. Traditional power plants must size for peak demand, leaving expensive infrastructure idle most of the time. Thermal storage decouples generation from consumption. A grid operator could charge batteries during the spring solar surplus, discharge them during winter evenings, and avoid building natural gas peaker plants that run only a few dozen days per year.

Beyond the Heating Season

Heating and cooling combined account for half of global energy consumption—a figure that surprises people who think of energy primarily as electricity. Most of that thermal demand currently comes from burning things: natural gas in furnaces, diesel in boilers, coal in district heating plants. A 2020 assessment calculated that widespread adoption of sand battery technology could eliminate 100 million tons of CO2 annually by 2030, roughly 3% of EU emissions.

The timeline matters. Unlike fusion power or green hydrogen, thermal batteries use proven materials and simple physics. The Kankaanpää installation cost about €500,000 and took months, not years, to build. Scaling up means replicating the design, not inventing new science.

What remains uncertain is whether thermal storage can expand beyond district heating networks. Individual buildings could theoretically install small-scale systems, but the economics favor size. A 100-ton sand battery spreads its insulation and infrastructure costs across massive storage capacity. A home-scale version would cost more per kilowatt-hour stored than simply burning natural gas. The technology works best where heat demand is concentrated and continuous—exactly the conditions found in northern cities and industrial parks.