Every spring, heating systems switch off and the question of staying warm fades from daily life. The warmth that carried us through winter simply seems to vanish.
But researchers at Kaunas University of Technology argue it does not have to. Their team has found that the ground beneath our buildings — the same soil under streets, parking lots, and foundations — can capture that heat and hold it until winter returns. The earth we walk on every day may be one of the most overlooked energy storage systems we already have.
Heat that vanishes — or could it be saved?
Every heating season, large amounts of thermal energy quietly escape into the ground and go to waste. Dr. Tadas Ždankus, a professor at Kaunas University of Technology, saw that loss differently — as an untapped opportunity.
The team’s research began in an unlikely place: wind energy. Rather than using a conventional generator, they built a hydraulic system to convert wind into heat. In the process, they noticed that hydraulic losses — typically dismissed as inefficiencies — were generating usable thermal energy. The catch: a portion of that heat disappeared into the ground before it could warm any building.
That observation reframed the challenge entirely. “The question became how to not only reduce heat loss to the ground but also store and retain it for future use,” says Dr. Ždankus. The ground was not just the problem. It might also be the solution.
Phase change: the physics that makes underground storage work
To test whether soil could function as a deliberate storage medium, the researchers placed an artificial heat source in surface soil layers and tracked how heat spread, how fast it moved, and how long it stayed.
One result stood out. When soil was heated sufficiently, moisture began to evaporate — triggering a phase change from liquid water to vapor. Phase changes store significantly more energy than a simple temperature rise, making them far more efficient for thermal accumulation. “The significantly higher amount of energy can be charged into the soil,” Dr. Ždankus explains.
As vapor migrated through the ground, it carried heat outward, raising temperatures wherever it reached. “We noticed a sharp temperature rise wherever the vapour flow reached. This means the energy is moving and can be controlled,” the professor notes. Controllability is what turns a passive observation into a workable engineering concept.
From laboratory prototype to a ‘ground energy cell’
Once feasibility was confirmed, the team moved from experiments to a structured prototype — a “ground energy cell” built alongside a dedicated testing setup that measured temperatures at various depths, including at the surface and in the surrounding air.
KTU master’s students joined the effort, conducting measurements and calculations across an entire year. That extended timeline captured seasonal temperature patterns and allowed comparisons against existing climatological data. Trends that shorter studies would have missed emerged clearly from the year-long dataset.
Numerical simulations then assessed heat loss over time and the effectiveness of underground storage beneath real buildings. The central finding was direct: even passive use of an insulated soil volume beneath a building reduces heat loss and improves energy efficiency. “Less heat loss means less energy needed for heating, which in turn leads to energy savings,” Dr. Ždankus says.
Where this technology could fit into everyday infrastructure
The potential applications span both individual buildings and larger urban systems. Thermal accumulators could be installed beneath residential buildings, streets, or parking lots — serving a single household or feeding into a district-level network.
At grid scale, underground heat storage could help balance district heating systems and relieve pressure during peak demand or power grid stress. The same physical principles also work in reverse: “Underground cold or coolness storage is also possible,” the team notes, pointing to a broader range of climate-control applications than heating alone.
The emissions implications are direct. Stored heat that displaces energy from burning fossil fuels or biomass produces a measurable reduction in CO₂ output — without requiring significant changes to existing building infrastructure.
Next steps: scaling up and integrating with existing systems
The KTU team is now developing scaled-down prototypes and refining methods to control how heat distributes through the ground. The work brings together geotechnical engineers, energy system specialists, and civil architects under a single research framework.
Their near-term goal is integration rather than reinvention. Boreholes, foundation piles, and heat exchange systems already sit beneath cities and buildings. The researchers want to connect these into a unified storage framework that serves industrial facilities and residential neighborhoods alike.
The broader direction points toward a heating infrastructure that works with the ground rather than against it. As energy systems face mounting pressure to decarbonize, solutions drawing on what is already there — the soil, the moisture, the physics of phase change — may prove more practical than approaches requiring entirely new materials or construction.
Carlos is an engineer with strong expertise in technical and industrial topics. He previously worked at international companies such as Siemens and speaks Spanish, German, English, and Italian.
