Heat Transition: A Thermos Made of Stone

A slender drilling rig rises just under five meters into the air. Like a forgotten, oversized toy, it stands at the edge of a site on the Ruhr University campus in Bochum. Its drill head bites its way piece by piece through the damp, cold ground, until in a few days it will have reached a depth of 120 meters.

There lies a shaft of the former Mansfeld mine. In a mud-splattered neon jacket, Stefan Klein walks past a bright yellow excavator to his colleagues at the drilling site. Were there problems? Is everything going as planned? By now, yes. In the beginning the power supply was not quite cooperative.

Klein is a geoscientist and works for the Fraunhofer Institute for Energy Infrastructures and Geotechnologies (IEG). Mine Thermal Energy Storage, or MTES for short, is the technology they are researching here. A pilot project. The aim: old mines should become a kind of stone battery for heat. In the summer they store warm energy to release it in winter.

Hundreds of kilometers long, beneath the Ruhr area runs a tangle of mines and shafts, the extent of which the mining authority in North Rhine-Westphalia can only estimate. From an estimated 60,000 surface entry points they start. Through cracks and fissures water has seeped into the cavities for decades. Reservoirs that the researchers now want to exploit.

Under the ground, in the former Mansfeld mine near Bochum, lie two large chambers, 10,000 cubic meters, enough for four Olympic swimming pools. In the spring of 2026 the researchers intend to test whether the chambers are connected, whether they are sealed, whether there are currents. The first has already been researched; now the second follows.

Like the Inside of a Marble Cake

The ground above the shaft vibrates. A box-shaped machine, the separator, loudly separates the water washed out of the borehole from the rock. Geoscientist Klein leads to a small white container into which the researchers repeatedly bring samples of the borehole water.

He rummages from a corner for oversized laminated sheets. “From this cross-section of the Bochum subsurface you can see why coal regions are so well suited for MTES,” he explains. Like the inside of a marble cake, light and dark layers alternate: sandstone and claystone, 300 million years old.

Sandstone is naturally porous, riddled with many hollow spaces, explains Klein. If you heat the mine water, it flows directly into the rock and heats it up. The reason MTES works lies in the surrounding claystone. This is a poor heat conductor and thus insulates the heated sandstone.

What forms is a kind of rock thermos, storing heat in the summer and delivering heat in the winter. The location is secondary. The geological conditions are nearly identical in places where coal was mined—whether in the Ruhr region, eastern Germany, or Spain.

The Heat Dissipates Unused

Klein stuffs the laminated sheets away and steps out of the container into the icy cold, despite the glaring sun. His gaze travels to a chimney in the background of the drilling site. Thick steam clouds billow into the blue sky. “Do you see that?” he asks, pointing to the white wisps. “That CHP plant up ahead heats the whole campus,” Klein explains.

And in the summer? “Cold water flows from here through a network of pipes across the campus, cooling lecture halls and laboratories. The water absorbs the heat.” The separator drones on as he speaks. He pauses. “Heat that would otherwise dissipate unused.”

Every summer, Stefan Klein calculates, so much energy is lost that it could heat 480 uninsulated single-family houses. It is this heat that will be stored in the mine in the future to heat precisely these lecture halls and laboratories in winter.

The MTES project is part of an international research collaboration worth 20 million euros. Since 2018 the researchers have been working to store heat underground. In mines, away from mines—in Germany, the Czech Republic, Slovakia.

They model subsoils, calculate, drill. 500 terawatt-hours could be stored in German mines according to their calculations—energy that could power millions of households. At least in theory.

Winter is the Problem

For decades, Thomas Kohl, professor of geophysics at the Karlsruhe Institute of Technology, has been researching renewable energy. His field is geothermal energy, tapping into the heat of the Earth’s core. Yes, Kohl says, MTES has potential. More than that. The mines could help solve a crucial problem of the energy transition: winter.

The sun stands high on this winter day in Bochum. Only far above, faint, icy cirrus clouds become visible. But even though the sky is clear, the sun barely melts the frozen dew from the grass blades. The performance of solar energy falls in winter, explains Thomas Kohl, the geophysicist. If the wind then wanes in between, that can become a problem.

From 2030, at least one third of the heat used by German households must come from renewable sources. In 2024 it was just 18.1 percent. After a long startup phase, the demand for heat pumps is now booming. But they require electricity, and the colder it gets, the more. Just when the sun produces less of it.

MTES, says geophysicist Thomas Kohl, could be a useful addition there. The summer overproduction could be stored deep underground and used in winter. Underground boilers could be created, which, when combined with heat pumps, could supply entire neighborhoods.

In Scandinavia They Work with Similar Technology

But is MTES really suitable for district heating networks? It could certainly heat individual buildings, but transporting the warmth through kilometer-long pipes, as with deep geothermal energy? If anything, probably only by storage of heat, Kohl notes.

He cautions that water for district heating must be about 90 degrees Celsius hot. Without additional energy sources, that is a substantial challenge. And heating water with heat pumps is extremely electricity-intensive. The electricity would itself have to be produced from renewable energy for a sustainable solution.

In Scandinavia and the Netherlands they have been working with a similar technology for longer. Up to 30 degrees Celsius can be stored there in aquifers, groundwater stores. More is not currently possible; the technology simply does not permit it, and the water could become contaminated.

Moreover, the geological constraints are greater than with MTES. Stefan Klein and his colleagues in Bochum want more: around 85 degrees is what the mine should store. Since they do not work with groundwater, the high temperatures in the mine are not a problem.

To push the energy transition forward, as the geophysicist Kohl and project leader Klein say, MTES can only be one building block. However, in the Ruhr area, besides the good underground conditions, there is also the fact that coal has made the region one of the most densely populated in Germany.

Stored energy can thus be used again with short distances. Exactly the remnants of the coal era, which fueled global warming, could now be used against the climate crisis there.