Deep beneath our feet, in the porous rock layers that hold ancient saltwater, lies a potential solution to one of the energy transition's most stubborn problems: how to store surplus wind and solar power for when the sun doesn't shine and the wind doesn't blow. Researchers in Portugal have been exploring this idea, and their findings suggest that the underground could act as a giant battery, capable of powering entire towns.
The concept is not new. Since the 1970s, engineers have experimented with storing compressed air in salt caverns, most notably in Germany. But the Portuguese team, led by Ricardo Pereira of the GeoBioTec research unit at NOVA University Lisbon, is looking at a more abundant and accessible geological formation: porous rock. These are the same types of rock that hold water and oil, but here they would be used to hold compressed air.
"One of the problems with the energy transition is that there is greater availability of energy from renewable sources which cannot be stored," Pereira explains. "It is possible to use that surplus electrical energy and store it temporarily in the rocks in the subsurface, at depths of 500, 1,000 or 2,000 metres."
The process is elegantly simple. Surplus electricity from wind turbines or solar panels drives air compressors, which push air down a well into a layer of porous rock. The air fills the spaces between the rock grains, displacing the non-drinkable saltwater that naturally resides there. The compressed air remains trapped under pressure, like a charged battery. When electricity is needed, the air is released, rising up the well and expanding to drive turbines that generate power—without burning any fuel.
"By using the ground, the rocks, as a kind of battery we can store amounts of energy capable of powering small towns, and that is the great advantage of this technology," Pereira says.
Scaling up to gigawatt-hours
The study, published in the journal Geoenergy, modelled different geological conditions and depths to estimate the maximum energy that could be stored. In the most favourable scenario—a single adiabatic-isobaric system at a maximum operating depth of 3,000 metres—the researchers calculated an energy density of up to 156 kWh per cubic metre and a total storage capacity of up to 1 TWh. That is roughly the annual electricity consumption of a medium-sized Portuguese city like Lisbon or Almada.
"Obviously this is the maximum scenario, where all the optimum conditions are in place," Pereira cautions, but he stresses that the figures give "an idea of the maximum scale and potential" of the technology. The study also shows that deeper reservoirs can store more energy, which could help decarbonise industrial coastal cities and reduce dependence on fossil fuels.
Europe's current storage capacity stands at about 55 gigawatts, but the European Commission has set a target of 200 gigawatts by 2030 to ease grid constraints and reduce the risk of blackouts. Compressed-air storage in porous rock is particularly suited to long-duration storage, unlike lithium-ion batteries, which are better for short-term balancing. This makes it a valuable complement to the continent's growing renewable fleet.
A safety net for the grid
One of the most intriguing features of this technology is its "black start" capability—the ability to restart a power grid without external power. Compressed air can be released almost instantly, allowing generators to switch on and off rapidly. This could have mitigated some of the effects of the Iberian blackout in 2025, Pereira suggests. "It is something that, perhaps, if it had already been up and running in 2025, could have avoided some of those problems," he says, though he acknowledges it would not be a "magic recipe" for such a widespread event.
The research is a collaboration between NOVA University Lisbon and the Instituto Dom Luiz at the University of Lisbon. The team is now mapping potential sites in Portugal, with Lisbon and Almada highlighted as areas with high population and industrial density that could benefit most.
But the biggest obstacle is not geological—it is legislative. In Portugal, the technology currently exists only on paper, and there is no regulatory framework to support its development. As Europe pushes forward with its energy independence plans, which have faced hurdles in grids and funding, such storage solutions could prove vital. The UN climate chief has urged Europe to lead the clean energy transition, and this underground battery might be a key part of that leadership.
For now, the research offers a clear roadmap for where and how to deploy this technology. As Pereira puts it, "This is really one of the innovative aspects of this study: it shows, first, that the deeper we can place our reservoir in porous rocks, the more energy can be released." The next step is to turn that potential into policy.
