A study, conducted today and yesterday, demonstrated, in theory, that porous rocks at depth can store enough energy to supply small cities, providing an alternative to dependence on fossil fuels such as gas.
The work, published this month in the Geoenergy specialty journal, was developed by researchers at the GeoBioTec Center of the Faculty of Sciences and Technology of the New University of Lisbon (FCT-UNL) and at the Dom Luíz Institute of the Faculty of Sciences of the University of Lisbon.
“The study analyzes, for the first time and in a systematic way, the influence of depth on underground energy storage conditions. As depth increases, pressure and temperature vary simultaneously, factors that directly condition the amount of energy that can be stored. Thus, rocks will function as a giant battery beneath our feet,” said, cited in a press release from FCT-UNL, the study coordinator, geologist Ricardo Pereira.
The investigation, which was based on theoretical models built from geological and thermodynamic data, aimed to evaluate the potential of energy storage by compressed air in porous media.
“Unlike conventional batteries, compressed-air storage uses the excess electricity produced by renewable sources to compress atmospheric air, which is subsequently injected into deep geological reservoirs. When there is a need to generate electricity, this air is released and used to drive turbines,” explains the press release from FCT-UNL, noting that the technology, by using only atmospheric air, “avoids many risks associated with underground storage of combustible or toxic gases, such as hydrogen or carbon dioxide.”
The study’s authors, who took into account various variables in the models, such as rock properties, storage conditions and compressed-air injection, pressure and energy storage volume at different depths, concluded that porous geological reservoirs at depths between 1,000 and 2,500 meters meet the best conditions for storing energy produced from clean sources (wind, solar or hydro).
In this scenario, a single reservoir could reach a storage capacity of one terawatt-hour, enough to supply a small city, Ricardo Pereira told Lusa.
In large cities, not allowing for energy self-sufficiency for consumption, the technology will allow “to reduce the consumption of fossil fuels such as gas” with five or ten storage units, exemplified the GeoBioTec researcher from FCT-UNL.
In a previous study, Ricardo Pereira and other scientists concluded that in Portugal the coastal strip between Torres Vedras and Aveiro has sedimentary rocks (with porous sands where it is possible to extract water and inject compressed air into the pores) with potential to store, at depth, the excess electricity produced by renewable sources.
Further studies will be required to assess the technology’s impact (including costs) on the decarbonization of the economy and on the efficient, and resilient, management of the electrical grid and consumption.
“Long-duration storage technologies are considered fundamental for reducing waste from renewable production, for increasing the stability of electrical grids and for ensuring critical services, such as the restoration of the grid after widespread failures, like the blackout that affected Portugal and Spain in April 2025,” the note from FCT-UNL highlights.
Countries such as the United States, China and Germany use the technology, but from salt caverns, which store “a lower volume” of energy, Ricardo Pereira told Lusa.
The geologist and the rest of the team plan, in a next step, to study the rocks in more detail to develop “more solid models,” which will require collecting samples for laboratory analysis.
“The success of this technology depends not only on engineering solutions but fundamentally on rigorous geological characterization. The size of the reservoir, porosity, permeability, pressure limits and geothermal conditions are determining factors for the technical and economic viability of these systems,” emphasizes the press release from the Faculty of Sciences and Technology of the New University of Lisbon.
Despite its potential, underground energy storage in geological reservoirs collides with a gap in Portugal: the law, which, according to Ricardo Pereira, does not allow doing it or even testing it.
“It needs to be regulated,” he noted.
