LISBON, Aug. 28 (Xinhua) -- Portuguese scientists have found that porous underground rock formations could store renewable energy on a scale sufficient to power entire cities, offering a potential solution to one of the greatest challenges of the energy transition, according to a study announced on Friday.
The research, published in the journal Geoenergy, and conducted by teams from the GeoBioTec center at the NOVA University Lisbon Faculty of Sciences and Technology and the Dom Luiz Institute at the University of Lisbon Faculty of Sciences, is the first systematic analysis of how depth influences the conditions for underground compressed air energy storage in porous media.
Countries including China and Germany already use underground compressed air energy storage, but from salt caverns, which store lower energy volumes. The Portuguese team plans to study the rock formations in greater detail through laboratory sample analysis to build more robust models.
"As depth increases, pressure and temperature vary simultaneously, directly conditioning the amount of energy that can be stored. The rocks will function as a giant battery beneath our feet," said lead researcher Ricardo Pereira of GeoBioTec.
The results show that geological reservoirs can reach storage capacities of the order of 1 terawatt-hour, enough to supply hundreds of thousands of homes. Key technical factors include reservoir size, porosity, permeability, pressure limits, and geothermal conditions.
The study also proposes a practical methodology to help identify and assess geological reservoirs suitable for future underground energy storage projects.
The research is particularly relevant for Portugal, which recorded 12 gigawatt-hours of unused renewable energy in 2025 due to insufficient absorption capacity. Because electricity cannot be stored directly, it must be converted into other forms of energy -- chemical, mechanical or thermal -- and reconverted when needed.
Experts said long-duration storage is currently one of the biggest challenges of the energy transition, and that technologies capable of delivering it are considered essential for grid resilience, including recovery from large-scale blackouts such as the one that affected Portugal and Spain in April 2025. ■
