Portuguese scientists studied options for storing energy as compressed air in deep porous rock. In the best case, one reservoir could power a small town.
Storing energy underground may sound like science fiction, but it is not exactly a new practice in geological research. The first experiments were carried out in salt caverns in Germany in the 1970s. The process opens up a world of possibilities for renewable energy and the storage needs that come with it.
“One of the problems with the energy transition is that there is greater availability of energy from renewable sources which cannot be stored,” Ricardo Pereira, a researcher at GeoBioTec at the NOVA School of Science and Technology in Lisbon, tells Euronews Earth. “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,” he explains.
“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.”
How can rocks be used to store energy?
In the case of porous rocks, the process consists of feeding air compressors with surplus electricity produced from renewable sources such as wind turbines or solar panels. The compressors then push air, without hydrogen or CO2 and therefore with no risk of explosion, down a well into a layer of porous rock at depth.
The air fills the spaces between the grains of rock, which are normally filled with non-drinkable salt water. This water is displaced without contaminating anything used for human consumption. The air remains trapped there, under pressure, like a “charged” battery that can be used as needed.
When required, the compressed air rises up the well and expands; that force drives turbines at the surface which generate electricity, just like a conventional power station but without burning fuel.
Pereira is one of the Portuguese scientists behind a study that set out to understand how much energy could be stored under different rock conditions.
“This study looks at how much energy could actually be stored under different geological conditions and at different depths using various physical or thermodynamic parameters,” he says, noting that “different scenarios were developed” to “give an idea of the maximum amount of energy that can be stored at different depths”.
Potential to store enough energy to power a city
Among the European Commission’s energy priorities is the increase in energy storage capacity, which it considers “indispensable”. Europe currently has about 55 gigawatts of storage capacity. The goal is to reach 200 gigawatts by 2030 in order to ease grid constraints, which increase the risk of blackouts and dependence on imported fossil fuels.
It is in this gigawatt deficit that compressed-air storage in porous rocks could play a role. Unlike lithium-ion batteries, which currently dominate the field, this technology is aimed at long-duration storage, making use of a natural resource that is abundantly available.
The research, a partnership between scientists at NOVA University Lisbon and the Instituto Dom Luiz at the Faculty of Sciences of the University of Lisbon, published in the journal Geoenergy, concluded that in the scenarios analysed “a single adiabatic-isobaric system is the most favourable, capable of providing up to 156 kWh m−3 of energy density and reaching up to 1 TWh of storage capacity at a maximum operating depth of 3,000 m”.
“Obviously this is the maximum scenario, where all the optimum conditions are in place,” stresses Pereira, noting that these figures nonetheless provide “an idea of the maximum scale and potential” of the process.
“Between an estimated minimum value and a theoretical estimated maximum value there is a whole range of possibilities, but what this study shows is that a single reservoir, and we are talking about a simulation for a single case, in the most favourable situation, this 1 TWh is very close to the consumption of some medium-sized cities in Portugal”.
‘Black start’: Could underground energy storage have mitigated the Iberian blackout?
Besides offering large storage capacity, this process works through what is known as “black start”, that is, the ability of an electrical system to start up without relying on the grid.
“Compressed air is almost immediate and therefore allows you to switch on and off almost instantly,” Pereira explains, thereby helping to stabilise the electricity grid, something that is useful in situations such as the Iberian blackout. “It is something that, perhaps, if it had already been up and running in 2025, could have avoided some of those problems,” says Pereira.
“Obviously the blackout problem was far more extensive and this would not be a magic recipe for solving an equivalent situation… but it would help to manage power on the grid.”
According to the researcher, work is currently under way to determine the areas and geographical characteristics in Portugal where processes of this kind could be implemented, but the study already allows clear conclusions to be drawn to support that mapping.
“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,” says Pereira. In this way, he adds, it is possible “to compare it with the consumption of certain coastal towns, especially some of the more industrialised cities, and to see what impact this could have on the decarbonisation of certain regions and on increasing energy independence from fossil fuel consumption”.
Lisbon and Almada, both with high population and industrial density, are among the areas highlighted as having great potential.
Legislation is the main obstacle
For now, in Portugal the technology exists only on paper, with a timeframe for deployment, according to the researcher, of five to 10 years, to cover the various phases required: geological study, drilling of wells, pilot project and start of operations.
“There is a whole set of studies that have to be carried out, starting with characterising the subsurface geology at a given site. Each project is in itself a technical challenge and therefore the preliminary studies alone already take some time,” he explains.
According to Pereira, in terms of financial viability, compressed-air energy storage emerges as a “very advantageous” solution, particularly when you compare the costs with other options such as green hydrogen, “which comes with very high costs”.
He notes that China is one of the main countries investing in this type of project, with several already under way and others in the pipeline. In Europe, Germany also has some projects, although without many results so far, not least because “these technologies take years to plan, to do all the engineering and to get up and running”.
In Portugal, the main obstacle is not economic but regulatory.
“So far, as far as I am aware, there is no specific legislation that would allow such projects to be implemented in Portugal. Everything beneath our feet belongs to the state, to all of us, and therefore there has to be specific regulation for this type of technology, just as there is now for carbon dioxide storage,” the researcher says.
“Even if a bold company, keen to invest, wanted to commission a study, then a pilot and eventually reach the production phase, without legislation it cannot be done… There is a very real barrier here that prevents companies in Portugal from even starting a minimally robust study.”
Huge potential for compressed-air energy storage – but questions remain
In Portugal, the researcher says, there is still a lack of in-depth knowledge about the impact of this technology on national energy consumption. “We can imagine that if in a given locality we are able to cut natural gas consumption during the night by 5, 10 or 15 per cent, what impact that would have at local or even national level,” he explains, stressing that the country has a vast geological area with considerable potential.
“From what we know of the geology, and we are talking about a first assessment of a strip running from Lisbon to Aveiro, in geological terms the so-called Lusitanian Basin, which has a whole range of rocks with the potential to serve as reservoirs. We are talking about a huge area and at the same time one of the most industrialised in the country, with extensive deployment of wind, solar and even hydropower, in other words with the energy available to feed this process.”
On the basis of studies carried out so far, and from the point of view of environmental sustainability, Pereira believes that this “methodology really has legs”. It could even be a solution for the power grid and, with a series of commitments, “perhaps in 10 years’ time, if all goes well, we will have compressed-air projects up and running, helping to manage the grid and to reduce energy problems in an affordable way and at reasonable cost,” he argues.
Source: www.euronews.com
