It is eleven meters under the asphalt. It doesn’t make noise, it doesn’t emit smoke and it doesn’t appear on the news. But while Zaragoza residents debate the electricity bill, under their feet there is a layer of underground water that remains at a stable 18 °C all year round – in the heat of the August heat wave or in the January frost – and that has been silently heating and cooling dozens of buildings in the city for almost three decades.
The existence of this “natural radiator” hidden under the streets of Zaragoza has returned to the news this week with a double reason: the consolidation of the city as a European benchmark in urban geothermal exploitation, and the presentation of a pioneering method – developed and tested there – to intelligently manage this resource before success destroys it.
In short. The team of the Advanced Hydrogeological and Geothermal Systems Group (SHGA) of the Geological and Mining Institute of Spain (IGME-CSIC) has presented the results of THERMAL, a new method of managing the urban aquifer that they have successfully tested in Zaragoza. The data is concrete: by better coordinating existing heat pumps – without drilling a single new well – more than 7,500 euros per year can be saved per installation and the emission of almost 15 tons of CO₂ can be avoided.
As Cristina de Santiago Buey, geologist and researcher at IGME-CSIC, details, the Aragonese capital is already a reference. “What makes Zaragoza a benchmark is not only the magnitude of the use, but the way in which it has been managed collectively through a model based on scientific knowledge and institutional coordination,” explains the scientist. “This total vision guarantees that geothermal exploitation does not compromise either the sustainability of the aquifer or public health, and turns the municipality into a pioneering example of urban subsoil governance.”
Why Zaragoza? The “mattress” of the Ebro. It is no coincidence that this happens here. Beneath the city lies what geologists call the aquifer “Ebro Alluvial: Zaragoza“: a mass of underground water between 20 and 30 meters thick, in direct connection with the riverbed, and with the water table about 11 meters deep. In simple terms, it is a cushion of water linked to the Ebro that acts as a natural thermostat.
The geothermal key to that mattress is its temperature. While the outside air oscillates between 35 °C in the Aragonese summer and 2 °C on a Cerro day, the groundwater remains stable at around 18 °C throughout the year. That consistency is exactly what a geothermal heat pump needs to work at maximum efficiency.
A giant refrigerator under the asphalt. To understand its mechanism, it is worth remembering how the home refrigerator works: it does not generate cold, it simply moves heat from the inside to the outside. The geothermal heat pump does the same, but on an urban scale and using the subsoil as a source or sink of energy. In winter, the system extracts water from the aquifer at 18 °C, “steals” part of that heat through an exchanger, and amplifies it to heat the building. Then, the water – now somewhat colder – is reinjected. In summer, the process is reversed: heat is extracted from the building and released to groundwater, which at 18°C is much colder than the outside air.
The advantage over aerothermal energy is substantial. Cristina de Santiago Buey illustrates it very clearly: if we want to keep a house at 22 °C and the outside air is at 5 °C in winter, an aerothermal pump has to overcome a large thermal jump of 17 degrees. “If instead of air we use the ground, which remains stable around 18 °C, the jump is much smaller and the pump works much more easily and efficiently,” details the expert. Less effort translates directly into less electricity consumed and a much lower bill.
Three decades and sixty installations. The geothermal use of the Zaragoza aquifer was growing progressively for almost thirty years. The result: about 60 large installations, mostly in public buildings, with an installed power of about 110 thermal megawatts only for cooling – the approximate equivalent of the energy needed to air-condition more than 15,000 homes.
Hospitals, university campuses, shopping centers and apartment blocks benefit from it. Highlights include the City Council’s Zero Emissions Building, which consumes 52% less energy than a conventional building, or the Saica paper mill, with a field of 12 holes integrated into its foundations. The managers of these properties agree: the peace of mind of not depending on the fluctuations of the electricity market to cool or heat huge surfaces compensates for any initial installation effort.
Although there is a B side. With so many wells extracting and reinjecting water, facilities can interfere with each other. If the aquifer becomes excessively hot in the long term by returning too much hot water, it is no longer useful. The current challenge is not the lack of resources, but rather coordinating their use among dozens of actors.
This is where the THERMAL method comes in. The system adjusts flow rates and temperatures so that no installation interferes with the others. The next step is already underway: incorporating artificial intelligence and machine learning to anticipate energy demand and climate changes in the subsoil, with the aim of exporting this model to other European cities.
From Zaragoza to Mieres: an exportable model. To measure the milestone of Zaragoza, it is advisable to look at international references. Paris, thanks to the large Dogger aquifer, has an immense underground air conditioning network; and near Helsinki, in Vantaa, the world’s largest seasonal thermal storage system is being built, designed to store summer heat and release it in winter.
In Spain, the other great example is Mieres (Asturias), where the Pozo Barredo – an abandoned and flooded coal mine – was converted into the largest geothermal network in the country. Today it heats a hospital, the university and hundreds of homes in a perfect example of a circular economy: from a polluting mine to a source of clean energy.
Why doesn’t the rest of Spain look down? Geothermal energy has a marketing problem: it is invisible. It doesn’t have the visual impact of a solar panel on a roof or a wind turbine on the horizon. Furthermore, although the initial investment in drilling scares many developers, experts insist that it pays for itself within five to eight years thanks to continuous monthly savings. With the current push for European funds and aid for energy efficiency, financial support is beginning to become a reality; Now all we need is will and specialized technical training.
There is something almost paradoxical in the history of the Zaragoza aquifer. For three decades, that water has been there, invisible, working silently. Now, with the help of science and artificial intelligence, that story is coming to the surface. Zaragoza has shown that an entire city can be air-conditioned by taking advantage of what is beneath its streets, without polluting and drastically reducing the cost. The question is no longer whether it works. The question is why the rest of Spain continues without looking down.
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