The Mediterranean Sea, with its endless blue, has always been the ultimate refuge from the heat when summer takes hold along the coast. However, its waters will now serve not only for a quick dip but also to regulate the temperature inside homes. The French city of Nice launched a major call for bids this week to build an urban heating and cooling network that will operate using marine thermal energy, a system known as thalassothermy. With an estimated budget of between 80 and 100 million euros, the project aims to supply thousands of homes, hotels, offices, and local businesses.
The initiative comes at a critical moment for adapting to intense heat waves and seeks to drastically reduce the use of fossil fuels in urban areas. Through a network of underground pipes and heat pumps, the system will harness the thermal stability of the sea to generate cooling during the hottest months and heating during the winter. The proposal promises stable, regulated utility bills for residents, turning a local and constant energy source into the solution for cooling buildings.
A Natural Thermostat Beneath the Asphalt
The secret of this system lies in a very simple physical principle: seawater maintains a fairly stable temperature throughout the year, ranging from 12 to 25 degrees Celsius , regardless of whether the wind is blowing or the sky is cloudy. To harness this, underground pipes will be installed to capture this temperature and transfer it to buildings via heat exchangers.
Although installing this technology requires a high initial investment—which the European Commission estimated in 2023 at between 1,200 and 1,800 euros per kilowatt installed—the economic return is significant. This is especially true for hotels, which welcome savings of between 15% and 20% on their energy bills, in addition to protection against unexpected gas price hikes.
Local Solutions and Ecological Balance
The mayor of Nice, Éric Ciotti, has defended the measure on social media, asserting that the project provides “concrete solutions for decarbonizing our region.” However, the installation of these pipes is also being closely monitored.
Various environmental groups, while initially supportive of the measure, have called for detailed environmental impact studies to be conducted. The goal is to rigorously ensure that the facilities and thermal flows do not disrupt marine biodiversity or harm protected areas along the Côte d’Azur coastline.
From neighboring pioneers to pilot projects in Spain
Nice is not alone in this marine venture. The neighboring principality of Monaco was a pioneer in the region, heating its Olympic-sized swimming pool with this method as early as 1963. Very close by, the city of Cannes is preparing a similar 10-kilometer network for 2027 that will connect some 4,500 homes, the Palais des Festivals, and about thirty luxury hotels, reducing carbon dioxide emissions by thousands of metric tons. Similarly, in the town of La Grande-Motte, a project launched in April 2024 is already providing a 10% reduction in heating costs for 16 public and residential buildings.
This model is particularly well-suited to the Mediterranean Sea due to its almost complete lack of tides, unlike the Atlantic coast, where tidal variations would require extending the pipelines many kilometers out to sea to ensure a continuous flow. On the Iberian Peninsula, in fact, this technology is still in its infancy and has only been implemented in two projects: a plant at the Meliá hotel in Palma de Mallorca, operational since 2018, and another port facility currently in the testing phase in Vigo.
Could thalassothermy be used in Barcelona?
So far, there have been no reports regarding the implementation of such a model in the city. Even so, the conditions in cities that have already implemented it are very similar to those in Barcelona, so it is reasonable to think that in the future this could be an option worth considering for a city that is experiencing higher temperatures every year.
