Thassalia, marine geothermal power for a DHC network in Marseille

With 40% of the population living less than 100 km from the coast, the sea offers enormous potential as an energy source, both in France and worldwide. In addition to energy from tides and currents, the ENGIE Group is also developing another innovative area of expertise that contributes to the energy transition: thermal energy from the sea. Marine geothermal energy makes use of the difference in temperature between warmer surface water and the cold water found at greater depths. Although this geothermal power plant system is already a reality in Paris using water from the River Seine, the Group is now developing two entirely new and unique projects using seawater in Marseille and Réunion. The Thassalia project in Marseille was inaugurated on 18 October.

A new solution has been developed in Marseille to take advantage of locally available renewable energy by using the thermal energy held in the Mediterranean Sea. As a partnership between the public sector, the Euroméditerranée Development Agency, local authorities, regional authorities and private enterprises (Constructa, Foncière des Régions and ENGIE), this project is an excellent example of how innovation is driving the energy transition and energy efficiency. It is therefore consistent with plans to turn the Euroméditerranée district into an example of a sustainable city.


The Thassalia marine geothermal project has been designed specifically to meet the needs of Marseille’s Euroméditerranée Eco-City business centre. This is the largest urban regeneration program in Southern Europe and is the first project of its kind to generate central heating, water heating and air conditioning services on such a scale using seawater, a fact that imposes significantly higher technical constraints, especially in terms of corrosion control.

Built at the Marseille-Fos Port, the Thassalia marine geothermal power station is the first in France, and even in Europe, to use the sea’s thermal energy to supply linked buildings in the city of Marseille with power for heating and cooling. The plant covers some 500,000 m² while reducing greenhouse gas emissions by 70% and water consumption by 65%.

Marine geothermal energy makes use of the difference in temperature between warmer surface water and the cold water found at greater depths. Water is pumped from the sea through 1 km-long pipelines to coastal facilities, where heat exchangers and heat pumps are used to meet heating or cooling needs. The heated or cooled water is then pumped to individual buildings.



A 3 km grid will provide energy to the buildings during the construction and renovation of the zone, spanning from the CMA-CGM Tower to Marseille Cathedral. Currently, the connected buildings are the Docks (Constructa), the Calypso and the Hermione (Euromedcenter), and the Golden Tulip. The Constructa towers will soon be connected: the Marseillaise, the Floreal (Euromedcenter), Castel, and the Parc Habité d’Arenc, whose main client will be Nexity.

ENGIE’s innovative solution has been made possible thanks to the expertise of its subsidiaries, ENGIE Cofely as regards thermal aspects, and Climespace for district cooling networks. Every technical element of the power station has been created by the company’s teams; Ineo and Cofely handled electricity, Axima and Cofely covered internal networks, with Axima also providing half of the refrigeration units.