Versailles Powers Its Historic Legacy with Deep Geothermal Energy: A Model for Sustainable Urban Heating

Versailles Embraces Deep Geothermal Energy to Decarbonize Its District Heating Network

The historic city of Versailles in France's Yvelines department is taking a bold step toward sustainability. On December 18, 2025, Mayor François de Mazières and Franck Lacroix, Deputy Managing Director of Engie, signed a 32-year concession agreement. This partnership entrusts Engie with transforming, modernizing, and greening the city's district heating network, aiming to slash carbon emissions while providing reliable, renewable heat to residents.

At the project's core is deep geothermal energy, harnessing the natural heat from underground aquifers. The site targets the Dogger aquifer, a limestone formation from the Middle Jurassic period, located about 1,500 meters below the Paris Basin. Water in this layer reaches temperatures of around 60°C, making it ideal for heating.

The plan involves drilling a geothermal doublet two wells: one to extract hot water and another to reinject cooled water—on the existing boiler site along Avenue du Maréchal-Juin. Four geothermal heat exchangers and eight heat pumps, totaling 24 MW in power, will be installed. The current gas-fired boiler plant, with three boilers, will be renovated to serve as backup during peak demand.

By 2030, geothermal energy is projected to supply 69% of the network's needs. The network will expand from its current 18-23 kilometers to 35 kilometers, adding 187 substations. Annual heat delivery will reach 145 GWh, serving nearly 19,600 equivalent housing units up from about 5,000 today. Iconic sites like the Château de Versailles, military barracks, and residential buildings will benefit.

Works will span a decade: the first phase (July 2026 to October 2031) focuses on the city center, followed by expansion to the Satory district (April 2027 to June 2035). Engie will invest €110 million, covering drilling, extensions, and upgrades.

Environmental gains are significant: a nearly 70% reduction in CO₂ emissions from the network, avoiding approximately 900,000 tons over the contract's duration equivalent to removing thousands of cars from roads. Geothermal also offers price stability, shielding users from fossil fuel volatility.

Mayor François de Mazières called it a "strategic project" for a sustainable historic city: "Geothermal energy is discreet and permanentthe ideal solution for our remarkable heritage." Franck Lacroix emphasized accelerating low-carbon networks in Île-de-France.

Operations will be handled by Verseo Geo, Engie's local subsidiary.

How Deep Geothermal Works

The system uses a closed-loop process: Hot water is pumped from the Dogger aquifer, transfers heat via exchangers to the urban network, and cooled water is reinjected. No combustion occurs, producing zero direct emissions. The resource is virtually inexhaustible on human timescales.

Île-de-France leads Europe in deep geothermal district heating, exploiting the Dogger's favorable conditions (55-80°C temperatures, high productivity). The region hosts over half of France's installations, heating more than 310,000 equivalent housing units and avoiding hundreds of thousands of tons of CO₂ annually. Engie operates 20 sites and 52 wells here.

Context in Île-de-France's Geothermal Landscape

Versailles joins a growing list of projects. Nearby, Le Chesnay-Rocquencourt and Vélizy-Villacoublay have recent geothermal developments. Region-wide, production reached about 2 TWh in recent years, with ambitions to double by 2050. New explorations, like the Géoscan campaign, map untapped potential in western and southern areas.

Challenges include managing aquifer sustainability—avoiding overexploitation that could create "cold bubbles"—and corrosion from saline waters, addressed through advanced materials and monitoring.

Benefits Beyond Environment

For users, geothermal means lower, more stable bills. The network's shift to low-pressure hot water improves efficiency and safety. Connecting iconic sites like the Château de Versailles underscores blending modernity with heritage preservation.

Future Outlook

As works begin in 2026, Versailles sets an example for historic cities transitioning to renewables. This project not only decarbonizes heating—a major emission source—but reinforces energy independence. In a region already geothermal pioneer, Versailles' initiative highlights scalable, local solutions for global climate goals.

With Engie's expertise and municipal commitment, this could inspire further expansions, proving that even palaces can run on Earth's ancient heat.

Source: Versailles