France is the second-largest producer of deep geothermal heat in the EU. It exploits less than 1% of it.
Meanwhile, Germany—a country specialists readily describe as a latecomer in this field—is currently drilling beneath its abandoned coal basins. The Fraunhofer IEG institute recently published the first results of its drilling at Weisweiler, in the Rhine basin. Germany is targeting 10 TWh per year by 2030 for its entire national geothermia. France, a global pioneer of the geothermal doublet fifty years ago, watches from afar.
The Essentials
- Fraunhofer IEG published results from its exploratory drilling at Weisweiler, a former coal plant in the Rhineland, as part of a national effort targeting 10 TWh/year of geothermal heat by 2030 for all of Germany.
- The BRGM estimates the potential for saving 100 TWh/year of gas through surface geothermia in France; less than 1% of total geothermal potential is currently being exploited.
- France was the first to industrialize the geothermal doublet in the Paris region in the 1970s, but successive programs stagnated after the 1990s.
- The main obstacle identified by experts is investment cost. A geothermal borehole pays for itself over thirty years, and financing struggles to cover this duration and the associated exploration risk.
Weisweiler, Laboratory of Industrial Reconversion
The Weisweiler plant burned lignite for sixty years. It closed in 2022, in line with Germany’s coal phase-out schedule. What it leaves behind is not vacant land: it is a partial infrastructure, an existing heat network, a skilled workforce in transition, and above all, a subsoil that decades of mining exploitation have mapped with rare precision.
It was on this invisible capital that Fraunhofer IEG bet. The Institute, based in Bochum and specialized in the energy transition of mining regions, has been drilling at Weisweiler since 2023. Current exploratory boreholes reach only 100 meters and 500 meters depth; deep boreholes, planned to reach 1,500 to 3,000 meters, constitute the next phase of the program. Recently published results feed prospects for supplying urban heat networks and low-temperature industrial processes. The project is part of a German national objective: to achieve 10 TWh per year of geothermal production nationwide by 2030.
This figure deserves to be contextualized. Ten TWh of heat is roughly equivalent to what 800,000 poorly insulated homes consume for their annual heating, or the heat output of approximately two nuclear reactors, all proportions considered regarding the nature of the energy. It is significant. It is not massive on the scale of the German energy system, which consumes several hundred TWh of heat each year. But it is a start, and the Germans know that starts matter.
What makes Weisweiler instructive is less the volume than the method. By relying on sites whose geology is already known, Germany reduces exploration risk, which has historically been the main economic obstacle to deep geothermia. An exploratory borehole in unknown terrain costs between 5 and 15 million euros and can prove unsuccessful. A borehole on an old coal basin benefits from decades of underground data. The cost of risk collapses.
France, the Pioneer That Fell Asleep on Its Laurels
There is a geological irony in the current situation. France is not a country that ignored geothermia. It invented it, in the industrial sense of the term.
In 1969, the first modern geothermal doublet is drilled at Melun-l’Almont, in Seine-et-Marne. The principle is elegant: two wells, one to extract hot water, the other to reinject it after use. During the 1970s and 1980s, spurred by the oil shock and a strong energy policy, several dozen doublets were installed in the Paris region. Entire neighborhoods in the southern and eastern suburbs of Paris are heated with geothermal water from the Dogger, an aquifer that extends beneath the entire Paris Basin at temperatures of 56 to 85 degrees.
By the end of the 1980s, France managed the largest park of geothermal doublets in the world for urban heat. It was an undeniable technical success, driven by the Bureau of Geological and Mining Research, which mapped resources, and by local public-private companies that operated the networks.
Then oil prices collapsed. The urgency disappeared. Programs stagnated. Between 1990 and 2010, the French geothermal park hardly evolved. A few incidents on poorly corroded doublets fueled a bad reputation. Public funding dried up. The sector survived, carried by existing operators, but without notable expansion.
Today, the BRGM estimates the potential for saving 100 TWh/year of gas that surface geothermia, assisted by heat pumps, would allow. Deep geothermal production reaches about 2 TWh of heat delivered in 2023. The gap between the possible and the actual is abysmal.
The Paradox of the Best Student Who No Longer Advances
The comparison with Germany is not cruel by chance. It illustrates a recurring dynamic in the energy transition: it is often the countries that start furthest behind that progress the fastest, because they have nothing to lose and everything to prove.
Germany, in the 1970s and 1980s, had no geothermal tradition. It had coal, plenty of coal, and a steel and chemical industry based on that resource. The question of underground heat did not arise. Today, the coal phase-out forced by law, abandoned coal basins, and the urgency of finding controllable alternatives to fossil heat create a pressure that did not exist.
France, conversely, had a functional park and recognized expertise. But this head start also functioned as an anesthetic. When you already have dozens of doublets running, when nuclear electricity remains competitive, when natural gas remained cheap until 2021, the sense of urgency does not form.
The war in Ukraine changed Europe’s energy landscape. But French deep geothermia has not experienced a revival comparable to that of heat pumps or photovoltaic solar. The reason is simple: solar gets installed in eighteen months. A geothermal doublet takes five to eight years between the decision, studies, drilling, and commissioning. In a political context where electoral cycles shape budgets, geothermia always arrives too late for the current mandate.
What France’s Subsoil Can Actually Offer
One hundred TWh per year: this figure needs to be broken down to be understood.
It does not designate a uniform resource accessible everywhere. It represents the sum of several deep aquifers distributed across the territory. The Paris Basin, already partially exploited, concentrates an important share. The Aquitaine Basin, whose deep waters reach 80 to 120 degrees, represents considerable thermal potential for the agri-food industry and agricultural greenhouses. The Alsatian Rhine graben, meanwhile, harbors resources at much higher temperatures, close to 200 degrees, which would allow considering electricity production in addition to heat.
To put this figure in perspective differently: 100 TWh of heat is approximately 40% of current heating needs of French residential buildings. It is a fraction, but a fraction that does not burn gas, produces no CO2, operates 8,000 hours per year without intermittence, and whose fuel is literally inexhaustible on a human scale.
Deep geothermia has a rare property in the renewable energy landscape: it is controllable. Unlike solar and wind, it does not depend on weather. A geothermal heat network delivers the same power in December as in August. For managers of urban heat networks, this is a quality worth its weight in gold, because it eliminates the need for fossil backup capacity.
This is precisely this stability that makes the gap between potential and exploitation so difficult to justify rationally. The resource is there, known, mapped, technically mastered. The obstacle is not an engineering question.
Institutional Patience as a Rare Resource
Economists of energy transition financing identify a structural problem that traditional finance does not easily resolve: investments with very long payback periods, even if profitable, do not find market financing. Deep geothermia is a textbook case.
A geothermal doublet costs between 10 and 25 million euros depending on depth and geology. It begins producing five to eight years after the investment decision. Its complete amortization spreads over twenty-five to thirty years. Experts state it plainly: the main brake is the investment cost. Over the entire project duration, the cost of thermal MWh is competitive with all fossil alternatives, and the installation consumes neither gas nor oil. But finding a private investor willing to immobilize 20 million euros for seven years before the first euro of revenue, in a sector perceived as risky on the resource, remains difficult.
France attempted to solve this problem with ADEME’s Heat Fund, created in 2009. This fund subsidizes renewable heat installations, geothermia included. It enabled a few additional projects. But its annual allocations are insufficient to cover exploration risk, which remains the main barrier. One failed drilling breakthrough on a project in which local communities and their financial partners had invested everything can discourage an entire region for a decade.
Germany chose a different approach for mining wastelands. By directly financing the geological reconnaissance phase through Fraunhofer IEG, a public applied research organization, it socializes the cost of risk. If the borehole is unsuccessful, the cost is absorbed by the institute. If the borehole is successful, the results are published and accessible to all potential operators. Private risk decreases; the number of bankable projects increases.
This mechanism is not new. It is exactly what the United States did in the 1980s for unconventional gas exploitation: federal research programs established resource mapping and validated extraction techniques. When geological risk was known, private capital arrived massively. We know what happened next.
Thirty-Year Horizon, A Few Years to Decide
The temporal arc of deep geothermia is incompatible with the logic of political mandates. A program launched today will give its first operational results around 2032, its first complete returns on investment around 2055. The elected officials who would make the decision today will probably not see its electoral fruits.
This is why deep geothermia, like forest investments or very long-term infrastructure, requires a particular type of public actor: one that sets generational objectives and equips itself with financing tools that traverse alternations of power. The Caisse des Dépôts, public investment banks, local public-private companies with stable capital: these are the structures that carried Parisian geothermia in the 1970s. They still exist.
There is a quiet revival movement in France today. Several local communities, notably in Île-de-France and Alsace, have relaunched preliminary studies on their subsurface resources. The Île-de-France region has announced a program aimed at significantly developing the number of geothermal doublets exploited on its territory. The Strasbourg Eurometropolis has been working for several years on a deep geothermia project in the Rhine graben, with prospects for high-temperature production.
These initiatives remain dispersed, without coordinated national vision. Germany does not have a national geothermia vision either, strictly speaking. But it has something France does not yet have: territorial urgency. Entire cities in the Ruhr and Rhine regions that heated with coal must find something else. This local constraint produces inventiveness and political pressure that the comfort of the French gas system has not yet generated.
The gas price in 2021 and 2022 could have been this trigger. It was not for geothermia, whose implementation timelines are too long to respond to a short-term crisis. But projects that would have been launched in 2022 would be operational by 2029. They were not.
The question is not to catch up with Germany. Germany was catching up with France eighteen months ago; it will probably surpass it on geothermia of industrial wastelands by 2030 solely through the force of its coal phase-out constraint. The question is whether the gigawatts sleeping beneath French soil will find institutions patient enough to awaken them before the decision window closes once again.
A geothermal doublet launched in 2026 will be paid for in 2056. The resource will still be available in 2200. Time plays in geothermia’s favor. We just need to decide to play along.
Sources
- Media24 / Fraunhofer IEG — Results of Drilling at Weisweiler, July 2026
- BRGM — Assessment of French Geothermal Potential, Synthesis Report (available on brgm.fr)
- ADEME — Heat Fund Assessment, 2024 Edition (available on ademe.fr)
- Fraunhofer IEG — Institute for Energy Infrastructure and Geothermia, Institutional Presentation and Research Publications (available on ieg.fraunhofer.de)
- Strasbourg Eurometropolis — Deep Geothermia Program in the Rhine Graben, Public Consultation Documents
- Île-de-France Region — Renewable Heat Plan, 2030 Objectives (available on iledefrance.fr)
- EDF - Geothermia in Figures
- Fraunhofer IEG - Official Press Page
- Fraunhofer IEG - Real Laboratory Geothermia Rhineland
- BRGM / HCP - Surface Geothermia Potential 100 TWh
- City of Melun - First Doublet 1969
- MTES - Key Figures on Renewable Energies 2025
- Knowledge of Energies - Main Brake on Geothermia
- ThinkGeoEnergy - 10 TWh Germany Objective 2030
- Strasbourg Eurometropolis - Deep Geothermia