What is geothermal?


    The term ‘geothermal’ refers to the process of recovering heat from the Earth, and encompasses all the applications used to recover that heat from hot rocks or reservoirs of steam or hot water deep underground. The temperature of subterranean rocks and water increases with depth at a rate of 3°C every 100 meters. Where the geothermal reservoir is at a moderate temperature, this resource can be exploited to generate heat that is then distributed via a heating network.


    Key figures


    At present, 90 countries use geothermal energy. For some, it contributes up to 22% of their national energy production. In France, geothermal is currently the third most important source of renewable energy in terms of energy generated, behind biomass and hydroelectric power.


    What is geothermal?


    How does geothermal work?


    Geothermal resources represent an inexhaustible store of energy. We take a closer look at the stages in the geothermal cycle, geothermal activity in France and the exploitation of this energy from the Earth that has been used by man throughout history.

    Chiffres clés

    France: rich in geothermal resources


    What is geothermal?


    France has an extremely rich and promising subterranean geothermal resource of which only a miniscule proportion is currently exploited.


    • An assessment of deep-level resources began in 1976 in the aftermath of the first oil crisis. The results revealed large quantities of hot water close to the country’s most densely populated areas
    • The majority of urban heating plants were constructed in the 1980s, and estimates suggest that they currently meet the needs of approximately 200,000, homes equivalent
    • Geothermal is particularly well developed for district heating applications in the Aquitaine and Paris Basins
    • After a large-scale development phase at the beginning of the 1980s driven by high hydrocarbon prices, the geothermal market lost some of its impetus in the 1990s, but is currently experiencing a strong revival in France as a result of policies to promote the use of renewable energy sources
    • Although the main producing countries are Japan, China, the former USSR, the countries of central and eastern Europe and the USA, France has nevertheless played a pioneering role in the development of geothermal, largely as a result of the doublet drilling technique (two wells, the first referred to as the production well, and the second as the re-injection well), and the Dogger aquifer in the Paris Region, which has the largest density of ongoing geothermal operations in the world


    Geothermal: integral to government thinking on sustainable development


    It was at the Rio Earth Summit in 1992 that world leaders expressed their awareness and concern about the advanced stage of environmental damage being caused worldwide (depletion of natural resources, sea and land pollution, the greenhouse effect, acid rain, etc.).


    Acutely aware of the need to reverse or at least slow this trend, most countries - including the member states of the European Union – are now putting sustainable development concepts into practice as part of their wider policies. As a result, geothermal is seen as an extremely promising option for sustainable development.


    Geothermal: a response to the Grenelle de l’Environnement


    • Commitments made as part of the Grenelle de l’Environnement environmental legislation in France (to achieve a 75% reduction in greenhouse gas emissions by 2020) are encouraging the development of geothermal: this means that the contribution made by geothermal heat to the overall energy mix will have to increase by a factor of six between now and 2020
    • To achieve this target, the government plans - amongst other solutions – to develop geothermal-powered heating for office buildings, public institutions and communal housing developments in those regions where the resource is plentiful, and especially in the Paris Region


    The strengths of this promising energy source


    Geothermal has a lot going for it!


    • A continuous resource
      Although it remains just as difficult to locate as oil, this basic form of energy does not suffer from the intermittent availability issues faced by wind power and solar power. It is available and usable 24/7. The heat generated from thermal energy requires no special storage, since the Earth itself is its store
    •  A clean and eco-friendly resource
      It has already been demonstrated that unlike fossil fuels, geothermal operations generate little waste. The average quantity of CO2 released to the atmosphere by geothermal power generating plants around the world is 10 times less than for natural gas powered plants. Better still, making use of this heat source uses very little in the way of fossil fuels
    • A renewable resource
      Unlike fossil fuel reserves, geothermal resources are not exhausted as a result of extraction. They are constantly and naturally renewed by the flow of surface water or, in some cases, by artificial injection. The heat is locked into the rock masses that represent 90% or more of this reserve
    • A universally available resource
      Unlike today’s most highly prized fossil fuels, geothermal reserves are not concentrated in a few specific – and sometimes difficult to access - locations. Subterranean heat is present on every continent. Technologies now exist to enable its development in areas with suitable geological formations and/or rock composition

    La géothermie dans le monde

    Les principaux pays producteurs sont le Japon, la Chine, la Russie, les pays d’Europe centrale et orientale et les Etats-Unis. La France a également joué un rôle de pionnier dans le développement de la géothermie, notamment grâce à la technique du doublet (deux puits, le premier puits est appelé puit de production, le deuxième est appelé puit réinjecteur) et grâce au Dogger, le réservoir du bassin parisien présentant la plus grande densité au monde d'opérations de géothermie en fonctionnement.

    Marine geothermal


    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, ENGIE is also developing another innovative area of expertise that contributes to the energy transition: thermal energy from the sea.



    How does marine geothermal work?


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


    Although this geothermal power plant system is already a reality in Paris using water from the River Seine, the Group has developed one entirely new and unique project using seawater, in Marseille.


    Thassalia: The first marine geothermal power plant in Marseille


    The Thassalia Marine geothermal project has been designed specifically to meet the needs of Marseille's Euroméditerranée eco-city business center - the largest urban regeneration program in Southern Europe - and is the first project of its kind to generate space heating, water heating and air conditioning services on such a scale. In practical terms, the Thassalia plant transform the Mediterranean Sea into a sustainable energy source for around 500,000 m2 of buildings in the city of Marseille.


    As a partnership between the public-sector Euroméditerranée development agency, local authorities, regional authorities and private enterprises, this project is an excellent example of how innovation is driving the energy transition and energy efficiency, and one to which Cofely Services has contributed its expertise in heating technology, whilst Climespace, the ENGIE Group specialist in urban cooling networks, has contributed its refrigeration expertise.

    Onshore geothermal


    Against a background of fossil fuel depletion and soaring prices, geothermal offers encouraging prospects for controlled energy costs in the medium to long terms. Still a low-profile technology at present, geothermal is defined as a renewable, local and eco-friendly source of energy that is simultaneously efficient and economical. It shows considerable potential for heat and power generation.