Geothermal energy draws hot water from below ground to produce heating and/or electricity before reinjecting the water back into the subsoil.

The hot water that comes out of the ground flows through a closed loop and releases its heat via a heat exchanger to another network, which then takes the hot water to radiators. A backup boiler system can provide additional heating when needed and can be used in cases where geothermal energy is unavailable.

Three types of geothermal energy

There are three types of geothermal energy, based on depth:

  • Low-temperature resources (at a depth of between 30 and 600 metres). Water drawn at these depths requires the use of heat pumps to provide heat directly to buildings. The water can also be used to heat swimming pools and greenhouses.
  • Moderate-temperature resources (between 600 and 2,500 metres). Here, the temperature (between 30 and 90°C) makes the water suitable for use in heating networks.
  • High-temperature resources (below 2,500 metres). The temperature of the water at these depths is above 100°C, which is enough to generate electricity.

The benefits of geothermal energy

The French environment and energy management agency (ADEME) and the country's geological survey* (BRGM) see geothermal power as an "outstanding energy source" since it offers a whole array of advantages with virtually no drawbacks.

  • Green
  • Reliable
  • Local
  • Inexpensive
  • Unlimited


Geothermal energy releases very little CO2 into the atmosphere (10 times less than a gas-fired facility). It requires no combustion process, using only electricity to operate facilities.


It is not dangerous and is not subject to the intermittent nature of other renewables like solar and wind power, or, in some cases, biomass supplies.


Geothermal provides a readily available source of on-site energy, right below our feet! It requires no transport and suffers no energy loss, with water drawn from the ground returned to the aquifer at a temperature in keeping with the environment to continuously regenerate natural resources and energy supplies.


Most of the heating costs come from the initial investment rather than the outlay required to purchase fuel. The price of the heating or cooling provided by geothermal energy is therefore stable over time and in the long term, with properly maintained geothermal wells offering a lifespan of up to 30 years or more. What is more, this energy is not vulnerable to the volatile nature of oil and gas prices.


Unlike oil and other fossil fuels, geothermal energy provides an unlimited supply that does not diminish as it is used. The water supply is naturally replenished by surface run-off and reinjection of the water drawn from the ground.

Energy that provides renewable heating

According to the French Environment and Energy Management Agency (Ademe), geothermal energy is used in some 70 countries around the world, providing total output of more than 27 GW, not including heat pumps.

It is one of the main sources of renewable energy used in France to provide heating, after biomass. France currently has 93 geothermal installations, including 63 geothermal heating networks in the Paris region.

Learning about geothermal energy

Geothermal energy is a renewable resource produced from “the earth’s heat" and is especially suited to meeting heating needs.

Dalkia and geothermal energy: time-honoured expertise

 Dalkia has over 40 years' experience in geothermal energy and currently operates 18 deep and 36 near-surface geothermal facilities in France. We manage 18 installations in the Île de France region alone. Dalkia has been operating the oldest deep geothermal sink in the Paris basin since its inception in the early 1970s for the Melun l'Almont heating network, 40 km south of the French capital.

We cover all aspects of this advanced technology: from knowledge of below-ground factors and comprehensive expertise in drilling methods to building and operating facilities and related heating networks.

Drawing on our time-honoured expertise, Dalkia ensures:

  • A secure supply of geothermal water through state-of-the-art monitoring, operation and maintenance.
  • Design and build solutions for production facilities.
  • A sustainable supply of clean energy.

The assets of geothermal energy

The Bagneux geothermal heating network…The energy we need, underneath our feet !