Geothermal energy: What is it?

Geothermal energy is the energy coming from the Earth’s underground.
The energy of the earth’s crust comes from the initial formation of the planet (for 20%) and from the radioactive decay of certain elements (for 80%), the lifetime of this energy is counted in billions of years (like the sun). The deeper we go, the more the temperature increases.

This thermal energy can be used directly, or it can be converted into electrical energy if the conditions allow it.

Different forms of exploitation

  • High-energy geothermal energy: for electricity production. Usually exploited in active volcanic and tectonic geological contexts, it is also called high temperature geothermal energy. Geothermal waters are generally at more than 150°C. High temperature geothermal energy can also be developed in other geological contexts such as the Massif Central or Alsace in France, but also elsewhere in Europe, at depths varying from 2500m to 5000m.
  • Low and medium energy geothermal energy: exploitation of hot water resources between 30° and 90°C, mainly used for district heating by heat networks and some industrial applications. It is widely used in the Paris basin, with some 40 plants and more than 100 wells drilled over the last 40 years, particularly in the last decade.
  • Very low energy geothermal energy: for individual or small collective heating, by heat pumps with vertical geothermal probes (100 to 200m deep wells) or horizontal probes.

Geothermal energy: Temperatures and uses


Geothermal energy comes from the earth’s underground. It is a primary energy that is renewable on a human scale and can be used everywhere

High temperature geothermal energy and its uses

High temperature geothermal energy can be used directly to produce heat or electricity, thanks to a geothermal power plant.

Many advantages

In terms of production:

  • Primary energy available 24 hours a day all year round
  • Stable and predictable production, not intermittent (baseload energy)
  • Availability rate of power plants between 75% and 95%, i.e. between 6,570 and 8,322 operating hours per year
  • Non-intermittent decentralized energy with a neutral impact on the existing electrical networks
  • Lifetime of geothermal reservoirs 40 to 50 years minimum

On the environmental level:

  • Clean and renewable energy on a human scale
  • No greenhouse gas emissions, no air pollution
  • Low land use
  • Available everywhere, with high concentration in some places (which should be exploited in priority)
  • Energy of substitution to coal, oil or gas, the most emitting of greenhouse gases
  • Energy independence of the territory or the country, no external supply

Competitive energy:

  • The cost of geothermal electricity ranges from €60 to €140 per MWh, and power plants, wells and reservoirs have lifetimes of more than 25 years.

Some disadvantages

The development and operation of a geothermal power plant can have some disadvantages that are however well controlled:

  • During the drilling phase of the wells, the drilling rig can generate noise pollution. In the Paris region, where more than 100 geothermal wells have been drilled in the last 40 years, all necessary measures have been taken to reduce this temporary nuisance. A connection of the drilling rig to the electrical network makes it possible to strongly reduce this nuisance by doing without the use of a generator.
  • Induced seismicity of very low intensity (not felt) is possible during operation depending on the geological context encountered. The implementation of a seismic monitoring system allows to monitor and reduce these events. It is therefore important to inform the population during the development phases of this energy, reminding them that the stakes linked to this induced seismicity remain very low (the potential impact on buildings is very low in case of occurrence)

Efficient power plants

There are 3 types of power plants depending on the nature of the geothermal fluid exploited.

According to the thermodynamic laws, it is a question of implementing the expansion of a working fluid (steam or other) through a turbine.

The fluid drops in pressure and condenses in contact with a cold source (water or air). The turbine drives an electric generator.