Under the cobblestones, energy

Drilling to recover the Earth's heat: that's the principle of geothermal energy. It's a renewable, low-carbon, non-intermittent energy source, but one that is struggling to establish itself in France alongside solar and wind power. Roger Soliva and Benoit Gibert, teacher-researchers at the Geosciences Montpellierlaboratory, take a closer look at the issue.

An inexhaustible source of energy lies within our reach, beneath our feet. A constant flow of heat naturally generated by the continuous cooling of the planet, but also by the decay of radioactive elements contained in rocks. Recovering this energy to heat our homes or generate electricity is what geothermal energy is all about. " A term that describes both the study of thermal phenomena in the Earth and all the techniques and processes used to extract heat from the subsoil," explains Benoit Gibert, a rock physics researcher at the Géosciences Montpellier laboratory. " Geothermal energy concerns all fields of the geosciences," adds Roger Soliva, a tectonist and specialist in geological resources.

And to recover this heat, there's no secret: you have to dig. Because the deeper you go, the higher the temperature. At a depth of just a few metres, the underground temperature corresponds to the average annual surface temperature, i.e. around 14°C," explain the researchers. Beyond this depth, the temperature rises by an average of 3°C every 100 meters". Geothermal installations therefore require the drilling of boreholes to bring the hot fluids contained in the subsoil to the surface.

Deep geothermal energy

And depending on the depth of these boreholes, there are several different types of geothermal energy. " At shallow depths, we talk about surface geothermal energy: water is brought up from the surface, from which the energy is extracted by heat pumps to heat buildings," explains Benoit Gibert. " This is a major challenge, given that 45% of the energy consumed in France is used in the form of heat, " adds Roger Soliva. Not only for domestic heating, but also for heating swimming pools, fish farms and other industrial uses.

But geothermal energy can also be used to generate electricity. " This is known as electrogenic geothermal energy," explains Roger Soliva. With deeper geothermal drilling, temperatures in excess of 120°C can be reached. The fluids extracted at these temperatures can then be used to generate electricity.

High-energy geothermal energy can also be harnessed in certain parts of the planet, usually in volcanic settings. Here, fluids are extracted at much higher temperatures, in excess of 200°C, making it possible to generate electricity from fluids that naturally contain water in vapour form," explains the researcher. The expansion of this pressurized steam turns a turbine, which in turn drives an electricity-generating alternator," explains Benoit Gibert. This is the case, for example, of the Bouillante geothermal power plant in Guadeloupe, which produces 6 to 7% of the island's total electricity consumption.

And while countries such as the United States, Iceland and New Zealand have banked on the Earth's heat, in mainland France, geothermal energy still accounts for a derisory share of energy production, and only a few tens of thousands of homes are heated by terrestrial energy, "even though the territory could be suitable from a geological point of view", explains Roger Soliva.

A place in the energy mix

"When we talk about renewable energies, we think of solar and wind power, but not geothermal energy," says Benoit Gibert. Yet geothermal energy offers unique advantages: "It's a source of energy that can be considered infinite, and its production doesn't depend on weather conditions like solar or wind power," explains Benoit Gibert. It can therefore be produced continuously, and can do away with storage problems, as production can be controlled "by opening or closing the tap according to need", says the geologist.

So why is geothermal energy struggling to develop despite a "much greater potential", as geologists put it? "There is very little communication on the subject, a lack of awareness on the part of political decision-makers, builders and the general public, but also European and national pressure for the development of intermittent renewable energies." And an image tarnished by the earthquakes in Vendenheim in 2019: "It was avoidable, because all the indicators were in the red, and these earthquakes were predictable for geologists," says Roger Soliva. Yet this event cast a pall over the technology, which has been the subject of renewed interest since 2022. With the war in Ukraine and the gas crisis, we're in greater demand, but there aren't enough of us working on this subject," points out Roger Soliva. Decision-makers have an important role to play in the development of geothermal energy, which could make a significant contribution to the energy mix, and at least to its use in the form of heat, which carries no risk of induced seismicity", concludes the geologist.