[LUM#19] Energy beneath the pavement

[LUM Magazine, Podcast] - Published on June 21, 2023 in Science-Society

Drilling to recover heat from the Earth is the principle behind geothermal energy. It is a renewable, carbon-free, and non-intermittent energy source, but one that is struggling to gain ground in France alongside solar and wind power. Roger Soliva and Benoit Gibert, Professors the Géosciences Montpellierlaboratory, explore the issue.

An inexhaustible source of energy lies within our reach, beneath our feet. A constant flow of heat generated naturally by the continuous cooling of the planet, but also by the decay of radioactive elements contained in rocks. Recovering this energy for heating or electricity generation is the principle behind geothermal energy. "It's 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 ground," explains Benoit Gibert, a rock physics researcher at the Montpellier Geosciences Laboratory. "Geothermal energy concerns all fields of geoscience," adds Roger Soliva, a tectonicist and geological resources specialist.

And to recover this heat, there is no secret: you have to dig. Because the deeper you go, the higher the temperature rises. "A few meters below the surface, the temperature of the subsoil corresponds to the average annual temperature at the surface, which is 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 drilling and bringing the hot fluids contained in the subsoil to the surface.

Deep geothermal energy

Depending on the depth of these boreholes, there are several types of geothermal energy. "At shallow depths, we talk about surface geothermal energy, where water is brought to the surface and energy is extracted using heat pumps to heat buildings," explains Benoit Gibert. " This is a major challenge when you consider that 45% of the energy consumed in France is used in the form of heat, " adds Roger Soliva. This includes domestic heating, but also heating swimming pools, fish farms, and other industrial uses.

But geothermal energy can also be used to generate electricity. "This is known as geothermal power, " explains Roger Soliva. Deep geothermal energy allows temperatures above 120°C to be reached by drilling deeper. The fluids extracted at these temperatures can be used to generate electricity through certain processes.

In certain parts of the world, most often in volcanic areas, it is also possible to use what is known as high-energy geothermal energy. "Here, fluids are extracted at much higher temperatures, above 200°C, which makes it possible to generate electricity from fluids that naturally contain water in a vapor state," explains the researcher. "The expansion of this pressurized steam drives a turbine, which in turn drives an alternator that generates electricity, " explains Benoit Gibert. This is the case, for example, at 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 invested in geothermal energy, in mainland France, geothermal energy still accounts for a negligible share of energy production, and only a few tens of thousands of homes are heated by geothermal energy, "even though the territory could be suitable from a geological point of view, " says Roger Soliva.

A place in the energy mix

"When we talk about renewable energy, we think of solar and wind power, but not geothermal energy," says Benoit Gibert. Yet it offers unique advantages: "It's an energy source that can be considered infinite and whose production does not depend on weather conditions like solar or wind power, " explains Benoit Gibert. It can therefore be produced continuously and does not pose storage issues because its production can be controlled "by turning the tap on or off as needed, " explains the geologist.

So why is geothermal energy struggling to develop despite its "much greater potential, " as geologists say? "There is very little communication on the subject, a lack of knowledge on the part of policy makers, builders, and the general public, but also European and national pressure to develop intermittent renewable energies. " And a tarnished image of geothermal power due to the earthquakes that occurred in Vendenheim in 2019. "It was avoidable, however, because all the indicators were flashing red; these earthquakes were predictable for geologists, " says Roger Soliva. This event nevertheless cast a chill over the technology, which has been enjoying renewed interest since 2022. "With the war in Ukraine and the gas crisis, we are in greater demand, but there are not enough of us working on this issue," points out Roger Soliva. "Decision-makers have an important role to play in participating in the development of geothermal energy, which could contribute significantly 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.

Keywords: Energy, Renewable energy, Geothermal energy, LUM#19, LumLu, LUM Magazine, Research, Science, Company