[LUM#19] Beneath the cobblestones, energy

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

Drilling to harness the Earth’s heat is the principle behind geothermal energy. It is a renewable, carbon-free, and reliable source of energy, but one that is struggling to gain a foothold in France alongside solar and wind power. Roger Soliva and Benoit Gibert, Professors the Géosciences Montpellier, delve into the issue.

An inexhaustible source of energy lies within our reach, right beneath our feet. A constant flow of heat generated naturally by the planet’s ongoing cooling, as well as by the decay of radioactive elements contained in rocks. Harnessing this energy for heating or electricity generation is the very principle of geothermal energy. “A term that describes both the study of thermal phenomena within the Earth and the full range of techniques and processes used to extract heat from the subsurface, ” explains Benoit Gibert, a researcher in rock physics at the Montpellier Geosciences Laboratory. “Geothermal energy touches on all fields of geosciences,” adds Roger Soliva, a tectonic geologist and specialist in geological resources.

And to capture this heat, there’s no secret to it—you have to dig. Because the deeper you go, the higher the temperature rises. “At a depth of just a few meters, the temperature underground matches the average annual surface temperature, which is about 14°C,” the researchers explain. “Beyond that depth, the temperature rises by an average of 3°C every 100 meters.” Geothermal systems therefore require drilling and bringing the hot fluids contained in the ground back to the surface.

Deep geothermal energy

And depending on the depth of these boreholes, there are several types of geothermal energy. “At shallow depths, we refer to surface geothermal energy; water is brought to the surface, and energy is extracted from it using heat pumps that heat buildings, ” explains Benoit Gibert. “ This is a major issue when you consider that 45% of the energy consumed in France is used in the form of heat, ” adds Roger Soliva. For home heating, but also for heating swimming pools, fish farms, and other industrial applications.

But geothermal energy can also be used to generate electricity. “This is what we call geothermal power generation, ” explains Roger Soliva. Deep geothermal energy allows us to reach temperatures above 120 °C by drilling deeper. The fluids extracted at these temperatures can be used, through certain processes, to generate electricity.

In certain parts of the world, most often in volcanic settings, it is also possible to utilize what is known as high-temperature geothermal energy. “In these cases, we extract fluids at much higher temperatures—above 200°C—which allows us to generate electricity from fluids that naturally contain water in the form of steam,” explains the researcher. “The expansion of this pressurized steam turns a turbine, which in turn drives a generator to produce 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.

While countries such as the United States, Iceland, or New Zealand have relied on the Earth’s heat, in mainland France, geothermal energy still accounts for a negligible share of energy production, and only a few tens of thousands of households are heated by geothermal energy, “even though the territory could be suitable for it from a geological standpoint, ” notes 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 with some surprise. Yet it offers unique advantages: “It’s an energy source 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 avoids storage issues because its production is controlled, “by turning the tap on or off as needed, ” the geologist illustrates.

So why is geothermal energy struggling to gain traction despite having “much greater potential, ” as geologists put it? “There is very little communication on the subject, a lack of understanding among policymakers, builders, and the general public, as well as European and national pressures to develop intermittent renewable energy sources. " And a tarnished image of geothermal power generation due to the earthquakes that occurred in Vendenheim in 2019 —'it was avoidable, however, because all the indicators were in the red; these earthquakes were predictable for geologists, ' says Roger Soliva. An event that nevertheless put a damper on this 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,” notes Roger Soliva. “Decision-makers have an important role to play in supporting the development of geothermal energy, which could significantly contribute to the energy mix—at least in its use for heating, which carries no risk of induced seismicity, ” concludes the geologist.

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