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Geothermal heat: the great forgotten renewable energy

Mikael Philippe
Head of the Geothermal and Energy Storage Unit, BRGM
Jérôme Vergne
physicist at the École et Observatoire des Sciences de la Terre in Strasbourg
Key takeaways
  • Geothermal energy uses the heat in the subsoil: it is captured and used in the form of electricity or heat.
  • It is particularly interesting for urban areas and will be an asset in the face of climate change.
  • However, a very small proportion of heat consumption is supplied by geothermal energy in France: in 2021, it will amount to 1% of final consumption.
  • This is because it is little known to the public and local authorities, and there are relatively few drilling companies in France.
  • Thanks to the many projects that are being set up, Europe should see a 270% increase in geothermal energy consumption between 2019 and 2024.

What if France were to rely part­ly on geot­her­mal ener­gy to achieve car­bon neu­tral­i­ty? Last Feb­ru­ary, the gov­ern­ment pre­sent­ed an action plan to accel­er­ate its deploy­ment. The objec­tive: “to pro­duce enough heat in 15 to 20 years to save 100 TWh/year of gas, i.e. more than Russ­ian gas imports before 2022.” The mul­ti-annu­al ener­gy pro­gramme aims to increase geot­her­mal heat pro­duc­tion facil­i­ties by a fac­tor of 2 or more between 2016 and 2028.

Geot­her­mal ener­gy relies on the heat in the sub­soil1: it is cap­tured and used in the form of elec­tric­i­ty or heat. Let’s take a look at heat pro­duc­tion, the gov­ern­men­t’s objective.

Geothermal energy to produce heat

At a depth of more than 200 metres, low-ener­gy deep geot­her­mal ener­gy – such as the Dog­ger aquifer in the Île-de-France region – con­sists of pump­ing and then rein­ject­ing water from aquifers, under­ground reser­voirs. It is heat­ed by the nat­ur­al decay of radioac­tive ele­ments in the sub­soil. High-ener­gy deep geot­her­mal ener­gy, as in Soultz-sous-Forêts in Alsace, exploits the water nat­u­ral­ly con­tained in frac­tured rocks. These sites are locat­ed in active vol­canic zones or in col­lapse trench­es (par­tic­u­lar geo­log­i­cal structures).

At a depth of less than 200 metres, sur­face geot­her­mal ener­gy exploits the iner­tia of the ground, which is almost unaf­fect­ed by vari­a­tions in atmos­pher­ic tem­per­a­ture: it remains con­stant at around 10–15°C in main­land France. The use of a geot­her­mal heat pump (PACg) is nec­es­sary: it exploits the tem­per­a­ture dif­fer­ence between the sur­face and the sub­soil. Heat is recov­ered by pump­ing (and then re-inject­ing) ground­wa­ter or by cir­cu­lat­ing a heat trans­fer flu­id through a pipe in the hot ground.

Geothermal energy in France

In France, the heat pro­duced by deep geot­her­mal ener­gy is main­ly used to sup­ply urban heat­ing net­works (cur­rent­ly 59) and is used in indus­tri­al process­es (2 TWh in total) or to heat green­hous­es. “With an ambi­tious deploy­ment plan, it would be pos­si­ble to pro­duce around ten TWh of addi­tion­al heat with­in 20 years,” adds Mikaël Philippe, head of BRGM’s Geot­her­mal and Ener­gy Stor­age unit. It is par­tic­u­lar­ly inter­est­ing for sup­ply­ing large conur­ba­tions: it requires the instal­la­tion of a heat­ing net­work and a pow­er sta­tion near­by, with a sur­face area of around 2 000 m2. “There are many untapped aquifers with very inter­est­ing resources,” says Mikaël Philippe. “We are start­ing new explo­ration and research pro­grammes to bet­ter assess their poten­tial.” These basins are locat­ed to the west of Paris, in south-east­ern France and in the Aquitaine basin. The lim­it? “Exploita­tion of the resource is only pos­si­ble if it is appro­pri­ate to the need: the aquifer must be locat­ed near a high­ly pop­u­lat­ed area,” replies Mikaël Philippe.

Anoth­er poten­tial to be devel­oped is sur­face geot­her­mal ener­gy. It accounts for most of the geot­her­mal heat pro­duced in France today (4.8 TWh). “We esti­mate the poten­tial to be reached with­in 20 years at 100 TWh, which is 10 times more than with deep geot­her­mal ener­gy,” says Mikaël Philippe. The major advan­tage? It is avail­able over almost the entire French ter­ri­to­ry, unlike deep geot­her­mal ener­gy. “Sur­face geot­her­mal ener­gy is par­tic­u­lar­ly inter­est­ing in areas with scat­tered, mod­er­ate­ly dense hous­ing,” adds Mikaël Philippe. “Its reversibil­i­ty to pro­duce cold, thanks to PACg, is a real asset in the con­text of cli­mate change.”

What are the limits of geothermal energy?

How­ev­er, a very small pro­por­tion of heat con­sump­tion is sup­plied by geot­her­mal ener­gy in France: in 2021, it will amount to 1% of final con­sump­tion2. Why? “It is lit­tle known by the gen­er­al pub­lic, and local author­i­ties,” describes Mikaël Philippe. “There are also very few drilling com­pa­nies in the coun­try. With the help of sev­er­al organ­i­sa­tions, we are work­ing to remove these obsta­cles.” The invest­ment costs are also high, even if the State sup­ports its deploy­ment through var­i­ous schemes (Fonds Chaleur, MaPrimeRen­ov’, Coup de pouce chauffage). For a sin­gle-fam­i­ly home, Ademe esti­mates3 that the cost (exclud­ing sub­si­dies) of a PACg is €2,731 per year (includ­ing instal­la­tion), com­pared with €2,236 for a gas boil­er or €4,429 for elec­tric heat­ing. How­ev­er, the cal­cu­la­tion is dif­fer­ent if the increase in the price of elec­tric­i­ty, gas and wood is tak­en into account: geot­her­mal ener­gy becomes the solu­tion with the low­est oper­at­ing cost. For col­lec­tive and ter­tiary build­ings, sur­face geot­her­mal ener­gy is cur­rent­ly the most expen­sive solution.

In 2021, geot­her­mal ener­gy account­ed for 1% of final heat consumption.

What about inter­na­tion­al­ly? Every­one has in mind the images of pow­er sta­tions sur­round­ed by steam in Ice­land. By feed­ing a tur­bine, geot­her­mal heat (above 110°C) is used here to pro­duce heat and elec­tric­i­ty in cogen­er­a­tion. In 2013, 29% of Ice­land’s elec­tric­i­ty was pro­duced using this tech­nique and 45% of build­ings were heat­ed4. But Ice­land is a mod­el: world­wide in 2022, only 0.37% of the heat con­sumed is of geot­her­mal ori­gin5. French heat pro­duc­tion amounts to 6.7 TWh, com­pared to 82.1 TWh on a Euro­pean scale6 and 26,000 TWh on a glob­al scale7. For elec­tric­i­ty, the Unit­ed States has the largest pro­duc­tion capac­i­ty (2.5 TWh), fol­lowed by Indone­sia and the Philip­pines8. In France, elec­tric­i­ty pro­duc­tion is essen­tial­ly lim­it­ed to the Bouil­lante plant in Guade­loupe (112 GWh/year) and the Soultz-sous-Forêts plant in Alsace (12 GWh/year).

“In France, we are see­ing an accel­er­a­tion in heat pro­duc­tion projects, par­tic­u­lar­ly for ter­tiary build­ings and heat­ing net­works,” says Mikaël Philippe. Accord­ing to the Inter­na­tion­al Ener­gy Agency, Chi­na and Turkey are respon­si­ble for most of the growth in geot­her­mal heat pro­duc­tion in recent years. In its pro­jec­tions, the agency esti­mates that Chi­nese growth should con­tin­ue, but also notes that Europe is one of the most active mar­kets: the con­ti­nent should record a 270% increase in geot­her­mal ener­gy con­sump­tion between 2019 and 2024.


Deep geot­her­mal exploita­tion is often accom­pa­nied by earth­quakes. “This is a well-known phe­nom­e­non, and oper­a­tors are oblig­ed to mon­i­tor this seis­mic­i­ty,” reports Jérôme Vergne, a physi­cist at the School and Obser­va­to­ry of Earth Sci­ences in Stras­bourg. The earth­quakes mea­sured are very often below mag­ni­tude 1.5 and are not felt by the pop­u­la­tion. “In some spe­cif­ic cas­es, a few earth­quakes of a high­er mag­ni­tude occur: for exam­ple, between 2019 and 2021, three earth­quakes of mag­ni­tude 3 to 3.9 were record­ed dur­ing the prepa­ra­tion phase of the Venden­heim very deep site, in the north of the Stras­bourg Eurome­trop­o­lis,” says Jérôme Vergne.

Most of the seis­mic activ­i­ty is gen­er­at­ed dur­ing the hydraulic stim­u­la­tion phas­es: a mix­ture of water and addi­tives is inject­ed under pres­sure to improve the cir­cu­la­tion of geot­her­mal flu­ids in the reser­voirs. “These reser­voirs are nat­u­ral­ly per­me­able due to pre-exist­ing cracks and faults, nat­ur­al frac­tures along which earth­quakes can occur,” explains Jérôme Vergne. Inject­ing water mod­i­fies the pres­sures on these frac­tures and can gen­er­ate seis­mic rup­tures. These are called induced earth­quakes. Some­times larg­er earth­quakes are record­ed, such as the Pohang earth­quake (South Korea) in 2017 with a mag­ni­tude of 5.4 (the largest asso­ci­at­ed with a geot­her­mal project). “In this case, geot­her­mal ener­gy did not induce an unprece­dent­ed earth­quake, but rather a trig­gered earth­quake,” com­ments Jérôme Vergne. Geot­her­mal exploita­tion accel­er­at­ed the occur­rence of an earth­quake that would have tak­en place nat­u­ral­ly lat­er, it was the last straw.  A pre­ven­tion sys­tem – called “traf­fic lights” – is put in place for each high-ener­gy deep geot­her­mal project. In Illkirch-Graf­fen­staden and Venden­heim, it pro­vides for the switch to rein­forced vig­i­lance as soon as an earth­quake of mag­ni­tude 1.5 is record­ed, and a grad­ual stop for any earth­quake reach­ing mag­ni­tude 2.

Anaïs Marechal 
1Web­site con­sult­ed on 30/03/2023: www​.geot​her​mies​.fr
2Min­istry of Ener­gy Tran­si­tion, 2 Feb­ru­ary 2023, Geot­her­mal ener­gy: an action plan to accel­er­ate.
3Ademe, Coûts des éner­gies renou­ve­lables et de récupéra­tion en France, édi­tion 2022.
4Web­site con­sult­ed on 31/03/2023: https://​nea​.is/​g​e​o​t​h​e​r​m​a​l​/​t​h​e​-​r​e​s​o​urce/
5IEA (2019d), World Ener­gy Sta­tis­tics and Bal­ances 2018 (data­base), www​.iea​.org/​s​t​a​t​i​s​tics/; IEA (forth­com­ing), World Ener­gy Out­look 2019.
6Web­site accessed on 30/03/2023: www​.geot​her​mies​.fr
7IEA (2019), Renew­ables 2019, IEA, Paris https://​www​.iea​.org/​r​e​p​o​r​t​s​/​r​e​n​e​w​a​b​l​e​s​-2019, License: CC BY 4.0
8Web­site accessed on 30/03/2023: www​.geot​her​mies​.fr

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