Geothermal heat: the great forgotten renewable energy

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.

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.

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.”

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.

Anaïs Marechal