[Un article de The Conversation écrit par Benjamin Brigaud – Professeur en géologie et géothermie, Université Paris-Saclay]
But turning to air conditioners to cool our buildings asks the question: these devices consume a lot of electricity.
Globally, the cooling of buildings represented, in 2016, 18.5 % of total electricity consumption, against only 13 % in 1990. The International Energy Agency (AIE) also tables a high increase in the number of air conditioners on a global scale, with a rate of household equipment which should drop from 30 % in 2016 to more than 60 % in 2050. electricity in the coming decades.
The other drawback of air conditioners is that, to produce cold, they reject heat outside – which contributes locally to urban overheating.
For example, in Paris, air conditioners are typically small equipment of this type, from 6 to 70 kW per unit. It has been shown that their use could increase the temperature in Paris in summer by 0.5 to 2 ° C, thus amplifying the well -known urban heat island effect in cities. During heat waves, this microclimate will be all the more exacerbated as the number of heat waves per year will increase in the future, going from nine identified in Paris between 1960 and 1990 where the maximum temperature reached 38 ° C, potentially one per year by 2100.
A solution to cool buildings without overheating the outdoor air would be to mobilize the freshness of our basement. Geothermal energy thus represents a serious track not only for heating in the cold season, but also for summer comfort.
An economical solution that does not generate heat island
A few meters deep, in France, the basement is at a constant temperature of 12 ° C all year round. Then on average, the temperature increases depending on the depth, by around 3.5 ° C every 100 meters. 100 meters deep, this temperature is therefore 15/16 ° C, and 200 meters deep, it is around 20 ° C. At less than 200 meters, the basement therefore represents a source of freshness, if a temperature is targeting less than 20 ° C.
This freshness is abundant, mobilized almost everywhere on our territory, and can thus be produced locally. It is a low carbon and economical cooling solution, not intermittent and almost inexhaustible. The solution would therefore be to better use this source of freshness in summer, and vice versa, heat in winter.
Mobilizing the freshness of the basement to produce cold and refreshing buildings is a mature technical solution, already deployed all over the world. It consists of transferring calories from the basement to buildings through one or more boreholes a few meters a hundred meters deep, supplying a geothermal heat pump (PACG). This geothermal energy is called “surface geothermal energy”, with drilling of a depth less than 200 meters.
How does a geothermal heat pump work?
Currently, this technology is mainly used for heating. However, if the PACG is reversible, it also makes it possible to produce cold in summer. However, this type of heat pump (CAP) is currently used negligible in France.
For pacg heating, under conventional surface geothermal energy, two techniques are mainly implemented.
The first (right-wing illustration on the diagram opposite) consists in capturing groundwater at around 12 to 15 ° C to produce hot. As calories are taken from this water for heating, colder water (a temperature between 5 and 10 ° C) is then reinjected at the same depth in another drilling located several tens of meters. We then speak of geothermal doublet on a water table, that is to say using groundwater.
The second technique (leftist illustration on the diagram above) is that of the PACG on vertical geothermal probe. It consists in placing in a drilling hole, at a depth of a few meters at a hundred meters, a geothermal probe. This is generally made up of a U U-density polyethylene tube and containing a heat transfer fluid made up of glycolé water. As the heat transfer fluid is reinjected at a temperature of approximately 0 ° C, the basement cools from a few degrees in depth, typically from 3 ° C to 1 meter from the drilling, which remains sufficient to warm the heat transfer fluid which feeds the PACG.
The two techniques both supply a PACG, whose performance coefficient (COP) is generally 4 to 5. That is to say that for 1 kWh of electricity consumed by the PACG, 4 to 5 kWh of hot or cold will be returned. It is much more effective than capturing outdoor hot air to supply an aerothermal (PACA)-that is to say, operating in the air-to produce cold. In addition, the heat emitted by the PACG is injected into the basement, and not in the outside air as with a PACA, avoiding creating urban heat islands (ICU).
Another technique is that of “geocooling”. The freshness of the basement is then directly used in the ventilation and cooling networks of the building (floor/refreshing ceiling, ventilo-converse, air treatment center, etc.), without supplying PACG. It is then a so-called passive refreshment insofar as it does not consume almost no electricity, apart from the circulation pumps of the heat transfer fluid in the basement and the building. It is characterized by unbeatable performance coefficients: from 30 to 50! That is to say, for 1 kWh consumed, 30 to 50 kWh of freshness can be returned.
Store heat in the basement for the winter
Another advantage of geothermal energy for the cooling of buildings is that the heat evacuated in the basement in summer by the PACG can be very useful a few months later, in the cold season. Indeed, this makes it possible to warm up the basement locally of a few degrees around the probes, or to reinject warmer water (up to 25 ° C) directly in the aquifer, which allows to store heat.
As this is a reversible heat pump, you can reverse the system between summer and winter. We can then “recover” in summer the calories lost in winter, which increases the annual yield of the geothermal installation.
This heat storage can still be amplified by the addition of additional resources, such as thermal solar panels (heated water can be directly reinjected in the wells) or photovoltaic (the electricity produced in excess the day in summer then feeds a resistance which heats water). In the case of industrial buildings, we can also recover the so -called fatal heat, produced by industrial processes (for example, by data centers) to heat the water reinjected in the wells.
It is therefore a question of reheating the basement actively, either by reinjecting hot water (at around 40-50 ° C) directly in the phreatic tablecloth, or in the basement via Geothermal probes. This “reversible” geothermal energy, with a coupling of heating in winter and refreshing in summer, is the great advantage of this resource, especially since this coupling allows the optimization of heating and refreshing systems.
Political ambitions and unexploited potential
Guest on June 29, 2025 during the Grand Jury RTL-Le Figaro-Public Senat-M6, Prime Minister François Bayrou recalled that geothermal energy allows you to refresh by saving more than 90 % of electricity. When he was a high commissioner, he himself recognized the interest of developing surface geothermal energy (less than 200 meters deep) to heat and cool buildings.
During his visit to the Operational Center for Crisis Management of the Ministry of the Interior on 1er July 2025, the Minister of Ecological Transition Agnès Pannier-Runacher recalled the issue to be refreshed without returning heat outside.
Despite its potential, geothermal energy for heating (excluding electricity production) has produced only a million terajoules (TJ) in 2020, or only 0.17 % of the world's annual energy production, which amounted to 580 million TJ. In France, geothermal energy produced 7 terawatt hours (TWH) of heat in 2023: 2.3 TWh for deep geothermal energy and 4.7 TWh for surface geothermal (used for PACG)… or 0.49 % of the 1,420 TWh of energy produced in France in 2023.
In our country, PACG today represent only 2 % of the PAC park. The cost of their installation can be a brake: it represents double that of a gas boiler. But once amortized, their operating cost is very low, and even lower than the operating cost of an air heat pump (PACA).
The installation of a PACG can also be complex, as it requires drilling the basement before setting up the heating and air conditioning system. Few professionals master this technology or offer it spontaneously, it is often implemented on the owner's initiative.
There is a real subject on how to develop geothermal energy and make it more incentive. There is no doubt that the next few years will be hinged to better develop the sector. On July 28, 2025, the Ministry of the Economy presented its seven measures to accelerate the development of renewable and local energy. For example, simplify administrative procedures and improve communication on geothermal energy.
Another of the measures also aims to reduce the confidentiality deadlines for certain geological exploration data: today, companies that make drilling to study the basement must keep their confidential results for 10 years. This would enhance them more quickly to launch new geothermal projects while reducing operating costs.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
