Geothermal energy is often associated with home heating and supplying district heating networks. However, it’s less well known that it can also be used to produce cooling. What are the different techniques? Let’s take a closer look.
Did you know that geothermal energy isn’t just used to produce heat and electricity, but also cooling? In the face of rising global temperatures and more frequent heat waves, geothermal energy offers an eco-friendly, cost-effective, and discreet way to cool or air-condition buildings. In particular, it could help cities combat urban heat islands, which are exacerbated by the use of air conditioners.
There are two main options for generating cool air or cooling using geothermal energy:
- Directly utilizing the coolness of the subsurface, which remains stable throughout the year (10 to 15 °C in Europe): this is known as free cooling or passive geothermal cooling (geocooling);
- Using the heat from the subsurface to power a geothermal heat pump (HP) in cooling mode: this is referred to as “active cooling.”
Free cooling
This passive cooling system is used primarily during the off-season, when only slight cooling is needed, and in the summer when temperatures are not extremely high. It is particularly well-suited for multi-unit housing and office buildings.
How does it work? At a certain depth below ground, the temperature remains stable year-round. In France, for example, at a depth of 10–20 meters, the temperature ranges between 8 and 16 °C. Free cooling involves directly using this underground coolness to cool buildings, without using a heat pump (see below).
This involves circulating a fluid designed to carry the coolness (known as a heat transfer fluid) through geothermal probes placed vertically or horizontally underground, then transferring this coolness directly into the building’s system (cooling ceilings or floors, convectors, etc.).
The advantage of free cooling is that it consumes very little energy. With certain systems, 1 kWh is enough to produce 50 to 60 kWh of cooling. Its coefficient of performance (COP) is said to be 50 to 60. Another advantage: it recharges the ground with heat in the summer, which can improve the efficiency of geothermal heating systems operating in parallel during the winter.
A super-simple cooling method: the Canadian well
This system operates without a heat pump (see below). It involves circulating air from a building through a pipe buried at a depth of more than one meter, where the ground is minimally affected by outdoor air temperature.Une méthode de rafraîchissement ultra simple : le puits provençal
Actively producing cooling: geothermal heat pumps
The geothermal heat pump (HP) relies on using underground heat to heat homes or supply district heating networks, but also to cool the air inside a building.
The principle is very simple: heat from the ground is transferred to the refrigerant circulating in the heat pump’s evaporator; it thus turns into a gas, is then compressed in the compressor, and becomes a heat source.
- Geothermal heat pumps operate either with horizontal collectors placed 1 m deep or with vertical collectors that extend deeper, up to 100 m (closed-loop system);
- Pond/lake heat pumps rely on a vertical collection system connected to the groundwater table comprising two wells: a pumping well to extract water, and a reinjection well where the water is returned (open-loop system).
To cool a building, so-called “reversible” geothermal heat pumps must be used. These are systems capable of reversing their operating cycle to produce cooling by capturing heat from the building to be cooled and then releasing it back into the ground. To simultaneously produce heat and cooling, heat pumps called “simultaneous heating and cooling systems” are used.
Key facts
- The COP of a geothermal heat pump is 4 to 5. In other words, for every 1 kWh of electricity consumed, 4 kWh of heat are produced.
- The energy efficiency of a heat pump is much higher than that of an air conditioner, as it uses energy from the air rather than generating it.
Two projects using geothermal energy to produce cooling
The Olympic Village and the Pleyel district in Saint-Denis, France
With rising summer temperatures, demand for air conditioning systems is skyrocketing. In December 2023, a geothermal power plant was commissioned in Saint-Denis to supply renewable energy to the district cooling and heating network, which serves 600,000 m2 of buildings, including the Athletes’ Village for the 2024 Olympic and Paralympic Games and the Pleyel district.
The plant is powered by 11 geothermal wells 50 to 70 meters deep, where water is maintained at a constant temperature of 14 °C. These wells are connected to three heat pumps that simultaneously produce:
- cooling (water at 5 °C supplying the cooling networks, particularly ground-source systems);
- heat (water at 65 °C supplying the heating and domestic hot water networks).
This solution would provide the neighborhood with 68% renewable energy and prevent the emission of nearly 5,000 tons of CO2 per year.
The Montreal Biodome in Canada
The Biodome is a “living museum” located in the former velodrome in Montreal, Canada. It features five different ecosystems within a single building: tropical rainforest, underwater environment, Canadian forest, the Canadian East Coast, and a sub-Antarctic island. It “feels like a walk in nature” and allows visitors to observe 200 animal species and 400 plant species.
To maintain very different environments (both tropical and polar) within the same building without skyrocketing energy costs, an energy recovery system and comprehensive heat flow management had to be implemented during renovation work completed in 2010. This was made possible by the installation of four geothermal heat pumps connecting the various ecosystems, utilizing a 16°C underground aquifer located 20 meters beneath the building. It is the largest open-loop geothermal system of its kind in Canada.
The extracted water heats or cools the heat pump refrigerant and is then returned to the aquifer 150 meters from the pumping point. During peak periods, geothermal energy thus provides the additional energy required. This heat recovery is sufficient to heat the building 50% of the time (on an annual basis).
The pools that must be kept at temperatures lower than the ambient air are mechanically cooled; the energy extracted is then released into the heating network or used to heat the warm pools.
This project enabled the Biodôme to reduce its energy bill by 52% and its greenhouse gas emissions by 80%.
Sources
- ADEME
- Connaissance des énergies
- Géothermies
- Syndicat des énergies renouvelables
- Les clés de la transition énergétique
- Espace pour la vie – Biodôme
- Voir Vert
By Véronique Molénat, science editor