Michèle CynaChair of the Geothermal Energy Committee at the French Renewable Energy Syndicate (SER)
"Home geothermal energy systems offer bright prospects for producing heat, refrigeration and cooling thanks to their remarkable efficiency, very low operating costs, and increasingly light regulatory framework."
Using Geothermal Energy to Produce Heat, Refrigeration and Cooling
Home geothermalDescribes the technology used to tap subsurface heat to produce energy... energy systems offer bright prospects for producing heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... , refrigeration and cooling thanks to their remarkable efficiency, very low operating costs, and increasingly light regulatory framework. While they are popular in new office buildings, they have a harder time convincing individual homeowners. Michèle Cyna, Chair of the Geothermal Energy Committee at the French Renewable EnergyEnergy sources that are naturally replenished so quickly that they can be considered inexhaustible on a human time scale... Syndicate (SER), reviews developments in the field.
Heating, refrigeration and – as we shall see – cooling from underground sources thanks to geothermal energy is an idea that no doubt deserves more attention. This article focuses on very low‑temperature geothermal energy, which relies on heat pumps and is thus a separate phenomenon from the geothermal energy that provides high-temperature heating or is used to generate electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... . The very low-temperature variety represents three-quarters of the geothermal energy produced in France, and its regulatory framework has been relaxed since 20141. It is currently possible to install systems between 10 and 200 meters deep, without too much red tape, in approximately 98% of France. The equipment can supply individual houses as well as large office buildings or city blocks.
Two Main Geothermal Techniques
The first technique used to harness geothermal energy is to pump water up from a phreatic zone. These zones can be found as little as 10 meters below the surface, because they are not underground “lakes”, but rather water that circulates, sometimes in small quantities, in the pores and fractures in the ground. The water exchanges its heat via a serpentine heat exchanger with the system installed in the building or the house in question.
The second technique, if there is no phreatic zone, is to install a probe in the subsoil. The probe creates a loop containing a heat transfer fluid which, as with the first technique, transfers its heat to the surface system via a heat exchanger.
The Rise of Geocooling
A heat pump connected to a circuit can produce heat in winter and cold air in summer, via an air conditioner. But very low-temperature geothermal energy can also provide “cool” temperatures. This new concept of geocooling has particularly high potential in the more temperate zones in France.
The water or heat transfer fluid is usually between 15°C and 20°C all year round. In the summer, the fluid brings the water in the building’s circuit to the same temperature, without even going through the heat pump. It will not provide true air conditioning, but in our climate it will guarantee a temperature about 5°C lower than the outside temperature. For example, when it’s 30°C outside it will be 25°C inside the offices. This is enough to create a feeling of coolness and comfort. Of course, if the temperature stays at 35°C for a long time, it might be necessary to use a more powerful air conditioner.
Geocooling – cool, not cold, that is – has an extraordinary coefficient of performance (COP) of about 60. The COP is the ratioIn mathematics, the relationship between two numbers (the ratio of x to y is equal to x/y). Often expressed as a percentage... of the electrical energy needed to run the pumps to the energy that the system produces. In order to lower the temperature by the same 5°C, a traditional air conditioner would have to use 60 times more electricity.
Another absolutely remarkable application is the tempered water loop. Instead of bringing water in just one building to a constant 15°C to 20°C, it is circulated around a loop that supplies an entire neighborhood. Decentralized heat pumps are added to the circuit and produce heating and cooling for an entire group of buildings. The new Airbus headquarters in Toulouse uses this system. After more than two years, the feedback is very positive. The back-up air conditioners, installed in case of severe heat waves, have never been used...
Very low-temperature geothermal energy is being used more and more in new office buildings, particularly in the Paris region, for example in the Aéroville mall near Roissy-Charles de Gaulle airport. In addition to the recognized efficiency factor, the developers benefit from the opportunity to receive an ecolabel. On the other hand, the market is generally sluggish for individual homes. The risk of drillingThe process of boring a hole into the ground using special equipment... a 10-to-20-meter well and not striking a phreatic zone often scares off homeowners. Lower fossil fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... prices and improved insulation for homes are also a disincentive for seeking less costly energy solutions. It’s a shame, because these installations might be an expensive upfront investment, but their operating costs are unbeatably low...
Michèle Cyna is a graduate of École Polytechnique, École Nationale des Ponts et Chaussées, and MIT. Formerly Vice President, International Business Development at Veolia Transport, she is currently the Chief Executive Officer of BURGEAP, an environmental engineering group specializing in air, water and soil pollution, energy, health, the industrial environment, and nuclear powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... and plant dismantling. Michèle chairs the Geothermal Energy Committee at the French Renewable Energy Syndicate (SER).
(1) In France, regulations allow “minimal-impact geothermal energy” projects to be carried out following registration or a streamlined approval process.