Feature Report: What Is Geothermal Energy?

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Creation date 20 May. 2019

Op-ed articles

How geothermal energy works in France

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 (COPThe "Conferences of Parties" (COP) bring together every year since 1995 ... ) 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.

Louis-Marie DenoyelProject leader for Renewable Heat at the French Ministry for the Ecological Transition
"Geothermal energy is recognized by the French public authorities as one of the most promising sources of renewable heat (...)"

The Outlook for Geothermal Energy in France

GeothermalDescribes the technology used to tap subsurface heat to produce energy... energy is recognized by the French public authorities as one of the most promising sources of renewable heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... , alongside wood fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... , heat pumps, solar heating and biogasA product of the methanation (anaerobic digestion) of organic waste... . It also shows significant potential for “renewable cooling”, offering a more virtuous solution than traditional air conditioners. Louis-Marie Denoyel, project leader for Renewable Heat at the French Ministry for the Ecological and Inclusive Transition, explains.

There are several different types of geothermal energy, categorized depending on depth and temperature. They can be harnessed for a wide variety of individual and collective uses.

Deep low-temperature geothermal energy, between 30°C and 90°C, is well suited for district heating networks supplying neighborhoods of several thousand people, or industrial parks. In France, this type of heat generally lies at depths of 1,000 to 2,500 meters in aquifers, which can be formed from the remainders of ancient seas trapped in the porous layers of sedimentary basins, for example. The Dogger aquiferAn aquifer is a geological formation (or rock) that acts as a reservoir... in the Paris Basin, and the Aquitaine and Lorraine Basins, have the most potential in the country. It would be beneficial to carry out precise exploratory drillingThe process of boring a hole into the ground using special equipment... tests in order to pinpoint the locations and characteristics of the aquifers.

Once a pumping station has been installed, water is brought to the surface and a heat exchanger transfers its heat to a district heating system. Then the water is returned to the aquifer. The advantage is that the pumping station, which the underground water flows through for a short time, takes up little space, discharges no pollutants or noise, and is easy to maintain. After the initial investment to drill the hole and set up the district heating system, the operating costs are practically nothing for a foreseeable period of at least 30 years and often as much as 50 or 60 years. The main challenge is not to drill the holes too close together, so as not to exhaust a depositAn accumulation of natural resources, such as oil, natural gas, coal, uranium, metal ore or another commodity... .

Deep high-temperature geothermal energy, with temperatures above 150°C, can be used to generate electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... . In France, this is already happening at Soultz-sous-Forêts in Alsace and Bouillante in Guadeloupe. There are also new projects, but the new multi-year energy plan has put the priority for public funding on recovering geothermal heat, which is less costly and more efficient than generating electricity.

Very Low-Temperature Geothermal Energy

When the water temperature is below 25°C, it is called very low-temperature geothermal energy. If drilling is not dangerous or harmful to the environment (shallow drilling, low-temperature water, low flow rate, etc.), it is referred to in France as “minimal-impact geothermal energy” and is subject to highly simplified regulation. Geothermal heat pumps are used.

This very low-temperature sector has very interesting potential: the resource can be used for heating and (or) cooling. This possibility will become more valuable in the future, as the European Union will soon be defining the concept of “renewable cooling”, which will most probably include the use of such geothermal resources.

Geothermal cooling has at least three major advantages over traditional air conditioning:

- Geothermal heat pumps are more efficient than air-to-air units and therefore consume less electricity. For this reason, there is less risk of overloading the 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... grid during a heat wave.

- A collective geothermal project in an office complex, for example, makes it possible to have a central system, so the refrigerants can be more efficiently managed than if they were in individual air-conditioning units. Refrigerants have a considerable effect on global warmingGlobal warming, also called planetary warming or climate change... . On a scale where CO₂See Carbon Dioxid is one and methane (ch4)The main component of natural gas deposits and oil deposit gas caps. Methane is produced naturally by landfills... is four, most refrigerants used today rate above 600! Given that the use of air conditioning is climbing exponentially worldwide, leaking refrigerants are no minor problem.

- Lastly, if everyone uses an air-to-air unit supplied with outside air, a “heat island” effect can cause a significant rise in local temperatures. With geothermal energy, the heat is re-injected into the ground and warms back up the resource, which can be used for heating in winter.

Geothermal heat systems, particularly large-scale projects, are supported by the public authorities through a “heat fund” managed by the French Environment and Energy Management Agency (ADEME) since 2009. Available funds rose by 20% in 2018, and 20% more was budgeted for 2019, bringing the total to €307 million. In 2018, 10% of the fund was dedicated to geothermal energy.

Louis-Marie Denoyel has been a project leader for Renewable Heat since September 2018 at the Directorate General for Energy and Climate, part of the French Ministry for the Ecological and Inclusive Transition. He is a graduate of École Polytechnique and the University of Cambridge, and is a Bridges, Water and Forests engineer. After studying engineering and economics, Louis-Marie chose to focus on European public energy policy and photovoltaic self-consumption. He is currently working to develop the renewable heat sector, among other things.