Feature Report: What Is Geothermal Energy?

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Deep Low-Temperature Geothermal Energy

The principle behind deep low-temperature geothermal energy is the same as for other low-temperature varieties, except that the wells descend further below the surface, to depths of 200 to 2,500 meters, and that the water temperatures are higher, generally between 30°C and 90°C. The water is captured via one well then reinjected underground through another, forming a “doublet”. This technique can be used to heat entire neighborhoods with thousands of inhabitants or industrial parks.

fr- La géothermie basse énergie : le chauffage
Geothermal energy can be used to heat facilities such as greenhouses, like this one in Aigueperse in France's Auvergne region. © STF / AFP

How Deep Geothermal Energy Works

Deep geothermal energy can be used to heat entire neighborhoods or industrial parks.

Deep geothermalDescribes the technology used to tap subsurface heat to produce energy... facilities capture groundwater and water from aquifers and bring it to the surface through a production well, before transferring its energy to the local system via a heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... exchanger. A network of pipes at the surface dispatches the heat to substations located at the base of each building in the neighborhood or industrial park. The water is then returned underground via an injectionSecondary oil recovery method in which water or gas is injected into a deposit to increase pressure in the reservoir... well.

The production well and the injection well must terminate several hundred meters apart underground to ensure that the cooled water is not introduced directly back into the hot spring, thereby depleting the resource. Measures must also often be taken to protect pipes from rapid corrosionThe gradual degradation of a material by a physical or physicochemical reaction... , as the water in deep aquifers typically contains high levels of salt.

This method has several advantages. These aboveground facilities are compact and quiet, and do not release any pollutants or consume water. They are also easily maintained and, after the initial investment to drill the wells and set up the district heating system, the operating costs are extremely low for a period of around 50 years.

Two Case Studies in the Paris Region

- The Clichy-Batignolles neighborhood in Paris (see Close-Up: Clichy-Batignolles: Eco-District Energy Solutions).
 To provide sustainable heat to the new eco-district, a geothermal facility draws water from the 600-meter-deep Albien aquiferAn aquifer is a geological formation (or rock) that acts as a reservoir... , which serves as Paris’ emergency drinking water supply. The water is extracted at only 30°C – putting it in the lower range of temperatures possible for this type of geothermal energy use – then heated to 65°C using heat pumps. Its temperature has dropped down to 12°C by the time it returns underground. The two wells terminate 650 meters apart in the aquifer.

- Orly Airport. Since 2010, a geothermal doublet has been operating in the Dogger aquifer, located nearly 1,800 meters below the surface. Water at 74°C is extracted at a rate of 300 cubic meters per hour and transfers its heat to a pipe network, which then supplies heating to a portion of the airport terminal and office buildings. The water is around 35°C when it is returned to the aquifer. The facility, which has a thermal output of 10 megawatts, reduces fossil fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... consumption by an estimated 4,000 metric ton of oil equivalent (toe)Unit of energy measurement corresponding to the energy produced by the combustion of a ton of oil... per year.

Medium-Temperature Geothermal Energy

Geothermal energy above 90°C (sometimes called “medium temperature”) can be used for specific industrial applications, including drying textiles, manufacturing paper pulp and processing biomassIn the energy sector, biomass is defined as all organic matter of plant or animal origin... .
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... generation becomes possible at temperatures of 150°C and above.
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