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Geothermal energy, a source of heat, cold and electricity
New energies 14/09/2021

Geothermal energy, a source of heat, cold and electricity

Geothermal energy is a persistent local resource, a mature, low-carbon technology that can not only produce heat and cold, but also be used to generate electricity.

An article from the Special Issue of Pour la Science magazine : "Energy up above, down below"

Geothermal energy is therefore an ideal part of the energy mix, since it mitigates the disadvantages of other, mainly intermittent, renewables such as wind or solar power.

Today the idea is to exploit this resource as best we can. Low temperature geothermal systems that tap into shallow aquifers can meet the heating and cooling needs of houses, several buildings and even an entire Eco district.

At the end of 2020 in France, drilling began on a deep well at the Cité Descartes (an ecodistrict in Champs-sur-Marne near Paris) with the objective of installing a geothermal heating plant within the coming year. Connected to a 19-kilometre distribution network, this installation will pro-vide heat to the equivalent of 10,000 homes. People previously thought that geothermal energy - heat that comes from the radioactive decay of elements naturally present in the Earth’s mantle and crust - was reserved exclusively for countries such as Iceland, but it now has the wind in its sails in France. 

The reason is that this source of energy is in fact both abundant and renewable. The principle is simple, as the temperature of the Earth increases with depth (with an average geo-thermal gradient of the order of 30° C per kilometre), the idea is to pump hot water and deep underground aquifers.

Cooling ? 

As strange as it might seem, the answer is yes: geothermal heat pumps provide a low-carbon cooling solution. In the case of geothermal fluid temperatures of at least 25° C, geothermal energy can be used as part of a wider scale urban heating network and even, if temperatures reach 110° C, be used to produce electricity. But more about that later.

In addition to being renewable, the undeniable advantage of geothermal energy is that it is available 24/7, in other words it is a base load energy source (to use the technical term), which means that it is not intermittent and can therefore continually supply the minimum level of demand of an electrical grid or heating network. Geothermal energy is therefore an  ideal part of the energy mix, since it mitigates the disadvantages of other, mainly intermittent, renewables such as wind or solar power.

The french experience

Like Champs-sur-Marne, several towns in France have opted to use this energy source to heat or cool public buildings and collective housing. An urban heating network is made up of one or several geothermal plants that distribute heat to users by means of a network of pipes running under the town, sometimes several kilometers long.

A typical geothermal facility includes a production well and an injection well (to reinject water back into the aquifer). On the surface, heat exchangers transfer heat from the geothermal fluids to a clean water network, which in turn heats the buildings and provides domestic hot water (see figure opposite). In other words, it isn’t the same water taken from deep underground that circulates through the pipes!

In a geothermal plant, water (in red) tapped from the ground transfers its heat to a distribution network (in the boiler room) before being reinjected (in blue).

The Île-de-France region (and the Val-de-Marne department in particular) has invested heavily in geothermal district heating. 

Val-de-Marne boasts a greater density of geothermal facilities in operation than anywhere else in the world and, with around 20 distribution networks, this source of energy represents more than a third of total heat production. The explanation lies in the favorable geological conditions: the area is situated above the 1.7-kilometre-deep Dogger aquifer with water temperatures between 60 and 75° C and particularly good hydrodynamic properties (see figure below).  

The so-called doublet system is used: hot water moves in a closed loop from the production well through the heat exchanger where its energy content is extracted (10 to 20 megawatts), after which, now cooled, it returns to the aquifer via the injection well.

This interest in geothermal energy in the Île-de-France region dates back to 1975-1986, i.e. the years fol-lowing the first oil crisis. In 2010, there was a renewal of interest, notably thanks to the support of the French environmental agency l’ADEME [1] , which works to facilitate the energy transition. Growth has been non-stop ever since and the French PPE (multiannual energy plan), created by the "loi de transition énergétique pour la croissance verte" (energy transition law in favor of green growth) aims to double the use of geothermal energy by towns and industry by 2028.

Agence De l'Environnement et de la Maîtrise de l'Energie

Thanks to its waters at a temperature between 50 and 85 °C, the Dogger aquifer (located at a depth of 1,500 to 2,000 metres) is an invaluable geothermal resource for the Île-de-France region. 

It is within this general context that, over the last ten years, ENGIE has developed ten new geo-thermal plants, the most recent of which have been installed (in addition to Champs-sur-Marne) in Vélizy and Bordeaux. The Bordeaux project was particularly innovative. The idea was to explore a Jurassic formation of unknown geothermal potential (actually non-existent as it is not a water-bearing formation) located at an approximate depth of 1.7 km, whilst having a back-up plan as the same well could tap geothermal energy from a 900 m deep Cretaceous aquifer, whose resources were already known. Admittedly the aquifer is not as deep and is less interesting as a source of geothermal energy, but at least, the energy recovery is guaranteed and satisfactory.

In addition to these new installations, around forty geothermal facilities commissioned in the 1980s have also been overhauled. In total, France possesses around one hundred installations of this type, of which 50 are in the Île-de-France region. Such a density is a guarantee of unique expertise, which French companies can export to the rest of Europe where there is certainly no lack of geothermal potential.

Geothermal electricity production

Geothermal energy not only provides heat and cold, but electricity as well. Italy has been leading the way since 1913 and notably in Tuscany, a region whose geology is highly favorable: magmatic activity and the thinning of the Earth’s crust in this region result in a higher geothermal gradient of around 100° C per kilometer. The first geothermal power plant in Larderello had an initial capacity of 20 kilowatts: today its capacity is in excess of 800 megawatts.

In the 20th century, the visibility of geothermal energy increased thanks to numerous sites all over the world where geological conditions favored the exploitation of this resource. Scientific progress and the most recent techniques would even offer the possibility of extracting lithium from some geothermal water, which can then be used for electric mobility solutions. ENGIE is contributing to this research in collaboration with its academic and institutional partners by designing and testing imaging technologies to be used in steam fields, as well as gas injection technologies.

In 2020, the world installed geothermal electricity generating capacity was 15.9 gigawatts (the equivalent of 16 nuclear reactors), in other words less than 1% of total production. Currently in use in 29 countries whose geothermal resources are both abundant and readily accessible, there is therefore a large potential for development.

Further afield, ENGIE and its partners have recently started production at the 85 MW Murah Laboh geothermal plant in Indonesia, whereas in France the objective is now to investigate other regions in mainland and overseas France and carry out the geoscientific studies that are a prerequisite for any project.

Ultimately, by developing urban heating and cooling networks and producing electricity using geothermal resources, the ambition is to contribute to the growth of a low-carbon energy mix.

GECO: a EU research program  to reduce greenhouse gas emissions  from geothermal energy production

Storengy is participating in the GECO (Geothermal Emission Control) project, a European project with 9 partner countries, aimed at developing techniques to reduce greenhouse gas emissions from geothermal power plants. As part of GECO, Storengy is developing a test unit to demonstrate the feasibility of total reinjection of non-condensable gases (NCG) into a high-temperature well (>200°C). 

Storengy is collaborating with Iceland GeoSurvey (ISOR) to implement a closed loop demonstration test unit and a specific well completion. This reinjection pilot will be tested at Hveragerði (Iceland).

In addition to the progress towards total control of emissions in geothermal energy, this technological development will allow on the one hand a better acceptability of geothermal electricity projects as it will allow a harmonious integration in the landscape, and will benefit on the other hand the production thanks to the maintenance of pressure in the exploited reservoir. 

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 818169.

This article was writen by 

Delphine PATRIARCHE - R&D Key Expert Storengy - Responsable du programme GeoEnergy Lab d'ENGIE
Olivier RACLE - Director District Heating & Cooling Solutions - ENGIE
Nicolas MONNEYRON - Geothermal Global Expert - ENGIE

For the 
Special Issue of Pour la Science magazine  :    "Energy up above, down below"  

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