What is geothermal energy

Geothermal energy, from the Greek “geo” (earth) and “thermos” (heat) corresponds to both the science that studies the earth’s internal phenomena, the industrial applications that can generate electricity and/or heat and the renewable energy.

There are two different kinds of geothermal energy:

• Shallow geothermal energy that exploits only sub-surface resources in order to heat/cool individual or collective buildings (with a heat pump)
• Deep geothermal energy that exploits deeper resources in specific contexts to produce electricity and/or heat for cities or industrial applications

GEODEEP members are specialized in the three main domains of deep geothermal energy and in specific applications of shallow geothermal energy.

Enhanced Geothermal System (EGS) is a technology that extracts hot water (150°C/200°C) from naturally fractured rocks in order to produce electricity and/or heat. According to the specificities of the context, the wells can reach between 2,000 and 5,000 meters). Some stimulations are necessary in order to facilitate the circulation of the fluid and the thermal exchanges between the production well and the injection’s one. Recently, companies are also trying to extract metals directly from the geothermal brine such as Lithium. Indeed, it corresponds to an additional resource that can be co-produced in addition to the production of electricity and or heat. Moreover, these metals are necessary to support the energy transition. A demo-site will be operating in France in the next years.

In volcanic areas (active or recent), the geothermal gradient can be very important. In a reservoir situated between 500 and 1500 meters depth, the temperature can reach 200°C/300°C, that is impossible in a common context. The geothermal brine is circulating through a highly fractured reservoir, close to melting rocks. The idea is to produce electricity directly from the geothermal water and vapor (flash power plant) that powers a turbine and an alternator. It is therefore possible to use heat as direct utilization.

In sedimentary context, the technic of doublets is the most common. It consists of two boreholes (or more): one is pumping hot water from a deep aquifer that has very high permeability (production well) and the other is injecting the warm water after utilization at the surface (reinjection well). It can be used to supply district heating, agricultural greenhouses or thermal centre. This technique has been proven since a long time. Indeed, since 1970’ more than 60 operations were carried out in Paris area. Nevertheless, there are still innovations like horizontal drilling. It allows to extract more water because the well is longer in the reservoir. It has been done in Cachan in 2017 and it corresponds to the first geothermal horizontal drilling in the world.

GEODEEP members are also working in shallow geothermal energy. Indeed, there are several temperate geothermal water closed-loop running in France and this concept is emerging (also known as thermal smart grid). It consists of a kind of district heating supplied by shallow resources (shallow aquifer, geothermal vertical loops, lake or sea water, wasted water, …). Therefore, the temperature in the loop is low (around 10-20°C) and there are heat pumps in each building/group of buildings. The idea is to be multi-energy, to share the energy between buildings, to evolve through the development of projects in order to be efficient and effective.