More and more countries around the world agree that deep geothermal energy is an essential alternative for producing both heat and electricity in a low-carbon way. What are its advantages? How does it work? Which geological contexts are the most suitable? What are the most promising innovations? We take stock.
Drilling down to depths of several hundred meters, or even several thousand, to extract hot fluids capable of powering turbines in power plants or supplying district heating networks. This is the simple principle behind deep geothermal energy.
Stimulated by the oil crisis of 1971, then put on hold in favor of more profitable energies from the 1990s onwards, geothermal energy has been the subject of renewed interest in recent years. It has to be said that deep geothermal energy combines a number of advantages.
An increasingly coveted energy source
First and foremost, it’s a renewable, local, low-carbon energy source that could contribute to the energy independence of nations, while helping the transition to cleaner energies. Technological advances in the field of deep geothermal energy suggest that it will soon be possible to access underground reservoirs of heat that were previously difficult to harness, and thus promote its deployment (read more below).
Secondly, unlike other renewable energies (photovoltaics and wind power), its production does not depend on the time of day or weather conditions; it is available 24 hours a day. Finally, geothermal energy requires very little land. Per gigawatt produced, geothermal power plants take up 8 times less space than coal-fired power plants or solar farms, and 3 times less than wind farms, making them easier to integrate into existing infrastructures.
On January 18, 2024, the European Parliament called for massive investment in geothermal energy to accelerate its deployment; the subject was top of the agenda at the Energy Ministers’ meeting in Budapest on July 17. In the United States, a bipartisan bill to support the development of the geothermal sector has been approved by the US Natural Resources Committee, and in mid-February the Department of Energy announced a $60 million investment to support enhanced geothermal systems.
Where does the earth’s heat used in geothermal energy come from?
Essentially radioactive elements present in the rocks making up the earth’s crust and mantle: uranium, thorium, potassium… The deeper you go underground, the higher the temperature. This is known as the geothermal gradient. On a global scale, this gradient averages 3°C every 100 meters. In volcanic zones, where magma rises, this gradient can reach 100°C every 100 meters.
Which areas are most suitable for deep geothermal energy?
Traditionally, deep geothermal energy uses hot water – in liquid or gaseous form – naturally present at depth in so-called aquifers. This type of hot water resource can be found in many different geological contexts.
First, in sedimentary basins located at a distance from tectonic plate boundaries. These are known as “intracratonic” basins. This is the case, for example, of the Paris Basin, which today boasts the largest concentration of geothermal power plants in the world. Most of these pump fluids from the Dogger aquifer, located at depths of between 1,500 and 2,000 meters.
Hot-water reservoirs are also found in areas known as ” collapse basins “. These are areas along which tectonic plates are moving apart. In this type of zone, often accompanied by volcanism when active, the Earth’s crust is thinned and the hot mantle is at a shallower depth than average; the geothermal gradient is therefore higher than average. Fluids are found in so-called “hot and humid” rocks, such as fractured granites. In Kenya and Ethiopia, for example, several geothermal power plants have been built along the South-East African rift. In France, the Rhine Basin, in which the famous Soulz-sous-Forêts geothermal power plant is located, is an ancient rift; the geothermal gradient here remains three times higher than average (8 to 10°C every 100 meters).
Finally, geothermal potential can be found in volcanic areas. These areas are particularly interesting because of their high thermal gradient. This makes it possible to access very high-temperature water at relatively shallow depths. In Bouillante, Guadeloupe, a geothermal power plant uses fluid at 250-260°C drawn from a reservoir at a depth of between 500 and 1,000 meters. Iceland, a volcanic island straddling two diverging tectonic plates, generates over 90% of its heat and 30% of its electricity from deep geothermal energy, and boasts several of the world’s largest geothermal power plants (Hellisheiði, Nesjavellir, Reykjanes…).
These three types of geological context are the ones most often targeted today, but there are others.
High or low energy?
For electricity generation, we speak of :
- medium-energy deep geothermal energy when the fluid is drawn from deep aquifers (800 to 4000 meters) at a temperature of 90 to 150°C. It is mainly used for projects located in sedimentary basins;
- high-energy geothermal energy, when the fluids used reach temperatures in excess of 150°C. They mainly concern collapse trenches and volcanic areas.
For heat production, we speak of low-energy geothermal energy. This uses fluids at temperatures between 30 and 90°C. They are found mainly at depths of 500 to 2,500 meters in widespread sedimentary formations.
How does a geothermal system work?
Deep geothermal energy is based on harnessing the hot water contained in an “aquifer” located at a depth of several hundred to thousands of meters. An aquifer is a naturally porous or fissured reservoir rock that is sufficiently permeable for water to circulate freely. This water, in liquid or vapor form, is extracted through one or more “production” wells, then conveyed to the turbines of a power plant or to an exchanger connected to a district heating network.
(Source: Syndicat des énergies renouvelables)
Once it has been stripped of its calories, the water can be discharged into the sea, into basins that become lakes, or into the air in the case of steam. However, to avoid any impact on the environment and guarantee the sustainability of the resource, cooled water is increasingly reinjected into the subsoil via “reinjection wells”.
Innovations that could accelerate the deployment of deep geothermal energy
In theory, geothermal energy can be accessed from anywhere. In reality, however, it requires drilling to great depths, which drives up costs and poses a host of technical problems. This is one of the reasons why geothermal energy has remained a “niche” energy source to this day. Today, it accounts for just 0.5% of total installed renewable energy capacity. But a number of innovations suggest that it will soon be possible to harness underground heat on a much larger scale. Here are just a few of them.
Enhanced Geothermal Systems (EGS)
Principle: Geothermal energy requires three essential elements: heat, a fluid and permeable rock (in which the fluid can circulate freely). In many areas, the rock is sufficiently hot, but insufficiently permeable and/or too poor in fluid. In a stimulated geothermal system, the controlled injection of a fluid underground will open up naturally occurring fractures, or create new ones. The aim is to improve the permeability of the rock, and hence the heating of a larger quantity of fluid. Drilling a production well then recovers the hot fluid to produce electricity and/or heat.
Why: EGS, which hasbeen tested for several years at the Soultz-sous-Forêts power plant, provides access to heat reservoirs that would otherwise remain unexploited.
Project in progress: Soultz-sous-Forêts is one of the pioneering power plants currently using EGS to generate electricity. The Utah FORGE experimental station in the USA is working to advance this technology and reduce the risks (mainly seismic) associated with the creation of such reservoirs. But many other projects around the world are exploring this approach.
Using geothermal lithium to reduce the cost of power plants?
Some of the geothermal waters that circulate deep within granites and sandstones contain substantial quantities of lithium. This mineral, essential for the manufacture of electric vehicle batteries, is expected to triple in demand in France between now and 2030. Its use in cogeneration with geothermal energy could therefore help improve the profitability of geothermal power plants. ES-Geothermie in partnership with ERAMET are the first to have extracted geothermal lithium in France, at the Soultz sous Forets power plant. If this process were to be extended to a dozen or so geothermal plants, it could meet current French demand for lithium.
Super Hot” and supercritical geothermal systems
Principle: This involves drawing fluids at temperatures above 400°C, i.e. hotter fluids. than conventional geothermal installations (by way of comparison, volcanic contexts feature fluids at “only” 200 to 300°C). At this temperature level, and given sufficient pressure, water becomes a “supercritical” fluid, i.e. almost as dense as a liquid, but which tends to behave like a gas, enabling geothermal energy to be transported more efficiently.
Benefits: A single well using supercritical fluids could produce 10 times more energy than a conventional well. In addition, rocks at 400°C have mechanical properties that are consistent and predictable: no matter where in the world you are, if the rock temperature is “super hot”, it will behave in the same way; this feature should make it possible to design common tools and considerably increase drilling predictability.
Challenges: These mainly concern drilling costs (the deeper the well, the more expensive it is), stimulation techniques (more complex at such depths), and the composition of brine from this type of well (highly corrosive to pipes).
Current projects: On this interactive map created by the Clean Air Task Force, it is possible to locate the various projects currently underway in the field of super-hot rocks. In Iceland, in 2009, a drilling company prospecting for new geothermal sites around the Krafla volcano accidentally stumbled upon magma. This discovery (a first) led to the setting up of the Krafla Magma Testbed project, one of whose aims is to experiment with harnessing this heat for geothermal projects. Drilling is scheduled to begin in 2025.
Closed loops (Eavor Loop)
Principle: This is a system for circulating a heat-transfer fluid (whose physical properties enable it to transport heat efficiently) within a closed-loop network of horizontal pipes located underground, at a depth of 3 or 4 km. Heat transfer is therefore not by convection, as is usually the case, but by conduction between the rock and the liquid. The hot liquid is then converted into electricity or transferred to a district heating network.
(Source: Eavor)
Advantages: This system does not require a natural reservoir of hot water, and can therefore be used in a greater number of sites, particularly in geological formations with low permeability. As the system is self-maintaining (cold water being denser than hot water, it pushes hot water towards the surface), there is no need to pump the fluid, which improves energy efficiency. Last but not least, the system is controllable, enabling energy production to be modulated according to demand.
Current projects: Eavor Loop is currently working on two projects, one in Geretsried, Bavaria (Germany) and one in Alberta, Canada. Find out more about Eavor Loop in this article article in Recharge magazine.
Giant underground batteries
Principle : based on the use of a “flexible” geothermal plant. This involves shutting down a power plant’s production well for a few hours or days to create an accumulation of pressurized liquid in the rock. The faults in the reservoir will expand and change shape under the effect of the water pressure, rather like balloons. When the valve reopens, the fractures revert to their original shape, creating an increase in water flow and thus in electricity production.
Benefits: By producing electricity on demand, these batteries could provide a solution to the intermittent nature of renewable energies (solar and wind). They would be put on standby during periods of abundant production, and activated when there is a shortage.
Current projects: Fervo is testing the underground battery concept in a desert region of Nevada, USA. Find out more in this MIT Technology Review article.
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Other innovations
A great deal of work is currently underway to break down the barriers that stand in the way of wider use of geothermal energy, in particular to improve performance and safety and reduce costs: reuse of oil rigs (onshore and offshore) as geothermal units, development of more efficient drilling techniques using microwaves (e.g. Quaise), techniques to combat pipe fouling in boreholes (e.g. Bluesparks technology), etc.
Innovations to keep an eye on!