Aquacalida

Geothermal energy, the path of the water

But where do the waters come from? To what do we owe the hot water that supplies the baths, spa treatments and geothermal power plant? A series of animated films retrace the water’s path from the clouds to the depths of the earth and back to the surface. The discovery of the mechanisms of geothermal energy opened the way for new solutions to the world’s energy needs.

Discover the origins and uses of La Léchère’s thermal waters through animated films.

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The thermal waters extracted by the Natacha well take thousands of years to percolate through the La Léchère fault.

This long underground journey takes the rainwater down to depths of up to 5000 m. The deeper the water percolates into the ground, the longer it takes to come back to the surface. Using isotopic analyses, geologists have calculated that the water at La Léchère is between 2000 and 10,000 years old. Thus, the water that emerges from the Earth today began its journey in the Stone Age, when our ancestors had only just started farming and domesticating livestock.

As it progresses through the ground, the water absorbs minerals. Helped by the heat, it dissolves minerals from the rock, which dissociate into ions such as Ca2+, Na2+ and S04. Every rock encountered contributes minerals. Nevertheless, La Léchère’s thermal water has a lower mineral content than the waters of neighbouring Brides-les-Bains, Salins-les-Bains and Aix-les-Bains.

The waters turn sour !

In 1930, an attempt was made to bottle La Léchère’s water. However, the venture quickly turned sour, as the mineral-rich thermal water does not keep. Since then, La Léchère’s weakly radioactive water has only been available for consumption under prescription. Today, it no longer has the right to call itself mineral water.

Géothermie
Hyperthermal waters

La Léchère has Savoie’s hottest thermal waters. With a temperature of 61°C, they are classified as hyperthermal waters, the term used to describe waters that are hotter than 50°C. The thermal waters extracted at Aix-les-Bains are between 22°C and 46°C. The temperature of the water at Brides-les-Bains is 36°C, at Salins-les-Bains it is 15°C and at Challes-les-Eaux it is a mere 11°C.

Heat from the depths

Our planet is a true power plant, a phenomenal energy reserve that is constantly fed by the natural radioactivity of rocks.

When the Earth formed, 4.55 billion years ago, it accumulated an enormous amount of energy in the form of heat. It was a true furnace! The temperature at the Earth’s core is about 4200°C, whereas the mantle is at between 1000°C and 3000°C.

All this heat diffuses slowly through the Earth’s crust, creating a geothermal gradient of 3.3°C per 100 m. Geothermal energy systems exploit this natural gradient by bringing hot water to the surface and recovering the energy stored in it.

Earth’s heat resources are renewable. The breakdown of naturally occurring radioactive elements in the rock provides a continuous source of heat. When the nuclei of unstable elements such as uranium, thorium and potassium decompose they give off energy that heats the rock. 90% of the heat extracted by geothermal energy systems is produced by this radioactivity and only 10% is a residue from the formation of our planet.

Geothermal energy across the world

Heat at plate boundaries

Mountainous and volcanic regions, such as the Andes, West Indies and Alps, have very favourable geological conditions for geothermal energy. The Earth’s crust in these areas is fractured and underlain by magma chambers, so the water within the rock is heated to very high temperatures. Geothermal power plants are built in these areas, which occur at the boundaries of tectonic plates.

Heat at plate boundaries

Mountainous and volcanic regions, such as the Andes, West Indies and Alps, have very favourable geological conditions for geothermal energy. The Earth’s crust in these areas is fractured and underlain by magma chambers, so the water within the rock is heated to very high temperatures. Geothermal power plants are built in these areas, which occur at the boundaries of tectonic plates.

Geothermal energy of the future

A new generation of geothermal installation has been built in Alsace. The Soultz-sous-Forêts geothermal power plant uses “Hot Dry Rock” technology. Unlike classic geothermal installations, geothermal waters are created artificially by injecting water into naturally cracked hot rocks. At Soultz-sous-Forêts, three boreholes have been drilled to a depth of 5000 m, where the rock is at 250°C. Cold water injected into one of the boreholes re-emerges from the other two boreholes at a temperature of 200°C.