Geothermal energy

When we dig deeply underground, for example in a mine or during a drilling; we discover that the temperature increases gradually with depth, on average by 3°C per 100m; that is what is called the geothermal gradient. The water contained in the underground reservoir rock is therefore hot water. And it is hotter the deeper the reservoir. The principle of geothermal energy is to use these reservoirs of hot water to recover a part of the heat, which can be used directly for heating purposes, or, when the water is sufficiently hot, for the production of electricity.

Geothermal hot water was used in ancient times, in China, Japan, and in Rome, for washing and bathing and it was recognised for its therapeutic virtues. The Romans even used it to heat the walls and floors of their houses.
Geothermal energy is renewable, as long as the underground hot water is exploited carefully, because it only reheats slowly.

Geothermal energy is classified in 3 types, according to the temperature of the water:

- Low level geothermal energy: the temperature of the water is between 30 and 100°C. The depths of the reservoirs attain 1000 to 2500 m. It is used for heating. This type of geothermal energy is present in all the major sedimentary basins of the world. Numerous regions benefit from this low level energy: the basins of the Mississippi, the Amazon, and the Rio de la Plata, artesian basins in Australia, China, central Asia, and in the Pannonian Basin in Hungary … In France, the basins in the Paris region and in the Aquitaine supply geothermal water.

- Medium-level geothermal energy: the temperature of the water is between 100 and 180°C. It is used for the production of electricity. It is found in volcanic regions in reservoirs less than 1000 m deep. It is also found in sedimentary basins, but in much deeper reservoirs (2500 to 4000m) than those of low-level thermal energy.