Deep geothermal energy accesses rock structures at least 400 meters into the earth, and various uses are possible depending on the temperatures in the subsoil.
Temperatures of between 20°C and 70°C are found in aquifers (layers of rock or soil that can absorb and hold water) at depths of 400 to 2,000 meters. The thermal water found at these depths is suitable not only for bathing but also for heat production, and systems that exploit the drainage water from tunnels can be put to a similar use (SFOE, Nutzung der Erdwärme - Überblick, Technologie, Visionen).
At depths of 4,000 to 6,000 meters, temperatures are between 150°C and 200°C. Geothermal energy projects that access these layers of rock generally use part of the energy recovered directly for power production, while the rest can be fed into a district heating grid.
A distinction is made between two systems for energy recovery at great depth: petrothermal and hydrothermal.