State of the Art in Deep Geothermal Energy in Europe: With Focus on Direct Heating

The distribution of heat in the Earth’s crust is determined by various geophysical and geological processes. As heat constantly flows from the Earth’s interior to the surface, temperature increases with depth. The rate of temperature change with depth is called geothermal gradient. A high geothermal gradient results in a high geothermal potential, thus making a region attractive for the use of geothermal energy. The geothermal gradient of a region depends on the predominant type of heat transfer in the subsurface, and the thermal and hydraulic rock properties. Based on these geological controls, a distinction is made between hydrothermal and petrothermal systems, both of which require different strategies for the technical development of the geothermal reservoir.