Geothermal systems can be designed as an open or closed system. Closed systems are often used in near-surface areas and open systems are commonly used for deep geothermal applications. At the surface the heated medium can be used for heating and for electricity generation.

In an open system hot water is extracted from an underground aquifer and pumped to the surface, where the heat is transferred to a heat exchanger. The cooled off water is then pumped back into the aquifer and the cycle starts over again. Other types of open systems like the hot dry rock technique pump a heat carrier under pressure into hot rock layers. The heat carrier absorbs the heat of the rock formation and is pumped back to the surface

In closed systems the heat carrier medium circulates through a closed pipe system. The pipe has a high heat permeability to facilitate the heat transfer between the subterranean earth layers to the heat carrier in the pipe system.

Based on the availability of deep water, water permeability and the system concept, a distinction can be made between hydrothermal and petrothermal geothermal energy as well as deep borehole heat exchangers

Petrothermal applications: Petrothermal geothermal energy comes from deep-lying heat reservoirs, which have no water flow or only negligible water flow, such as dry layers of rock with temperatures of more than 150°. Through an injection borehole, water is transferred under the earth for heating.

Hydrothermal applications: Hydrothermal applications tap into already existing hot water reservoirs some 400 meters below the surface. In this case, the hot water is used for driving a steam turbine to create electricity and heat simultaneously.

Borehole heat applications: Deep borehole heat exchange technology refers to a closed system of energy production comprising a single borehole at depths of 400 m to several thousand meters. Water circulates through double pipe exchangers in a closed circuit. The heat from the water heated at these depths is then extracted at the surface and delivered to a heat pump circuit. In the case of high temperatures, the recovered energy can be used, for example, as process heat for industrial applications or for agricultural applications in the case of low temperatures. .