Geothermal probe

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Drilling of a geothermal probe (the actual probe rolled up on the left in the foreground)
The U-shaped probe base, the lower end of a geothermal probe. The connecting pipes were cut.
The pipes of a geothermal probe protrude from the ground, are marked in red and must still be connected to the heat transfer circuit.

A geothermal probe (EWS) is a geothermal heat exchanger in which a heat transfer fluid circulates. In contrast to the horizontally laid geothermal heat collector , the pipe system is installed in a vertical or inclined borehole . With the geothermal probe, heat is extracted or supplied to the ground. With the help of a heat pump heater , the temperature level of the near-surface geothermal energy can be increased in order to be able to use the heat gains for heating the building . Apart from that, geothermal probes are also used to feed cold local heating networks and for (seasonal) storage of thermal energy in the ground.


The most common type of geothermal probe consists of parallel polyethylene plastic pipes , two of which are connected at the lower end via a U-shaped base. One speaks of U-probes or double U-probes when two pairs of pipes are used per borehole. Coaxial probes are also possible in which the flow and return of the heat transfer fluid take place in the inner pipe and in the annular space between the inner and outer pipe of the coaxial probe.

If a pile foundation is planned for a structure , the probes can be designed as so-called energy piles . Similar to thermal component activation , the plastic pipes for the heat exchange fluid are then concreted into the support or foundation piles.

A brine , a mixture of water and antifreeze, usually flows through the pipes in a closed circuit . Brine-filled geothermal probes are often not permitted in water management-sensitive areas. Alternatively, carbon dioxide can be used as a heat transfer medium. The probe then works on the principle of the heat pipe (two-phase thermosiphon) and is usually made of stainless steel.

Installation and function

Double-U probes are most commonly used today. Before that, a mobile drilling rig is used to drill a bore using the flushing or dry drilling method, with or without casing , depending on the rock . When using the usual double-U probes, the drilling diameter is around 140 to 180 mm. After the drilling of the bore up to the planned depth, the probe beam is (U-probes and Verpressrohr, all loaded with a draw weight at the probe together, if necessary) into the well introduced. The remaining cavity of the borehole is grouted with a filling material ( bentonite- cement suspension or grout ), if possible with good thermal conductivity, using the contractor method via the grouting pipe carried with the probe bundle from bottom to top. Any pipework that may have been installed during drilling is pulled out again during grouting. The setting grouting material ensures good heat transfer from the surrounding rock to the probe pipes and serves as a safeguard between the groundwater levels. At the same time, this prevents the heat transfer fluid ( brine ) from leaking into the groundwater.

Twin pipe probes do not require a drilling. They are brought into the ground using a flushing process.

After the rest of the probes in the field have been set up and the final work (such as the pressure test of the individual probes), the probe flow and return lines are connected to the heat pump by means of horizontal connection pipes laid in a frost-proof manner and the system is filled with the heat transfer fluid and vented. Except for inspection shafts, no installations are visible above ground level after completion.

In operation, the heat transfer fluid, which is in a closed circuit, is pumped through the geothermal probe with the aid of a circulating pump and heated by the geothermal energy via the wall on its way to the deepest point and back . The geothermal probe thus forms a large-area heat exchanger . The large surface area is also achieved by bundling tubes (principle of the tube bundle heat exchanger ), whereby in practice mostly 2 pairs of tubes are used per borehole.

So that heat can be transferred, the absorbing heat transfer fluid must be cooler than the rock temperature. This need is ensured beforehand by a heat pump . The heat transfer fluid heats up in the probe, but cannot get warmer than the mountains.

The heated heat transfer fluid flows into a heat exchanger of the heat pump in order to extract the heat it contains by means of evaporative cooling . The downstream heat pump is used to raise it to the temperature level required for heating. The greater the temperature difference between the earth temperature and the desired heating medium temperature, the more mechanical pump energy is required. Therefore, low temperature heating systems such as underfloor heating are beneficial.


Planning for geothermal probes requires an extensive calculation including geological and heating parameters. Professional support from a geologist with experience in the dimensioning of geothermal probes is strongly recommended. The heat demand of the building to be determined (= heat sink) is offset by the productivity of the subsoil (= heat source). To avoid damage to the probe circuit, e.g. The local meteorological (including annual average temperature), geological (rock parameters, including thermal conductivity), hydrogeological (including the presence of groundwater over the length of the probe) and heating should be avoided, for example by icing the subsoil near the probe, and other undesirable effects during the operation of the system (including heat demand and flow temperature of the building to be heated) parameters must be included in the calculations.

Downloadable simulation models can be used for dimensioning and performance calculation. With such models, comparisons with geothermal collectors are easily possible. These calculations can give a rough overview. More precise calculations can only be made based on knowledge of the geological subsurface. In the case of large systems (> 30 kW), more precise geothermal heat output of the subsurface can be determined through special tests such as the Thermal Response Test (TRT). For this purpose, a first probe hole is provisionally drilled as a test hole and expanded into a test probe; Based on the TRT result of this probe, the layout planning of the remaining probes or the probe field is carried out.

Geothermally unfavorable subsoil (e.g. dry sands) require more drilling meters (= geothermal probe meters), which accordingly leads to higher investments for the development of the heat source. Additional temperature control of the building in summer is possible and may lead to a lower number of drilling meters, since the subsoil is thermally regenerated in the summer months. Values ​​given in various sources for extraction rates from the subsurface should be treated with great caution, as each location is (geologically) different. Therefore, a professional calculation of the necessary geothermal probe lengths should always be carried out, taking into account the geological conditions. Ideally as part of a feasibility study that also considers the economic viability of a geothermal probe system.

Drilling depths

From a depth of around 10 meters, the temperature remains practically unchanged over the year and is around 11 ° C in the low mountain range . In Central Europe, the temperature increases by 1 ° C on average every 30 meters. Therefore, the geothermal probe is more efficient than the geothermal collector . The depth of a borehole varies according to the geological nature of the subsoil and is between 50 and 300 meters in normal residential construction. Depending on local conditions and performance requirements, it can also be 400 meters and more. Occasionally there are experimental drilling depths over 400 meters (= scientific or industrial deep geothermal projects), whereby the effort here usually exceeds the benefit.

In private residential construction (single-family houses) in Germany, geothermal probes rarely reach deeper than 100 m. In other countries, greater depths are also common. In Switzerland, for example, drilling is regularly carried out to a depth of around 300 meters. In addition to the high costs for the drilling rig (drilling costs), a corresponding permit (including water protection regulations) must be obtained and mining law must be observed for depths greater than 100 meters .

If larger heat transfer surfaces are required, several holes are usually drilled next to each other at a distance of a few meters. Since the drilling is deep, the space requirement is small compared to the geothermal collector . According to VDI 4640, a minimum distance of 6 meters between neighboring probe holes and 3 meters from the property line is recommended in order to avoid negative influences between the probes. The LAWA recommendation for water management requirements for geothermal probes and geothermal collectors recommends a distance between two geothermal probe systems of 10 meters with a distance of 5 meters from the property line.


Mainly, geothermal probes are used to obtain ambient heat using heat pumps. But the variant for cooling can also be implemented using geothermal probes. Here, heat from buildings is transferred into the ground via the heat transfer fluid. So the ground serves to cool down. The heat transfer fluid cannot get colder than the floor temperature. If lower temperatures are required, a downstream refrigeration machine is required.

Deep geothermal probes are used exclusively for heating. If the cooling case is also to be covered, the drilling depths can be reduced due to the storage application.

Geothermal probes are also used to supply heat to cold local heating systems . Seasonal storage of excess energy is also possible, for example by storing heat energy from solar thermal systems or industrial waste heat that cannot be used in summer for the winter half-year. The probes are used both for loading (heating) the soil and for removing the heat again.

Large-scale plants

For the first time, extensive research is to be carried out on the use of geothermal energy in the geothermal park in Neuweiler in the northern Black Forest, a construction area in which only geothermal energy is used for the purposes of heating and cooling buildings. As part of a model project, the heating and cooling of the existing roads are to be implemented for the first time.

The Prenzlau geothermal depth probe with a depth of 2790 meters and a continuous heat output with a heat pump of 520 kW at a rock temperature of 108 ° C has been in operation since November 10, 1994 . The heat output without a heat pump is 150 kW. The depth probe is characterized by practically trouble-free operation over the years, with occasional interruptions of a few hours.

During the construction of the SuperC building in November 2004, RWTH Aachen University reached a depth of 2500 m with a geothermal probe. The rock temperatures reach 70 to 100 degrees Celsius. The geothermal probe should deliver an output of approx. 450 kW. This would have saved around 300 t of CO 2 annually for heating the building. The thermal performance, however, remained far below expectations.

Legal situation


According to the German Water Resources Act (WHG), drilling work that may affect the groundwater is notifiable (Section 49 WHG). The introduction of geothermal probes into the groundwater-bearing layers is an element of use within the meaning of Section 9 WHG, which in some cases requires a permit or permit under the WHG and the respective state water laws of the individual federal states. The use of substances hazardous to water (for example cooling brine with WGK 1) in underground parts of the system can also constitute an offense under water law.

In the protection zones of designated water protection areas or mineral spring protection areas, drilling is often restricted or prohibited.

In the case of boreholes over 100 m deep, the provisions of Section 127 (1) of the Federal Mining Act must be observed. After that, mining law applies with certain provisions.

Web links

Individual evidence

  1. Dirk Gebhardt, Stephan Peters: CO2 geothermal pipe as a monoprobe from the point of view of groundwater protection . Ed .: BRUGG Rohrsysteme GmbH. Magdeburg 2010 ( [PDF; 2.6 MB ; accessed on February 23, 2014] DKV conference). ( Memento of the original dated November 3, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot /
  2. . Simulation model for geothermal probes, 2008
  3. Simulation model for geothermal collectors, 2008
  4. Recommendation for water management requirements for geothermal probes and geothermal collectors of the Federal Government / State Working Group on Water (LAWA) , December 2011, page 7 (PDF document)
  5. Simone Buffa et al .: 5th generation district heating and cooling systems: A review of existing cases in Europe . In: Renewable and Sustainable Energy Reviews . tape 104 , 2019, pp. 504-522 , doi : 10.1016 / j.rser.2018.12.059 .
  6. Technical realization of the geothermal well RWTH-1.  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Website of the Institute for Extraction of Raw Materials and Drilling Technology at RWTH Aachen University@1@ 2Template: Toter Link /