In the fields of commercial buildings, multi-family houses, inner-city refurbishment of old buildings, but also of living groups, a thermal output between 20 and 200 kW is often necessary. The use of near-surface probe fields is due to their space requirement and the approval right in these fields rather problematic.
Larger drilling depths are principally associated with higher drilling costs. These additional costs must be compensated in order to enable an economic implementation of medium-depth probes. Most important is, in this respect, the higher energy yield due to higher rock temperatures (20 – 40 °C). The abstraction capacities (in W/m) of medium-depth probes are thus significantly higher than those of near-surface probes. Depending on the actual drilling depth and the operation of the heat pump, COP values of clearly above 5 are possible. Another important advantage of medium-depth probes is their base load capacity (> 8,000 h). Model calculations have shown that there is only a 10% loss of performance compared to the typical operation length (up to 2,400 h). A clear disadvantage is that the cooling operation is not economically feasible due to the higher temperature level. A combined heating and cooling mode is possible with a combination with near-surface probes. Compared to near-surface probe fields, the low space requirement of medium-depth probes is a clear advantage. Current project examples have shown that probe fields are not eligible for approval in critical hydrogeological areas. However, under certain circumstances, one or few medium-depth drillings may be accepted because challenging drilling techniques have to be applied (for example the complete piping of the borehole). In soil-mechanically sensitive areas, it may also be advantageous to not destabilize the soil ground through a dense network of drillings. Especially in the area of the North German Basin, medium-depth probes are suitable for the development of the great geothermal potential of salt domes. Due to above-average temperatures, higher thermal outputs can here be achieved. In the North German Basin, but also in other regions of Germany, the occurrence of thermal water horizons in depths up to 1,000 m is not unusual. Medium-depth probes thus often also provide the opportunity, as it has been shown in Osnabrück and Nienburg, to find thermal water horizons and to use them thermally.
The term “medium-depth geothermics” shall here stand for the depth range of 400 to 1,000 meters. This clarification of the term exclusively aims at enabling a consistent communication. The usual separation from near-surface and deep geothermics according to VDI guideline 4610131 shall thus not be rejected. The PK Tiefe Geothermie recommends: “Deep geothermics should only apply to depths of more than 1,000 meters”. The PK also states that “deep geothermics apply to systems, where geothermal energy is developed through deep drillings and can indirectly be used (without level control)”. A direct use without the activation of a heat pump is, however, in depths up to 1,000 m only possible in exceptional cases and only at low temperatures. A definition of medium-depth geothermics at 400 m is not in conflict with the definition of near-surface geothermics, but takes account of the fact that the financial support per drilling meter according to the market incentive program (MAP) begins at 400 m. The MAP-financing rate of 375 €/m precisely covers the range of medium-depth geothermics. The availability of drilling systems also speaks for a limitation of the abovementioned depth range. Mobile drilling system are able to develop this depth range with a maximum hook load of 50 tons. The costs per drilling meter significantly increase for drilling depths of > 1,000 m and it is often necessary to use overdimensioned drilling systems. The range of necessary approval procedures for “medium-depth” drillings is defined by the conditions under and above the surface, as well as the planned drilling depth, and the expansion of the borehole at the drilling location. The responsible mining authority decides on an individual basis whether the drilling requires a management plan in accordance to § 51 ff. BBergG or not.
Medium-depth coaxial probes can be chosen for a heat requirement between 20 and 200 kW as a low-risk development option. With multiple probes, theoretically even larger systems or network structures are realizable. Ideally, open medium-depth duplication systems can be used to achieve significantly higher power ranges. At comparatively moderate drilling costs and in consideration of financial support through the market incentive program, economic geothermal installations can, with manageable planning efforts, be realized. Medium-depth probes are not suitable for the provision of refrigeration. Here, a combination with near-surface probes may be considered. A great advantage of medium-depth geothermal energy is its basic load capacity. It is therefore particularly for low-temperature systems with a high number of hours of the year an attractive alternative.
The author of this text is Prof. Dr. Dieter Michalzik,
Executive board member of GeoEnergy Celle e.V. and head of the working group “Mitteltiefe Geothermie” (medium-depth geothermics), as well as
Managing director of the GeoDienste GmbH.
The text has been published in “Geothermische Energie”, issue 76 // 2013 / 2