Impact of Freeze-Thaw Cycles on hydraulic conductivity of Borehole Heat Exchangers

For several reasons, the inlet temperature of the heat carrier fluid in borehole heat exchangers (BHE) may temporarily fall below the freezing point of water. Consequently, freezing and thawing processes are induced in the grouting material and the surrounding subsoil. However, these processes also influence the transition zonecontact area between pipes and grout as well as the skin zone between the grout and the undisturbed subsoil. This can lead to significant impairment damage of the grouting material or the subsoil structure and cause detachment phenomena between the pipe and the grout or in the skin zone. As a result, the hydraulic integrity of the system cannot be sustained and possible secondary flow paths may occur. Previous research on this issue focused on the grout only or on the compound simplified system of grout and BHE-pipes, but completely omitted the subsoil as well as the skin zone. For this reason, a large-scale experiment was developed, which allows for a simulation of freeze-thaw cycles, considering a realistic BHE configuration of a double-U-Pipe embedded in a grout cylinder and surrounded by compacted soil material. During the experiment, the hydraulic conductivity and temperature distribution areis measured constantly. The experimental results illustrate the extent of freeze-thaw damage processes under in situ conditions., Furthermore, the transition zonecontact area between the pipes and the grout material is clearly identified as the major weak point of the system compound. The findings are used to develop improvement measures in order to maintain the integrity of the system even under the impact of numerous freeze-thaw cycles.