Impact of Freeze-Thaw Cycles on Borehole Heat Exchangers -- a Holistic Experimental Approach

Abstract

For various reasons, the inlet temperature of the heat carrier fluid in borehole heat exchangers (BHE) may fall below the freezing point of water. Consequently, temporary freezing and thawing processes occur in the grouting material and the surrounding rock formation. However, the transition zone between pipes and grout as well as the skin zone between the grout and the undisturbed subsoil are also influenced by these processes. This can lead to significant damage of the grouting material or the subsoil structure as well as 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. In aquitards or aquicludes, such secondary flow paths may act as hydraulic short circuits. Previous research on this issue focused either on the grout only or on the compound of grout and BHE-pipes, but completely omitted the subsoil as well as the skin zone. For this reason, a laboratory and a technical scale experiment were developed. The laboratory approach improves an existing measuring method, which considers a simplified geometry with just a single tube and the grout, by integrating the surrounding soil. In contrast, the large-scale experiment allows for the simulation of freeze-thaw cycles (FTC) considering a realistic BHE geometry with a double-U-Pipe embedded in a grout cylinder, which is also surrounded by compacted soil material. A sealed cylindrical steel tank with a height of 100 cm and a diameter of 50 cm encases the experiment. The device is operated like a triaxial cell. This facilitates measurements of the system’s hydraulic conductivity under defined pressure conditions such as hydraulic backpressure as well as a confining pressure. The BHE pipes are connected to an external heat pump, which enables a simulation of realistic fluid temperature variations including freeze-thaw cycles. Moreover, the shell of the tank is tempered to define an outer thermal boundary condition. A radially symmetrical arrangement of temperature sensors in the cell renders a monitoring of the temperature field as well as the detection of the ice penetration depth possible. The main objective of the experimental series is the identification of critical factors that control the integrity of a BHE systems in situ, when numerous freeze-thaw cycles are applied.