A large share of the primary energy is consumed to provide space heating. Geothermal energy offers a regenerative alternative. For reasons of efficiency and environmental protection, it is important to ensure the system integrity of a borehole heat exchanger (BHE). Previous investigations have focused on the individual components of the BHE or on the grout and pipe systems’ integrity. This study focused on the analysis of the hydraulic system integrity of the complete subsoil–grout–pipe system as well as possible thermally induced changes. For this purpose, a pilot-scale experiment was built to test a 1-m section of a typical BHE under in situ pressure, hydraulic and temperature conditions. During the tests the hydraulic system permeability of the soil and the BHE was measured continuously and separately from each other. In addition, the temperature monitoring array was installed in a 50-cm cross-sectional area. Significant temperature-related fluctuations in the sealing performance could be observed. Hydraulic conductivity limits required by VDI 4640-2 (Thermal use of the underground—ground source heat pump systems, 2019) were exceeded without frost action. The succeeding application of freeze–thaw cycles further enhances the system permeability. The study shows that the thermally induced effects on the system integrity of the BHE are larger and more significant than the subsequent frost-induced effects. The hydrophobic character of the high-density polyethylene (PE-HD) pipes as well as its high coefficient of thermal expansion seem to be the main points of weakness in the system. Optimization research should focus on the interface connection between grout and pipe, whereby hydrophilic pipe materials such as stainless steel or aluminum should also be considered as well as manipulation of the pipe surface properties of PE-HD.
<p>Freezing and thawing in the subsurface is often related to complex technical handling of possible influences on the engineered structures (e.g. permafrost or geothermal heat pumps). Freeze-thaw processes in the vicinity of borehole heat exchangers can significantly impair the system. However, for groundwater protection and thermal efficiency, the hydraulic and thermal integrity of such systems must be permanently ensured for the complete operation time. Detailed knowledge on freeze-thaw processes in porous media, such as soils or geotechnical grouts, and the driven parameters, is still pending. Freezing in porous media does not occur as a sudden transition from pure liquid water to the ice phase, but rather within a freezing interval strongly depending on various boundary conditions such as soil type or pore water chemistry. As the content of frozen and unfrozen water has a strong impact on material properties, it is essential to have suitable information about the different factors influencing freezing processes as well as the thermo-hydraulic-mechanical (THM) effects on porous media due to phase change. Thus, a THM laboratory experiment was developed and built to gain more knowledge on freeze-thaw processes and their effects on soil and grouting materials. The experiment consists of a modified triaxial test, enabling for controlled temperature and hydraulic flow conditions, that is combined with an ultrasonic measurement device to determine the unfrozen water content.</p><p>In this contribution, results of the THM experiment are presented, whereas the following parameters were investigated: The freezing interval using P-wave velocity, freezing pressure as well as axial and radial volume expansion due to ice formation as well as the influence of hydraulic flow on the ice formation. First, benchmark experiments were conducted on well-characterized solid rock samples to avoid any influence of a variable sample pore structure during the experiments. Further experiments focused on the investigation of soil samples of different texture classes. For upscaling to real scale applications, the experimental findings will be implemented in numerical models.</p>
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