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.

中文翻译：

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工作温度变化和冻融循环对埋管换热器水力传导率的影响


大部分一次能源被消耗用于提供空间供暖。地热能提供了一种可再生的替代方案。出于效率和环境保护的考虑，确保埋管换热器 (BHE) 的系统完整性非常重要。之前的调查主要集中在 BHE 的各个组件或灌浆和管道系统的完整性上。这项研究的重点是分析整个地下-灌浆-管道系统的液压系统完整性以及可能的热引起的变化。为此，我们建立了一个中试规模的实验，以在原位压力、液压和温度条件下测试典型 BHE 的 1 米截面。在测试过程中，连续且独立地测量土壤和 BHE 的液压系统渗透性。此外，温度监测阵列安装在50厘米的横截面积内。可以观察到密封性能与温度相关的显着波动。在没有霜冻作用的情况下，超出了 VDI 4640-2（地下地源热泵系统的热利用，2019 年）要求的水力传导率限制。冻融循环的成功应用进一步增强了系统的渗透性。研究表明，热引起的对 BHE 系统完整性的影响比随后的霜引起的影响更大、更显着。高密度聚乙烯（PE-HD）管道的疏水特性及其高热膨胀系数似乎是该系统的主要弱点。 优化研究应重点关注灌浆和管道之间的界面连接，其中还应考虑不锈钢或铝等亲水性管道材料以及PE-HD管道表面性能的控制。