Freeze damage of grouting materials for borehole heat exchanger: Experimental and analytical evaluations

The closed-loop ground source heat pump (GSHP) systems can be operated below the freezing point of water with anti-freeze mixture through the installed borehole heat exchangers (BHE) in the ground for the heating purposes of buildings. During the operations, the BHE is exposed by the thermal stresses due to heat loading. In addition if the porous grout material is immerged in water, the freezing-induced ice pressure may damage the grout. In this paper, the freezing impact on BHEs is investigated analytically and experimentally. For the theoretical approach, an analytical solution is developed by using the hollow cylinder model that accounts for both the high density polyethylene (HDPE) pipe and the grout material. Firstly, the frozen pore water pressure is incorporated in the generalized Hooke's law equations in 2D plane stress, and secondly the model is solved for the considered boundary conditions. In order to validate the developed model, the experimental setup is conducted in agreement with the geometry of the considered analytical model and the BHE specimens are prepared with three different grout materials having large difference in the thermal and hydraulic characteristics (i.e. silica-sand based, calcite based and homemade thermally enhanced with graphite). According to the experiments for 50 h of freezing operation, the calcite based grout and the homemade grout, having lower permeability and relatively higher porosity, are fractured. In contrast, the silica-sand based grout having higher permeability did not exhibit any damage. Compared with the theoretical results, the observations from the experiments are consistent. The effective tangential stress induced by the frozen pore water pressure causes the crack development and agrees with the crack patterns. It is concluded that the porosity and the permeability play significant role on the grout failure upon freezing. © 2015 Elsevier Ltd.