Vertical hydraulic conductivity of borehole heat exchanger systems before and after freeze-thaw cycle stress

Abstract

Quality assurance is a very important aspect in the planning, implementation and operation of borehole heat exchangers (BHE). Within the research project Quality Assurance for Borehole Heat Exchangers (QEWS) the hydraulic sealing behaviour of industrial grouting materials was investigated. Important aspects therein were the capability for sealing between different aquifers and aging resistance against freeze-thaw cycle stress. For this purpose, a large-scale test rig for determining the hydraulic conductivity of borehole heat exchanger systems (BHES) was developed and put into operation. Compared to samples of the pure grouting material, the system samples exhibited significantly increased permeability, which was further increased by freeze-thaw cycle stress. It could be shown that the first freeze-thaw cycle caused the most significant aging effect. Aging caused by further freeze-thaw cycles could be partly compensated by regeneration effects. But the large-scale test rig has a substantial disadvantage: due to the length of the system sample, depending on the hydraulic permeability of the grouting material, the experimental program is very complex and time-consuming. Therefore, a downscaled test rig will be developed within the current project Quality Assurance for Borehole Heat Exchangers II (QEWS II).