Temperature evolution around four laboratory-scale borehole heat exchangers grouted with phase change materials subjected to heating–cooling cycles: An experimental study

Abstract

This article presents the experimental results from a unique laboratory test-setup used to comparative study of heat transfer conditions in borehole heat exchangers with different grouts under a controlled environment. The work was part of a larger, EU-funded project on advanced materials for borehole heat exchangers pipes and grout, GEOCOND. Four grout columns with different formulations were cast and tested under cyclic heating and cooling. One grout column was cast using high thermal conductivity grout; two columns were cast from high thermal conductivity grout with microencapsulated phase change material (MC-PCM) and one with shape-stabilised phase-change material (SS-PCM). The objective of the test was to comparatively evaluate performance of the borehole heat exchanger under a cyclic temperature regime and to investigate if the selected phase change materials (PCM) embedded in grout can be activated by cyclic heating and function steadily. Twenty-five heating-cooling cycles were performed, each lasted 24 h. The results showed clear cooling delay in grouts containing PCM associated with the crystallization heat release. The cooling delay was better expressed in grouts with SS-PCM. The PCM related cooling delay was stable throughout all the cycling in SS-PCM containing grouts, however, the effect vanished in grouts with MC-PCM.