Relevance of computing freeze-thaw effects for borehole heat exchanger modelling: A comparative case study

In closed-loop geothermal systems the underground may freeze and thaw due to intensive heat extraction. Phase change alters the sediments' mechanical and hydraulic properties and their thermal behaviour, thus affecting the thermal plume propagation and the area of potential changes. By means of a finite element model, this paper evaluates the thermal plume in a particular case study in several conditions, comparing the results obtained when considering or disregarding the freeze-thaw processes by applying a benchmarked plug-in. The model is based on experimental input data. The results show that the frost front propagates significantly faster when phase change occurs.