| Title | Modelling Thermo-Hydraulic Coupled Phase Change Processes of BHE Induced Ground Freezing |
|---|---|
| Authors | Dalla Santa, G; Galgaro, A; Farina, Z; Di Sipio, E; Anbergen, H; Rühaak, W |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Ground source heat pump; operating temperature; FEFLOW; freeze-thaw cycles; thermal conductivity; thermal plume |
| Abstract | The heat exchange with the subsoil related to a ground source heat pump closed-loop borehole heat exchanger (BHE) system has been shown to alter the natural thermal status of the surrounding sediments (Banks, 2012). When it is necessary to improve the rate of heat extraction per meter from the underground, anti-freezing solutions are added to the carrier fluid, in order to lower its working temperature. In such cases, the induced thermal anomaly is more intense and can cause freeze-thaw cycles (FTCs) in the surrounding sediments. FTCs in cohesive sediments affect irreversibly the soil structure, resulting in a change of sediments’ mechanical and hydraulic properties. Hence, it is important to estimate correctly the maximum propagation of the frost front induced by a BHE system, in order to evaluate the volume involved in the critical thermal processes. This work presents a modelling evaluation of the BHE thermal impact on the underground, considering the case of induced FTCs. A newly developed plug-in is applied to FEFLOW finite element modelling. The plug-in, not only incorporates the sediments’ thermal properties in frozen state, but also considers the phase change of the pore water by taking into account the release of latent heat. In this case the plug-in is applied to a fully discretized model representing a transversal section of a double-U BHE working with a high percentage glycol fluid. A real case scenario is considered where the loads are considerably unbalanced towards heating. The propagation of the thermal plume is simulated with and without application of the plug-in; finally the results are compared. |