When BHE are in operation, freeze-thaw cycles can lead to a damage in the annular space filling of the BHE and the surrounding formation. In Baden-Württemberg, the guideline “Qualitätssicherung Erdwärmesonden (LQS EWS)” regulates how to built BHE. In order to prevent potential frosting in the backfilling and formation, the minimum inlet temperature in the probes of the BHE is limited to minus 3 °C. This minimum temperature, determined on the basis of empirical values, was experimentally investigated in the research project “Influence of the Operation of Heat Pumps on Potential Freezing of BHE – EWS-Frost” in large-scale experiments considering the determinations in the guideline.
The results of the large-scale tests from the EWS-Frost research project show that using the “minus 3 °C” rule does not prevent the BHE from a freezing hazard in certain operation scenarios.
For a reliable evaluation of the “minus 3 °C” limit, further investigations of different geometric as well as material- and operation-related boundary conditions of a BHE are necessary. The interaction of various parameters such as inlet temperature, heat removal times, borehole geometry, grouting materials, surrounding formation, material of the probe tube, type of flow in the probe tubes, initial temperature of the formation etc. results in a large number of possible scenarios. An experimental investigation of all possible parameter combinations is neither economical nor feasible within an acceptable time frame.
Solites therefore creates a suitable simulation model and carries out the numerical determination of the minimum permissible BHE inlet temperature to prevent freezing. Critical operation scenarios are identified, which are investigated by the project partner VEGAS in medium scale frost experiments.