Validate Frost

Detailed Evaluation of the “Minus 3 °C-Limit” for Borehole Heat Exchanger Systems (BHE) while Operation


  • Geothermal energy


  • Research
  • Knowledge transfer


  • Public funding


09/2018 – 08/2020 (2 years)


606,000 EUR


Validate Frost evaluates the „minus 3 °C limit“ as a minimal inlet temperature for borehole heat exchanger systems (BHE) while operation to prevent freezing of the backfill material as well as in the underground as it is determined in the guideline for quality assurance of borehole heat exchanger systems “LQS EWS” of Baden- Württemberg.


Federal state of Baden-Württemberg


  • Steinbeis Research Institute Solites
  • University Stuttgart – Institute of Geotechnical Engineering
  • University Stuttgart – Institute for Modelling Hydraulic and Environmental Systems


The aim of the Validate Frost joint research project is to determine the minimum permissible inlet temperature of geothermal probes. This is done in particular under the premise of frost prevention in the surrounding formation and in the backfill material, as well as a sufficiently low system permeability in the backfill material and in the contact zones between backfill material and borehole wall or probe pipes. The results obtained in the large-scale test of the previous research project “Influence of the Operation of Heat Pumps on Potential Freezing of BHE – EWS-Frost” are going to be transferred to the real geological situation.


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.


Expected results are:
  • Creation and selection of a suitable simulation model for the thermal conditions in a BHE and the surrounding formation
  • Identification of critical operation scenarios for the integrity of the BHE, if the minimum inlet temperature is -3 °C

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Yannick Reduth

This project has received funding from

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