Numerical simulation of a freeze–thaw testing procedure for borehole heat exchanger grouts

The amount of research conducted on geothermal energy as a source for heating and cooling demands of buildings, as well as for electrical energy production, has increased substantially in the past decades. The simulation of freezing and thawing is important for geothermal applications involving ground coupled heat pumps. One area of research is the development of grout cements for borehole heat exchangers (BHE). In many cases, BHEs are operated at temperatures below 0 degrees C due to manifold reasons. Hence, the simulation of freezing and thawing cycles (FTC) is important for such geothermal applications, especially in cold regions. Recently, a testing device for measuring and quantifying the influence of FTC stresses on the mechanical integrity and hydraulic properties of BHE grouts was developed (Anbergen, published in 2014). The testing procedure simulates the downhole in situ conditions as confining radial earth pressure, freezing, and thawing directions from the inside to the outside and under saturated conditions. The hydraulic conductivity can be measured in axial flow direction. Thus, statements regarding the susceptibility of grouts against cyclic freezing and thawing stresses can be made. These results differ substantially from earlier findings, as in situ boundary conditions were not simulated sufficiently. For the verification of the procedure's thermal process, temperature logs were recorded using thermocouples and thermography imaging. The thermal process was simulated using the finite element method (FEM) groundwater, heat, and mass modeling software FEFLOW. FEFLOW is a common software solution for thermohydraulic coupled groundwater applications with mass transport, as well as geothermal applications. However, up until now, the program could not yet simulate phase changes between solid and liquid phases. To enable the program for such simulations, a plug-in was developed. To do this, a C++ code was written and coupled to the simulation routine of the FEM software. The code is based on a modification of the material parameters of fluid and the incorporation of the latent heat effects in the fluid heat capacity. A linear and an exponential approach for the latent heat release were implemented and benchmarked. The code was verified using different analytical solutions and other FEM codes. Finally, the experimental results of the test procedure could be successfully computed using the new plug-in. Thus, it is now possible to compute phase changes with FEFLOW for geothermal applications as well as other applications like permafrost research.