Numerical Simulation of Freezing-Thawing-Cycles in the Grout of Borehole Heat Exchangers

Abstract

Borehole heat exchangers (BHE) are a popular and efficient way of regenerative heating. Due to hydrogeological legal constraints, in Europe most BHEs have to be grouted and boreholes must remain sealed under all operating conditions. Borehole heat exchangers operated with fluid temperatures below zero degrees Celsius are required to be grouted with a material that guarantees adequate sealing also under thaw-freezing-stresses. A testing device was developed, that measures the hydraulic conductivity of grout specimens after a freely selectable number of cyclic freeze-thaw-stresses. Simulating radial earth-pressure and following the freezing directions as in-situ, results differ from earlier investigations substantially. Consequently the frost-resistance of the grouts can be assessed. The temperature distribution during the testing procedure and in-situ are highly influenced by phase change and latent heat effects. For numerical modeling of such processes using the FEM flow and heat-transport simulation software FEFLOW was extended by a phase change plug-in. The plug-in was validated by numerical benchmarks with SHEMAT and analytical solutions as well as with experimental data.