A numerical study on ground heat exchangers and the effect of their design on the performance of geothermal systems

In this study, OpenFOAM was used to model two different designs of ground heat exchanger (GHE). GHEs are used in geothermal applications such as ground source heat pumps and geothermal storage, both of which provide sustainable, low-carbon space heating and domestic hot water. The u-tube and coaxial ground heat exchanger designs were simulated in order to investigate differences in performance. Both designs consist of tubing placed inside of a well and surrounded by grout. A working fluid flows downwards into the tubing, exchanging heat with the ground, before returning upwards towards the outlet. The tubing in the u-tube design consists of a u-shaped loop, while the coaxial heat exchanger consists of two concentric pipes. While many studies in the literature have sought to compare the performance of these designs, the results thus far have been mixed. Existing experimental and analytical studies have faced methodological challenges. With experimental studies, the range parameters possible to investigate has been limited by the high cost of installing a GHE. Where analytical or semi-analytical models were used, the complex conjugate nature of the problem at a wide range of time scales could not be thoroughly accounted for. Therefore, this study used detailed numerical modelling in OpenFOAM to explore the performance of these designs. This was accomplished using the chtMultiRegionFoam solver, which coupled the heat transfer within the flowing fluid to the solid tubing, grout, and rock. The solver was adapted to allow for 1D fluid flow with empirical heat transfer coefficients, and a novel circular mesh was developed to model 1D flow within a pipe. The resulting u-tube and coaxial models were compared against existing experimental results with good agreement attained.