Modelling of helical coil heat exchangers for heat pump applications: Analysis of operating modes and distance between heat exchangers

Highlights: • A 3D finite element model is developed for Geothermal Heat Exchangers (GHE). • Non-isothermal pipe flow theory is coupled with 3D heat transfer theory. • The model considers the helical GHE as well as fluid flow and heat exchanges. • Effects of operating schedules on the extracted heat are shown for single device. • For multiple devices, suitable distances to improve the performances are proposed. - Abstract: Very shallow geothermal baskets are an advantageous alternative to conventional borehole and horizontal ground heat exchangers. This study proposes 3D finite element modelling of helical coil Ground Heat Exchangers (GHE) focusing on the real geometry of the heat exchanger and the energy extracted from the ground. The developed model is used to analyse a single GHE and multiple GHEs. The results show that an intermittent short time operating mode improves the GHE's performance. Concerning the multiple GHEs, the analysis in different configurations has shown that the optimal distance between the GHEs (of 1 m diameter) is at least equal to 4 times the GHEs diameter for line or square configurations and 6 times the GHEs diameter for hexagonal configurations. The developed 3D Finite Element model could be helpful in investigating helical coil GHE problems outside of the basic assumptions of the usual 2D axisymmetric models.