The use of low-enthalpy geothermal energy through vertical borehole heat exchangers (BHE) represents a low-impact solution, based on a renewable energy source that is widespread worldwide. This study focuses on studying the most significant parameters that influence the energy performances and the environmental impact of vertical borehole heat exchangers (BHE) installed in different geological contexts.
The aim of the study is to develop a design guide for designers indicating the optimal distances between a row of BHE arranged perpendicular to groundwater flow and varying operational condition of the energy system, geometry of BHE and the hydrogeological parameters of the aquifer. The study was performed through the implementation of synthetic numerical models into MODFLOW-USG code. The vertical U-shape pipes of BHE were implemented into the model adapting the Connected Linear Network (CLN) Package, such as discussed in Antelmi et al., 2021. Specific flow and thermal boundary conditions were introduced to reproduce the real operation of a vertical BHE and various physical and hydrogeological properties of the aquifer were set to simulate a wide range of porous materials characteristics (from clay to sand and gravel).
Starting from numerical models representing a single BHE, having the aim to determine operating conditions without interferences, different borefield of 3 and 5 BHE were implemented to study in detail how their closeness can influence the energy performance of the central BHE and the extension of the thermal plume. Fixed a threshold (95%) of acceptance related to the heat rate exchanged reduction between BHE and surrounding subsoil, the optimal distances between geothermal exchangers were carried out for each parameter combination: results show an optimal distance ranging from 8 meter for fine materials and low groundwater flow velocity to 2 meters for coarse materials and higher groundwater flow velocity.