3D modeling to evaluate the thermal interferences between borehole heat exchangers in a Mediterranean area

Abstract

Thermal interference between Borehole Heat Exchangers (BHEs) results in a loss of performance of a geothermal installation. An analysis of this phenomenon can provide key information for enhance and optimize the design of a closed-loop geothermal system. To study this thermal effect in the subsoil, a simulation tool is needed to solve numerically the equations of flow and heat transport in a porous medium. The main objective of this research is to analyze and compare the influence of the distance between different BHE in three types of geometrical arrays using a 3D finite element modelling software. By fixing the heating and cooling demand of a group of single-family houses, different numbers of vertical BHEs in a closedloop system with simple parallel U-tubes were simulated. The 3D finite elements model was performed including the geological and hydrogeological settings of the northern part of Valencia city (SE, Spain) situated in the Mediterranean area. The applied geological, hydrogeological and thermal conceptual model is based on previously available information. The thermal model was calibrated in transient state with two different datasets: a set of operating data of a monitored shallow geothermal installation located in the ETSII-UPV (Escuela Técnica Superior de Ingenieros Industriales - Universidad Politécnica de Valencia - UPV), and data of a Thermal Response Test (TRT) did at the same area. The resulting model set the initial conditions to model the thermal interference between the different BHEs arrays. The studied configurations were matrices of 2, 4 and 9 BHEs, with variable distances between 3 and 20 m. The radius of thermal perturbation on the subsoil for each array of BHEs simulated was determined and the variation of the performance was characterized. The simulation results showed that there was a slight increase in temperature in the subsoil between the BHEs and its surroundings. Thermal interference was usually not visible in the first year, but in a long-term analysis, for a certain range of distances between BHEs, it was observed that a hot area appeared. The temperature increase calculated in one layer at an average depth was reproduced for different types of arrays. Even though the experimental data and the simulated scenarios corresponds to a specific place in Valencia City, the methodology scheme applied could be used in other cases, helping to understand the subsoil and BHEs behavior from a long-term point of view.