Understanding numerically generated g-functions: A study case for a 6x6 borehole field

Abstract: The Ground Source Heat Pump systems (GSHP) are an emerging technology used to exchange heat with the ground through the use of some buried heat exchangers. The thermal response of a borehole field can be characterized by its g-function. It is a non-dimensional temperature response factor, which can be calculated using either numerical or analytical solutions. Eskilson developed the first study made for the calculation of these g-functionts. Lamarche and Beauchamp proposed another analytical approach based on the Finite Line Source (FLS). Generally, both solutions present similar results with some small differences. They could be attributed to the boundary condition performed in both researches: the FLS solution considers uniform heat flux along the borehole wall in all the heat exchangers, while Eskilson’s model defines as a condition, uniform temperature at the borehole wall within all the pipes in the field. In this Master of Science Thesis, the temperature response factors (g-functions) of a 6x6 borehole field with 36 heat exchangers (BHE) arranged in a squared configuration are obtained from new numerical models, mainly based on the use of a highly conductive material composing the BHE. For this purpose, a commercial software called Comsol Multyphisics© is employed. The aim of this thesis is to get larger knowledge in generating the g-function in relation to the boundary condition performed in the model trying to reach better approximations to the reality. Some strategies with respect to the geometry, size of the model and mesh are performed to reduce the computing time. The influence of the geothermal heat flux and the influence of the highly conductive material (HCM) composing the BHEs are also studied in our model. Going further, the thermal behavior of the ground is also studied by imposing variable heating and cooling loads during seasonal periods over a time of 25 years. Finally, the g-functions obtained from our numerical models are compared to the one generated with the commercial software, Earth Energy Design (EED), which represents the numerical solution proposed by Eskilson, and the one generated with FLS approach. The results may explain in a closer approximation to the reality the thermal response for large borehole fields.