A Cost-Effective 3D Finite Element Model for Predicting Transient Heat Transfer in U-tube Ground Heat Exchangers

Ground Source Heat Pumps (GSHPs) offer an energy-efficient and eco-friendly alternative to conventional heating and cooling systems by utilizing stable ground temperatures. This study presents a cost-effective and computationally efficient three-dimensional Finite Element Model (FEM) in COMSOL Multiphysics for predicting outlet temperatures and heat transfer rates in U-tube Ground Heat Exchangers (GHE), crucial for evaluating GSHP performance. The model examines three boundary condition scenarios: (1) uniform initial ground temperatures with constant surface temperature, (2) a Kasuda-based non-uniform profile, assuming constant average soil thermal properties, and (3) time- and depth-varying ground and surface temperatures from experimental data at Peniel Farm, Lexington, VA, with AI-based soil property predictions. Validated against reported laboratory data, the model accurately captures transient behaviors for both constant and interrupted heat inputs. By employing a detailed grid generation methodology, our approach achieves high accuracy with minimal computational costs, establishing the basic COMSOL package as a practical alternative to expensive Computational Fluid Dynamics (CFD) models. Results show that while constant temperature assumptions have minimal impact on outlet temperatures, heat transfer rates vary significantly near the borehole surface, emphasizing the importance of realistic boundary conditions in GHE simulations.