Comparison of 2D Conduction Models for Vertical Ground Coupled Heat Exchangers

The effect of the infinite length approximation on evaluating the temperature Tb of the external surface of Borehole Heat Exchangers (BHEs) is determined by means of COMSOL Multiphysics (© COMSOL AB.). In detail, two 2D models of a BHE are compared: in the first model, the computational domain is represented by a cross-section of the geometry, where the BHE is replaced by a hole subjected to a constant heat flux. In the second model, the computational domain is represented by an axial-section of the geometry and, thus, the actual length of the BHE is considered. Also in this case, the internal structure of the BHE is not modeled and a constant heat flux on its surface is prescribed. The discrepancies in Tb due to these two models are investigated as a function of time and a dimensionless correlation is proposed to estimate the error made by neglecting the BHE length.