Finite-Element Evaluation of Thermal Response Tests Performed on U-Tube Borehole Heat Exchangers

The results of two thermal response tests recently performed on two vertical borehole heat exchangers (BHEs) are presented. The BHEs are located in North Italy; they have the same cross section and a depth of 100 m and 120 m respectively. The evaluation of the thermal properties of the ground and the grout is performed by a finite-element simulation method, developed through the software package COMSOL Multiphysics 3.4. The problem is sketched as two dimensional and the true geometry of the BHE cross section is considered. The convective thermal resistance is reproduced through an effective (reduced) conductivity of the tube walls. Water is considered as a solid in which a uniform heat generation occurs. The heat capacity of the water contained in the circuit is represented through a time dependent effective density of the water contained in the BHE. The method allows to reproduce accurately the time evolution of the mean temperature of water, even in the initial part of the heating process, and to obtain a reliable evaluation of the thermal properties of both ground and grout.