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

The results of two thermal re- sponse tests recently performed on two ver- tical 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 flnite-element simulation method, devel- oped through the software package COM- SOL Multiphysics 3.4. The problem is sketched as two dimensional and the true geometry of the BHE cross section is con- sidered. The convective thermal resistance is reproduced through an efiective (reduced) conductivity of the tube walls. Water is con- sidered 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 efiective 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.