Finite Element Simulation of Coaxial Borehole Heat Exchangers

Finite-element simulations of Coaxial Borehole Heat Exchangers (CBHEs) are performed by means of the software package COMSOL Multiphysics 3.4 (© Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered, both for winter and for summer working conditions. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the weak form boundary condition; the forced-convection heat transfer in the outer annular passage is simulated directly. The geometrical parameters of a CBHE usually employed in Italy, with a length of 100 m, are employed. Three different materials of the inner circular tube are examined: PE 100, PPR 80, and an ideal material with a vanishing thermal conductivity. Two different grounds and three different water flow rates are considered. The results point out that the effects of thermal short-circuiting and of flow rate on the performance of long CBHEs are important.