Effects of flow direction and thermal short-circuiting on the performance of coaxial ground heat exchangers

The effects of flow direction and thermal short-circuiting on the performance of coaxial ground heat exchangers are studied by finite-element simulations, performed through COMSOL Multiphysics 3.4 (©Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the “weak boundary form” boundary condition available in COMSOL Multiphysics; the laminar convective heat transfer in the outer annular passage is simulated directly. Two Coaxial Ground Heat Exchangers (CGHEs) with the same length but different cross sections are examined; moreover, two values of the ground thermal conductivity, as well as two materials for the inner tube wall are considered. The results point out that the annulus-in flow direction (fluid inlet in the outer annular passage) is more efficient than the center-in flow direction (fluid inlet in the inner circular tube) and that the effect of short-circuiting is not very important, if the annulus-in flow direction is employed. Indeed, with this inlet configuration the energy loss for thermal short-circuiting is less than 1% for time intervals longer than 1 hour.