ABSTRACT
Most of the existing methods and studies to predict thermal performance of GHEs have considered purely conductive heat transfer in the ground or conductiveconvective heat transfer with pressure-driven convection, which occurs when there is groundwater flow (Chiasson et al., 2000, Diao et al., 2004). However, even in the absence of ground water flow in saturated soils, the temperature gradient induced by heating/cooling operations of GSHP systems may result in buoyant flow of groundwater due to density changes of (ground) water with temperature and consequently induced natural heat convection in the ground may occur. To investigate the effects of natural convection on thermal performance of GHEs, a 3D numerical model is developed.The model considers different carrier fluid velocities in the GHEs and the efficiency of GSHP systems subjected to natural heat convection are compared to the cases where only pure conductive heat transfer is considered in the ground. The governing thermo-hydro equations are implemented in the finite element package COMSOL Multiphysics to couple the heat transfer and fluid flow in the ground (porous medium) and the GHEs to the fluid flow and heat transfer in the carrier fluid circulating in the pipes of GHEs (Comsol, 2015).
