Shallow geothermal energy: emerging convective phenomena in permeable saturated soils

Abstract

Ground source heat pump (GSHP) systems efficiently heat and cool buildings using sustainable geothermal energy accessed by way of ground heat exchangers (GHEs). Thermal performance of GSHP systems is typically investigated considering either pure conduction in the surrounding ground or the hydrogeological conditions (i.e. groundwater flow). However, in saturated soils, the temperature gradient in the ground induced by the GSHP systems heating/cooling operations may result in natural movement of groundwater due to the changes in water density, which leads to an emerging natural convective heat transfer in the ground. This usually ignored convection may influence the thermal performance of GHEs; therefore, to capture and quantify this effect, a GHE field consisting of 16, 30 m-long GHEs installed in a fully saturated soil is modelled using a state-of-the-art three-dimensional finite-element model. The effect of carrier fluid velocity on natural convective heat flux in the saturated soil is investigated in this study and is compared with cases where buoyancy-driven convection in the ground is ignored. Results show that for saturated soils with relatively high hydraulic conductivity, natural convection affects the thermal performance of GHEs significantly and ignoring this effect may lead to overdesign of GSHP systems.