Experimental study on heat transfer of soil with different moisture contents and seepage for ground source heat pump

Abstract

Geothermal energy is a renewable energy source for heating, ventilating and air-conditioning (HVAC) systems, and is leveraged by ground heat exchanger (GHE) which is a key element of ground source heat pump (GSHP). The performance of GHE is significantly affected by soil thermal properties. To explore the heat transfer characteristics of soils with different moisture contents (MC) and seepage, a well-controlled sandbox was established and a series of experiments were performed. The results demonstrated that the soil with higher MC had stronger heat dissipation capacity, especially when seepage occurred. The temperature sensor 25 cm from the sandbox-centre detected temperature changes at the 452^nd, 206^th and 297^th min under dry, unsaturated and saturated conditions, respectively. Moreover, the dominant mode of heat transfer in soils with different MCs was conduction, and the elevated heat-releasing rate enhanced heat propagation by moisture transfer. Furthermore, the size of the thermally affected zone was proportional to the thermal diffusivity, which was 2.14 × 10⁻⁷, 5.18 × 10⁻⁷ and 4.74 × 10⁻⁷ m²/s under dry, unsaturated and saturated conditions, respectively. Overall, these results indicate that the MC is a critical factor in characterizing soil heat transfer. This study provides guidance in choosing the installation environment for GHE and confirms the importance of matching soil characteristics with the GHE load.