Heat transfer energy efficiency characteristics of U-tube ground heat exchangers in stratified soil

Based on soil stratification, introduces an energy efficiency coefficient index to evaluate heat transfer characteristics of ground heat exchangers (GHEs). By dynamic simulation, investigates the impact of soil stratification in the radial direction and axial direction on the heat transfer characteristics of GHEs. The results show that the thermal conductivity of backfill material, and position and thickness of soil layers with different thermal conductivities have impact on the heat transfer characteristics of GHEs. The backfill material with higher thermal conductivity can weaken or counteract the heat accumulation effect in early stage but may strengthen short circuit of heat in later time. The section efficiency coefficient can be improved when the soil layer with higher thermal conductivity is on the upper part of borehole. The energy efficiency may increase with the thickness of the soil layer with high thermal conductivity.