Performance of a horizontal heat exchanger for ground heat pump system: Effects of groundwater level drop with soil–water thermal characteristics

Abstract

Horizontal Ground Heat Exchangers (HGHEs) are generally embedded at a shallow depth above the GroundWater Table (GWT), where unsaturated soil exists. The change in the GWT strongly affects the thermal properties of the soil, and therefore influences the heat transfer efficiency of the HGHEs. This study evaluated the heat transfer performances of HGHEs with different backfill materials by considering the level drop of the GWT. In the experimental study, the soil–water characteristic curves and unsaturated thermal properties of backfill materials (natural sand, weathered granite soil, and controlled low-strength materials) were defined using modified 150-SWCC devices and a water extractor, coupled with a thermal conductivity measurement system. Subsequently, the influence of the GWT level change on the moisture content of the backfill materials was investigated using a seepage analysis (SEEP/W). Furthermore, finite element analyses were conducted to examine the influence of GWT level change on the thermal performances of the HGHEs. Based on thermal conductivity values experimentally obtained with corresponding volumetric water contents, the results from the experimental study indicate that thermal conductivity and volumetric water content have a linear relationship for all of the considered backfill materials. The results from the heat exchanger performance suggest that a HGHE backfilled with natural sand is the most affected by the GWT level drop among the three backfill materials, as it has the lowest air entry value and high-water content. In contrast, the HGHE backfilled with a controlled low-strength material is the least affected by the GWT level drop owing to its high air entry value and lowest water content. It is concluded a decrease in GWT level has a significant negative effect on the performances of HGHEs, especially when the decrease in the GWT level results in an increase in the soil suction that exceeds the air entry value of the backfill material.