Simulation of temperature and moisture fields around a single borehole ground heat exchanger: effects of moisture migration and groundwater seepage

Abstract

This paper aims to investigate the effects of moisture migration and groundwater seepage on the heat transfer capacity of ground heat exchangers in stratified soils. A three-dimensional unsteady groundwater flow and heat transport model was established using finite volume method. Sixteen cases with different model considerations and initial soil conditions were simulated based on the proposed model. A group of 8 cases considering only transverse moisture migration and another group considering both transverse and longitudinal moisture migration were compared. The heat and moisture fields after 30 days of operation reveal that considering the change of saturation caused by vertical moisture transfer, the soil temperature field will be affected, but borehole outlet temperature was less influenced. The absolute value of outlet temperature difference between corresponding cases in the two groups is only about 0.2 °C. The position of groundwater seepage and arrangement of unsaturated soil layers with different degrees of saturation on heat transfer capacity of vertical ground heat exchanger were further explored. The results show that the longitudinal moisture migration would be made more influential by the existence of seepage layer, because the average relative deviation of inlet and outlet temperature difference between the corresponding cases of Group 1 and Group 2 was 1.34% when setting seepage layer and was 0.44% when without seepage layer. Heat transfer performance of borehole heat exchanger is also affected by the location of seepage layer. The average relative deviation of inlet and outlet temperature difference between the reference case and cases with seepage in the top, middle and bottom layers is 34.18%, 25.08% and 16.82%, respectively. The arrangement of unsaturated soil layers also has a certain effect. When the soil layer with low degree of saturation is located in the upper layer of soil, heat transfer capacity is better.