Experimental study on heat and moisture transfer in soil during soil heat charging for solar-soil source heat pump compound system

Due to the imbalance of heating and cooling demand, the soil temperature decreases after several yearsʹ operation for soil source heat pump system, and consequently the coefficient of performance (COP) decreases. Soil heat charging with solar energy was proposed for solar-soil source heat pump compound system. Many studies have been carried out to investigate the heat and moisture transfer in soil for soil heat charging, but they failed to provide a clear description and explanation of the coupled and mutual effect between heat and moisture transfer. This paper presents an experimental study on the heat and moisture transfer in soil heat charging. The simplified testing rig was constructed to investigate the coupled and mutual effect of one-dimension horizontal heat and moisture transfer in soil heat charging. The results show that, peak volumetric water content (VWC) occurs firstly at the position near heat source and moves towards the other cold end of the tested soil pillar, but it is limited in an area near heat source. The time consumption and value for peak VWC increase with the increasing heat source temperature at the same initial VWC and increase with the increasing initial VWC at the same heat source temperature as well. The heat source temperature is a main factor affecting the peak soil temperature and peak VWC, while the initial VWC has little impact on them.