Research on thermal storage and release capacity of freezing sandy soil with phase change

•Thermal storage and release capacity of freezing sandy soil is studied in test.•Considering phase change of water in sand, the numerical model of GHEs is built.•Effect of continuous running on heat transfer performance of GHEs is analyzed.•Heat transfer per unit length of GHEs is calculated for saturated freezing sand. In cold coastal areas, the ground heat exchangers of solar assisted ground source heat pump system were buried in sandy soil and the soil contains water inevitably. When the heat pump supplied heat for buildings, geothermal energy will be extracted, which caused the soil temperature dropping below the freezing point of water. The aqueous sandy soil would be frozen and then the solar heat would be used for thawing. This paper studied the thermal storage and release capacity of freezing sandy soil with phase change by using experimental and numerical methods, respectively. The conclusions could be obtained that the heat transfer per unit length of GHEs and average COPs of heat pump under freezing sandy soil condition were both higher than those under the dry and no freezing sandy soil. The numerical model of the GHEs in soil was built and validated by experimental results. Based on the verified numerical model, the flow rate in pipe, the burial depth of GHEs, the moisture content of soil and the kinds of water in soil were reset to simulate the heat transfer performance of the experimental system for three sunny days. The results showed that the average heat transfer per unit length was calculated as 59.6 W/m in the heat storage stage and 44.4 W/m in the heat release stage, respectively.