Numerical evaluation on the effects of soil freezing on underground temperature variations of soil around ground heat exchangers

Freezing of soil around ground heat exchangers (GHE) is a common problem for a ground coupled heat pump (GCHP) operated in the cold district, which will affect the underground soil temperature variation and its heat transfer performance. In this paper, a two-dimensional model for heat transfer with freezing-thawing phase change in soil around GHE is developed and numerically analyzed by apparent heat capacity method. The influences of soil freezing, soil water content, and soil type on soil temperature variations and long term underground thermal imbalance of GCHP with GHE array are investigated. The results indicate that the soil freezing can lessen the soil temperature drop and thus increase the temperature difference between the fluid inside GHE and far-field soil. This helps to shorten the GHE design length and cut down the initial system cost. The soil freezing characteristics are mostly affected by the soil thermal diffusivity. From the view of the ability in delaying soil temperature drop, the sandstone is preferable to sand, and sand is better than clay. Additionally, increasing water content can reduce the drop degree and speed of the soil temperature, as well as, accordingly alleviate the underground thermal imbalance of GCHP.