Characteristics of ground source heat pump considering soil freezing process in cold regions

In the cold northern region of China, when the ground source heat pump (GSHP) system runs in extremely cold weather,the soil near the surface is easy to undergo phase change and freeze, which affects the performance of the GSHP system. In this study, a residential building in Harbin, China, is taken as a case building. Based on fractal theory and TRNSYS, the changes of near-surface soil phase transition and performance of GSHP system under extremely cold conditions were studied. The results showed that the change of soil temperature has an impact on the underground circulating water and the indoor supply and return water of the heat pump system. The temperature of the underground circulating water fluctuates significantly with the change of soil temperature, and the indoor supply and return water is relatively little affected. The coefficient of heating performance ( COPh ) of GSHP system decreased by 5.6% after a whole heating season. When the GSHP system continues to operate for a long time, the soil temperature in the different depths of the buried pipe decreases, and the decline rate decreases with the increase of the depth. The soil near the earth surface (10 m, 15 m and 20 m deep) undergoes phase change freezing, and the soil at 25 m depth does not reach the phase change point. Therefore, the insulation of buried pipes near the ground is very necessary. The study will contribute to the application of GSHP system in practical engineering.