Numerical investigation on the underground thermal imbalance of ground-coupled heat pump operated in cooling-dominated district

Underground thermal imbalance is a common problem in ground-coupled heat pump (GCHP) system operated in cooling-dominated district, which will cause the average underground soil temperature to rise, and inevitably deteriorate the operation performance of GCHP system. In this paper, a two-dimensional heat transfer model with groundwater advection was developed for modeling the heat transfer of underground GHE array. The influences of underground thermal imbalance rate, soil type, borehole layout and groundwater advection on the underground soil temperature distribution were analyzed. The results indicate that the soil temperature increase with the increase of the ratio of annual heat released into the ground to that extracted from the ground, and thus the underground thermal imbalance becomes worse. The better the soil thermal conductivity and diffusivity are, the faster the soil heat diffusion is, and thus the underground heat buildup can be alleviated. At the same time, the underground thermal imbalance can be controlled or eliminated by decreasing the intensive degree of borehole layout through increasing the borehole spacing or using strip type and block layout. Additionally, the underground heat buildup can be efficiently removed by the groundwater advection.