Study On Underground Thermal Unbalance Of Ground Coupled Heat Pump Operated In Districts With Unbalanced Cooling And Heating Load

Ground thermal imbalance is a key problem to be solved in the application of ground source heat pump (GSHP), which directly affects the proper promotion and healthy development of the GSHP. To control the underground thermal imbalance, the dynamic variation characteristics of underground soil temperature of GSHP operated in cooling-dorminated district were studied through theoretical analysis, numerical simulation and experiments. The effects of different factors on underground thermal imbalance of GSHP under unbalanced cold and heat load were investigated. This study can provide a theoretical base and technological guidance.In the aspect of theory, a GHE array heat transfer model with groundwater advection was developed, a computation program was written by Matlab language to solve numerically the model. Based on the numerical solution of the model, the factors that include underground thermal imbalance rate, soil type, GHE layout style and groundwater advection on the underground soil temperature distribution characteristics were analyzed for ten years operation. The results indicate that the soil temperature increase with the increase of the ratio of heat release to heat extract, and thus the ground thermal imbalance becomes worse. The better the soil thermal conductivity and diffusivity are, the faster the soil heat diffusion velocity are, and thus the soil heat buildup can be alleviated. At the same time, the ground thermal imbalance can be controlled or eliminated by decreasing the intensive degree of GHEs layout through increasing the spacing between different boreholes and using a strip type and block layout. Additionally, the heat buildup can be efficiently removed by the groundwater advection, and the GCHP will lose its original working principle.In order to balance annual heat release and heat extraction of GHE into/from ground, an external auxiliary cooling heat dissipation mode was presented in this article to control thoroughly underground thermal imbalance. The effects of control time and heat release intensity of external auxiliary cooling equipment on underground soil temperature variation regularity for ten years were simulated. The results show that the soil temperature can recover completely and timely by properly adjusting heat dissipation time and intensity of auxiliary cooling installation, and thus.the underground thermal imbalance can be alleviated or even eliminated.In the aspect of the experiments, the experiments operated in different mode of cooling tower-GHE combination system were conducted in a set of multifunctional composite GSHP experimental system. The soil temperature at different depth and radius direction were tested. The inlet and outlet water temperature and flow rate of GHE and cooling tower were monitored. The soil temperature resumme characteristics under different operation mode were obtained by the analysis of experimental data. The results indicate that the rapid increase of soil temperature can be effectively alleviated, the heat accumulation of GHE can be reduced, and thus the recovery rate of soil temperature can be improved by the external auxiliary cooling. Due to the energy storage characteristic of soil, the heat transfer along radius direction appears a certain delay phenomenon. For the experimental conditions used in this article, the thermal interference radius of GHE is about0.5m for an operation period of24hours. The altenate operation mode that the heat stored by GHE during day time is extracted to release by the cooling tower at night can effectively improve the heat storage effect of GHE in the second day. This is a good choice for the building that the cooling is only needed during daytime.This research can provide a theoretical and experimental foundation for further research of underground thermal imbalance control of GSHP. It can also provide a guidance for GSHP practical engineering application.