A NUMERICAL MODEL FOR THE SIMULATION OF A VERTICAL U-BEND GROUND HEAT EXCHANGER USED IN A GROUND-COUPLED HEAT PUMP

Heat transfer between the ground heat exchanger (GHE) and surrounding soil is a common problem for the optimum design and simulation of ground-coupled heat pump (GCHP). In this article, a quasi-three-dimensional numerical model of vertical U-bend GHE was proposed and developed by coupling the one-dimensional model of fluid in the depth direction with the two-dimensional heat transfer model of soil in the level direction but considering the soil temperature variation along depth. In the model, the soil in depth direction was divided into saturated and unsaturated regions by water table, and the influence of groundwater advection on the heat transfer performance of GHE was considered for the saturated soil region. Based on the model, the variations of heat rejection and fluid outlet temperature of GHE with time were discussed, and the influences of soil thermal properties and groundwater advection on the heat transfer performance of GHE were analyzed. The results show that the heat rejection in cooling mode will decrease with time, and an intermittent heat rejection mode can improve the heat transfer performance. Meanwhile, the increases of soil thermal conductivity and heat capacity can all enhance the heat transfer characteristics, and the exist of groundwater advection can improve the heat exchange between the GHE and surrounding soil. Furthermore, an experimental validation was performed in a solar-geothermal multifunctional heat pump experiment system. The results indicate the predicted values of heat rejection rate of GHE are agreed well with the corresponding measured data, and the guess average relative error for the daily heat rejection is about 5.5%. This means the numerical model proposed in this article is feasible and thus can be used to simulate the heat transfer of GHE.