Numerical Analysis of the Heat Transfer Characteristics of Vertical U-Tube Ground Heat Exchanger Used in Ground Coupled Heat Pump

A heat transfer model of vertical U-tube ground heat exchanger (GHE) used in ground coupled heat pump (GCHP) is developed based on the energy conservation. The model takes the temperature variation of fluid along the depth into consider, and the thermal interference between the two adjacent legs of U-tube is also incorporated by introducing the idea of thermal interact angle and equivalent distance. Based on the model, the numerical analysis on the heat transfer characteristics of GHE is performed, the results show that the increase of the conductivity of the soil and grout, the flow rate and the decrease of the thermal interact angle can all improve the heat transfer performance of GHE, and the influence of the soil conductivity is predominate among these parameters. However, the increase of grout conductivity should have a limit, the enhancement of thermal interference between the two adjacent legs resulting from the increase of grout conductivity should be considered in the determination of grout conductivity. At the same time, the increase of flow rate should be constrained by the flow resistance, and could be optimized by a variable flow design.