Performance study of a ground heat exchanger based on the multipole theory heat transfer model

•A heat transfer model based upon the multipole theory inside the borehole is developed.•The experimental validation of the multipole model has been performed.•The effects of some factors on the thermal effectiveness coefficient are analyzed.•The model could analyze the temperature field in complex profile and boundary.

A vertical U-tube ground heat exchanger can be utilized to exchange heat with the soil in ground source heat pump systems. The outlet temperature of the working fluid through the U-tube is an important characteristic parameter of heat transfer process. It is quantified by an effectiveness coefficient of ground heat exchanger in this paper. A multipole model based upon the multipole theory is developed inside the borehole with a cylindrical heat source outside the borehole, considering the working fluid temperature variety along the U-tube and its heat convection. The model overcomes the boundary shape limitation of traditional analytic model and could analyze the temperature field in complex profile and boundary, such as the two branch pipes of GHE. The experimental validation of the multipole model has been performed. A dynamic simulation was conducted to evaluate the influence of a number of parameters on thermal effectiveness. The effect of some important characteristic factors on the heat transfer performance of ground heat exchangers is analyzed. The results may provide the references for design of ground source heat pump system in actual projects.