Heat transfer analysis of pile geothermal heat exchangers with spiral coils

► The heat transfer process of the buried spiral coils in piles are described. ► An analytical transient model and the relative temperature algorithm are derived. ► The coil pitch has an influence on the heat transfer performance of PGHE. ► Comparisons between the ring-coil and cylindrical models are conducted. ► The ring-coil model can better describe the heat transfer process of the PGHE. This paper investigates the transient heat conduction around the buried spiral coils which could be applied in the ground-coupled heat pump systems with the pile foundation as a geothermal heat exchanger. A transient ring-coil heat source model is developed, and the explicit analytical solutions for the temperature response are derived by means of the Green’s function theory and the image method. The influences of the coil pitch and locations are evaluated and discussed according to the solutions. In addition, comparisons between the ring-coil and cylindrical source models give that the improved finite ring-coil source model can accurately describe the heat transfer process of the pile geothermal heat exchanger (PGHE). The analytical solutions may provide a desirable and better tool for the PGHE simulation/design.