It is critical to understand the heat transfer characteristics of pile heat exchanger (PHE) with double spiral pipes buried in parallel so that it can be designed feasibly and operated efficiently. Therefore

a 3-D dynamic simulation model as established to study the heat transfer processes of a pile heat exchanger in which double spiral pipes and the return risers were buried in parallel. And the numerical simulation results were validated by the data obtained in an in-situ test and analytical solution of the solid cylindrical heat source model. The temperature distributions along the pipes

in the pile body and soil surrounding are illustrated and analyzed. A continuous and three intermittent heat transfer processes of this PHE were simulated dynamically. Such parameters as the dynamic outlet water temperatures of the buried parallel double spiral pipes and the heat transfer rate per unit pipe length are revealed. The temperature variations at different locations on the pile wall and in the soil surrounding along the pile depth and in radial directions are analyzed. The index of pile wall temperature recovery percentage is proposed to quantify the temperature recovery degree under different intermittent operational modes. It is found that the smaller the on-off ratio

the more significantly the temperature recovery degree along the pile wall can be achieved. But the recovery degree in each intermittent period tends to decrease as the operational cyclic increases.