Analysis on the transient heat transfer process inside and outside the borehole for a vertical U-tube ground heat exchanger under short-term heat storage

• A three-dimensional unsteady model is established for the vertical U-tube ground heat exchanger.
• The soil temperature field in the direction of the depth is distributed in a “narrow belt shape”.
• The temperature field inside the borehole is dominated by the inlet branch of the U-tube.
• The heat storage time has significant impact on the temperature difference between inside and outside the borehole.

A three-dimensional unsteady model is established to study the heat transfer performance for a vertical U-tube ground heat exchanger (GHE). The transient heat transfer process between the inside and outside of the borehole under short-term heat storage is analyzed. The results indicate that the soil temperature field in the depth direction at the center section is distributed in a “narrow belt shape”. The thermal interference distance of the ground heat exchanger under short-term heat storage is within a radius of 1 m, while the main heat transfer field is within a radius of 0.4 m. The inside borehole temperature field is dominated by the inlet branch of the U-tube, and it gradually becomes uniform as the heat storage time increases. For the temperature difference between the inside and outside of the borehole, the longer the heat storage time is, the greater the temperature difference between the borehole wall and the surrounding soil is and the lower the temperature difference between the fluid in the U-tube and the borehole wall is. At the same time, the change in temperature difference between the surrounding soil and the more distant boundary soil is not obvious.