This paper presents the development of a hybrid model for simulating a vertical U-tube geothermal heat exchanger under actual climate. The model consists of a 3D model representing a pair of U-tubes buried in symmetric boreholes and a supplementary 2D model that can be applied to real site scenarios. The temperature series at the wall of the borehole and initial soil temperatures at different depths derived from the simulation results of the 3D model serve as the input boundary conditions of the 2D model. Hence

the hybrid model shares the respective advantages of both subordinate models and therefore provides comprehensive simulation results that span approximately 10 months at a slight cost in implementation complexity. Meanwhile

the model is limited by numerous assumptions such as the consistency between the ground surface and air temperatures and physical uniformity of the materials. Simulations were performed based on the conditions obtained at a real site in the city of Hangzhou. The simulation time range was June 12

2015

to March 17

2016

which covered an entire cooling

transition

and heating season. The collected data revealed that the soil within an effective radius of 2 m gained an absolute increase in internal energy of 0.292 MJ after the simulation. The relative increases in the internal energy and average temperature were 1.037 MJ and 0.28 K

respectively

compared to those affected only by climate

which indicated an unwanted accumulation of heat beneath the ground. Moreover

the temperature at a deeper soil depth showed a lag in phase behind that of a shallower depth. This lag was approximately 2 weeks between a point at the center of the entrance and exit of the U-tubes and a point 1 m below it. Moreover

the fluctuation patterns of the inlet/outlet fluid temperatures showed a significant shift from the air temperature toward the annual averages

which were -8.313 K in the cooling season and 9.077 K in the heating season. Finally

the temperature distribution among pipe-clusters at different depths showed that the heat accumulation mainly occurred at a depth greater than 7 m

and the accumulation was more severe at a shallower depth and at the corners of clusters.