Experimental and numerical investigation of heat transfer performance and sustainability of deep borehole heat exchangers coupled with ground source heat pump systems

•A numerical model for DBHEs considering multilayer properties was developed.•The performance under different operating periods and run-stop ratios was examined.•The operation sustainability of DBHEs under typical operation modes was evaluated.•Soil temperature profiles under continuous and intermittent operations were presented. Deep borehole heat exchangers (DBHEs) are a state-of-the-art and feasible apparatus for building heating and renewable energy utilization. Conventional BHEs with long-term operation may experience notable performance degradation, DBHE may provide an alternative way to overcome this issue. In this paper, a numerical model was developed by considering the ground temperature gradient in the axial direction and multilayer thermal properties of rock and soil and was used to simulate the temperature distribution and the performance characteristics of coaxial DBHEs. The effectiveness of the model was validated against the experimental data collected from a demonstration project. It was shown that the simulation results agreed well with the experimental data. The DBHEs could offer a higher heat exchange capacity in comparison to conventional BHEs. In the intermittent operation for 10 years, the decreasing proportions of the outlet temperature under four different operation modes with the run-stop ratio (i.e. the ratio of the running time to the stopping time in a day) of 8:16, 12:12, 16:8 and 24:0, were no more than 3.57%. The rock temperature profiles in the heating mode of both commercial and residential buildings were presented and the annual decreasing proportions were less than 4.0%. The findings obtained from this study could be used as a reference for sustainability research of DBHEs under different working conditions and operation modes.