Numerical research on the heat transfer model and performance of deep borehole heat exchangers combined with geothermal wells

For the purpose of carbon neutrality, deep borehole heat exchanger (DBHE) is widely utilized in building heating by efficiently developing hydrothermal geothermal resources. In this study, a DBHE combined with the geothermal wells is proposed given that there are abundant hydrothermal geothermal resources in the BoHai Bay basin, China. A three-dimensional unsteady heat and mass transfer model coupled with DBHE and thermal reservoir is developed taking into account the forced seepage. Based on the moving finite line source model, the transient dimensionless analytical solutions are derived by applying Fourier and Laplace transform. The proposed model and solving method are validated against the experimental data determined from a field test and the previous numerical simulations, respectively. The calculation results indicate that the heat transfer power per unit buried depth increases by 39.60 W/m and its decline rate throughout the operating period decreases by 7.17% when the exploitation quantity of geothermal water increases from 0 m(3)/h to 120 m(3)/h. With the increase of exploitation quantity, the thermal diffusion radius ranges from 10.11 m to 20.34 m at the downstream, however, the thermal diffusion radius decreases from 10.11 m to 9.01 m at the upstream. When the exploitation quantity of geothermal water is 0 m(3)/h, 60 m(3)/h and 120 m(3)/h, the average heat transfer power increases by 139.37 kW, 174.20 kW and 200.80 kW respectively with the increase of circular quantity of DBHE from 10 m(3)/h to 60 m(3)/h. When the exploitation quantity of geothermal water increases, the forced seepage (Darcy flow rate) in the thermal reservoir is enhanced, and the influence of the circular quantity of the DBHE on the heat transfer performance is improved. The DBHE combined with the geothermal wells can be utilized in the case of existing geothermal well construction, which will enhance the heat transfer performance of DBHE substantially. The conclusions obtained from this research will be a reference to guide further research and application of the DBHE or DBHE array combined with the geothermal wells under different geological conditions and operation modes.