Impact of thermal dispersion on full-scale heat transfer of borehole heat exchangers

Research on the heat transfer of borehole heat exchanger (BHE) is the key to enhance the heat transfer efficiency of the ground source heat pump(GSHP) system. Most of the BHE models proposed in the current literature ignore the influence of thermal dispersion, which is caused by spatial heterogeneity of underground aquifer. In order to consider thermal dispersion effect, an improved full-scale heat transfer model is developed by including thermal dispersion coefficient. The main findings are summarized as follows. The appropriate range of seepage velocity applied in the model is from 1×10-8 m/s to 1×10-6 m/s. The thermal dispersion effect is mainly reflected in the medium and long time scale. Seepage velocity, thermal dispersivity and porosity are the main factors affecting the heat transfer process. High values of the seepage velocity and thermal dispersivity of groundwater, as well as low values of the porosity, may lead to strong thermal dispersion effect. Seepage velocity has the strongest impact on the overall heat transfer around boreholes, followed by the thermal dispersivity, and the porosity has the weakest effect. As far as the borehole group is concerned, thermal dispersion has the greatest impact on the heat transfer process of the upstream borehole, followed by the middle reaches, and the downstream is the least. Within the parameters studied in this paper, thermal dispersion can increase the steady-state heat transfer capacity of the borehole by 5.52% to 49.93%. © 2021, Editorial Board of CIESC Journal. All right reserved.