Full-scale Study To Predict Heat Transfer Of Borehole Heat Exchangers Based On Thermal Dispersion

Borehole heat exchanger is an important component of the ground-source heat pump system.Research on the heat transfer of borehole heat exchanger（BHE）is the key to enhance the heat transfer efficiency of the 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.Based on the full-scale heat transfer model（CMLS-MFLS）built in the previous work,this research develop an improved full time scale model(CMLS-MFLS_TD)including thermal dispersion effect.The model is further applied to estimate the impact of thermal dispersion on the overall heat transfer in the cases of single borehole and multiple boreholes.Two conditions,which are constant unit load and variable building loads,are considered in both cases.The impact factors related to the thermal dispersion effect are also discussed.Such analysis provide theoretical basis and references to the optimal design of BHEs.In order to build the full-scale heat transfer model of BHE with thermal dispersion,the thermal dispersion coefficient is correlated with seepage velocity.By combining the thermal dispersion coefficient with stagnation heat transfer coefficient,it is able to evaluate the impact caused by thermal dispersion in the heat transfer procedure around boreholes.The impact of full time scale is indicated by a simultaneous consideration of the transient thermal conduction related to the internal structure and the axial thermal conduction related to the boundaries of the borehole.The proposed model is validated with numerical models in different time length.Using this improved model(CMLS-MFLS_TD),heat transfer analysis is applied in the cases with single borehole and multiple boreholes.The main findings are summarized as follows.The appropriate range of seepage velocity applied in the model is from 1e-8m/s to1e-6m/s.Under constant unit load,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 stronger thermal dispersion effect.Based on a comprehensive consideration of the three parameters,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.For a single borehole,within the range given in this study,it is found that under constant load,thermal dispersion can increase the steady-state heat transfer ability of the borehole by 6.07%to 49.71%;under annual building loads,combined with the heat transfer analysis of the effect of thermal dispersion,the required tube length of the BHE can be reduced compared to the traditional design,which exclude thermal dispersion effect.For multiple boreholes,the upstream position are highly affect by thermal dispersion effect,followed by the midstream and the downstream;under the annual building loads,the fluid temperature derived in the upstream position of BHE is closest to the soil temperature;by considering the operation conditions of cooling and heating,combined with the heat dispersion effect,the required tube length of the upstream,midstream and downstream BHEs can be reduced successively when compared to the traditional design without thermal dispersion effect.The lengths of the tubes in the upstream boreholes reduce most among the proposed three positions.