| In order to achieve the goal of “peak carbon dioxide emission and carbon neutrality”,China vigorously promotes the application of geothermal energy and other kinds of renewable energies in the field of construction.In recent years,more and more buildings adopted middeep geothermal energy for building heating.The mid-deep ground source heat pumps(MDGSHP),which takes heat but not takes water,has gradually become the main mid-deep geothermal energy heating technology.Due to the huge calculation workload,the current research on the heat extraction characteristics of multiple deep borehole heat exchanger(MDBHE)is seldom reported,and little is known about the influence of the borehole layout on the long-term operation characteristics of MDBHE,which makes the lack of theoretical guide for borehole layout design in the engineering design stage.In view of the above situation,the heat transfer model of MDBHE is established in this thesis.Based on the model,the effects of six kinds of borehole layouts on the operating characteristics of MDBHE are studied with a nine borehole MDBHE.The influence of circulation fluid flow rate,terrestrial heat flow rate,thermal conductivity and borehole spacing on the operating characteristics of MDBHE is also analyzed.The results show that under the same conditions the heat extraction capacity of the singleline layout MDBHE is the largest,and that of the square layout is the smallest.The MDBHE with the borehole layout which brings larger heat extraction capacity makes less disturbance to the geotechnical temperature field,while that brings less heat extraction capacity causes greater disturbance to the geotechnical temperature field.Since the thermal disturbance among boreholes is minor at the end of the first heating season,the operating characteristics of the BHE of different borehole layout are the same.With the extension of the operating time,the mutual thermal influence among boreholes gradually increases,and the operating characteristics of different borehole layout are increasingly different from each other.The impact of the borehole layout on MDBHE operating characteristics increases with the increase of terrestrial heat flow rate and circulation fluid flow rate.For all kinds of borehole layouts,the heat extraction capacity and inlet circulation fluid temperature of MDBHE increases with the increase of circulation fluid flow rate and terrestrial heat flow rate,but the increase degree gradually decreases.The influence of the geotechnical thermal conductivity on MDBHE heat capacity is closely related to the value of the terrestrial heat flow rate.When the terrestrial heat flow rate is small,the MDBHE heat capacity increase with the increase of geotechnical thermal conductivity.When the terrestrial heat flow rate is large,the MDBHE heat capacity decrease with the increase of geotechnical thermal conductivity.The reason is that the initial ground temperature gradient along the depth direction increases with the increase of the terrestrial heat flow rate,but it decreases with the increase of the geotechnical thermal conductivity.The mutual thermal influence among boreholes decreases with the increase of borehole spacing.Therefore,increasing the borehole spacing can increase the heat extraction capacity and inlet fluid temperature.However,the influence of the borehole spacing on the MDBHE operating characteristics becomes weaker and weaker as the borehole spacing gradually increases.The parameter of heat extraction capacity enhancement rate is introduced to quantify the effect of the borehole spacing increase on the heat extraction capacity of different borehole layout.The results show that,for the MDBHE with the borehole layout which brings larger heat extraction capacity,the increase of the heat extraction capacity enhancement rate is smaller than that brings smaller heat extraction capacity.To compare and optimize borehole layouts,the parameter of Equivalent Borehole Spacing(EBS)is introduced to fast evaluate the heat extraction capacity of MDBHE with different borehole layouts.For different borehole layouts of MDBHE with the same number of boreholes,the heat extraction capacity of the MDBHE increases with the increase of EBS.At the beginning stage of the engineering design of MDBHE layout,the preliminary borehole layout can be determined by just comparing EBS of different borehole layouts,and then detailed design calculation can be carried out.By this,it can significantly reduce the designing calculation workload. |
