| In this paper,the heat transfer performance of single-U type borehole heat exchanger?BHE?,downhole coaxial heat exchanger?DCHE?and groundwater source heat pump?GWHP?system are analyzed by using FEFLOW software.The main contents are as follows:Single-U type BHE:Without considering the temperature change of aquifer,the influence of inlet velocity of BHE and the location,thickness,seepage speed of aquifer on the heat transfer performance of BHE are researched.The heat transfer performance of the BHE increases with the increase of the inlet velocity.Under the condition of this paper,the optimum inlet velocity of the BHE is 0.8m/s.The location of aquifer is influential on the temperature distribution in depth direction of BHE but has little effect on the heat transfer performance.The critical seepage speed?Pe?,which determines whether groundwater seepage affects the heat transfer performance of BHE,are 1.89×10-6m/s?Pe=25.2?,1.02×10-6m/s?Pe=27.2?,6.98×10-7m/s?Pe=27.9?when the thickness are 10,20,30m,respectively.When the seasonal temperature change in the groundwater recharge area is considered,for the sites near the recharge area of groundwater,the seasonal temperature fluctuation of aquifers around the BHE becomes larger,where the heat transfer performance of the BHE tends to be determined by the coupled effect of both groundwater temperature fluctuation and the seepage velocity.The heat transfer rate per borehole depth increases linearly with the increase of the thickness of aquifers.GWHP system:The heat transfer performance and the change of temperature field in the aquifer around the heat exchanger influenced by the well spacing,natural hydraulic gradient,seasonal use pattern,recharge mode are simulated and analyzed.The results show that the increase of well spacing and natural hydraulic gradient is conducive to delaying the occurrence time of heat breakthrough,decreasing the degree of thermal breakthrough and improving the energy recovery efficiency of the system.Under the simulated condition in this paper,the best spacing between wells is 100m.When the well spacing is large,the heat transfer efficiency under the seasonal alternate use pattern is higher.Under the condition of the same well spacing,the phenomenon of heat breakthrough is relatively late and in a lower degree in the different layer recharge mode,the energy efficiency of the system is higher.DCHE:The influence of inlet flow rate,inner pipe material,circulating fluid flow mode,thickness and seepage speed of aquifer and geothermal gradient on heat exchanger performance are simulated and analyzed in the shallow and deep geothermal utilization.The results show that in the shallow geothermal utilization,when the PPR and PE are used as the inner pipe material,the heat loss in the return stage are respectively 25.60%and32.56%.The circulation mode has little influence on the heat transfer performance.In the deep geothermal utilization,when the PPR and PE are used as the inner pipe material,the heat loss in the return stage are respectively 21.03%and 34.90%.The heat transfer rate of reverse circulation mode is 15.45%higher than the forward circulation mode.The heat transfer rate per borehole depth increases linearly with the increase of the ground temperature gradient.The critical seepage speed?Pe?are 1.10×10-6m/s?Pe=14.7?,5.71×10-7m/s?Pe=15.2?,4.11×10-7m/s?Pe=16.4?when the thicknes are 10,20,30m,respectively. |
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