Numerical Simulation Study On Heat Transfer Characteristics Of Ground Heat Exchangers And Seasonal Effect

In recent years, ground source heat pumps (GSHPs) have been used increasingly in the fieldof renewable energy and building energy saving in China. Borehole heat exchanger (BHE) whichlargely determines the performance of the whole system is crucial for ground source heat pumps'air-conditioning system. Studies are carried out to research the heat-transfer performance of theBHE by means of numerical simulation; the following contents can be included:Three-dimensional models of the heat-transfer performance of the BHE with single anddouble U-type were set up, and the numerical calculations in steady-state were present. Then, thedifferent operating parameters influencing the heat-transfer performance were analyzed. Theresults show that the heat-transfer capacity, under these simulation conditions, was increasingwith the increase of the inlet velocity and the the thermal conductivity of backfilling. Besides,the heat-transfer capacity presented unobvious increase when the flow rate is greater than0.6-0.8m / s and 1.8-2.2W/m·K. The heat-transfer performance of the BHE with single anddouble U-type was calculated in unsteady-state under different heating power. Taking advantageof the UDF function, the change of initial soil temperature in winter and summer was described.Moreover, this paper laid special stress on studying the seasonal effects on thermal response test.The simulated results show that the BHE with double U-type tends to be stable takes thelongest time, but the thermal conductivity of soil determined by thermal response test (TRT) wasthe best fit with the theoretical value. To the BHE of single U-type with the condition of 3kwheating power, the thermal conductivity of soil in winter was 1.5 times to that in summer.Constantly increasing the heating power can improve the accuracy of the thermal conductivity ofsoil determined by TRT. By contrast, to the BHE of double U-type with the condition of 6kwheating power, the thermal conductivity of soil in winter was 1.12 times to that in summer. The results of this study help to further understanding of the heat-transfer performance between theBHE and its surrounding ground, and they provide significant guidance for the thermal responsetest.