| Ground Source Heat Pump（GSHP）air conditioning systems are widely used as a new type of HVAC system because of their advantages of safety,cleanliness,and efficiency.The reasonable design of Borehole Heat Exchangers（BHEs）is conducive to the economical and efficient operation of GSHP systems.Therefore,accurately establishing the heat transfer model of multiple BHEs in complex soil and obtaining its heat transfer performance in complex soil can provide a reference basis for the optimal design of GSHP systems.In this study,an analytical model of coupled heat transfer inside and outside the borehole for multiple-layer multiple BHEs considering groundwater seepage was proposed based on the Moving Finite Line Source（MFLS）model and quasi three-dimensional model in the borehole.The calculation results of the model in stratified and homogeneous soil were compared,and the calculation method of borehole wall temperature was determined.Finally,the influence of stratified geological parameters on the soil temperature response around multiple BHEs was explored.The results show that for the analytical model of multiple-layer multiple BHEs under seepage action,treating the stratified rock as homogeneous would result in a 6.8%deviation in the edge middle BHE outlet temperature.The borehole wall temperature should be selected by taking into account both the average value of double integration of the circumferential position of the borehole and the borehole depth direction.The analytical model proposed in this study is conducive to the rapid calculation of the heat transfer performance of large-scale BHEs.Compared with the analytical model,the numerical model comprehensively considers the effects of backfill materials in the borehole and fluid heat transfer in the pipe.Therefore,a three-dimensional layered numerical model of multiple BHEs considering groundwater seepage was established.Two indexes were introduced to evaluate the thermal performance,including the regional thermal efficiency（E）and Dynamic Performance Loss（DPL）.The effects of different factors on the heat transfer performance of multiple BHEs with different arrangements were investigated.For the best arrangement of multiple BHEs under seepage conditions,the effects of different factors on E of multiple BHEs were explored.The results show that when the seepage velocity is in the range of 1×10-6m·s-1to 1×10-5m·s-1and the thickness of the seepage layer is large,the downstream DPL of the staggered arrangements multiple BHEs with 3 m borehole spacing was only 1.27 percentage points higher than that of the aligned arrangements multiple BHEs with 6 m borehole spacing.In this case,it is suggested that the borehole spacing of aligned and staggered arrangements should be 4 and 3 m,respectively.The heat transfer of multiple BHEs in the layered soil was simplified as that in homogeneous soil under this condition.Finally,the thermal interaction coefficient（δ）is introduced to evaluate the thermal interaction between boreholes,and an empirical method was proposed for quickly obtaining the total heat transfer rate of a BHE field in stratified soil.The accuracy of the empirical formula was verified,and the effects of different factors onδbetween boreholes were investigated.The results show that in soils with thermal conductivities of 1 W?m-1?K-1to 4 W?m-1?K-1and seepage velocities of 5×10-7m·s-1,where the thickness of each layer does not vary significantly,theδvalue for multiple BHEs is largest in the first layer of soil.Considering comprehensively,for the heat transfer of multiple BHEs in layered soil under a seepage velocity of 5×10-7m·s-1,the inlet temperature should be set at 30℃to 35℃and the inlet velocity set between 0.6 m·s-1and 0.8m·s-1. |
