热渗耦合的分层土壤中管群换热器热性能分析

Thermal performance analysis of multiple borehole heat exchangers in the heat conduction and advection coupled layered soil

准确获取管群换热器在复杂土壤中的传热情况,有利于合理设计地源热泵系统。该研究建立了考虑地下水渗流的三维管群分层数值模型,通过引入区域热效率(E)和动态性能损失(Dynamic Performance Loss,DPL)2个指标,评估不同因素对渗流作用下管群在分层岩土中传热的影响。结果表明:当渗流速度为100 m/a且渗流层厚度较大时,管群在分层岩土中的传热可以简化为在均质岩土中的传热;通过调整不同导热系数的土壤层的位置和厚度,管群区域热效率的变化可以忽略不计;变等效入口流速和变入口温度的变化对管群区域热效率的影响可以忽略。在渗流速度为100 m/a、连续运行2000 h、管间距为3 m的叉排管群,下游DPL仅比管间距为6 m的顺排管群高1.27个百分点。因此,当渗流速度为100 m/a且渗流层厚度较大时,建议顺排和叉排的管间距分别为4和3 m。此外,渗流速度越大,管群的区域热效率越大,DPL越低,且趋于稳定的速度越快。研究结果可为合理设计热渗耦合分层土壤中的地埋管管群以及有效缓解土壤热堆积提供参考依据。

Safe,clean,and efficient geothermal energy has been widely popular in various countries,particularly for reducing carbon dioxide emissions and achieving the transition from fossil energy to renewable energy.A rational design of heat pump systems depends mainly on the accurate acquisition for the heat transfer of multiple Borehole Heat Exchangers(BHE)in complex soil.In the present study,a multiple-layered numerical model was established for three-dimensional multiple BHE considering the groundwater advection using Fluent software.The accuracy of the model was validated using a 48h thermal response test data.Two indexes were introduced to evaluate the performance,including the regional thermal efficiency(E)and Dynamic Performance Loss(DPL).Furthermore,an analysis was made to clarify the influence of the location and thickness of the advection layer on the heat transfer of multiple BHE in layered soil.The result indicated that there was a small difference of E between the layered and homogeneous models,when the advection velocity was 100 m/a,and the thickness of the advection layer was large.The heat transfer of multiple BHE in the layered soil was simplified as that in homogeneous soil under this condition.The E of multiple BHE was ignored,only to adjust the position and thickness of the soil layer with different thermal conductivities at a groundwater advection velocity of 100 m/a.It infers that there was a small effect of the variation in thermal conductivity in different depths of soil layers on the heat transfer of multiple BHE.Therefore,the backfill material with higher thermal conductivity should be balanced to use in the backfill layer of buried pipe in the multiple BHE,when the advection velocity of groundwater was 100 m/a.There was no influence of variable equivalent inlet velocity(0.115,0.158,and 0.237 m/s)and inlet temperature(300,310,and 320 K)on E the multiple BHE.The downstream DPL of the staggered arrangement multiple BHE with 3 m pipe spacing was only 1.27 percentage points higher than that of the aligned multiple BHE with 6 m pipe spacing,after 2000 h of continuous operation under the groundwater advection velocity of 100 m/a.It was suggested that the borehole spacing of aligned and staggered arrangement should be 4 and 3 m,respectively,when advection velocity was 100m/a and the thickness of the advection layer was large.The average unit heat transfer rate of multiple BHE with 105 m advection layers increased by 24.4%,compared with that with 35 m advection layer.When the advection velocity was 100 m/a,the inlet temperature was 300 K,and the equivalent velocity in the pipe was 0.115 m/s,there was no effect of the advection layer location on the heat transfer of multiple BHE,whereas there was an obvious effect of the advection layer thickness on the heat transfer of multiple BHE.In addition,the E of multiple BHE was greater,while the DPL was lower,and the rate of stabilization was faster,as the advection velocity was larger.The findings can provide a sound reference for the rational design of multiple BHE in the layered soil with groundwater advection.