Abstract In designing and sizing of borehole thermal energy system, natural groundwater movement and temperature driven flow have a great importance on the borehole heat exchanger efficiency. The efficiency of double U – tube arrangement in gravel – backfilled borehole installed in a fractured limestone aquifer has been analyzed by means of three – dimensional numerical simulations. The numerical model is representative of 1 m deep of gravel back – filled borehole surrounded by the fractured aquifer. Several simulations have been carried out in order to evaluate the effect of aquifer parameters and boundary conditions on heat exchange efficiency by varying the mean temperature within the double U - tube. The fractured limestone aquifer of the industrial area of Bari (Italy) has been chosen as field site in order to identify the aquifer parameter range and the respective combinations. The results highlight that borehole thermal energy system efficiency is strictly dependent on aquifer transmissivity and groundwater Darcian velocity. The conducted analysis shows that, under lower Darcian groundwater flow and lower aquifer transmissivity, heat transfer efficiency increases at least by 25% compared to stagnant water, whereas heat transfer in the aquifer is governed by heat conduction. The increase of aquifer transmissivity induces the thermosiphon effect enhancing heat transfer processes both in the gravel back-filled borehole and aquifer. At higher values of groundwater Darcian velocity (> 0.1 m/d) advection due to groundwater flow is not negligible and mixed with free convection enhancing heat transfer further. Based on the results, discussion on the performance and environmental constraint of gravel back – filled borehole at field site has been presented.

中文翻译：

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岩溶裂隙石灰岩含水层局部尺度砾石回填钻孔换热器分析

摘要 在钻孔热能系统的设计和选型中，地下水的自然运动和温度驱动的流动对钻孔换热器的效率具有重要意义。通过三维数值模拟分析了安装在裂缝性石灰岩含水层中的砾石回填钻孔中双U管布置的效率。数值模型代表了 1 m 深的砾石回填钻孔，被裂缝含水层包围。为了通过改变双 U 形管内的平均温度来评估含水层参数和边界条件对热交换效率的影响，已经进行了多次模拟。为了确定含水层参数范围和各自的组合，选择巴里（意大利）工业区的裂隙石灰岩含水层作为现场场地。结果强调，钻孔热能系统效率严格依赖于含水层透射率和地下水达西速度。进行的分析表明，在较低的 Darcian 地下水流量和较低的含水层传导率下，与死水相比，传热效率至少提高 25%，而含水层中的传热受热传导控制。含水层透射率的增加引起热虹吸效应，增强了砾石回填钻孔和含水层中的传热过程。在较高的地下水达西速度值 (> 0. 1 m/d) 由地下水流引起的平流不可忽略，并与自由对流混合，进一步增强了传热。在此基础上，讨论了现场砾石回填钻孔的性能和环境约束。