Borehole heat exchanger (BHE) is a very suitable and cost-effective technology for heat storage and extraction of heat from the ground. In this paper, comprehensive analysis of major influencing parameters on thermal performance of single U-tube BHE (sBHE) and double U-tube BHE (dBHE) was performed by using validated numerical heat transfer model. Combined effect of increasing borehole size and soil or grout thermal conductivity (for different cases of shank spacing) on performance of sBHE and dBHE was studied. Detailed comparative analysis of thermal performance was made among sBHE and dBHE with small and large borehole sizes. The impact of borehole depth and diameter, fluid inlet temperature, mass flow rate, U-tube pipe material type and diameter on total heat transfer per unit borehole depth, borehole thermal resistance and thermal effectiveness was also investigated. The simulation result revealed that when the shank spacing is kept at high or low value in sBHE and dBHE (both with small and large borehole size), dBHE with larger borehole size has the highest thermal performance while sBHE with smaller borehole size has the lowest performance; and the highest and the lowest thermal resistance is obtained for sBHE with smaller borehole size and dBHE with larger borehole size, respectively. Grout with high thermal conductivity is preferred in terms of improving thermal effectiveness and reducing thermal resistance of the BHE when the soil thermal conductivity is sufficiently high; while low grout thermal conductivity is preferred with large borehole size and high shank spacing. For high soil thermal conductivity (4W/m.K), the heat transfer rate per unit borehole depth increases with grout thermal conductivity to a maximum value; and then further increasing of the grout conductivity results in smooth declining of the heat transfer in the BHE. Thus, ground and grout thermal conductivity have an optimal range to achieve a lower value of the BHE thermal resistance, and consequently for better BHE sizing. With increasing fluid inlet temperature (15^oC to 42^oC), the total heat transfer is improved by 51.5 W/m and 36.2 W/m for dBHE and sBHE respectively; and more heat is injected into the borehole when dBHE is used than sBHE, particularly at higher inlet temperature showing that dBHE is preferable than sBHE for system that works with higher fluid inlet temperature. The result obtained can be used as a quick reference for the design, operation optimization and performance study of ground coupled heat pump system where BHE is used as aborehole thermal energy storage.

钻孔换热器 (BHE) 是一种非常适合且具有成本效益的技术，用于储热和从地下提取热量。本文利用经过验证的数值传热模型，对影响单U型管BHE（sBHE）和双U型管BHE（dBHE）热性能的主要影响参数进行了综合分析。研究了增加钻孔尺寸和土壤或灌浆热导率（对于不同的柄间距情况）对 sBHE 和 dBHE 性能的综合影响。对 sBHE 和 dBHE 的热性能进行了详细的比较分析，具有小井眼和大井眼。钻孔深度和直径、流体入口温度、质量流量、U 型管材料类型和直径对单位钻孔深度总传热的影响，还研究了钻孔热阻和热效率。仿真结果表明，在sBHE和dBHE（无论是小孔径还是大孔径）中，当柄间距保持在高值或低值时，孔径较大的dBHE热性能最高，而孔径较小的sBHE的热性能最低; 孔径较小的 sBHE 和孔径较大的 dBHE 分别获得最高和最低的热阻。当土壤热导率足够高时，从提高热效率和降低 BHE 热阻的角度考虑，优选具有高热导率的灌浆；而大钻孔尺寸和大柄间距优选低灌浆导热率。对于高土壤热导率 (4W/mK)，单位钻孔深度的传热率随灌浆热导率增加到最大值；然后进一步增加灌浆电导率导致 BHE 中的传热平稳下降。因此，地基和灌浆热导率具有最佳范围以实现较低的 BHE 热阻值，从而获得更好的 BHE 尺寸。随着流体入口温度的升高 (15^o C 至 42 ^o C)，dBHE 和 sBHE 的总传热分别提高了 51.5 W/m 和 36.2 W/m；与 sBHE 相比，使用 dBHE 时注入钻孔的热量更多，特别是在较高的入口温度下，这表明对于在较高流体入口温度下工作的系统，dBHE 比 sBHE 更可取。所得结果可为以BHE作为井下热能储存的地面耦合热泵系统的设计、运行优化和性能研究提供快速参考。