桩基螺旋型地埋管换热器换热性能的数值模拟与验证

为了探讨不同因素对桩基螺旋型地埋管换热性能的影响,建立了桩基螺旋型地埋管换热器的传热数学模型,分析了桩基直径、桩基深度、螺旋管组数、土壤类型对桩基螺旋型地埋管换热量及土壤温度分布的影响,结果表明:增加桩基直径有利于改善桩基的蓄热能力、提高螺旋型埋管的换热性能,但是单位管长换热量会减小,因此,桩基直径不可无限制增加;桩基深度的增加有利于提高桩基螺旋型埋管换热器的换热量,而且对单位长度桩基的换热量影响很小,因此,可以通过增加桩基深度来提高换热量;同样条件下,黏土、砂土、砂岩中砂岩最有利于桩基换热器换热,土壤温度上升速率和幅度最低,而黏土换热效果最差,土壤温度上升速率最快;此外,螺旋管组数越多,换热器换热量越大,但是单位管长换热量会大幅下降。试验验证表明:所建桩基螺旋埋管模型预测出的换热量与土壤温度值与对应试验值吻合较好,其最大相对误差分别在9.7%与9.2%以内。

Numerical simulation and validation on heat exchange performance of pile spiral coil ground heat exchanger

Abstract: The new trends in energy savings and greenhouse gas reductions are expecting to explore the utilization of shallow geothermal energy. The most popular way to exploit shallow geothermal energy resources is the ground coupled heat pump (GCHP) system with using ground as a heat source. Because underground temperature is rather constant compared with ambient air temperature, the GCHP could achieve higher efficiency as well as more stable performance compared with traditional air source heat pumps. Thus the GCHP system becomes increasingly popular in commercial and institutional buildings. In general, a vertical borehole with ground heat exchanger (GHE) is used as the mainstream of GCHP system. However, the wide application of this type of GCHP technology has been limited by its higher initial cost and substantial land areas required to install the GHE. For this reason, the foundation piles of buildings have been used as part of GHE in recent years to reduce the cost of drilling borehole and save the required land area. This innovative idea of utilizing what are usually called "energy piles", has led to notable progress in the field of GCHP systems. It has become particularly attractive because it lowers total cost and spatial requirements, and offers the higher renewable contribution. In this paper, a novel configuration of an energy pile with a spiral coil was proposed. In order to investigate the effects of various factors on heat exchange performance of the pile spiral coil GHE, a numerical model of the pile with a spiral coil was developed. Based on the numerical solution of the model, the effects of pile diameter, pile depth, spiral coil group number and soil type on the heat exchange rate and soil temperature distribution of the spiral pile GHE were analyzed. The results indicated that increasing foundation pile diameter can improve the thermal storage capacity and thus enhance heat exchange rate of pile. But increase in foundation pile diameter can also result in the decrease of heat exchange rate per unit pipe length. So the pile diameter cannot be increased unlimitedly. At the same time, increasing the pile depth can improve the heat exchange rate of pile, and have little influence on heat exchange rate per unit pipe length. Thus the thermal performance of pile foundation can be improved by increasing pile depth. As for soil type, among clay, sand and sandstone, the sandstone was most conductive to the pile heat transfer and thus the soil temperature rise rate was minimum. On the contrary, the clay was the worst for heat transfer of pile foundation and soil temperature rise rate was the fastest among the three soil types. Additionally, increasing spiral pipe group number helped to improve heat exchange rate, but the heat exchange rate per unit length can be reduced largely. The experimental validation showed that the heat transfer rate and soil temperature predicted by the model were in good agreement with the corresponding experimental data, and the maximum relative errors were within 9.7% and 9.2% for heat transfer rate and soil temperature, respectively.