Research on the efficient development of shallow geothermal energy: Experimental and numerical investigations on the heat transfer characteristics of the soil-cement energy pile
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摘要:
土壤源热泵供热制冷系统是当前浅层地热能最主要的开发利用方式。为了解决土壤源热泵系统中传统地埋管热效率低、占地面积大等瓶颈问题，本研究提出了一种新方案：将地埋管与水泥土桩复合地基相耦合，即一边钻进一边将水泥与原土在桩孔内现场搅拌成桩，利用地基处理材料良好的热力特性提高地埋管热效率。通过COMSOL Multiphysic5.0软件的数值模拟以及相似比为1∶2的相似模型试验，对比研究了地埋管分别在水泥土桩和土壤的热传递机理，探讨了温度场分布的时空变化规律，初步建立了水泥土能源桩的计算方法。研究表明：水泥土能源桩有效减小了能源桩传热系统中的最大热阻部分，极大提高了能源桩的热效率；在制冷或制热工况下，水泥土能源桩单位孔深换热量比常规地埋管分别提升了22.88%～34.20%和21.20%～55.60%；但是，仅靠提高埋管内的流速并不能进一步提高水泥土能源桩的换热能力，需要选择合适的流速与管径。水泥土能源桩系统这种建筑物绿色供热与制冷新模式，充分发挥了土壤源热泵系统适用性广和水泥土桩环保降本等各自的主要优势，为浅层地热能的高效开发提供了新的途径。
Abstract:The heating and cooling system of ground-source heat pump is currently the most important development and utilization method of shallow geothermal resources. In order to solve the bottleneck problems of low thermodynamic efficiency and large footprint of the traditional ground heat exchanger in ground-source heat pump system, this study proposes a new scheme of coupling the ground heat exchangers with the soil-cement pile composite foundation, a type of pile foundation where cement and the native soil are mixed on-site in the pile hole while drilling to form a pile, to improve the thermodynamic efficiency of the ground heat exchanger by utilizing the good thermodynamic properties of the ground treatment materials. By means of numerical simulation using COMSOL Multiphysic5.0 software and similarity model experiment with a similarity ratio of 1/2, the heat transfer mechanisms of the ground heat exchanger in the soil-cement pile and the soil are compared and studied, the spatio-temporal variation rules of temperature field distribution are discussed, and the calculation method of the soil-cement energy pile is established preliminarily. The research indicates that the soil-cement energy piles can effectively reduce the maximum thermal resistance in the heat transfer system of energy piles, greatly improving the thermodynamic efficiency of energy piles; the heat exchange unit length hole depth of the soil-cement energy pile is 22.88%～34.20% and 21.20%～55.60% higher than that of the ground heat exchanger in soil when cooling in summer and heating in winter, respectively; However, simply increasing the flow velocity inside the ground heat exchanger cannot further enhance the heat exchange capacity of the soil-cement energy piles, the effective methods are to choose the appropriate flow velocity and pipe diameter. This new model of green heating and cooling for buildings, the soil-cement energy pile system, fully leverages the main advantages of both the wide applicability of the ground-source heat pump systems and the environmental protection and cost reduction of the soil-cement piles, providing a theoretical basis and experimental support for the efficient development of shallow geothermal energy.
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表 1 相似模型试验的换热量
Table 1. Heat exchange of the similarity model experiment
工况 流速
/（m·s−1）土中单U管
单位孔深换热量
/（W·m−1）水泥土能源桩
单位孔深换
热量/（W·m−1）单位孔深换
热量增量/%夏季制冷 0.19 25.67 34.45 34.20 冬季制热 0.15 4.19 5.90 40.81 0.19 5.44 6.80 25.00 0.40 5.27 8.20 55.60 0.50 3.97 5.91 48.87 
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表 2 主要材料热物性参数
Table 2. Main material thermophysical parameters
材料 密度/（kg·m−3） 比热容/（J·kg−1·K−1） 导热系数/（W·m−1·K−1） 水 998.2 4182 0.60 PE管 950.0 2 000 0.44 粉土 1921.0 1230 1.41 水泥土 2026.0 1403 1.55
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表 3 管道参数
Table 3. Pipe parameters
PE管规格 PE管外径/mm PE管壁厚/mm PE管内径/mm DN15 20 2.3 15.4 DN20 25 2.3 20.4 DN25 32 2.7 26.6 DN32 40 3.0 34.0 DN40 50 3.7 42.6
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表 4 公式法计算能源桩单位孔深换热量
Table 4. Heat exchange per linear meter of the energy piles calculated with the equation method
工况 流速
/（m·s−1）土中单U管单位孔深
换热量/（W·m−1）水泥土能源桩单位孔深
换热量/（W·m−1）单位孔深换
热量增量/%夏季制冷 0.19 21.90 26.91 22.88 冬季制热 0.15 4.09 4.96 21.30 0.19 4.58 5.55 21.20
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