Analysis of heat transfer characteristics and thermal-permeability coupling characteristics of single U-tube borehole heat exchangers

doi:10.3969/j.issn.2097-0706.2023.04.012

Abstract

Abstract:

To achieve the "dual carbon" target， shallow geothermal energy heating（cooling），a new utilization form of clean energy， has been widely applied. To improve the efficiency of geothermal energy， taking the ground source heat pump of an integrated energy heating（cooling） system in northern China as the research object， a 3D unsteady-state heat transfer model of a single U-tube borehole heat exchanger （BHE）was constructed， and its heat transfer characteristics and coupling characteristics thermal conduction and groundwater seepage were analyzed by the software Fluent. Comparative analyses on the total transfer heat and transfer heat per unit length were made on the BHE with three different spacing distances（4.0，4.5 and 5.0 m） and three different tube lengths（106.0，113.0 and 120.0 m），respectively. The analysis results show that the BHE with 4.0 m spacing distance and 106.0 m tube length demonstrates the optimal performance. Comparing the temperature fields of the BHE with and without seepage flow， the results show that seepage flow exerts positive effect on the BHE heat transfer efficiency， and the faster the seepage flows， the higher the heat transfer efficiency is. The analyses above provide references for the design of practical examples of BHE projects.

Key words: geothermal energy, ground-source heat pump, borehole heat exchanger, heat transfer characteristics, coupling of thermal conduction and groundwater seepage, clean energy, dual carbon

CLC Number:

TK529

LIU Yuanyuan, GENG Zhi, ZHANG Yuanfeng, ZHANG Liang, HAN Zhao, ZHANG Bin. Analysis of heat transfer characteristics and thermal-permeability coupling characteristics of single U-tube borehole heat exchangers[J]. Integrated Intelligent Energy, 2023, 45(4): 81-88.

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模型参数	数值
埋管深度（l)/m	106.0/113.0/120.0
埋管间距(s)/m	4.0/4.5/5.0
钻孔直径/mm	168
U型管进、出口管间距/mm	64
U型管内/外径/mm	26/32

材料	密度/(kg·m^-3)	比热容/[J·(kg·K)^-1]	导热系数/[W·(m·K)^-1]
水	1 000	4 182	0.60
PE100	950	2 300	0.45
回填材料	2 600	330	2.00
土壤	1 600	1 824	2.26

项目	网格数量
项目	2 412 542	2 449 066	2 744 906
出口温度/K	296.748	296.749	296.749
温度变化百分比/%	0	0.000 3	0.000 3

项目	埋管间距/m
项目	4.0	4.5	5.0
入口温度/K	299.81	299.81	299.81
出口温度/K	296.75	296.80	296.86
换热总量/J	8 640.828	8 499.638	8 330.210
单位井深换热量/(W·m^-1)	72.01	70.83	69.42

项目	埋管深度/m
项目	106.0	113.0	120.0
进口温度/K	299.81	299.81	299.81
出口温度/K	296.96	296.87	296.75
换热总量/J	8 047.830	8 301.972	8 640.828
单位井深换热量/(W·m^-1)	75.92	73.47	72.01