Heat Transfer Performance And Heat Transfer Efficiency Of Vertical Ground Heat Exchanger In Layered Subsurface

As a significant component of geothermal energy,shallow geothermal energy is mainly exploited by ground source heat pump?GSHP?.Compared with conventional fossil energy,it is characterized by cleanness,high efficiency and energy conservation.By 2016,the area of buildings applying GSHPs in China has reached 478 million m².During the shallow geothermal energy exploitation process,thermal is transformed with the surrounding rock-soil through the borehole heat exchanger?BHE?.The processes and mechanisms of heat transfer in rock-soil media are quite complex.Different lithology,different moisture contents in identical lithology,different strata and presence or absence of groundwater flow have a great impact on the heat transfer performance of BHE.From the perspective of geology and hydrogeology,this paper mainly considers the influence of different strata and groundwater flow on heat transfer performance,so as to evaluate its influence on heat transfer efficiency.In this paper,through laboratory testing and data collection,the influence of mineral composition and lithology on thermal conductivity was analyzed,and the thermal conductivity of sand was tested and analyzed.The statistical analysis of the eigenvalues of the representative lithology in North China Plain was carried out,and the statistical eigenvalues of the thermophysical parameter of different lithologies were studied to provide basic data for subsequent research.The author set up a single medium?medium sand?thermal response test?TRT?platform and established a numerical simulation model to analyze the influence of thermal conductivity of rock-soil on heat transfer performance.The author selected the southern part of the Hebei Plain as the study area,conducted TRTs based on the constant heat flux method in three types of areas:the piedmont alluvial plain,the central lacustrine plain and the coastal marine plain and studied the impact of groundwater flow on the heat transfer performance of BHE.The author selected Shijiazhuang area to carry out a group of TRTs based on constant temperature method so as to carry out quantitative analysis of the impact of groundwater flow rate on heat transfer performance.Based on the actual stratum structure,the variation characteristics of the heat transfer capacity in the upper,middle and lower layers of the BHE under different layered subsurface were studied.A method for estimating the proportion of layered heat transfer capacity was proposed,and the relationship between the rate of change of the layered thermal conductivity and the rate of change of heat transfer amount was established.By introducing the equivalent thermal conductivity and the modified Peclet number,the author proposed the evaluation method of heat transfer performance of BHE in layered subsurface and the method was applied in a project.Following are major conclusions:?1?The thermophysical properties of rock-soil are the key factors affecting the heat transfer capacity of BHEs.The results of 1515 test data in North China show that for unconsolidated formation,the thermal conductivity increases with the increase of sediment average particle size.The thermal conductivity of rock is generally higher than that of the quaternary unconsolidated formation.The median thermal conductivity of carbonate rock is the largest,followed by sandstone and mudstone.Specific heat capacity and the thermal conductivity show opposite trends,and the trend of thermal diffusivity is consistent with the thermal conductivity.Thermal conductivity increases with the increase of water content.When the water content is<6.9%,the thermal conductivity increases slightly.When the water content is greater than 6.9%and less than 32%,the thermal conductivity increases linearly and reaches the maximum value under saturation.?2?In the uniform subsurface,the larger the thermal conductivity of the rock-soil,the larger the average heat transfer coefficient of the BHE.However,there is not a single linear relationship between these two parameters.As the thermal conductivity increases,the increasing rate of average heat transfer coefficient decreases.The fitted equation between the thermal conductivity and the average heat transfer coefficient is:K=1.9029+0.3943?-0.0169?².There is a difference in the heat transfer at different depths.The heat transfer capacity of the upper layer accounts for 27.94%,the middle layer 34.58%,and the lower layer 37.48%.?3?In the non-uniform layered subsurface,the change of thermal conductivity of the upper layer,the middle layer and the lower layer has different effects on the heat transfer capacity of each layer,and the redistribution ratio of the heat transfer capacity caused by the change of the thermal conductivity of the lower layer is more significant.A method for estimating the average heat transfer coefficient of non-uniform layered subsurface was proposed.It is recommended to adopt the formula to calculate the average heat transfer coefficient:K=?n₁/K₁+n₂/K₂+n₃/K ₃?^-1..?4?Affected by the groundwater flow rate,the average heat transfer coefficient of 5boreholes in the piedmont plain and the central plain of the Hebei Plain is greater than 6W/?m·K?,and 9 boreholes located in the coastal plain is less than 6 W/?m·K?.Under different groundwater flow rates but identical geological setting in the Shijiazhuang area,the fitted equation between the dimensionless Peclet number and the average heat transfer coefficient is:K=2.1692+2.1004Pe-0.5138Pe².When the diameter of the borehole is 0.2m,and when the Pe value is greater than 0.11,the groundwater flow has a significant effect on the heat transfer performance of the BHE.?5?By introducing the equivalent thermal conductivity(?_eff)and the modified Peclet number,the author proposed a method for evaluating the average heat transfer coefficient under groundwater flow conditions in layered subsurface.The average heat transfer coefficient of the single-hole borehole heat exchanger at the specific site in Shandong Province is 3.06 W/?m.K?,and the heat transfer efficiency increased by 15%compared with the case without considering the groundwater flow.When the long-term working operation with groundwater flow is compared to without groundwater seepage,the heat exchange rate increases by about 10%.When the groundwater and the system's intermittent working condition is compared to the long-term working condition without groundwater,the heat exchange rate increases by about 17%.Considering cold accumulation phenomenon in the existing projects in the study area,it is recommended that intermittent operation strategy should be adopted,and the ratio of operation and intermittent time should be less than 16:8.