Research On Heat Transfer And System Operation Characteristics Of Single U-type Buried Pipe And Its Group

Ground-coupled heat pump（GCHP）,which can replace fossil energy for building heating,cooling and hot water supply,has been widely used and developed for many years.Among them,the shallow ground heat exchanger（GHE）system integrates energy storage and energy supply,which is a firm guarantee for achieving nearly zero energy consumption building,and promoting carbon peak and carbon neutrality.GHE is the key component of GCHP system.There are many factors complicatedly affecting its heat transfer capacity.Due to the limited heat transfer and specific heat capacity of soil,the temperature of soil around GHE changes with time during the operation.Therefore,determining the key factors affecting its heat transfer capacity,clarifying the relationship between them,and combining them with the heat transfer law in soil to optimize its operation mode are the premise to use GHE rationally and efficiently.Firstly,based on the similarity theory,this thesis built a sandbox set-up more accord with the actual situation,which can break through the limit space and position of existing sandbox,embody the temperature difference of U-type branch,and simulate GHE with different buried depths.According to the experimental results,a3D dynamic numerical model of U-type GHE was built and verified for the further numerical study.Secondly,this thesis studied the heat transfer performance of GHE and the variation characteristics of soil temperature field for short-term operation through the sandbox set-up,confirmed the validity of the set-up.The results showed soil moisture content,inlet velocity and inlet temperature are the important factors which affect the heat transfer of GHE.When the inlet temperature increased from 25?C to 45?C（T_?=18.5?C）,the average heat exchange per unit length whithin 4h increased from 35.5W/m to 127.5 W/m,which improved by 2.6 times.When the inlet temperature was35?C and the flow rates increased from 0.12 m/s to 0.61 m/s（Re=1338～6752）,the average heat exchange per unit length of GHE increased by 48.0%.Thirdly,the experimental study showed that in the short-term heat storage-extraction process,the heat storage improves the heat extraction.For a comparative study of single GHE among the operating modes“injection（I）”,“extraction（E）”,“IEIE”,“ISES”,“ISIS”,“ESES”and“EIEI”,it has been found that the best operating mode of GHE is the mode with alternating demand.In actual operation,the first recommended operation mode is“IEIE”,and the second recommended mode is“ISES”.Then,Spearman correlation analysis was adopted to select 9 key factors from 15factors which influenced heat transfer capacity of GHE.The order of correlation coefficient by heat transfer capacity is:inlet temperature>Reynolds number>thermal conductivity of soil>borehole depth>soil density>specific heat capacity of soil>wall spacing between two U-shaped branches>thermal conductivity of backfill>pipe diameter of GHE.The Kriging response surface data model was used to obtain the quantitative relationship between the thermal conductivity of backfill and soil,as well as the calculation method of the optimal Reynolds number.Finally,based on the key factors that affect GHE heat transfer ability and the rule of energy migration in soil,this thesis proposed the bionic structure GHE group:root shape and serrated shape.We established a 3D dynamic load calculation model of GHE group and studied the performance of GCHP system in single-family building for heating,cooling and hot water supply by 10 years.We found that GHE group with shorter depths should be preferentially applied if possible.The COP_sys of heat injection and extraction of 20m GHE group system are 4.9 and 3.1,that of the 93.4m GHE group are 4.0 and 2.5 respectively.In seasonal“IEIE”operation mode（cooling first,which means heat injection to soil first by GHE group）,GHE group with small borehole space has a higher COP in the early stage of injection and extraction,but a lower COP in the late stage.Compared to 6 m borehole space,2 m borehole space can take a 10%increase both in the early stage of injection and extraction,but a 14.5%and 6.8%reduce in the late stage of injection and extraction respectively.The COP_syscan be improved by optimizing the Reynolds number.When the flow rate is 0.27m/s～0.56 m/s,the COP_sys of heat injection is 4.7～5.6,and the COP_sys of extraction is2.9～3.4;which decreased to 3.6 and 2.2 when the flow rate is 1.0 m/s.Compared with the GHE group in equal depth,the bionic structure GHE group changed the square distribution of soil temperature around GHE into a cone distribution.Serrated and rooted bionic GHE group coped with sudden load change better,and COP of GCHP can be increased by 0.7%～4.0%.In conclusion,based on the physical experiment,numerical experiment and theoretical analysis of the GHE and GHE group,this thesis expounded the soil heat taransfer law around GHE,revealed the optimization direction of heat transfer capacity of GHE and pointed out the optimum operation mode of GHE,which can guide the design and operation of the actual GHE and GHE group.This work is a part of the National Key R&D Program of China for the 13th Five-Year Plan（No.2017YFC0702600）...