The Study On Heat Transfer Performance Of Ground Heat Exchangers With Typical Geological Structure In Hot Summer And Cold Winter Area

The ground source heat pump system has been a relatively mature new energy development mode after decades of research and development.At present,the ground source heat pump system has been studied from many angles at home and abroad,but the ground source heat pump system has not entered the perfect stage in research and popularization.There are still some shortcomings in geological stratification and seepage research.The ground heat exchangers(GHEs)is the core component of the ground source heat pump system,and it is the core of the whole system.To identify the heat transfer performance of the ground pipe under different conditions is the key to the ground source heat pump system.Because our country has vast territory and various geological types,there are significant differences in heat transfer performance between geotechnical and underground pipes with different geological structures,even in the same area.The existing research has not focused on the difference of heat transfer performance in the typical geological structure of hot summer and cold winter areas.Because there are geological differences between different cities and different geological areas of the same city in the hot summer and cold winter area,that will affect the heat exchange efficiency of the ground source heat pump system.The analysis of specific projects can not effectively represent the heat transfer capacity of the buried pipe in the typical geological condition of the region.Therefore,it is necessary to evaluate and analyze the influence of the heat transfer capacity of buried pipes under different typical geological conditions.This project takes the vertical buried pipe in ground source heat pump system as the research object,and focuses on the influence of different geological structures in the hot summer and cold winter zone on the heat exchanger performance of ground heat exchanger.Through the previous survey data,the typical geological structure of hot summer and cold winter area is summed up,and a three-dimensional heat transfer model of multi-layer tube group is established.Using the ground source heat pump based on the similarity theory to build similar experimental platform to verify the correctness of numerical model of ground heat exchanger and the construction method,then,using the typical geological structure of hot summer and cold winter area as the boundary conditions.Using the hourly building load as input source for numerical calculation,finally,getting the inlet and outlet temperature of GHEs and geotechnical dynamic temperature field to quantificationally analyze the differences between the various parameters under different geological structure.First,according to the difference of geological types,the typical geological structures of five cities in Chongqing,Chengdu,Guiyang,Wuhan and Shanghai are selected to represent the typical geological structure in the hot summer and cold winter zone.Through the investigation and collecting the test report,the geological survey data and accessing to a large number of thermal response test in hot summer and cold winter area,the geological structure type of Chongqing,Chengdu,Wuhan,Shanghai,Guiyang are summed up.Then reasonableness of the geological structure is demonstrated from two aspects of geotechnical geological data and project measurement.On the basis of the original buried pipe heat transfer model,a three-dimensional multi-layer tube group heat transfer model is established by using the typical geological structure.Then based on Darcy’s law and heat similarity theory,build 1:8 buried pipe experimental platform,and establish the numerical model of the same according to the experimental platform.The measured value compared with the numerical solution,verifying the heat transfer model and the correctness of the established method,numerical calculation model and method can be applied to the actual engineering.An office building in Chongqing as the basis,establish the load calculation model to calculate the hourly building load of the whole year and to choose the appropriate heat pump.Then,the hourly output load is introduced into the numerical calculation of heat transfer through the UDF program after obtaining the hourly output load of the heat pump set by MATLAB,so that the inlet and outlet temperature of the buried pipe can be coupled with the unit load at the same time.Finally,through the heat transfer model established coupling force load of the unit,we can obtain the dynamic inlet and outlet temperature of GHEs and the surrounding rock dynamic temperature field with the typical geological structure in Chongqing,Chengdu,Shanghai,Wuhan,Guiyang.Then,we can use these to calculate the excess temperature,the average heat transfer coefficient,the change ratio of soil excess temperature,the change rate of soil excess temperature,thermal unbalance rate so that they can use to quantitatively evaluate the heat transfer performance of GHEs.Through the comparison and analysis of the above evaluation parameters and sorting of five kinds of geology in various parameters,The order of heat transfer performance of GHEs under different geological structures is obtained as follows:Guiyang > Wuhan > Chongqing > Chengdu > Shanghai.The specific parameters of Guiyang and Wuhan are close,The specific parameters of Chengdu and Shanghai are close too.In addition,the rock and soil of five geological structures can basically recover to the initial temperature of rock and soil during the seasonal intermittence in the first year of the GHEs operation.The whole year heat unbalance rates of Five geological structures have no significant difference,all in the vicinity of 49%,that prove there exists obvious thermal imbalance in hot summer and cold winter area.If geothermal recovery period is not well recovered,it will lead to geothermal heat storage imbalance in the long run,which will affect the operation and even paralysis of ground source heat pump system.The above conclusions are of reference value for evaluating the heat transfer performance of GHEs in hot summer and cold winter areas and providing preliminary guidance for the corresponding project.At the same time,they also provide the basis to solve the problem of the unbalance of soil energy storage in GSHP system with multi parameter conditions,such as geological structure,load characteristic and dynamic energy saving rate.And they can help to establish rock and soil energy storage maladjustment model.