| The middle-deep ground source heat pump system has more abundant resource reserves and higher heat extraction efficiency than the shallow ground source heat pump system.It is an important utilization mode of geothermal energy.Its development and application have positive practical significance for promoting sustainable development and realizing the dual carbon strategy in China.In the middle-deep ground source heat pump system,backfill material,as the heat exchange medium between the buried pipe and surrounding rock and soil,has a significant impact on the heat efficiency of the system.In the current practical projects,many backfill materials for buried pipes of medium and deep layer geothermal heat pump have low heat exchange efficiency due to improper material selection and poor performance,which makes the heat extraction efficiency of the system difficult to reach the expectation.Therefore,it is necessary and important to carry out research on the heat transfer between backfill materials of medium and deep layer geothermal heat pump and rock soil mass.Considering that there are relatively few studies on the thermal performance of backfill materials on the heat transfer of rock mass in the existing studies,and the influence law of heat transfer needs to be deepened and improved,this paper carried out the test and simulation research on the thermal impact of backfill materials in the middle-deep ground source heat pump system,and investigated the impact of different backfill materials on the heat efficiency of the medium and deep geothermal heat pump according to the change of formation temperature.Firstly,samples of graphite and iron powder modified concrete backfill materials were prepared,and the thermal conductivity of the samples was measured.Then,a one-dimensional heat transfer test platform for backfill materials and rock masses was built to study the effects of environmental temperature,cold source temperature,and temperature difference of the backfill materials on heat transfer efficiency;Finally,a heat transfer simulation study of backfill materials and rock masses was conducted using Fluent software,and the results were compared and verified with experimental results;The main research conclusions of this article are as follows:(1)The heat transfer test results indicate that the lower the cold source temperature,the longer it takes for the temperature field of the stone to reach thermal equilibrium again,and the more intense the temperature change in the area near the backfill material.Under the same temperature difference,the higher the thermal conductivity of the modified concrete backfill material,the greater the temperature drop value of the stone,but the effect also weakens with the increase of the thermal conductivity.When the thermal conductivity reaches a certain value,its impact on heat transfer begins to stabilize.When the ambient temperature is 70℃and 60℃,the temperature drop curve of the stone material shows a significant turning point when the thermal conductivity of the backfill material is 1.61 W/(m·K)and 1.77 W/(m·K),and shows a trend of steep front and slow back.The change in temperature field during heat transfer is greatly influenced by the thermal conductivity of the backfill material and the temperature difference between the environment and the cold source.A high thermal conductivity is beneficial for heat transfer,but the temperature drop caused by fast heat transfer is also large,the temperature difference between the observation point and the cold source decreases,the driving force of heat transfer decreases,and the heat transfer slows down.Therefore,heat transfer should comprehensively consider the interaction and joint influence of thermal conductivity and temperature difference.(2)Simulation studies have shown that when the environmental temperature is 50℃ and the temperature difference is 20℃,a thermal conductivity of 2.32 W/(m·K)is more suitable for modified backfill materials;If the thermal conductivity of the backfill material is less than the optimal value,its heat transfer will be significantly reduced.When the environmental temperature is 70℃,the thermal conductivity of the backfill material decreases from 1.61W/(m·K)to 0.63 W/(m·K),that is,when the backfill material changes from concrete backfill to original soil backfill,the temperature drop value of point 1 decreases by 2.54℃.In practical engineering,if materials with poor thermal conductivity,the heating effect of the system will be significantly reduced,which is not conducive to energy conservation and efficiency enhancement of the system.(3)When the contact area between the backfill material and the stone is less than 80%,it has a significant impact on the temperature drop of the stone,causing the system to take less heat.Therefore,when the middle-deep ground source heat pump system is backfilled,the backfill materials shall be closely contacted with the surrounding stratum as far as possible.This paper mainly analyzes and discusses the thermal conductivity of backfill materials and the heat transfer law of rock mass in the middle-deep ground source heat pump system through the method of combining experiment and numerical simulation.The research conclusions can provide reference and reference for the improvement and optimization of heat extraction efficiency of medium and deep geothermal heat pump. |
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