Study On Heat Exchange System Of Geothermal Bushing Buried Pipe Heat Pump In Middle And Deep Layer

Geothermal energy is a kind of clean and renewable energy.Middle-deep geothermal energy refers to the energy in rock and soil with a depth of 500-3000m,which is not affected by the external environment.The extraction of geothermal energy from the middle and deep layers through the bushing buried pipe heat exchanger has the characteristics of high efficiency and stability.In order to further study the operation characteristics of geothermal energy utilization system.This paper based on the use of TRNSYS software to establish a simulation system.Based on the middle and deep geothermal bushing buried pipe heat pump heat exchange system in Qingdao,the experimental research was carried out.The accuracy of the model was verified by the measured data,and the operating characteristics of the system were analyzed according to the model.The main research work of this paper is as follows:Firstly,based on TRNSYS,the middle and deep casing buried pipe heat pump heating system is established.The main modules of the system include the casing buried pipe module,the heat pump module and the water pump module.The heat exchange model of the buried pipe,the theoretical model of the heat pump unit and the mathematical model of the working efficiency of the water pump are introduced.Second,the geothermal well area of hydrological geology and geothermal are introduced and measurement,through the survey of underground measurement has been clear about the project drilling geothermal Wells geothermal distribution area,the heat removal well within 0-1500m rock and soil temperature increased with the increase of the depth of the present law of linear increase,and 1500 m below the temperature gradient,temperature increasing faster.In addition,based on the middle and deep casing type ground pipe heat exchanger heat exchange heating system in Qingdao,the system has two operating modes,namely the heat exchanger direct supply system and the heat exchanger coupled heat pump heating system.According to the above operation mode of the system,two operating conditions were analyzed experimentally,including single-well direct supply system operation experiment,single-well heat pump heat exchange system operation experiment.The experiment shows that the geothermal heating in the middle and deep depth can meet the heating demand of actual engineering.The inlet and outlet temperature of the heat source side decreases with the increase of the operating time in the initial and middle heating period.The average performance coefficient of ground source heat pump unit and the average performance coefficient of the system are respectively COP=6.9 and COP_S=4.3.When the system is directly provided,the heat transfer power of the extension meter is 110 W·m^-1,and the heat transfer efficiency is 12.5%.When the heat pump system is running,the heat transfer power of the extension meter is 171 W·m^-1,and the heat transfer efficiency is 23%.In addition,the established simulation system is used to analyze the effects of geothermal gradient,thermal conductivity coefficient of rock and soil,heat extraction power of extension meter,depth of heat extraction well,inlet and outlet mode on the operation characteristics of the system from qualitative and quantitative perspectives.The analysis shows that the larger the geothermal gradient,the deeper the heat extraction well and the greater the geotechnical thermal conductivity,the higher the outlet temperature at the source side,the more conducive to the long-term operation of the system.When the inlet and outlet of the fluid in the heat exchanger adopts the operation mode of"in and out",the heat transfer effect is better than that of"in and out".The simulation results of the system under different heat extraction power of the extension meter show that the heat extraction power of the extension meter is less than200 W·m^-1 and should not exceed 150 W·m^-1.The smaller the heat extraction power is,the smaller the decrease range of inlet and outlet temperature and soil temperature will be,and the shorter the time required for thermal recovery and thermal recovery to reach equilibrium.Finally,the operating time ratio is defined.Through the analysis of the simulation system under different operating modes,the following results are obtained:the smaller the operating time ratio is,the greater the average length of heat removal is,the larger the COP of the heat pump unit and the COP_sof the system are,the smaller the system energy consumption is.In addition,by comparing the three operation modes,the intermittent operation of the system has the best energy saving effect.