Study On Mechanism And Characteristics Of Heat Transfer Enhancement Of Ground Heat Exchangers

With the increasingly serious problems of environment and energy consumption emerging,geothermal energy,one of the renewable energy sources,has attracted more and more attention.Ground source heat pump(GSHP)technology can effectively reduce the consumption of primary energy and plays an important role in energy saving and emission reduction.The thermal performance of the source side of the ground source heat pump system is greatly affected by the ground heat exchangers(GHE)and the system operation control strategy.It is particularly important to study the mechanism and characteristics of the heat transfer enhancement of the ground heat exchangers,the application of the ground heat exchangers and the system operation control strategy.In order to enhance the thermal performance of the vertical ground heat exchangers of the ground source heat pump system,the thermal resistance of the ground heat exchangers in the heat transfer process is analyzes in this paper.The heat transfer can be enhanced by adding outer fins on the outer wall of the buried pipes,due to the thermal conductivity resistance of the buried pipes wall is the largest in the heat transfer process between the fluid in the buried pipes to the outer wall of the buried pipes.A U-tube with radial rectangular fins is proposed,in this paper.The heat transfer model is developed for predicting the thermal performance of the U-tube with radial rectangular fins,based on the analysis of underground heat transfer process of buried pipes.The influence of the construction of radial rectangular fins and operation parameters on thermal performance of single-U tube and double-U tube are discussed,utilizing the MATLAB.The simulation results show that the heat transfer quantity of single-U and double-U PE tube with 8 fins,is higher than the normal smooth U-tube 19.12% and 18.28%,under the condition of the inlet fluid temperature is 32 ℃,the flow rate is 0.6 m/s and the buried depth is 100 m.According to the correlation analysis between parameters and the heat transfer quantity,the effect of fin height and number of fins on thermal performance are most significant.Aiming at studying the application of the ground heat exchanger and the system operation control strategy,the annual hourly energy consumption of a geological data center building in Hangzhou was simulated.The TRNSYS system simulation model of a hybrid ground source heat pump system aided by cooling tower was established based on the characteristics of cooling and heat load of the building.The operating performance of the hybrid ground source heat pump with singel-U tube with 8 fins,double-U tube with 8 fins and smooth double-U tube respectively is simulated,without system operation control strategy.The simulation results show that compared with smooth double-U tube,the boreholes quantity of the single-U tube with 8 fins is 5.64% more,the total flow is 47.19% less,the mean annual temperature of soil is lowest and the coefficient of performance(COP)of the system is highest.The system under running time control strategy,highest temperature control strategy and temperature difference control strategy respectively are simulated,in this paper.The results show that cooling tower running time is 12:00-15:00,highest temperature is 32 ℃ and temperature difference value is 4 ℃ is the optimal solution among each operation control strategy.The annual mean energy consumption and annual mean soil temperature of the system under temperature difference of 4 ℃ is 22029.56 MJ and 29376.38 MJ,0.22 ℃ and 0.31 ℃ less compared with other operation control strategies.