Study Of Heat Transfer Performance And Distributed Thermal Response Test For Ground Source Heat Pump Coaxial Borehole Heat Exchanger

As the most important shallow geothermal energy utilization technology,ground source heat pump has attracted extensive attention all over the world due to its advantages of energy saving,environmental protection and stable operation.However,due to the high initial investment cost of ground source heat pump,the rational design of ground source heat pump system determines the success of ground source heat pump installation.The lack of uniform and precise design guidance limits the further promotion of ground source heat pump technology.The heat transfer model of heat exchanger can predict the heat transfer performance of the heat exchanger and calculate the required pipe length of the heat exchanger to guide the installation of the ground source heat pump system.As the input parameters of the heat transfer model of the buried heat exchanger,the thermal properties of the formation can also improve the prediction accuracy of the model.However,the current heat transfer model and thermal property acquisition methods have yet to be optimized.In this paper,a transient numerical model for a coaxial borehole heat exchanger which considered the impact of borehole specific heat capacity was proposed.The model introduced a new thermal response equation and connected it with the energy equation and the momentum equation of heat exchanger.The solution of fluid vertical temperature distribution inside the coaxial BHE had been predicted based on MATLAB and compared with other transient models.Validated by measured data from a thermal response test,the built model agreed better than other models,especially in short times,with a relative error of 3.63%in 2h.Then,the quantitative influences of parameters such as borehole specific heat capacity and thermal conductivity of formation,thermal conductivity of internal pipe and flow of circulating fluid on thermal performance of borehole heat exchangers were specified.The high-precision test method for the thermal properties of the formation was studied based on the heat transfer model of the borehole heat exchanger.Firstly,the sensitivity of the thermal properties of the formation to the heat transfer power of the heat exchanger and the temperature of the outlet fluid were analyzed by the single parameter sensitivity analysis method and the Spearman rank correlation coefficient method.The study found that the sensitivity of the formation thermal conductivity was stronger than the specific heat capacity of the formation.Then,based on the analysis of thermal sensitivity of the formation,the Monte Carlo random algorithm was used to perform the two-parameter grading inversion of the thermal properties of the formation by combining the established fluid temperature distribution model with the distributed thermal response test.The predicted results showed that the specific heat capacity of the formation and the thermal conductivity of the formation were 3.65W·K^-1·m^-1 and2.89×106 J·K^-1?m^-3,respectively,which were close to the reference value.The program for inverting the geothermal property parameters was compiled by Matlab software and the data obtained from the inversion prediction were substituted into the heat transfer model and showed that the simulated temperature agreed well with the experimental temperature distribution and thus the accuracy of the inversion prediction model was further proved.