Improved Line-source Method For Ground Thermal Response Test

GSHP systems are often coupled to the ground by circulating a fluid through a buried U-tube loop. As one of the core components of ground source heat pump system, the rationality of ground heat exchanger design is directly related to the efficiency, reliability and the initial capital of the whole system. Thermal tests are often performed on vertical boreholes.These in-situ tests estimate soil thermal conductivity, which is an important parameter in the design of these systems.The heat rate of thermal response test(TRT) should be kept constant but it is difficult for practice tests. In analyzing the thermal response test(TRT), one commonly uses the mean of the measured inlet and outlet temperatures of the circulating fluid as a representative fluid temperature along the entire ground loop. This assumption is convenient but not rigorous. A new method should be proposed concidering the variable heat rate case.And it is necessary to modify the results with the mean temperature approximation.A series of work has been done in this paper.The mathematical model of three dimensional unsteady heat exchanger was built for a practical ground heat exchanger, which is coupled heat transfer in the ground.The heat transfer process of ground heat exchanger to ground is simulated. The results of the tests and the numerical simulations were compared and the numerical simulation results shown well consistency with the experimental ones.If an electrical power loss or other equipment failure interrupts the test, the interruption greatly complicates the analysis of the test data. Waiting for the temperature distribution in the ground to approach its undisturbed uniform temperature, which may require a long delay before restarting the test.When thermal response test(TRT) is applied to the same borehole without ground temperature recovery between the successive two tests, the second test results obtained using conventional data processing method will remarkably differ from the first test because the ground temperature has been disturbed before the second test. A linear regression model which based on the heat source superposition principle is presented in this paper. Both of conventional line-source analysis and the new method are used to process the TRT data obtained with different heat flux. The analysis results based on superposition principle shows a good agreement with the measurement.Several authors have proposed empirical and theoretical relations to evaluate these resistances as well as methods to evaluate them experimentally. Two-dimensional numerical simulations are used to evaluate the different methods.The paper compares the different approaches and proposes good practice to evaluate the resistances. The impact of the different approaches on the design of heat exchanger is also examined.The best equation of borehole resistance and internal resisitance was proposed. In this paper an analytical model of the actual vertical temperature profile in the ground loop has been developed for the late-time period of the in-situ test. It is validated that the method is reliable which evaluate soil thermal conductivity with the arithmetic average temperature. A new method is also proposed to evaluate the borehole resistances.The correct borehole resistance can be calculated that an other correction factor should be added to the borehole resistance computed with the mean temperature.