Parameter Analysis Of Thermal Response Test And Research Of Heat Transfer For Ground Source Heat Exchanger

As one of the core components of ground source heat pump system, the rationalityof ground heat exchanger design is directly related to the efficiency, reliability and theinitial capital of the whole system. Rock-soil thermal properties are significantparameters in the design of ground heat exchanger for the reason of its tremendousinfluence on the heat transfer of ground heat exchanger underground. At the moment,the constant heat flux method of rock-soil thermal response test (TRT) is a popularmethod to obtain the parameters of rock-soil thermal properties.However, the carrier fluid inevitably takes a heat exchange with the surrounding airwhen it flows through test devices overground in the engineering application of TRT,and it will influence the test results. The numerical model of single U-tube ground heatexchanger is established，and the influence of overground heat transfer on TRT has beensystematically studied under different conditions. The simulation results show that theshorter the buried pipe, the larger the obtained error under the same conditions, and thefluctuation of test results comes with the fluctuation air temperature. In order toimprove the accuracy of TRT, the overground heat transfer between carrier fluid andambient should be carefully restrained.Though CFD model accurately simulate the underground transient temperaturefield of ground heat exchanger, it has some problems such as the high demand forcalculation, extensive computation time and heavy workload. As a consequence, CFDnumerical simulation has some limitations when processing numerous problems ofground source heat pump.For a conventional three-thermal-resistance model within a borehole of singleU-tube ground heat exchanger, a uniform temperature distribution is usually assumedalong the perimeter of borehole to simplify the heat conduction of ground in thehorizontal direction. Unfortunately, the uniform temperature distribution along boreholeperimeter could lead to a heat flux from cold pipe to hot pipe (i.e., the heat flow fromlower temperature region to higher temperature region) under certain conditions in theconventional three-thermal-resistance model, which is physically not possible andcorrect due to the negative thermal resistance.In this paper, a four-thermal-resistance model is adopted for the horizontal heattransfer within borehole for single U-tube ground heat exchanger, in which the borehole perimeter is divided into two equal parts along the symmetrical line and thenon-uniform temperature distribution along borehole perimeter could be taken intoaccount. Based on this four-thermal-resistance model within borehole in the horizontaldirection and combined the fluid flow and heat conduction in the vertical direction, anew transient3D thermal resistance-capacitance model is developed and solved for avertical single U-tube ground heat exchanger. With advantages such as the highsimulation accuracy, less time-consuming, easy modeling, strong versatility andconvenience, the proposed model makes the research for heat transfer of single U-tubeground heat exchanger much more available.