New Thermal Response Test Based On Thermal Resistance-Capacitance Network Model Of Vertical Ground Heat Exchanger

Ground heat exchanger(GHE) is the most important component in a ground-source heat pump(GSHP) system, which is designed to extract or inject thermal energy from/into ground soil for space heating and cooling in residential and commercial buildings. The heat transfer performance of ground heat exchanger directly determine whether the whole ground source heat pump system work with reliability and economy. The thermal physical property parameters such as the effective rock-soil thermal conductivity, the rock-soil heat capacity value and the thermal conductivity of grout material are the key factors to influence the heat transfer performance of GHE. Therefore, one of the most measures and approaches to study the heat transfer performance of ground heat exchanger and obtain accurate and effective thermal physical parameters is to establish a reasonable heat exchanger heat transfer model of GHE.Through reading the relevant literature, the calculation results of fitting formulas based on three-thermal-resistance model and four-thermal-resistance model within borehole have been compared in this paper. In particular, the thermal resistance of pipe-pipe based on three-thermal-resistance model within borehole is calculated to be negative, while new correlations of thermal resistances based on four-thermal-resistance model of our team could be able to overcome this defect. At the same time, two transient thermal-resistance-capacitance models of single U-tube ground heat exchanger based on there-thermal-resistance model and four-thermal-resistance model were established. By comparing the calculation results between the two transient models and CFD reference model, it has been found that the established analysis model based on four-thermal-resistance could be better to reflect the heat transfer of ground heat exchanger, and confirmed that it's necessary to introduce reasonable borehole-to-borehole thermal resistance borehole to describe the non-uniform temperature distribution along borehole perimeter.In order to further improve the calculation accuracy of the transient analysis model of GHE, reliable modification with borehole based on four-thermal-resistance model has been proposed. Improved transient thermal-resistance-capacitance model could give better simulation results of GHE, especially the early stages of unsteady heat transfer process. In addition, the calculation program of transient model could be able to greatly shorten the calculation time and improve calculation efficiency by compared with CFD modeling.In this paper, a new method to evaluate the thermal physical parameters of soil and grout material combined this improved transient thermal-resistance-capacitance model of ground heat exchanger is proposed. Through Nelder-Mead optimization algorithm for parameter-estimation, it is easy to determine those thermal physical parameters of soil and grout material rapidly and accurately including not only more accurate rock-soil equivalent thermal conductivity, but also the rock-soil heat capacity value and the thermal conductivity of grout material. Unlike the traditional data evaluating methods based on line-source model, this new way to evaluate the data from Thermal Response Test have significant advantages such as shortening test-time and being used for un-constant heating power condition. In addition, new method could be used to make up for the defect of abandoning previous testing data based on line source model, develop suitable short-period thermal response test process and save test cost, which have significant reference value to guide the design for ground heat exchanger.