Determining parameters of rock-soil thermal properties in thermal response test of ground-coupled heat pump under working condition of transient heat flow

Abstract: The ground-coupled heat pump system (GCHPs) has been recognized as being among the most energy efficient systems for space heating and cooling in residential and commercial buildings. GCHPs consist of a conventional heat pump coupled with ground heat exchanger (GHE). The knowledge of underground thermal properties is a prerequisite for correct design of GHE. For GHE, the two important parameters are ground thermal conductivity and volumetric heat capacity of the rock-soil on the project site. The thermal response test (TRT) experiment is often performed on a test borehole for larger commercial installations, and it has been required in the GCHPs project whose building area is more than 5000m2 according to technical code for GCHPs in China. Based on the national regulations, it is necessary to hold the input power at a constant rate in the in-situ TRT, and power outages or high voltage fluctuations are not allowed. However, a constant supply of electricity is generally very difficult to achieve in the actual project. Although the regulator may be installed and the power stability is improved, the effect is limited. Therefore, it is significant for unstable heat power TRT to determine the true value of the two important parameters. For the GHE mathematical model of the TRT in the previous study, the heat transfer in the borehole is generally treated approximately by a line-source model which ignores the thermal capacity of the circulating fluid, the grout, and the differences in the properties of the grout that depart from the soil properties. The approximation may result in some errors, especially in the unstable heat power TRT, because the thermal capacity of grout, tube, and fluid in the tube has influence on the heat transfer in the borehole even though it is relatively small. In general, the test and data processing in a situation with a large input voltage fluctuation (>5%) need to be further studied. This paper presents a simulation-optimization method based on the duct storage system (DST) model of the GHE in which the unsteady state heat transfer was considered in the borehole. As an objective function, the temperature difference quadratic sum of the simulated average water temperature in the GHE from the system model and the testing value from the TRT was calculated. The ground thermal conductivity and volumetric heat capacity can be determined when the objective function reached the minimum value in the process of optimization. Then, a calculating sample based on unstable heat power TRT was conducted to validate the simulation-optimization approach. In the sample, the two parameters based on the simulation-optimization method make the square of difference between calculating average water temperature and experiment data is less than 0.14 after 10 hours. The relative errors of ground thermal conductivity and volumetric heat capacity are 1.2% and 0.7%, respectively, compared to the true value calculated based on the national regulations on the same in-situ measuring site. Finally, the duration of the unstable heat power TRT is discussed according to the optimization results from different measuring times. In general, the simulation-optimization method applied in the unstable heat power TRT is proved to be successful, and the study is helpful for the design and application of GCHPs.