土壤源热泵非稳态热流热响应试验中岩土热物性参数的确定

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

在土壤源热泵系统现场热响应试验时,复杂的现场状况会影响热响应试验中恒加热功率的实现,结合测试现场的实际状况,该文提出了非稳态热流工况下确定岩土热物性参数的方法。通过建立非稳态热流热响应试验系统模型,实施系统优化,使地埋管换热器进出水平均温度计算值和实测值的平方和最小,确定最优的岩土导热系数和容积比热容2个参数。对比同一测试地点的恒热流和非稳态热流热响应试验确定的2个热物性参数的结果,非稳态热流工况系统优化方法确定的岩土导热系数的相对误差为1.2%,容积比热容的相对误差为0.7%。同时,在非稳态热流工况下,利用系统优化方法确定热物性参数可适当缩短热响应试验的测试时间,降低了测试成本,为土壤源热泵系统热响应试验的实施和岩土热物性参数的确定提供了重要参考。

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.