Research On Factors Affecting The Accuracy Of Thermal Response Test Of The Ground-coupled Heat Pump Systems

In the context of sustainable development and energy structure transformation,the shallow geothermal energy can become a key driving force for the deep decarbonization of the energy system.The ground-coupled heat pump systems is a main way to utilize the shallow geothermal energy.Accurate thermal properties of rock-soil,especially thermal conductivity,is the prerequisites for design and application of the ground-coupled heat pump systems.The thermal response test（TRT）is a conventional method to obtain the thermal physical properties of rock-soil in the test area.However,the initial ground temperature,the heating power,the flow rate of the medium in the tube,the test duration during the test and the data processing will affect the test results.At present,the initial ground temperature test has the limitation of being affected by the heat transfer of the pump.The current research basically focuses on the influence of a single factor and lacks the analysis of the comprehensive influence of each factor.When processing the thermal response test data based on the line heat source model,there are a few studies involving discarding the test data.In fact,to improve the accuracy of thermal response test results,the complete test process and data processing should be fully considered.For the above problems,this paper built an on-site thermal response test bench in Raoyang County,Hengshui City,Hebei Province,China.Nine groups of thermal response tests under different conditions were carried out,and the entire thermal response test process was studied and analyzed.Including the initial ground temperature test,the inflluence on test results of different heating powers,different medium flow rates and different test durations,and the temperature recovery of each layer after the heat removal test.When processing the test data,the differences between the line heat source model and the cylindrical heat source model were compared,and the optimal diacard time of the line heat source model was explored.The results show that:The initial ground temperature obtained by the T-type thermocouple measurement method was 16.9°C,it obtained by the 24 h cycle method was 18°C,and it obtained by the reactive cycle method was 17.4°C.The reactive power cycle method is better than the 24 h cycle method in measuring the initial ground temperature.The influence degree of each factor on the thermal conductivity obtained by the thermal response test is heating power>test duration>medium flow rate.The different geological characteristics of each stratum have the greatest impact on the recovery of the ground temperature of each stratum within 0-24 h after the end of the test.After more than 48hours after the end of thermal response test,the temperature recovery degree of each layer can reach more than 80%.Compared with the cylindrical heat source model,the line heat source model has a time advance in the heat transfer process.The thermal conductivity obtained by discarding the test data of the first 10 h is 1.63 W/（m·K）,and the relative error with the calculation result of the cylindrical heat source model is 8.0%.The thermal conductivity obtained by discarding the first 20 h test data is 1.74W/（m·K）,however the relative error is 14.9%..F₀>1.79 can meet the accuracy requirement,and the relative error of the line heat source model can be controlled within8.87%instead to achieve better results.This study optimizes the initial ground temperature test method,clarifies the primary and secondary degrees of the influence of various factors on the test results,improves the accuracy of data processing of the line heat source model,and provides more choices and experience for future engineering practice.