Improvement Of Thermal Response Test For Ground Source Heat Pump System

Utilizing the ground as heat source and sink, ground source heat pump (GSHP) system has the advantage of lower carbon dioxide emissions and higher energy efficiency compared with conventional system. As a result, it is widely used at present in China.In practice, the thermal performance and economical efficiency of GSHP system is largely dependent on the local ground thermal conductivity.“Technical Code for Ground Source Heat Pump System”(GB50366-2005) recommends using the in-situ thermal response test (TRT) to determine this thermal property which is based on injecting or extracting a constant heating power to or from the ground and measuring the inlet and outlet fluid temperatures continuously during the test, and then the ground thermal conductivity can be calcalated from the measured temperature history.The average temperature of circulating fluid in BHE is the basic test data for computing the ground thermal conductivity. Conventionally, the arithmetic mean value of inlet and outlet temperatures is taken as the average fluid temperature, this method is simple but lack of theoretical foundation. This paper used the CFD software FLUENT to simulate the three-dimensional transient temperature field of circulating fluid in a typical TRT and found that the arithmetic mean temperature has little difference with the volume integration temperature. So, in practice, it can be used to represent the average temperature of circulating fluid.Conventional test equipment can only conduct the heat injection test. Focus on the situation, this paper constructed a new kind of test equipment for TRT. Compared with the conventional test equipments, this test equipment can conduct the heat injection and extraction tests simultaneously, in addition, it is more stable while running and requires much less test duration.Moreover, this paper proposed a new data analysis method based on the two-dimensional finite line source model. With the new test equipment a TRT was conducted in Tianjin, and the ground thermal conductivity was determined by the data analysis method. By comparison, it was found that this method is more accurate than conventional methods, and the resulting ground thermal conductivity can reflect the annual operation condition for BHE. Besides, for the convenience of engineering application, the paper developed a computing interface corresponding to the data analysis method based on MATLAB. The users can input relative information and get the ground thermal conductivity in this interface.Finally, it should be noted that the data analysis method based on the two-dimensional finite line model has some limitations, so this paper also proposed a parameter estimated method based on the one-dimensional numerical model to evaluate the thermal conductivity of ground and grout. Applying this method to the TRT mentioned above, the resulting ground thermal conductivity was compared with that resulted from the analytical method, and it was found that there was a bit difference between the results. The reasons for this difference were also analyzed.