Research Of Mechanisim And Influential Factors On In Situ Ground Thermal Conductivity Measurement

Energy crisis and environmental pollution problem are getting worse. Ground source heat pump technology with the advantages of energy conservation and almost no pollution has being widely used. The operation performance of ground source heat pump systems depend on ground heat exchangers which design needs accurate ground thermal properties. The accuracy of the thermal properties influences the boreholes number and thereby influence the initial cost of GSHP systems, which will inevitably affect the application of ground source heat pump technology. So it is necessary to study the influential factors on in situ ground thermal conductivity measurement, which include thermal conductivities of the backfill material and buried pipe wall, shank spacing, borehole diameter, the temperature variation of the circulating water, and the heating power, etc.This paper has built a thermal response test apparatus and established a3D numerical ground heat exchanger model to simulate and analysis the heat transfer process of ground heat exchangers. With the solid cylinder source model and the parameter estimation method, a computer program was compiled to calculate the ground thermal parameters from the data collected by thermal response tests. Based on this, it has investigated the influence of the measurement accuracy of these factors on the ground thermal conductivity measurement. The results showed that the measured value of the backfill thermal conductivity has greater impact on the measured ground thermal conductivity. The measured thermal conductivity significantly greater than the true value when backfill material thermal conductivity less than its true value. The accuracy of shank spacing measured has greater impact on the measured ground thermal conductivity. The ground thermal conductivity determined significantly less than its true value if the measured shank spacing is measured less. On the contrary, the measured ground thermal conductivity will be smaller. The accuracy of measured heating power also has greater impact on the ground thermal conductivity measurement. When the measured heating power is higher than the actual, the ground thermal conductivity is over estimated, and vice versa, the ground thermal conductivity will be calculated less. However, the measuring errors of buried pipe wall thermal conductivity, borehole diameter, the circulating water temperature have relatively little effect on the measurement accuracy of the ground thermal conductivity.