Optimization of the thermal response test under voltage fluctuations based on the infinite line source model

Abstract

Obtaining accurate geotechnical thermal parameters is essential for the efficient use of geothermal energy because their accuracy directly affects the economy and reliability of a heat pump system. When determining geotechnical thermal property parameters using thermal response tests (TRTs), voltage fluctuations, uncertain test duration, and discard time are often encountered. Based on the linear source theory, the Fourier number (Fo) was selected as the reference standard in this study to accurately determine the most appropriate data discard time and test duration. Then, to obtain an accurate initial ground temperature (T0), a new cycling without power method was proposed and demonstrated to be accurate and feasible. The errors caused by voltage fluctuations during the test were handled by using data segmentation to select the best-fitted data for solving the thermal property parameters, with a good fitting accuracy (R2) of 0.996 and root mean square error (RMSE) as low as 0.006. The most accurate value of the comprehensive geotechnical thermal conductivity (λs) was determined to be 1.88 W/(m·°C). The thermal resistance (Rb) was determined to be 0.14 °C/W by combining T0 and parameters from model fitting. In conclusion, the test process and data processing should be considered to improve the accuracy of the TRT.