Governing factors for actively heated fiber optics based thermal response tests

Abstract

The accurate measurement of ground thermal conductivity is crucial for the evaluation of shallow geothermal energy potential. Actively heated fiber optics-based thermal response test (ATRT) is a recently developed novel method for the measurement of ground thermal conductivity, yet the governing factors for ATRT remain unclear. This study used numerical and physical models to examine the ATRT heat transfer process and the factors that may affect the measurement accuracy. The results showed that ATRT has a typical heat transfer process with three stages of composition, revealing diverse heat transfer in the cable, grout, and surrounding ground. The minimum heating duration of ATRT is approximately 150 min under a typical setup. The heating power is recommended to be within the range of 15–40 W·m−1, considering the temperature measurement accuracy. Using grout with a slightly higher thermal conductivity than the surrounding ground is beneficial. Actively heated fiber optic cable positions have a negligible effect as long as the heat duration is sufficient. A small borehole radius improves test accuracy by minimizing grout thickness effects. Understanding the ATRT heat transfer process and governing factors guarantees the validity of its in-situ applications and enables the standardization of ATRT.