Uncertainty analysis of wireless temperature measurement (WTM) in borehole heat exchangers

Abstract

The reliability of temperature measurements in open and closed geothermal systems is closely related to their design, quality control and performance evaluation. Thus, wireless and miniaturized probes, which provide highly resolved temperature profiles in borehole heat exchangers (BHEs), experience a growing interest in research. To ensure quality assurance and reliability of these emerging technologies, errors and uncertainties relating to wireless temperature measurements (WTMs) must be determined. Thus, we provide a laboratory analysis of random, systematic and dynamic measurement errors, which lead to the measurement uncertainties of WTMs. For the first time, we subsequently transfer the calculated uncertainties to temperature profiles of the undisturbed ground measured at a BHE site in Karlsruhe, Germany. The resulting precision of 0.011 K and accuracy of -0.11 K ensure a high reliability of the WTMs. The largest uncertainty is obtained within the first five meters of descent and results from the thermal time constant of 4 s. The fast and convenient measurement procedure results in substantial advantages over Distributed Temperature Sensing (DTS) measurements using fiber optics, whose recorded temperature profiles at the site serve as qualitative comparison. We additionally provide recommendations for technical implementations of future measurement probes. Our work will contribute to an improved understanding and further development of WTMs.