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

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
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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.