Oscillatory thermal response test using heating cables: A novel method for in situ thermal property analysis

In situ subsurface thermal conductivity (lambda) and thermal diffusivity (alpha) are critical parameters to design ground-coupled heat pump systems (GCHP). lambda is conventionally determined from thermal response tests (TRT), whereas alpha is commonly selected from literature data or with laboratory analyses. Alternatively, both thermal properties can be assessed independently with an oscillatory thermal response test (O-TRT), which consists of heating the subsurface at an oscillatory rate and monitoring the resulting thermal perturbation. Due to their flexibility and relatively small radius, heating cables can be used to carry out tests in shallow unconsolidated deposits in the scope of horizontal GCHP design. The duration of the tests is short compared to vertical ground heat exchangers (fractions of a day) since the thermal resistance be-tween the heating cable and the soil is negligible. The objective of this work was to develop an apparatus and an analytical approach to assess lambda and alpha from an O-TRT carried out in shallow unconsolidated de-posits. Nine different O-TRTs were performed in the lab for a dry standard material and five O-TRTs were performed in situ in a natural sandy deposit. After validation via numerical modelling, results show that the analytical approach using correction factors allowed the estimation of lambda and alpha with an accuracy of < 2%. The study demonstrates that O-TRTs are a useful method to estimate both lambda and alpha. Further test-ing in other reference materials and with different apparatus configurations is needed to enhance the experimental setup and extend O-TRTs towards new applications.(c) 2022 Elsevier Ltd. All rights reserved.