Effects of temperature, test duration and heat flux in thermal conductivity measurements under transient conditions in dry and fully saturated states

¹ LNEC - National Laboratory for Civil Engineering, Geotechnical Department, 1700-006 Lisbon, Portugal
² University of Aveiro, Civil Engineering Department, 3810-193 Aveiro, Portugal
³ CERIS/IST-Instituto Superior Técnico, University of Lisbon, 1049-001 Lisbon, Portugal

^* Corresponding author: kaljundi@lnec.pt

Abstract

In shallow geothermal energy systems (SGES) thermal conduction can be considered the dominant process in the heat transfer between the primary circuit (borehole heat exchanger or thermoactive geostructure) and the surrounding ground. Thus, a proper characterization of soil thermal properties, namely of its thermal conductivity, is mandatory for evaluating this energy exchange. There are difficulties associated to the assessment of soil thermal conductivity by laboratory methods related, among other factors, to the samples’ quality and to the measuring method itself. The purpose of this work is to analyse the effect of changing test control parameters in thermal conductivity measurements in transient conditions by means of a high accuracy thermal probe in both dry and fully saturated states. In order to eliminate potential measurements’ deviations and errors due to sample variability the same reconstituted samples were used several times. In each condition the sand samples were systematically tested under different ambient temperatures (10ºC, 20ºC, and 40ºC) controlled by means of a climatic chamber. The effects of changing the tests heating time and imposed thermal fluxes were also analysed.