Heat transfer in quaternary deposits: how thermal conductivity is affected by moisture content

The performance of very shallow geothermal systems, as horizontal collector systems or special forms (i.e. heat baskets), interesting the first 2 m of depth from ground level, is strongly correlated to the kind of sediment locally available.
In detail, the thermal properties (i.e. thermal conductivity) of quaternary deposits are greatly affected by two parameters, the soil moisture content and the grain size.
A key challenge for very shallow geothermal systems is to understand how to enhance the heat transfer of the sediments surrounding the pipes, taking into account the interactions between the soil, the horizontal heat exchangers and the surrounding environment and the daily and seasonal variability of weather conditions, able to modify the moisture and water content in the first ground meters’ depth.
Trying to answer these questions, the ITER Project (Improving Thermal Efficiency of horizontal ground heat exchangers, European project funded by European Commission under the Marie Skłodowska Curie Fellowship Program Horizon 2014-2020) focuses on testing thermally enhanced backfilling material (TEBM), created using natural sediments and acting on their ability to preserve a constant thermal behavior in the long term.
In fact, the heat transfer exchange between the ground, the collectors and the ground heat pumps is improved in water saturated soil, where the presence of water in the pore size favors the heat transfer continuity in the medium.
Given the heterogeneity of sedimentary deposits in alluvial plain and the uncertainties related to the estimation of thermal parameters for unconsolidated material affected by thermal use, here are presented the preliminary results determined for the same material (sand and loamy sand) both on a field test site and in laboratory, under different degree of water saturation.