Relationship between thermal conductivity and saturation conditions in soils interested by very shallow geothermal systems: an overview of ITER Project

摘要

The performance of very shallow geothermal systems, as horizontal collector systems or special forms, interesting the first 2 m of depth from ground level, is strongly correlated to the kind of sediment / grain size distribution at disposal and suddenly decreases in case of dry-unsaturated conditions in the surrounding soil [1-2]. Among all the parameters influencing the thermal conductivity of unconsolidated sediments, the soil water content and the grain size are considered as the most influencing factors on the performance of the collectors and the ground heat pumps. Therefore, a better knowledge of the relationship between thermal conductivity and water content is required, given the heterogeneity of sedimentary deposits in alluvial plain and since measured soil thermal property data are currently not always readily available [3-4]. In this study, some outcomes of ITER Project (Improving Thermal Efficiency of hoRizontal ground heat exchangers, http://iter-geo.eu) funded by European Union, are shown [5]. An overview of physical-thermal properties variations under different moisture and load conditions for different mixtures of natural material is presented, based on laboratory data. In detail the physical-thermal properties of two natural sediments (fine sand and sand), both as pure and mixed material have been analyzed. Two commercial products readily available on the market (bentonite and clay powder) have been mixed separately to the sand in different percentages. In this way, more than 100 samples have been prepared, gradually varying the reference material (pure or with additive), the kind of additive (bentonite or clay), the water content (fresh water added gradually to the dry unconsolidated sediment in incremental steps) and the pressure applied.