The work performed during the project consists mainly of three research phases:
1. laboratory activities: the physical-thermal properties of more than 15 soil mixtures [(i) natural soil, (ii) pure sand and (iii) mixtures of pure sand and clay additives] have been tested under different water content percentages and different consolidation degree. The collected data constitute a valuable reference to support modelling for very shallow geothermal applications and to validate heat and water transport model in soil and, due to the specific methodological approach selected, can be directly related to in situ field test measurements. In general, bulk density and moisture content affect both the thermal conductivity of soils, responsible for the transfer of heat to the helix system. On one hand, an increase of soil density for the same moisture content lead to an increase of the thermal conductivity; on the other, for the same soil density, an increase in thermal conductivity is related to an increase of the moisture content;
2. field test monitoring concerns (a) monthly measurement of thermal conductivity and moisture content on surface; (b) continuous recording of air and ground temperature (T); (c) continuous climatological and ground volumetric water content (VWC) data acquisition.
In all soils tested in situ, when the system is not running, the amplitude of Tground waves is already reduced at 0.60 m depth compared to Tair. When the heat pump is running, Tground variations induced by the helix are no longer noticed at 0.30 m from the top and 0.50 m from the lateral margins of the helix itself. In situ, a decrease of VWC with depth is registered (no groundwater flow is present), followed by a reduction of thermal conductivity with depth. The influence of precipitation on soil moisture content is limited to the first 20-30 cm depth below the ground level (bgl), while at 80 cm bgl the main changes are related to T variations induced by running the VSG system in heating mode. In detail, in coarse sand material a gradual decrease of moisture content implies a rapid decrease of thermal conductivity, while on bentonite mixtures or loamy sands, the reduction is more gradual. Then, these materials are promising for a better performance of the helix if initial adequate moisture conditions (> 12.5%) are provided and maintained over time.
3. numerical simulation (simplified 2D) for the helix has been set up using the finite element interactive modeling system code FEFLOW according to the laboratory and monitoring data. However, due to the complex geometry of the helix and the contribution of several factors to the heat transfer (i.e. conduction and latent heat…) the model can still be improved, so the research will continue in order to refine this topic. In fact, when T drops below 0 °C, the VWC is noticeably reduced and the soil is dried out. However, even if the thermal conductivity of the unfrozen soil decreases, the helix take advantage from the release of latent heat during the phase change process. The greater latent heat storage capacity belongs to the fine sand with 15% bentonite, that shows in situ always a volumetric water content equals or greater than 25%, corresponding to the ideal amount of water to reach the maximum thermal conductivity for a given material and the best heat transfer with the VGS system. Anyway, periods of not operating condition must be respected to allow soil to regenerate its thermal energy content and thermal properties.
