Effetti sulle proprietà meccaniche, idrauliche e termiche prodotti da scambiatori geotermici nei terreni argillosi: il caso studio della città di Venezia
In Ground Source Heat Pump systems (GSHP) a continuous circulation of a fluid inside the exchangers installed in the ground transfers heat between the ground and the buildingâs conditioning system. The heat exchange connected to a GSHP has been shown to alter the natural thermal status of the surrounding subsoil (Banks, 2012). Often the carrier fluids are brines consisting of a mixture of water and anti-freezing solutions, which lower their working temperature in order to improve the heat extraction from the ground during the cold season. The international community has already pointed out the importance of assessing a minimum temperature threshold for the brines inside the probes, in order to constrain the thermal anomalies induced in the soil (Haehnlein et al., 2010; Haehnlein et al., 2013).
This research analyzes how the cyclic thermal stress induced by a borehole heat exchanger (BHE) in the subsoil could change the sediments' properties, if the BHE works in extreme running conditions which induce freeze-thaw cycles (FTCs) and heating processes in the ground.
The study case of Venice (Italy) is considered, where GSHP systems could be a very interesting solution for the issues related to the particular configuration of the city center, to the density of historical buildings and to the local regulations. Venice represents an example of a densely urbanized area with the subsoil characterized by a continuous alternation between cohesive and sandy layers, as in most lowland plains.
A large laboratory program is undertaken in order to measure how the thermal anomaly affects the mechanical, hydraulic and thermal properties of deposits surrounding a BHE, if FTCs occur. In addition, a first evaluation of the thermal impact on the subground is carried out using finite element modelling (Feflow FEMCode), considering a typical building and the geological context of the study case.
The freezing point of sediments is some degrees below 0°C, and varies depending on the kind of sediment, water content, salt content and imposed load (Bing e Ma, 2011; Marion, 1995). While coarse materials display very few effects to temperature changes, FTCs induce a thermal consolidation process affecting irreversibly the cohesive sediments texture, due to the important role that water molecules play in their structure (Konrad e Morgenstern, 1980; Qi et al., 2008)). After 5-7 FTCs, the cohesive samples achieve a new state of equilibrium, characterized by a lower void ratio and a higher state of compaction (Konrad, 1989c).
Experimental results show that a significant settlement is induced in normal-consolidated cohesive layers, while, in the case of overconsolidated layers, a negligible expansion occurs. The effects are intensified in more active clayey sediments characterized by a higher plasticity index and with the presence of smectite minerals, which are more sensitive to temperature changes. The induced thermal settlement is measured considering several conditions of thermal and mechanical loads, degree of overconsolidation and interstitial water salinity, by means of a special device consisting in a thermostatically controlled oedometer.
The irreversible compaction effect induced on cohesive sediments increases with higher salinity concentration, despite the fact that the increasing salt content lowers the sediment freezing point, thereby protecting the soil from freezing processes.
The thermal induced consolidation is achieved in clayey layers with different intensity along the probe, decreasing with increasing applied mechanical stress corresponding to increasing depth. These layers will display hereafter a higher stiffness to higher loads and a sort of insensitivity to further thermal stress.
The obtained results also demonstrated that the BHEâs thermal stress can significantly increase the vertical hydraulic conductivity in cohesive layers, if FTCs are established. The effect is higher in shallow deposits and in overconsolidated layers.
Therefore, it is important to estimate the propagation of the frost front induced by a GSHP system in terms of time and position, in order to evaluate the volume involved in the critical thermal processes.
The propagation of the thermal plume induced in the ground is gained from several modelling simulations performed considering different conditions. A first model represents a 100m length BHE inserted into the ground, characterized consistently with the urban features and the geological context of the study case considered. A real case scenario is analyzed where the thermal requests are unbalanced towards heating. The results show that the volume of ground involved in the freezing processes is very constrained next to the probe; hence a correct representation of the studied phenomenon needs a new and more defined modelling mesh.
For this purpose, another fully discretized model of a double-U BHE was performed, in order to increase the accuracy of the representation of the heat transfer process in frozen ground conditions, providing a more reliable evaluation of the induced thermal anomaly. The sediments phase change is considered by means of a recently developed plug-in(Anbergen et al., 2014 ),, which takes into account the release of latent heat and incorporates the sedimentsâ thermal properties in frozen state, which affect the extension of the induced thermal anomaly. Hence, specific experimental measures of thermal properties of cohesive sediments sampled in the Venetian area are performed in both frozen and unfrozen conditions. Four different 50cm deep slices of the probe-ground system are analyzed by using the fully discretized model, characterized with the boundary conditions provided by the total-length model and by the experimental measures.
Results show that the freezing front is very constrained around the probe (with a radius <20cm from the filling grout) in the studied conditions, decreasing with increasing depth along the probe.
Despite the fact that only a limited volume close to the probe will experience FT cycles, the consequences of the thermal alteration on cohesive layers cannot be neglected. A significant settlement could occur next to the probe, derived from the compaction gained in the clayey layers present in the local stratigraphic sequence. Furthermore, the increased vertical permeability of the BHE surrounding cohesive layers could constitute a possible hydraulic connection of different aquifers previously separated. These occurrences have to be taken into account in the boreholes field design and during the running phase, because their relevance increases with the abundance of clayey layers and with the number of BHE in the array.
The issues studied are particularly hazardous in dense urbanized areas, characterized by abundance of cohesive layers in the stratigraphic sequence, where the lack of external spaces implies that the BHEs have to be bored under or close to the buildingsâ foundations.
In order to regulate the installation of new BHE fields and their exploitation avoiding the highlighted issues, it is important to identify the areas more suitable for this application. Finally this work presents the map of geological sensibility to the thermal stress induced by a BHE of Veniceâs historical center, which is based on the distribution of sensitive cohesive sediments in the subsoil, obtained by a high density stratigraphic data-base.
In conclusion, low enthalpy geothermal systems should be designed taking into account the thermal sensitivity of the subsoil. The obtained results could contribute to the definition of the environmental hazards connected to the use of GSHP systems
