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

Laboratory thermal conductivity measurements on gravel sample

Modern Ground Source Heat Pumps (GSHPs) systems must be designed by taking into account the ground thermal properties, in order to properly plan the capability of the heat pumps to transfer calories through the Ground Source Heat Exchangers (GSHE) to the subsoil (and vice versa). [...

Laboratory Measurements of Gravel Thermal Conductivity: An Update Methodological Approach

Abstract Direct measurements of gravel thermal properties are usually quite challenging to be performed in laboratory, due to the very coarse sediments size. As a consequence, the reference thermal values provided by literature for gravels are quite limited and dispersed. A guarded hot plate Taurus Instruments TLP 800, usually used for measuring the thermal conductivity of buildings materials, was slightly modified in order to measure the thermal conductivity of some gravel samples. The tests were performed both in dry and wet conditions. The paper presents the first obtained results

Experimental setup to measure the heat-exchange processes by controlling thermal and hydraulic conditions

The design of a Borehole Heat Exchanger (BHE) is based on the evaluation of the thermal exchange capacity of the whole system constituted by the probes and the surrounding ground. The energy performance of a BHE mainly depends on the thermal properties of the sediments, the possible groundwater flow and the changes in the thermal gradient in the probe's surroundings due to the continuous heat exchange with the subsoil. The interpretation of the in-field applications is often difficult because in many instances the information needed is unavailable due to difficulties of in-field measurements. An experimental device was built in order to assess, under controlled conditions, the evolution in time and space of the energetic processes that occur between a thermal probe and the surrounding ground. A copper probe was placed into a soil control volume of 1m3 and 24 high precision temperature sensors were distributed inside this volume at different distances from the probe. The configuration of the experimental settings was built to allow alterations in terms of sediments, groundwater flow conditions, thermal probe properties and operations, in order to simulate different physical conditions and to better understand the complex physical processes involved. Another goal of the experimental research was to produce reliable experimental data that can be used for the calibration and set up of numerical models. This paper describes the experimental apparatus and two experiments performed in order to assess its capability to satisfy the design requirements

Soil thermal conductivity from early TRT logs using an active hybrid optic fibre system

At the Molinella test site in Italy, a monitoring well has been equipped with a hybrid active optical fibre cable down to the depth of 100 m sealed with geothermal grouting. The cored borehole provided a full stratigraphic sequence of unconsolidated alluvial deposits. The cored material has been classified from a geotechnical point of view, and the thermal parameters of the most relevant lithologies have been directly measured. Active heating of the optical fibre cable has been provided by a constant heating power injected through copper wires contained within the cable structure. This way, not only the equivalent thermal conductivity of the entire stratigraphy but also the thermal conductivity at a spatial resolution of 1 m have been obtained. For each investigated layer, the thermal conductivity obtained from the distributed temperature measurements and the ones acquired using direct measurements are interpreted and compared

New tools to support the designing of efficient and reliable ground source heat exchangers: the Cheap-GSHPs databases and maps

Abstract. The final aim of the EU funded Cheap-GSHPs project is to reduce the total installation cost of closed-loop shallow geothermal systems. As part of the project a Decision Support System (DSS) has been developed and released on the web, in order to support the design of new closed-loop geo-exchange systems. The Cheap-GSHP project addresses all the aspects involved in planning and dimensioning a new borefield and therefore, the DSS is composed of several databases and tools that collect and elaborate the preliminary data and information that are necessary during the sizing phase, such as the geological and drilling aspects as well as the heating and cooling building demand. This paper briefly introduces the content of the databases and the mapping methodology developed for the Cheap-GSHPs DSS. All these researches are further deepen in the EU project GEO4CIVHIC, with a special attention to the application of shallow geothermal systems for building conditioning to historical buildings.</p

EU project "Cheap-GSHPs": the geoexchange field laboratory

Abstract The Molinella test site is the open-air laboratory of the EU project entitled "Cheap-GSHPs: Cheap and Efficient Application of Reliable Ground Source Heat Exchangers and Pumps". Here, innovative helical heat baskets and steel coaxial probes are installed next to the traditional double-U. The tests involve the probes design as well as materials and drilling techniques and machines, therefore the newly developed GSHEs can be directly compared with the traditional ones with respect to technical issues and energetic performances. The Molinella test site therefore represents a very extraordinary possibility to improve the knowledge of heat transfer processes in shallow geo-exchange systems

An updated ground thermal properties database for GSHP applications

Abstract When a new ground source heat exchanger field is planned, underground thermal properties input data are necessary for the correct sizing of the geo-exchange system. To support the design, the EU founded Cheap-GSHPs project developed a Decision Support System, that comprises a new database of thermal properties for both rocks and unconsolidated sediments. The thermal properties database has been developed by integrating and comparing data (1) provided by the most important international guidelines, (2) acquired from a wide literature review and (3) obtained from more than 400 direct measurements. The data are mainly thermal conductivity data, hence the convective contribution provided by groundwater flow to heat transfer is not included. This paper presents and analyses the collected database

A European Database of Building Energy Profiles to Support the Design of Ground Source Heat Pumps

[EN] The design of ground source heat pumps is a fundamental step to ensure the high energy efficiency of heat pump systems throughout their operating years. To enhance the diffusion of ground source heat pump systems, two different tools are developed in the H2020 research project named, Cheap GSHPs: A design tool and a decision support system. In both cases, the energy demand of the buildings may not be calculated by the user. The main input data, to evaluate the size of the borehole heat exchangers, is the building energy demand. This paper presents a methodology to correlate energy demand, building typologies, and climatic conditions for different types of residential buildings. Rather than envelope properties, three insulation levels have been considered in different climatic conditions to set up a database of energy profiles. Analyzing European climatic test reference years, 23 locations have been considered. For each location, the overall energy and the mean hourly monthly energy profiles for heating and cooling have been calculated. Pre-calculated profiles are needed to size generation systems and, in particular, ground source heat pumps. For this reason, correlations based on the degree days for heating and cooling demand have been found in order to generalize the results for different buildings. These correlations depend on the Koppen-Geiger climate scale.This work received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 657982.Carnieletto, L.; Badenes Badenes, B.; Belliardi, M.; Bernardi, A.; Graci, S.; Emmi, G.; Urchueguía Schölzel, JF.... (2019). A European Database of Building Energy Profiles to Support the Design of Ground Source Heat Pumps. Energies. 12(13):1-23. https://doi.org/10.3390/en12132496S1231213De Carli, M., Tonon, M., Zarrella, A., & Zecchin, R. (2010). 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