Energy Procedia 40 ( 2013 ) 87 – 94

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of the GFZ German Research Centre for Geosciencesdoi: 10.1016/j.egypro.2013.08.011

ScienceDirect

European Geosciences Union General Assembly 2013, EGU

Division Energy, Resources & the Environment, ERE

Rock Thermal Conductivity as Key Parameter for Geothermal Numerical Models

Eloisa Di Sipioa*, Sergio Chiesab, Elisa Destroa, Antonio Galgaroa-c, Aurelio Giarettaa, Gianluca Golad, Adele Manzellad

aInstitute of Geosciences and Earth Resources IGG-CNR, Via Gradenigo 6, 35131 Padova, Italy bInstitute for the Dynamics of Environmental Processes IDPA-CNR, Via Pasubio 5, 24044 Dalmine, Italy

cDepartment of Geosciences - University of Padua, Via Gradenigo 6, 35131 Padova, Italy dInstitute of Geosciences and Earth Resources IGG-CNR, Via Moruzzi 1, 56124 Pisa, Italy

Abstract

The geothermal energy applications are undergoing a rapid development. However, there are still several challenges in the successful exploitation of geothermal energy resources. A special effort is required to characterize the thermal properties of the ground and to implement the thermal energy transfer technologies. Aim of this study is to provide original heat conductivity values for rocks and sediments in regions included in the VIGOR Project (southern Italy), to overcome the existing lack of data. Thermal properties tests were performed on several samples, both in dry and wet conditions, using thermal analyzer operating following the Modified Transient Plane Source method. © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the GFZ German Research Centre for Geosciences Keywords: thermal conductivity; geothermal energy; low enthalpy resources; thermal conductivity analyzer

* Corresponding author. Tel.: +39-049-8279123; fax: +39-049-8279134. E-mail address: eloisa.disipio@gmail.com; eloisa.disipio@igg.cnr.it

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of the GFZ German Research Centre for Geosciences

88 Eloisa Di Sipio et al. / Energy Procedia 40 ( 2013 ) 87 – 94

1. Introduction

According to the European Vision for the Renewable Heating & Cooling (RHC) sector in Europe, which aims to produce over 25% of heat consumed in the EU with renewable energy technologies by 2020 and over 50% by 2030, the potential for the production of geothermal energy is huge [1].

The exploitation of low (<30°C), medium (30°<T<100°C) and high temperature (>100°C) resources, together with the underground storage of heat (UTES), involves both the residential sector (residential and non-residential building) and the industrial processes. A wide variety of application for heating and cooling, ranging from shallow geothermal plants (geothermal heat pump - GHP) to district heating is able to reduce energy consumption and limit greenhouse gas (GHG) emissions. In addition, the development of technologies for electrical power generation and for combined heat and power installation (CHP), as Enhanced Geothermal Systems (EGS) technology, provides interesting energetic solutions for urban areas and industrial district [2].

Next future the geothermal market is expected to increase, involving mainly the shallow geothermal systems for conditioning of buildings, the construction of new district heating and cooling networks and finally the installations of cogeneration geothermal systems. Therefore, research and development actions are necessary to optimize and to innovate these sectors [3-4].

In this frame, one of the main challenge is improving the understanding of shallow and deep geothermal reservoirs through the characterization of their thermal, hydrogeological and environmental properties [5]. In detail, the knowledge of thermal conductivity of materials is one of the main input parameters in geothermal modeling since it directly controls the steady state temperature field. An evaluation of this thermal property is required in several fields, such as Thermo-Hydro-Mechanical multiphysics analysis of frozen soils, designing ground source heat pumps plant, modeling the deep geothermal reservoirs structure, assessing the geothermal potential of subsoil.

This paper focuses on understanding the quantitative contribution that geosciences can receive from the characterization of rock thermal conductivity. Aim of this study is to provide original rock thermal conductivity values useful for the evaluation of both low and high enthalpy resources at regional or local scale. To overcome the existing lack of thermal conductivity data of sedimentary, igneous and metamorphic rocks, a series of laboratory measurements has been performed on several samples, collected in outcrop, representative of the main lithologies of the regions included in the VIGOR Project (southern Italy).

The VIGOR Project is a national project coordinated by the Institute of Geosciences and Earth Resources (CNR-IGG) and sponsored by the Ministry of Economic Development (MiSE), dedicated to the evaluation of geothermal potential in the regions of the Convergence Objective in Italy (Puglia, Calabria, Campania and Sicily). One of the main outcomes expected is to evaluate the ability of the territory to exchange heat with the ground for air conditioning of buildings. To identify the conditions for the development of low enthalpy geothermal systems, geological and stratigraphic data have been collected and organized on a regional scale and specific thematic maps, able to represent in a synergistic and simplified way the physical parameters (geological, lithostratigraphic, hydrogeological, thermodynamic) that most influence the subsoil behaviour for thermal exchange, have been created [6-7-8]. In detail, this article describes the results obtained for the Calabria region, considered as a representative case study, given the opportunity to determine the heat conductivity variability according to different weathering degree for samples of the same lithology (Fig.1).

In fact, the weathering of crystalline rocks in the Sila Grande Massif, located in the centre of the Calabria territory, is a well known phenomenon. In situ disintegration of granitoid/gneissic rocks is due to chemical decomposition and mechanical weathering processes related both to climate conditions and to tectonic evolution of the territory [9-10].

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Fig. 1. (a) the four regions of Southern Italy involved in the VIGOR Project (Puglia, Calabria, Campania and Sicily); (b) arealdistribution of the igneous, metamorphic and sedimentary rocks outcrops in Calabria (scale 1:250.000) highlighting in green the

areal extent of the Granites and Granodiorites lithological unit (2004 km2) and in yellow the location of sampling point (77)

Granites and granodiorites, for example, may occur as fresh (boulder or bedrock), methamorphosed or completely altered (saprolite) rocks (Fig.2). The thickness of weathering process can reach a great depth,ranging from one to some tens of metres, up to about 100 m [11-12]. Therefore, for a better design of shallow heat exchangers, which can reach in general 100-150 m of depth, it is necessary to discriminatethe thermal conductivity values belonging to the different degrees previously identified.

Fig. 2. Overview of granite outcrops in Calabria: (a-c) fresh granite; (b) slightly methamorphosed granite; (d-e) granitic saprolite (completely weathered); (f) boulder of granite

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2. Materials and methods

An extensive sampling campaign was carried out in Calabria during the summer of 2012. About 77 specimens belonging to the main representative geological units, that is those characterized by the greatest areal extent and the highest density of potential users, have been collected. In detail, 21 different lithologies belonging to sedimentary, metamorphic and igneous rocks can be recognized (Fig.1b).

Thermal properties tests were carried out both in dry and wet conditions, using a thermal analyzer operating following the Modified Transient Plane Source method (MTPS, Fig.3). This technique is non-invasive, high accurate (precision within 1% RSD, accuracy within 5% error) and quick, compared to other measurement methods [13-14]. The operational principle of the C-Therm TCi system consists of applying a constant current heat source to the sample using a one-sided interfacial sensor. In this way, the sample absorbs some of the heat depending on its thermal conductivity, and the rest causes a temperature rise at the sensor interface (1-3°C) and a rapid voltage decrease at the heating source. The rate of voltage decrease is inversely proportional to the ability of the sample to transfer heat. Thermal conductivity and effusivity are measured from the slope of the voltage drop plot, providing a detailed overview of the thermal nature of the sample material [15-16]. The system requires a smooth contact surface, so every sample must be cut and smoothed before to register the measurements. Moreover, a contact agent (silica gel or deionized water used in dry or wet condition, respectively) is applied between the sample and the sensor to reduce the contact resistance to a negligible level [13].

Measurements were made at standard laboratory conditions on samples both dehydrated and water saturated. Dry specimens have been prepared with a fan-forced drying oven at 70 °C for 24 hr, for preserving the mineral assemblage and preventing the change of effective porosity. Subsequently, the samples have been stored in an air-conditioned room (t>24h, T= 21-23 °C, 30-50% RH) to reach the thermal equilibrium with the sensor, while bulk density, solid volume and porosity were detected. Then, once the samples have been hydrated for 96h, the wet heat conductivity value was registered [6].

Fig. 3. Thermal conductivity analyzer (C-Therm TCi) composed of interfacial sensor, computer software and controller electronics

The measured thermal conductivity values of rocks and loose materials have been validated by comparison with data published in the international literature. In fact, a general review of heat

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conductivity range obtained by direct laboratory tests described in different authors has been performed [17-18-19-20] and data regarding about 90 lithologies were organized in a database and used as reference.

3. Result and discussions

In Calabria the Granites and granodiorites unit cover large part of the territory. Due to the different degree of weathering, the outcrops are in the form of fresh rock, granitic saprolite or metamorphosed rock (Fig.4), characterized by thermal conductivity values similar to those obtained from literature respectively for granite (3.0 Wm-1K-1) or dry sand (0.5 Wm-1K-1), as shown in Table 1.

Therefore, direct thermal measurements performed in situ or in laboratory are necessary and strict recommended in order to create a regional database representative of the actual condition of the territories, useful for planners, public administrations and operators involved in the geothermal sector.

In detail, the heat conductivity value assigned to every sample (dry or wet) derives by two series of measurements, constituted each by 8 recordings. The first two data are generally discarded due to possible contact problem between the probe and the surface, and the remaining six are analyzed. The weighted average of the 12 thermal conductivity values obtained in this way is representative of the sample, while the median of the values defined for every samples belonging to the same lithology corresponds to the heat conductivity of that lithology.

In Calabria, ten Sila Grande Massif granite specimens have been analyzed resulting in a total of 110 values treated statistically. In dry condition the thermal conductivity ranges from 0.4±0.01 Wm-1K-1 for the most weathered granite (CAL30.2, granitic saprolite) till 3.5 0.01 Wm-1K-1 for the hardest samples (CAL22.1, metamorphosed), with an intermediate value of 2.8±0.04 Wm-1K-1 recorded for fresh samples (CAL19.2, CAL 29.2, CAL 31.3).

As expected (Table 1) the dry assigned are always lower than those derived from the literature screening. In fact, in the subsoil the water saturates the porosity of rocks by increasing their ability to transfer heat. Therefore, according to the procedure described above, some tests on saturated materials were carried out for 6 Calabria granites resulting in more than 60 direct records. In the first case the final

wet derived from the processing of data belonging to granite increases up to 3.0 Wm-1K-1, comparable with the bibliographic one. The samples characterized by the minimum and maximum value of wet are always the granitic saprolite (CAL30.1, 1.3±0.01 Wm-1K-1) and the fresh granite (CAL19.2, 3.9±0.02 Wm-1K-1).

Fig. 4. Calabria granite samples analyzed in laboratory: (a) fresh granite; (b) granitic saprolite; (c) metamorphosed granite

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Table 1. List of granite samples and their thermal-physical properties Sample code dry (W/mK) wet (W/mK) biblio

(W/mK) density (g/cm3)

porosity (%)

CAL 19.1 1.8 3.2 3.0 2.67 2.88 fresh granite

CAL 19.2 2.8 3.9 3.0 2.28 3.87 fresh granite

CAL 22.1 3.5 - 3.0 2.76 1.85 metamorphosed

CAL 29.1 2.5 - 3.0 2.54 3.75 fresh granite

CAL 29.2 2.8 - 3.0 2.55 3.92 fresh granite

CAL 30.1 1.3 - 3.0 2.00 26.76 granitic saprolite

CAL 30.2 0.4 1.3 3.0 - - granitic saprolite

CAL 31.1 2.5 2.8 3.0 2.63 3.30 fresh granite

CAL 31.2 2.1 2.9 3.0 2.67 2.22 fresh granite

CAL 31.3 2.8 2.8 3.0 2.62 2.57 fresh granite

4. Conclusion

The local knowledge of the geological, structural and hydrogeological features of a territory at local scale is essential to characterize the thermal properties of the rocks.

On this regard, the Calabria region is a perfect case study to understand the influence of physical-chemical weathering processes affecting the material. The combined action of weathering and fracturing phenomena determine a strong variation of open porosity and water content, both able to strongly influence the heat conductivity parameter. In a single rock type, as granite, where the mineralogical content is considered constant, the range of the thermal conductivity value detected is directly related to the rock weathering degree and to its water saturation level, able to considerably improves the heat transfer conduction process. Considering that in Calabria the thickness of granite unit can reach a depth of tens of meters, it is essential to differentiate the real value over the entire stratigraphic column. In this way, the design of geothermal plants is suited to the actual conditions of the underground.

A direct thermal measurement performed on representative specimen is necessary and strict recommended in order to give real data to planners, public administrations and operators involved in the geothermal sector. The use of dry thermal conductivity values ( dry) in modelling is a precautionary measure, while the use of the wet record ( wet) is recommended once determined the presence of groundwater in the underground.

The modified transient plane source method (MTPS) used to determine the sample thermal conductivities gives results comparable with those obtained in literature and provides a quick reliable evaluation of data. Moreover, the creation of a database dedicated to the measured thermal properties is useful for the making of geothermal maps and in the evaluation of geothermal assessment at different enthalpy levels (also medium and high temperature resources).

Future and ongoing developments for this research concerns the evaluation of variation with depth in agreement with the entire stratigraphic sequence and comparison and validation of laboratory data with on site Thermal Response Test (TRT) outcomes.

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Acknowledgements

The present activity has been performed in the frame of the VIGOR Project aimed at assessing the geothermal potential and exploring geothermal resources of four regions in southern Italy. VIGOR is part of the activities of the Interregi -2013 the management of VIGOR Project. and in particular Dr. Piezzo of MiSE-DGENRE (Directorate General for Nuclear Energy. Renewable Energy and Energy Efficiency of the Ministry for Economic Development) and Dr. Brugnoli. Director of CNR-DTA (National Research Council of Italy. Department of Sciences of the Earth System and Environmental Technologies).

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