Rend. Online Soc. Geol. It., Vol. 46 (2018), pp. 59-66, 4 figs., 2 tabs. hiips://doi.org/10.3301/ROL.2018.53( ) © Società Geologica Italiana, Roma 2018

Ground Source Heat Pumps in Valley (NW ): assessment of existing systems and planning tools for future installations

Alessandro Casasso (a), Simone Della Valentina (a), Andrea Filippo Di Feo (a), Pietro Capodaglio (b), Raoul Cavorsin (c), Rosalia Guglielminotti (d) & Rajandrea Sethi (a)

(a) Department of Environment, Land and Infrastructure Engineering, (DIATI), Politecnico di Torino, corso Duca degli Abruzzi 24, 10129, Turin, Italy. (b) ARPA Valle d’Aosta, località Grande Charrière 44, 11020, Saint-Cristophe (AO), Italy. (c) Regione Valle d’Aosta - Assessorato Attività Produttive, Energia, Politiche del lavoro e Ambiente, piazza della Repubblica 15, 11100, Aosta, Italy. (d) COA Finaosta, via B. Festaz 22, 11100, Aosta, Italy. Corresponding author e-mail: [email protected].

Document type: Short note. Manuscript submitted 16 October 2017; accepted 15 January 2018; editorial responsibility and handling by G. De Filippis.

ABSTRACT design and technical issues of different shallow geothermal The economic viability of shallow geothermal systems with techniques with a focus on specific Alpine conditions Borehole Heat Exchangers (BHEs) strongly depends on the thermal (Bottig et al., 2016; 2017), and iii) tools to include shallow load which can be efficiently and sustainably exchanged with the geothermal energy in local energy plans of three pilot areas ground. This quantity is usually defined as geothermal potential and, - Oberallgäu (Germany), Cerkno (Slovenia) and as reported in literature, it mostly depends on the thermal conductivity (Italy) - where the shallow geothermal potential for closed- and the undisturbed temperatures of the ground. The GRETA Project funded by the EU Interreg Program Alpine loop and/or open-loop systems is assessed and mapped. Space aims to produce maps of the geothermal potential in pilot The geothermal potential is usually defined as the areas across the Alpine territory to identify the most suitable areas for thermal load which can sustainably be exchanged by a shallow geothermal installations. This paper presents the case study GSHP with certain characteristics, depending on the local of the Aosta Valley, where the recently developed G.POT (Geothermal POTential) method was adopted. It describes the data sources used thermal and/or hydrogeological properties of the ground and the assumptions made to derive input parameters (ground (Ondreka et al., 2007; Gemelli et al., 2011; De Filippis et thermal properties, usage profile, etc.). In addition, the results of a al., 2015; Galgaro et al., 2015; Casasso & Sethi, 2016; Viesi survey on existing geothermal installations are presented. et al., 2018). It therefore allows a quick estimation of the installation cost, which is probably the strongest barrier to KEY WORDS: shallow geothermal energy, ground source heat the spread of GSHPs. Although it should not be considered pumps, geothermal potential, borehole heat exchanger, Aosta as a design method, the shallow geothermal potential is Valley. a useful tool to identify the most suitable areas for the diffusion of these systems. Studies and models for the assessment of shallow geothermal potential mostly focused INTRODUCTION on the Borehole Heat Exchanger (BHE), since it is the most adopted technique. Numerous studies adopted the well- Shallow geothermal systems can be divided into two known method proposed by the German standard VDI main groups: open-loop, exchanging heat with groundwater, 4640 (VDI, 2010), such as reported in Ondreka et al. (2007); and closed-loop systems, where the heat exchange occurs Gemelli et al. (2011); De Filippis et al. (2015). This method through the circulation of a heat carrier fluid in a closed provides tables of specific power extraction values (W/m) pipe loop buried in the ground horizontally (earth coils) or for a few rocks and sediments classes, and for two different vertically (Borehole Heat Exchangers and geothermal piles) utilisation profiles (1800 and 2400 full-load equivalent (Sarbu & Sebarchievici, 2014). The utilization of shallow hours per year), but it does not take ground temperature geothermal energy is growing in Europe due to its low carbon into account. For this reason, VDI 4640 provides unrealistic emissions and the saving margins on operational costs estimates where this parameter is highly variable, e.g. (Antics et al., 2016). However, Ground Source Heat Pumps in mountaineous areas. The method proposed by UK (GSHPs) still represent a marginal sector of renewable Department of Energy and Climate Change (DECC, 2011), heat sources (Bayer et al., 2012). In order to promote this which also provides tabulated values, overcomes this issue, sustainable energy source, the on-going cooperation project introducing ground temperature as an input parameter GRETA (near-surface Geothermal REsources in the Territory on BHE sizing tables. In addition, lithology classes are of the Alpine space) is working to overcome some of the main replaced by thermal conductivity values. More recently, barriers to the diffusion of GSHPs (Casasso et al., 2017b), the G.POT method was developed by Casasso & Sethi among which i) the simplification of existing regulation and (2016), which takes into account a wide range of ground authorisation procedures, based on best practices identified thermal properties and plant parameters. This method among existing ones (Prestor et al., 2016), ii) addressing can be applied both to only-heating or only-cooling usage 60 A. Casasso ET AL. profiles, thus being suitable also for hot climates in which Data from different sources were harmonised, trying the heating energy consumption is negligible compared to extract basic information (closed/open-loop, year of to cooling needs. The method has already been applied in installation, thermal power, spatial coordinates) and another pilot area of the project, the Slovenian mountain remove errors and duplicates. A total of 67 installations municipality of Cerkno (Casasso et al., 2017a). was identified, among which 43 are closed-loop (64%) and In this paper, we present the assessment and mapping of 24 are open-loop (36%) (Fig. 1A). The total installed power existing GSHPs and of the shallow geothermal potential in the is 3.9 MW with a total renewable energy production of 6.9 Aosta Valley (NW Italy). The methods adopted for the survey GWh. Although open-loop systems are a minor part of the on GSHPs are explained, identifying possible error margins total installations, they cover most of the installed power of the resulting data. Geological and climatic data are then and of the heat production (2,903 kW and 5.1 GWh, equal analysed and integrated with laboratory measurements on to 74%) since very large systems are installed in Aosta and field samples to derive the thermal properties of the ground Pont-Saint-Martin. Most of the geothermal installations on which the shallow geothermal potential depends. Finally, are used only or mainly for heating purposes, due to the the map of the closed-loop geothermal potential is presented Alpine climate of the Valley, in agreement with data on and commented, identifying further improvement margins the HVAC (Heating, Ventilation and Air Cooling) systems and possible developments of this work. in this region, where only 1.5% of families has a home cooling system (ISTAT, 2014). The survey is a first attempt to assess the current diffusion of GSHP in the Aosta ASSESSMENT OF EXISTING GSHP INSTALLATIONS Valley. The resulting data would set this region above the national average: in Italy, a total of 13,200 installations Concerning shallow geothermal energy, one of the main (1 on 4,590 inhabitants) with a capacity of 531 MW issues of energy planning is the lack of trusted, consistent (8.7 kW every 1,000 inhabitants) (Antics, et al., 2016); for and complete data sets on existing installations. Depending the Aosta Valley, 1 installation on 2,069 inhabitants and on the country, and often on the region or province, different 29 kW every 1,000 inhabitants result from this survey. This authorisation procedures are set (Prestor et al., 2016), result can be attributed to the high GDP per capita (+27% and different authorities are managing data on shallow compared to national average, according to ISTAT, 2016) geothermal utilisations. Most countries have set public and the highest expense for heating among all Italian or confidential databases on GSHPs, on their positions, regions (2,000 €/year per family, i.e. +22% compared to the installed power etc. In Italy, public authorities usually the national average, according to ISTAT, 2014) due to its have information on open-loop geothermal installations, cold climate (Heating Degree-Days range between 2,700 since a water abstraction permit is required. In this case, and 4,955). Also, renewable energy sources are popular the main issue can be the absence of specific information in this region, since 23.4% of families use biomasses for on the geothermal use of groundwater: especially in the heating, almost the double of the national average (14.5%) past, a number of open-loop systems was considered as (ISTAT, 2014). industrial use. On the other hand, data on closed-loop This survey can be considered as a starting basis, with installations, i.e. most of GSHP installations, are generally some error margins related to: estimated based on communications from market practitioners, while only Region Lombardia set a public –– mistakes in the building performance certificates, database with compulsory registration of existing BHEs where air-source and ground-source heat pumps may and communication of forthcoming installations (Regione have been wrongly identified; Lombardia, 2010). –– missing responses to the survey on the building The Aosta Valley is one of the regions for which data on performance certificates (about half of the professional closed-loop systems are missing. For this reason, a survey contacted); on existing geothermal heat pumps was conducted, based –– missing or wrong position of installations on four data sets: communicated by practitioners; –– double-counting of the same installations when no –– 32 systems were identified from the regional dataset detailed data on its position are available. on building energy assessment, introduced by the EU directives 2010/31/EU and 2012/27/EU, covering buildings which were built, rent or sold from 2011 ASSESSMENT OF SHALLOW GEOTHERMAL POTENTIAL onwards. These data were corrected by contacting the authors of energy assessments of buildings equipped As reported in the previous chapter, GSHPs are still with a heat pump, since no distinction on the heat a niche market in Valle d’Aosta, although the diffusion source was made in the registry; is higher than the national average. To support planning –– 9 systems were included in the database of renewable future expansion of this technology, we developed the energy and energy saving interventions on buildings map of closed-loop shallow geothermal potential in this funded by fiscal incentives since 2007, held by the region, identifying the areas which are most suitable for national energy agency ENEA; the installation of Borehole Heat Exchangers. In this –– information on 37 plants from communications chapter, the territory surveyed is first described from by installers and designers of GSHPs operating in the geological and climatic points of view, thus deriving Northern Italy; the input parameters for the G.POT method, which is –– 17 open-loop systems were identified in the database of shortly explained. The resulting map is then discussed, well permits, where the geothermal use is not always and examples are shown to explain how the geothermal explicitly mentioned. potential influences the economic feasibility of GSHPs. Ground Source Heat Pumps in Aosta Valley (NW Italy) 61

Fig. 1 - A) GSHPs in Aosta Valley, according to the survey presented, divided into closed-loop (red crosses) and open-loop systems (blue circles). B) Simplified tectonic map of the Aosta Valley, derived from the geological map by Regione Valle d’Aosta (2005).

THE TERRITORY SURVEYED serpentinites and metabasalts) and associated metasediments (mainly calceschists) and iii) the outer The Aosta Valley is the smallest Italian region, with a domain of the Briançonnais zone, consisting of surface of 3,200 km2 and a population of about 128,000 various kind of metasedimentary rocks. inhabitants. The region coincides with the highest part of –– the Austroalpine Domain represents the continental the basin of the Dora Baltea river (about 100 km long), crust of the Adriatic tectonic plate; it can be divided and the main towns (among which Aosta) are concentrated in i) a lower unit in the south-western portion of along the bottom valley at a average altitude of 350-600 the region (Sesia Lanzo zone, composed mainly by m asl, however some major villages are located at much eclogitic micascists and gneiss with metabasites) higher elevations, such as the well-known ski resorts if and ii) an upper one in the central part of the region (1,224 m a.s.l.), (1,528 m (Dent Blanche unit composed mainly by kinzingites, a.s.l.), and (1,524 m a.s.l.). amphibolites, and marbles). The Aosta Valley is a mountainous region with an average altitude of 2,100 m a.s.l. and bounded by the Quaternary alluvial sediments (sandy gravels) in the highest peaks of the Alpine chain, such as Mont Blanc bottom valley are of great hydrogeological importance (4,810 m a.s.l.), the Monte Rosa Massif (4,554 to 4,634 m since they host very thick and permeable aquifers, exploited a.s.l.) and Cervino/Matterhorn (4,478 m a.s.l.). The region mainly for industrial and drinking use, and lately even for is characterised by an Alpine climate, with cold winter and geothermal use. Their recharge is provided by seasonal short summer. snowfall melting, in addition to several glaciers covering about 5% of the total regional area.

GEOLOGY THE G.POT METHOD The Aosta Valley is located in the heart of the Europe-vergent belt of the Alps; three main tectonic The shallow geothermal potential BHE is hereby domains are represented (Dal Piaz et al., 2003; Schmid defined as the thermal load for which an imposed et al., 2004; Pfiffner, 2014) as shown in Fig. 1B: maximum thermal alteration is reached overǬ the lifetime of the BHE in a certain location. In order to draw the map –– the Helvetic Domain, in the north-western portion of the shallow geothermal potential of the Aosta Valley, of the region, is the only sector not undergone to the algorithm G.POT (Geothermal POTential) was used metamorphism, representing - in the evolution of (Casasso & Sethi, 2016). This algorithm is based on the the continental collision - the European passive assumption that the application of a cyclic sinusoidal continental margin. It consists of granite and thermal load induces a time-varying thermal alteration of migmatites (i.e. the basement of the Mont Blanc the ground, thus reaching a threshold fluid temperature massif) and a poorly preserved liassic sedimentary (minimum or maximum, depending on the use), which cover. depends on the following parameters: –– the Pennidic Domain refers to a broad set of rocks of originally different geological genesis and –– Geological: ground thermal conductivity λ (Wm-1K-1) paleogeographic position, later all heavily deformed and thermal capacity ρc (Jm-3K-1); during the orogenesis. It can be subdivided in –– Average undisturbed ground temperature (°C).

i) the inner domain of Grand Paradis and Mont –– Annual working rate of the plant: t'c=tc⁄ty, where is the Rose massifs (mainly gneiss) ii) the paleo-oceanic length of the heating season (s), and ty is the length of Piemontais zone, consisting of ophiolites (mainly the year (s). 62 A. Casasso ET AL.

–– BHE characteristics: length L (m); life time ts (s); threshold temperature Tlim (°C); thermal resistance Rb (mKW-1).

CASASSO ETThe AL. 2017alteration of the fluid temperatureT f(t) is 4 calculated with the Infinite Line Source solution (Carslaw & Jaeger, 1959), applying the superposition principle in order to consider the variable thermal load. The shallow 4 geothermal4 use. Their recharge is CASASSOprovided ET AL.byCASASSO 2017 seasonal ET AL. 2017 geothermal potential BHE (expressed in MWh/y) is then snowfall melting, in addition to several glaciers coveringdescribed by Eq. 1: / 0.0701 · (𝑇𝑇+ − 𝑇𝑇567 ) · 𝜆𝜆 · 𝐿𝐿 · 𝑡𝑡. about 5% of the total regional area. #$%Ǭ / / / Eq. 1 geothermal use.geothermal Their recharge use. Their is provided recharge byis seasonalprovided by seasonal 𝑄𝑄" = 7DE 4 . . 9 snowfall melting, in addition to several glaciers covering where represents𝐺𝐺/ the(𝑢𝑢 , 𝑢𝑢aforementioned, /𝑡𝑡 ) + 4𝜋𝜋𝜋𝜋 · 𝑅𝑅 Eq.maximum 1 snowfall melting, in addition to several glaciers covering + 567 . 0.0701 · (𝑇𝑇+ −0𝑇𝑇.0701567 ) · ·𝜆𝜆(·𝑇𝑇𝐿𝐿 ·−𝑡𝑡.𝑇𝑇 ) · 𝜆𝜆 · 𝐿𝐿 · 𝑡𝑡 about 5% of theabout total 5% regional of the area total. regional area. #$% thermal" #$%alteration/ / / and/ / / Eq. 1 is Eq.a 1function of three The G.POT method 𝑄𝑄" = 𝑄𝑄 + = 5677DE 4 . . 9 wherewhere 7DE T( –represents4 T . represents.𝐺𝐺) the(𝑢𝑢 , 𝑢𝑢aforementioned9, 𝑡𝑡 the) + aforementioned4𝜋𝜋𝜋𝜋 · 𝑅𝑅 maximum maximum where representsnon𝐺𝐺-dimensional the0𝑇𝑇𝑢𝑢 , −𝑢𝑢aforementionedlim,𝑇𝑇𝑡𝑡 +parameters 4𝜋𝜋𝜋𝜋 · 𝑅𝑅 maximum ,/ / and/ : The shallow geothermal potential is herebythermal alteration and 7DE is a 4function. . of three The G.POT methodThe G .POT method thermal alterationthermal and + alteration567 andis a functionG (u' of,u'( three,t' ) is a) function of three + 567 𝑇𝑇 − 𝑇𝑇 max𝐺𝐺 s /𝑢𝑢c , 𝑢𝑢c/ , 𝑡𝑡 / defined as the thermal load for which an imposednon -maximumdimensional𝑇𝑇 − 𝑇𝑇nonnon-dimensional parameters -dimensional ,/parameters / and /parameters : ,/ / and/ t' ,:. u' and. u' :4 A The shallow Thegeothermal shallow potentialgeothermal potential is hereby #$% is hereby 7DE 4 . .𝐺𝐺7DE(𝑢𝑢4, 𝑢𝑢., 𝑡𝑡.) c𝑡𝑡 c𝑢𝑢 𝑢𝑢s 𝑄𝑄" 𝐺𝐺 (/𝑢𝑢 , 𝑢𝑢/ , 𝑡𝑡 ) / / / / thermaldefined alteration as the isthermal reached load overfor which the anlifetime imposed of maximum the BHE in . . 4 . . 4 defined as the thermal load for which an imposed maximum#$% 𝑡𝑡 𝑢𝑢 𝑢𝑢 "#$% 𝑄𝑄" 𝑡𝑡 N 𝑢𝑢N N 𝑢𝑢 / / / thermala alterationcertainthermal location is reached alteration. over In is orderthe reached lifetime to𝑄𝑄 overdraw of the the lifetimeBHE map in of of the the BHE shallow in 7DEKLM,LO,POQ . 4 . Eq. 2 N N N 𝐺𝐺N N N / / = −/ 0.619// · 𝑡𝑡 · 𝑙𝑙𝑙𝑙𝑙𝑙(𝑢𝑢/ ) + (0.532 · 𝑡𝑡 − 0.962) a certain location. In order to draw the map of the shallowM O O 7DEKLM,LO,POQ . 4 . Eq. 2 a certaingeothermal location. In potential order to draw of thethe map Aosta of the shallowValley, the𝐺𝐺7DE algorithmKL ,L ,P Q =𝐺𝐺 −0.619 · 𝑡𝑡. =· 𝑙𝑙𝑙𝑙𝑙𝑙−0(.𝑢𝑢6194) +· 𝑡𝑡(0·.532𝑙𝑙𝑙𝑙𝑙𝑙(·𝑢𝑢𝑡𝑡.)−/+0(.0962.532) · 𝑡𝑡 − 0/ .962) geothermal potential of the Aosta Valley, the algorithm / // ./ Eq. 2 . Eq. 2 geothermal potential of the Aosta Valley, the algorithm wherewhere. .. · 𝑙𝑙𝑙𝑙𝑙𝑙(𝑢𝑢 .), − 0.455 · 𝑡𝑡 − 1.619 ,are are two two non - G.POTG.POT (Geotherm (Geothermal alP OTPOTential)ential) was usedused ( Casassowhere(Casasso & &where · 𝑙𝑙𝑙𝑙𝑙𝑙 (𝑢𝑢 ), − 0.455· 𝑙𝑙𝑙𝑙𝑙𝑙·(𝑡𝑡𝑢𝑢 −), −1.0619.455 are· 𝑡𝑡two− non1.619- are two non- G.POT (Geothermal POTential) was used (Casasso & non-dimensional/ parameters.[ / [ dimensional/ parametersdimensional[ dimensional/ . parameters/ [ parameters. [ / . [ Sethi, Sethi,2016). Th2016Sethi,is algorithm) 2016. Th)is. Th isalgorithm isbased algorithm on the is is assumptionbased based onon the thethat assumption assumption that that . .9 . 9 4 . 9 4 4 9 4 . 9 ⁄For(𝑢𝑢 = .the)𝜌𝜌𝜌𝜌𝑢𝑢4· 𝑟𝑟definition=⁄(4𝜌𝜌𝜌𝜌𝜆𝜆𝑡𝑡9·)⁄𝑟𝑟(𝑢𝑢⁄ =(4of4)𝜌𝜌𝜌𝜌𝜆𝜆 the𝑡𝑡· 𝑟𝑟) ⁄ 𝑢𝑢(characteristics4𝜆𝜆=𝑡𝑡 )𝜌𝜌𝜌𝜌 · 𝑟𝑟 ⁄(4𝜆𝜆 𝑡𝑡of) the BHE, the application of a cyclic sinusoidal thermal load inducesFor𝑢𝑢 the a= definition𝜌𝜌𝜌𝜌 · 𝑟𝑟For 4ofthe𝜆𝜆 𝑡𝑡the Fordefinition characteristics𝑢𝑢 the= 𝜌𝜌𝜌𝜌 definition of· 𝑟𝑟the characteristics4of𝜆𝜆 𝑡𝑡the of BHE, the characteristicstypical of the BHE, typical of the BHE, typical the applicationthe application of a cyclic of sinusoidal a cyclic thermal sinusoidal load induces thermal a load induces typicala values for a standard plant were set: length time-varying thermaltime-varying alteration thermal of the alteration ground, ofthus the reaching ground, thusvalues reaching for a vstandardalues valuesfor planta standard forwere a set:plantstandard length were plantset: length were, set: length, , time-varying thermal alteration of the ground, thus reachingboreholeL = 100 mradius, borehole radius, lifetime r = 0.075 of them , plantlifetime of the plant a threshold fluida threshold temperature fluid (minimumtemperature or (minimummaximum, or boreholemaximum, radius borehole, radiuslifetime of the bplant , lifetime of the plant a threshold fluid temperature (minimum or maximum,, thresholdt = 50 ,temperature yearsthreshold, threshold temperature temperature ,𝐿𝐿 =borehole100𝑚𝑚 T,𝐿𝐿 =borehole=100 –3°𝑚𝑚 C , borehole depending on dependingthe use), whichon the depends use), which on the depends following on the following s 9 9 4 lim 4 𝑟𝑟, =threshold0.075𝑚𝑚 temperature–1 𝑡𝑡 = ,𝐿𝐿 =borehole100𝑚𝑚 dependingparameters on the: use), which depends on thethermal following resistancethermalthermal 𝑟𝑟 = resistance0.075 resistance𝑚𝑚 . While R =9 these0.1 mKW .parameters While𝑡𝑡 .these =While parameters these parameters 4 parameters: 567b𝑟𝑟 = 0.075567 𝑚𝑚 𝑡𝑡 = - 50are 𝑦𝑦𝑦𝑦𝑦𝑦uniform𝑦𝑦𝑦𝑦 - on50are the 𝑦𝑦𝑦𝑦𝑦𝑦uniform territorythermal𝑦𝑦𝑦𝑦 on surveyed, the resistance territoryef spatial𝑇𝑇 surveyed,= distributions−3°e𝐶𝐶f spatial𝑇𝑇 = have distributions−3° 𝐶𝐶 . While have these parameters parametersGeological:: groundGeological: thermal ground conductivity thermal conductivity (Wm (Wm are uniform9 on 9the territory surveyed, spatial567 distributions 1 -1 1 -1 -3 -1 -3 -1 been 𝑅𝑅derived50= 𝑦𝑦𝑦𝑦𝑦𝑦0.1 𝑚𝑚𝑚𝑚for𝑦𝑦𝑦𝑦 𝑊𝑊the𝑅𝑅 =remaining0.1 𝑚𝑚𝑚𝑚𝑊𝑊 parameters (ground𝑇𝑇 thermal= − 3°𝐶𝐶 K ) andGeological: thermalK capacity) and groundthermal (Jm capacity thermalK ); conductivity(Jm K ); been derived (Wm forhave- theare beenremaining uniform derived parameters on thefor territory the(ground remaining thermalsurveyed, e parametersf spatial distributions (ground have - properties𝜆𝜆 andproperties length ofand the leng heatingth of season),the 9heating which season), are which are - Average1 undisturbed -1 Average ground undisturbed temperature ground 𝜆𝜆 temperature (°C). -3 -1 (°C). thermalbeen propertiesderived for andthe𝑅𝑅 =remaininglength0.1 𝑚𝑚𝑚𝑚 of𝑊𝑊 parametersthe heating (ground season), thermal K ) and thermal capacity (Jm K ); characterized by a high spatial variability and hence determine Annual working Annual rate workingof𝜌𝜌𝜌𝜌 the plantrate :of𝜌𝜌𝜌𝜌 the plant, : characterized, bywhich a high arespatial characterized variability and hence by determinea high spatial variability and - - + the𝑇𝑇+ distribution𝜆𝜆 the of distributiontheproperties shallow ofgeothermal the and shallow lengpotential geothermalth . of thepotential heating. season), which are - where Aisverage thewhere length undisturbed ofis thethe lengthheating ground of season/ 𝑇𝑇the temperature heating (s), season/ (s) (°C), . . . 1 . . 1 hencecharacterized determine theby adistribution high spatial ofvariability the shallow and geothermalhence determine - Annual- and working is the length rate of theof 𝑡𝑡 year𝜌𝜌𝜌𝜌 =the𝑡𝑡 (s)⁄ 𝑡𝑡. plant 𝑡𝑡 : = 𝑡𝑡 ⁄𝑡𝑡 , and is the length of the. year (s). potential. . 𝑡𝑡 + the distribution of the shallow geothermal potential. BHE- characteristics:𝑡𝑡 BHE characteristics: length (m); lifelength time (m); (s );life time/ 𝑇𝑇 (s); B 1 where 1is the length of the heating season. . (s)1, 𝑡𝑡 threshold𝑡𝑡 temperature (°C); thermal ⁄ threshold- temperature (°C); thermal 𝑡𝑡 =4 𝑡𝑡 𝑡𝑡 and is the length of -1the year4 (s). Ground thermal conductivity and capacity Fig. 2 - Spatial distributions of the estimated thermal conductivity (A) - resistance - (mKWresistance. -1). (mKW𝐿𝐿 ). 𝐿𝐿 𝑡𝑡 𝑡𝑡 Ground GROUNDthermal conductivity THERMAL and CONDUCTIVITYcapacity AND CAPACITY 𝑡𝑡 567 567 and capacity (B) of the ground. The alteration Theof BHE thealteration fluid 1characteristics: temperature of the 𝑇𝑇fluid temperature length is calculated𝑇𝑇 (m); lifeis Groundcalculated time thermal (sGround); parameters thermal valuesparameters were valuesassigned werebased assigned based 9 𝑡𝑡 9 on the geological map 1:500,000 of ISPRA, included in the with thethreshold𝑅𝑅 Infinite Linetemperature𝑅𝑅 Source solution (Carslaw (°C); & onJaeger, thermalthe geological map 1:500,000 of ISPRA, included in the with the Infinite Line Source solution (Carslaw: & Jaeger,: project 4 OneGeologyprojectGround (ISPRA,OneGeology thermal 2009 ).( ISPRA, parameters13 main2009 ). lithotypes 13values main were lithotypes assigned based 𝑇𝑇 -(1𝑡𝑡) 𝑇𝑇 (𝑡𝑡) 1959), applying1959- the) superposition,resistance applying the principlesuperposition (mKW in order). principle to consider𝐿𝐿 in order to considerwere 𝑡𝑡identified werein theidentified AostaGround inValley, the thermaleachAosta oneValley, conductivitycomposed each oneof composedand capacity of 567 on the geological map 1:500,000 of ISPRA, included in the the variable thermalthe variable load. thermalThe shallow load. geothermalThe shallow 𝑇𝑇potential geothermal potentialup to 5 lithologies,up to according5 lithologies, to theaccording classification to the adoptedclassification adopted TABLE 1 The alteration of the fluid temperature is calculatedproject by GroundOneGeology OneGeology .thermal Values (ISPRA,ofparameters thermal 2009).conductivityvalues 13 ( were)main and lithotypesassigned based (expressed in MWh/y)9 is then described by Eq. 1: by OneGeology. Values of thermal conductivity ( ) and (expressed in MWh/y) is then𝑅𝑅 described by Eq. 1: thermal werecapacitythermal identified on (thecapacity) geological in are the ( )Aosta mapreported are1:500,000 Valley, inreported each of ISPRA,one incomposed included inof the with the Infinite Line Source solution (Carslaw: & Jaeger, project OneGeology (ISPRA,𝜆𝜆 2009).𝜆𝜆 13 main lithotypes Thermal conductivities λ (Wm-1K-1) and capacities ρc #$% "#$% 𝑇𝑇 (𝑡𝑡) up to 5 lithologies, according to the classification adopted 𝑄𝑄" 1959), applying𝑄𝑄 the superposition principle in order to consider were𝜌𝜌𝜌𝜌 identified in𝜌𝜌𝜌𝜌 the Aosta Valley, each one composed of (MJm-3K-1) derived from measurements on field samples by OneGeology. Values of thermal conductivity (λ) and theTab. variable 1 and Tab.thermaltheir 1spatial andload. theirdistributions The spatial shallow aredistributions shown,geothermal are shown, potential up to 5 lithologies, according to the classification adopted (indicated with *) and from the Italian standard UNI thermalby capacity OneGeology (ρc.) areValues reported of thermal in Tab. conductivity 1 and their( ) and respectively, (expressed inrespectively, Fig. 2A in and MWh/y) in Fig. Fig. 2 B2.A isThermal and then Fig. describedcapacity 2B. Thermal and by capacity Eq. 1: and (2012). conductivity valuesconductivity were attributed values were to the attributed main lithotypes to the main lithotypes spatial distributionsthermal capacity are shown, (respectively,) are in Fig.reported 2A and in based on the resultsbased onof thelaboratory results testof laboratorys performed test withs performed a with a Fig. 2B. Thermal capacity and conductivity values were𝜆𝜆 Lithotype Secondary lithologies λ ρc 𝑄𝑄"#$% 𝜌𝜌𝜌𝜌 Thermal ConducivityThermal Scanner Conducivity (Popov Scanner et al., 2003(Popov) on et rockal., 2003) on rock attributed to the main lithotypes based on the results of Alluvial deposits (sat.) Silt + Sand + Gravel 1.90 2.00 samples from thesamples Aosta from Valley. the ForAosta a few Valley. minor For formations a few minor formations laboratory tests performed with a Thermal Conducivity Tab. 1 and their spatial distributions are shown, Glacial deposits (dry) Diamicton + Clay + Silt 1.70 2.50 and forrespectively, quaternaryand for in deposists, quaternaryFig. 2A values anddeposists, Fig. were 2 Bvaluesassigned. Thermal were capacityassigned andScanner (Popov et al., 2003) on rock samples from the Aosta according toa ccordingUNI (2012to ).UNI The (2012highest). Thethermal highest thermal Ophiolites* Peridotite/Gabbro/Basalt 2.94 3.22 conductivity values were attributed to the main lithotypesValley. For a few minor formations and for quaternary conductivities conductivitieswere found in were the highfound-grade in the metamorphic high-grade metamorphic deposits, values were assigned according to UNI (2012). Limestone Dolomite 2.40 3.00 based on the results of laboratory-1 -1 test-s1 performed-1 with a rocks samplesrocks (Micaschists, samples 3.52(Micaschists, Wm K ;3.52 Gneiss, Wm 3.43K ; Gneiss, 3.43 Conglomerate* Sandstone+Mudstone 2.82 2.52 -1 -1 Wm-1K-1), and lower values attributed for glacial and The highest thermal conductivities were found in the high- Wm KThermal), and lowerConducivity values attributed Scanner for (Popov glacial et and al., 2003) on rock - alluvial sedimentsalluvial (respectively sediments 1.7 (respectively and 1.9 Wm 1.7-1 Kand-1). 1.9The Wm -1K-1). The grade metamorphic rocks samples (Micaschists, 3.52 Wm Andesite Rhyolite 2.70 3.10 samples from the Aosta Valley. For a few minor formations1 -1 -1 -1 thermal capacitythermal is less capacity variable: is lessthe lowervariable: values the (1.93lower values (1.93 K ; Gneiss, 3.43 Wm K ), and lower values attributed for Granite* Granodiorite 3.12 1.93 and for quaternary-3 deposists, values were A assigned A - MJm-3) was foundMJm in) wasGranit founde samples in Granit ande attributedsamples andto attributed to glacial and alluvial sediments (respectively 1.7 and 1.9 Wm Monzonite Quartz diorite 3.20 2.80 accordingsediments, to whileUNI the highe(2012st values). The (up-3 to 3.3highest MJm- 3) werethermal 1 -1 sediments, while the highest values (up to 3.3 MJm ) were K ). The thermal capacity is less variable: the lower values Schist* Quartzite 3.32 2.75 found conductivitiesin Micascfoundhists. in Micaweresc hists.found in the high-grade metamorphic(1.93 MJm-3) was found in Granite samples and attributed Mica schist* Gneiss (+ Phillyte) 3.52 3.30 rocks samples (Micaschists, 3.52 Wm-1K-1; Gneiss, 3.43to sediments, while the highest values (up to 3.3 MJm-3) Eclogite* Schist 3.26 2.18 Wm-1K-1), and lower values attributed for glacial andwere found in Micaschists. Gneiss* Migmatite + Schist 3.43 2.50 alluvial sediments (respectively 1.7 and 1.9 Wm-1K-1). The Granulite* Amphibolite 3.39 2.34 thermal capacity is less variable: the lower values (1.93UNDISTURBED GROUND TEMPERATURE AND LENGTH OF -3 A MJm ) was found in Granite samples and attributed to THE HEATING SEASON sediments, while the highest values (up to 3.3 MJm-3) were found in Micaschists. The ground temperature is strongly correlated to (2004), the ground is 1 to 2°C warmer than the air, and this the yearly mean of the air temperature, which in turns difference increases with the altitude due to the isolating depends on the altitude. According to Signorelli & Kohl effect of the snow cover during winter. Yearly average air

GROUND SOURCE HEAT PUMPS IN AOSTA VALLEY (NW ITALY): ASSESSMENT OF EXISTING SYSTEMS AND PLANNING TOOLS 5 FOR FUTURE INSTALLATIONS

B

Fig. 2 – Spatial distributions of the estimated thermal conductivity (A) and capacity (B) of the ground.

Ground Source Heat Pumps in Aosta Valley (NW Italy) 63

temperatures Tmed air (°C) were therefore used to derive between tc and the altitude was found (Fig. 3C) using the ground temperatures T0 (°C), assuming a difference of 1°C: same meteorological data utilised for the evaluation of the undisturbed ground temperature. The resulting spatial Tab. 1 – Thermal conductivities and capacities Eq. 3 distribution is shown in Fig. 3D. from measurements on field- 1samples-1 (indicated with *) and- 3from-1 Eq. 3 𝜆𝜆 (Wm K ) 𝜌𝜌𝜌𝜌 (MJm K ) the Italian standard UNI (2012). 𝑇𝑇+ =Values𝑇𝑇7qr D6s of +were1°𝐶𝐶 derived from the time series recorded derived Values of were derived from the time series Lithotype Secondary lithologies recorded in the periodin the 2006 period-201 52006-2015 by 38 meteorological by 38 meteorological stations RESULTS AND DISCUSSION 7qr D6s Alluvial deposits (sat.) Silt + Sand + Gravel 1.90 2.00 𝑇𝑇 managed by ARPA Valle d’Aosta, discarding all years for 𝜆𝜆 𝜌𝜌𝜌𝜌 stations managed by ARPA Valle d’Aosta, discarding all Glacial deposits (dry) Diamicton + Clay + Silt 1.70 2.50 years for which morewhich than more3% of thanrecordings 3% of w recordingsas missing. Awas missing. A linear The raster map of the geothermal potential of the Ophiolites* Peridotite/Gabbro/Basalt 2.94 3.22 linear correlation wascorrelation found (Fig. was 3 Afound) and used(Fig. to3A) derive and used to derive the Aosta valley (below 2000 m a.s.l.), obtained with the G.POT Limestone Dolomite 2.40 3.00 spatial distribution (Fig. 3B) of the undisturbed ground method, is shown in Fig. 4 (cell size 30x30m). This map Conglomerate* Sandstone+Mudstone 2.82 2.52 the spatial distribution (Fig. 3B) of the undisturbed ground temperature T0 (Eq. 3), using ground elevations from a indicates punctual values for a single borehole installed Andesite Rhyolite 2.70 3.10 temperature (Eq.global 3), using30m-grid ground Digital elevations Elevation from Model a (DEM) of USGS with the above mentioned characteristics and it has been Granite* Granodiorite 3.12 1.93 global 30m-grid Digital Elevation Model (DEM) of USGS + & NASA (2011). Values of T0 below 5°C (corresponding to published in a web GIS at https://goo.gl/AR722W (Casasso Monzonite Quartz diorite 3.20 2.80 & NASA (2011𝑇𝑇 ). Valuesaltitudes of abovebelow 2000 5°C m(corresponding a.s.l.) were excludedto from the map et al., 2017b). The shallow geothermal potential is highly Schist* Quartzite 3.32 2.75 altitudes above 200reported0 m a.s.l.) in were because excluded at these from altitudesthe map the snow coverage variable and it is mainly driven by the ground temperature Mica schist* Gneiss (+ Phillyte) 3.52 3.30 lasts for𝑇𝑇 +a long time and hence Eq. 3 would strongly Eclogite* Schist 3.26 2.18 reported in because at these altitudes the snow coverage (ranging from 15°C to 5°C, respectively between 300 and underestimate the value of T (Signorelli & Kohl, 2004). Gneiss* Migmatite + Schist 3.43 2.50 lasts for a long time and hence Eq. 3 would strongly0 2000 m a.s.l. of elevation) and the thermal conductivity Since the average elevation of the Aosta Valley is 2100 m -1 -1 Granulite* Amphibolite 3.39 2.34 underestimate the value of (Signorelli & Kohl, 2004). (ranging between 1.7 and 3.52 Wm K ). The thermal a.s.l., this means that about half of the territory is therefore capacity (ρc) and the length of the heating period (t ), Since the average elevation of the Aosta Valley is 2100 m c excluded 𝑇𝑇from+ the mapping of the ground temperature, instead, have a marginal role on , also due to their lower Undisturbed ground temperature and length of the a.s.l., this means thatand about hence half of of the the shallow territory geothermalis therefore potential. However, variability. heating season excluded from the onlymapping a few of isolated the ground buildings temperature, are present and at such elevations. The highest values of geothermal potential (13 to hence of the shallow geothermal potential. However, only a The ground temperature is strongly correlated to the According to UNI (1987), the length of the heating 15 MWh/year) are found in the piedmont areas below 1000m few isolated buildingsseason are present (t ) was at suchdefined elevations. as the number of days with an of elevation, close to the alluvial plains of Aosta, Verres and yearly mean of the air temperature, which in turns depends c According to UNI average(1987), thetemperature length of the inferior heating to season 12°C. A linear correlation Pont-Saint-Martin. This is due to the combination of a high on the altitude. According to Signorelli & Kohl (2004), the ( ground is 1 to 2°C warmer than the air, and this difference ) was defined as the number of days with an average temperature inferior to 12°C. A linear correlation between increases with the altitude due to the isolating effect of the 𝑡𝑡. snow cover during winter. Yearly average air temperatures and the altitude was found (Fig. 3C) using the same meteorological data utilised for the evaluation of the (°C) were therefore used to derive ground . 𝑡𝑡 temperatures (°C), assuming a difference of 1°C: undisturbed ground temperature. The resulting spatial 7qr D6s 𝑇𝑇 distribution is shown in Fig. 3D. + 𝑇𝑇

A B

A B

C D

Fig. 3 - From top to bottom, from left to right: correlation with altitude and spatial distribution of the ground temperature (A,B) and of the length of the heating season (C,D). 64 A. Casasso ET AL.

Fig. 4 - Map of the closed-loop shallow geothermal potential (MWh/y) in the Aosta Valley. thermal conductivity of the schists and mica shists (3.32- borehole pipes are usually delivered with such standard 3.52 Wm-1K-1) and a relatively high ground temperature lengths. The number of boreholes is the ratio between the (above 10°C). Other areas with a good geothermal potential energetic demand and the geothermal potential of the town (11-13 MWh/year) are in the highest part of the Aosta Valley and is rounded to the upper unit. The total cost is considered (Courmayeur, La Thuile, La Salle) and in lateral valleys such as the sum of both mentioned costs. Cost variations (Δ cost as , Valgrisenche, Valpelline and Valtournenche in Tab. 2) are calculated compared to the most favourable (see Chamois). In Valtournenche, a GSHP system is installed case. These variations deeply influence the economic return at 2,400 m a.s.l. on a ski track, which is the top elevation for a of a GSHP in these locations, thus driving the choice of geothermal plant in Italy (Fabrizio et al., 2015). the heating technology. In the Aosta plain, for example, The alluvial plains of Aosta (from Sarre to Quart, due to the presence of a productive and thick aquifer, the see Fig. 4), Verres and Pont-Saint-Martin have a lower installation of open loop systems can be an economic thermal conductivity (1.90 Wm-1K-1) and hence the alternative: considering a cost of 15 k€ per well, the total geothermal potential is much lower (8 to 9 MWh/year), cost for the heat pump and the well doublet can be finally which may be considered as a medium value compared estimated in 85 k€ (-37% than the closed loop solution). to other areas where G.POT was applied (Casasso, et al., 2017a, Casasso & Sethi, 2017). On the other hand, the plain of Aosta hosts a thick and conductive shallow CONCLUSIONS unconfined aquifer (Bonomi et al., 2013) and hence it is suitable for the installation of groundwater heat pumps Shallow geothermal potential maps are useful tools for (which have not been analysed in this work). the evaluation of the installation costs, and hence of the economic feasibility of GSHP at a certain installation site. ECONOMIC FEASIBILITY OF A GSHP: IMPACT OF THE In this paper, we presented the assessment and GEOTHERMAL POTENTIAL ON THE CASE OF A BLOCK OF mapping of the closed-loop shallow geothermal potential FLATS in the Aosta Valley depending on the ground thermal properties and on the utilisation profile. According to the In Tab. 2 a brief example is provided to explain how results, the following conclusions can be made: the shallow geothermal potential affects the costs of an installation in some town centres of the Aosta Valley. A –– the closed-loop geothermal potential of the Aosta Valley block of flats with an annual energetic heating demand is highly variable, due to the wide range of variability of 140 MWh/y is considered. The cost of the heat pump of the thermal conductivity (from 1.70 to 3.52 Wm-1K-1) is constant for all the interested sites, while the number and the undisturbed temperature of the ground (from of required boreholes depends on their length and on the 5 to 15°C in the elevation range considered); geothermal potential of the town. –– the highest values of geothermal potential (12 to The cost of a 70 kW power heat pump (considering 15 MWh/year with a 100 m long BHE) are achieved 2,000 equivalent full load working hours per year) can be in schists and mica schists at elevations below 1000 estimated at 55 k€; the cost of drilling and installation of m a.s.l., due to the contemporarily high thermal each borehole is considered 5 k€, considering a fixed length conductivity (up to 3.52 Wm-1K-1) and the intermediate of 100m, which is a common installation practice since value of ground temperature (over 10°C); Ground Source Heat Pumps in Aosta Valley (NW Italy) 65

TABLE 2 Carslaw H.G. & Jaeger J.C. (1959) - Conduction of heat in solids. Cambridge University Press, Cambridge (UK), pp. Installation costs of a standard GSHP plant in different Casasso A., Pestotnik S., Rajver D., Jež J., Prestor J. & Sethi R. (2017a) - Municipalities. The number of required 100m-deep Assessment and mapping of the closed-loop shallow geothermal potential in Cerkno (Slovenia). Energy Procedia, 125C, 335-344. boreholes is reported, along with the exact required Casasso A., Piga B., Sethi R., Prestor J., Pestotnik S., Bottig M., Goetzl length (in brackets). G., Zambelli P., D’Alonzo V., Vaccaro R., Capodaglio P., Olmedo M., Baietto A., Maragna C., Boettcher F. & Zosseder K. (2017b) - Number The GRETA project: the contribution of near-surface geothermal P (and exact Total Municipalities BHE Δ cost energy for the energetic self-sufficiency of Alpine regions. Acque (MWhy-1) length) of cost (k€) Sotterranee - Italian Journal of Groundwater, 6, 1-12. 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