Groundwater cooling of a supercomputer in Perth, Western Australia:

hydrogeological simulations and thermal sustainability

Heather A. Sheldon & Peter M. Schaubs &

Praveen K. Rachakonda & Michael G. Trefry &

Lynn B. Reid & Daniel R. Lester & Guy Metcalfe &

Thomas Poulet & Klaus Regenauer-Lieb

Abstract Groundwater cooling (GWC) is a sustainablealternative to conventional cooling technologies forsupercomputers. A GWC system has been implementedfor the Pawsey Supercomputing Centre in Perth, WesternAustralia. Groundwater is extracted from the Mullaloo

Aquifer at 20.8 °C and passes through a heat exchangerbefore returning to the same aquifer. Hydrogeologicalsimulations of the GWC system were used to assess itsperformance and sustainability. Simulations were run withcooling capacities of 0.5 or 2.5 Mega Watts thermal(MWth), with scenarios representing various combinationsof pumping ra te , in jec t ion tempera ture andhydrogeological parameter values. The simulated systemgenerates a thermal plume in the Mullaloo Aquifer andoverlying Superficial Aquifer. Thermal breakthrough(transfer of heat from injection to production wells)occurred in 2.7–4.3 years for a 2.5 MWth system.Shielding (reinjection of cool groundwater between theinjection and production wells) resulted in earlier thermalbreakthrough but reduced the rate of temperature increaseafter breakthrough, such that shielding was beneficial afterapproximately 5 years pumping. Increasing injectiontemperature was preferable to increasing flow rate formaintaining cooling capacity after thermal breakthrough.Thermal impacts on existing wells were small, with up to10 wells experiencing a temperature increase≥0.1 °C(largest increase 6 °C).

Keywords Groundwater cooling . Shielding . Numericalmodelling . Geothermal . Australia

Introduction

Supercomputers and data centres use large amounts ofelectricity, most of which is ultimately converted to heat.Cooling is therefore essential to maintain the temperaturewithin a suitable range for the processors and othercomponents to operate (Ebrahimi et al. 2014; McDonnell2013). In recent years there has been growing concernabout the energy required for both running and coolingsupercomputers (McDonnell 2013; Wu-chun and Camer-on 2007). Water consumption is another cause for concernas conventional cooling technologies rely on evaporationto transfer heat to the atmosphere. Thus, the developmentof sustainable, energy- and water-efficient cooling solu-tions for supercomputers is of broad interest. Groundwatercan play a role in addressing this problem.

Received: 9 November 2014 /Accepted: 7 June 2015Published online: 8 August 2015

* Springer-Verlag Berlin Heidelberg 2015

H. A. Sheldon ()) I P. M. Schaubs I K. Regenauer-LiebCommonwealth Scientific and Industrial Research Organisation(CSIRO) Mineral Resources Flagship, PO Box 1130, Bentley, WA6102, Australiae-mail: heather.sheldon@csiro.au

P. K. RachakondaCSIRO Energy Flagship, PO Box 1130, Bentley,, WA6102, Australia

M. G. TrefryCSIRO Land and Water Flagship, Private Bag 5, Wembley, WA6913, Australia

L. B. ReidCDM Smith Australia, 11/300 Rokeby Road, Subiaco, WA6008, Australia

L. B. ReidSchool of Civil, Environmental and Mining Engineering, 35 StirlingHighway, Crawley, WA 6009, Australia

D. R. LesterCSIRO Mineral Resources Flagship, Box 56, Highett, VIC3190, Australia

D. R. LesterNow at School of Civil, Environmental and Chemical Engineering,Royal Melbourne Institute of Technology, 124 La Trobe Street,Melbourne, VIC 3000, Australia

G. MetcalfeCSIRO Manufacturing Flagship, Box 56, Highett, VIC3190, Australia

T. PouletCSIRO Mineral Resources Flagship, Riverside Corporate Park, 11Julius Avenue, North Ryde, NSW 2113, Australia

K. Regenauer-LiebNow at School of Petroleum Engineering,University of New South Wales, Sydney, NSW 2052, Australia

Hydrogeology Journal (2015) 23: 1831–1849DOI 10.1007/s10040-015-1280-z

Conventional cooling solutions for supercomputersinclude cooling towers and evaporative air conditioners,both of which consume large amounts of water (e.g.,Sharma et al. 2010) and may be ineffective duringextremes of atmospheric heat or humidity. An alternativeapproach that has received some attention in recent yearsis to use the waste heat for other purposes such as local ordistrict heating systems or to drive an adsorption chiller(Ebrahimi et al. 2014; Meijer 2010; Meyer et al. 2013;Romero et al. 2014; Zimmermann et al. 2012). This heatrecovery approach reduces the net energy and waterconsumption of the supercomputer, but it may not beeffective in all seasons due to fluctuations in ambienttemperature and in the demand for heating and cooling.Another alternative to cooling towers is to reject the wasteheat underground, using groundwater to transfer heat fromthe supercomputer to an aquifer. This groundwater cooling(GWC) approach can operate effectively in all seasons,thanks to the stable temperature of groundwater.

This paper describes the use of GWC for temperaturecontrol of a supercomputer. In the GWC system, coolgroundwater is pumped from an aquifer through a heatexchanger, where it is warmed by heat from thesupercomputer, before being returned to the same aquiferat some distance from the point of extraction. Thus, theGWC system involves no net loss of water. Schemes ofthis type may be viable even in locations where theaquifer is fully allocated for other purposes. GWCtherefore has great potential for cooling supercomputersand other large buildings (e.g., Liu et al. 2013), especiallyin areas where water supplies are limited and extremeweather conditions make cooling towers ineffective forpart of the year.

To the best of the authors’ knowledge, the firstsupercomputer to use GWC was Olympus at the PacificNorthwest National Laboratory in Washington state, USA.Olympus uses groundwater at 18 °C. This paper concernsa GWC system for the Pawsey Supercomputing Centre inPerth, Western Australia, which uses groundwater atapproximately 20.8 °C. The water-saving features ofGWC are particularly attractive in Perth’s hot, dry climate.

The success of a GWC system depends on a number offactors. Of particular importance is the longevity of thesystem, which depends primarily on the time to Bthermalbreakthrough^; that is, how long it takes for heat from theinjected water to reach the production wells (Banks 2009).When thermal breakthrough occurs, either the pumpingrate or the temperature of the injected water must beincreased in order to maintain the same cooling capacity.Another important issue is the impact of the system (dueto changes in water level, temperature and biogeochem-istry) on existing groundwater users and groundwater-dependent ecosystems in the area (Bonte 2015). Theseissues must be considered when designing a GWC system.

Research into ground-sourced heating and coolingsystems, of which GWC is an example, dates back tothe 1970s (e.g., Gringarten 1978; Gringarten and Sauty1975; Tsang et al. 1977). These pioneering studiesprovided equations for estimating drawdown and the time

to thermal breakthrough in idealised and geometricallysimple systems. While these equations are useful for afirst-order analysis, they do not take into account thethree-dimensional (3D) complexity of real systems.Hydrogeological simulations can be used to provide amore realistic assessment of the behaviour, performanceand impacts of a GWC system in a real 3Dhydrogeological context. Hydrogeological simulationsare used in this paper to simulate the GWC system ofthe Pawsey Centre.

The Pawsey Centre was built in 2013 to supportresearch conducted on the Square Kilometre Array radiotelescope and other science activities. The CommonwealthScientific and Industrial Research Organisation (CSIRO)has designed and implemented a GWC system for thePawsey Centre, with the aim of providing a sustainableand reliable cooling solution that is suited to Perth’sclimate. The system was designed to support Phase 1 ofthe Pawsey supercomputer, which has a 5-year design life.The GWC system utilises a six-well layout comprisingtwo production wells, two warm water injection wells andtwo Bshielding^ wells. The shielding wells are used forreinjection of some of the cool water from the productionwells with the aim of delaying thermal breakthrough(Trefry et al. 2014). Assessing the effectiveness of thisnovel shielding approach was a key aim of this study.Hydrogeological simulations are used here to explore thebehaviour and performance of the Pawsey Centre GWCsystem, focusing on the following key questions:

1. What is the effect of varying injection temperature andpumping rate for a given cooling capacity?

2. Is shielding effective at delaying thermal breakthroughand/or slowing the rate of temperature increase at theproduction wells after thermal breakthrough?

3. What is the best method for maintaining the coolingcapacity of the system after thermal breakthrough:increasing pumping rate or increasing injectiontemperature?

4. What is the thermal impact of the system on existinggroundwater extraction wells in the area?

Potential impacts of the GWC system on neighbouringecosystems are not considered here, but have beenassessed as part of regulatory compliance activities(Sheldon et al. 2014), whereas biogeochemical impactshave been addressed by Douglas et al. (2015). While thedetails of this study relate to a specific system and itscorresponding hydrogeology, the findings are of broadinterest because this type of GWC system has the potentialto be widely applicable in similar urban environments.

Principles of GWC

The use of groundwater for above-ground temperaturecontrol is not a new concept (Banks 2012; 2009; Sarbuand Sebarchievici 2014), although its application tocooling a supercomputer is unusual. The GWC system

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considered in this paper is an open-loop balanced system,in which all of the extracted water is reinjected into thesame aquifer, such that there is no net loss of water. Thistype of system operates as follows (Fig. 1):

1. Cool groundwater (temperature T0) is extracted fromone or more production wells in a shallow aquifer.

2. Groundwater is passed through an above-ground heatexchanger, where it is warmed by heat from thesupercomputer (or other heat source).

3. Warm groundwater (temperature T1) is returned to thesame shallow aquifer in one or more injection wells, atsome distance from the point of extraction.

The simplest balanced GWC system comprises oneproduction well and one injection well. It may bepreferable to have multiple injection and extraction wellsin order to spread the rejected heat over a wider area andto reduce the pumping rate per well. However, increasingthe number of wells increases the cost and complexity ofdeveloping and running the system. Ideally the productionwells should be located upstream from the injection wellsrelative to the background groundwater flow, such thatheat tends to be naturally transported away from theproduction wells (Banks 2009).

The overall pumping rate (i.e., the total rate of waterextraction across all production wells) required to satisfy agiven cooling capacity is determined by the followingequation (Banks 2009):

Q ¼ H

ΔTcvð1Þ

where Q is the flow rate (L/s), H is the cooling capacity ofthe system (i.e., the amount of cooling that is required;W), ΔT=T1 – T0 is the temperature difference betweeninjected and produced water (°C), and cv is the volumetricheat capacity of the groundwater (4,180 J/L/K for purewater at 20 °C).

Heat from the warm injected groundwater is dissipatedin the aquifer by conduction, advection and dispersion.Eventually some of the heat will reach the productionwell(s) (that is, thermal breakthrough will occur) unlessthe following condition is satisfied (Banks 2009):

L >2Q

T tπið2Þ

where L is the distance between the injection andproduction wells, Tt is the aquifer transmissivity and i isthe hydraulic gradient. Note that Eq. (2) assumes auniform aquifer with a single well doublet aligned in thedirection of the head gradient; this ideal situation isunlikely to be satisfied, but the inequality is a useful rule-of-thumb for determining the likelihood of thermalbreakthrough. After thermal breakthrough has occurred,

the pumping rate or injection temperature must beincreased in order to maintain the cooling capacity of thesystem (Banks 2009). At some point the system may ceaseto be viable, if the required pumping rate becomes toohigh to be sustained or if ΔT drops to zero. A keyperformance indicator for a GWC system is the time tothermal breakthrough; that is, the time at which temper-ature at the production well(s) starts to increase due toarrival of heat from the injection wells. The time tothermal breakthrough, and the rate of temperature increasefollowing thermal breakthrough, depend on: (1) thethermal properties of the aquifer; (2) the pumping rate;(3) the rate and direction of background groundwater flowrelative to the well layout; and (4) the distance betweeninjection and production wells (Banks 2012; 2009; Clydeand Madabhushi 1983; Gringarten 1978; Tsang et al.1977).

It may be possible to delay thermal breakthrough, and/or reduce the rate of temperature increase after thermalbreakthrough, by introducing Bshielding^ wells betweenthe injection and production wells (Trefry et al. 2014).When shielding wells are used, the pumping rate from theproduction wells is increased and the additional cool wateris reinjected into the shielding well(s) without passingthrough the heat exchanger (Fig. 1). This reinjection ofcool water has the simultaneous effects of: (1) creating ahydraulic barrier between the production and injectionwells, which increases the effective path length for warmwater attempting to reach the production wells; and (2)creating a continuously replenished low-temperature ther-mal barrier which can take up heat carried by the injectedwarm water. The net effect of the hydraulic and thermalbarriers produced by shielding is expected to be areduction in the rate of heat transport from the injectionwells to the production wells. Quantification of theshielding benefit is site-specific and depends on thehydraulic and thermal properties of the aquifer, and onthe background groundwater flow direction and magni-tude. The application of shielding to a GWC system is anovel concept which is explored in this paper.

The Pawsey Centre GWC system

The Pawsey Centre is located approximately 4.5 kmsouth-southeast of the Perth city centre in WesternAustralia, and approximately 12 km inland from theIndian Ocean (Fig. 2). Geologically, the area is situatedin the Perth Basin, a north–south-trending rift basin thatextends more than 1,000 km along the western margin ofAustralia (Playford et al. 1976).

The hydrogeology of the study area is illustrated inFig. 3. The Pawsey Centre GWC system uses water fromthe semi-confined Mullaloo Aquifer, comprising theMullaloo Sandstone Member of the Kings Park Forma-tion. The Mullaloo Sandstone Member is a localised unitdominated by unconsolidated quartz sands deposited in apaleochannel incised into the underlying shale of theKings Park Formation (Davidson 1995; Davidson and Yu

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2007). The Mullaloo Aquifer and Kings Park Formationare overlain by the Superficial Aquifer: a thin, unconfinedaquifer covering much of the Perth area, consistingpredominantly of the Bassendean Sands and TamalaLimestone in the vicinity of the Pawsey Centre (Davidson1995; Davidson and Yu 2007). The Kings Park Formationis underlain by the Osborne Formation (primarily anaquitard in the study area) and Leederville Formation (anaquifer).

A thin, localised confining layer, comprising fine-grained organic-rich material, has been identified at thetop of the Mullaloo Sandstone Member in some wellsclose to the Pawsey Centre, representing a leaky aquitard(hereafter called the Mullaloo Aquitard) between theSuperficial and Mullaloo Aquifers (Rockwater Pty Ltd2011, 2013a, 2012). This aquitard is laterally discontinu-ous, being present in some but not all of the PawseyCentre GWC and monitoring wells, and in two ground-water monitoring wells located approximately 500 mnortheast of the site. Its extent, thickness and propertiesare unknown beyond the immediate vicinity of the PawseyCentre.

The area around the Pawsey Centre is bounded by theSwan and Canning rivers to the north, west and south(Fig. 2). The Superficial Aquifer in this area dischargesinto the estuarine river system, and is recharged fromrainfall. The water table has a shallow slope across thestudy area from 11 m relative to the Australian HeightDatum (mAHD) on the east side to 0 mAHD on the westside. The depth of the water table ranges from 10 to 30 mbelow ground level in the vicinity of the Pawsey Centredue to undulating topography. The Mullaloo Aquifer isrecharged below ground from the Superficial Aquifer tothe northeast, and is believed to discharge into theSuperficial Aquifer beneath the Indian Ocean (Davidson

1995), approximately 12 km west of the Pawsey Centre.Thus, the direction of groundwater flow in the MullalooAquifer is broadly towards the west.

Pumping well locations for the Pawsey Centre GWCsystem are shown in Fig. 2. There are two injection wellson the west side, two production wells on the east side,and two shielding wells in the middle. The wells arescreened at depths of 45 to 130 m.

The Pawsey Centre GWC system was originally con-ceived to provide a cooling capacity of 2.4 MWth (MegaWatts thermal); however, the actual thermal load generatedby the supercomputer is considerably smaller, expected to beonly ∼0.5 MWth in the first planned supercomputer stage,due to technological improvements that occurred betweenthe original design phase and commissioning of the stage 1supercomputer. It is likely that the thermal load will increaseover the next few years as further computational and/or datastorage capacity is added to the Pawsey Centre; therefore,simulations have been performed to assess the performanceand impacts of the system with cooling capacities rangingfrom 0.5 to 2.5 MWth.

The current Pawsey Centre supercomputer can operatewith inlet water temperatures up to 24 °C. For thepurposes of the present investigation, it is assumed thatthe maximum allowable temperature of produced ground-water (hereafter described as the cut-off temperature) is23.5 °C, to allow for inefficiencies in the heat exchangeprocess.

The Groundwater Licence Operating Strategy (GLOS)for the Pawsey Centre (Rockwater Pty Ltd 2013b) sets outthe operating limits for the GWC system as agreed withthe Western Australia Department of Water (DoW). TheGLOS allows the injected water to be up to 10 °C hotterthan the ambient groundwater temperature in the MullalooAquifer, with no net abstraction of water. The operating

Fig. 1 Cross section view of a simple balanced GWC scheme, comprising an injection well, a production well and an optional shieldingwell (Sheldon et al. 2014). Blue arrows indicate movement of cool water, red arrows indicate movement of warm water. Background flowand shielding (if used) deflect the thermal plume away from the production well

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rules specify maximum flow rates of 45 L/s in theproduction wells, 30 L/s in the injection wells and 15 L/sin the shield wells. In the simulations reported in this paper,the required cooling capacity is achieved by adjusting ΔTand pumping rate within these bounds.

Hydrogeological model

Hydrogeological simulations of the Pawsey Centre GWCsystem were performed using FEFLOW, a finite-elementmodelling package that solves the equations of

Fig. 2 Location and layout of the Pawsey Centre GWC system. a Location map and boundary of the hydrogeological model. b Site mapshowing GWC well locations. See blue box in a for location of b

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groundwater flow and heat transport in two or threedimensions (Diersch 2014). Fluid flow is assumed to obeyDarcy’s law, and heat is transported by advection,conduction and dispersion.

Model constructionThe hydrogeological model represents an area centred onthe Pawsey Centre, bounded by the Swan and Canningrivers to the north, west and south (Fig. 2a). Both riversare estuarine, and are therefore in close communicationwith the Indian Ocean. The rivers have a minor tidalfluctuation of approximately 0.5 m at the model bound-aries, and are known to be hydraulically connected to theshallow aquifers (Smith 1999); thus, they form naturalhydraulic boundaries for the Superficial Aquifer in thearea. The model area is truncated to the east by a north–south line located approximately 4 km east of the PawseyCentre, which is at a sufficient distance from the GWCsystem to minimise boundary effects. There would be nobenefit in extending the model further to the east becausethe eastern extent of the Mullaloo Aquifer is largelyunknown, and there are few wells to provide furtherinformation on hydraulic conditions in the aquifer to theeast. Similarly, there is little information about theMullaloo Aquifer to the west of the model area; extendingthe model to the supposed offshore discharge area of theMullaloo Aquifer (Davidson 1995) would result in anunfeasibly large model without any obvious benefit to thepresent investigation.

The model encompasses the Superficial Aquifer (10–50 m thick), Mullaloo Aquitard (0–25 m thick), MullalooAquifer (0–130 m thick), Kings Park Formation (0–300 mthick) and Osborne Formation (0–180 m thick). Thebottom of the model corresponds to the top of theLeederville Formation. It was not necessary to includethe Leederville Formation in the model because the KingsPark and Osborne Formations act as an aquitard separat-ing it from the Mullaloo Aquifer. The top of the modelrepresents the topographic surface defined by a digitalelevation model (DEM; Geoscience Australia 2010).

The geologic model (Fig. 4) was constructed usingLeapfrog Hydro™, which employs a radial basis functionand implicit modelling technique to construct its models(Krom and Lane 2012). Volumes representing the

hydrogeological units were created using stratigraphicpicks from well logs obtained from the DoW andRockwater Pty Ltd, and logs from the pumping andmonitoring wells around the Pawsey Centre site.

Having constructed the geologic model, the next stepwas to discretise the model to create a finite element mesh.The usual procedure for constructing a 3D mesh inFEFLOW involves generating a 2D mesh (of 3- or 4-sided elements) on the top surface of the model, thenprojecting this mesh down vertically through a series oflayers that represent the hydrogeologic units. Thisprocedure works well when the units are continuousacross the model domain, but requires some tedious anderror-prone manual intervention where units pinch out,because FEFLOW requires every layer of elements to becontinuous across the model domain. Such difficultieswere avoided in the present study by generating a 3Dmesh from the geologic model in Leapfrog Hydro™, thenimporting the resulting mesh back into FEFLOW.Pinching out is dealt with automatically in this approach.A further advantage of generating the mesh in LeapfrogHydro™ is the ease of regenerating the mesh if changesare made to the geologic model (e.g., due to a newconstraint on a surface in the model), or if a different meshresolution is required. The resulting mesh (Fig. 5) com-prised 2,235,288 triangular prismatic elements withhorizontal mesh resolution of 20 m around the modelboundaries, 2 m around the GWC wells and 50 melsewhere. Mesh sensitivity was assessed by running oneof the simulation scenarios on a refined mesh. Results ofthe mesh comparison are presented in the ‘Appendix’.

PropertiesThe hydraulic and thermal properties of the model wereassumed to be spatially and temporally constant withineach hydrogeological unit, with the exception of hydraulicconductivity within the Mullaloo Aquifer, which wasvaried in some scenarios (see the following). Hydraulicconductivities were assumed to be isotropic in thehor izon ta l p lane and have a ra t io of 10 :1horizontal:vertical. Property values and data sources arelisted in Table 1. The values were guided by publisheddata for the hydrogeological units where such information

Fig. 3 Cross-section through the study area. Vertical exaggeration×5

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was available or, otherwise, by representative values forthe relevant rock types.

Hydraulic conductivity of the Mullaloo Aquifer islikely to influence the behaviour of the GWC systembecause it influences the groundwater flow rate, which inturn influences the evolution and migration of the thermalplume; therefore, it was considered important to investi-gate the effect of varying this parameter. Estimates of theaverage regional hydraulic conductivity of the MullalooAquifer range from 8 to 44 m/day (Davidson and Yu2007; Rockwater Pty Ltd 2012); however, pumping testsin the GWC wells suggested that it could be much higher(up to 92 m/day), at least in the vicinity of the GWCsystem (Rockwater Pty Ltd 2013a). High hydraulicconductivity is consistent with the lithology of the aquiferat the site, comprising medium- to coarse-grained uncon-solidated sand. However, given the channel depositionalsetting of the aquifer (Davidson 1995), it is likely that thehydraulic properties are heterogeneous and this highhydraulic conductivity may be quite localised (RockwaterPty Ltd 2012). Consequently, some simulations were runwith elevated hydraulic conductivity (55 m/day) in part ofthe Mullaloo Aquifer extending from the GWC system tothe western boundary of the model (Fig. 6). The shape andextent of this area is quite arbitrary and is not constrainedby any measurements to the west of the GWC site, but it isgeologically plausible.

Another area of uncertainty in the hydrogeologicalmodel is the extent and hydraulic conductivity of theMullaloo Aquitard. The impact of the aquitard on thebehaviour of the GWC system was assessed by runningsome simulations with the aquitard having the samehydraulic and thermal properties as the Mullaloo Aquifer,effectively representing the case where the MullalooAquitard is absent.

Boundary conditions

Fluid flow boundary conditionsA phreatic boundary condition with vertical recharge(representing infiltration of rainfall) was applied at thetop of the model, allowing the water table to move withinthe Superficial Aquifer. Vertical flux modelling carried outby Xu et al. (2008) indicates a rainfall recharge rate ofapproximately 8×10−4 m/day in the vicinity of the PawseyCentre, based on data from 1997–2003. Using this valueas a guide, the recharge rate on the top boundary wasallowed to vary spatially based on soil type andgeomorphology, with three areas being defined (Fig. 7).The recharge rates for these areas were determined duringhydrogeological calibration (see the following). Seasonalvariations in recharge were not considered; therefore, thesimulated depth of the water table should be considered as

Fig. 4 Three-dimensional structure of the hydrogeological model: a complete model, and b model with Superficial Aquifer removed,highlighting the lateral extent of the Mullaloo Aquifer. Models viewed from the southeast (top) and west (bottom). White circle indicateslocation of GWC system. Vertical exaggeration×5

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an annual average rather than representative of the actualdepth at any point in time.

The following boundary conditions were applied wherethe Superficial and Mullaloo aquifers intersect the verticalsides of the model:

& A fixed head boundary condition (first kind/Dirichlettype) of 0.5 m in the top two slices of the SuperficialAquifer where it intersects the Swan and Canningrivers (i.e., some nodes on the north, west and southboundaries). This boundary condition reflects dis-charge from the Superficial Aquifer into the riversystem.

& A fluid-transfer (third kind/Cauchy type) boundarycondition along the eastern boundary of the SuperficialAquifer. The reference heads were interpolated from2003 groundwater elevation contours published by the

DoW in the Perth Groundwater Atlas (Department ofWater 2004) along a line located 1 km east of themodel boundary. This atlas was the most comprehen-sive data source on groundwater levels in the Super-ficial Aquifer at the time of constructing the model, butit is likely that water levels have changed since 2003.The fluid-transfer condition was used instead of a fixedhead condition for this boundary because it providedimproved numerical stability when combined with heattransport.

& A fixed head boundary condition (first kind/Dirichlettype) of 13 m where the Mullaloo Aquifer intersectsthe eastern edge of the model domain. This water levelwas determined by extrapolating the measured hydrau-lic gradient in the Mullaloo Aquifer to the east.

& A fluid-transfer boundary condition (third kind/Cauchytype) where the Mullaloo Aquifer intersects the

Fig. 5 Finite element mesh on top surface of the model, illustrating refinement around model boundaries and GWC wells (green dots)

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western edge of the model domain, representinggroundwater flowing towards the Indian Ocean. Thisfluid-transfer boundary condition was based on areference head of 0 m measured in bores locatedapproximately 1 km west of the model boundary.

These boundary conditions drive flow from east to westwith a flow rate of ∼0.01 m/day in the Mullaloo Aquifer.All other boundaries (sides and bottom) were treated asno-flow boundaries. It should be noted that the hydraulicboundary conditions for the Mullaloo Aquifer are rather

poorly constrained due to the limited number of wells withwater level measurements in the Mullaloo Aquifer.

Existing groundwater extraction wellsGroundwater extraction wells are treated as internalboundary conditions in FEFLOW. Groundwater is ex-tracted from hundreds of wells in the Superficial Aquiferand a few wells in the Mullaloo Aquifer within the modelarea, including both licensed and unlicensed wells. TheDoW provided information on well locations and licensedmaximum extraction rate for all groundwater licences inthe model area. This information was used to estimate theextraction rate in each licensed well, assuming that actualextraction equalled the licensed amount and was uniform-ly distributed between all wells covered by a single

Table 1 Calibrated property values

Horizontal hydraulicconductivity (m/day)

Specific storage(10−4 m−1)

Specificyield

Porosity(%)

Thermal conductivityof solid (W/m/K)

Specific heat capacityof solid (106 J/m3/K)

Superficial Tamala 15a 10a 0.15a 25b 3.75c 2.10d

Superficial Bassendean 18a 10a 0.15a 25b 3.75c 2.10d

Mullaloo Aquitard 0.29a 0.1a N/A 5.0e 2.90c 2.30d

Mullaloo Aquifer 11a 5a N/A 25a 4.00c 2.05d

Kings Park Formation 0.10f 1g N/A 5.0e 2.25h 2.30d

Osborne Formation 2.0f 1g N/A 20b 3.20h 2.05d

a Rockwater Pty Ltd (2012)b Davidson (1995)c Estimated from mineral compositions of formations provided by Rockwater Pty Ltd (2012) combined with mineral thermal conductivitiesd Values for sandstone, siltstone and limestone (Waples and Waples 2004)e Representative value for shalef Davidson and Yu (2007)g FEFLOW default value, representative for clastic sedimentary rock (Domenico and Mifflin 1965)h Hot Dry Rocks Pty Ltd (2008)

Fig. 6 View of top of Mullaloo Aquifer (green and red) showinghigh hydraulic conductivity area (red) used in some simulations.Grey area is outside Mullaloo Aquifer (Osborne Formation andKings Park Formation). Black box indicates location of GWCsystem

Fig. 7 Soil types defining recharge areas. Black box indicateslocation of GWC system

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groundwater license. The effect of unlicensed wells, andany differences between licensed and actual extractionrates in licensed wells, was effectively accounted for byadjusting the recharge rate during model calibration.

Thermal boundary conditionsThe top of the model was assigned a fixed temperatureand the bottom was assigned a fixed heat flux. The valuesof the fixed temperature and fixed heat flux weredetermined during thermal calibration (see the following).Seasonal variations in surface temperature were notrepresented in the model. To assess the impact of seasonaltemperature variations, the calibrated model was run for10 years with surface temperature varying on a seasonalcycle between 13 and 25 °C, which is representative of therange of average daily temperature in Perth (Bureau ofMeteorology 2013). This simulation showed that theseasonal variation decays with depth such that thetemperature at the top of the Mullaloo Aquifer varies byless than 0.4 °C annually. Therefore, simulated tempera-tures in the Superficial Aquifer should be considered asannual averages, whereas simulated temperatures in theMullaloo Aquifer may be representative of the tempera-ture at a given point in time.

CalibrationSome of the property values and boundary conditionswere adjusted through a two-stage calibration process,involving:

1. Hydrogeological calibration. The model was run insteady-state mode without heat transport. Hydraulicconductivity and recharge rates were adjusted until themodel provided a satisfactory match to water levels in10 wells in the model area (seven in the SuperficialAquifer and three in the Mullaloo Aquifer).

2. Thermal calibration. The model was run in steady-statemode with fluid flow and heat transport. Thermalconductivity, specific heat capacity and thermal bound-ary conditions were adjusted to obtain a satisfactorymatch to temperature-depth logs in four wells close tothe Pawsey Centre.

The calibrated property values and boundary condi-tions are listed in Tables 1, 2 and 3, along with notes on

their sources. Further details of the hydrogeological andthermal calibration are given in the following.

Hydrogeological calibrationThe wells used for hydrogeological calibration are shownin Fig. 8, along with the measured and simulatedhydraulic head in each well. Measured heads wereobtained from the DoW and Rockwater Pty Ltd(Rockwater Pty Ltd 2012). The measured values are fromend-of-winter (EOW) and end-of-summer (EOS) 2011,which was the most comprehensive dataset available at thetime the model was constructed. Wells MPB01, MB01,MB02, MB03 and MB04A did not have EOS 2011measurements. The calibrated property values and bound-ary conditions are listed in Tables 1 and 3. Summarystatistics for the calibration are listed in Table 4. The meanerror, scaled root mean square (RMS) error and standarddeviation of the modelled water levels are calculatedrelative to the average of the EOS and EOW water-levelmeasurements. The scaled RMS error is below therecommended maximum of 5 %, and the overall waterbalance error of the model is well within the recommend-ed maximum of 1 % (Middlemis 2000).

The largest errors occur in the Superficial Aquifer wellsthat are furthest from the GWC site (SCC-2308, 149 and2729); the other wells display a good match to measuredwater levels, particularly those on and close to the site.Discrepancies between measured and simulated hydraulichead in the Superficial Aquifer could be due to localunlicensed extraction from the Superficial Aquifer, and/orlocal variations in hydraulic properties or recharge.

It should be noted that very little is known about thegroundwater flow regime in the Mullaloo Aquifer due tothe very small number of wells in the model area withwater level measurements in this aquifer. The accuracy ofthe hydrogeological calibration is limited by this lack ofdata. The distribution of hydraulic head in the calibratedmodel is illustrated in Fig. 9. Note the gradient from east

Table 2 Properties treated as constants throughout the model

Property Value Source

Longitudinal thermal dispersivity 1 m Representative of values derived from field-scale pumpingtests (Anderson 2005; de Marsily 1986; Molina-Giraldoet al. 2011; Molson et al. 1992; Papadopulos and Larson1978; Sauty et al. 1982; Vandenbohede et al. 2009, 2011)

Transverse thermal dispersivity 0.1 m

Specific heat capacity of groundwater 4.185×106 J/m3/K Wagner et al. (2000)Thermal conductivity of groundwater 0.6 W/m/KThermal expansion coefficient of groundwater 0.00021 /K

Table 3 Boundary conditions after calibration

Boundary conditions Value

Top boundary: temperature 20 °CBottom boundary: heat flux 0.075 W/m2

Recharge: Bassendean Sand 1.05×10−3 m/dRecharge: Tamala Limestone 5.50×10−4 m/dayRecharge: wetlands 0 m/day

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to west, driving groundwater flow towards the west in theMullaloo Aquifer.

Thermal calibrationThere are only four wells in the study area withtemperature-depth logs that could be used for thermalcalibration (Table 5); furthermore, the distance betweenthese wells is small (MB04A and MPB01/11 are on thePawsey Centre site, while AM40 and AM40A are located∼500 m northeast of the site); therefore, the quality of thethermal calibration is limited.

The measured and modelled temperatures are shown inFig. 10. The match to AM40 and AM40A is good, except inthe top 20–40mwhere the logged temperature is considerablylower than the modelled temperature. This is a seasonal effect;note that these wells were logged in winter (see Table 5).

In contrast, the match to MB04A and MPB01/11 ispoor. These wells display higher temperatures at thesurface which is a seasonal effect (logged in Autumnand Spring, respectively), but the elevated temperaturesextend into the Mullaloo Aquifer. It is unlikely that theseasonal effect would penetrate to such depths (Kasenow2001); this was confirmed by running a simulation withoutpumping in the GWC wells but using a seasonaltemperature variation at the surface, which showed only0.15 °C seasonal variation at the top of the MullalooAquifer at MPB01/11. Possible causes of the discrepancybetween measured and modelled temperatures at thesewells include: (1) localised advective heat transport

associated with locally high hydraulic conductivity and/or locally high hydraulic gradient within the Mullaloo orSuperficial Aquifers; (2) local variations in thermalconductivity; (3) errors in the temperature measurements;or (4) disturbance of the temperature profile by drilling.Measurement errors are considered unlikely becausesimilar curvature of the temperature logs was observedin logs taken at different times in these wells. Disturbanceof the temperature profile also seems unlikely as sufficient

Fig. 8 Wells used for hydrogeological calibration. a Well locations. Green dots indicate wells in the Superficial Aquifer, blue dots indicatewells in the Mullaloo Aquifer. Enlarged black box shows detail around GWC system. b Measured and modelled hydraulic heads incalibration wells

Table 4 Summary statistics for hydrogeological calibration. Errorsin modelled water levels relative to measured water levels in cali-bration wells. RMSroot mean square

Statistic Value

Scaled RMS error (%) 4.2Mean error (m) −0.09Standard deviation (m) 0.57Water balance error (%) 0.001

Fig. 9 Hydraulic head (m) at top of Mullaloo Aquifer prior toonset of pumping in GWC system. Black box indicates location ofGWC system. Grey area is outside Mullaloo Aquifer (OsborneFormation and Kings Park Formation)

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time had elapsed between completion of the wells and thetemperature logging for such effects to dissipate. Theclose match between measured and modelled temperatureat AM40 and AM40A suggests that the assumed thermalconductivities of the aquifers (see Table 1) are reasonable;however. a local variation in thermal conductivity and/orhydraulic conductivity (in the vicinity of MB04A andMPB01/11) is plausible. This issue requires furtherinvestigation. The best surface temperature for thermalcalibration was found to be 20 °C; this value produced theclosest match to temperature logs in AM40 and AM40A,which is consistent with other indicators of the averagesurface temperature in the area (Bureau of Meteorology2013; Horowitz and Regenauer-Lieb 2009).

Simulation strategyStarting from the steady-state conditions obtained duringmodel calibration, pumping in the GWC wells wassimulated for a period of 5 or 10 years, followed by afurther 100 years of simulation after the cessation ofpumping to monitor migration and decay of the thermalplume. Table 6 lists the scenarios that were investigated.Scenarios 1 to 3 have the same cooling capacity (0.5MWth) achieved with varying pumping rate and ΔT.Scenarios 4 and 5 have a cooling capacity of 2.5 MWth,with the addition of shielding in scenario 5. Scenarios 4aand 4b are the same as scenario 4, but with the coolingcapacity maintained after thermal breakthrough by

increasing the pumping rate (4a) or the injection temper-ature (4b).

To explore the sensitivity of the system to reasonablevariations in hydraulic and thermal properties, scenarios 1,3, 4 and 5 were repeated with: (1) higher hydraulicconductivity (increased from 11 to 55 m/day) in part of theMullaloo Aquifer (see Fig. 6); and (2) the MullalooAquitard having the same hydraulic and thermal proper-ties as the Mullaloo Aquifer. For brevity the results ofthese alternative hydraulic conductivity scenarios are notdescribed in detail, but are compared with the results ofthe original scenarios at the end of the results section.

Results

Characteristics of the thermal plumeInjection of hot water creates a thermal plume in theMullaloo Aquifer, which spreads into the SuperficialAquifer by thermal conduction, advection and dispersion(Fig. 11). The maximum temperature in the SuperficialAquifer ranges from 21 to 30 °C (Table 7). The plumegrows during pumping, then decays and migrates to thewest after cessation of pumping (Fig. 12). The size of theplume at a given time is proportional to the flow rate inthe injection wells; hence, the plume has the same size andshape in scenarios 1 and 4 (Fig. 11), although themaximum temperature of the plume differs between thesescenarios due to their different injection temperatures.Shielding alters the shape of the plume (compare scenarios4 and 5 in Fig. 11), creating cool areas within the plumeand causing the heat to spread laterally (north–south) overa slightly wider area.

Impact on existing wellsSeven wells in the Superficial Aquifer and one well in theMulla loo Aquifer exper ienced a temperatureincrease≥0.1 °C in one or more scenarios (Fig. 13). Thelargest temperature increase at an existing well after5 years pumping was 1.5 °C, and the largest increase

Table 5 Wells with temperature logs used for thermal calibration

Name Easting(m)a

Northing(m)a

Distance toPawseyCentre (m)

Date (andseason) whenlogged

AM40 394,744 6,460,282 550 12/07/2010(Winter)

AM40A 394,768 6,460,258 530 22/06/2010(Winter)

MB04A 394,566 6,459,753 20 18/04/2012(Autumn)

MPB01/11 394,576 6,459,764 0 09/11/2011(Spring)

aMGA94 zone 50

Fig. 10 Comparison between modelled and measured temperatures at four well locations: a AM40, b AM40A, c MB04A, d MPB01

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after 10 years pumping was 6.0 °C. The largest temper-ature increases occurred in scenario 5.

Effect of varying ΔT and flow rateScenarios 1–3 illustrate the effect of varying ΔT and flowrate while maintaining the same cooling capacity (0.5MWth). The smallest ΔT produces the smallest tempera-ture increase in the Mullaloo and Superficial aquifers(Fig. 11); however, this reduced thermal impact comes atthe expense of greater drawdown (Fig. 14), and a morespatially extensive thermal plume (Fig. 11), due to thehigher pumping rate.

Thermal breakthroughThree aspects of thermal breakthrough are considered: (1)the time to thermal breakthrough; (2) the rate oftemperature increase following thermal breakthrough;and (3) the temperature increase at the production wellsafter 10 years pumping. Aspects 1 and 3 are summarisedin Table 7, and the rate of temperature increase isillustrated in Fig. 15.

Scenario 1 was the only 0.5-MWth scenario todemonstrate thermal breakthrough (due to having thehighest pumping rate), with breakthrough occurring after4.3 years. The temperature increase at the productionwells in this scenario was only 0.5 °C after 10 yearspumping. Thermal breakthrough occurred earlier in the2.5-MWth scenarios (3 to 3.5 years), with a temperatureincrease of 1.7 to 3.1 °C after 10 years.

Comparison of the temperature curves for scenarios 4and 5 in Fig. 15 illustrates the effect of shielding onthermal breakthrough. Shielding reduces the temperatureat the production wells after 10 years pumping from 23.3to 22.4 °C; note, however, the crossover of the thermalbreakthrough curves from scenarios 4 and 5 at approxi-mately 5 years. Shielding resulted in earlier thermalbreakthrough (see Table 7) but a lower rate of temperatureincrease at the production wells after breakthrough. Thenet effect is that shielding results in slightly highertemperatures at the production wells in the first 5 years,but lower temperatures at the production wells between 5and 10 years.

The temperature curves for scenarios 4a and 4b(Fig. 15) illustrate the effect of different strategies formaintaining cooling capacity after thermal breakthrough.

Increasing the injection temperature after thermal break-through (scenario 4b) resulted in very little change in thetemperature evolution at the production wells comparedwith scenario 4, whereas increasing the flow rate (scenario4a) resulted in a significantly faster temperature increase atthe production wells. Scenario 4a was the only scenario toreach the cut-off temperature (23.5 °C) within 10 years.

Parameter sensitivityThe effects of varying the hydraulic and thermal proper-ties in the model are summarised in Table 7. Removingthe Mullaloo Aquitard resulted in drawdown increasing by0.1–0.2 m in the Superficial Aquifer and decreasing by asimilar amount in the Mullaloo Aquifer. Thermal break-through was slowed, with the temperature change at theproduction wells after 10 years decreasing by 0.1–0.2 °Cand the time to thermal breakthrough increasing by 0.8 to1.4 years. This delay in thermal breakthrough reflectsincreased transfer of heat from the Mullaloo Aquifer intothe Superficial Aquifer, due to improved hydraulic andthermal connectivity between the aquifers.

Increased hydraulic conductivity in the MullalooAquifer resulted in faster migration and dispersion of thethermal plume. This in turn resulted in a larger number ofex i s t i ng we l l s expe r i en c i ng a t empe r a t u r eincrease>0.1 °C, although the magnitude of the tempera-ture increase at existing wells was smaller. Drawdown wasreduced by up to 0.4 m in the Superficial Aquifer and upto 2.1 m in the Mullaloo Aquifer, but was spread over awider area. The maximum temperature in the SuperficialAquifer was reduced by 1–5 °C. The impact on thermalbreakthrough was relatively small, with breakthroughoccurring slightly earlier in scenarios 1 and 5 and slightlylater in scenario 4, and there was little change in thetemperature at production wells after 10 years pumping.

Discussion

Optimum flow rate and ΔT for a given coolingcapacityFor a given cooling capacity, the simulation resultsshowed that a smaller ΔT and larger flow rate results in alarger thermal plume, greater drawdown and earlierthermal breakthrough. The optimum combination of flowrate and ΔT therefore depends on the relative importance

Table 6 Hydrogeological model scenarios

Scenario Thermal load (MWth) Shielding ΔT (°C) Injection temperature (°C) Flow rate per well (L/s)P S I

1 0.5 No 2 22.8 30 0 302 0.5 No 5 25.8 12 0 123 0.5 No 10 30.8 6 0 64 2.5 No 10 30.8 30 0 305 2.5 Yes 10 30.8 45 15 304a 2.5 No 10 30.5 ≥30 0 ≥304b 2.5 No 10 ≥30.5 30 0 30

Pproduction, Sshielding, Iinjection

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of minimising temperature versus minimising drawdown,minimising plume size and delaying thermal

breakthrough. For the system considered here, maximisingΔT within the bounds of the GLOS results in the most

Fig. 11 Temperature (°C) on a horizontal plane through the Mullaloo Aquifer (−70 mAHD) and Superficial Aquifer (0 mAHD) after10 years pumping. Numbers (1, 2, 3, etc.) indicate the scenario. Blue box on map shows area covered by contour plots; black outlineindicates the hydrogeological model boundary; dots indicate GWC well locations

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favourable outcome. In other locations it may be moreimportant to minimise ΔT.

Shielding and thermal breakthroughShielding was shown to be effective at reducing therate of temperature increase at the production wellsafter thermal breakthrough. However, thermal break-through occurred earlier with shielding, such that thetemperature at the production wells was slightly higherwith shielding in the first 5 years of pumping, but waslower between 5 and 10 years. This subtle interplaybetween the timing of thermal breakthrough and therate of subsequent temperature increase reflects thebalance between two competing effects—injection ofcool water in the shielding wells pushes the thermalplume away from the production wells (the desiredeffect), but shielding requires a higher extraction ratefrom the production wells which tends to pull thethermal plume towards the production wells (undesir-able effect). The simulation results suggest that thesecond effect results in earlier thermal breakthrough,while the first effect causes the heat to spread over awider area, thus reducing the rate of temperatureincrease after thermal breakthrough.

This result suggests that shielding may be counterpro-ductive if the lifetime of the system is less thanapproximately 5 years, at least for the 3D hydrogeologicalsetting considered here. The timing of this crossover ineffectiveness of shielding is likely to be system specificand would be different in other hydrogeological settings.Simulations should therefore be used during the designstage of a GWC system to assess the usefulness ofshielding.

The beneficial effect of shielding should also beweighed against the increased capital and running costs

associated with implementing and running theshielding wells, and the increased drawdown associatedwith shielding (Fig. 14) due to the higher pumpingrate in the production wells. Shielding also resulted inhigher temperatures in some existing groundwaterwells in the surrounding area (Fig. 13), due todeflection of heat away from the GWC system. Again,this effect is highly site specific and would depend onthe location of existing wells around the GWC system.

Thermal impact on existing groundwater usersThe predicted temperature changes in existing ground-water wells in the Superficial Aquifer are similar inmagnitude to seasonal variations at shallow depths inthis aquifer. One well in the Mullaloo Aquifer ispredicted to experience a temperature increase of up to6 °C; however, while this is larger than the seasonaleffect at the screen depth of this well, it is unlikely tocause a problem with use of the water for irrigationpurposes.

Maintaining cooling capacity after thermalbreakthroughOnce thermal breakthrough has occurred, the coolingcapacity can be maintained by increasing either theinjection temperature or the flow rate. The simulationresults indicate that increasing the flow rate increasesthe rate of temperature increase at the production wellsand results in considerably greater drawdown, whereasincreasing the injection temperature does not havethese negative effects. Therefore increasing the injec-tion temperature is likely to be the optimum methodfor maintaining cooling capacity after thermal break-through; however, both approaches take the system

Table 7 Summary of key results (10 years pumping)

Scenario Time tothermalbreakthrough(years)

Increase in Tat productionwells (°C)

Maximum Tin superficialaquifer (°C)

Maximumdrawdown inSuperficial Aquifer(m)

Maximumdrawdown inMullaloo Aquifer(m)

Number ofwells with Tincrease≥0.1 °C

Maximum Tincrease at awell (°C)

Original scenarios: parameter values as in Table 11 4.3 0.5 22 0.5 2.6 4 0.72 – 0.0 23 0.2 1.0 1 0.13 – 0.0 23 <0.1 0.5 1 0.14 3.5 2.5 29 0.5 2.6 7 4.45 3.0 1.7 29 0.7 3.7 8 6.04a 3.5 3.1 29 0.7 4.7 8 5.84b 3.5 2.6 30 0.5 2.6 7 4.5No Mullaloo Aquitard (same properties as Mullaloo Aquifer)1 5.7* 0.4 22 0.6* 2.4 5* 1.0*3 – 0.0 24* 0.1* 0.5 1 0.2*4 4.3* 2.3 29 0.6* 2.5 7 4.05 3.9* 1.5 29 0.9* 3.5 9* 5.8High hydraulic conductivity in part of Mullaloo Aquifer1 3.9 0.5 21 0.2 0.5 5* 0.53 – 0.0 22 <0.1 0.1 1 0.14 3.7* 2.4 24 0.2 0.6 9* 1.85 2.7 1.7 24 0.3 0.7 10* 5.5

Ttemperature. Values with an asterisk (*) or italics indicate an increase or decrease, respectively, relative to corresponding original scenarios

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outside the bounds specified in the GLOS for thePawsey Centre and would therefore require approval

from the DoW before they could be implemented. Thisissue should be considered when determining the

Fig. 12 Growth, migration and decay of thermal plume during and after pumping in the Mullaloo Aquifer (−70 mAHD) and theSuperficial Aquifer (0 mAHD), scenario 5. Numbers (2, 5, 10, etc.) indicate time (years) from start of pumping. White dots indicate GWCwell locations. Temperature in °C. Blue box on map shows area covered by contour plots; black outline indicates hydrogeological modelboundary

Fig. 13 Licensed groundwater extraction wells experiencing a temperature increase≥0.1 °C in one or more scenarios. Temperatureincrease (°C) in wells after: a 5 years pumping plus 100 years post-pumping; b 10 years pumping plus 100 years post-pumping. Blank cellsindicate temperature increase was<0.1 °C. All wells are screened in the Superficial Aquifer, except AG2 which is screened in the MullalooAquifer. c Location of groundwater extraction wells (green dots) and GWC wells (red dots). Grey shading indicates extent of model. Areashown in c is the same as in Fig. 12

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operating restrictions for a GWC system, or whenconsidering the viability of a GWC system under agiven set of restrictions on flow rate and injectiontemperature.

Robustness to parameter variationsWhile the exact details of the simulation results (e.g.,time to thermal breakthrough) are sensitive to changesin hydraulic and thermal properties, the generalconclusions of this study (e.g., the effectiveness ofshielding for slowing temperature increase at theproduction wells) are unaffected by these changes inparameter values. Therefore, it is concluded that theresults reported here are robust to reasonable changesin hydraulic and thermal properties in the MullalooAquifer and Mullaloo Aquitard. Changes in propertiesof the other units would have less influence on theresults and were not investigated.

Conclusions

A GWC system has been implemented for the PawseySupercomputing Centre in Kensington, Western

Australia. This study used hydrogeological simulationsto assess the behaviour of the Pawsey Centre GWCsystem under varying conditions with a simulatedcooling capacity of 0.5 or 2.5 MWth. The simulationresults show that:

1. For a given cooling capacity, maximising ΔT andminimising flow rate results in the most favourableoutcome in terms of delaying thermal breakthrough,minimising drawdown and minimising the size of thethermal plume.

2. Shielding results in slightly earlier thermal break-through but a reduced rate of temperature increase atthe production wells after breakthrough, such thatshielding is most likely to be beneficial after the first5 years pumping.

3. Increasing the injection temperature is preferable toincreasing flow rate for maintaining cooling capacityafter thermal breakthrough.

4. The thermal impact on existing wells is relatively smalland unlikely to cause a problem with the use of thesewells for irrigation.

These findings are robust to reasonable changes inhydraulic and thermal parameters of the MullalooAquifer and Mullaloo Aquitard. This study has shownthat GWC is a sustainable and effective coolingsolution for the Pawsey Centre, and demonstrates thepotential for application of GWC in similar urbanenvironments, especially in hot, dry climates wherewater supplies are limited and cooling towers may beineffective for at least part of the year.

Acknowledgements This research was undertaken by the CSIRO(Australia) as technical support to capital infrastructure activitiesfunded by a Commonwealth Government Grant under the Educa-tion Investment Fund awarded to CSIRO in 2010.

Appendix

To assess mesh sensitivity, scenario 4 was repeated on arefined mesh comprising 5,344,008 elements with a meshresolution of 0.75 m around the GWC wells. A compar-ison of results obtained using the original and refinedmeshes is presented in Table 8. The results are verysimilar; hence, it was concluded that the original meshresolution is sufficient to resolve the behaviour of theGWC system.

Fig. 14 Maximum drawdown in Mullaloo and Superficial aquifersduring 10 years pumping

Fig. 15 Temperature at production wells in scenarios thatexperienced thermal breakthrough

Table 8 Mesh sensitivity—comparison of results on normal andrefined mesh, 10 years pumping, scenario 4

Criterion Difference

Maximum drawdown in Superficial Aquifer 0.001 mMaximum drawdown in Mullaloo Aquifer 0.008 mTemperature at an existing groundwater well 0.2 °CTemperature at production wells 0.03 °C

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