ties

b TV Rheinland, LGA Bautechnik GmbH, Tillystr. 2, 90431 Nuremberg, Germanyc Three Gorges Research Center for Geo-Hazard, Faculty of Engineering, China University of Geosciences, 430074 Wuhan, China

heat exof thed moderansferto the

nalysis of the heat transfer of a BHE with different geological layers is impor-HE.

liness and low maintenance GSHP systems have attracted moreand more attention [1]. The major costs of vertical GSHP systemsdepend on the length, diameter and numbers of borehole heatexchangers (BHEs). Analysis on heat transfer in subsurface isimportant to size the BHE that optimum performance is achievedwith minimum costs [2,3].

d is needed. Theivity and sf BHE [4]oratory me

ments or eld tests. In laboratory, thermal conductivity is gemeasured using the specimens gathered from the boreholDrilling cores are often collected during the drilling worused for the preparation of specimens. More often, thermal proper-ties are estimated by in situ Thermal Response Test (TRT) [5]. For aTRT, a constant amount of energy is injected into or extracted fromthe ground [6]. The temperature response of the heat carrier uidis recorded during the testing process. Interpretation of TRTrecordings follows commonly Kelvins line source theory [7]. Thereare two major parameters that describe heat transfer of BHE:

Corresponding author. Tel.: +49 91318522685; fax: +49 91318522688.E-mail address: luojin-84@hotmail.com (J. Luo).

Applied Energy 123 (2014) 5565

Contents lists availab

Applied

lseIn recent years, ground source heat pump (GSHP) systems havebeen widely applied in residential and commercial buildings. Dueto the superiority of high energy-efciency, environmental friend-

knowledge of thermal properties of the grounground thermal properties such as heat conductheat capacity play a vital role in heat transfer otwo parameters are commonly assessed via labhttp://dx.doi.org/10.1016/j.apenergy.2014.02.0440306-2619/ 2014 Elsevier Ltd. All rights reserved.pecic. Theseasure-nerallye eld.ks andAvailable online 12 March 2014

Keywords:Ground source heat pump (GSHP) systemBorehole heat exchanger (BHE)Ground propertiesLayered subsurfaceNumerical model

In this paper, we examine thermal performance of BHEs settled in a ground with ve bedded sedimen-tary layers. Thermal and hydraulic properties of the borehole eld are rstly investigated. Based on theexperimental data, a numerical model is developed to examine thermal exchange of the BHEs in this lay-ered subsurface. Two modeling approaches, where the ground properties considered to be homogenousor stratied, are implemented. Numerical results from both approaches are validated and they give sim-ilar thermal output of the BHEs. The model with a stratied subsurface further indicates that there is only74.1% amount of heat transferred in the basal layer with negligible groundwater ow as compared to thatof aquifer layers.

2014 Elsevier Ltd. All rights reserved.

1. Introduction In order to estimate thermal performance of BHE accuratelyReceived in revised form 7 January 2014Accepted 11 February 2014

different strata. Thereby, atant for optimal sizing of Bh i g h l i g h t s

We examine performance of borehole Both thermal and hydraulic properties Both homogenous and stratied groun Stratied model indicates less heat is t Length of the BHEs can be reduced due

a r t i c l e i n f o

Article history:Received 30 August 2013changer with a layered subsurface.borehole eld are investigated.ls give similar thermal output of BHE.red in aquiclude than aquifers.lower heat performance of basal layer.

a b s t r a c t

In the current design of borehole heat exchangers (BHEs), the ground is commonly considered to behomogenous. However, in a layered subsurface, thermal performance of BHE can deviate drastically withaGeoZentrum Nordbayern, Friedrich-Alexander-University-Erlangen-Nuremberg, Bayern, GermanyAnalysis on performance of borehole heasubsurface

Jin Luo a,, Joachim Rohn a, Manfred Bayer b, Anna Pr

journal homepage: www.eexchanger in a layered

s a, Wei Xiang c

le at ScienceDirect

Energy

vier .com/locate /apenergy

nergeffective thermal conductivity of the ground, keff, and effectivethermal resistance inside the borehole, Rb [8]. Furthermore, theheat capacity of the grouting material also affects short term heattransfer of BHE.

Based on the ground investigations heat transfer of BHE in sub-surface can then be analyzed [9,10]. Performance of BHE can be af-fected by many factors such as conguration of BHE and thermalproperties of the ground [1115]. Acua and Palm presented astudy concerning distributed TRTs on pipe-in-pipe BHEs [16]. Re-sults obtained from TRTs indicated that the effective boreholeresistance of pipe-in-pipe BHEs is almost negligible as comparedto U-pipe BHEs. However, the linear source theory assumes a con-stant heat injection rate, which does not reproduce the effect ofwater ow along the pipe of BHEs. In order to overcome such ashortcoming, Raymond and Lamarche [17] performed a numericalstudy to examine the temporal variation rate under a constant heatinjection at the surface. Results indicated that the analysis of syn-thetic data generated with numerical simulations can be improvedby accounting for variable heat injection rates determined insideBHE. Estimation of both ground thermal conductivity and boreholeresistance was within 20% difference of the expected values, exceptwhen the thermal conductivity was as low as 1.0 W/(m K). Micho-poulos et al. [18] demonstrated an experimental setup of a GSHP

Nomenclature

T temperature (C)r borehole radius (m)Q amount of energy (J)q thermal transfer rate (W)H length (m)W water pumping rate (m3/h)c specic heat capacity (J/kg/K)i hydraulic gradient ()h water head (m)KD hydraulic transmissivity (m2/day)t time (s)

Greek symbolsq density (g/m3)

56 J. Luo et al. / Applied Esystem in Northern Greece. Thermal load of the BHEs was esti-mated by measuring uid inlet and outlet temperature of theU-pipes. Three years results showed that the thermal load of theBHE is strongly affected by ambient temperature.

Hydraulic properties of the ground are also reported can affectheat transfer of BHE [19]. Hydraulic conductivity is one of the mostimportant parameters to evaluate properties of the ground. In aeld investigation, pumping test is often implemented to deter-mine the hydraulic conductivity [20]. Due to the anisotropy of geo-logical material, laboratory tests are also necessary. Permeabilitytest is a typical method to determine hydraulic conductivity inthe laboratory by application of Darcys law [21]. The inuence ofgroundwater ow on thermal performance of BHE can be exam-ined based on the investigated hydraulic properties. Wang et al.[22] implemented an experimental measurement to study thetemperature prole of BHE in subsurface. The measuring resultsshowed that temperature of BHE was drastically affected by stronggroundwater ow. Due to the strong ground-water advection thethermal performance of the BHE is enhanced. Therefore, it isimportant to investigate hydraulic ground conditions for the anal-ysis of heat transfer of BHE. Capozza et al. [23] investigated theinuence of aquifers on the ground temperature in ground-sourceheat pump operation. The groundwater ow was assessed to havepositive effects upon performance of heat pump plant. Further-more, the total length of BHEs is evaluated to be reduced when ta-ken into account the effects of groundwater ow.

The former studies [1618] either evaluated thermal perfor-mance of BHE by considering of homogenous ground, or someother studies [22,23] examined groundwater effects for the nalthermal output of BHE. However, in a layered ground, the ground-water ow is often limited to aquifers. Thermal properties arecommonly changes with the strata and the ground can then notconsider being homogenous. Thermal performance can deviatedrastically with the strata which have different ground propertiessuch as thickness of the layers, thermal property and hydraulicproperty. Therefore, a further study aiming to quantitatively esti-mate the heat transfer of BHE in different strata is needed.

In this paper, we analyze thermal performance of three groupsof BHEs settled in a layered subsurface. The investigating works areimplemented on the borehole eld where the BHEs are installed.We rstly examine thermal properties as well as hydraulic proper-ties for the different geological layers both in laboratory and in theborehole eld. Then, a numerical model is developed using FE-FLOW to examine heat transfer in subsurface of the BHEs. Thenumerical results will be compared with the in situ measured uidtemperature of BHEs. Furthermore, salting tracer is used in GWM-1

k thermal conductivity (W/(m K))a dispersivity

Subscriptsin inlet owout outlet owf uids ground surroundingeff effectiveb boreholew watero initial conditionp pipe

y 123 (2014) 5565to investigate the behavior of groundwater. The ground tempera-ture distribution is also measured in the two investigation wellsto analyze the implication of heat transfer along the total lengthof the BHEs. Finally, the heat transfer of the BHEs for ve differentstrata will be quantitatively examined. The following works of thispaper are organized as follows: Section 2 describes methodology ofthis study, containing the investigation of the ground conditionsand numerical modeling of heat transfer of the BHEs in a layeredsubsurface; then, the measuring results and numerical ndingsare analyzed and discussed in Section 3; nally, conclusions of thispaper are included in Section 4.

2. Methodology

2.1. Experimental measurements

This work is conducted on a GSHP system installed in an ofcebuilding in Nuremberg, Germany. Fig. 1 shows a schematic dia-gram of the borehole eld. There are 18 boreholes installed inthe GSHP system that can be classied into 3 groups according tothe borehole diameter: 121 mm, 165 mm and 180 mm. In orderto investigate properties of the layered subsurface, two additionalwells were drilled at the borehole eld. One well, in the upstream

of the BHE-eld, is named as groundwater measuring points one(GWM-1). The other, in the downstream of the BHE-eld, is

referred as groundwater measuring point two (GWM-2). All thoseBHEs and the two investigation wells (GWM-1 and GWM-2) weredrilled with the same depth of 80 m.

Fig. 2 shows the geological prole of the borehole eld within80 m depth. It is observed that the ground consists of ve geolog-ical layers: (1) Quaternary, 04 m, is a river deposit consisting ofmiddle sand, ne sand, clay and gravels; (2) Blasensanstein layer,425 m, is a sandstone layer with a good hydraulic conductivity.This layer is considered to be an aquifer. (3) Lehrbergschichtenlayer, 2555 m, is bedded claystone with sandstone. This layerhas a low vertical hydraulic conductivity but a high horizontal con-ductivity. Therefore, this layer is horizontally considered to be anaquifer in this study; (4) Schilfsandstein layer, 5562 m, is a sand-stone layer with a good hydraulic conductivity. This layer is also anaquifer layer; (5) Estherienschichten layer, 6280 m, is a claystonelayer. This layer has a very low hydraulic conductivity and it is con-sidered to be an aquiclude.

2.1.1. Thermal propertiesThermal properties such as thermal conductivity and specic

thermal capacity are measured by the samples collected from

Fig. 1. Position of the investigation wells (GWM-1 and GWM-2) and ThermalResponse tests (T1, T2 and T3) in the borehole eld.

J. Luo et al. / Applied Energy 123 (2014) 5565 57Fig. 2. Prole of the layered subsurface. Within a depth of 80 m, the ground consists of represents their positions and third one is the descriptions.ve different geological layers. The rst row is the name of the layers, the second one

Note that the uid temperature development is mainly con-trolled by the borehole lling rather than by the surrounding geo-logical layers in the rst few hours. Eq. (4) is only valid after therecommended minimum testing time as follows:

t >5r2

a5

The borehole thermal resistance, Rb, is estimated by the line-source model [2627] by the expression:

Fig. 3. Recordings of the three performed TRTs. T1 represents the TRT for BHEs with121 mm diameter, T2 denotes TRT for BHEs with 165 mm diameter and T3 for BHEwith 180 mm diameter.

nergGWM-1. The thermal properties are continuously investigated foreach geological layer. In this work, the instrument ISOMET 2104(Applied Precision Ltd., Stavitelska, Slovakia) is selected for themeasurements. Surface probe is used to measure heat propertiesof the collected hard rock samples. A linear shaped metal is embed-ded in the surface probe to produce heating power. Plexiglas with avery low thermal conductivity of 0.16 W/(m K) is used in theopposite of the sample to force the heat produced by the heatingelement almost completely to the half-space of the samples. Byfollowing linear source theory [24], thermal conductivity of themeasured samples can be formulated as:

k 2 q4p

lnt2 lnt1TQ t2 TQ t1 1

where k is the thermal conductivity (W/(m K)), q is the heating rate(W), T is the temperature (C), Q is the amount of energy (J), t is thetime (s), the subscripts 2 and 1 represent the measurements in twodifferent times. To ensure the accuracy of the measuring output, themeasurements for each sample are repeated 4 times in this work.With known the temperature varying with the time thermal diffu-sivity is also estimated. Thermal diffusivity is the ratio of the timederivative of temperature to its curvature, as shown in Eq. (2). Then,volumetric heat capacity is derived using thermal conductivity todivide thermal diffusivity.

@T@t

ar2T 2

where r is the rst derivative, a is thermal diffusivity (m2/s).Effective thermal conductivity of the ground over the length of

the BHEs is also estimated by TRTs. As it is depicted in Fig. 1, threeTRTs were performed in the BHE with three different boreholediameters. The rst TRT was implemented in the BHE with a121 mm diameter with a testing period of 7 days (between May16, 2012 and May 23, 2012). The second one was implementedin the BHE with a diameter of 165 mm for 7 days (started fromMay 23, 2012 till May 30, 2012). Finally, the third TRT was per-formed in the BHE with a diameter of 180 mmwith the testing per-iod of 5 days (between May 30, 2012 and June 5, 2012).

Fig. 3 shows the recordings of the performed three TRTs for thethree groups of BHEs. It is observed that uid inlet and outlet tem-perature keeps increasing with the testing time. Heat input re-mains almost constant during the whole testing period,excepting for a few minutes at the beginning of the test. However,the ambient temperature was observed to change abruptly. Itranges from 12 C to 25 C during the testing period, implying thatthe abrupt change in ambient temperature can affect seriously thetesting results.

The TRTs are interpreted by following the linear source theorywhich was proposed by Hellstrm [25]. In the linear source theory,the mean uid temperature (Tf = (Tin + Tout)/2) can be formulatedby following equation:

T f qH1

4pkeffln

4atr2b

c

Rb

Ts 3

where Tf is the uid temperature (C), Ts is the undisturbed groundtemperature (C), Q is the power input (W), H is the length of theborehole (m), rb is the borehole radius (m), t is the time (s), a isthe thermal diffusivity (m2/s) and c is the Euler constant (0.5772),Rb is the thermal resistance of the borehole ((m K)/W).

When linear regression is presented between mean uid tem-perature versus logarithmic time, Eq. (3) can then be simplied as:

T f m lnt n 4

58 J. Luo et al. / Applied Ewhere m is the linear regression of mean uid temperature versuslogarithmic time.y 123 (2014) 5565Rb 14pkeffT f Ts

m ln 4at

cr2b

6

2.1.2. Hydraulic propertiesIn the present work the hydraulic conductivity of the different

geological layers is investigated by pumping tests. GWM-1 is se-lected as the pumping well and GWM-2 is used for monitoringthe drawdown. The water was pumped with a constant ow ratefrom GWM-1 and the drawdown for both GWM-1 and GWM-2are recorded. Three pumping tests are implemented separatelyfor the three potential aquifers (Blasensandstein, Lehrbergschich-ten and Schilfsandstein), as shown in Fig. 4. Packers are used toblock the groundwater leaking between the adjacent aquifers, asshown in Fig. 4.

In cases when only one observation well at a distance, r, fromthe pumping well is available, Thiems method can be used forthe interpretation of the horizontal pumping tests. With the mea-suring data, the well discharge can be formulated as:

W 2pKDsw s1lnr1=rw 7

whereW is the pumping rate (m3/s), K is the hydraulic conductivity

J. Luo et al. / Applied Energof the aquifer (m/day), D is the thickness of the aquifer (m), sw ands1 are the steady-state elevations of the water levels in the piezom-eters (m), r1 is the distance between pumping well and observationwell (m), rw is the radius of the pumping well (m) [28]. In this work,the experimental results of three potential aquifers are separatelyrecorded. Thiems method is used for estimating hydraulic conduc-tivity for each investigated layer.

2.2. Groundwater measurements

In order to analyze heat transfer over the length of the BHEs(80 m), the ground temperature is measured in the aforemen-tioned two investigation wells (GWM-1 and GWM-2). We select3 days (July 18, 2012, September 12, 2012 and March 27, 2013)to examine the temperature difference of those two wells. Themeasurements are implemented using a temperature sensor (TLCMeter, Solinst Ltd., Ontario, Canada) with a measuring accuracyof 2.0%.

Furthermore, salting tracer is input in GWM-1 to investigate theowing behavior of the groundwater. 100 kg salt is dissolved inGWM-1 which is located upstream of the BHEs. The electric con-ductivity is measured in this well for two times: one was on July31, 2013 and the other was on August 08, 2013. The measuring re-sults are plotted along the depth of this well to show the saltingFig. 4. Schematic diagram of the implemented three pumping tests, the hydraulicconductivity has been investigated for three geological layers. GWM-1 is used as apumping well and GWM-2 is utilized as an observation well.concentration. Then the behavior of the groundwater ow is ana-lyzed based on the measurements. In this work, the conductivitysensor (TLC Meter, Solinst Ltd., Ontario, Canada) with a measuringrange from 0 to 80,000 ls/cm and a measuring accuracy of 5%is selected for measuring the electric conductivity of thegroundwater.

2.3. Numerical modeling

To evaluate heat transfer of the BHEs in the layered subsurface,a numerical model is developed using FEFLOW. In FEFLOW, theBHE can be modeled by fully discretized meshes. This methodhas been practically proved to be a suitable way for simulatingheat transfer of BHE [29]. However, this method takes largeramount of time in parameters setting and matrix operating dueto the needed numerous meshes. In order to overcome such short-comings, a 1D BHE model has been developed in the release of theFEFLOW version 6.0X [30]. The BHE is simulated as vertical 1D dis-crete element in the nite-element matrix system which is similarto fracture elements. In the 1D element, the uid temperature ofthe U-pipe is expressed as a superposition of temperature responseby the heat uxes per unit length. This 1D discrete element hasbeen proved to t the discretized mesh very well in modeling ofheat transfer of BHE, especially in long-term performance predic-tion [31]. Compared to the fully discretized method this approachhas an obvious advantage in parameters setting and in saving ofcomputing time due to fewer meshes are needed. However, thisapproach can not reect the real conguration of in situ installa-tions because the gap between heat carrier uid and borehole wallis represented by thermal resistances.

Within the framework of the present study, 1D BHE is selectedto model the heat transfer of the BHEs. The entire model has a sizeof 200 200 90 m (length width depth, Fig. 5), which is ver-tically discretized in accordance with geological congurations.The numerical model is set to match the BHEs on site, with a dis-tance of 6 m between the adjacent boreholes. In order to obtainmore accurate results the meshes in the neighborhood of the BHEsare rened.

Groundwater ow is simulated by a rst-type boundary condi-tion (1st BC-Dirichlet) that assigns a known water head [8,32]. Thegroundwater tables are measured in both the monitoring wells(GWM-1 and GWM-2). These two wells are drilled along thegroundwater ow direction, which is based on many observationwells distributed in this area [33]. The hydraulic gradient (i) is thencalculated by use of the difference of the water heads (h1 h2) di-vided by their distance (s). It can be formulated as: I = (h1 h2)/s.In the simulation we used the so called local coordinate system inFEFLOW. In this coordinate system we set the ground surface as0 m. Then, the measured value can be extrapolated using thesetwo measured values of the water head (h1, h2) and the calculatedhydraulic gradient (i) in the boundaries, as shown in Table 1. Thetemperature of the groundwater, which enters the models domainis treated also as a rst-type BC. In FEFLOW, a single BHE or a seriesof BHEs can be set at nodes on the top slice of the 3D to simulateheat transport by the 1D BHE model. Settings of borehole resis-tance of BHE can be done by computational results or bypre-setting of the measured value from TRT tests [30]. Comparedto the fully discretized model, the 1D BHE model has been provedto t the uid outlet temperature very well after dozens of min-utes simulation [31]. Therefore, in order to obtain valid outputfrom the 1D BHE model the time interval between two differentoperating modes (heating and cooling) should be larger enough.In the present work, borehole thermal resistances measured by

y 123 (2014) 5565 59TRTs are used: 121 mm diameter of 0.093 (m K)/W, 165 mm diam-eter of 0.105 (m K)/W and 180 mm diameter of 0.110 (m K)/W, aslisted in Table 3.

nerg60 J. Luo et al. / Applied EInput properties for the homogenous groundmodel are summa-rized in Table 1. Thermal properties of pipe, grouting material aswell as heat carrier uid are obtained from the product specica-tions. Parameters such as hydraulic conductivity and initial groundtemperature are obtained from the eld tests. The initial groundtemperature is measured by the pumping test and therefore a

Fig. 5. Left: 3D overview of the numerical model and its discretization. Right: 2D top viewis vertically discretized in accordance with the geological layers. The mesh is rened ar

Table 1Parameters input for the numerical model with humongous ground.

Parameter Component

Hydraulic gradient Aquifer

Heat conductivity WaterPipeBack llsHeat carrier uidGround

Volumetric heat capacity WaterPipeHeat carrier uidBackllsGround

Initial/upstream boundary temperature Ground/1st heat BC

Porosity Ground

Fluid inlet temperature (winter) Heat (1st BC)

Fluid inlet temperature (summer) Heat (1st BC)

Table 2Parameters input for the numerical model with ve bedded layers. The parameters are prthrough the material.

Layer Depth (m) Heat conductivity (W/(m K)) He

Quaternary 04 1.6 0.8Blasensandstein 425 2.72 2.1Lehrbergschichten 2555 2.77 2.1Schilfsandstein 5562 2.22 2.2Estherienschichten 6280 2.24 2.2y 123 (2014) 5565homogenous value is obtained. Due to the Quaternary layer is lo-cated above the groundwater table, there has no groundwater owin this layer. The hydraulic conductivity of this layer is obtainedfrom the local environmental report, in which the value of5.0 104m/s is used for sand deposits [33]. Thermal propertiesof the ground are determined by both the TRTs and laboratory

of model domain and the setting of boundary conditions (BCs). The model domainound the boundary of the geological layers.

Symbol Value Unit

i 3.7 103 ()k 0.65 (W/(m K))

0.38 (W/(m K))2.35 (W/(m K))3.80 (W/(m K))2.54 (W/(m K))

qwcw 4.2 (MJ/(m3 K))qpcp 1.6 (MJ/(m3 K))qfcf 0.6 (MJ/(m3 K))qbcb 2.5 (MJ/(m3 K))qgcg 2.2 (MJ/(m3 K))

T0 11.9 (C)

n 0.25 ()

Tin 6.5 (C)

Tin 16 (C)

esented for each geological layer. The porosity is the effective porosity for water ow

at capacity (MJ/(m3 K)) Hydraulic conductivity (m/s) Porosity ()

2 5 104 [33] 0.352 1.74 105 0.39 1.27 105 0.207 9.8 106 0.33 6.74 109 0.01

this is because of two reasons: the rst one is the valid time andthe second one is the measuring errors of TRT2 (T2). As mentionedin Eq. (4), the valid time is related to the borehole radius. Withincreasing the drillhole diameter the valid time for estimatingthe output is also increased. The measuring error in T2 is causedby the power input. During the second day of this measurement(T2), the electricity is automatically stopped. This causes abruptchanges in uid inlet and outlet temperature, as shown in Fig. 3.In order to estimate the output correctly, we have removed thedata recorded during that time period away.

The estimated thermal conductivities for the three differentborehole diameters match nicely, indicating similar ground condi-tions, as listed in Table 3. The mean effective thermal conductivity,keff, over the length of the borehole is estimated to be 2.54 W/(m K), as shown in Table 4. By comparison, the arithmetic mean va-lue of the laboratory-measured thermal conductivities (2.50 W/

nergmeasurements. The numerical results based both on in situ inves-tigations and laboratory measurements will be analyzed andcompared.

Table 2 lists the input parameters for the layered ground model.The setting of the parameters of the layers was done according tothe investigated geological prole, as shown in Fig. 2. The layeredmodel has the Z axis discretization in accordance with geologicalsetting in the borehole eld. The meshes are vertically renedaround the boundaries of the geological layers, as shown inFig. 5. Therefore, the mesh is not evenly distributed in verticaldirection. The layered ground model has the same boundary condi-tions as the homogenous ground model, as shown in Table 1.

The numerical model is applied to simulate the BHEs, which in-ject energy of a known temperature into the subsurface. The uidinlet temperature set in the numerical model is measured by thedata logging system. To evaluate heat transfer of the BHEs, theresulting synthetic time series of the outlet temperature of the heatcarrier uid is recorded. Thermal transfer rate, q, of the simulatedBHE can be calculated based on the difference inlet temperature,Tin, and outlet temperature, Tout, the volume of the heat carrieruid, W, and the volumetric heat capacity of the heat carrier uid,qfcf. Conclusively, this can be formulated as:

q WqfcfT in ToutH

8

According to the system operation, 1800 h (75 days) for theyearly heating period and 850 h (35 days) for the yearly coolingperiod have been simulated.

3. Results and discussion

3.1. Ground properties

In this study, both thermal properties and hydraulic propertiesare investigated for ve different bedded layers in the boreholeeld. Thermal conductivity of the ground is investigated by bothlaboratory measurements and eld tests. Hydraulic properties ofthe aquifers are examined by pumping tests. The obtained resultswill be analyzed in the following parts of this paper.

3.2. Thermal conductivity

Table 3The estimated effective thermal conductivity and borehole thermal resistance byTRTs.

Boreholediameter (mm)

Effective thermalconductivity (W/(m K))

Borehole thermalresistance ((m K)/W)

121 2.54 0.093165 2.53 0.105180 2.55 0.110

J. Luo et al. / Applied EIn laboratory, thermal properties of the collected samples aremeasured using the instrument ISOMET 2104 (Applied PrecisionLtd., Stavitelska, Slovakia) which has a measuring accuracy of10%. Due to the groundwater table is lying about 4 m below theground surface these samples are saturated before themeasurements.

Fig. 6 presents the measured thermal conductivities and ther-mal capacities of the drilling cores collected from GWM-1. It is ob-served that the measured thermal conductivities range from1.6 W/(m K) to 3.7 W/(m K), as shown in Fig. 6. The changes ofthe measured thermal conductivities are due to the geologicalstructure of the layers. In the borehole eld, the layers consistmainly of Triassic river deposits which are inter-bedded sandstoneand claystone, as shown in Fig. 2. Samples containing more clayhave a lower thermal conductivity, while samples containing moresand have a comparatively higher thermal conductivity. On aver-age, the measured thermal conductivity has a mean value of2.5 W/(m K) and the mean value of thermal capacity is estimatedto be 2.2 MJ/(m3 K).

Effective thermal conductivity of the ground is also examinedby three TRTs. Recordings of the TRTs are interpreted by followingthe linear theory proposed by Hellstrm [28], as shown in Fig. 7.Note that the three TRTs have different evaluation intervals and

Fig. 6. Thermal properties measured using instrument ISOMET 2104. Thermalconductivity and volumetric thermal capacity are measured using the core samplescollected from the well GWM-1.

y 123 (2014) 5565 61(m K)) ts with the TRTs results very well. This nding indicatesthat the TRT test is a suitable way to determine effective thermalconductivity of the ground.

3.2.1. Hydraulic propertiesThe geological prole indicates that there are three potential

aquifers in the subsurface, as shown in Fig. 8. The hydraulic prop-erties for these three layers are determined by pumping tests. Fur-thermore, the basal layer with pure claystone at the bottom of theprole is measured by permeability test.

Hydraulic properties of the three potential aquifers measuredby pumping tests are listed in Fig. 8. The interpretation of the accu-mulated water-head response is followed by Thiems method [28].

nerg62 J. Luo et al. / Applied EThe upper aquifer consists of sandstone and shows the biggest va-lue of hydraulic conductivity (1.74 105 m/s). Then, the hydrau-lic conductivity of the middle aquifer with inter-bedded sandstoneand claystone is measured to be 1.27 105 m/s. The lower aquiferconsisting of sandstone is measured to be 9.8 106 m/s. These re-sults indicate good hydraulic conductivity of the investigated threeaquifers. Furthermore, results obtained from permeability tests

Fig. 7. Results obtained from the performed three Thermal Response Tests. Theinterpretation of the measurements is conducted by following the linear sourcetheory. T1 is corresponding to the BHE with a 121 mm diameter, T2 is for 165 mmdiameter and T3 is for 180 mm diameter.show that the sample collected from the basal claystone layerhas a hydraulic conductivity of 6.74 109 m/s. This layer is there-fore considered to be an aquiclude.

3.3. Groundwater properties

3.3.1. Electric conductivityIn order to examine the groundwater ow a salting tracer was

used in GWM-1 and the electric conductivity of the groundwaterwas measured along the depth. The measurements were imple-mented on two different days: one was measured immediatelyafter the tracer input in the groundwater; the other was carriedout 8 days after the tracer input. The measured electric conductiv-ity of the groundwater on July 31, 2013 shows that the salt in thegroundwater is not evenly distributed over the length, as depictedin Fig. 9. However, 8 days after the salt input, the electric conduc-tivity was measured to remain at a constant value of 870 ls/cmwithin the depth between 4 and 62 m. This nding suggests thatthere exists groundwater ow within this depth. The velocity ofthe groundwater is nally estimated to be 25 cm/day by the tracertest.

Below the depth of 62 m, the electric conductivity keeps con-stant at 30,000 ls/cm. This value is close to the rst measure-ments, implying that there is a weak or negligible groundwaterow in the basal claystone layer. This can be attributed to thelow hydraulic conductivity of the claystone layer. As it is veriedin Fig. 8 the hydraulic conductivity of the claystone layer was mea-sured to be 6.74 109 m/s,

3.3.2. Ground temperatureThis GSHP system was started to operate at the beginning of

2009. After three years operation of the system, the response ofthe ground temperature is examined. The groundwater tempera-ture is investigated to analyze the implication of energy exchangein the layered subsurface. Fig. 10 shows the temperature distribu-tion over the length of 80 m in GWM-1 and GWM-2. The measure-ments have been implemented on three different months whichrepresent different operational conditions, including heating mode(July, 2012), cooling mode (March, 2013) and inter-seasonal mode(September, 2012). The temperature prole can be classied intothree parts according to the vertical temperature distribution:upper part, middle part and lower part. The upper part, between0 and 10 m, was inuenced by the ambient temperature and therehas been no obvious temperature difference between the twoinvestigation wells (GWM-1 and GWM-2). The middle part, be-tween 10 m and 62 m, consists of three layers with obviousgroundwater ow. In this part, the ground temperature in GWM-1 is around 1 C higher than that of GWM-2 during the typical cool-ing and inter-seasonal period. However, this difference becomesnegligible during the typical winter day. This can be well explainedby location of two wells and the amount of energy is extractedfrom the subsurface. GWM-1 is located upstream of the BHEsand GWM-2 is located downstream of the BHEs, as aforementionedin Section 2.1. When the heating load of the building is larger thanthe cooling load more energy is extracted for heating from theground in winter than that is injected in summer. The amount ofenergy for yearly heating is monitored to be 47.03 MW h and foryearly cooling is 25.03 MW h. Therefore, the ground temperaturedownstream of the BHEs is observed to decrease in long-term per-iod. The lower part, between 62 m and 80 m, consists of claystonelayers which have a negligible groundwater ow. In this part, thereis a smaller ground temperature difference in the two investigationwells as compared to the middle part. This is because the ground

y 123 (2014) 5565temperature drops sharply around the BHE (GWM-1) due to largeamounts of energy are extracted from the ground. These resultsshow that the temperature distribution of the ground within the

three aquifers is more affected by ground water ow as comparedto the lower claystone layer with negligible groundwater ow.

3.4. Thermal exchange over the length of the BHEs

The numerical simulation is implemented for two cases: onemodeling of homogenous ground, based on the effective thermalconductivity obtained from TRTs; the other modeling of a layered

subsurface by the parameters obtained from the measurementsin laboratory and the pumping tests. Negligible difference of uidoutlet temperature is found among the BHEs with three differentdiameters (121 mm, 165 mm and 180 mm). As aforementioned inSection 2.3, the 1D BHE model can only obviously affect the begin-ning dozens minutes of the operation. Afterwards, the performanceof the BHE is depended both by thermal resistance of BHE and theground. Due to the operational period for both modes (heating andcooling mode) last for several months and measured Rb values forthree groups BHEs are quite similar, the numerical output can notreect the difference in uid outlet temperature. As it is also veri-ed in the experimental measurements, the difference of the ther-mal performance among the BHEs with three different diameters isnegligible [34]. Therefore, in this study, the mean uid outlet tem-perature of all BHEs is used for the validation of the model. Thecomparison of the uid outlet temperature between those twocases shows a good agreement with each other, as depicted inFig. 11. Then, the numerical results are compared with the eld

Table 4Comparison of thermal conductivity obtained from lab measurements and TRTs.

Layer Depth(m)

Arithmetic labmeasured heatconductivity(W/(m K))

Estimatedheatconductivityby TRTs (W/(m K))

Difference(%)

Quaternary 04 2.50 2.54 1.6Blasensandstein 425Lehrbergschichten 2555Schilfsandstein 5562Estherienschichten 6280

Fig. 8. The geological layers and their measured hydraulic conductivity by pumpingtests. The hydraulic conductivity of the claystone layer is measured by permeabilitytest in laboratory. Three pumping tests have been performed separately in the threepotential layers.

J. Luo et al. / Applied Energy 123 (2014) 5565 63Fig. 9. Measured electric conductivity in GWM-1. The measurements wereconducted twice: one was conducted immediately after the salt was input in thewater and the other was 8 days after the tracer input.measurements for the validation of the model. The uid outlettemperature has been collected for two years under the operationof the GSHP system. By comparison, the modeled uid outlet tem-perature ts very well with themeasurements, as shown in Table 5.Due to the measuring uncertainties and data noise, there has noperfect tting can be obtained instead of one optimal parametercombination [8]. Therefore, it is desirable to evaluate the validparameters of uid outlet temperature. In present work root meansquared error (RMSE) is used to minimize the mist. Consideringthe impreciseness caused by ow meter temperature sensor anddata noise the acceptable error is estimated to be 2.0% of the mea-suring uid outlet temperature. The evaluated value of RMSE isthen set to be 0.2 C. Table 6 lists modeled, measured uid outlettemperature and estimated RMSE values. It is observed that bothhomogenous model and layered model have similar RMSE values,indicating that the modeling based on both cases give the similarenergy output. Both estimated RMSE values are within the accept-able tolerance, suggesting both models are reasonable.

Table 6 lists the thermal output obtained from experimentalmeasurements and numerical modeling. It is observed that themeasured heating output of the BHEs is 22.53 W/m which is simi-lar to the simulated thermal output of the homogenous groundmodel (22.52 W/m) and the layered ground model (22.61 W/m).The similar phenomenon is also observed in the cooling period,as shown in Table 6. Both numerical models (homogenous and lay-ered ground model) give the same output of the energy, indicatingthat the overall performance of BHE can be simply estimated usingFig. 10. Ground temperature distribution over length of 80 m in GWM-1 andGWM-2.

4. Conclusions

This paper examines heat transfer of three groups of BHEs set-tled in a layered subsurface. The BHEs are installed in a GSHP sys-tem of an ofce building in Nuremberg, Germany. Thermal

Table 6Comparison of the simulated thermal output with the measurements.

Methods Thermal output of BHE (W/m)

Heating Cooling

Measurement 22.53 17.99Modeling (Homogenous) 22.52 18.14Modeling (Layered) 22.61 18.12

Fig. 12. Thermal exchange over the total length of BHEs: Case (1) modeling on thethermal exchange of the BHEs in a homogenous ground; Case (2) modeling on thethermal exchange of the BHEs in an investigated layered subsurface. For the top10 m (from the top of the model), the results presented by two parts: 4 m and 6 m.Below 10 m depth, the results presented in each 5 m.

64 J. Luo et al. / Applied Energy 123 (2014) 5565the mean or effective value of ground parameters. However, heattransfer rate can change with different strata in a layered subsur-face and thereby the total length of the BHE might be reduced.Hence, it is necessary to further examine heat transfer for differentstrata in a layered subsurface.

Thermal exchange over the total length of the BHEs is examinedusing the layered ground model. Fig. 12 shows the energy ex-change of the BHE for both the homogenous ground and the lay-ered ground. Note that the rst 4 m in the top are the zone overthe groundwater table and this is an unsaturated zone. For thehomogenous ground model, the efciency of energy exchange is al-most evenly distributed over the length below the unsaturatedzone. It is also observed that slightly smaller heat transfer capabil-ities at the bottom of BHE. This effect is due to the uid tempera-ture decreases with depth, which result into 1.2% lower energyefciency at the bottom layer. On the other hand, in the modelwith a layered subsurface, the efciency of thermal exchange is ob-

Fig. 11. Comparison of measured mean uid temperature and the simulated uidtemperature. Two scenarios of the numerical simulation: Case (1) humongousground with using the effective thermal conductivity obtained by TRTs; Case (2)layered subsurface with using the laboratory measured thermal properties and theeld tested hydraulic properties.served to be obviously reduced in the basal claystone layer. Com-pared to the aquifers the basal claystone layer has only 74.1% ofthe efciency of the heat transfer of the BHEs. This nding indi-cates that heat transfer efciency of the BHEs is obviously higherwithin the aquifer layers as compared to the basal aquiclude. Theanalysis implies also that total length of the BHEs could be reduceddue to the lower heat transfer efciency in the basal claystonelayer.

Table 5Comparison of the modeled uid outlet temperature with the measured results. The evaluaan RMSE value smaller than 0.2 C are presented.

Date (day) Fluid outlet temperature (C)

Homogenous model Layered model

1 7.50 7.5610 7.41 7.4730 7.38 7.4450 7.32 7.3775 7.28 7.33

100 15.18 15.12110 15.21 15.16130 7.32 7.38150 7.28 7.34185 7.24 7.29200 15.08 15.03220 15.17 15.12properties as well as hydraulic properties of the ground with 5 dif-ferent geological layers have been investigated. Results obtainedfrom those investigations have been analyzed and discussed. Fur-thermore, the temperature distribution of the ground has beenmeasured over the total length of the BHEs. Based on the measure-ments, a numerical model is developed using FEFLOW. Then, thenumerical examination of thermal exchange of the BHEs has beenanalyzed. Main ndings of this study include:

ted time interval lasts for 220 days (2 years system operation) and one parameter with

RMSE (C)

Measurements DTHom-Mea DTLay-Mea

7.54 0.18 0.167.367.327.667.34

14.8015.007.367.327.26

14.8215.05

J. Luo et al. / Applied Energ Laboratory tests and eld TRTs are implemented to investigatethe thermal conductivity of the ground. Effective thermal con-ductivity, keff, over the total length of the borehole is estimatedto be 2.54 W/(m K). In comparison with laboratory measure-ments, the results of the TRTs t very well with the arithmeticmean value of the laboratory-measured thermal conductivities(2.5 W/(m K)). The slight difference between TRTs and labora-tory measurements is 1.6% which can be possible caused bythe measuring uncertainties. This nding indicates that theTRT test is a suitable way to determine effective thermal con-ductivity of the ground.

Hydraulic properties of the three potential aquifers in the lay-ered subsurface are investigated by three pumping tests. Thehydraulic conductivity of the basal claystone layer is measuredby permeability tests in laboratory. Based on the measuringresults, this layered subsurface is classied into differenthydraulic units: one upper unsaturated Quaternary layer, threeTriasssic layers are horizontally considered to be aquifers andone Triassic basal claystone layer is regarded as an aquiclude.

Salting tracer is used in GWM-1 and the electric conductivity ofthe groundwater is measured to analyze the behavior ofgroundwater ow. Results indicate that there exists groundwa-ter ow within the depth between 4 m and 62 m and there isnegligible groundwater ow in the basal claystone layer(6280 m). The groundwater temperature is also investigatedto study the implication of thermal exchange in the layered sub-surface. The ground temperature prole is sub-divided intothree parts: upper part, middle part and lower part. Comparedto the lower part which has a negligible groundwater ow,the ground temperature distribution in the middle parts withthe three horizontal aquifers is strongly inuenced by thegroundwater ow.

Two scenarios are modeled: one is homogenous ground mod-eled based on the effective thermal conductivity obtained fromTRTs; the other layered subsurface model which is imple-mented using the parameters obtained from both laboratorymeasurements and pumping tests. The comparison of the uidoutlet temperatures between those two cases gives a similarthermal output. Then, these numerical modes are validatedwith eld measurements. Furthermore, thermal exchange alongthe total length of the BHEs is examined using the validatedground models. Results obtained from the homogenous groundmodel suggest that BHE has an even performance over itslength. The model with layered subsurface further indicatesthat there is only 74.1% of the amount of heat transferred inthe basal layer with negligible groundwater ow as comparedto that of the aquifer layers. Therefore, total length of the BHEscould be reduced due to the lower heat transfer efciency in thebasal claystone layer.

Acknowledgements

This work is nanced by Bayerisches Staatsministerium fr Um-welt und Gesundheit. The rst author sincerely thanks to Bayeri-sche Forschungsstiftung for providing a scholarship. Furthermore,we would like to thank Ochs Company and especially Mr. Sittnerfor his help during the eld investigations. The fruitful commentsfrom two anonymous reviewers are also gratefully acknowledged.

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y 123 (2014) 5565 65

Analysis on performance of borehole heat exchanger in a layered subsurface1 Introduction2 Methodology2.1 Experimental measurements2.1.1 Thermal properties2.1.2 Hydraulic properties

2.2 Groundwater measurements2.3 Numerical modeling

3 Results and discussion3.1 Ground properties3.2 Thermal conductivity3.2.1 Hydraulic properties

3.3 Groundwater properties3.3.1 Electric conductivity3.3.2 Ground temperature

3.4 Thermal exchange over the length of the BHEs

4 ConclusionsAcknowledgementsReferences