Numerical Simulation of Freezing-Thawing-Cycles in the Grout of Borehole Heat Exchangers

Transcription

1 Proceedngs World Geothermal Congress 2015 Melbourne, Australa, Aprl 2015 Numercal Smulaton of Freezng-Thawng-Cycles n the Grout of Borehole Heat Exchangers Hauke Anbergen 1,2, Wolfram Rühaak 1, Jens Frank², Lutz Müller³ and Ingo Sass 1 1 Technsche Unverstät Darmstadt, Geothermal Scence and Technology, Schnttspahnstrasse 9, Darmstadt, Germany 2 Frank GeoConsult GmbH, Hofweg 31, Hamburg, Germany 3 Unversty of Appled Scence Ostwestfalen-Lppe, Geotechncs and Geothermcs, An der Wlhelmshöhe 44, Höxter, Germany anbergen@geo.tu-darmstadt.de Keywords: Freeze-Thaw-Cycles, Borehole Heat Exchanger, Grout, Phase Change ABSTRACT Borehole heat exchangers (BHE) are a popular and effcent way of regeneratve heatng. Protecton of groundwater has led to legal constrants, n many European countres whch requre BHEs to be grouted and boreholes to reman sealed under all operatng condtons. Borehole heat exchangers operated wth flud temperatures below zero degrees Celsus are requred to be grouted wth a materal that guarantees adequate sealng also under freeze-thaw-stresses. A testng devce was developed that measures the hydraulc conductvty of grout specmens after a freely selectable number of cyclc freeze-thaw-stresses. When smulatng radal earth-pressure and followng n-stu freezng drectons the results dffer from earler nvestgatons substantally. Consequently the frost-resstance of the grouts can be assessed. The temperature dstrbuton durng the testng procedure and n-stu are hghly nfluenced by phase change and latent heat effects. For numercal modelng of such processes the FEM flow and heat-transport smulaton software FEFLOW was used and extended by a phase change plug-n. The plug-n was valdated by numercal benchmarks and comparson wth an analytcal soluton as well as wth expermental data. 1. INTRODUCTION There s a dversty of shallow geothermal system applcatons. The most common applcaton s the ground source heat pump (GSHP) combned wth borehole heat exchangers (BHE). A workng flud streams through a closed loop system made of polyethylene (PE) ppes. For heatng cold flud leaves the heat pump through the return ppe, enters the BHE and collects heat from the surroundng ground. The ncrease n temperature of the workng flud s used by the heat pump mostly for domestc heatng and warm water producton. Most borehole heat exchangers are constructed vertcally (Fgure 1). Common depths are up to 100 m, but there are shallower and as well as deeper BHEs. Due to the depth - typcally between 50 m to 100 m - t s very lkely that the BHE s n contact wth aqufers and eventually penetrates aqucludes. As the workng flud s often a water-monoethyleneglycol-mxture there s a potental hazard f a leakage of the closed ppe system occurs (Mehnert, 2004). Ths s one of the reasons why the borehole of the BHE needs to be grouted, as the grout embeds the ppes and thus can be understood as a protecton layer. Fgure 1: Vertcal borehole heat exchanger executed wth a Double-U-Ppe (DGG and DGGT, 2014). Borehole heat exchangers n unconsoldated rocks requre grouted boreholes (see Fgure 1) because of borehole stablty ssues as well as thermal and hydraulc needs. In many countres groutng of BHEs s regulated by law. The permsson procedures and 1

2 requrements vary strongly. However the groutng of a BHE n specfc geologcal condton s very useful n terms of systems effcency and safety aganst contamnaton hazards. The grout seals the borehole and s a potental protecton layer for the event of ppe leakage. Furthermore t prevents unwanted vertcal water flow (Anbergen et al. 2014). Fgure 2 shows three potental events of damage due to an unwanted vertcal flud and mass transport caused by mproper sealng of the BHE. Fgure 2: Consequences of an mproper sealng. (A) Upstream due to artesan ground water condtons, (B) unwanted water connecton of two dfferent aqufers, and (C) vertcal contamnaton from the surface (DGG and DGGT, 2014). In artesan ground water condtons an up streamng flow path through the borehole mght lead to eroson effects and eventual settlements n consequence (A). The hydraulc connecton of two dfferent aqufers (B) mght affect the hydrochemcal behavor of the ground water. In several regons ground water provdes drnkng water supply and needs to be protected from any contamnatons or unwanted mxng. Furthermore a vertcal contamnaton from the surface along the borehole mght occur (C) n case of loss of sealng propertes of the groutng materal. These need to be prevented by the hydraulc ntegrty of the groutng materal. Ths s the man task for the groutng from a hydrogeologcal pont of vew. Beneath the hydraulc propertes the grout needs to secure a suffcent heat transport. The use of neat cement mxtures leads to hgh thermal resstances for the heat flow. Thus t s recommended to use thermally enhanced grouts. Furthermore the grouts need to be resstant aganst shrnkage or crackng as ths affects the bondng between grout and probes (Allan and Phlppacopoulos, 2000). A loss of bondng leads to reducton of conductve heat transport und thus reduces the effcency of the whole GSHP system. Ths paper focusses on the sealng propertes of groutng materals, as they are the most crtcal from a hydrogeologcal and approval pont of vew. To revew hydraulc propertes of the grout n-stu flow paths have to be taken nto account. In the event of ppng system leakage the hydraulc conductvty of the bulk grout materal has to be low n order to hnder the workng flud from enterng the surroundng aqufer. Ths s a foremost radal flow path. Most drectves prescrbe a bulk hydraulc conductvty of grouts of k 10-9 m s -1 (Mehnert, 2004, Skouby, 2010, Anbergen, 2014). Most groutng materals fulfll ths requrement. Dfferent to the bulk hydraulc conductvty vertcal hydraulc conductvty of the system (Fgure 2) s more crtcal. Ths axal flow path needs to be assessed n order to quantfy the vertcal sealng propertes. Allan and Phlppacopoulos (2000) stated that the system s hydraulc conductvty s substantally hgher than the bulk hydraulc conductvty. The system s hydraulc conductvty has to reman on a suffcent low level n order to prevent the descrbed events of damage. The low hydraulc conductvty has to be provded under every potental operaton status. The most crtcal operaton statuses for BHEs are the potental freezng and thawng processes of the grout durng tmes of extensve heat extracton. In order to cover heatng demands durng tmes of long heat extracton perods several BHEs run wth negatve workng flud temperatures. In ths case the workng flud must not be water, but an antfreeze flud (lke water - monoethyleneglycol-mxtures). These thermal stresses and ce buldng processes may affect the hydraulc ntegrty of a BHE. In order to assess ths most crtcal stress on the grout a testng devce was developed that s capable to measure the systems hydraulc conductvty before and after cyclc freeze-thaw-stresses (Fgure 3). Fgure 3: Freeze-Thaw-Testng-Devce (left) and specmen composed of a Polyethylene ppe and hollow cylndrcal grout body (rght) (Anbergen et al., 2014a) 2

3 The testng procedure provdes three-dmensonal mechancal boundary condton and follows the freezng drecton from the nsde to the outsde. Hence a realstc assessment of the hydraulc conductvty before and after cyclc freeze-thaw-stresses s possble. The testng procedure was descrbed n detal by Anbergen et al. (2014a). In order to observe the temperature dstrbuton durng the testng procedure the Fnte Element Smulaton Method (FEM) software FEFLOW was used. FEFLOW s a common software for heat and mass transport smulaton. Unfortunately FEFLOW does not take effects of phase change nto account. The change of the thermal propertes of water and the latent heat s not ncorporated accordng to the knowledge of the authors. Ths s why a plug-n for FEFLOW was wrtten that enables the FEM software to smulate freezethaw-cycles n porous meda. The relevant thermal background and a benchmark of the programmed plug-n wll be presented n ths paper. Furthermore the applcaton of the plug-n for the smulaton of the temperature dstrbuton n the testng devce wll be provded. 2. BACKGROUND 2.1 Grouted Borehole Heat Exchangers Groutng materals for BHEs mostly contan cement, water and thermal enhancements. Cement s necessary for the preventon of eroson and constant volume propertes. Furthermore cement bulds a rgd protecton layer for PE ppes. For the enhanced thermal propertes addtves such as slca sand are added because of ts thermal conductvty ncreasng effects. The enhanced grouts have thermal conductvtes around 2 W m -1 K -1 or even hgher. Unfortunately most of these mxtures are prone to cyclc freeze-thawstresses (Fgure 4), wet-dry-cycles, and other mechancal deformatons as they tend to crack. The cracks become hydraulc relevant and the sealng effect of the grout body decreases, eventually to an nacceptable level. Fgure 4: Oven dred grout specmen of a cement-sand-mxture (water/sold substance rato 0.82) frozen and thawed out radally n the developed testng devce (Anbergen et al., 2014a). The vertcal cracks ncreased the axal hydraulc conductvty. As seen n Fgure 4, the cracks are foremost vertcal. These are typcal frost nduced cracks, as structural dsruptons form perpendcular to the drecton of the frost front. In order to reduce the bleedng of the cementtous suspenson swellng clays are added to the mxtures (Allan and Phlppacopoulos, 2000). Besde the stablzaton of the suspenson, the bentonte partcles are capable to reduce the water flow through the flow tubes n the structure. Thus the system hydraulc conductvty s reduced substantally. Small amounts of bentonte are suffcent for a substantal reducton of hydraulc conductvty. When crackng occurs there s enough swellng potental that the bentonte reduces the loss of sealng to an acceptable level. These mxtures of a rgd cement matrx wth sealng bentonte contents are generally sutable for the groutng of BHEs. For the assessment of the hydraulc ntegrty of BHE grouts t s necessary to consder the n-stu flow drectons nsde the system of the backfllng. The dealzed system of a BHE backfll s axal ppe system wth a surroundng grout body (Fgure 3, rght). It s obvous that the system s hydraulc conductvty s ansotropc. The axal hydraulc conductvty wll be hgher than the radal hydraulc conductvty. The contact area between grout and ppe s a preferred flow path. In general the radal flow path corresponds wth the bulk materal hydraulc conductvty. The axal flow path (here referred to as system hydraulc conductvty) corresponds wth a hydraulc conductvty that s about two orders of magntude hgher than the bulk materal conductvty (Allan and Phlppacopoulos, 2000, Anbergen, 2014). Hence a regular hydraulc conductvty test of the bulk materal wll not reflect the hydraulc ntegrty of the n-stu system. The developed testng devce (Fgure 3, left) consders the hgher system hydraulc conductvty correctly and s able to smulate a wde range of thermal stresses on the grout specmens, smulatng the operaton modes of GSHPs. Furthermore the n-stu heat flow drectons are respected as they dffer substantally from exstng freeze-thawtest procedures for fne graned sols, concrete or natural stones (Anbergen et al., 2014a). 2.2 Freeze-Thaw-Cycles n Borehole Heat Exchangers In cold clmates, especally n regons wth long heatng perods t s reasonable to run the GSHP wth temperatures below 0 C. Wth an ncreasng amount of heat extracton the temperature of the surroundng ground of the BHE wll decrease. The decrease s strongly dependent on the geologcal boundary condtons and the total workng flud temperatures n the BHE. In sols wth low permeablty the heat transport s foremost conductve, so s the heat recharge. Due to the lowered sol temperature the GSHP needs to lower the return flow temperatures as well n order to extract suffcent thermal energy. For most heat pump devces the dfference between return flow and flow of the GSHP needs to be approx. 3-4 K. By usng return flow temperatures below 0 C t s possble to reach the requred temperature dfference even when the surroundng sol temperature s far below 10 C. A plot of a GSHP wth BHEs s shown n Fgure 5. 3

4 Fgure 5: Temperature log of a GSHP nstallaton wth BHEs, February 2014 (Anbergen, 2014) Usually a heat pump runs n relatve short operaton cycles. In ths case the demand for heatng and warm water s so hgh that the heat pump runs constantly. Even though ths s not a proper desgn for a heat pump, such GSHP behavor happens qute often. In consequence there s a nearly constant frost mleu n the vcnty of the return ppes. The grout eventually freezes. Phase change of the pore water wll start drectly beneath the return ppes and the frost front wll propagate n radal drecton. The postve temperature value of the flow ppe wll cause an asymmetrc propagaton of the frost front. As there s not enough water supply for the formaton of bg ce lenses, the ce lenses beneath the return ppe wll reman small. Wth ncreasng dstance from the return ppe the dameter of ce lenses wll ncrease as well. The formng mechansms of ce lenses n fne graned sols (Konrad and Morgenstern, 1980) and groutng materals correspond (Anbergen et al., 2014a). As the heat flow s prmary of a radal shape, the ce lenses form perpendcular to the heat flow whch leads to axs parallel crackng (Fgure 4). 2.3 Heat Transport The effectve heat transport n porous meda s determned by the convectve and conductve part of heat flow. The general heat equaton s wrtten as T c T c (1) q T eff t eff flud where ρ, c,, t, λ, q are densty, gravmetrc heat capacty, temperature, tme, thermal conductvty and flow rate, respectvely. The thermal conductvty and the flow rate are drected. The ndces eff and flud defne the effectve (bulk) specfc heat or thermal conductvty and the flud specfc heat, respectvely. Even f a BHE s a hghly dynamc thermal system, most analytcal and numercal solutons smplfy the model to an axsymmetrc calculaton, such as the lne source approach or cylnder source approach (Sass and Lehr, 2011). Whle observng the heat transport n a grouted BHE t s mportant to defne n whch geologcal formatons the observaton takes place. The hydraulc ntegrty of the grout body s of specal mportance n aqucludes (Fgure 2). There, only very lmted convectve heat transport occurs. Thus the heat transport s foremost conductve. Fgure 6 shows the dmensons of drected heat flow. Fgure 6: Dmensons of drected heat flow (Anbergen, 2014) Followng the common calculatons of BHEs the heat flow s two-dmensonal (or radal). Observng the BHE pecewse n test volumes of only small vertcal length, ths approach s reasonable. Consequently a testng procedure has to follow ths heat flow drecton as well. For the numercal mplementaton of such a model, a two-dmensonal axsymmetrc approach s suffcent, as the change n z-axs s small. In earler studes about the hydraulc behavor of groutng materals under cyclc freeze-thaw-stress the heat flow drectons of BHEs were not respected suffcently. One-dmensonal or three-dmensonal freezng and thawng leads to dfferent ce lens 4

5 formatons whch do not reflect the n-stu process. Thus only two-dmensonal freezng from the nsde to the outsde can lead to realstc results (Anbergen and Sass, 2013) 2.4 FEFLOW The fnte element method (FEM) software FEFLOW s a tool for ground water smulaton n porous meda ncludng heat and mass transport. The code s often used for numercal smulatons of shallow geothermal systems n every sze. FEFLOW uses lnear nterpolatons for the effectve thermal conductvty and heat capacty as shown n Eq. (2) and (3). Assumng fully water saturated condtons the thermal conductvty s calculated weghted by the porosty of the matrx. eff flud 1 - sold (2) where λ and ε are drected thermal conductvty and porosty, respectvely. The ndces defne the effectve, flud and sold components. In general flud thermal conductvty s sotropc; n dfference the thermal conductvty of the sold structures can be hghly ansotropc. The specfc heat capacty s calculated respectvely. c c 1 - c eff flud sold (3) By default FEFLOW does not take phase changes nto account. Nether t s capable of defnng amounts of water and ce n the model parameters. In order to enable FELOW for such smulatons a plug-n was developed usng FEFLOW s programmable nterface IFM. The presented plug-n s freely avalable from the authors upon request. 3. MATERIAL AND METHODS 3.1 Incorporaton of Phase Change Effects There exst several approaches for the ncorporaton of phase change effects n the equatons for heat and mass transport. One frequently appled method s the modfcaton of the flud heat capacty. A temperature nterval s defned, n whch the phase changes from lqud to sold or vce versa. Durng ths transformaton process thermal energy s released n form of an apparent heat capacty. The apparent heat capacty ncorporates the weghted heat capacty of the phases and an amount of crystallzaton heat (latent heat). The ncrease n heat capacty reduces the velocty of heat transport n the transton zone of phase change. Thus the thermal mpact of the phase change s consdered mathematcally. For the transton zone of phase change a relatve saturaton of lqud water content can be appled. The boundary temperatures of the transton zone are defned as T w (temperature water) where pore water s fully lqud, T (temperature ce) where pore water s fully sold. In between these two temperatures there exst both, lqud and sold pore water (water and ce). For now t s assumed that all pore water freezes to sold, so there s no resdual water content even though t s easy to mplement. In a fully saturated envronment the content of lqud water S w at T w s equal to 1. At the Temperature T the saturaton of lqud water S w s equal to 0. In between a transton functon s mplemented; for example lnear or exponental transton functons. Detaled nformaton for such mplementatons s publshed by Mottaghy and Rath (2006), McKenze et al. (2007), Anbergen et al. (2014b), and others. Besdes the effect of latent heat release, the thermal propertes of water and ce dffer substantally. So do the hydraulc propertes. The temperature dependency of both, thermal and hydraulc propertes needs to be taken nto account n order to smulate the effects of phase change correctly. 3.2 Temperature Dependent Thermal Propertes The thermal conductvty and heat capacty of water are hghly temperature dependent as seen n Fgure 7. The thermal conductvty of frozen water s about four tmes hgher than the thermal conductvty of lqud water. In dfference the heat capacty of ce s about 50% lower than the heat capacty of water. For the mplementaton n FEFLOW lterature data was evaluated and the formulatons of Alexades and Solomon (1993) were found to have a nearly perfect ft. Thus FEFLOW s thermal propertes of the flud were modfed accordng to Fgure 7. In ths case a lnear ramp functon was appled. Fgure 7: Temperature dependent flud thermal conductvty (left) and flud gravmetrc heat capacty (rght). Comparson of dfferent approaches (Anbergen et al., 2014b) 5

6 As the thermal propertes of the sold structure (here: sol or rock) are nearly constant n the evaluated temperature regons, they are not consdered here n order to keep the calculaton effort to a low level. The resultng falure s neglgble. 3.3 Temperature Dependent Hydraulc Propertes The hydraulc propertes of water and ce change wth temperature as well. Thus FEFLOW s flud densty and vscosty are modfed, as well as the systems hydraulc conductvty. Agan dfferent approaches were revewed and mplemented (see Anbergen et al., 2014b). For the modfcaton of the hydraulc conductvty t s assumed that flow s reduced to nearly zero when water starts freezng. Even though there s stll lmted water flow even n frozen regons, ths s neglected for the mplementaton. The water movement n frozen sols very low, so s the convectve heat transport. By assumng a no flow restrcton at temperature below T w, heat flow s under estmated slghtly. Thus a conservatve freezng process s smulated. 4. BENCHMARK AND APPLICATION 4.1 Benchmark wth Analytcal and Numercal Solutons The plug-n was compared to dfferent numercal and analytcal solutons for freezng and thawng processes. Here a benchmark to the analytcal Neumann Soluton and a numercal soluton wth SHEMAT s presented. The Neumann Soluton s frequently consdered for freezng and thawng processes. The calculatons are based on the thawng mechansms of ce n oceans descrbed by Stefan (1891). As the mathematcal formulatons are based on Neumann, the soluton s often referred to as the Neumann Soluton (Mottaghy and Rath, 2006). For the benchmark a calbraton model was used accordng to McKenze et al. (2007) as shown n Fgure 8. Fgure 8: Model for the benchmark of the FEFLOW plug-n for freeze-thaw-cyclng based on McKenze et al. (2007) For the numercal model a sem-nfnte half-space s chosen n order to fulfll the requrements of the analytcal soluton. The model has an ntal Temperature of T w. At tme t = 0, a temperature of T 0 < T s appled at the left sde (x = 0). Thus the temperature of the pore water decreases and the frost front (locaton of T ) propagates from left to rght wth ncreasng tme. The propagaton of the frost front can be calculated as a functon of tme. X t 2 a t (4) where X, γ, a, t are Locaton of the frost front, a form factor, thermal dffusvty of ce, and tme, respectvely. The form factor γ s calculated accordng to Eq. (5) e 2 (a - a w ) a w erfc ( erfc ( ) a a w ) = (T (T w - T ) - T 0 s ) a a w (5) where a, a w, γ, Tw, T, T0, λ, λs, are thermal dffusvtes of ce and water, the form factor, boundary temperatures of fully lqud and fully frozen phase, temperature of heat snk, and thermal conductvtes of sold and ce, respectvely. The thermal dffusvtes can be calculates weghted arthmetcally or geometrcally. In ths case the geometrc mean was appled. For the benchmark not only the analytcal soluton s compared, but also a numercal soluton usng SHEMAT. SHEMAT s a smulaton code for heat and mass transport as well. It s commonly used and s capable to calculate phase change processes (Mottaghy and Rath, 2006). The nput parameters for the SHEMAT soluton are publshed by Mottagh and Rath (2006), the FEFLOW nput date s shown n Table 1. Table 1: Parameters for the benchmark smulaton n FEFLOW (Anbergen et al., 2014b) Parameter FEFLOW Unt Flud specfc heat (grav.) J kg -1 K -1 Flud thermal conductvty varable J s -1 m -1 K -1 Densty of flud varable kg m -3 Ice specfc heat varable J kg -1 K -1 6

7 Ice thermal conductvty varable J s -1 m -1 K -1 Densty of ce 920 kg m -3 Sold gran specfc heat (vol.) 2.06 MJ m -3 K -1 Sold gran thermal conductvty 2.9 J s -1 m -1 K -1 Porosty Permeablty / Darcy flux 6.5 x 10-4 c m s -1 Latent heat of fuson J kg -1 Maxmum Step Sze 0.25 h Total Smulaton Tme 2 d The results of the three solutons are compared n Fgure 9. Fgure 9: Propagaton of the frost front n the benchmark model. Comparson of the analytcal Neumann Soluton, the numercal Soluton wth SHEMAT, and FEFLOW. It can be observed, that the three solutons (Neumann, SHEMAT, and FEFLOW) are n good ft. The propagaton of the frost front s calculated wth only lttle errors and thus t can be stated that the plug-n provdes good predctons on the temperature dstrbutons under phase change condtons. Further benchmarks of the model are publshed n Anbergen (2014) and Anbergen et al. (2014b). 4.2 Applcaton of the Plug-In and Expermental Observatons As the benchmark wth the lterature worked well, a comparson wth expermental data was executed. Therefore three specmens (see Fgure 3, rght) were casted, cured under exact same condtons, transferred to the testng devces, and saturated. After full saturaton and the determnaton of the hydraulc conductvty the freeze-thaw-cycle (FTC) was commenced. One FTC s composed of 20 hours of freezng wth workng flud temperatures of 10 C and 20 hours of thawng wth workng flud temperatures of + 8 C. The workng flud flows through the axal ppes of the specmens whlst the specmen remans nsde the testng devce. Durng the FTC smulaton the temperature at the outer edges of the specmens was recorded. The thermal process of the FTC smulaton was smulated usng FEFLOW. The nput data s shown n Table 2. Therefore a model of the testng devce and the specmen was set up. On the one hand the model was smulated wthout any modfcatons of the FEFLOW code, on the other hand the developed plug-n was actvated. The testng devce was modeled as a two-dmensonal axsymmetrc setup accordng to the components shown n Fgure 3 (left). Table 2: Parameters for the smulaton of the freezng and thawng process of the testng devce (Anbergen and Sass, 2013) Materal Porosty [-] Thermal Conductvty [W m -1 K -1 ] Vol. Heat Capacty [MJ m -3 K -1 ] Probe Grout Flter plate Upper and lower plate Membrane Insulaton A comparson of three temperature logs of the freeze-thaw-process, the FEFLOW smulaton wthout applcaton of the plug-n, and the FEFLOW smulaton wth the plug-n s shown n Fgure 10. It can be observed that the smulaton wth latent heat ncorporaton fts the measured temperature date very well. The freezng and thawng temperatures are smulated correctly and the smulated and measured latent heat effects correspond. In dfference the smulaton wthout phase change modfcatons does not reflect the expermental results. 7

8 Fgure 10: Temperature at the lateral surface of a specmen durng FTC smulaton. Measured temperature (black), smulated temperature wth/wthout latent heat ncorporaton (red / blue). 4. DISCUSSION The FEFLOW plug-n calculates satsfactorly the temperature profles of the benchmark model and the testng devce. The comparson to the expermental data shows a shft of the temperature log after 0.8 d of freezng before thawng fully starts. Ths s because of the reacton tme of the heat pump. The heat pump cannot ncrease the workng flud temperature nstantaneously as the model does. The heat pump needs tme to ncrease the temperature slowly n order not to damage the nternal heat exchanger. Consequently the measured temperature at the lateral surface of the specmen remans on a low level untl the heat flow reaches the measurement pont. Despte ths shft, the ft between temperature smulaton and expermental date s very good. Further verfcatons wth thermography measurements of frozen specmens are publshed n Anbergen et al. (2014b). 5. CONCLUSION Wth the developed testng procedure the hydraulc ntegrty of BHE grouts can be assessed whle smulatng the relevant n-stu boundary condtons. Measurng the system hydraulc conductvty whle provdng consstent radal earth pressure smulatons and followng the n-stu freezng drectons, the results dffer substantally from earler nvestgatons on the freeze-thaw-behavor of BHE grouts. The thermal processes of the testng procedure and BHEs under operaton correspond. By modfyng the FEM software FEFLOW wth a latent heat plug-n, the temperature dstrbuton nsde the testng devce under operaton could be successfully smulated. The apparent heat capacty approach of the plug-n leads to good benchmark results wth numercal and analytcal solutons. Besdes the study of FTCs on BHE grouts a potental future applcaton of the plug-n s the modelng of permafrost. The rsng atmospherc temperatures reduce the extent of permafrost globally, causng reduced slope stablty due to thawng ground. The plug-n could also be combned wth regonal ground water models to mprove predctons, for nstance wth respect to recharge. A more advanced applcaton of the plug-n are ntegrated models coupled wth approprate sol-vegetaton-atmosphere-transfer (SVAT) schemes (Mölders and Romanovsky, 2006), surface run-off and atmospherc models (Mölders and Rühaak, 2002) whch are valuable tools for dentfyng the most relevant parameters and processes. An extenson to the plug-n wll be developed based on Rühaak et al. (2014) n order to ncorporate mechancal stress nto the FEFLOW smulatons. REFERENCES Alexades, V. and Solomon, A.D.: Mathematcal Modelng of Meltng and Freezng Processes. Hemsphere Publshng Corporaton, Washngton, USA (1993) Allan, M.L., and Phlppacopoulos, A.J.: Performance Characterstcs and Modellng of Cementtous Grouts for Geothermal Heat Pumps, Proceedngs World Geothermal Congress 2000, Kyushu - Tohoku, Japan, May 28 - June 10, 2000, (2000) Anbergen, H., and Sass, I.: Freeze-Thaw-Behavour: Observatons n Grouted Borehole Heat Exchangers, 38 th Workshop on Reservor Engneerng, Stanford, CA, (2013) Anbergen, H.: Prüfverfahren zur Bestmmung des Frost-Tau-Wechsel-Enflusses auf Hnterfüllbaustoffe für Erdwärmesonden, Dssertaton, Technsche Unverstät Darmstadt, Darmstadt, (2014) Anbergen, H., Frank, J., Mueller, L., and Sass, I.: Freeze-Thaw-Cycles on Borehole Heat Exchanger Grouts: Impact on the Hydraulc Propertes, Geotechncal Testng Journal, 37, (2014a) Anbergen, H., Rühaak, W., Frank, J., and Sass, I.: Numercal Smulaton of a Freeze-Thaw-Testng Procedure for Borehole Heat Exchanger Grouts wth FEFLOW, Canadan Geotechncal Journal, (2014b, under revew) DGG and DGGT: Empfehlungen des Arbetskreses Geotherme Oberflächennahe Geotherme Planung, Bau, Betreb, Qualtätsscherung, Deutsche Gesellschaft für Geowssenschaft e.v. (DGG) und Deutsche Gesellschaft für Geotechnk e.v. (DGGT) (Ed.), Ernst & Sohn, Berln (2014) 8

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