Aquifer Thermal Energy Storage'

Transcription

1 WATER RESOURCES RESEARCH, VOL. 19, NO. 4, PAGES , AUGUST 1983 Aqufer Thermal Energy Strage' An Attempt t Cunter Free Thermal Cnvectn F. J. MOLZ, J. G. MELVLLE, O. GOVEN, AND A.D. PARR 1 Cvl Engneerng Department, Auburn Unversty n prevus Aqufer Thermal Energy Strage (ATES) experments, apprecable free thermal cnvectn was bserved. n an attempt t cunter the detrmental effects f cnvectn, a dual recvery well system was cnstructed at the Mble ste and a thrd njectn-strage-recvery cycle perfrmed. Usng a partally penetratng well, cycle 3-3 njectn began n Aprl 7, A ttal f 56;680 m 3 f 79øC water were njected. After 57 days f strage, prductn began wth a dual recvery well system. Due t the dmnatng effect f nnhmgenetes, the dual well system dd nt wrk partcularly well, and a recvery factr f 0.42 was acheved. The degree f aqufer hetergenety at the lcatn f the present experments was nt apparent durng prevus experments at a lcatn nly 109 m away, althugh pumpng tests ndcated smlar values f transmssvty. Therefre aqufers wth the same transmssvty can behave qute dfferently n a thermal sense. Heat cnductn t the upper aqutard was a majr energy lss mechansm. Water sample analyses ndcated that there were n mprtant changes n the chemcal cnsttuents durng the thrd set f experments. There was a 19% ncrease n ttal dsslved slds. At the end f njectn, the land surface near the njectn well had rsen 1.39 cm wth respect t bench marks lcated 70 m away. NTRODUCTON Begnnng n 1975, Auburn Unversty cnducted a seres f aqufer thermal energy strage (ATES) experments n a cnfned aqufer near Mble, Alabama [Mlz et al., 1978, 1979, 1981]. The bjectves f these experments were t demnstrate the techncal feasblty f the ATES cncept, t dentfy and reslve nherent peratnal prblems, and t acqure a data base fr develpng and testng mathematcal mdels. Fr the mst part, these bjectves have been met. ATES s techncally feasble at the Mble ste, and nherent peratnal prblems that were encuntered have been largely reslved [Mlz et al., 1983; Parr et al., 1983]. The cllected data have served as a partal bass fr testng several mathematcal mdels f varyng degrees f cmplexty, and the resultng studes have prved t be llumnatng [Papadpuls and Larsn, 1978; Tsang et al., 1981; Sauty et al., 1982; Dughty et al., 1982; Sykes et al., 1982; Buscheck et al., 1983]. The mst recent feld experments (thrd set) were based n the gemetry shwn n Fgure 1 and cnssted f three njectn-strage-recvery cycles f 3 mnths, 7.3 mnths, and 8 mnths duratn respectvely. Durng cycle 3-1, 25,402 m 3 f water were njected at an average temperature f 58.5øC. After strage and recvery, ths was fllwed by cycle 3-2 njectn f 58,063 m 3 at an average temperature f 81øC. Bth f these cycles are descrbed n detal by Mlz et al. [1983] and smulated by Buscheck et al. [1983] usng a cmputer mdel called PT. A summary f all three sets f experments perfrmed at the Mble ste s presented n Table 1. Durng the strage phase f cycle 3-1, t became apparent that a relatvely large amunt f free thermal cnvectn was ccurrng n the cnfned aqufer. Such a phenmenn was nt bserved t a sgnfcant extent durng cycles 2-1 and 2-2, Nw at the Department f Cvl Engneerng, Unversty f Kansas. Cpyrght 1983 by the Amercan Gephyscal Unn. Paper number 3W /83/003 W whch tk place n a dfferent strage zne [Mlz et al., 1979, 1981; Sykes et al., 1982]. CnvectOn leads t thermal stratfcatn n the strage aqufer (relatvely ht n tp and cld n the bttm) whch causes mxng f ht and cld water durng recvery. Thermal lsses by cnductn nt the upper aqutard are maxmzed als. Bth effects act t lwer the recvery factr. At the hgher njectn temperature (81øC) f cycle 3-2, free thermal cnvectn was mre prnunced and the ntal recvery temperature was nly 55.1øC. By 2 weeks nt the prductn perd, water abve 45øC had mgrated t the tp half f the strage aqufer. At ths tme t was decded t mdfy the recvery well n an attempt t mprve energy recvery. The bttm half f the well was flled wth sand and a fgure k packer was placed abve the sand. After ths mdfcatn was cmplete, pumpng resumed, and ultmately the recvery factr was f the mdfcatn had nt been made, t s estmated that the recvery factr wuld have been 0.40 [Mlz et al., 1983]. After cnsderatn f the free thermal cnvectn prblem and ts negatve effect n recvery temperature, t was cncluded that a dual recvery well system mght result n mprved energy recvery. The tw wells wuld be lcated as clse tgether as pssble, wth ne well screened n the upper half f the strage aqufer and the ther screened n the lwer half. Upn ntatn f recvery pumpng, bth wells wuld be pumped smultaneusly. n a thermally stratfed and hmgeneus strage aqufer ths wuld mantan radal flw apprxmately, wth clder water enterng the lwer screen and warmer water enterng the upper screen. The clder water culd then be renjected r wasted at an apprprate lcatn. The effect f nnhmgenetes, whch we knw exst at the Mble ste, cannt be predcted n detal but wuld prbably act t reduce the effectveness f a dual well system. At the Mble ste, cnstructn f a dual recvery well system was cmpleted n Aprl 1, 1982, and cycle 3-3 njectn began n Aprl 7. The tw majr bjectves f ths paper are t reprt the resultng cycle 3-3 data and t dscuss the effectveness f the dual recvery well system. A thrd 922

2 MOLZ ET AL.' AQUFER THERMAL ENERGY STORAGE 923 T T +AT LAND SURFACE SUPPLY WELL WELL UNCONFNED AQUFER / / / / / / / /CONFNNG LAYER 'l '-- CONFNED AQUFER -.40M ---20M _.ll //// 11 ß 244 M (800 FT) ll Jll Fg. 1. Dagram f the subsurface gemetry and dublet well system at the Mble feld ste. bjectve s t cmpare sme f the results wth thse f prevus cycles and prevus experments. DESCRPTON OF EXPERMENTS The prject ste s lcated n a sl brrw area at the Barry Steam Plant f the Alabama Pwer Cmpany, abut 32 km nrth f Mble, Alabama (see Mlz et al. [1978, 1983] fr detals). The strage aqufer s cmpsed f a medum sand f Quaternary age cntanng ntersttal slts and clays [Mlz et al., 1978]. n rder t cnduct cycles 3-1, 3-2, and 3-3, the well feld shwn n Fgure 2 was cnstructed. Fr cycle 3-3, well 2 was used fr njectn f heated water and fr prductn. Grundwater temperatures were nly recrded at 6 elevatns n wells 12, 11, 4, 5, and 6. There was n tracer njected durng cycle 3-3, but heads were recrded n each head bservatn well. Land elevatn changes and grundwater chemstry data were recrded als. Wells 2 and R1 cnsttute the dual recvery well system shwn schematcally n Fgure 3. Durng recvery 2 s called the prductn well and R1 s called the rejectn well. The wells are separated hrzntally by 1.8 m, wth 2 screened n the tp 9.1 m f the strage aqufer. The rejectn well screen s 9.1 m n length als and begns 1.5 m belw the bttm f the upper screen. Cycle 3-3 njectn began n Aprl 7, 1982 and cntnued ntermttently untl July 14, 1982 when a ttal f 56,680 m 3 f water had been njected. The average njectn temperature was 79øC. Shwn n Fgures 4 and 5 are the cumulatve njectn vlume and the njectn temperature as functns f tme, respectvely. njectn prceeded smthly except fr falure f a fuel pump (large hrzntal segment n Fgure 4) at abut 1000 hurs nt the experment. A 52-day strage perd ended n September 9, 1982, and prductn pumpng wth the dual recvery system began. Plts f cumulatve prductn and rejectn vlumes versus tme are shwn n Fgures 6 and 7, respectvely. Recvery pumpng was ffcally ended n Nvember 16, At ths tme 64,140 m 3 f water had been prduced and 19,300 m 3 rejected. At regular ntervals durng cycle 3-3, careful level measurements were made s that data culd cntnue t be btaned n land surface elevatn changes caused by ATES [Mlz et al., 1981]. As descrbed n Mlz [1983], tw reference pads, tw measurement pads, and ne bservatn pad were cnstructed f renfrced cncrete wth surveyng markers embedded n the center f each. A level was placed n the bservatn pad, and frm ths lcatn relatve elevatns f the markers n the reference and measurement pads were recrded. RESULTS AND DSCUSSON OF CYCLE 3-3 The temperature hstry f the prductn and rejectn wells durng recvery pumpng s shwn n Fgure 8. After a few mnutes f pumpng the prductn temperature stablzed at 51.5øC, whch s well belw the average njectn temperature f 79øC. t was sn dscvered that varatns n the rejectn pumpng rate had very lttle effect n the prductn temperature. Evdently, the nnhmgenety n the strage aqufer was exertng a dmnant nfluence n the velcty dstrbutn. Further evdence fr the sgnfcant effect f hetergeneus and temperature-dependent hydraulc cnductvty n the strage aqufer can be btaned by examnng the vertcal temperature dstrbutn curve b- TABLE 1. Summary f Sx njectn-strage-recvery Experments Perfrmed at the Mble Ste Over the Past Seven Years Cycle njectn njectn njectn Strage Recvery Duratn, Vlume, Temperature, Duratn, Duratn, Recvery days m 3 øc days days Factr , , , , ' 58, , The recvery duratn ndcated resulted n recvery vlume equalng njectn vlume. *27 days f early dwn tme were remved t facltate cmparsn.

3 , 924 MOLZ ET AL.' AQUFER THERMAL ENERGY STORAGE Flter Sl : Bler, $21 $20 ' XO3 ' t Temperature e Tracer Supply njectn Rejectn )8 09 Fg. 2. Plan vew f the wellfeld at the Mble ste shwng the dfferent types f wells., taned frm the 15-m bservatn well (well 4) and shwn n Fgure 9. At varus tmes durng prductn the average temperature n the upper 50% t 60% f the aqufer s wthn a degree r tw f the prductn temperature. Hwever, the average temperature n the bttm half f the aqufer s 10øC t 12øC belw the bserved rejectn temperature. Ths data mples a preferental flw n the upper half f the aqufer, at least wthn a 15-m radus f the prductn well. Such a flw wuld be nduced by the hgh ntrnsc perme- R1 CASNG m' xxx w, G ablty zne smewhere near the center f the aqufer and a temperature-nduced permeablty ncrease (knematc vscsty f water decreases by 50% between 30øC and 70øC) due t htter water n the upper part f the aqufer, sme f whch remaned after cycle 3-2. The magntude f a temperature-nduced permeablty change s cmparable t the ntrnsc permeablty dfferences selected by Buscheck et al. [1983] n ther smulatns f cycles 3-1 and 3-2. The prevusly mentned relatnshp between average aqufer temperature, prductn temperature, and rejectn temperature held even when the prductn pumpng rate was fve tmes greater than the rejectn rate. Pumpng the rejectn well at a hgher rate relatve t the prductn well resulted n smply rasng the rejectn temperature wth lttle r n effect n the prductn temperature. t appears, therefre, that bth wells are pullng water frm the mddle t upper prtn f the strage aqufer where a hgh ntrnsc permeablty zne exsts and where the httest water resdes. Relatvely lttle water s mvng hrzntally thrugh the bttm thrd f the aqufer n the vcnty f the rejectn well where the ntrnsc permeablty s lwer and the water vscsty s hgher due t lwer temperatures. --m- TUR81Nœ VERTCAL 60- ' ' V//11111/ 50- CONF NED AQUFER ////////////////// 4 "Dw.Sm T '. 9. ra [ ' ' m ) / //11/11/////1///////// ;/ 9.1m m Fg. 3. Schematc dagram f the dual recvery well system cn- structed at the Mble ste. x O O O 260 TME ('x hrs.) Fg. 4. Cumulatve njectn vlume versus tme fr cycle 3-3. The large hrzntal segment was due t the unexpected falure f the bler fuel pump.

4 MOLZ ET AL..' AQUFER THERMAL ENERGY STORAGE OO 3O 8 2O <: D_ 4-10 >0, O 8' TME (101x hrs.) Fg. 5. njectn temperature versus tme fr cycle 3-3. An ndcatn f the gemetry f the hgh ntrnsc permeablty zne detected at the present strage lcatn may be btaned by examnng Fgures 10 and 11. Shwn n these fgures are frst arrval tmes f the thermal frnt recrded n the temperature bservatn wells durng cycle 3-1. Fr each aqufer crss sectn the lcatns f the tw smallest arrval tmes n each bservatn well are cnnected t thse f neghbrng wells by straght lnes. The lne segments ndcate apprxmate bundares f a hgh permeablty zne near the mddle f the aqufer. Ths apprxmatn crrelates well wth the temperature dstrbutns shwn n Fgures 13 and 14 f Mlz et al. [1983]. t supprts als the three layer aqufer permeablty mdel used by Buscheck et al. [1983] n ther cmputer smulatns f cycles 3-1 and 3-2. t s mprtant t nte that the degree f aqufer nnhmgenety ndcated by the present experments was nt bserved n experment set 2 (cycles 2-1 and 2-2) whch utlzed a zne f the aqufer apprxmately 109 m frm the present strage zne. Examnatn f Fgures 7, 8, and 9 f Mlz et al. [ 1979] ndcates a relatvely symmetrc temperature dstrbutn. Fr cmparsn an average radal sectn f the stherms at the end f cycle 2-1 njectn s shwn n Fgure ' 4,,0 ' : 0 ' 560 TME (10hr.) Fg. 7. Cumulatve rejectn pumpng vlume as a functn f tme. 12 alng wth an sthermal plt fr a vertcal aqufer crss sectn at the end f cycle 3-1 njectn. The central fngerng apparent at the new lcatn durng cycle 3-1 was nt bserved durng cycle 2-1 at the ld lcatn. Whle stll playng an mprtant rle, the effects f the nnhmgenety n the temperature stherms was nt as apparent durng cycles 3-2 [Mlz et al., 1983] and 3-3 (Fgure 13) because the relatvely strng buyancy flw at the hgher njectn temperatures smears ut the bvus effect f the nnhmgenety [Buscheck et al., 1983]. The nferred dfferences n strage aqufer hydraulc prpertes at tw lcatns nly 109 m apart have mprtant mplcatns fr testng prgrams whse purpse s t select aqufers sutable fr ATES. Often t may nt be suffcent t smply measure average hrzntal hydraulc cnductvty, whch s what results (we hpe!) frm standard pumpng tests. t s nt easy t detect layerng n uncnsldated meda wthut varable screen length pumpng tests r tracer Ll 55- n- l,- LL Z 45 LL 4O Z Z F , TME ( 10xhr. ) Fg. 6. Cycle 3-3 cumulatve prductn pumpng vlume as a functn f tme. a_ TMr (hrs,'10) Fg. 8. Prductn temperature (curve A) and rejectn temperature (curve B) versus tme fr cycle 3-3.

5 926 MOLZ ET AL.' AQUFER THERMAL ENERGY STORAGE E 12 [_. 8 [// 9/28/82 ) ),.,..., 9/9/ '0 TEMPERATURE (øc) Fg. 9. Vertcal temperature prfles at the 15-m bservatn well durng the frst 3 weeks f recvery pumpng. tests, and these prcedures can be qute tedus. Smulatn mdels are useful but nly after gd data have been btaned. t may be that mderate-scale ht water njectn testng wll be an ecnmcal prcedure fr makng an verall and fnal evaluatn f an aqufer's sutablty fr ATES. Shwn n Fgure 13 are radal sthermal plts f the aqufer temperature dstrbutn at the end f njectn and strage respectvely. There s clear vsual evdence f free thermal cnvectn durng the strage perd. Based n a mre detaled versn f Fgure 13 and a smple numercal ntegratn scheme, t s pssble t estmate the percentage f stred heat that was lst due t cnvectn and cnduc- tn frm varus znes f the aqufer durng the strage perd. The result ndcated that 45% f the thermal energy stred n a cylnder f aqufer cncentrc wth the njectn well and f m radus was lst frm that zne durng strage. An estmate based n average njectn temperature and ntal prductn temperature alne wuld have been 47% [[(79-20) - ( )]/(79-20)]. Ths energy lss s qute large and lttle r nthng can be dne durng strage t cunteract t. Heat lsses n smlar aqufer vlumes f m radus and 45.7-m radus were estmated t be 32% and 22%, respectvely. Obvusly, free thermal cnvectn ccurred durng cycle 3-3 just as t dd durng cycle 3-2. The dual well recvery system was benefcal but nt hghly effectve n cunteract- ng negatve cnvectn effects. Shwn n Fgure 14 are plts f recvery fractn versus cumulatve recvery vlume wth (curve A) and wthut (curve B) the dual well system. (T get curve B we smply cmbned the heat flws and pumpng vlumes frm the prductn and rejectn wells as f they were a sngle well.) Wth the dual well system peratng, we btaned a recvery factr f A sngle prductn well wuld have yelded a recvery factr f abut Further understandng f the therm-hydrdynamcs at the Mble ste may be btaned by examnng the radal stherm plts at the end f the varus recvery perds (defned as tme when water vlumes pumped n and ut are equal) dsplayed n Fgure 15. These plts shw the prgressve effects f free thermal cnvectn and emphasze the rle played by the hgh ntrnsc permeablty zne near the center f the aqufer. At the end f cycle 3-1 recvery, there s a large dfference n temperature between the bttm and tp f the strage aqufer. Fr the hmgeneus case wthut buyancy flw, the httest water wuld be lcated symmetrcally alng the upper and lwer aqutards. Even cycle 3-1 wth a 58.5øC njectn temperature devates dramatcally frm ths pattern, wth 40øC water lcated at the very tp f the aqufer and 22øC water at the bttm. Cycle 3-2 recvery ended wth a prductn temperature f 39.5øC. At ths tme the temperature n the upper thrd f the aqufer vared frm 50øC t abut 62øC at the tp. The temperature near the aqufer bttm was nly 2 r 3 degrees abve ambent (20øC). Due t the hgh-permeablty zne near the aqufer center, a relatvely steep temperature gradent f abut 9øC/m s created at the tp f the mddle thrd f the strage aqufer. Presum- ably, ths s due t a relatvely hgh dscharge f cler water carryng away heat frm the upper thrd f the aqufer. By the end f cycle 3-3 recvery, the aqufer s almst perfectly stratfed thermally wthn a 45-m radus f the njectn well. # ft NF 10 L 36 )3F [ O ft -' #4 5.NF ' ' ' NF _. NF _ O RADUS(ft.) Fg. 10. Scale dagram f a west t east vertcal aqufer prfle shwng frst thermal arrval tmes (n hurs) at the varus thermstrs durng cycle 3-1 njectn. The early arrval tmes, mstly near the aqufer center, ndcate a hgh ntrnsc permeablty zne. The ntatn 'NF' means that a defnte thermal frnt dd nt arrve. The dashed lnes ndcate the upper and lwer aqufer bundares.

6 MOLZ ET AL' AQUFER THERMAL ENERGY STORAGE 927 #8 7 2 #1 # ff , "" ' ' ,745 NF 100 do ' 0 0 do 100 RADUS (ft.) Fg. 11. Scale dagram f a suth t nrth vertcal aqufer prfle shwng frst thermal arrval tmes (n hurs) the varus thermstrs durng cycle 3-1 njectn. Thearly arrval tmes, mstly near the aqufer center, ndcate hgh ntrnsc permeablty zne. The ntatn NF means that a defnte thermal frnt dd nt arrve. The dashed lnes ndcate the upper and lwer aqufer bundares. The upper prtn f the aqufer s cler cmpared t cycle equal t the ttal energy njectedurng the cycle. Due t the 3-2 because f the lnger strage perd and lwer njectn nteractn f buyancy flw and nnhmgenetes, heat temperature. Hwever, the steep temperature gradent zne cnductn nt the caprck thus emerges as a majr energy s stll evdent. lss mechansm at the Mble ste. Based n the temperature dstrbutns shwn n Fgure 15 Use f a partally penetratng njectn well durng cycle 3- and the measured njectn and recvery energes, t s 3 dd nt have a sgnfcant effect n the verall aqufer pssble t develp an energy budget fr the thrd set f temperature dstrbutn when cmpared t cycle 3-2, n experments. Ths budget, whch s lsted n Table 2, reflects whch a fully penetratng well was emplyed. Shwn n the grss energy dstrbutn and heat strage changes Fgure 13 are radal sthermal plts at the end f njectn thrughut the thrd set f experments. The mst nterestng fr cycles 3-2 and 3-3. A cmplete cmparsn cannt be fgures relate t heat strage n the aqutards. Prbably 95% made because f thermstr falures at hgher temperatures r mre f ths energy resdes n the upper aqutard, and by durng cycle 3-2. [Mlz et al., 1983]. Hwever, the 25øC and the end f cycle 3-3 recvery (.e., when the prductn plus 35øC stherms are qute smlar, ndcatng n grss dfferrejectn vlumes were equal t the njectn vlume) the ences wthn 50 m f the njectn well. Ths bservatn s cumulatve heat cntent ncrease f the caprck was nearly als cnsstent wth the prpsed hgh ntrnsc permeablty END OF 3-1 NJECTON 3 #2 ' 55 6 "-- 10m ---"1 Fg. 12. Plt f stherms n degrees Celsus n a vertcal aqufer crss sectn at thend f njectn fr cycles 2-1 and 3-1, respectvely. stherm 'fngerng' durng cycle 3-1 njectn suggests thexstence f a hgh permeablty zne. Lttle fngerng was bserved durng cycle 2-1 whch was cnducted at a dfferent lcatn n the same aqufer.

7 928 MOLZ ET AL.' AQUFER THERMAL ENERGY STORAGE RADAL SOTHERMS 40, 25 ; ',l ' - -- q ' END Of NJECTON (CYCLE 3-2) END OF PRODUCTON (CYCLE 3-1) 15.25m 30.5m 45.75m 2 WELL WELL WELL r NJECTON 15.25m 30.5m 45.75m WELL WELL WELL WELL ', -- ' END OF PRODUCTON (CYCLE 3-2) 65 ø 25ø END Of NJECTON (CYCLE 3-3) 6 END OF PRODUCTON ( CYCLE 3-3) Fg. 15. Plt f stherms n degrees Celsus n a vertcal aqufer crssectn at the end f prductn fr cycles 3-1, 3-2, and 3-3, respectvely. Numercal ntegratn f these fgures enabled us ' t estmate the quantty f njected energy remanng the aqufer END OF STORAGE (CYCLE 3-3) at the end f the varus prductn perds. Fg. 13. Plt f stherms n degrees Celsus n a vertcal aqufer crssectn at the end f njectn fr cycles 3-2 and 3-3. A smlar plt s gven fr the end f cycle 3-3 strage. The best verall measure f change s prbably the ttal dsslved slds (TDS), and the data n Table 2 ndcate a trend frm 274 t 284 t 299 mg/1. Thrughut mst f the zne. Flw frm bth the partally and fully penetratng experment, TDS at the njectn/prductn well was meanjectn wells wuld tend t fllw the hgh permeablty sured n a weekly bass. Durng cycle 3-1 the TDS averaged nnhmgenety. 280 mg/1. Ths average held fr 3-2 njectn, but durng At the end f cycle 3-3 njectn, t was decded t perfrm prductn the average TDS ncreased t 320 mg/1. Durng addtnal water chemstry analyses t determne f the cycle 3-3 njectn the average ncreased agan t 333 mg/1. flushng f heated water thrugh the strage aqufer had Mst lkely ths ncrease was due t mnr dsslutn f the caused changes n the cncentratns f varus dsslved aqufer matrx by the htter water durng cycles 3-2 and 3-3. materals. Accrdngly, samples were taken n July 12, 1982 Als, by cycle 3-2 recvery the ht water had been n frm wells S2 and 22. The water btaned frm well 22 was at cntact wth the aqufer matrx fr an extended perd f a temperature f 62øC and had been flushed thrugh the tme. strage zne durng cycle 3-2 and 3-3. That btaned frm S2 Land surface elevatn measurements were perfrmed was clser t a sample f the natve grundwater but stll durng the later part f cycle 3-3, and the results are shwn n subject t sme flushng. Analyses f bth samples alng Fgure 16. At the end f njectn, the relatve elevatn wth prevus measurements utlzng natve grundwater ncrease peaked at 1.39 cm and then began a steady fall are dsplayed n Table 3. Ths data supprts the cnclusn durng strage and recvery. The maxmum elevatn gradthat there were n majr changes n the chemcal cnsttu- ent between pads A and B ccurred at the end f cycle 3-2 ents f the grundwater durng the thrd set f experments. njectn and was The maxmum average gradent between pad A and the reference pads were at the end f cycle 3-3 njectn. Such elevatn changes are nt 0.60 neglgble, and ther ptental effect n fundatns wuld z have t be cnsdered, especally f an ATES system were beng desgned fr an urban envrnment. Dependng n 0.40 lcal stratgraphy, njectn temperature (assumed <100øC) and njectn vlume, elevatn changes 2 r 3 tmes greater than thse bserved at the Mble ste seem pssble SUMMARY AND CONCLUSONS RECOVERY VOLUME (m3x 10 3) Fg. 14. Cycle 3-3 energy recvery fractn as a functn f recvery vlume. Curve A represents the dual recvery well system, and curve B s the predcted result fr a sngle equvalent well. Ths paper deals prmarly wth the thrd njectn-stragerecvery cycle f the thrd set f aqufer stragexperments (cycle 3-3) t be cnducted by Auburn Unversty at the Mble, Alabama feld test faclty. Usng a partally penetratng well, cycle 3-3 njectn began n Aprl 7, By 98 days later a ttal f 56,680 m 3 f water at an average

8 MOLZ ET AL' AQUFER THERMAL ENERGY STORAGE 929 TABLE 2. Energy Summary f the Energy Budget fr the Thrd Set f Experments Energy Cumulatve Energy Left, J Energy Added, J njected, Recvered, Cycle J J Aqufer Aqutards Aqufer Aqutards X X x X x X x x x x x x x x x x x x 1012 The cumulatve energy left behnd n the aqufer was calculated by numercal ntegratn f the temperature dstrbutns shwn n Fgure 15. temperature f 79øC had been njected. After a 57-day strage perd, prductn began wth a dual recvery well system. The bjectve was t pull the htter water n the upper prtn f the strage aqufer nt the prductn well and the clder water n the bttm prtn nt the rejectn well. By varyng the pumpng rates f the tw wells, t was hped that a near-ptmum energy recvery frm a thermally stratfed aqufer culd be acheved. Shrtly after prductn began, t became bvus that the dual recvery well system was nt gng t wrk as well as had been ntended. The nadequate cntrl was due t the fllwng nteractng effects. 1. Durng njectn, much f the flw ccurred near the center f the aqufer whch caused sgnfcant lateral spreadng f the njected vlume. 2. Durng strage, thermal cnvectn n and abve the hgh permeablty zne was dramatc, causng ncreased lateral spreadng f the heat. 3. Ht water n the tp f the aqufer havng spread ver a large area, allwed fr maxmum cnductve heat lss t the upper cnfnng layer. 4. The selectve recvery system, althugh t s estmated t have ncreased the recvery factr frm 0.40 t 0.42, dd nt have a dramatc effect because the aqufer anstrpy and nnhmgenety cntrlled the velcty dstrbutn t a sgnfcant extent. Because f the abve phenmena, the ntal prductn temperature was $1.$øC, well belw the average njectn temperature f 79øC. Durng strage, apprxmately 45% f the thermal energy stred n a cylnder f aqufer f m radus was lst. The degree f aqufer nnhmgenety nferred at the lcatn f the present experments was nt apparent durng prevus experments at a lcatn nly 109 m away. Therefre aqufers wth the same transmssvty can behave qute dfferently n a thermal energy strage sense. Vertcal varatns f hrzntal hydraulc cnductvty are dtfcult t detect, and mderate-scale ht water njectn testng alng wth cmputer smulatn may be an ecnmcal prcedure fr makng an verall and fnal evaluatn f an aqufer's sutablty fr ATES. Chemcal analyses f water samples ver the curse f the Mble experments ndcated that there were n mprtant changes n the chemcal cnsttuents durng the thrd set f experments. Hwever, due t the flushng f heated water thrugh the system the ttal dsslved slds cntent ncreased frm 280 mg/l t 333 mg/1. At the end f 3-3 njectn the land surface near the njectn well had rsen 1.39 cm wth respect t bench marks lcated 70 m away. The average elevatn gradent was Dependng n lcal stratgraphy, njectn temperature (assumed <100øC), and njectn vlume, elevatn changes 2 r 3 tmes greater than thse bserved at the Mble ste seem pssble. t s safe t say that the varus experments at the Mble ste have demnstrated the techncal feasblty f lw b.l 1.2 <.d 1.1 Z 1.O Z.8 > ll.7 ß PAD PaD a B TME (WEEKS) Fg. 16. ncrease n land surface elevatn near the njectn well as a functn f tme. The elevatn was measured relatve t a benchmark apprxmately 70 m away. TABLE 3. Results f Chemcal Analyses Made at Varus Tmes Durng the ATES Experments at the Mble Ste Parameter Well S-2, Well 22, Well -l, July 12, July 12, June 20, ph Dsslved slds, mg/ Ca 2+, mg/l Mg 2+, mg/l 0 0 Alkalnty, mg/l as CaCO Hardness, mg/1 as CaCO Na +, mg/1 85 * 85 * 9.4 NH4 + N, mg/ NO3- N, mg/l Fe 3 +, mg/l <0.1 <0.1 <0.1 Mn 2+, mg/l <0.05 <0.05 SO42-, mg/l <1.0 <1.0 K +, mg/l OD, mg/ , mg/l Zn 2+, mg/l <0.1 <0.1 Cu+, mg/l <0.1 <0.1 S, mg/l Br, mg/l 10 * 9.5* <0.4 *Elevated levels due t NaBr tracer njectn durng cycles 3-1 and 3-2.

9 930 MOLZ ET AL..' AQUFER THERMAL ENERGY STORAGE temperature ATES but nt necessarly the ecnmc feasblty. Hwever, several applcatns f ATES technlgy currently underway n Canada, Denmark, Sweden, and ther lcatns n Eurpe wll sn cntrbute t reslutn f the ecnmc questn. Sme f the mre nterestng appraches nvlve the use f heat pump systems t extract heat frm warm aqufer water and prduce a useful temperature fr space heatng, water heatng, and ther applcatns. Wth the cmbned ad f feld tests and cmputer mdelng technques that have been perfected ver the past decade, t s nw relatvely straghtfrward t develp the ntal desgn f an ATES system. Hwever, the useful lfetme and lng-term mantenance csts are mre dffcult t defne. The mst subtle prblems are chemcal n nature. They result manly frm mxng waters havng dfferent temperature and chemcal prpertes (ph, n cncentratn, etc.) durng the njectn prcess. Ths wll ccur t sme degree even when the supply and njectn wells are lcated n the same aqufer. Deleterus gechemcal and/r clld chemcal effects can be mmedate and dramatc, serusly mparng njectn wthn a few days, r they can be f a very gradual, lng-term nature. Fr bvus reasns, the latter stuatn has receved the least amunt f study. Acknwledgments. Ths wrk was made pssble thrugh supprt f the U.S. Department f Energy and the Battelle Pacfc Nrthwest Labratres (subcntract B A-0, admnstered by the Auburn Unversfy Water Resurces Research nsttute). The Alabama Pwer Cmpany allwed us t cnduct the experment n ther land and als mantaned the rads leadng t the ste. Ths help s gratefully acknwledged. Feld engneers Mke T. Hpf and Davd A. Kng and graduate research assstants Rnald W. Falta, Jr., and Charles V. Camp perfrmed much f the feld wrk. Wthut ther cntrbutns ths paper wuld nt have been wrtten. n addtn, the wrters wuld lke t thank Rd Jenkns, Lance Bell, and Larry Benefeld, Asscate Prfessrs f Cvl Engneerng, Auburn Unversty, fr perfrmng the land surface elevatn and chemcal measurements reprted heren. Many frutful dscussns wth staff members f the Battelle Pacfc Nrthwest Labratry and the Lawrence-Berkeley Labratry are acknwledged als. REFERENCES Buscheck, T., C. Dughty and C. F. Tsang, Predctn and analyss f a feld experment n a mult-layered aqufer thermal energy strage system wth strng buyancy flw, submtted t Water Resur. Res., Dughty, C., G. Hellstrm, C. F. Tsang, and J. Claessn, A dmensnless parameter apprach t the thermal behavr f an aqufer thermal energy strage system, Water Resur. Res., 18, , Mlz, F. J., J. C. Warman, and T. E. Jnes, Aqufer strage f heated water, 1, A feld experment, Grund Water, 16, , Mlz, F. J., A.D. Parr, P. F. Andersen, V. D. Lucd, and J. C. Warman, Thermal energy strage n a cnfned aqufer: Expermental results, Water Resur. Res., 15, , Mlz, F. J., A.D. Parr, and P. F. Andersen, Thermal energy strage n a cnfned aqufer: secnd cycle, Water Resur. Res., 17, , Mlz, F. J., J. G. Melvlle, A.D. Parr, D. A. Kng, and M. T. Hpf, Aqufer thermal energy strage: A well dublet experment at ncreased temperatures, Water Resur. Res., 19, , Papadpuls, S.S., and S. P. Larsn, Aqufer strage f heated water, 2, Numercal smulatn f feld results, Grund Water, 16, , Parr, A.D., F. J. Mlz, and J. G. Melvlle, Aqufer testng fr thermal energy strage, Grund Water, 21, 22-35, Sauty, J.P., A. C. Grngarten, H. Fabrs, D. Thery, A. Menjz, and P. A. Landel, Sensble heat strage n aqufers, 2, Feld experments and cmparsn wth theretcal results, Water Resur. Res., 18, , Sykes, J. F., R. B. Lantz, S. B. Pahwa, and D. S. Ward, Numercal smulatn f thermal energy strage experment cnducted by Auburn Unversty, Grund Water, 20, , Tsang, C. F., T. Buscheck, and C. Dughty, Aqufer thermal energy strage: A numercal smulatn f Auburn Unversty feld experments, Water Resur. Res., 17, , O. Gven, J. G. Melvlle, and F. J. Mlz, Cvl Engneerng Department, Auburn Unversty, AL A.D. Parr, Department f Cvl Engneerng, Unversty f Kansas, Lawrence, KS (Receved January 31, 1983; revsed Aprl 21, 1983; accepted May 13, 1983.)