Double Generator Abs Ammoniac Eau (Ref Imp) (1)

8/3/2019 Double Generator Abs Ammoniac Eau (Ref Imp) (1)

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E L S E V I E R Desalination 168 (2004) 1 37-144DESALINATION

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S o l a r c o o l i n g w i t h th e a b s o r p t io n p r i n c ip l e :F ir st a n d S e c o n d L a w a n a ly s is o f a n a m m o n i a - w a t e rd o u b l e - g e n e r a t o r a b s o r p t i o n c h i l l e rN. B en E zz ine* , M . Barhoumi , Kh . Mejbri , S . Chemkhi , A . B e l lag i

Unit6 de Re cher che Therm ique et Thermodyn amique des Proc6d6s Industriels, D ~partement E nerg6tique,Ecole Na tionale d ' Ing6nieurs de Monastir , lb n E l Jazzar 50 19 Monastir , TunisieTel . +216 (97) 256585; Fax +216 (73) 500514 ; email: n_benezzine@yahoofr

Received 16 February 2004; accepted 24 February 2004

A b s t r a c tThis article deals with the modelling, therm odyna mic simulation and Secon d Law analysis o f an am mo nia-w aterdouble-effect, do uble-gen erator absorption chiller. T he analysis of the unit established a simulation thermo dynam ic

mo del as well as the limits o f the operating conditions. Com puter simulation was carried out in orde r to determine itsstream properties an d the am ount o f heat and w ork exchan ged with the surroundings. Simulation results w ere then usedto study the influ ence of the various operating parameters on the pe rform ance coefficient of the chiller. To quantifythe irreversibility of each com pon ent of the chiller and to determine the potential for each co mp onen t to contribute toovera l l sys tem energy eff ic iency, he Seco nd Law analysis of thermodynamicswa s applied, Results indicated that theabsorber, solution hea t exchangers, and c onde nser have the most potential to improve chiller ene rgy efficiency.K e y w or ds : Solar cooling; Refrigeration; Double-effectabsorption chiller; Modeling; Simulation; Am mo nia-w ater;Exergy; Entropy; S econd Law

1 . I n t r o d u c t i o nT o d a y e n e r g y r e s e a r c h p u r s u e s t h e d o u b l e

g o a l o f t he r e s o l u t i o n o f b e t t e r e n e r g y m a n a g e -m e n t a n d r e s e a r c h f o r n e w s o u r c e s . I n d e e d , t h e*Corresponding author.

p o s s i bi l it y o f p r o d u c i n g c o l d b y d i r e c t u s e o f th ep r i m a r y e n e r g y , i n p a r t i c u l a r s o l a r a n d n a t u r a lg a s , c r e a t e d i n t e r e s t i n a b s o r p t i o n e q u i p m e n t f o ra i r -c o n d i t io n i n g . I n a d d i t i on , t h e i m p r o v e m e n t o ft h e p e r f o r m a n c e o f i n d u s t ri a l p r o c e s s e s i s a n o n -g o i n g o b j e c t i v e o f r e s e a r c h e r s .

Presen ted at the Eu roM ed 2004 conference on Desalination Strategies in South Mediterran ean Countries: Cooperationbetween Med iterranean Countries o f Eu rop e and the Southern Rim o f the Mediterranean. Spon sored by the Europ eanDesalination Society an d Off ice Natio nal de l 'Ea u Potable, Marrakech, Mo rocco, 3 0 M ay -2 June, 2004.0 0 1 1 - 9 1 6 4 / 0 4 / $ - See front matter 200 4 Elsevier B.V. A ll rights reserveddoi; 10.1016/j desa l.2004.06.179


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138 N . B e n E z z i n e e t al . / D e s a l i n a t i o n 1 6 8 ( 2 0 0 4 ) 1 3 7 - 1 4 4Absorpt ion ref rigera tors and heat pum ps have

been for a long t ime l im i ted to v ery speci f ic andmarginal uses ( regenera tion , engine er ing dev i ces )[ 1 because o f t he i r low p e r fo rmance coe f f i c i en t(PC ) compared t o t ha t o f vapor compres s i onequ i pmen t . O ve r t he l a s t f ew decades , how ever ,the f ie ld has ex per ienc ed a resurgence o f in teres tw h i ch i s l a rge l y marke t -d r i ven by l ow -grade hea tor solar energy. Unfor tunate ly , s imp le cycles areno t ab l e t o p roduce PC s neces sa ry fo r compe-t it ion w i t h vapor co mpres s i on cyc l e s even w henthe d i f ferent ia l cos t for he at and e lec t r ic pow er i scons idered [2] . This means tha t the energyadvantage o f th i s k ind o f chi l le rs remains ins igni -f icant unless mo di f ied s truc tures are develop ed.This i s why m ost s tudies on absorpt ion chi l l e rswere concerned wi th mul t ip le-ef fec t s t ruc turesdes i gned by com bi n i ng s ing l e - st age componen t sand c ycles [2,3] .

The p r esen t w ork i s pa r t o f ou r r e sea r ch onabsorpt ion chi l l ing and so lar ref r igera t ion [4-6] .I t is conc erne d with a part icular s t ructure: thedouble-ef fec t , dou ble-gen era tor absorpt ion chi l -l e r , opera t ing wi th an ammonia-water mixture .The com mo n w ay t o spec i fy t he ope ra t ing ene rgyef f ic iency of an absorpt ion chi l l e r i s to pro videt he PC, de t e rmi ned by t he F i r st Law o f The rmo -dynami cs. Th i s pa r ame t e r, how ever , makes nore f e r ence t o t he bes t pos s i b l e pe r fo rmance o rg i ves any i n fo rma t i on r ega rd i ng w here i ne f f i -c iencies occur in the chi l le r . Also , i t cann ot beused t o d e t e rmi ne t he po t en t ia l f o r each comp o-nent of the chi l l e r to cont r ibute to the overa l le f f ic i ency o f t he sys t em. The Second Law ana ly -s i s addresses the ene rgy balance and the ent ropybalance for the sys tem. I t can be us ed to analyzeent ropy genera t ion and the i r revers ib i l i ty (orexe rgy des t ruc t i on ) o f each componen t , andprov i des much m ore i n fo rma t i on than a F i rs t Lawanalysis [7-9] .Th i s w o rk app l i ed t he rmo dynam i c ana l ysi s,inc luding Fi r s t and S econ d Law analyses , to anam mo ni a -w a t e r doub l e -e f f ec t, doub l e -gene ra to r

absorpt ion chi l l e r . The i r revers ib i l i ty of eachcom pon ent in the chi l l e r was quant i f ied and thepotent ia l of each component to cont r ibute tot he ove ra l l sys t em ' s ene rgy e f f i c i ency w asde t e rmi ned .2. C ycle description

Fi g . 1 show s schemat ica l ly an amm oni a -w a t e rdouble-effec t, do uble-gene ra tor absorpt ion chi l le rformin g an ex tens ion for the s ingle ef fec t cycle[4,5] . I t i s comp osed esse nt ia l ly o f two conden -sers; an evaporator; two steam generators , e a c hone provided wi th a boi ler and di s t i l l a t ioncolumn ; an absorber ; four expans ion valves ; twopump s and an ad j us tab l e t h ree -w ay va l ve. Thes team (3) , f lowing f rom the evapora tor to theabsorber , is absorbed b y the w eak solut ion (9) .The absorpt ion heat i s re jec ted towards theenv i ronm ent ( coo l ing w a t e r o r a i r) w h i ch a l soreceives energy re leased by the f i rs t condenser .The s t rong solut ion (4) i s d i s t r ibuted by anad j us t ab l e t h r ee -w ay va l ve be t w een t he t w ostages of the uni t , each one w i th a genera tor , acondenser and a solut ion heat exchanger . Thefract ion t~ o f s t rong so lut ion (4) is se nt towardsthe f i r s t genera tor . Evap ora t ion o f the ref rigeranti s per formed a t two tempera ture levels : theene rgy need ed i n t he f ir s t bo i le r i s supp l i ed by t hesecond condense r (C ond2) . La rge t empera t u r ed i f fe r ences be t w een hea t exchang i ng s tr eams a rethus avoided. The su m ref r igerants thus formedand accum ul a t ed in t he f ir st condense r (C on d l )are forwarded to the evapora tor . The s t rongsolut ion (16) sup plying the s econ d genera tor is acold source o f d is t i ll a t ion colum n tha t de terminesthe pur i ty l imi t o f the ref r igerant (23) , l eaving thelas t s tage of th i s column. This solut ion i s seeninsuf f ic ient to be a ra ther pu re ref rigerant. T o th i send an in ternal rec t i fi e r was incorpora ted in thechi l l e r . For a ra t ional use of energy, threecoun t e r- cu r ren t hea t exchange r s (H EX 1, H EX 2,H EX 3) w ere used .


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N. Ben Ezzine et al. / Desalination 168 (2004) 137-144 1392O

D i stil la tio n ~ _ ~ _ _ _ ~olumn L I5 ~ 1 4

21 ' Qco~a 1 7> > ~ _ _E V 3


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140 N. Ben Ezzine et al . /Desal inat ion 168 (2004) 137-144. Energy balance:

Entropy balance:OKSge.,~--E "LS.-E "~:,- rK

Irreversibility:

(2)

(3)

/= TOSgen,K (4 )

where QK is the heat added to c omponen t K attemperature TK, Tx is the entropic average temp-erature at which heat flows across the compo nent,K [7,11 ] and Sge,,K is the entropy generation ofeach component K of the chiller.

4.2. Cycle analysisThe PC of the chiller is defin ed as

with the same h eat sinks (environ ment, air). Theirentropic average temperatures are fixed equal tothe en vi ro nm en t tem per atu re (Tco,d = TAb~). Asecond boiler exchange s its heat at varying temp-eratures. Calculation o f the exac t entropic averagetemperature for this desorbing process is com-plex, and was therefore estimated as its hightemp erature of a we ak solution. TEvap was esti-mated as the numerical average of the enteringand exiting temperatures. The error introduced bythis estimation is minimal, as demonstrated byAle feld [7,11 ].

When Eq. (7) is multiplied by TAb~and sub-tracted from Eq. (6), the f ollowin g equation isproduced:

Qs2- TAb~_TEva s2 Ts2 )

rAb~- rEv"p Q.2 )P C - Qcv~p (5)Q~2For the chiller, direct Second L aw analysis beginsby applying the First and Second Law to theentire system, with on ly heat crossing the chiller,as shown in Fig. 1.

(6 )

/ (,2-,bs)Evap is th e PCrev for athermally-driven refrigerator operating amongthree temperature reservoirs. Eq. (8) may thus berewritten as follows:PC = PC re v - ~ PCdeg radiation,K (9)Kwhere

OEv., + + 0cod, 0Abs+ - - E ge.,K (7) (10)

rE vap , TB2 TCond 1 and TAb are defined as theentropic average temperatures at which heatflows across the define d com pone nt [7,1 ! ]. Thefirst condenser and the absorber exchange heat

Eq. (10) shows how much the entropy gene-ration of each component K of the chiller de-grades the reversible PC to the actual irreversiblePC.


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Se con d La w ef f i c i en cy 1"1 i s de f ine d as th er a ti o o f the a c t ua l P C t o t h e m a x i m u m p o s s ib l e( r ever s ib le) P C u n d e r t h e s a m e o p e r a t i n g c o n -di t ions :

P C (11)r I - P C r e v

5 . R e s u lt s o f s i m u l a t i o nPE (bar )

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . r . . . . . . . . . . . . . . . . . i . . . . . .

3.1 3.6 4.1

M a s s a n d e n e r g y b a l a n c e e q u a t i o n s a n d t h ev a r io u s c o m p l e m e n t a r y r e la t io n s c o n s t i tu t e t h es i m u l a ti o n m o d e l o f t h e c h i ll e r. T o s o l v e t h e l a r g es e t o f n o n l i n e a r e q u a t io n s s i m u l t a n e o u s l y i n t h ec h i ll e r t h e r m o d y n a m i c m o d e l , t h e C o n i e s a l g o -r i th m a v a i la b l e a s a F O R T R A N 7 7 c o d e [ 1 2] w a su s e d . C o m p u t e r s i m u l a t i o n w a s c a r r i e d o u t i no r d e r t o d e t e r m i n e t h e v a r i o u s s t r e a m p r o p e r ti e sa n d t h e a m o u n t o f h e a t a n d w o r k e x c h a n g e d b ym a i n e q u i p m e n t s o f t h e c h i l le r .

Tab l e 1 p r e s en t s t he e s s en t i a l re s u l t s o f a s t udyw h e r e w e r e q u i re d a c o o l i n g e f fe c t o f 1 7.5 k W a t1 2 - 7 C . T h e p i n c h e s o f a ll h e a t e x c h a n g e r s a r eequa l to 5 C , t he abs o r be r an d t he f i rs t con den s e rt e m p e r a t u r e i s 4 0 C a n d t h e t w o s u b - c o o l i n g a rebo t h equa l t o 3 C .

5 .1 . E f f e c t o f th e e v a p o r a to r p r e s s u r eF i g . 2 s h o w s t h e e v o l u t i o n o f th e c h i l l e r 's P C

w i t h t h e e v a p o r a t o r p r e s s u r e . W e n o t e t h e e x i s -t e n c e o f a m a x i m u m v a l u e o f r o u g h l y 0 . 7 6 1c o r r e s p o n d i n g to a p r e s s u r e o f 3 .5 b a r . S u c hb e h a v i o r, e n c o u n t e r e d a l s o i n t h e c a s e o f s im p l ee f f ec t ch il l e rs [ 4 ] , i s due t o t he e x i s t ence o f t w oo p p o s i t e e ff e c ts . W h e n t h e e v a p o r a t o r p r e s s u rei nc r eases , the eva por a t i on t em per a t u r e inc r eas est o o. T h e t e m p e r a t u r e T2 be i n g f i xed , t he ave r agee v a p o r a t i o n te m p e r a t u r e i n c r e a s e s a n d i n c re a s e st he ch i l l e r ' s P C . Fi g . 3 s h ow s t ha t the r e f r i ge r an tf l ow ra t e i nc r eas es w i t h t he ev apo r a t o r p r e s s u r e( w h i c h h a s a p o s i t i v e e f fe c t o n t h e a m o u n t o f h e a tt o o f f e r t o t he bo i l e rs ) and t h us t he P C dec r eas es .

0.79

0.77

0.75

0.73

0.71

0.69

0.67

0.652. 6

C O PN . B e n E z z i n e e t a l. / D e s a l i n a t i o n 1 6 8 ( 2 0 0 4 ) 1 3 7 - 1 4 4 1 4 1

Fig. 2. Performance coefficient (COP) vs. evaporatorpressure (Pc).1.81. 71. 61. 51. 41. 31. 21.11

0 .90 .8

2 .5

m f ( m o l / s )

P E ( b a r )3 3 .5 4 4 .5

Fig. 3. T otal refrigerant flowrate vs. e vaporator pressure(PE).5 .2 . E f fec t o f the par t i t ion coe f f ic ien t , tr , be twe eng e n e r a to r s

Fi g . 4 s how s t he P C of t he ch i l l e r as a f unc -t i on o f t he pa r t i t i on coe f f i c i en t , a , f o r va r iousevapor a t o r p r e s s u r e va l ues , PE . To r e s pec t t heS e c o n d L a w o f T h e r m o d y n a m i c s , th is c o e ff ic i en tt h e n h a s a l o w e r l i m i t , w h i c h c o r r e s p o n d s t o av a l u e f r o m w h i c h t h e t e m p e r a t u r e T7 d r i v i ng t hef i r s t bo i l e r exceeds t he s t eam t em per a t u r e T2o


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142 N. Ben Ezzine et al. / Desalination 168 (2004) 13 7-1 44Table 1Results of analysis of the First and Se cond La w f or the chillerComponent First Law ~ Second Law b

Q, kW St~,,Ic,W/K ] , W PCa~,,adi=io,,r ]/]to~, %Evapo rator 17.5 0.461 141.982 0.040 3.076HEX 1 2.544 0.436 134.257 0.038 2.909EV 1 0.00 0.181 55.707 0.016 1.2071 t con den ser(Co nd 1) - 18.31 1.915 589.823 0.167 12.78Absorber - 25.925 3.854 1186.95 0.337 25.72EV2 0.00 0.165 50.971 0.014 1.104HE X2 30.311 2.591 797.98 0.226 17.288Mixer 0.00 0.168 51.863 0.015 1.124HEX 3 30.374 2.216 682.587 0.194 14.79EV3 0.00 0.454 139.929 0.039 3.0312nd boiler (B2) 22.9965 1.361 419.27 0.119 9.084Rec tifier 3.535 0.975 300.183 0.085 6.504Coupling (Cond 2-1 st boiler) 3.514 0.208 64.21 0.018 1.391aPCr~ = 2.292; P C = 0.761.bt) = 0.332.0.8 COP , 3.5bar 1 m~(mol/s)0.78 -- 3.8 bar 0.98

0.76 1~ 11 ~. ~ . ,~ ~ , ~ . . . ~ 0.96O.74 . _ 0.940.72 ~ 0.92

0.7 0.90.68 0.88 . . . . . ~(%)0.66 , o~(%) 5% 15% 25% 35, 45%

5% 15% 25% 35% 45% Fig. 5. Second stage refrigerant flow rate vs. partitionFig. 4. Perform ance coeffic ient (COP ) vs. partition coefficie nt ct.coefficien t 0c or different evap orator pressures (PE).

s u p p l y i n g t h e s e c o n d c o n d e n s e r ( C o n d 2 ) . T h ei n c r ea s e in c o e f f i c i e n t a i s a c c o m p a n i e d b y am o r e i m p o r t a n t f l o w o f r e f r i g e r a n t l e a v in g t h es eco n d g en e r a t o r , r ep r e s en t ed i n F i g . 5 , an d t h u sa m o r e s i g n if i c a n t a m o u n t o f h e a t to o f f e r t o t hes eco n d b o i l e r .

T h e i n c r ea s e i n t h e r e f r i g e r an t f l o w ( F i g . 5 ) isc o m p l e t e l y n o r m a l s i n c e a m o r e s i g n if i c a n tam o u n t o f s t r o n g s o l u t i o n t o d e s o r b i n t h e f i rs tg e n e r a t o r n e c e s s i ta t e s a m o r e s i g n if i c a n t a m o u n to f h ea t r e l e a s e d b y t h e s e c o n d c o n d e n s e r( C o n d 2 ) .


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N. Ben E zzine et al. / Desalination 168 (2004) 137-144 143o.81 COP [ I l ~ ~O'70~650730"69~ l ~S I " f~" ba0.61 I / 3.8 bar0.57 T17(K)0.53420 440 460 480 500Fig. 6. Performance coeff icient (COP) vs. chiller drivin gtemperature for dif ferent values ofP e.

given on the basis of the fundamental data. Simu-lation enabled the study of the influence of thevarious operating parameters on chiller perfor-mance . An ana lysis o f the Second Law ofThermodynamics was app l ied to quan t i fy theirrevers ibil i ty of each compo nent . Results indi-cate that the absorber, solution heat exchangers ,and condenser have the greates t potentia l toimprove the chiller's energy efficiency. Also theyindicate that focusin g on irrevers ibil i ty is a moredirect way of analyzing the potentia l forimp ro v in g th e e f f ic i e n cy o f a n a mm o n ia -wa te rabsorption chiller.

5.3. E ffect o f tempera ture 1"17driving the chillerThe evo lu t ion o f the P C as a function o f the

dr iv ing tempera ture fo r va r ious va lues o f PE,g iven by F ig . 6 , is monoton ous . W hen the coo l ingeffect QE is f ixed , the s team amou nt p rod uced bygenerators is calculated too. T hen the increase inthis temperature s tart ing from a certa in l imit ison ly used to overhea t the s team re fr ige rant . Thenthe P C becomes inc reas ing ly s low a f ter hav ing as ignificant evolution.

Fur ther examin ing the i r revers ib i li ty o f eachcomp onent o f the ch i l le r revea ls tha t the absorber,solution heat exchangers , f irs t condenser, andsecond bo i le r represen t mos t o f the to ta l i r re -vers ibil i ty of the chil ler (Table 1). For the idealprocess , the revers ible P C is 2 .292. Inclusion ofthe degrada t ions o f a ll the c omp onen ts resu l ts inan actual P C of 0 .761 . The absorber genera to rcon tr ibu tes the la rges t i r revers ib le de gr ad a t io n-0.337. Therefore , the absorber had the greates tpotentia l to impr ove the chil ler energy effic iency.

6. ConclusionsIn th is work we deve loped a the rmodynamic

s imula t ion mode l o f an anamonia -wate r double -effect, double-generator absorption chiller. Thel imits o f the va r ious op era t ing paramete rs a re

References[1] M. Duminil, Syst~mes ~ absorption, h adsorption etthermochimiques en vue de la climatisation, Revue

g6n6rale de thermique, 362 (1992) 19-25.[2] L.A.Shaefer and S.V. Shelton, Modelling and analy-

sis of the air cooled ammonia-water triple effectcycle, Georgia Institute of Technology, 1998.

[3] K. Nasri, Frigo-pompes/t absorption multi-6tag6esdehaute performance, Th6se, Institut National Poly-technique de Lorraine, 1997.[4] N. Ben Ezzine, A. Snoussi, M. Barhoumi and A.Bellagi, Simulation thermodynamique d'un clima-tiseur ~ absorption, Revue G6n6rale Froid, 1036(2003) 23-27.

[5] N. Ben Ezzine, A. Snoussi, M. Barhoumi and A.Bellagi, Optimisation du fonctionnement d'unclimatiseur ~ absorption, Revue G6n6rale Froid, 1038(2003) 15-19,[6] N. Ben Ezzine, Kh. Mejbri, A. Snoussi andA. Bellagi, Simulation thermodynamique desmachines frigorifiques ~ absorption op6rant aucouple ammoniac/eau: mod61isationet optimisation,Congr6s frangais de thermique SFT2003, Grenoble,2003.[7] L.A. Schaefer, A. Delano and S.V. Shelton, SecondLaw Study of the Einstein Refrigeration Cycle,GWW School of Mechanical Engineering, GeorgiaInstitute of Techno logy, 1999.[8] T.J. Kotas, The Exergy Method of Thermal PlantAnalysis, Lavoisier, Paris, 1987.


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144 17. Ben Ezzine et aL / Desalination 168 (2004) 1 37-1 44[9] J. Jeong, K. Saito and S. Kawai, Optimum design

method for a single effect absorption refrigeratorbased on the First and S econd La w analysis, 21 st IIRInternational Congress o f Refrigeration, Washington,DC, 2003.

[10] J.L. Cauchepin, Le s pompe s h ehaleur h absorption:recherches, drveloppement et perspectives. PYC,1983.

[11] K.E Herold, R. Radermaeher and S.A. Klein,Absorption Chillers and Heat Pumps, CRC Press,Boca Raton, 1996.

[ 12] M. Shacham, A Fortran subrouti ne for the numericalsolution of system s of nonlinear equations, with andwitho ut constraints, Intern. J. Num erical Metho dsEngn., 23 (1986) 1455-1481.

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Double Generator Abs Ammoniac Eau (Ref Imp) (1)