Experimental and theoretical study of a humidification dehumidification water desalination system using solar energy

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Deslintion 168 (2004) 151 159 Experimentl nd theoreticl study of humidifiction dehumidifiction wter deslintion system using solr energy J. Orfi *, M. Lplnte b, H. Mrmouch, N. Glnis b, B. Benhmou c, S.

tn b Déprtement de Génie Mécnique, Université de Sherbrooke, Sherbrooke, Cnd Tel. +1 (819) 821-7144; Fx +1 (819) 821-7163; emil: Nicols.glnis@gme.usherb.

1 Deslintion 168 (2004) Experimentl nd theoreticl study of humidifiction dehumidifiction wter deslintion system using solr energy J. Orfi *, M. Lplnte b, H. Mrmouch, N. Glnis b, B. Benhmou c, S. Ben Nsrllh, C.T. Nguyen d Lbortoire des Etudes des Systèmes Thermiques et Energétiques, ENIM, Monstir, Tunisi Tel (73) ; Fx +216 (73) ; emil: Jmel.orfi@fsm.rnu.tn b Déprtement de Génie Mécnique, Université de Sherbrooke, Sherbrooke, Cnd Tel. +1 (819) ; Fx +1 (819) ; emil: Nicols.glnis@gme.usherb.c c LMFE, Déprtement de Physique, Fculté des Sciences Semlli, Mrrkech, Morocco Fx +212 (44) ; emil: bbenhmou@ucm.c.m d Déprtement de Génie Mécnique, Université de Moncton, Moncton, NB,Cnd Tel. +1 (506) ; Fx +1 (506) ; emil: nguyen@umoncton.c Received 16 Februry 2004; ccepted 25 Februry 2004 Abstrct This pper presents the chrcteristics of solr deslintion system bsed on the humidifiction dehumidifiction principle. The system which, cn be used in n open or closed cycle for ir, is modulr one. It hs the following independent components: two solr collectors, n evportor nd condenser. The first prt of the study dels with the presenttion of the designed nd constructed components. Some experimentl results re nlysed. They present generl picture of the evolution of the temperture nd the humidity between the entry nd the exit of the solr collector, the evportor nd the condenser. In the second prt, generl mthemticl model including the mechnism of het nd mss trnsfer in the vrious components of the present deslintion system ws developed. The theoreticl results show tht there exists n optimum mss flow rte rtio corresponding to mximum fresh wter product. Keywords: Solr deslintion; Humidifiction-dehumidifiction; Evportor; Modelling; Het nd mss trnsfer, Air-wter interfce *Corresponding uthor. Presented t the EuroMed 2004 conference on Deslintion Strtegies in South Mediterrnen Countries: Coopertion between Mediterrnen Countries of Europe nd the Southern Rim of the Mediterrnen. Sponsored by the Europen Deslintion Society nd Office Ntionl de l Eu Potble, Mrrkech, Morocco, 30 My 2 June, /04/$ See front mtter 2004 Elsevier B.V. All rights reserved

2 152 J. Orfi et l. / Deslintion 168 (2004) Introduction Solr deslintion with humidifiction dehumidifiction processes hs proven to be n efficient mens of production of fresh wter in remote nd sunny regions [1]. Numerous solr deslintion instlltions concerned with smll nd medium production hve been developed nd studied. Exmples of such instlltions include pssive nd ctive bsin type solr stills nd systems using humidifiction dehumidifiction principle. Goosen et l. [2] presented n overll review of recent developments in solr deslintion systems. They reported tht the solr stills, known to hve simple principle nd low operting costs, hve however low efficiency nd consequently low production of fresh wter (less thn 5 l/m 2 d). Systems bsed on the humidifiction dehumidifiction principle produce fresh wter t rtes higher thn those obtined from single solr stills under similr solr rdition [3]. The thermodynmic bckground for this process is clerly explined by using the psychrometric chrt [4,5]. Nwyseh et l.[3] constructed two different units in Jordn nd Mlysi. These units consist of two verticl ducts connected end to end to form closed loop for ir circultion. The uthors conducted simultion study in which the effects of vrious operting prmeters such s the ir nd wter flow rtes on the fresh wter production re nlysed. It ws shown tht the vrition of the dily production with the wter flow rte exhibits mximum. On the other hnd, the uthors lso developed pproprite correltions for het nd mss trnsfer coefficients in the humidifier s well s in the condenser. Al Hllj et l. [6] studied the performnce of closed ir cycle humidifiction-dehumidifiction process using solr energy. They reported tht the constructed units were lso cpble of producing lrge quntity of sline wrm wter for domestic use. It is observed tht the wter flow rte hs strong effect on unit production. Their experiments included results for night opertion which cn serve to keep the unit t elevted temperture. Recently, Chfik [4] presented procedure which consists of severl steps (stges) of ir heting nd humidifiction nd leds to high vpour concentrtion in the irflow. The present work dels with study of wter deslintion system by solr energy using the humidifiction dehumidifiction principle. The proposed system is modulr one nd hs the following independent components: two solr collectors, n evportor nd condenser. The study hs two prts. The first one concerns the presenttion of the chrcteristics of the designed nd constructed components of the deslintion system. Some experimentl results re nlysed. In the second prt, generl mthemticl model for the system is presented. It includes the models of the different components of the developed system from the governing het nd mss trnsfer equtions. 2. Presenttion of the studied system The studied solr deslintion system hs seprte evportor nd condenser. It comprises ir nd wter solr collectors, humidifier nd condenser. Fig. 1 presents the configurtion of this modulr system. Se wter which is preheted in the condenser nd heted in the solr collector enters in contct with heted ir strem in the evportive chmber. Becuse of the het nd mss exchnge between the hot wter nd the ir in the evportor, the ltter is loded by humidity. An efficient process in the humidifier is to obtin t its outlet sturted moist ir with high mount of vpour. The next opertion consists of condensing this vpour in the condenser cooled by the feed wter nd recupertes the fresh wter. This system offers the following principl chrcteristics: The ir cycle cn be open or closed one. The ltent het of condenstion is recovered nd used to prehet the feed wter. The moist ir t the outlet of the evportor cn be returned to the solr collector, heted

J. Orfi et l. / Deslintion 168 (2004) 151 159 153 Fig. 1. Schemtic digrm of the solr deslintion system. nd humidified two or more times. Thus the ir cn be chrged with n importnt mount of vpour.

Experimentl study The experimentl prt of this project concerns the design nd the construction of the principl components of the bove described solr deslintion system.

3 J. Orfi et l. / Deslintion 168 (2004) Fig. 1. Schemtic digrm of the solr deslintion system. nd humidified two or more times. Thus the ir cn be chrged with n importnt mount of vpour. The globl efficiency of the unit depends on the efficiency of ech component. Consequently, it is importnt to design nd construct performent components. 3. Experimentl study The experimentl prt of this project concerns the design nd the construction of the principl components of the bove described solr deslintion system. The constructed ir solr collector consists of single glss plte with 3 mm of thickness nd 2 m 2 of surfce (Fig. 2). The evportor which is horizontl nd hs rectngulr cross section is constructed with wood. The fibber glss is used for insultion (Fig. 2b). In order to improve the het nd mss exchnge, five prllel pltes mde with wood nd covered with textile (cotton) re fixed in the evportor. The heted ir enters in contct with the hot wter by different wys. The horizontl surfce of the evportor is covered by the hot wter. The verticl pltes re wetted by cpillrity nd finlly the wter is spryed by mens of tubes with smll holes set in them. These tubes re Fig. 2. Schemtic of the constructed solr collector. Fig. 2b. Schemtic of the evportor. plced on the verticl pltes. The feed wter nd the ir re counter current. In the experimentl set-up, the wter solr collector hs been replced by n electric wter heter. The condenser consists of chmber with rectngulr cross section. It contins two rows of long cylinders mde of copper in which the feed wter flows. Longitudinl fins were soldered to

4 154 J. Orfi et l. / Deslintion 168 (2004) the outer surfce of the cylinders. The condenser is chrcterized by n exchnge surfce, 1.5 m 2 nd 28 m s totl length. Vrious vribles chrcterizing the present problem were mesured. Thus, the temperture of wter nd ir nd the humidity t the inlet nd the outlet of ech component nd the wter nd ir mss flow rtes re mesured. The wind nd rdition dt were tken for the town of Monstir from [7]. Tble 1 gives n exmple of the experimentl results obtined for typicl dy of July t Monstir, in Tunisi. It presents generl picture of the evolution of the temperture nd the humidity between the entry nd the exit of the solr collector, the evportor nd the condenser. These results indicte the importnt effect of the feed wter temperture on the quntity ω 3 ω 4 which is proportionl to the produced fresh wter. Fig. 3 presents the time vrition of the humidifiction efficiency of the evportor, e w, for typicl dy of July in Monstir. This efficiency is defined s: e w ( w w ) w in in out w in (1) where w in nd w out re respectively the humidity of ir t the inlet nd the outlet of the evportor. w in is the sturtion humidity corresponding to the ctul humidifiction process. This definition Fig. 3. Time vrition of the efficiency of the evportor. of the efficiency, lso used by Chfik [4], is nlogous to tht used in the evportive coolers [8]. 4. Theoreticl study The elbortion of generl mthemticl model is bsed on the conservtion principles of mss nd energy in ech component of the deslintion unit. For the solr collectors, nlyticl solutions for the stedy stte regime giving the distribution of the tempertures of ir (or wter) long the collector were obtined [9,10]. In the evportor nd the condenser, we consider counter flow ir wter system. The Tble 1 Mesured vlues t the inlet nd the outlet of ech component of the system for July 21, 2003 in Monstir (ir mss flow rte kg/s; wter mss flow rte kg/s) Time T T 1 T 2 ϕ 1, % ω 1, g/kg T 2 T 3 T 7 T 8 ϕ 3, % ω 3, g/kg T 4 ω 4, g/kg ω 3 ω 4 8: : : : : : : :

5 J. Orfi et l. / Deslintion 168 (2004) One cn note tht the solution of Eq. (1) is strightforwrd: l ( ) + ( ω ω ) m x m m (7) 5 3 The boundry conditions corresponding to the bove equtions re: (), ( 0 ), (), ( 0) m l m ω ω T l T T T (8) l 5 2 l 7 2 Fig. 4. Sketch of the evportor. model cn lso be pplied for prllel flow with minor modifictions. The principl ssumptions used to obtin the mthemticl model for the humidifier re (see Fig. 4): The liquid nd gs flows re stedy stte nd one-dimensionl. A very thin lyer of sturted ir exists between the liquid nd the gs strems. This lyer is supposed t the temperture T f. The following equtions were derived to describe the het nd mss blnces in the evportor: d l m m dω (2) ( ) f dω m Um, evpev ω st _ t _ T ω (3) dtl ( ) U P T T mc lev, ev l f l pl ( ) ( +ω ) dt U P T T m C C ev, ev f pl pv (4) (5) dω m ivl Ul, evpev ( Tl Tf ) + U, evpev ( T Tf ) (6) To describe the het nd mss exchnges between the ir nd the feed wter which re not in contct direct in the condenser, equtions re obtined in similr mnner to tht of the evportor. dm c dω m (9) ir ( ) f dω m ir Um, cond Pcond ω st _ t _ T ω (10) dtl ( ) U P T T m C lcond, cond l f 5 pl, cond cond ( f ) ( +ω ) + dt U P T T m C C m C ir p pv c pc dω m ivl Ul, cond Pcond Tl Tf + U P T T ( ) ( ), cond cond f Eq. (9) gives: m m ( ω ω) c 3 (11) (12) (13) The bove equtions re subjected to the following boundry conditions: ( 0) 0, ( 0 ), ( 0 ), ( ) m ω ω T T T l T (14) c 3 3 l 5 The evlution of the different coefficients chrcterizing the het nd mss trnsfer in the

6 156 J. Orfi et l. / Deslintion 168 (2004) evportor nd the condenser is n importnt tsk in the simultion problem. Bsed on the experimentl results, the vlues of the het trnsfer coefficients re s follows: U,ev 11.2 W/m 2 K, U l,ev 640 W/m 2 K, U,cond 4360 W/m 2 K. The following expression ws used to evlute the mss trnsfer coefficient: U m U 2/3 ( Cp +ω Cpv ) Le where Le is the Lewis number. The ltent het of vporiztion ws evluted by using the Clpeyron reltion from tbulted For the open system, ω 1 ω mb T 1 T mb dt, found in [11], of the pressure nd temperture t the sturtion of the slt wter. A generl computer progrm including the mechnism of het nd mss trnsfer in the vrious components of the present deslintion system ws developed. Fig. 5 gives generl picture of the flow chrt of the computer model. In order to evlute the ccurcy nd the precision of this generl model, severl tests hve been performed. These tests were concerned for exmple with: The ppliction of the globl blnce of het nd mss trnsfer on ech component nd lso on the entire cycle. The comprison with results of Ibrhim et l. [12], who studied similr problem in tht of the evportor. For the closed system ω 1 ω 4, T 1 T 4 T 1, ω 1 T 2, ω 2 ω 1 Procedure for the ir solr collector Initilistion of T 6 Procedure for the wter solr collector Feed wter temperture T 8 T 5 T 7 Procedure for the evportor Procedure for the condenser T 3, ω 3 T 6 T 4, ω 4 Fig. 5. Flow chrt for the entire system. Fresh wter

J. Orfi et l. / Deslintion 168 (2004) 151 159 157 The cpcity of the model to predict esily the results corresponding to the limiting cses.

Lplnte [9] hs shown tht the most relevnt non dimensionl prmeters tht chrcterize the system re: m A A U N N N N N 5 c cl ev, ev 1, 2, 3, 4, m Aev Aev m Cp U A N ω ω, cond cond 5, 6 mcp 3 4 Fig.

7 J. Orfi et l. / Deslintion 168 (2004) The cpcity of the model to predict esily the results corresponding to the limiting cses. More detils concerning the vlidtion of the model re found in [9]. Lplnte [9] hs shown tht the most relevnt non dimensionl prmeters tht chrcterize the system re: m A A U N N N N N 5 c cl ev, ev 1, 2, 3, 4, m Aev Aev m Cp U A N ω ω, cond cond 5, 6 mcp 3 4 Fig. 6 shows the different sttes in the psychrometric chrt tht moist ir experiences respectively for the cses of closed system nd open one. These results re obtined for the vlues of 0.05, 0.05, 8 nd 24 respectively for the non-dimensionl prmeters N2, N3, N4 nd N5. The mss flow rte rtio, N1 tkes the vlues of 0.8, 1.6, 2.4 nd. The fresh wter production is obtined by A mens of the stte vrition of ir in the evportor nd in the condenser (i.e. ω 3 ω 4 ). One cn observe tht for cse of closed system, the ir is loded with n importnt mount of vpour t the outlet of the humidifier. These figures show tht the closed system is more efficient thn the open system. It is lso of interest to note the strong effect of the normlized liquid flow rte on the performnce of the system. For the limiting cse, corresponding to very lrge liquid mss flow rte, the wter temperture in nerly constnt in the circuit nd the potentil of evportion nd condenstion is nerly zero. Also, very low mss flow rte of the feed wter is not beneficil to increse the productivity of the system. Fig. 7 indictes the vrition of the quntity ω 3 ω 4 with the chnge of the normlized mss flow rte of the sewter. It shows tht there exists n optimum mss flow rte rtio corresponding to mximum fresh wter production. The mbient conditions lso hve effects on the performnce of the deslintion system. Fig. 8 Fig. 6. Evolution of the sttes of the humid ir in the open system (left) nd the closed system (right).

8 158 J. Orfi et l. / Deslintion 168 (2004) Fig. 7. Effect of the wter flow rte on the production rte of the fresh wter. demonstrtes the impct of the externl conditions on the fresh wter production nd on the optimum non-dimensionl liquid flow rte. It is observed tht the ltter vries from the vlue of 1.2 to lmost 2 for the different cses considered in this figure. 5. Conclusion The present work concerns n experimentl nd theoreticl study of wter deslintion system by solr energy. The system bsed on the humidifiction-dehumidifiction principle is modulr one nd hs the following independent components: two solr collectors, n evportor, nd condenser. The first prt of the study dels with the presenttion of the designed nd constructed components. Some experimentl results re nlysed. They show the evolutions of the temperture of ir nd wter nd the humidity t the inlet nd the outlet of ech component of the system. In the second prt, generl mthemticl model bsed on the het nd mss blnces for ir nd wter is presented. The theoreticl results show tht there exists n optimum mss flow rte rtio corresponding to mximum fresh wter product. Acknowledgment The finncil support of the AUF, Agence Universitire de l Frncophonie is cknowledged. Symbols A Surfce re, m 2 C p Specific het t constnt pressure, J/(kg.K) D Mss diffusion coefficient, m²/s e Humidifiction efficiency i Specific enthlpy, kj/kg I Solr insoltion rte, W/m² L Length, m Le Lewis number α/d m 5 /m C 20 C 30 C 40 C ω 3 -ω C 20 C 30 C 40 C IA/m i vl IA/m i vl Fig. 8. Effect of the mbient conditions on the optimum normlized wter flow rte nd on the fresh wter production.

9 J. Orfi et l. / Deslintion 168 (2004) l Width, m m Mss flow rte, kg/s P Wetted perimeter, m, pressure, P T Temperture, K, C U Het (or mss) trnsfer coefficient, W/m²K (or kg/m²s) V Wind velocity, m/s x Coordinte in the flow direction, m α Het diffusion coefficient, m²/s ω Specific humidity, kg vp /kg dryir Subscripts Air c Condenste c Air collector cl Wter collector cond Condenser ev Evportor f Vpor film g Mixture (ir+wter vpor) l Liquid m Mss st Sturtion v Vpor References [1] L.G. Rodriguez, Solr energy, 75 (2003) 381. [2] M.F.A. Goosen, S.S. Sblni, W.H. Shyy, C. Pton nd H.A. Al-Hini, Deslintion, 129 (2000) 63. [3] N.K. Nwyseh, M.M. Frid, A.Z. Omr nd A. Sbrin, Energy Conversion nd Mngement, 40 (1999) [4] E. Chfik, Deslintion, 153 (2002) 25. [5] Y.J. Di, R.Z. Wng nd H.F. Zhng, Deslintion, 142 (2002) 107. [6] S. Al Hllj, M.M. Frid nd A.R. Tmimi, Deslintion, 120 (1998) 273. [7] [8] ASHRAE Hndbook: Fundmentls, SI Edition, Atlnt, [9] M. Lplnte, Étude numérique et expérimentle d un distillteur d eu slée à énergie solire, M.Sc. Thesis, Université de Sherbrooke, Cnd, [10] J. Orfi, M. Mthieu, H. Mrmouch, N. Glnis, W. Dridi nd S. Ben Nsrlh, Proc. L gouvernnce de l eu et le développement durble, Sousse, Tunisi, 9 10 October [11] E. Deliginni nd V. Belestehis, Méthode et Systèmes de Desslement (Version originle grecque), Athènes, [12] G.A. Ibrhim, H.B. Nbhn nd M.Z. Anbtni, Int. J. Refrigertion,18 (8) (1995) 558.

Psychrometric Applications

Psychrometric Applications Psychrometric Applictions The reminder of this presenttion centers on systems involving moist ir. A condensed wter phse my lso be present in such systems. The term moist irrefers to mixture of dry ir nd