Outdoor air state that may cause moisture condensation in membrane-type total heat exchangers using different membrane materials

Shen, Z. J.; Min, J. C. A icle Ou doo ai s a e ha may cause mois u e condensa ion in memb ane- ype o al hea exchange s using di e en memb ane ma e ials Ene gy Repo s P o ided in Coope a ion wi h: Else ie Sugges ed Ci a ion: Shen, Z. J.; Min, J. C. (2020) : Ou doo ai s a e ha may cause mois u e condensa ion in memb ane- ype o al hea exchange s using di e en memb ane ma e ials, Ene gy Repo s, ISSN 2352-4847, Else ie , Ams e dam, Vol. 6, Iss. 1, pp. 227-233, h ps://doi.o g/10.1016/j.egy .2019.08.049 This Ve sion is a ailable a : h ps://hdl.handle.ne /10419/243737 S anda d-Nu zungsbedingungen: Die Dokumen e au EconS o dü en zu eigenen wissenscha lichen Zwecken und zum P i a geb auch gespeiche und kopie we den. Sie dü en die Dokumen e nich ü ö en liche ode komme zielle Zwecke e iel äl igen, ö en lich auss ellen, ö en lich zugänglich machen, e eiben ode ande wei ig nu zen. So e n die Ve asse die Dokumen e un e Open-Con en -Lizenzen (insbesonde e CC-Lizenzen) zu Ve ügung ges ell haben soll en, gel en abweichend on diesen Nu zungsbedingungen die in de do genann en Lizenz gewäh en Nu zungs ech e. Te ms o use: Documen s in EconS o may be sa ed and copied o you pe sonal and schola ly pu poses. You a e no o copy documen s o public o comme cial pu poses, o exhibi he documen s publicly, o make hem publicly a ailable on he in e ne , o o dis ibu e o o he wise use he documen s in public. I he documen s ha e been made a ailable unde an Open Con en Licence (especially C ea i e Commons Licences), you may exe cise u he usage igh s as speci ied in he indica ed licence. h ps://c ea i ecommons.o g/licenses/by-nc-nd/4.0/ A ailable online a www.sciencedi ec .com ScienceDi ec Ene gy Repo s 6 (2020) 227–233 www.else ie .com/loca e/egy 6 h In e na ional Con e ence on Ene gy and En i onmen Resea ch, ICEER 2019, 22–25 July, Uni e si y o A ei o, Po ugal Ou doo ai s a e ha may cause mois u e condensa ion in memb ane- ype o al hea exchange s using di e en memb ane ma e ials Z.J. Shen, J.C. Min∗ Depa men o Enginee ing Mechanics, Tsinghua Uni e si y, Beijing 100084, China Recei ed 5 Augus 2019; accep ed 25 Augus 2019 Abs ac To al hea exchange (THX) is an ai - o-ai hea exchange wi h a hea /mois u e exchanging co e made o wa e apo pe meable memb ane, i is used o educe he building ene gy consump ion by eco e ing bo h hea and mois u e om en ila ion ai . In ho and humid clima e, he memb ane su ace may ge a empe a u e below he dew poin o supply ou doo ai , causing mois u e condensa ion a he memb ane su ace, which may deg ade he exchange pe o mance and inc ease he heal h isk. In his esea ch, a ma hema ical model is buil o desc ibe he hea and mois u e ans e in he co e o a THX, he e ec s o memb ane p ope ies on mois u e condensa ion a e analyzed, and he c i ical ou doo ai s a es o occu ence o he mois u e condensa ion a e calcula ed. The esul s show ha la ge memb ane adso p ion cons an and la ge maximum mois u e up ake may es ain he mois u e condensa ion, which occu s mo e easily a he in e sec ion o he supply and exhaus ai s eams. The op imal memb ane p ope ies a e ecommended based on a comp ehensi e conside a ion o he sa e y and economy. c 2019 Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/). Pee - e iew unde esponsibili y o he scien i ic commi ee o he 6 h In e na ional Con e ence on Ene gy and En i onmen Resea ch, ICEER, 2019. Keywo ds: To al hea exchange ; Memb ane p ope y; Mois u e condensa ion; Ou doo ai s a e 1. In oduc ion The building ene gy consump ion accoun s o app oxima ely 40% o he global ene gy consump ion, as epo ed by Ome [1]. Wi h he inc easing conce n o indoo ai quali y and ene gy sa ing, he memb ane- ype o al hea exchange (THX), which has a hea /mois u e exchanging co e made o wa e apo pe meable memb ane ha ans e s bo h hea and mois u e, is o en employed o educe he building ene gy consump ion associa ed wi h he o ced en ila ion. The co e con ains nume ous al e na ely s acked la channels con ined by memb ane shee s, supply ou doo ai and exhaus indoo ai low h ough he channels and exchange hea and mois u e ac oss he memb anes. Min and Duan [2,3] compa ed ou di e en me hods o e alua e he pe o mance o a THX, and ∗Co esponding au ho . E-mail add ess: [email p o ec ed] (J.C. Min). h ps://doi.o g/10.1016/j.egy .2019.08.049 2352-4847/ c 2019 Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/ licenses/by-nc-nd/4.0/). Pee - e iew unde esponsibili y o he scien i ic commi ee o he 6 h In e na ional Con e ence on Ene gy and En i onmen Resea ch, ICEER, 2019. 228 Z.J. Shen and J.C. Min / Ene gy Repo s 6 (2020) 227–233 Nomencla u e Cabso p ion cons an o memb ane cpspeci ic hea o ai (J kg−1K−1) Dwm mois u e di usi i y in memb ane (kg m−1s−1) Hen halpy (kJ kg−1) hcon ec i e hea ans e coe icien (W m−2K−1) dchannel heigh (m) Jmass lux (kg m−2s−1) kcon ec i e mass ans e coe icien (kg m−2s−1) Lwadso p ion hea (J kg−1) Le Lewis numbe m ai mass low a e (kg s−1) mmin minimum o supply and exhaus ai mass low a e (kg s−1) Nu Nussel numbe Nnumbe o channels q o al hea lux h ough memb ane (W m−2) Tabsolu e empe a u e (K) Whumidi y a io (kg kg−1) wmax maximum mois u e up ake (kg/kg) xFchannel leng h in xdi ec ion (m) yFchannel leng h in ydi ec ion (m) G eek symbols δ hickness o memb ane (m) εe ec i eness θmois u e con en o memb ane su ace (kg kg−1) λhea conduc i i y (W m−1K−1) φ ela i e humidi y Subsc ip s e exhaus ai H o al e ec i eness L la en e ec i eness m memb ane S sensible e ec i eness s supply ai sa sa u a ion poin ed ou ha he nume ical me hod wi h conside a ion o he adso p ion hea was he bes me hod o calcula e such pe o mance, especially when he hea and mois u e ans e in di e en di ec ions. Al-Waked e al. [4] in es iga ed he he mal pe o mance enhancemen o memb ane-based ene gy eco e y en ila o s o di e en numbe s o low channels, low con igu a ions, ai low a es, and wea he condi ions. Lee e al. [5] unde ook expe imen al in es iga ions on he cha ac e is ics o mois u e ans e wi h a THX co e made o di e en pape memb anes. Enga ne is e al. [6] de eloped a heo e ical model o cu en gene a ion asymme ic composi e memb anes used in he THX. On a ho and humid day in he summe , he supply ou doo ai has a high ela i e humidi y, so he memb ane su ace in con ac wi h i is ap o ge a empe a u e below i s dew poin , causing mois u e condensa ion a he Z.J. Shen and J.C. Min / Ene gy Repo s 6 (2020) 227–233 229 memb ane su ace. The condensa e wa e may we he memb ane and change i s p ope y, leading o a deg aded THX pe o mance. Ra a i Nas e al. [7] s a e ha he p oblems caused by he mois u e condensa ion in a THX may include: (1) inc eases in he p essu e loss and o e all hea /mois u e ans e esis ances, and (2) inc ease in he heal h isk. So, i is necessa y o ake measu es o es ain o e en elimina e he possible occu ence o mois u e condensa ion. In his esea ch, a ma hema ical model is buil o desc ibe he hea and mois u e ans e in he co e o a THX, and he e ec s o he memb ane p ope ies including he memb ane adso p ion cons an and maximum mois u e up ake on he mois u e condensa ion a e in es iga ed and analyzed. The c i ical ou doo ai s a es o he occu ence o mois u e condensa ion a e calcula ed and summa ized o di e en memb ane p ope ies. The ela i e humidi y con ou s a he memb ane su ace a e p esen ed, and he op imal memb ane p ope ies a e ecommended based on a comp ehensi e conside a ion o he sa e y and economy. 2. Theo e ical model Fig. 1 shows he basic model o he co e o a ypical THX. Because o he pe iodici y and symme y in geome y, hal he olume o he exhaus ai s eam, hal he olume o he supply ai s eam and in e media e memb ane a e aken o cons i u e he compu a ional domain. Fig. 1. Basic model. The model assumes ha he physical p ope ies o he ai luid and memb ane a e cons an , and he hea and mois u e ans e s a e bo h a s eady s a e. Supply ai : mscp N yF ∂Ts ∂x+2hs(Ts−Tms)=0,ms N yF ∂Ws ∂x+2ks(Ws−Wms)=0 (1) Exhaus ai : mecp N xF ∂Te ∂y+2he(Te−Tme)=0,me N xF ∂We ∂y+2ke(We−Wme)=0 (2) Memb ane: q= −λm ∂Tm ∂z=λm Tms −Tme δ,J= −Dwm ∂θm ∂z=Dwm θms −θme δ(3) u he , q=hs(Ts−Tms)+J Lw=he(Te−Tme)+J Lw,J=ks(Ws−Wms)=ke(We−Wme)(4) whe e mis he ai mass low a e, Wis he humidi y a io, hand ka e he con ec i e hea and mass ans e coe icien s, qand Ja e he hea and mois u e luxes ac oss memb ane, Nis he numbe o channels, xFand yF a e he channel leng hs in he xand ydi ec ions, λmis he memb ane he mal conduc i i y, Dwm is he mois u e 230 Z.J. Shen and J.C. Min / Ene gy Repo s 6 (2020) 227–233 di usi i y in memb ane, θis he memb ane mois u e con en , δis he memb ane hickness, and Lwis he hea o adso p ion o wa e apo on memb ane, which is assumed o be equal o he hea o apo iza ion o wa e . The subsc ip s s, e, m and w e e o he supply ai , exhaus ai , memb ane and wa e apo , espec i ely. The equa ion desc ibing he mois u e adso p ion cha ac e is ics o wa e apo a he memb ane su ace can be ep esen ed by Inc ope a [8] θ=wmax 1−C+C/φ (5) whe e φis he ai ela i e humidi y, wmax is he maximum mois u e con en o he memb ane ma e ial, and Cis he memb ane adso p ion cons an , which de e mines he shape o he adso p ion cu e. Eq. (5) is widely used o desc ibe he mois u e adso p ion cha ac e is ics o he memb ane ma e ials used in a THX, as in Niu & Zhang [9], Min & Su [10–12], and Min & Duan [2,3]. The con ec i e hea and mass ans e coe icien s be ween he ai and memb ane a e gi en by h=hs=he=Nuλ 2d(6a) k=ks=ke=h cpLe2/3(6b) whe e dis he channel heigh , Nu is he Nussel numbe , which has a alue o 7.54 o ully de eloped lamina low be ween pa allel pla es wi h a cons an empe a u e bounda y condi ion [13], and Le is he Lewis numbe , which is app oxima ely 0.85 o a empe a u e ange o 0–40 oC [14]. The THX pe o mance is e alua ed using he sensible, la en and o al e ec i enesses, gi en by εS=ms(Tsi −Tso)+me(Teo −Tei) 2mmin (Tsi −Tei)(7a) εL=ms(Wsi −Wso)+me(Weo −Wei) 2mmin (Wsi −Wei)(7b) εH=ms(Hsi −Hso)+me(Heo −Hei) 2mmin (Hsi −Hei)(7c) The equa ion o judging he c i ical ai s a e o no occu ence o mois u e condensa ion is as below: Wms <Wsa (8) whe e Wms is he humidi y a io o supply ai , and Wsa is he sa u a ion humidi y a io a he memb ane su ace, which co esponds o a ela i e humidi y o 100%. 3. Resul s and discussion The model has al eady been alida ed in ou p e ious esea ches (e.g., Min e al. [15], Min & Duan [2]). The alues aken o he THX co e dimensions and memb ane pa ame e s a e p esen ed in Table 1. The indoo ai empe a u e and ela i e humidi y a e se as 26 oC and 50%, while he ai eloci y is se as 2 m/s, which co esponds o an ai mass low a e o 0.2 kg/s. Table 1. THX co e dimensions and memb ane pa ame e s. THX co e dimensions Memb ane pa ame e s xF,yF[m] 0.25 λm[W/m K] 0.1 d[mm] 2 Dwm [10−7kg/m s] 0.1 N180 C0.1–10 δ[mm] 0.1 wmax [kg/kg] 0.15–0.35 Fig. 2 illus a es he c i ical ou doo ai s a es o he occu ence o mois u e condensa ion o memb ane ma e ials wi h di e en adso p ion cons an s (C). No mois u e condensa ion akes place i he ou doo ai s a e alls in he lowe -le egion in Fig. 2a, which is con ined by he cu e o each adso p ion cons an . All cu es a e almos linea and end o shi owa ds he uppe - igh co ne when he adso p ion cons an is inc eased, implying ha a la ge Z.J. Shen and J.C. Min / Ene gy Repo s 6 (2020) 227–233 231 Fig. 2. C i ical ou doo ai s a es o di e en memb ane adso p ion cons an s. adso p ion cons an allows a highe empe a u e and ela i e humidi y a which no mois u e condensa ion akes place, so he memb ane ma e ial wi h a la ge adso p ion cons an has a be e an i-mois u e condensa ion ea u e. Fig. 2b depic s he c i ical empe a u es co esponding o 100% ela i e humidi y o di e en adso p ion cons an s in pa icula , i indica es ha he la ge he memb ane adso p ion cons an he highe he ou doo ai empe a u e o no occu ence o he mois u e condensa ion. Fig. 3 shows he a ia ions o a ious e ec i enesses wi h he memb ane adso p ion cons an . Wi h inc easing adso p ion cons an , he sensible e ec i eness changes sligh ly while he la en and o al e ec i enesses i s inc ease, a e expe iencing a slow change s age, u n o dec ease. The maximum appea s a an adso p ion cons an ha is la ge han bu e y close o he alue o uni y. Fig. 3. Va ious e ec i enesses o di e en memb ane adso p ion cons an s. Comp ehensi e conside a ion o Figs. 2 and 3 esul s sugges s ha C=2.5 would be a good choice o he memb ane adso p ion cons an , because he ele an memb ane yields no only a good an i-condensa ion pe o mance bu also a high e ec i eness. Figs. 4 and 5a e simila o Figs. 2 and 3, hey a e o he maximum mois u e up ake o he memb ane. The esul s in Fig. 4 look e y simila o hose in Fig. 2, a la ge maximum mois u e up ake yields a highe ela i e humidi y and empe a u e a which no mois u e condensa ion akes place, meaning ha he memb ane ma e ial wi h a la ge maximum mois u e up ake has be e an i-condensa ion pe o mance. Fig. 5 shows ha , as he maximum mois u e up ake inc eases he sensible e ec i eness dec eases e y sligh ly while he la en and o al e ec i enesses inc ease signi ican ly, so he inc ease in he maximum mois u e up ake may con ibu e o he economic imp o emen in gene al. Comp ehensi e conside a ion o Figs. 4 and 5sugges s ha a la ge alue should be aken o he maximum mois u e up ake in o de o ge a be e e ec i eness pe o mance and less heal h isk. 232 Z.J. Shen and J.C. Min / Ene gy Repo s 6 (2020) 227–233 Fig. 4. C i ical ou doo ai s a es o di e en maximum mois u e up akes. Fig. 5. Va ious e ec i enesses o di e en maximum mois u e up akes. Fig. 6 illus a es he ela i e humidi y con ou s a he memb ane su ace o 35 oC ou doo ai empe a u e and 70% ou doo ai ela i e humidi y o wo di e en memb ane ma e ials. One memb ane has a p ope y o C=1 and wmax =0.25 kg/kg, called he no mal memb ane, and he o he has a p ope y o C=2.5 and wmax =0.35 kg/kg, called he ecommended memb ane. Fo bo h memb anes, he zone wi h a highe ela i e humidi y appea s in he lowe -le egion, which is he in e sec ion o he supply ou doo and exhaus indoo ai s eams, a which Fig. 6. Rela i e humidi y con ou s a he memb ane su ace. Z.J. Shen and J.C. Min / Ene gy Repo s 6 (2020) 227–233 233 he mois u e condensa ion occu s mo e easily because o he highe ela i e humidi y. Compa ison o Figs. 6a and 6b sugges s ha he ecommended memb ane has educed humidi ies as compa ed wi h he no mal memb ane, implying ha he mois u e condensa ion occu s mo e di icul ly a he ecommended memb ane su ace han a he no mal memb ane su ace, which may in e p e he eason why he memb ane wi h a la ge adso p ion cons an and a la ge maximum mois u e up ake ha e a be e an i-condensa ion pe o mance. Fu he , he e ec i enesses o he THX using he wo di e en memb ane ma e ials a e also calcula ed and he esul s show ha he ecommended memb ane yields almos he same sensible e ec i eness as he no mal memb ane bu gi es 3.7 pe cen age poin highe la en e ec i eness and 2.9 pe cen age poin highe o al e ec i eness han he no mal memb ane. 4. Conclusions •The ou doo ai ela i e humidi y and empe a u e anges o no occu ence o mois u e condensa ion o di e en memb ane p ope ies a e p o ided, i is ound ha he la ge he memb ane adso p ion cons an and maximum mois u e up ake he be e he an i-mois u e condensa ion pe o mance. •The memb ane ha has a mode a e adso p ion cons an and a la ge maximum mois u e up ake may yield no only a good an i-mois u e condensa ion pe o mance bu also a high e ec i eness pe o mance. The ecommended memb ane ha has a p ope y o C=2.5 and wmax =0.35 kg/kg can no only es ain he occu ence o mois u e condensa ion bu also gi e a good economy as compa ed o he no mal memb ane ha has a p ope y o C=1 and wmax =0.25 kg/kg. •The mois u e condensa ion akes place mo e easily a he in e sec ion o he supply and exhaus ai s eams because o he highe ela i e humidi y in ha egion. Acknowledgmen This s udy is unded by Beijing Municipal Na u al Science Founda ion (NO. 3182015). Re e ences [1] Ome AM. Ene gy, en i onmen and sus ainable de elopmen . Renew Sus ain Ene gy Re 2008;12(9):2265–300. [2] Min JC, Duan JF. Compa ison o a ious me hods o e alua ing he memb ane- ype o al hea exchange pe o mance. In J Hea Mass T ans e 2016;100:758–66. [3] Min JC, Duan JF. Memb ane- ype o al hea exchange pe o mance wi h hea and mois u e ans e ing in di e en di ec ions ac oss memb anes. Appl The m Eng 2015;91:1040–7. [4] Al-Waked R, Nasi MS. Enhancing he pe o mance o ene gy eco e y en ila o s. Ene gy Con e s Manage 2018;171:196–210. [5] Lee EJ, e al. Modeling and e i ica ion o hea and mois u e ans e in an en halpy exchange made o pape memb ane. In J Ai -Condi ion Re ig 2012;20(03):1–13. [6] Enga ne is A, Huizing R, G een S, Rogak S. Hea and mois u e ans e modeling in en halpy exchange s using asymme ic composi e memb anes. J Memb Sci 2018. S0376738817334658. [7] Ra a i Nas M, e al. A e iew o os ing in ai - o-ai ene gy exchange s. Renew Sus ain Ene gy Re 2014;30:538–54. [8] Inc ope a FP. 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[15] Min JC, Su M. Expe imen al and heo e ical in es iga ions o memb ane-based ene gy eco e y en ila o pe o mance. In J Ai -Condi ion Re ig 2012;20(1). 1150004.