EXERGY ANALYSIS OF A DUAL-MODE REFRIGERATION SYSTEM FOR ICE STORAGE AIR CONDITIONING

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1 Intrnatinal Jurnal n Architctural Scinc, Vlum 6, Numbr 1, p.1-6, 005 EXERGY ANALYSIS OF A DUAL-MODE REFRIGERATION SYSTEM FOR ICE STORAGE AIR CONDITIONING Guiyin Fang*, Lin Xing*, Fan Yang* and Hui Li # *Dpartmnt f Physics, Nanjing Univrsity, Nanjing 10093, China # Dpartmnt f Matrial Scinc and Enginring, Nanjing Univrsity, Nanjing 10093, China Rcivd 6 August 004; Accptd 5 Octbr 004 ABSTRACT This papr intrducs th dual-md rfrigratin systm fr ic strag air cnditining. A cmputatinal mdl rlating t th xrgy lss amunt and lss rat is dvlpd in ach cmpnnt f th rfrigratin cycl lp. A cas study is prsntd t dmnstrat th applicability f th mdl dvlpd. Exrgy analysis rsults f th rfrigratin systm in ic strag prating md ar cmpard with that f th rfrigratin systm in air cnditining prating md. Th rsults shw that th xrgy fficincy f th rfrigratin systm in ic strag prating md is 17.63% lwr than that f th rfrigratin systm in air cnditining prating md. Th nrgy saving and xrgy lss rductin pssibilitis ar als analyzd and discussd. 1. INTRODUCTION In rcnt yars, a numbr f ic strag systms hav bn cnsidrd and dvlpd, such as ic harvstr, ncapsulatd ic, ic n cil, tc. Th applicatin f ic strag tchnlgy shifts daytim air cnditining lads t nighttim hurs, thus cutting csts fr lctric utilitis by shifting daytim pwr dmand t th mst fficint baslad gnrating plants. Ic strag systms hav th ptntial t sav building wnrs a substantial prtin f thir air cnditining csts. Ic thrmal nrgy strag uss th latnt hat f fusin f watr. Ic thrmal strag strs cling in th frm f ic at its frzing pint 0 C. T str this nrgy, th chillr must prat at tmpraturs wll blw th nrmal prating rang fr air cnditining applicatins rsulting in lwr cfficint f prfrmanc COP f th chillr. Exrgy fficincy is a vry ffctiv paramtr t valuat an actual thrmdynamic prcss and prfrmanc f rfrigratin systm. Exrgy analysis is a mthd that uss th cnsrvatin f mass and cnsrvatin f nrgy principls tgthr with th scnd law f thrmdynamics fr th dsign and analysis f nrgy systms. Exrgy analysis rsults can ffctivly rval th prfrmanc and imprving ptntial f ach cmpnnt in th rfrigratin systm. In rcnt yars, xrgy analysis has bn cnsidrd as th mr pwrful mthd fr prfrmanc valuatins and dsign calculatins f rfrigratin systms and thrmal nrgy strag systms. Yumrutas t al. [1] prsntd a cmputatinal mdl basd n th xrgy analysis fr th invstigatin f th ffcts f th vaprating and cndnsing tmpraturs n th prssur lsss, th xrgy lsss, th scnd law f fficincy, and th cfficint f prfrmanc COP f a vapr cmprssin rfrigratin cycl. Th rsults shwd that whn th scnd law fficincy and th COP incrass, th ttal xrgy lss dcrass with dcrasing tmpratur diffrnc btwn th vapratr and rfrigratd spac and btwn th cndnsr and utsid air. Stgu-Sagia and Paignigiannis [] dscribd irrvrsibilitis in n stag rfrigrating prcss fr vapr cmprssin cycl with rfrigrant mixturs R-404A, R-410A, R-410B and R-507 as wrking fluids. Dtaild infrmatin n th variatin f cycl s xrgy fficincy with vaprating and cndnsing tmpraturs was givn. Bilgn and Takahashi [3] carrid ut xrgy analysis f hat pump-air cnditinr systms. Th xrgy dstructins in varius cmpnnts wr dtrmind fr furthr study and imprvmnt f its prfrmanc. Dincr t al. [4] studid snsibl hat strag SHS systms and prfrmanc valuatin tchnius. In additin t nrgy and xrgy analysis, svral dfinitins f nrgy and xrgy fficincy fr th prfrmanc f SHS systms wr prvidd with an illustrativ xampl. El-Dssuky and Al-Juwayhl [5] cnductd scnd law analysis fr a phas-chang thrmal nrgy strag systm. Th ntrpy gnratin numbr was dtrmind as a functin f th Rynlds numbr f th prating fluid, spcific hat transfr ara, prating fluid inlt tmpraturs during th cyclic prcsss and th thrm-physical prprtis f bth th strag matrials and th hat transfr fluids. Dincr [6] prsntd an nrgy and xrgy analysis f thrmal nrgy strag systms and thir sub-prcsss fr 1

2 Intrnatinal Jurnal n Architctural Scinc systm dsign and ptimizatin. An illustrativ xampl was als givn t dmnstrat hw xrgy analysis prvidd a mr ralistic and maningful assssmnt than th cnvntinal nrgy analysis f th fficincy and prfrmanc f a thrmal nrgy strag systm. ic-watr frm th ic strag tank thrugh th discharging pump. Ic strag unit incrass cling capacity in situatins whr th chillr and lctrical srvic capacity is fully utilizd, and crrspndingly rducs prating csts. Th bjctiv f this wrk is t carry ut xrgy analysis n th dual-md rfrigratin systm fr ic strag air cnditining. Exrgy analysis rsults f th rfrigratin systm in ic strag pratin md ar cmpard with that f th rfrigratin systm in air cnditining pratin md. Th nrgy saving and xrgy lss rductin pssibilitis ar als analyzd and discussd.. DUAL-MODE REFRIGERATION SYSTEM FOR ICE STORAGE AIR CONDITIONING Fig. 1 prsnts a schmatic diagram f th dualmd rfrigratin systm. This systm is cmpsd f tw parts: th cnvntinal air cnditining pratin unit and th ic strag pratin unit. Th cnvntinal air cnditining pratin unit cnsists f a cmprssr, a cndnsr, a rcivr, an lctrmagntic valv A, an xpansin valv A, an vapratr A, a fan cil and a chilld watr pump. Th ic strag pratin unit cnsists f a cmprssr, a cndnsr, a rcivr, an lctrmagntic valv B, an xpansin valv B, an vapratr B, a ic strag tank, a hat xchangr, a charging pump, a discharging pump, a fan cil and a chilld watr pump. At nighttim f ff-pak lctricity dmand, th rfrigratin systm starts t prat in ic strag md. Th rfrigrant flws insid th fllwing cmpnnts: cmprssr, cndnsr, rcivr, lctrmagntic valv B, xpansin valv B and vapratr B whr it xchangs hat with th clant fluid 5% glycl slutin. Th clant fluid chilld is circulatd btwn vapratr B and th ic strag tank thrugh th charging pump, and watr in th ic strag tank changs phas frm liuid t slid. Th phas chang taks plac at th cnstant frzing tmpratur f watr, and th cling is strd in th ic strag tank. At daytim f n-pak lctricity dmand, th rfrigratin systm prats in cnvntinal air cnditining md. Th rfrigrant flws insid th fllwing cmpnnts: cmprssr, cndnsr, rcivr, lctrmagntic valv A, xpansin valv A and vapratr A whr it xchangs hat with th clant fluid chilld watr. Th chilld watr is circulatd btwn vapratr A and th fan cil thrugh th chilld watr pump and th hat xchangr whr it xchangs hat with th mlting 1 - cmprssr 9 - vapratr B - cndnsr 10 - ic strag tank 3 - rcivr 11 - charging pump 4 - lctrmagntic valv A 1 - discharging pump 5 - lctrmagntic valv B 13 - hat xchangr 6 - xpansin valv A 14 - chilld watr pump 7 - xpansin valv B 15 - fan cil 8 - vapratr A Fig. 1: Schmatic diagram f th dual-md rfrigratin systm 3. EXERGY ANALYSIS OF THE DUAL- MODE REFEIGERATION SYSTEM Th xrgy analysis prsntd in this wrk is basd n th fllwing assumptins: Th utr surfac f th rfrigratin systm tubs is wll insulatd, s th hat absrbd r rlasd f th tubs is nglctd. Th vaprating and cndnsing tmpraturs f th rfrigrant ar cnstant du t th prssur drps f th rfrigrant in th vapratr and cndnsr ar ngligibl. Fig. shws T-s diagram f th rfrigratin cycl. 1- rprsnts th suprhating prcss in th vapratr, -3 rprsnts th cmprssin prcss in th cmprssr, 3-5 rprsnts th cndnsing prcss in th cndnsr, 5-6 rprsnts th subcling prcss in th cndnsr, 6-7 rprsnts th thrttling prcss, and 7-1 rprsnts th vaprating prcss in th vapratr.

3 Intrnatinal Jurnal n Architctural Scinc whr is xrgy lss amunt f rfrigrant in th vapratr, and is xrgy incrmnt f th clant fluid in th vapratr. by: and may b writtn = = h h T s a s7 Fig. : T-s diagram f th rfrigratin cycl Th xrgy f th nthalpy f a prcss fluid.g. a rfrigrant is calculatd accrding t th first thrmdynamic law and th scnd thrmdynamic law: = h h T s s 1 a a a As th rfrigrant fluid changs frm stat 1 t stat, th xrgy variatin f th nthalpy f a prcss fluid is dtrmind by: = h h Ta s 1 1 s1 Exrgy f an amunt f hat may b dfind as: Ta = 1 3 T whr T a is th ambint tmpratur and T is th tmpratur at which is absrbd hat sink r rjctd by th nvirnmnt hat surc. Blw ambint tmpratur T < T a, th hat amunt will hav a ngativ sign, as it is rmvd frm th spac r substanc that is cld fr xampl a cl strag. Thus, always has a psitiv sign. If th tmpratur T f hat surc r hat sink changs frm T 1 t T, th xrgy f an amunt f hat may b xprssd by: Ta lnt1 lnt = [1 ] 4 T T 1 Using xrgy analysis mthd, th xrgy lsss ccurring in all fur cmpnnts f th rfrigratin cycl ar calculatd as fllws: 1 Th xrgy lss in th vapratr is: l, v = 5 T lnt lnt ] a fi f = [1 7 T fi T f Th xrgy lss in th cmprssr is: = T s 8 l, cp a 3 s 3 Th xrgy lss in th cndnsr is: l, cd a 3 s6 = = h h T s 9 4 Th xrgy lss in th xpansin valv is: l, x a 7 s6 = T s 10 5 Th ttal xrgy lss in th rfrigratin cycl is: = l l, v l, cp l, cd + l, x 6 Th xrgy fficincy f th rfrigratin systm is: l i =, η = = 1 1 η 1 i w w 4. CASE STUDY In this sctin, a dmnstratin f th xrgy analysis is prsntd. Th rfrigratin systm studid is a dual-md rfrigratin cycl prcss. This indicats that th rfrigratin systm prats nt nly in cnvntinal air cnditining md during daytim, but als in ic strag md during nighttim. Th rfrigrant usd in th rfrigratin systm is R134a, and th clant fluid in th ic strag tubs is 5% mass rati glycl slutin. Th subcling dgr f rfrigrant in th cndnsr is st at 5K, and th suprhating dgr f rfrigrant is st at 10K. Th isntrpic cmprssin fficincy f th cmprssr is ual t In cnvntinal air cnditining md during daytim, th ambint tmpratur is 308K. Th cling mdium is watr, s th cndnsing tmpratur f th rfrigrant in th cndnsr is st at 313K, and th vaprating tmpratur f th 3

4 Intrnatinal Jurnal n Architctural Scinc rfrigrant in th vapratr A is st at 78K. Taking accunt f th tmpratur diffrnc f hat transfr btwn th rfrigrant and th chilld watr in th vapratr A, th inlt tmpratur f th clant fluid chilld watr in th vapratr A is st at 85K, and utlt tmpratur f th clant fluid chilld watr in th vapratr A is st at 80K. In ic strag md during nighttim, th ambint tmpratur is 303K du t th ambint tmpratur in th nighttim is lwr than that in th daytim, th cndnsing tmpratur f th rfrigrant in th cndnsr is st at 308K du t th lwr ambint tmpratur, and th vaprating tmpratur f th rfrigrant in th vapratr B is st at 63K du t ic strag systm strs cling in th frm f ic at its frzing pint 0 C. Cnsidring th tmpratur diffrnc f hat transfr btwn th rfrigrant and th clant fluid 5% glycl slutin in th vapratr B, th inlt tmpratur f th clant fluid 5% glycl slutin in th vapratr B is st at 71K, and utlt tmpratur f th clant fluid in th vapratr B is st at 66K. Th prating paramtrs f th rfrigratin systm in tw mds ar listd in Tabl 1. Tabl 1: Th prating paramtrs f th rfrigratin systm in tw mds Paramtrs Air-cnditining Ic strag pratin md pratin md T a K T cd K T v K T f K T f K T sc K 5 5 T sh K RESULTS AND DISCUSSION Th xrgy analysis rsults f th rfrigratin systm with prating mds ar listd in Tabl. Th xrgy flw diagrams f th rfrigratin systm with tw prating mds ar shwn in Fig. 3. Frm Tabl and Fig. 3, it is fund that th xrgy fficincy f th rfrigratin systm in ic strag pratin md is 17.63% lwr than that f th rfrigratin systm in air cnditining pratin md. This is du t th lwr vaprating tmpratur in ic strag md rsults in th lwr xrgy fficincy f th rfrigratin systm. Th highr vaprating tmpratur can imprv th xrgy fficincy f th rfrigratin systm. Th highr th frzing tmpratur f phas chang matrials, th highr th vaprating tmpratur f th rfrigratin systm. Thus, sm mixturs f inrganic salt hydrats ar usually usd as cl strag matrials. Tabl : Th xrgy analysis rsults f th rfrigratin systm with tw prating mds Paramtrs Air-cnditining Ic strag pratin md pratin md 0 kjkg w kjkg COP l,v kjkg l,cp kjkg l,cd kjkg l,x kjkg l kjkg η 1 = l,v /w % η = l,cp /w % η 3 = l,cd /w % η 4 = l,x /w % η = 1 - Ση i % It is sn that th xrgy lss rat in th cmprssr in tw pratin mds is th largst. This is du t th cmprssin prcss is irrvrsibl. Th irrvrsibility in th cmprssr rsults in xrgy lss. Th cmprssr with th highr isntrpic cmprssin fficincy may rduc xrgy lss, s scrw r scrll cmprssrs ar usually chsn in th rfrigratin systm. Excpt xrgy lss in th cmprssr, th xrgy lss in th cndnsr is largr than that in th xpansin valv and th vapratr. Thr ar tw rasns rsulting in th xrgy lss in th cndnsr. On rasn is th irrvrsibility f hat transfr tmpratur diffrnc btwn th rfrigrant and th cling mdia cling watr r utsid air, th thr is that th xrgy f mittd hat by th cndnsr is nt utilizd. Rducing thrmal rsistanc f hat transfr and th tmpratur diffrnc f hat transfr can dcras xrgy lss in th cndnsr. If pssibl, utilizing xrgy f mittd hat by th cndnsr can als rduc xrgy lss in th cndnsr. Th xrgy lss in th xpansin valv is du t thrttling prcss is irrvrsibl. Incrasing subcling dgr f th rfrigrant bfr ntring int th xpansin valv may rduc flw rsistanc f th rfrigrant in th xpansin valv. Thus, th xrgy lss in th xpansin valv is crrspndingly rducd. 4

5 Intrnatinal Jurnal n Architctural Scinc a air cnditining pratin md b ic strag pratin md Fig. 3: Exrgy flw diagrams f th rfrigratin systm Th xrgy lss in th vapratr is smallr du t th xrgy f th hat absrbd by th vapratr is utilizd. Th hat transfr tmpratur diffrnc btwn th rfrigrant and th clant fluid 5% glycl slutin r chilld watr rsults in th xrgy lss in th vapratr. Rducing thrmal rsistanc f hat transfr and th tmpratur diffrnc f hat transfr may als dcras xrgy lss in th vapratr. 6. CONCLUSIONS A cmputatinal mdl basd n xrgy analysis is prsntd fr a dual-md rfrigratin systm. It is knwn that th xrgy fficincy f th rfrigratin systm in ic strag pratin md is 17.63% lwr than that f th rfrigratin systm in air cnditining pratin md. Th xrgy lss in th cmprssr is th largst in all cmpnnts, s scrw r scrll cmprssrs with th highr isntrpic cmprssin fficincy may b chsn in th rfrigratin systm. Th xrgy lss in th cndnsr may b rducd by rducing thrmal rsistanc f hat transfr and th tmpratur diffrnc f hat transfr r utilizing xrgy f hat mittd by th cndnsr. Th xrgy lss in th xpansin valv is rducd thrugh incrasing subbing dgr f th rfrigrant bfr ntring int th xpansin valv. Th xrgy lss in th vapratr is als dcrasd thrugh rducing thrmal rsistanc f hat transfr and th tmpratur diffrnc f hat transfr. ACKNOWDGEMENTS This wrk is supprtd by Natural Scinc Fundatin f Jiangsu Prvinc undr th prjct N. BK00307 and High-Tchnlgy Dvlpmnt Prgram f Jiangsu Prvinc Univrsity undr th prjct N. JHB NOMENCLATURE COP cfficint f prfrmanc in th rfrigratin cycl spcific xrgy, Jkg -1 l ttal xrgy lss amunt f rfrigratin systm, Jkg -1 l,i xrgy lss amunt in ach cmpnnt, Jkg -1 l,cd xrgy lss amunt in th cndnsr, Jkg -1 l,cp xrgy lss amunt in th cmprssr, Jkg -1 l,v xrgy lss amunt in th vapratr, Jkg -1 l,x xrgy lss amunt in th xpansin valv, Jkg -1 spcific xrgy f an amunt f hat, Jkg -1 spcific xrgy f an amunt f hat 0, Jkg -1 h spcific nthalpy, Jkg -1 hat amunt absrbd hat sink r rjctd hat surc, Jkg -1 hat amunt absrbd by th vapratr pr unit mass f rfrigrant, Jkg -1 s spcific ntrpy, Jkg -1 K -1 T tmpratur, K w cmprssin wrk pr unit rfrigrant mass, Jkg -1 xrgy lss amunt f rfrigrant in th fluid in th vapratr, Jkg -1 xrgy incrmnt f th clant vapratr, Jkg -1 T sc subcling dgr f rfrigrant in th cndnsr, K T sh suprhating dgr f rfrigrant in th vapratr, K 5

6 Intrnatinal Jurnal n Architctural Scinc Grk symbls η xrgy fficincy f th rfrigratin systm, % η xrgy lss rat in ach cmpnnt, % i Subscripts a ambint cd cndnsr v vapratr fi inlt f th clant fluid at th vapratr f utlt f th clant fluid at th vapratr utlt f th vapratr 3 utlt f th cmprssr 6 utlt f th cndnsr 7 utlt f th xpansin valv REFERENCES 1. R. Yumrutas, M. Kunduz and M. Kanglu, Exrgy analysis f vapr cmprssin rfrigratin systms, Exrgy, Vl., pp A. Stgu-Sagia amd N. Paignigiannis, Exrgy lsss in rfrigrating systms. A study fr prfrmanc cmparisns in cmprssr and cndnsr, Intrnatinal Jurnal f Enrgy Rsarch, Vl. 7, pp E. Bilgn and H. Takahashi, Exrgy analysis and xprimntal study f hat pump systms, Exrgy, Vl., pp I. Dincr, S. Dst and X. Li, Prfrmanc analysis f snsibl hat strag systms fr thrmal applicatins, Intrnatinal Jurnal f Enrgy Rsarch, Vl. 1, pp H. El-Dssuky and F. Al-Juwayhl, Effctivnss f a thrmal nrgy strag systm using phaschang matrials, Enrgy Cnvrsin & Managmnt, Vl. 38, pp I. Dincr, On thrmal nrgy strag systms and applicatins in buildings, Enrgy and Building, Vl. 34, pp