Exergy Analysis of Organic Rankine Cycle with Ejector Using Dry Fluids

Transcription

1 Inrnaional Journal of Mining, Mallurgy & Mchanical Enginring (IJMMME) Volum 3, Issu 4 (21) ISSN (Onlin) Exrgy Analys of Organic Rankin Cycl wih Ejcor Ung Dry Fluids Hyung Jong Ko and Kyoung Hoon Kim Absrac Exrgy prformanc of an organic Rankin cycl combind wih an jcor rfrigraion cycl is hrmodynamically invsigad for h uilizaion of low mpraur ha sourcs. Th cycl is drivn by h snbl ha of war and h fiv kinds of dry fluids ar condrd as working fluid. Th xrgy consumpion a ach cycl lmns and h xrgy and scond-law fficincis of h cycl ar mulad for varying urbin inl prssur (TIP). Rsuls show ha h xrgy loss of sourc xhaus and xrgy dsrucions of boilr, jcor, condnsr, and vaporaor ar dominan ovr hos of coolan xhaus, urbin, pump, and xpanon valv. For ach working fluid h xrgy fficincy has a pak valu wih rspc o TIP, whil h scond-law fficincy incrass monoonically wih TIP. Th xrgy fficincy is highr for h working fluid having lowr criical mpraur. Kywords jcor, xrgy dsrucion, xrgy fficincy, organic Rankin cycl, scond-law fficincy S I. INTRODUCTION INCE i is difficul o fficinly convr low-grad hrmal nrgy ino lcriciy by convnional mhods, hr has bn an incrang inrs in h mhods o ffcivly convr h low-grad hrmal nrgy ino usful forms of nrgy. Th organic Rankin cycl (ORC) has aracd much anion as i is provn o b h mos fabl mhod o achiv high fficincy in convring h low-grad hrmal nrgy o mor usful forms of nrgy [1-4]. Imporan rviws on h sa of h ar of rsarch in nrgy convron from low-grad nrgy sourcs ar among ohrs hos of Raj al. [], Tchanch al. [6] and Chn al. [7]. A rviw on a sa of h ar rpor of ORC applicaions by Schusr al. [8] is also worhy o ci, in which gohrmal powr plans, biomass fird cognraion plans, and solar dsalinaion plans ar includd. Th slcion of working fluid maching wih h availabl ha sourc is ssnial o is succssful convron ino usful nrgy. Vlz al. [9] compard h working fluids usd for h ORCs opraing a low mpraur. Wang al. [] proposd a horical hrmal fficincy modl basd on an idal ORC o analyz h influnc of working fluid propris. Mago al. [11] prsnd an analys of rgnraiv ORCs ung dry organic fluids. An xrgy-basd fluid slcion mhod was Hyung Jong Ko is wih h Dparmn of Mchanical Enginring, Kumoh Naional Insiu of Tchnology, Gumi, Gyongbuk 39177, Kora (phon: ; fax: ; -mail: kohj@kumoh.ac.kr). Corrsponding auhor. Kyoung Hoon Kim is wih h Dparmn of Mchanical Enginring, Kumoh Naional Insiu of Tchnology, Gumi, Gyongbuk 39177, Kora (-mail: khkim@kumoh.ac.kr). suggsd by Hbrl and Bruggmann [12]. Ejcors ar fascinaing dvics which hav no moving pars. Bcaus of his faur, jcor rfrigraion cycl (ERC) is comparaivly mpl and can b drivn by low-grad nrgy sourcs [13]. ERC can b combind wih various powr cycls for h cognraion of powr and rfrigraion. Dai al. [14] proposd a novl cycl which cognras powr and rfrigraion, by adding a urbin bwn h boilr and h jcor of ERC and ung as working fluid. Th sam cycl was invsigad by Zhng and Wng [1], Ko and Kim [16], and Ko al. [17] for h working fluid of R24fa, and by Kim al. [18] for svral working fluids. Morovr, is modifid vron in which only a par of vapor was xracd from h urbin o h jcor was sudid by Wang al. [19] for h sam fluid and by Habibzadh al. [2] for svral working fluids. Li al. [21] proposd an ORC wih jcor for h purpos of incrang h powr oupu capaciy and improving is fficincy. In his sudy xrgy prformanc of an ORC combind wih an ERC is hrmodynamically invsigad. Exrgy analys is rgardd as an appropria ool for improving h prformanc of a hrmodynamic cycl nc i can loca and sima h loss of usful nrgy of a cycl [22]. Th cycl is drivn by h snbl ha of war a 1 o C and h fiv kinds of dry organic fluids ar condrd as working fluid. Th consumpion of xrgy a h cycl lmns and h xrgy and scond-law fficincis of h cycl ar mulad for varying urbin inl prssur. II. SYSTEM ANALYSIS Th sysm condrd in his sudy is an organic Rankin cycl combind wih an jcor rfrigraion cycl and is shown schmaically in Fig. 1. Th combind cycl is dividd ino wo sub-cycls: a powr sub-cycl and a rfrigraion sub-cycl. Two sub-cycls shar an jcor and a condnsr. Th primary flow in h jcor coms from h urbin and inducs h scondary flow from h vaporaor. Th nir sysm is drivn by h snbl ha supplid by a low-mpraur ha sourc. Boh h sourc fluid and h coolan for h condnsr ar condrd as war. Fiv fluids ar condrd as working fluid of h cycl. For h mpliciy of invsigaion h following assumpions ar usd: Th sysm opras in a sady sa. Prssur drop and ha loss during h sady procss in h sysm ar ngligibl. Th working fluid nrs h urbin as suprhad vapor, and lavs h condnsr and vaporaor as saurad 213

2 Inrnaional Journal of Mining, Mallurgy & Mchanical Enginring (IJMMME) Volum 3, Issu 4 (21) ISSN (Onlin) ha sourc m s 1 pump m 2 boilr urbin jcor condnsr 6 m c 3 4 m + m 7 9 m vaporaor xpanon valv Fig. 1 Schmaic diagram of h sysm. liquid and saurad vapor, rspcivly. Th minimum mpraur diffrnc bwn h ho and cold srams in h boilr and condnsr is qual o a prscribd valu of pinch poin mpraur diffrnc. Th isnropic fficincis of pump and urbin ar consan. Th flow in h jcor is on-dimnonal and h mixing of primary and scondary flows occurs a consan prssur. In addiion h irrvrbl ffcs of h nozzl, mixing, and diffusr scions of h jcor ar akn ino accoun by ung hir fficincis [14], [16]. Th procss in h xpanon valv is isnhalpic. Th hrmodynamic sas a 1 o 9 of Fig. 1 can b drmind if h urbin inl prssur and mpraur, urbin oul prssur, and condnsr and vaporaor mpraur ar spcifid. In h procss h mass flow ra nraind from h vaporaor m which is h mass flow ra of rfrigraion sub-cycl and h xi sa 4 of h jcor ar drmind iraivly from h balanc quaions of mass, momnum and nrgy, along wih h quaion of sa. For a spcifid mass flow ra of sourc fluid m s, h mass flow ras of h working fluid hrough h urbin and h coolan, dnod by m and m c, rspcivly, can b drmind from h quaions of nrgy balanc and h pinch poin condiions [18]. No ha h mass flow ra hrough h condnsr is qual o h sum of hos hrough h urbin and h vaporaor: m cd m m m 1 r, (1) whr r is h nrainmn raio of jcor. Th ra of ha inpu, n powr producion, and h ra of rfrigraion oupu ar obaind as Q W in n m h 2 h 1, (2) W W m h h h, (3) p h6 Q m h 9 h 8. (4) 8 Th ras of xrgy flow of h working fluid, xrgy inpu o h sysm by h sourc fluid, and xrgy oupu associad wih rfrigraion can b wrin as ln E m h h, () E T s s s pst T T T / m c T, (6) T Tcs Tcs E Q /. (7) Hr m, h, s, and T dno h mass flow ra, spcific nhalpy and nropy, and mpraur of h fluid. Th subscrip rfrs o h nvironmn sa and c ps is h isobaric spcific ha of h sourc fluid. T and T cs ar h mpraurs of h sourc inl and h spac coold by h vaporaor, rspcivly. Th ra of xrgy loss du o sourc xhaus, E so, can b obaind by rplacing T wih h sourc oul mpraur T so. Th ra of xrgy flow of h coolan can b obaind milarly by ung h isobaric spcific ha c pc and h mpraur T ci or T co. Th xrgy dsrucion of adiabaic sysm is qual o h diffrnc of h inpu and oupu of xrgy. Th xrgy dsrucion or loss raio of ach componn is dfind as is valu dividd by h xrgy inpu by h sourc fluid. For xampl, xrgy dsrucion raio of boilr D b and xrgy loss raio du o sourc xhaus D so ar calculad as b E E so E 1 E2 E D /, (8) D E / E. (9) so so Th xrgy fficincy and h scond-law fficincy of h cycl ar dfind as follows: ( W E )/ E, () x II n n so ( W E ) / E E. (11) η x is basd on h xrgy inpu and rprsns an simaion by assuming ha h xrgy of xhaus sourc is mrly dischargd o h nvironmn. Whil η II is basd on h n xrgy dlivrd o h sysm hrough h boilr and rprsns an opimisic simaion wih a hop ha h xrgy of xhaus can b fully rcovrd. No ha wo fficincis ar qual whn h sourc xhaus mpraur is qual o ha of nvironmn. In addiion, if h coolan inl sa is akn as h dad sa, h sum of h xrgy fficincy and all xrgy dsrucion and loss raios of h sysm bcoms uniy. III. RESULTS AND DISCUSSIONS An xrgical prformanc analys basd on h scond law of hrmodynamics is carrid ou for an organic Rankin cycl (ORC) combind wih an jcor rfrigraion cycl (ERC) drivn by h low-mpraur ha sourc. Th condrd sourc fluid is war flowing a a ra of m s = 1 kg/s wih inl mpraur of 1 o C. Fiv dry fluids ar condrd as working fluid of h cycl and can b squncd by hir criical mpraur: normal pnan (nc H l2 ), isopnan (ic H l2 ), 214

3 Inrnaional Journal of Mining, Mallurgy & Mchanical Enginring (IJMMME) Volum 3, Issu 4 (21) ISSN (Onlin) TABLE I BASIC DATA FOR THE WORKING FLUIDS Subsanc M (kg/kmol) T cr (K) P cr (bar) normal pnan isopnan R24fa (CHCl 2 CF 3 ), R24fa (CHF 2 CH 2 CF 3 ), and isobun (ic 4 H ). Thir hrmodynamic propris ar calculad by h Pal-Tja quaion of sa [3], [23], [24]. Th bac daa of ach fluid ndd o apply h Pal-Tja quaion ar givn in TABLE I, whr M, T cr, P cr, and ω ar molar mass, criical mpraur, criical prssur, and acnric facor, rspcivly [2]. Th mos imporan paramr of h ORC combind wih ERC is h urbin inl prssur (TIP) bcaus i affcs boh powr gnraion and rfrigraion oupu. In addiion rfrigraion prformanc is influncd by h urbin oul prssur (TOP), condnng mpraur, and vaporaor mpraur in a complicad mannr. Th main rason is h srong dpndnc of h nrainmn raio of h jcor on hs paramrs. In his sudy h xrgical prformanc of h sysm is invsigad by varying TIP in a rang of 6 o bar. Whil h TOP, h urbin inl and condnng mpraur ar fixd a 6 bar, 1 o C and 2 o C, rspcivly. Ohr bac calculaion daa ar summarizd as follows. Coolan mpraur: 1 o C, nvironmn mpraur: 1 o C, vaporaor mpraur: -2 o C, mpraur of coold spac: - o C, pinch poin mpraur diffrnc: o C, isnropic fficincy of pump:.7, isnropic fficincy of urbin:.8, nozzl fficincy:.9, mixing fficincy:.9, diffusr fficincy:.9. Only a par of h xrgy supplid o h sysm by h sourc fluid is rcovrd as n powr producion and rfrigraion xrgy. Th rs of i ar ihr dsroyd a h sysm componns or los by h sourc and coolan xhaus. Fig. 2 shows all prcnag consumpions of supplid xrgy rlaiv o h sourc inpu xrgy, E, for which has mdium criical mpraur of h fiv working fluids condrd. Rcall ha sum of all xrgy consumpion raios is uniy. For TIP is rsricd in a rang of abou 6. o 14. bar. Th lowr limi corrsponds o h minimum workabl TIP which mus b grar han h TOP of 6 bar. Th uppr limi is h TIP whn h urbin inl qualiy is jus on. Th rcovrd xrgy ( x ), los xrgy du o sourc xhaus (D so ), dsroyd xrgy a h boilr (D b ), condnsr (D cd ), jcor (D j ), and vaporaor (D ) ar prvaln. Th xrgy loss du o coolan xhaus (D co ) and h xrgy dsrucion a h urbin (D ), pump (D p ), and xpanon valv (D v ) ar rlaivly small. This sima of ordr of magniud of xrgy consumpion is milar o h ohr working fluids. Thrfor h dpndncs of h xrgy consumpion raios on TIP ar invsigad only for h major lmns. Fig. 3 shows h raio of xrgy loss du o sourc xhaus vrsus TIP for various working fluids. Excp TIP is limid abov in ordr o saisfy h suprhad inl condiion. For ach fluid, xrgy loss du o sourc xhaus incrass almos linarly wih TIP. I is h rsul of h incrasd sourc oul mpraur bcaus h ha load of h boilr bcoms smallr as h TIP gs highr for h sam inl mpraur. For h sam rason of rducd ha load, xrgy loss du o sourc xhaus is highr for h fluids wih highr criical mpraur. Raios of xrgy dsrucion of boilr and condnsr ar dmonsrad in Fig. 4 wih rspc o TIP for various working fluids. Exrgy dsrucions of boilr and condnsr ar comparabl in magniud and dcras wih TIP for all working fluids. This is mainly h rsul of h ffc of rducd mass Exrgy consumpion raios (%) 1.1 x Dso Db Dj Dcd D Dco D Dp Dv Exrgy loss raio du o sourc xhaus (%) normal pnan isopnan R24fa Turbin inl prssur (bar) Turbin inl prssur (bar) Fig. 2 Exrgy consumpion raio of ach lmn vrsus urbin inl prssur for. Fig. 3 Exrgy loss raio du o sourc xhaus vrsus urbin inl prssur for various working fluids. 21

4 Inrnaional Journal of Mining, Mallurgy & Mchanical Enginring (IJMMME) Volum 3, Issu 4 (21) ISSN (Onlin) Exrgy dsrucion raios of boilr & condnsr (%) Db Dcd normal pnan isopnan R24fa Turbin inl prssur (bar) Exrgy fficincy (%) normal pnan isopnan R24fa Turbin inl prssur (bar) Fig. 4 Exrgy dsrucion raios of boilr and condnsr vrsus urbin inl prssur for various working fluids. flow ras o which h ha ransfr ras a h boilr and h condnsr ar dircly proporional. For a sourc mass flow ra of m s = 1 kg/s, h mass flow ra of working fluid hrough h urbin rangs roughly from m =.3 o 1.3 kg/s dpnding on h working fluid and dcrass wih TIP. Th rason why m dcrass wih TIP is ha lss fluid can b circulad saisfying h pinch poin condiion a h boilr. Th nrainmn raio of jcor also dcrass slighly wih TIP. Exrgy dsrucion of h working fluid wih lowr criical mpraur is biggr boh a h boilr and h condnsr. Fig. dmonsras h xrgy dsrucion raios of jcor and vaporaor wih rspc o TIP for various working fluids. Th xrgy dsrucion of jcor is mor han wo ims of ha of vaporaor nc h mass flow ra of h jcor is largr Fig. 6 Dpndnc of xrgy fficincy on urbin inl prssur for various working fluids. han h vaporaor. Exrgy dsrucion monoonically dcrass as TIP incrass boh in h jcor and h vaporaor nc h mass flow dcrass wih TIP. Th dpndnc of xrgy dsrucion on h criical mpraur of h working fluid is no uniform. Howvr, h ra of chang of h xrgy dsrucion raio wih rspc o TIP is fasr for h fluid wih highr criical mpraur. Fig. 6 shows h dpndnc of xrgy fficincy of h sysm on TIP for various working fluids. Toal rcovrd xrgy is h sum of n powr producion and rfrigraion xrgy oupu. Th ffc of raing TIP on h powr cycl is wofold: incras of nhalpy drop and dcras of mass flow ra. Bcaus of hs comping ffcs of TIP, h n powr producion curv shows a hill shap bhavior wih rspc o Exrgy dsrucion raios of jcor & vaporaor (%) Dj D normal pnan isopnan R24fa Turbin inl prssur (bar) Turbin inl prssur a pak of x (bar) TIP x i normal pnan i i isopnan i i i Turbin oul prssur (bar) R24fa Pak valu of x (%) Fig. Exrgy dsrucion raios of jcor and vaporaor vrsus urbin inl prssur for various working fluids. Fig. 7 Dpndncs of urbin inl prssur and xrgy fficincy a h paks of x on urbin oul prssur for various working fluids. 216

5 Inrnaional Journal of Mining, Mallurgy & Mchanical Enginring (IJMMME) Volum 3, Issu 4 (21) ISSN (Onlin) fficincis of h working fluids. Scond-law fficincy (%) 4 normal pnan isopnan 3 R24fa Turbin inl prssur (bar) Fig. 8 Dpndnc of scond-law fficincy on urbin inl prssur for various working fluids. TIP. Whil h xrgy oupu associad wih rfrigraion monoonically dcrass wih TIP, mainly du o h dcras of h mass flow ra hrough h urbin. For mos of h condrd TIP rang h n powr producion ouwighs h rfrigraion xrgy oupu, so ha h xrgy fficincy curv has a pak for ach working fluid. Th xrgy fficincy of h working fluid wih lowr criical mpraur is highr han h fluid wih highr criical mpraur. Dpndncs of TIP and xrgy fficincy a h paks of h xrgy fficincy curv on TOP is plod in Fig. 7 for various working fluids. Th valu of TIP giving a pak of xrgy fficincy incrass almos linarly wih TOP. Th rason is bcaus h highr inl prssur is rquird in h urbin o produc h sam or mor powr maching wih h raisd oul prssur. On h conrary, h pak valus of h xrgy fficincy dcras wih TOP. Th implicaion of his rsul is ha h TOP should b kp as low as posbl o achiv br xrgy fficincy of h sysm. Th TIP and h xrgy fficincy a h paks of xrgy fficincy curv ar highr for h fluid wih low criical mpraur for ach TOP. Fig. 8 is h plo of h dpndnc of h scond-law fficincy of h sysm on TIP for various working fluids. Whil h xrgy fficincy is dfind as h raio of h rcovrd xrgy o h xrgy inpu by h sourc fluid, h scond-law fficincy is dfind as is raio o h n xrgy inpu. Hnc h rlaion of II = x / (1 - D so ) holds bwn h wo fficincis and h scond-law fficincy is highr han h xrgy fficincy for any sysm opraing condiion. Th scond-law fficincy incrass monoonically wih rspc o TIP for all working fluids, o which h incrang of D so has conribud. For h TIPs sufficinly highr han 6 bar which is qual o h TOP, givs h highs scond-law fficincy of h condrd working fluids. Excp for hr is only a sligh diffrnc among h scond-law IV. CONCLUSIONS Analys of h xrgy prformanc basd on h scond law of hrmodynamics is carrid ou for an organic Rankin cycl combind wih an xrgy rfrigraion cycl which cognras powr and rfrigraion. Th sysm is drivn by h snbl ha of sourc fluid which is assumd as war wih inl mpraur of 1 o C. Fiv dry fluids ar condrd as h working fluid. Spcial anion is paid o h ffcs of urbin inl prssur on h xrgy loss or dsrucion a h componns of h sysm and h xrgy and h scond-law fficincis of h cycl. Th rsuls of h mulaion can b summarizd as follows. 1) Th rcovrd xrgy as powr and rfrigraion, los xrgy du o sourc xhaus, dsroyd xrgy a h boilr, condnsr, jcor, and vaporaor ar dominan ovr h xrgy loss du o coolan xhaus, xrgy dsrucion a h urbin, pump, and xpanon valv. 2) For ach fluid, h xrgy loss du o sourc xhaus incrass almos linarly wih urbin inl prssur (TIP). On h conrary, h xrgy dsrucions of boilr, condnsr, jcor, and vaporaor dcras wih TIP. 3) Th xrgy fficincy curv has a pak for ach working fluid and h xrgy fficincy of h working fluid wih lowr criical mpraur is highr han h fluid wih highr criical mpraur. 4) For ach fluid h valu of TIP giving pak xrgy fficincis incrass almos linarly wih urbin oul prssur (TOP), bu h pak valu islf dcrass wih TOP. ) Th scond-law fficincy incrass monoonically wih rspc o TIP for all working fluids condrd. ACKNOWLEDGMENT This papr was suppord by Rsarch Fund, Kumoh Naional Insiu of Tchnology. REFERENCES [1] T. C. Hung, T. Y. Shai, and S. K. Wang, A rviw of organic Rankin cycls (ORCs) for h rcovry of low-grad was ha, Enrgy, vol. 22, pp , [2] K. H. Kim, A scond law assssmn of organic Rankin cycl dpnding on sourc mpraur analys, WASET, vol. 8, pp , 214. [3] K. H. Kim, and C. H. Han, Analys of ranscriical organic Rankin cycls for low-grad ha convron, Adv. Sci. L., vol. 8, pp , 212. [4] B. F. Tchanch, M. Périssans, and G. Papadakis, Ha rsourcs and organic Rankin cycl machins, Rnw. Susain. Enrgy Rv., vol. 39, pp , 214. [] N. T. Raj, S. Iniyan, and R. Goic, A rviw of rnwabl nrgy basd cognraion chnologis, Rnw. Susain. Enrgy Rv., vol. 1, pp , 211. [6] B. F. Tchanch, Gr. Lambrinos, A. Frangoudakis, and G. Papadakis, Low-grad ha convron ino powr ung organic Rankin cycls - A rviw of various applicaions, Rnw. Susain. Enrgy Rv., vol. 1, pp , 211. [7] H. Chn, D. Y. Goswami, and E. Sfanakos, A rviw of hrmodynamic cycls and working fluids for h convron of low-grad ha, Rnw. Susain. Enrgy Rv., vol. 14, pp. 9-67,

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