WASTE HEAT RECOVERY USING DIRECT THERMODYNAMIC CYCLE

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1 Bulltin of th Transilvania Univrsity of Braşov Vol. 8 (57) No Sris I: Enginring Scincs WASTE HEAT RECOVERY USING DIRECT THERMODYNAMIC CYCLE I. COSTIUC 1 L. COSTIUC 2 S. RADU 3 Abstract: For wast hat rcovry th thrmodynamic cycls is a solution by slcting a fluid with corrsponding working tmpratur. Th wast nrgy is usd for cognration of hat nrgy and lctrical nrgy. Th analysis of th thrmodynamic cycl has bn carrid out considring vaporation tmpratur and condnsation tmpratur of pur cologic working rfrigrant R245fa. Basd on th thrmodynamic mathmatical modl with quation of stat for working fluid, th ffcts of dsign paramtrs on th systm prformanc ar invstigatd from th viw of both thrmodynamics fficincy and xrgtic fficincy. Ky words: Rankin Cycl, rfrigrants, wast hat rcovry, cognration, thrmodynamic analysis. 1. Introduction Th rcovry of th nrgy from wast rsourcs is a modrn mthod for nrgy convrsion that cam from th all most tchnological procsss. Chang t al. [2] hav highlightd that th 60% of lowtmpratur wast hat is not rcovrd in US and th Rankin cycl is a promising thrmodynamic cycl for nrgy convrsion. Th dirct thrmodynamic cycl, calld also Rankin cycl, allows nrgy rcovry of wast hat strams that ar modrat in tmpratur to b convrtd into lctrical powr by convntional stam cycls. Th us of organic rfrigrants with high molcular mass nabls cost-ffctiv lctrical powr rcovry of wast hat, and ths thrmodynamic cycls ar calld Organic Rankin Cycls, ORC. A promising way to incras th cycl fficincy is th us of th wast nrgy to obtain lctricity combind with hating and/or cooling as has studid in [2-5]. Th thrmodynamic cycl fficincy of th Rankin cycl is influncd by th choic of th working rfrigrant. Th Montral protocol forcd industry to us thrmodynamically lss fficint rfrigrants which ar not containing chlorin in thir composition, namd HFC rfrigrants. Th non-toxic rfrigrants usd in th past, i.. CFC114, CFC11 and HCFC123, wr outlawd sinc ar not chlorin fr. This papr invstigats th cologic working rfrigrants R245fa and R600a (isobuthan) in a Rankin cycl systm to produc work to a turbin which is dirctly connctd to an lctrical gnrator. Th wast nrgy cam from gas xhaust of an automotiv ngin. Th tmpraturs of 1 Klimavnt SRL Braşov. 2 Mchanical Enginring Dpt., Transilvania Univrsity of Braşov. 3 Automotiv Dpt., Transilvania Univrsity of Braşov.

2 2 Bulltin of th Transilvania Univrsity of Braşov Sris I Vol. 8 (57) No xhaust gass dpnd on th functioning rgim of th ngin and hav about 350 to 450 C. Th study considrd th basic Rankin cycl with diffrnt vaporation and condnsation tmpraturs which ar dtrmind by th hat sourc and th hat sink. 2. Basic Thrmodynamic Cycl Th working fluid, in liquid stat, is prssurizd and circulatd through th systm by a pump. On th discharg sid of th pump, th highly prssurizd liquid is hatd in a hat xchangr, calld vaporator or boilr, until it absorbs th wast hat and boils at high prssur and modrat tmpratur. Th vapor stat rfrigrant thn passs through a prssur lowring dvic also calld an xpansion dvic. This may b an xpansion valv, or commonly usd a work-xtracting dvic such as a turbin. Th low prssur vapor rfrigrant thn ntrs to anothr hat xchangr, th condnsr, in which th fluid rlas hat and condnss. Th condnsd liquid rfrigrant thn rturns to th pump and th thrmodynamic cycl is rpatd. Th tmpratur ntropy diagram of th basic Rankin cycl is shown in Figur 1. Th corrlation btwn thrmodynamic stat points and th functioning cycl prsntd in Figur 1 is: th saturatd liquid rfrigrant lavs th condnsr at low-prssur with thrmodynamic stat point 1. Th pump raiss th prssur of th liquid to stat point 2 and fds th vaporator/boilr. In th vaporator th rfrigrant is hatd in liquid stat to stat point 3, vaporatd to stat point 4 and supr-hatd to stat point 5 which is th ntring stat to th turbin. Th adiabatic xpansion to low-prssur vapor in th turbin at stat point 6. Aftr that th vapor ntrs in th condnsr whr hat is rjctd from th systm during condnsation. Th turbin is usually connctd to lctric gnrator Thrmodynamic Modlling Th Rankin cycl modlling is basd on th consrvation quations for nrgy and mass. To analyz this thrmodynamic systm, th mass and nrgy balanc must b prformd for ach plant componnt. Bcaus this plant is a closd systm and th turbin has on inlt and on outlt port, th mass flow circulating in th plant is th sam for all dvics. For nrgy balanc for all dvics th spcific nthalpy diffrnc of inlt and outlt stat dfins th rat of spcific nrgy xchang at givn prssur and tmpratur using th quation of stat modl. Fig. 1. Rankin schmatics and T-S thrmodynamic cycl

3 Costiuc, I., t al.: Wast Hat Rcovry Using Dirct Thrmodynamic Cycl 3 Pump modlling Mass balanc: m rf m 2 m 1, (1) Enrgy balanc: P m ( h 2 h1), (2) p whr m rf is th mass flow [kg/s], P p is pump powr [kw], h 1, h 2 ar th inlt and outlt nthalpis. rf Boilr/Evaporator modlling Mass balanc: m 2 m 5, (3) Enrgy balanc: h 5 h ), (4) m rf ( 2 whr is th boilr hat flow [kw], h 2, h 5 ar th inlt and outlt nthalpis. Turbin modlling Mass balanc: m, (5) 6 m 5 Enrgy balanc: P m h 5 h ), (6) T rf ( 6 whr P T is th turbin powr [kw], h 5, h 6 ar th inlt and outlt nthalpis. Condnsr modlling Mass balanc: m, (7) 6 m 1 Enrgy balanc: h 1 h ), (8) c m rf ( 6 whr c is th condnsr hat flow [kw], h 6, h 1 ar th inlt and outlt nthalpis. Th ovrall nrgy balanc is: P P (9) p c T. Thrmodynamic fficincy or cofficint of prformanc is dfind as th ratio of usful nrgy or powr and consumd nrgy or powr and computd with th rlation: COP P P T p nt. (10) P Exrgtic fficincy is: x Ex( Pnt ) Ex( ) P nt T Fm TFm T amb, (11) whr T amb is th ambint tmpratur and T Fm is th avrag tmpratur in th vaporator calculatd with: ( h5 h2 ) T Fm as ratio of nthalpy and ( s5 s2) ntropy diffrnc for boilr. Rfrigrant modlling Th thrmodynamic proprtis for prssur, nthalpy, ntropy at saturation stat, subcoold or suprhatd stats ar calculatd using CoolProp prcision Hlmholtz thrmodynamic mathmatical modl for working fluid prsntd in [1]. This modl was implmntd for simulation in EES (Equation Enginring Solvr) [7] rsulting a systm with 52 algbraic quations, 11 quations for mass and nrgy balanc and 41 quations for prssur, spcific volum, tmpratur, nthalpy and ntropy of th stat points. Th thrmodynamic proprtis usd for R254fa and R600a rfrigrants according ASHRAE [6] ar: R245fa - th molcular mass M = kg/kmol, boiling tmpratur at normal prssur t bp = C, critical tmpratur t crit = C, critical prssur p crit = 3.6 MPa, and for R600a th molcular mass M = kg/kmol, boiling tmpratur at normal prssur t bp = C, critical tmpratur t crit = C, critical prssur p crit = 3.6 MPa. Th Rankin cycl systm calculation was mad undr th following conditions: - th maximum boiling tmpratur in th vaporator t,max = +100 C; - th ambint tmpratur t amb = +25 C; - th condnsing tmpratur t c = t amb + 10 C; - th wast hat sourc tmpratur t ws = C; - th turbin fficincy η T = 0.85; - th pump fficincy η p = Th variabl paramtr is considrd th ntring rfrigrant tmpratur in th turbin t 5 = C.

4 4 Bulltin of th Transilvania Univrsity of Braşov Sris I Vol. 8 (57) No Rsults and Discussions Th prformanc valuation of th Rankin systm rsulting from simulation for th 2 rfrigrants invstigatd is prsntd in function of inlt turbin tmpratur for: - th cofficint of prformanc, COP, th xrgtic fficincy, x, in Figur 2 and Figur 4; - boilr hat, Q in, condnsr hat, Q out and turbin work, W T, in Figur 3 and Figur 5. Th COP variation for R245fa dpnding on th turbin inlt tmpratur is dcrasing sinc turbin inlt tmpratur is incrasing, whil for R600a th COP hav a maximum around 110 C (s Figur 4). Th bhavior for R245fa and R600a of th xrgtic fficincy (s Figur 2 and Figur 4) is th sam as th thrmal fficincy. Eta x, COP [-] Rankin cycl fficincy Rfrigrant R245fa x COP 0.12 Inlt Turbin tmpratur T 5 [ C] Fig. 2. COP and x vs. tmpratur Rfrigrant R245fa Q in Qin, Qout, Wnt [kj/kg] Q out W nt 0 Inlt Turbin tmpratur T 5 [ C] Fig. 3. Q in, Q out, W T vs. tmpratur

5 Costiuc, I., t al.: Wast Hat Rcovry Using Dirct Thrmodynamic Cycl Rfrigrant R600a Rankin cycl fficincy Eta x, COP [-] x COP 0.12 Inlt Turbin tmpratur T 5 [ C] Fig. 4. COP and x vs. tmpratur Qin, Qout, Wnt [kj/kg] Rfrigrant R600a Q in Q out W nt 0 Inlt Turbin tmpratur T 5 [ C] Fig. 5. Q in, Q out, W T vs. tmpratur According to Figur 3 th turbin work which can b convrtd in lctrical work is incrasing with inlt turbin tmpratur incrasing for both rfrigrants. At maximum turbin inlt tmpratur th work producd by R245fa is about 37.8 kj/kg and hat introducd in boilr is kj/kg sinc for R600a th work producd is 66 kj/kg and hat introducd in boilr is kj/kg. If on considr a mass flow about 1 kg/s in plant an amount of 37.8 kw turbin powr is producd from kw of hat introducd in th systm for R245fa and 66 kw turbin powr is producd from kw of hat introducd for R600a. So, th turbin powr producd by R600a is 57% highr than powr producd by R245fa in th sam simulation conditions. Th fficincy of th invstigatd Rankin cycl using rcovrabl nrgy wasts is similar with othr plants i..

6 6 Bulltin of th Transilvania Univrsity of Braşov Sris I Vol. 8 (57) No using solar absorption thrmodynamic cycl as prsntd in [2], [3] or xhaust gas rcovry from disl ngins in [4], [5]. Evn th valu of thrmal fficincy is about 13% th valu of producd turbin powr is not ngligibl. 3. Conclusions 1. This papr has prsntd th Rankin cycl simulation using organic rfrigrants R245fa and R600a. 2. Th analysis has bn mad for thrmodynamic fficincy, xrgtic fficincy and turbin rsultd work using high accuracy quation of stat using Hlmholtz thrmodynamic mathmatical modl. 3. Th papr has dmonstratd that th nrgy wast rcovry can b usd for lctrical nrgy production. Acknowldgmnts This papr is supportd by th Sctoral Oprational Programm Human Rsourcs Dvlopmnt (SOP HRD), financd from th Europan Social Fund and by th Romanian Govrnmnt undr th projct numbr POSDRU/159/1.5/S/ Rfrncs 1. Bll, Ian H., Wronski, J., Quoilin, S., t al.: Pur and Psudo-pur Fluid Thrmophysical Proprty Evaluation and th Opn-Sourc Thrmophysical Proprty Library CoolProp. In: Industrial & Enginring Chmistry Rsarch 53 (2014), p Chang, J.-C., Hung, T.-C., H, Y.-L., t al.: Exprimntal Study on Low- Tmpratur Organic Rankin Cycl Utilizing Scroll Typ Expandr. In: Applid Enrgy 155 (2015), p Costiuc, L.: Solar Powrd Absorbtion Systm. In: Bulltin of th Transilvania Univrsity of Braşov (2013) Vol. 6 (55), Sris I, p Zhang, Y.-Q., Wu, Y.-T., Xia, G.-D., t al.: Dvlopmnt and Exprimntal Study on Organic Rankin Cycl Systm with Singl-Scrw Expandr for Wast Hat Rcovry from Exhaust of Disl Engin. In: Enrgy 77 (2014), p Yang, M.-S., Yh, R.-H.: Thrmodynamic and Economic Prformancs Optimization of an Organic Rankin Cycl Systm Utilizing Exhaust Gas of a Larg Marin Disl Engin. In: Appl. Enrgy 149 (2015), p ANSI/ASHRAE Standard , Dsignation and Safty Classification of Rfrigrants. Amrican Socity of Hating, Rfrigrating and Air- Conditioning Enginrs, Inc Tulli Circl NE, Atlanta, F-Chart Softwar: Equation Enginring Solvr v.8.40.

COMPARATIVE ANALYSIS OF AN ORGANIC RANKINE CYCLE FOR WASTE COGENERATION PLANTS

Bulltin of th Transilvania Univrsity of Braşov Vol. 10 (59) No. 2-2017 Sris I: Enginring Scincs COMPARATIVE ANALYSIS OF AN ORGANIC RANKINE CYCLE FOR WASTE COGENERATION PLANTS L. COSTIUC 1 I. COSTIUC 2