Effects of Source Temperature on Thermodynamic Performance of Transcritical Organic Rankine Cycle

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1 International Journal of Material, Mehani and Manufaturg, Vol. 1, No. 1, February 13 Effet of Soure Temperature on Thermodynami Performane of Tranritial Organi Ranke Cyle Hyung Jong Ko, Se Woong Kim, Chul Ho Han, and Kyoung Hoon Kim Abtrat Organi Ranke Cyle (ORC) ha attrated muh attention a a promig tehnology for an effiient onverion of low-grade energy to eletriity. In thi tudy the thermodynami performane of tranritial ORC with varyg oure temperature i vetigated. The ytem ue R134a a workg fluid and it performane uh a the ratio of ma flow rate, peifi work, and thermal and exergy effiienie are parametrially vetigated. Reult for the oure temperature rangg - o C and the redued turbe let preure (TIP) up to 3 how that the ratio of ma flow rate reae with TIP for fixed oure temperature. It i alo hown that the peifi work, and thermal and exergy effiienie reae with TIP the ubritial region and have a peak the uperritial region for eah oure temperature. Thermal effiieny an be raied by adoptg uperritial yle with uffiiently high oure temperature. Index Term Organi ranke yle (ORC), oure temperature, tranritial, turbe let preure. I. INTRODUCTION Se the traditional team Ranke yle doe not give a atifatory performane generatg eletriity from low-grade heat, mot of the low-temperature energy oure uh a geothermal energy, exhaut ga, bioma ombution, and wate heat are merely diarded. Therefore, it ha beome an iue how to effiiently onvert the low-grade energy to eletriity. Chen et al. [1] reviewed on a variety of thermodynami yle for the onverion of low-grade heat and the effet of workg fluid on the yle. Organi Ranke yle (ORC) whih ue organi fluid tead of water a workg fluid i onidered a one of the promig tehnologie for uh purpoe beaue of the great flexibility, high afety and low matenane requirement [2]. One of the hallenge of ORC i the hoie of organi workg fluid. It hould provide a high thermal effiieny and a high utilization of available heat oure. Saleh et al. [3] made a thermodynami reeng of pure omponent workg fluid for low-temperature ORC view of the thermal effiieny and heat tranfer harateriti. Mago et al. [4] preented a performane analyi of different ORC onfiguration whih ue dry organi workg fluid. They howed that regenerative ORC give higher firt and eond law effiienie and require le heat to produe the ame Manuript reeived Deember 5, 13; revied February 7, 13. Hyung Jong Ko, Chul Ho Han, and Kyoung Hoon Kim are with the Department of Mehanial Engeerg, Kumoh National Intitute of Tehnology, Gumi, Gyeongbuk 7-71, Korea ( {kohj, hhan, khkim}@kumoh.a.kr). Se Woong Kim i with the Department of Mehanial Engeerg, Kumoh National Intitute of Tehnology, Gumi, Gyeongbuk 7-71, Korea ( kw@kumoh.a.kr). power. One of the key feature of ORC i the heat tranfer harateriti between the oure fluid and workg fluid. An important limitation of the ORC with pure workg fluid i the ontant temperature boilg proe, whih reate a bad thermal math between the fluid. Thi problem an be irumvented if uperritial yle or mixture workg fluid i ued [5]-[9]. When zeotropi mixture workg fluid i ued, both the boilg and ondeng proee take plae with varyg temperature, thu a better thermal math i reated. Chen et al. [6] found that a uperritial Ranke yle ug zeotropi mixture workg fluid reult a remarkable improvement of thermal effiieny than a onventional ORC. Chen et al. [7] arried out a omparative tudy of the arbon dioxide tranritial power yle with an ORC with R123 a workg fluid and howed that the tranritial yle give a lightly higher power. Baik et al. [8] arried out a power-baed omparion between CO 2 and R125 tranritial yle. The latter yle wa reommended for heat oure about o C. Kim and Han [9] vetigated the thermodynami performane of tranritial ORC with and without ternal heat exhanger for variou workg fluid. It wa hown that uperritial yle ould provide better performane than ubritial yle. In thi tudy, the thermodynami performane of tranritial ORC with the redued turbe let preure (TIP) up to 3 i vetigated baed on the Patel-Teja equation of tate and the law of thermodynami. The ytem ue R134a a workg fluid and i driven by the enible heat of oure fluid with temperature rangg - o C. A an exploration to an effiient onverion of oure heat to ueful work, the performane of yle uh a the ratio of ma flow rate, peifi work, and thermal and exergy effiienie are parametrially vetigated term of the oure temperature and TIP. II. SYSTEM ANALYSIS Fig. 1. Shemati diagram of the ytem. Fig. 1 how the hemati diagram of the ytem DOI:.7763/IJMMM.13.V

2 International Journal of Material, Mehani and Manufaturg, Vol. 1, No. 1, February 13 onidered thi tudy, whih i onited of ondener, pump, heat exhanger (preheater, boiler, uperheater), and turbe. Heat i upplied to the ytem by the oure fluid at the heat exhanger and rejeted to the oolant at the ondener. The oure fluid and workg fluid onidered thi tudy are air and R134a, repetively. Water i ued a a oolant fluid. The thermodynami propertie of the workg fluid are alulated by the Patel-Teja equation of tate [], [11], whih i written a RT a P (1) v b v( v + b) + ( v b) Here, P, T, v and R are preure, abolute temperature, peifi volume and ga ontant, repetively, and a, b, and are determed from T and the parameter of the ritial pot. The bai data of the workg fluid whih are needed to alulate a, b, and are given TABLE I, where M, T r, P r and ω are moleular weight, ritial temperature, ritial preure, and aentri fator, repetively []-[12]. By the way organi fluid an be laified to three group by the ign of dt/d on the aturated vapor le, that i, wet, dry, and ientropi fluid. R134a belong to ientropi fluid [13]. TABLE I: BASIC DATA OF THE WORKING FLUID Subtane M (kg/kmol) T r (K) P r (bar) ω R134a In order to vetigate the thermodynami performane of the ytem the followg implifiation are utilized. 1) The oure fluid i tandard air and flow to the ytem at a ontant temperature of T S. 2) The workg fluid leave the ondener a aturated liquid at temperature of T L. 3) The turbe let temperature i lower than T S by T H, that i, T H T S - T H. 4) The temperature differene between the hot and old tream the heat exhanger i higher than a preribed value of ph pot temperature differene. 5) The heat tranfer proe the heat exhanger i modeled ug a heat tranfer effetivene. 6) The oolant fluid i water with let temperature of T. 7) The pump and turbe have ontant ientropi effiienie of η p and η t, repetively. 8) There i no preure drop or heat lo the ytem. 9) Heat upply and rejetion proe take plae at ontant preure of and P L, repetively. When the ytem i operated a a uperritial yle, there i no apparent phae hange proe the heat hanger. Therefore the identifiation of loation 3 and 4 Fig. 1 i not poible thi ae. However if the ytem i operated a a ubritial yle, pot 3 and 4 orrepond to aturated liquid and aturated vapor, repetively. The thermodynami tate at 1, 2, 5 and 6 are determed a follow. At pot 1, the fluid i aturated liquid at temperature T L and the orrepondg aturation preure P L i the low preure of the ytem. When the TIP i, pot 2 i determed uh that the preure i and the ientropi effiieny of the pump i equal to η p. At pot 5, the fluid ha preure and temperature T H. Fally, pot 6 i determed uh that the turbe ha outlet preure P L and ientropi effiieny η t. It i deirable to produe eletriity a muh a poible with the energy oure available the form of enible heat. Therefore it i neeary to irulate more workg fluid and le oolant fluid for a unit ma flow rate of oure fluid, unle the ph pot ondition i violated. The ratio of ma flow rate of workg fluid and oolant fluid to that of the oure fluid, r and r, an be determed from the energy balane and the heat tranfer effetivene a follow: r T m ( T T ) p h h (2) T η ( T 2) (3) 3 he T m( T T ) ΔT (4) r oure 6 p( T, out PP h h1 r T ) oolant PP (5) m( T T ) ΔT (6) where ubript,, and refer to the workg fluid, oolant fluid, and oure fluid, repetively, and the ma flow rate, h the peifi enthalpy, p the peifi heat at ontant preure. η he and T PP mean the heat tranfer effetivene of the heat exhanger and the ph pot temperature differene. The rate of heat put and work prodution an be alulated from ( ) Q h 5 h (7) 2 [( h h ) ( h )] W W W (8) t p h1 where ubript t and p refer to the turbe and pump, repetively. The exergy whih i a property of ubtane i defed a the maximum ueful work available when the ytem evolve reveribly to reah equilibrium with the environment whih i aid to be dead tate. When a ytem undergoe a teady tate operation, the thermodynami propertie of workg fluid at any referene tate an be aigned zero. It i onvenient to take the ambient ondition or dead tate a the referene tate for the enthalpy, entropy, and exergy. The peifi exergy e and the rate of exergy put to the ytem by oure fluid an be alulated a [14] h h T ( ) { T T T ln( T T )} e (9) E () p / where i the peifi entropy and ubript refer to the dead tate The overall performane of the ytem an be aeed with the thermal effiieny, η th, and the exergy effiieny, η ex, whih are defed a the ratio of work to heat put and exergy put, repetively, a follow. 56

3 International Journal of Material, Mehani and Manufaturg, Vol. 1, No. 1, February 13 η W / Q (11) th ex η W / E (12) It i to be noted that the exergy effiieny i a meaure of how loe the ytem i to a reveribly operatg ytem. III. RESULTS AND DISCUSSIONS In thi tudy the effet of oure fluid temperature on the thermodynami performane of R134a tranritial yle i parametrially vetigated. The redued turbe let preure, P R /P r, i varied up to 3 (about 1 bar for ), while the temperature rangg from o C to o C with an terval of o C i aumed for the oure temperature T S. Other bai data for analyi are a follow; T L o C, T 15 o C, T 15 o C, T H o C, T PP o C, η p.8, η t.8, η he.7. Se it i important organi Ranke yle to effiiently onvert the enible heat to eletriity, the performane of the ytem ludg the ma flow rate of the workg fluid and the power prodution per unit ma flow rate of oure fluid i vetigated for the varyg value of P R and T S. When the TIP i lower than the ritial preure, the yle i a ubritial yle. In thi ae boilg our at ontant temperature and there exit lear phae hange proee the heat exhanger. Fig. 2 i an aement of heat tranfer the heat exhanger, where the perentage of heat tranfer the three etion i figured out. Se the temperature at the exit of uperheater i equal to T S - T H, the portion of heat tranfer the uperheater i larger for higher T S. The ret of heat tranfer i ued to raie the temperature of workg fluid to boilg temperature and then to vaporize it. The former portion i domant for high TIP, while the latter portion i domant for low TIP. Thi i beaue the boilg temperature reae with and the latent heat of vaporization dereae with. In a limit ae of P R 1 there i no heat tranfer the boiler. In ontrat to the nearly lear dependene the preheater and boiler, the variation of the perentage heat tranfer with repet to i relatively mall the uperheater. Perentage of heat tranfer [%] Preheater Boiler Superheater Ratio of ma flow rate, r Fig. 3. Ratio of ma flow rate, r. Fig. 3 how the dependene of the ratio of ma flow rate, r, on the redued turbe let preure, P R, and oure temperature, T S. Notie a vertial le dividg the ubritial and uperritial yle. The ratio of ma flow rate of workg fluid to that of oure fluid reae with P R, and the reag rate beome higher a the oure temperature get lower. For fixed P R, r i larger for higher T S the ubritial ae. However, there exit a reveral of trend the uperritial region. A higher value of r implie that more workg fluid ould be irulated, whih i favorable to produe more power with the ame enthalpy drop. The volume flow rate at the exit of turbe for the prodution of 1 kw of power i hown Fig. 4 a a funtion of the redued TIP for varyg oure temperature. It i an important fator for the eletion of workg fluid of the power plant e it i diretly related with the ize and ot of the turbe. When the ytem i operated a a ubritial yle the turbe exit volume flow rate i not enitive to oure temperature and dereae with the TIP. For relatively high oure temperature, thi trend extend to the uperritial region pite of the lowdown of dereag rate. However the trend hange if the oure temperature i relatively low. The volume flow rate beg to reae pat a bottom pot. It eem, therefore, undeirable to operate a ytem at high preure when the oure fluid temperature i low. Turbe exit flow rate [(m 3 /)/kw] Fig. 2. Perentage of heat tranfer the heat exhanger. Fig. 4. Turbe exit volume flow rate per unit power prodution. 57

4 International Journal of Material, Mehani and Manufaturg, Vol. 1, No. 1, February 13 One of the important apet whih mut be onidered i the peifi power prodution of a power yle. In view of the optimal utilization of low-grade heat oure, work per unit ma of oure fluid i an appropriate variable for uh purpoe and i plotted Fig. 5 with repet to P R and T S. A expeted, the work i larger for higher oure temperature for the ame TIP irrepetive of ubritial or uperritial. When the oure temperature i fixed, the work reae with the TIP a ubritial region. In a uperritial region the reag rate low down and beg to dereae pat a peak pot. The peak pot hift to the right with the reae of T S although they are viible for T S > o C beaue of the preure range limitation. It hould be noted that the peifi work of the uperritial yle are everal time larger than that of the ubritial yle. Net work [kj/kg(oure)] Fig. 5. Net work prodution per unit ma of oure fluid. Fally we will onider the overall performane of the ytem term of the firt and eond law effiienie of thermodynami. Fig. 6 and Fig. 7 how the variation of the thermal effiieny and the exergy effiieny of the yle, repetively, with repet to P R and T S. Thermal effiieny of both ubritial and uperritial yle, a een Fig. 6, i higher if the oure temperature i higher. In ontrat to a margal differene the ubritial yle, it how a trong dependene on T S the uperritial yle. For eah fixed T S, thermal effiieny ha a peak value a uperritial region where the peak pot hift to the right with reag T S. Thi behavior i qualitatively imilar to that of peifi work. In pite of relatively low value ompared to the onventional power yle, thermal effiieny an be raied by adoptg uperritial yle with uffiiently high oure temperature from 12% maximum to 15-17%. The exergy effiieny of the yle i plotted with repet to the redued turbe let preure and oure temperature Fig. 7. Like peifi work Fig. 5 and thermal effiieny Fig. 6, the exergy effiieny reae with the redued TIP the ubritial region and atta a peak value and then dereae the uperritial region for eah fixed oure temperature. The peak value range from 38% to 4% the preure range onidered. Contrary to thermal effiieny, the exergy effiieny get lower a oure temperature get higher until the peak pot appear. In thi range uperritial yle give higher exergy effiieny than ubritial yle. Se a high oure temperature reult a high exhaut temperature, the reuperation of exhaut heat may ontribute to an improvement of exergy effiieny. Exergy effiieny [%] Thermal effiieny [%] Fig. 6. Thermal effiieny of the yle Fig. 7. Exergy effiieny of the yle. IV. CONCLUSIONS In thi tudy, the effet of oure fluid temperature on the thermodynami performane of the R134a tranritial yle i parametrially vetigated for the redued TIP up to 3. The ma reult an be ummarized a follow. The ratio of ma flow rate of workg fluid to oure fluid reae with TIP. For fixed TIP, it i larger for higher oure temperature the ubritial ae. In ubritial yle the turbe exit volume flow rate i not enitive to oure temperature and dereae with the TIP. It ha a bottom pot the uperritial range. The work prodution per unit ma of oure fluid i larger for higher oure temperature for the ame TIP irrepetive of ubritial or uperritial. For fixed oure temperature, it ha a aendg-deendg pattern with the TIP. Thermal effiieny of yle i higher if the oure temperature i higher. For fixed oure temperature, it ha a peak value a uperritial region. It an be raied by adoptg uperritial yle with uffiiently high oure temperature from 12% maximum to 15-17%. The exergy effiieny of yle reae with the redued TIP the ubritial region and ha a peak value the uperritial region for eah oure temperature. ACKNOWLEDGMENT Thi reearh wa upported by Bai Siene Reearh 58

5 International Journal of Material, Mehani and Manufaturg, Vol. 1, No. 1, February 13 Program through the National Reearh Foundation of Korea (NRF) funded by the Mitry of Eduation, Siene and Tehnology (No ). REFERENCES [1] H. Chen, D. Y. Gowami, and E. K. Stefanako, A review of thermodynami yle and workg fluid for the onverion of low-grade heat, Renewable and Sutaable Energy Review, vol. 14, pp ,. [2] N. A. Lai, M. Wendland, and J. Fiher, Workg fluid for high temperature organi Ranke yle, Energy, vol. 36, pp , 11. [3] B. Saleh, G. Koglbauer, M. Wendland, and H. Fiher, Workg fluid for low-temperature organi Ranke yle, Energy, vol. 32, pp , 7. [4] P. J. Mago, L. M. Charma, K. Srivaan, and C. Somayaji, An examation of regenerative organi Ranke yle ug dry fluid, Applied Thermal Eng., vol. 28, pp , 8. [5] K. H. Kim, C. H. Han, and K. Kim, Effet of ammonia onentration on the thermodynami performane of ammonia-water baed power yle, Thermohimia Ata, vol. 5, pp. 7-16, 12. [6] H. Chen, D. Y. Gowami, M. M. Rahman, and E. K. Stefanako, A uperritial Ranke yle ug zeotropi mixture workg fluid for the onverion of low-grade heat to power, Energy, vol. 36, pp , 11. [7] Y. Chen, P. Lundqvit, A. Johanon, and P. Platell, A omparative tudy of the arbon dioxide tranritial power yle ompared with an organi Ranke yle with R123 a workg fluid wate heat reovery, Applied Thermal Eng., vol. 26, pp , 6. [8] Y. J. Baik, M. S. Kim, K. C. Chang, and S. J. Kim, Power-baed performane omparion between arbon dioxide and R125 tranritial yle for a low-grade heat oure, Applied Energy, vol. 88, pp , 11. [9] K. H. Kim and C. H. Han, Analyi of tranritial organi Ranke yle for low-grade heat onverion, Adv. Si. Lett., vol. 8, pp , 12. [] T. Yang, G. J. Chen, and T. M. Guo, Extenion of the Wong- Sandler mixg rule to the three-parameter Patel-Teja equation of tate: Appliation up to the near-ritial region, Chem. Eng. J., vol. 67, pp , [11] L. D. Gao, Z. Y. Li, S. G. Zhu, and S. G. Ru, Vapor-liquid equilibria alulation for aymmetri ytem ug Patel-Teja equation of tate with a new mixg rule, Fluid Phae Equilibria, vol. 224, pp , 4. [12] C. L. Yaw, Chemial propertie handbook, MGraw- Hill, [13] K. H. Kim, Thermodynami performane of regenerative organi Ranke yle, WASET, vol. 59, pp , 11. [14] K. H. Kim and H. J. Ko, Exergetial performane aement of organi Ranke yle with uperheatg, App. Meh. Material, vol. 234, pp , 12. Hyung Jong Ko reeived the Ph.D. degree mehanial engeerg from Korea Advaned Intitute of Siene and Tehnology (KAIST). He i urrently a Profeor the Department of Mehanial Engeerg at Kumoh National Intitute of Tehnology, Korea. Hi reearh teret are the area of modelg of imultaneou heat and ma tranfer, and analyi of magi fluid flow. Se Woong Kim reeived the Ph.D. degree mehanial engeerg from Seoul National Univerity. He i urrently a Profeor the Department of Mehanial Engeerg at Kumoh National Intitute of Tehnology, Korea. Hi reearh teret are the area of automotive engeerg and new energy ytem. Chul Ho Han reeived the Ph.D. degree prodution engeerg from Korea Advaned Intitute of Siene and Tehnology (KAIST). He i urrently a Profeor the Department of Intelligent Mehanial Engeerg at Kumoh National Intitute of Tehnology, Korea. Hi reearh teret are the area of reliability engeerg of ytem and evaluation of formability tet. Kyoung Hoon Kim reeived the Ph.D. degree mehanial engeerg from Korea Advaned Intitute of Siene and Tehnology (KAIST). He i urrently a Profeor the Department of Mehanial Engeerg at Kumoh National Intitute of Tehnology, Korea. Hi reearh teret are the area of modelg and deign of energy ytem. 59