Exergy Analysis of Organic Rankine Cycle with Internal Heat Exchanger

Transcription

1 International Journal of Material, Mechanic and Manufacturing, Vol. 1, No. 1, February 21 Exergy Analyi of Organic Rankine Cycle with Internal Heat Exchanger Kyoung Hoon Kim, Hyung Jong Ko, and Se Woong Kim Abtract In recent year Organic Rankine Cycle (ORC) ha become a field of intene reearch and appear a promiing technology for converion of heat into ueful work or electricity. In thi work thermodynamic performance of ORC with internal heat exchanger i comparatively aeed for variou working fluid baed on the econd law of thermodynamic. Special attention i paid to the effect of turbine inlet preure on the exergy detruction (anergie) at variou ytem component and the exergy efficiency of ytem. Reult how that for a given ource the component at which the greatet anergy occur differ with working fluid. A turbine inlet preure increae, exergy efficiency increae for working fluid uch a ammonia or, but decreae for working fluid with low critical preure uch a io-pentane or n-pentane. Index erm Organic rankine cycle (ORC), internal heat exchanger, exergy, anergy. I. INRODUCION Statitical invetigation indicate that the low-grade wate heat account for % or more of the total heat generated in indutry. Due to the lack of efficient recovery method, a lot of the low-grade energy i merely dicarded. Since the worldwide energy demand ha been rapidly increaing but the foil fuel to meet the demand i being drained, an efficient ue of the low-temperature energy ource uch a geothermal energy, exhaut ga from ga turbine ytem, bioma combution, or wate heat from variou indutrial procee become more and more important. ORC i a Rankine cycle where an organic fluid i ued intead of water a working fluid and appear a a promiing technology for converion of low-grade heat into ueful work or electricity, ince it exhibit great flexibility, high afety and low maintenance requirement [1]-[2]. In an ORC the aturation vapor curve i the mot crucial characteritic of a working fluid. hi characteritic affect the fluid applicability, cycle efficiency, and arrangement of aociated equipment in a power generation ytem []. Drecher and Bruggemann [] invetigate the ORC in olid bioma power and heat plant, and they propoe a method to find uitable thermodynamic fluid for ORC in bioma plant. hey aert that the family of alkylbenzene how the highet efficiency. Dai et al [] ue a generic optimization algorithm identifying iobutane and R26ea a efficient working fluid. Heberle and Brueggemann [6] invetigate the combined heat and power generation for geothermal Manucript received December, 212; revied February 6, 21. he author are with the Department of Mechanical Engineering, Kumoh National Intitute of echnology, Gumi, Gyeongbuk 7-71, Korea ( {khkim, kohj, kw}@ kumoh.ac.kr). reource with erie and parallel circuit of an ORC. ranche et al. [7] invetigate comparatively the performance of olar organic Rankine cycle uing variou working fluid. Volume flow rate, ma flow rate and power ratio a well a thermal efficiency are ued for comparion. Hung et al. [8] examine Rankine cycle uing organic fluid which are categorized into three group of wet, dry and ientropic fluid. hey point out that dry fluid have diadvantage of reduction of net work due to uperheated vapor at turbine exit, and wet fluid of the moiture content at turbine inlet, o ientropic fluid are to be preferred. Kim [9], [1] examine comparatively the thermodynamic performance of ORC with uperheater or internal heat exchanger for variou working fluid including wet, dry and ientropic fluid. He point out that in election of working fluid it i required to conider variou criteria of performance characteritic a well a thermal efficiency. Kim and Han [11] invetigate the thermodynamic performance of trancritical ORC with and without internal heat exchanger for variou working fluid. hey point out that operation with upercritical cycle can provide better performance than that of ubcritical cycle becaue of better thermal match between the working fluid and the enible heat ource. Kim and Ko [12] carry out exergy performance aement of ORC with uperheating comparatively for variou organic fluid. hey how that for a given ource both the anergie and exergy efficiency may have a peak value or monotonically increae with evaporating temperature. In thi paper, the thermodynamic exergetical performance of the organic Rakine cycle with internal heat exchanger i comparatively and parametrically invetigated baed on the econd law of thermodynamic for variou working fluid. he exergy detruction (anergie) at variou component in ORC including ource exchanger, exhaut, condener, and internal exchanger, a well a exergy efficiency are invetigated in term of the ytem parameter uch a turbine inlet preure. II. SYSEM ANALYSIS he chematic diagram of the ytem i hown in Fig. 1. he ytem conidered in thi work conit of condener, pump, turbine, regenerator, preheater, boiler, and uperheater. A low-grade energy in the form of enible energy i upplied to the ytem. he working fluid conidered in thi work are nine fluid of NH (ammonia),,, i (io-butane), R12,, (butane), ic (io-pentane), nc (normal pentane). he thermodynamic propertie of the working fluid are calculated by Patel-eja equation of tate [12], [1]. DOI: 1.776/IJMMM.21.V1.9 1

2 emperature [ o C] International Journal of Material, Mechanic and Manufacturing, Vol. 1, No. 1, February 21 Pump Preheater Internal heat exchanger 2 1 Evaporator L Superheater Condener 6 Heat ource H urbine G greater than the other o the whole temperature-entropy diagram for NH i not hown in the figure [12]. ABLE I: BASIC DAA FOR HE WORKING FLUIDS Subtance M(kg/kmol) c (K) P c (bar) NH i ic nc Fig. 1. Schematic diagram of the ytem. R a( ) P (1) v b v( v b) c( v b) a 2 2 ( ) R c a ( ) (2) P c R b c () b P c R c c () c P c 1 NH 2 io io-c Entropy [kj/kgk] Fig. 2. emperature-entropy diagram of working fluid. he baic data of the fluid which are needed to calculate Patel-eja equation are hown in ABLE I, where M, c, P c and ω are molecular weight, critical temperature, critical preure, and accentric factor, repectively [1]. he molecular weight of NH and i are mall, and thoe of R12 and C 8 are large among the fluid. he critical temperature of and are low and thoe of nc and ic are high. he critical preure of nc and ic are low and thoe of NH and are high. he temperature-entropy diagram for the fluid are hown in Fig. 2. It can be een from the figure that i,, ic and nc belong to dry fluid, and to ientropic fluid, and NH, and to wet fluid. Epecially, the latent heat of vaporization of NH i much 1 A low-grade enible energy i upplied to the ytem and important aumption ued in thi work are a follow. 1) he energy ource i air at temperature of S. 2) he working fluid leave the condener a aturated liquid at temperature of L. ) he evaporating temperature, E i lower than the critical temperature of the fluid and the turbine inlet temperate become S - H by the uperheater. ) he minimum temperature difference between the hot and cold tream in the regenerator i operated at a precribed pinch point, PP. ) Preure drop and heat lo of the ytem are negligible. At point 1, the fluid i aturated liquid at L and the correponding aturated preure P L i the low preure of the ytem. When the turbine inlet preure i P H, the correponding aturation temperature E i the evaporation temperature of the ytem. he thermodynamic propertie at point are determined with the temperature H and the preure P H. he thermodynamic propertie at point 2 and are determined with the ientropic efficiencie of pump and turbine, η p and η t, repectively. A the ma flow rate of working fluid for a given energy ource increae, the temperature of ource flow at preheater exit decreae, and finally the temperature difference between the ource and the working fluid reache the pinch point value PP when the ma flow rate of working fluid i increaed to it maximum value. hen the ratio of ma flow rate of a working fluid to that of the ource, r m, can be determined a r m m m c p h h, out (), out (6) PP where ubcript and denote the working fluid and the ource fluid, repectively, and m the ma flow rate, the temperature, h the pecific enthalpy, c p the contant preure pecific heat of ource fluid, and PP the pinch point temperature difference of the heat exchanger. he rate of heat input and net work are obtained a Q in m h h (7) h h h W W W m (8) net t p 2 h1 2

3 Anergy ratio at heat exchanger [%] Anergy ratio of exhaut [%] Anergy ratio at regenerator [%] International Journal of Material, Mechanic and Manufacturing, Vol. 1, No. 1, February 21 where ubcript t and p denote turbine and pump, repectively [1]. When a ytem undergoe a teady tate operation, the thermodynamic propertie of working fluid can be arbitrarily aigned to be zero a reference value. herefore the thermo-mechanical enthalpy, entropy, and exergy at the ambient condition or dead tate can be neglected regardle of it chemical compoition. he pecific exergy e and the rate of exergy input to the ytem by ource fluid can be calculated a [12] e h h (9) ln / E m c (1) in p where i the pecific entropy and ubcript mean the dead tate. he exergy efficiency of the ytem η ex, which i defined a the ratio of net work to exergy input, can be written a follow. W / E (11) ex he exergy detruction or anergy of the adiabatic ytem i calculated a the difference of exergy input and output. he anergy ratio at the ytem component uch a preheater i defined a the ratio of anergy there to exergy input by ource fluid. net in anergy ratio at ource heat exchanger for variou working fluid. Becaue of the limitation of turbine inlet preure lower than the critical preure, the range of the turbine inlet preure i narrower for ome fluid uch a io-pentane or normal pentane. he anergy ratio at heat exchanger decreae monotonically with increaing turbine inlet preure except for the cae of whoe critical temperature i the lowet, at leat for the pecified condition of the work. For a pecified value of the turbine inlet preure, the anergy ratio for ammonia or which ha high critical preure i high, while that of io-pentane or normal pentane which ha low critical preure i low. For ammonia, the anergy ratio at heat exchanger i the greatet among the component of the ytem. he anergy ratio at internal heat exchanger or regenerator are plotted with repect to turbine inlet preure in Fig. for each fluid. he anergy ratio i a monotonically decreaing function of turbine inlet preure for all fluid. hi i mainly becaue a the turbine inlet preure increae, the turbine exit temperature decreae due to higher preure ratio, and it lead maller temperature difference of the hot and cold tream inide the internal heat exchanger. For a pecified value of the turbine inlet preure, the anergy ratio for or i high, while that of io-pentane or normal pentane which ha low critical preure i low. For, the anergy ratio at regenerator i the greatet among the component of the ytem. III. RESULS AND DISCUSSIONS In thi work the baic data for analyi are L = 2 o C, S = 2 o C and H = 1 o C, o the turbine inlet temperature in thi work i fixed at H = S - H = 18 o C. he turbine inlet preure P H i varied from 2 to bar under the retriction that P H i lower than the critical preure of the working fluid and the minimum temperature difference between the tream in the heat exchanger i equal to the precribed pinch point temperature difference, PP. Since the cycle performance i trongly dependent on the ource temperature level, the exergetical performance of ORC with uperheating i aeed by invetigating the dependence of anergy ratio (Fig. ~6) and exergy efficiency (Fig. 7) on the turbine inlet preure. NH io- io-c Fig.. Anergy ratio at regenerator. 7 6 NH io- io-c NH io- io-c Fig.. Anergy ratio at heat exchanger. Fig. how the effect of turbine inlet preure on the 1 2 Fig.. Anergy ratio of exhaut. Fig. how the effect of turbine inlet preure on the

4 Exergy efficiency [%] Anergy ratio of turbine and pump [%] International Journal of Material, Mechanic and Manufacturing, Vol. 1, No. 1, February 21 anergy ratio due to exhaut of ource fluid for the working fluid. A turbine inlet preure increae, the anergy ratio increae at a certain rate, and after a certain point it increaing rate become lower. hi can be explained a follow. A the turbine inlet preure increae, the correponding aturated temperature alo increae, which caue the exit temperature of ource fluid higher. hen, the temperature difference between fluid tream increae in the heat exchanger, o exergy detruction doe. However, when the turbine inlet preure increae to a certain value at which the working fluid entering the heat exchanger a aturated liquid, the increaing rate of exit temperature of ource fluid become maller or even minu. For a pecified value of the turbine inlet preure, the anergy ratio for io-pentane or normal pentane i high, but that of or ammonia i relatively low. he anergy ratio due to exhaut of ource fluid i the greatet among the component of the ytem. 1 1 NH io- io-c 1 2 Fig. 6. Anergy ratio of turbine and pump NH io- io-c 1 2 Fig. 7. Exergy efficiency For variou working fluid, Fig. 6 and Fig. 7 how the effect of turbine inlet preure on the anergy ratio of turbine/pump and exergy efficiency, repectively. A it i een in the figure, the anergy ratio of mechanical work of turbine and pump i approximately proportional to the exergy efficiency, ince in thi work ientropic efficiencie of turbine and pump are aumed to be contant for variou value of ytem parameter. he increae in the turbine inlet preure ha poitive or negative effect on the exergy efficiency, which i dependent on the working fluid. A turbine inlet preure increae, the exergy efficiency increae for ammonia,,, io-butane,,, and butane. But it decreae for io-pentane and normal pentane. Fig. 7 alo how that the working fluid which ha the maximum exergy efficiency varie with turbine inlet preure. IV. CONCLUSIONS In thi paper, the exergetical performance of organic Rankine cycle with internal heat exchanger ha been analyzed baed on the econd law of thermodynamic. he anergy ratio at ource heat exchanger or regenerator decreae monotonically with increaing turbine inlet preure for all fluid. For ammonia, the anergy ratio at heat exchanger i the greatet among the component of the ytem. However, the anergy ratio at regenerator i the greatet for, while the anergy ratio of exhaut i the greatet for io-pentane or normal pentane. Exergy efficiency generally increae with turbine inlet preure for uch a ammonia or, but decreae for io-pentane and normal pentane. For a given ource temperature, working fluid which ha the maximum exergy efficiency varie with turbine inlet preure. ACKNOWLEDGMEN hi paper wa upported by Reearch Fund, Kumoh National Intitute of echnology. REFERENCES [1] N. A. Lai, M. Wendland, and J. Fiher, Working fluid for high temperature organic Rankine cycle, Energy, vol. 6, pp , 211. [2] K. H. Kim, C. H. Han, and K. Kim, Effect of ammonia concentration on the thermodynamic performance of ammonia-water baed power cycle, hermochimica Acta, vol., pp. 7-16, 212. []. C. Hung,. Y. Shai, and S. K. Wang, A review of organic Rankine cycle (ORC) for the recovery of low-grade wate heat, Energy, vol. 22, pp , [] U. Drecher and D. Brueggemann, Fluid election for the organic Rankine cycle (ORC) in bioma power and heat plant, Applied hermal Eng., vol. 27, pp , 27. [] Y. Dai, J. Wang, and L. Gao, Parametric optimization and comparative tudy of organic Rankine cycle (ORC) for low grade wate heat recovery, Energy Conv. Mgmt., vol., pp , 29. [6] F. Heberle and D. Brueggemann, Exergy baed fluid election for a geothermal organic Rankine cycle for combined heat and power generation, Applied hermal Eng., vol., pp , 21. [7] B. F. chanche, G. Papadaki, and A. Frangoudaki, Fluid election for a low- temperature olar organic Rankine cycle, Applied hermal Eng., vol. 29, pp , 29. [8]. C. Hung, S. K. Wang, C. H. Kuo, B. S. Pei, and K. F. ai, A tudy of organic working fluid on ytem efficiency of an ORC uing low-grade energy ource, Energy, vol., pp , 21. [9] K. H. Kim, Effect of uperheating on thermodynamic performance of organic Rankine cycle, WASE, vol. 78, pp , 211. [1] K. H. Kim, hermodynamic performance of regenerative organic Rankine cycle, WASE, vol. 9, pp , 211. [11] K. H. Kim and C. H. Han, Analyi of trancritical organic Rankine cycle for low-grade heat converion, Adv. Sci. Lett., vol. 8, pp , 212. [12] K. H. Kim and H. J. Ko, Exergetical performance aement of organic Rankine cycle with uperheating, App. Mech. Material, vol. 2, pp. 69-7, 212.

5 International Journal of Material, Mechanic and Manufacturing, Vol. 1, No. 1, February 21 [1]. Yang, G. J. Chen, and. M. Guo, Extenion of the Wong-Sandler mixing rule to the three-parameter Patel-eja equation of tate: Application up to the near-critical region, Chem. Eng. J., vol. 67, pp. 27-6, [1] L. D. Gao, Z. Y. Li, S. G. Zhu, and S. G. Ru, Vapor-liquid equilibria calculation for aymmetric ytem uing Patel-eja equation of tate with a new mixing rule, Fluid Phae Equilibria, vol. 22, pp , 2. [1] C. L. Yaw, Chemical propertie handbook, McGraw-Hill, 1999 Kyoung Hoon Kim received the Ph.D. degree in mechanical engineering from Korea Advanced Intitute of Science and echnology (KAIS). He i currently a Profeor in the Department of Mechanical Engineering at Kumoh National Intitute of echnology, Korea. Hi reearch interet are in the area of modeling and deign of energy ytem. Hyung Jong Ko received the Ph.D. degree in mechanical engineering from Korea Advanced Intitute of Science and echnology (KAIS). He i currently a Profeor in the Department of Mechanical Engineering at Kumoh National Intitute of echnology, Korea. Hi reearch interet are in the area of modeling of imultaneou heat and ma tranfer, and analyi of magnetic fluid flow. Se Woong Kim received the Ph.D. degree in mechanical engineering from Seoul National Univerity. He i currently a Profeor in the Department of Mechanical Engineering at Kumoh National Intitute of echnology, Korea. Hi reearch interet are in the area of automotive engineering and new energy ytem.