Computational modelling of an Organic Rankine Cycle (ORC) waste heat recovery system for an aircraft engine

Transcription

1 Computatonal modellng of an Organc Rankne Cycle ORC) waste heat recovery system for an arcraft engne S. Saadon 1 1 Department of Aerospace Engneerng Faculty of Engneerng Unversty Putra Malaysa Malaysa Abstract. Escalatng fuel prces and carbon doxde emsson are causng new nterest n methods to ncrease the thrust force of an arcraft engne wth lmtaton of fuel consumpton. One vable means s the converson of exhaust engne waste heat to a more useful form of energy or to be used n the arcraft envronmental system. A onedmensonal analyss method has been proposed for the organc Rankne cycle ORC) waste heat recovery system for turbofan engne n ths paper. The paper contans two man parts: valdaton of the numercal model and a performance predcton of turbofan engne ntegrated to an ORC system. The cycle s compared wth ndustral waste heat recovery system from Hangzhou Chnen Steam Turbne Power CO. Ltd. The results show that thrust specfc fuel consumpton TSFC) of the turbofan engne reach lowest value at 0.91 lbm/lbf.h for 7000 lbf of thrust force. When the system nstallaton weght s appled the system results n a.0% reducton n fuel burn. Hence mplementaton of ORC system for waste heat recovery to an arcraft engne can brng a great potental to the avaton ndustry. 1 Introducton Growng consumpton of prmary fossl fuels and massve dscharge of pollutants are some of the results caused by the world s growng populaton and eventually the enlargng energy demand. It s therefore the man concerns that the developng world must face nowadays are the energy shortfall and the envronmental destructon. Snce 1973 the world energy consumpton has been crucally ncreased and the world energy demands are growng up to 89% startng from 006 tll ths year [1]. Ths affects sgnfcantly those ndustres whch waste a huge amount of energy. And for these vald reasons the awareness of the use of the low-grade heat sources has captvated researchers around the world n recent years. To manage ths matter approprate regulatons should be establshed to further utlze the fossl energy and mnmze the msuse energy n a more effectve way. Partcularly there s extra effort n the avaton feld to reach a hgher ualty of propulson system as the fuel cost s ncreasng and the future law s becomng more severe. It was recorded that the waste fuel energy from exhaust engne s over 30-40% and only a small fracton of ths fuel energy whch s roughly 1-5% were converted to useful work []. Apart from creatng a downturn n fossl fuels market waste heat recovery wll also lead to reducton n greenhouse gases and hence makng t the dea of a better future envronment more promsng. There are a lot of dscussons and researches emerge tryng to prove that ths waste heat recovery s a practcal resource of energy due to ts large uantty [3]. An example of steam-based waste heat recovery system appled to several ndustres s Heat Recovery Steam Generators HRSGs). However t s not recommended for a smaller gas turbne engne due to problems of weght as well as supply of water [4]. Hence an Organc Rankne Cycle ORC) system has been proven to be one of the benefcal exhaust heat recovery technologes due to ts small-scale and ts undoubtedly potental ntegraton n the next few years power supply systems. A Rankne cycle s a heat engne thermodynamc cycle whch converts the heat from a devce for example turbne nto mechancal work. In a Rankne cycle the system apples the heat to rse up the and of an organc flud. Thus the name organc Rankne cycle comes from ts use of an organc flud whch has a characterstc of a lud-vapor phase change at a lower than the phase change n water-steam. Ths allows heat recuperaton of Rankne cycle from a lower. Then ths heat recovery can be converted nto useful work whch can be subseuently transforms nto electrcty. Concernng the mplementaton of the ORC technology varous low-grade waste thermal energy ndustres such as solar energy bomass energy waste heat energy and geothermal energy have consdered ths system [5]. In power plant applcatons and marne desel engne there have been several researches concernng the thermal analyss and desgn optmzaton of an ORC usng waste heat source [6-8] and recently a study had also been demonstrated expermentally [9]. Even so to The Authors publshed by EDP Scences. Ths s an open access artcle dstrbuted under the terms of the Creatve Commons Attrbuton Lcense 4.0

2 MATEC Web of Conferences ntegrate the system to an arcraft engne few factors must be taken nto consderaton. Prncpally the am of the system for a power plant s to recuperate a great amount of energy from the exhaust engne to supply electrcty. However n an arcraft engne the fundamental pont s not to produce power but to produce thrust and to reduce a consderable amount of fuel consumpton. Bronck et al. [4] also come up wth the bass reasons for an ORC to be ntegrated to an arcraft gas turbne engne szed whch marks about 16% of the gas turbne performance s mprovement. Conventonally a small amount of ar at a very hgh s bled off from an arcraft engne and used to pressurze the cabn by provdng ar to the envronmental control system. Bleed ar s fundamentally loss of engne thrust as some ar cannot be employed to produce thrust. Therefore development of bleed less arcraft such as an engne drven electrc compressor has started to nfluence the current technologes. However ths s ute msleadng as electrc devces need hgher electrcty power from engne. Ths s where the ORC system comes as soluton. The system draws out waste heat from the turbofan s engne exhaust system and use t to supply electrcty to operate the external ar compressors to supply flow to the arcraft envronmental control system or other arcraft systems. It s a recovery system that ntends to ncrease the overall performance of the engne. As a result a smaller amount of electrcty power s needed from the arcraft engne hence less fuel consumpton and ths lower the engne bleed ar and therefore a hgher thrust could be acheved. However researches on desgn and performance analyss of an ORC waste heat recovery system ntegrated to arcraft engne and ther possble advantages are stll very few. Only one assessment study so far done by C A Perullo et al. [10]. They examned the feasblty and the benefts of an ORC heat recovery system to be used for nflght arcraft power generaton. In ths study they utlze the Envronmental Desgn Space EDS) as the smulaton tool [11 1]. The method apples a refne boler stuated nsde the nozzle walls of an arcraft engne to extract heat from the engne exhaust. The organc flud chosen to evaluate the system was R45fa because of ts hghest cycle energy performance across multple operatng compare to other three possble fluds. Nevertheless because of lmted tme and resources they decded to make several modelng assumptons. The researchers decded to use a fxed heat transfer coeffcent to search for the off-desgn condtons and they estmated a loss only n the turbne and although the engne flud flow progress durng the engne runnng they assumed a fxed core exhaust flow of 30% that lnks to the evaporator. Conseuently about 0.9% fuel burn savngs s possble but t depends on the whole engne system weght. However ther am s not to analyze the thermal performance n terms of how the heat transfer beng produced and the overall performance of the system; hence a very bref presentaton of the model was provded. A more recent study done by Saadon et al. [13] tred to understand clearly the thermodynamc behavor of the ORC system when connected to exhaust turbofan engne. However ther analyss s stll very loose n the am to understand clearly the man cause of such behavor and the mpact of the ORC system to the arcraft was not done. Therefore ths study dffers from the prevous n the followng: the detaled study on the heat transfer process of the ORC system conducted and ts algorthm; the analytcal study of ther thermal performance; the evaluaton of the Thrust Specfc Fuel Consumpton TSFC) of the arcraft engne and the fuel burn. Numercal model of ORC ntegrated to an arcraft exhaust engne An ORC system ntegrated wth an arcraft exhaust engne s presented n a dagram n Fg. 1 whch conssts of an evaporator a turbne a condenser a workng flud and a regenerator that s used to heat up the organc flud before gettng nto the evaporator to acheve a better thermal effcency of the system. Here the workng flud s the organc flud. The Rankne cycle s connected to the arcraft engne at the mddle of the low- turbne ext duct and the core nozzle. Fg. 1 below shows the lnk between the arcraft engne and the ORC system whch s based at the evaporator. The system starts at the outlet of the lud sde of the regenerator staton 1). An nlet of the workng flud to the evaporator s assumed and the system s performed. T ) WF _ In Fgure 1. Schematc dagram of the ORC system wth regenerator ntegrated to an arcraft engne. [13] The ORC system s presented n a T-s entropy) dagram as n Fg. below. A shell-tube heat exchanger s chosen as the evaporator as t s the most demanded n many ndustres and s convenent for hgher- applcatons. It conssts of a shell wth several tubes nsde. The heat s transferred between the two fluds through the tube wall wthn the shell by enterng one flud nsde the tubes whle the other flud flows outsde the tubes.

3 To model the evaporator there are three methods of modellng that can be appled to evaluate the heat transfer of the evaporator. They are dstrbuted modellng zone modellng and sngle node lump modellng methods J Sun and W L [8]. The sngle node lump modellng approach assumes that the dfference nsde each node s neglgble and ths mples that the constant specfc heat nsde s unchanged too. Hence t s normally not possble for ths ORC applcaton where lud-vapor phase change occurs. Meanwhle wth zone modellng the evaporator s separated nto three dfferent zones whch are super-heat two-phase and sub-cool zones. Conseuently the sngle node lump method can be appled to each zone. However a vgorous teratve algorthm s reured to attan the heat transfer surface area of each zone. Otherwse a dstrbuted modellng method treats the evaporator as small dvsons startng from where the flow enters untl towards the outlet. Then eventually the sngle node lump method here t s NTU Number of Transfer Unts) method can be appled n each dvson for evaluaton of heat and mass transfers. Ths dstrbuted modellng method s more accurate compared to the frst two methods. In ths paper the dstrbuted method s appled to model the evaporator. WF WF _ Out WF _ In m WF h h ) 1) Accordng to the E. 1 WF m WF h WF _ Out and h WF _ In are heat transfer rate mass flow rate and nlet and outlet enthalpes respectvely of the organc flud for the secton. Smlarly the heat transfer rate for the exhaust gas HF s noted as T HF p HF HF _ In HF _ Out Here C p HF m HF T HF _ In HF _ Out m HF C T T ) ) ) and are specfc heat mass flow rate and entrance and ext s respectvely of the exhaust gas for the secton. The maxmum heat transfer rate ) throughout the evaporator can be wrtten as MAX MAX MIN HF _ In WF _ In wth C T T ) 3) Fgure. T-s dagram of the ORC system wth regenerator. Fg. 3 below shows the three consecutve parts noted -1 and +1. By usng the dstrbuted modellng method t s adeuate to assume that the heat capacty of the waste heat engne and organc flud are roughly nvarable n each dscrete dvson. Therefore to construct a numercal model for each sngle segment the NTU method s appled. For each segment the enthalpy of the organc flud s supposed to be ncreased because the heat s absorbed from the exhaust engne. Conseuently the heat transfer rate of each segment s sum up to get the total heat transfer of the evaporator. Fgure 3. Dscrete segments of evaporator. [13] C MINm C m C 4) MIN WF p WF HF p HF Meanwhle the effectveness can be defned as [14] 5) where exp NTU 1 C V 1 Cr 1 C r 1 exp NTU 1 C r C MIN Cr 6) C C MAX MAX m WF C NTU s computed as MAX p WF m HF C p HF 1 ) 7) U A NTU 8) C ) MIN The total heat transfer rate of the evaporator where Q N 1 Q N s the total of evaporator segments. The power comsumpton n process 5 to 6 s defned by 9) 3

4 MATEC Web of Conferences where W P P 1 m WF Cp WF ) s the effcency of the and s the specfc heat rato of the organc flud. When the organc flud exts the as a saturated or superheated flud and flows through the turbne the power s produced and exts from the turbne as low superheat flud. Then the turbne power output s obtaned from W exp mwf CpWF exptexp_ n ) where s T exp_ n the turbne nlet and exp s the effcency of the turbne. The net power output s then W W exp 1) net W And the thermal effcency of the ORC system s net W W exp Q 13) The total of fuel kerosene) that could be eventually saved s Q m fuel HV fuel 14) where HVfuel s the heatng value of kerosene HVfuel = kj/kg. As the thermal model of the evaporator depends on the effectveness of the heat exchanger and ths effectveness tself depends on the NTU the total heat transfer rate of the evaporator s found numercally by teraton. The procedure s llustrated by the flow chart n Fg. 4. The calculaton commences wth an ntal estmaton of NTU. The correspondng organc flud mass flow rate and heat capacty of both fluds are then calculated whch allows determne the new value of NTU and C r for each zone. The evaporator effectveness s then evaluated for each zone. For the frst zone the ntal organc flud mass flow rate s taken as the nlet value. For subseuent zones the nlet mass flow rate s gven by the outlet mass flow rate of the upstream zone. These steps are repeated untl the dfference between the effectveness calculated for two consecutve teratons s less than 0.1%. Fnally the heat transfer rate for each zone s calculated and the total heat transfer of the evaporator s attaned. Fgure 4. Algorthm of calculaton of the total heat transfer rate. 3 Valdaton of a thermal model Wth the ntenton to valdate the numercal model constructed the ORC model was frst compared to the one for ndustral waste heat recovery [15] wth R13 at coolng water Tcn of 93 K and 303 K. The selected waste heat source s the low steam from Hangzhou Chnen Steam Turbne Power CO. Ltd and s depcted n Table 1 below. The specfc heat of R13 n the range of the desgn value s kj/kg.k whle for kerosene t s sad to be.01 kj/kg.k. The smulaton software s ndependently developed by the researchers through MATLAB. A smulaton s then performed by usng the mathematcal euatons defned above wth nlet of R13 at 100 K. The model was valdated by comparng the results of net power output and system thermal effcency wth the prevously publshed results and these results are presented n Fg. 5 and Fg. 6. It s noted that the net power output and thermal effcency vary wth the nlet heat source. It can be observed that there s a slght devaton compared to the results acheved by Jan Song et al. [15] occurred at hgher level of the heat source. These may be resulted from the constant nlet organc flud assumed n the smulaton. Bascally the nlet of R13 wll vary wth the envronmental and factory condtons. The relatve error of the results obtaned compared to works done by Jan et al. [15] for Tcn = 93 K s 0.43% whle for Tcn = 303 K the dscrepances s a bt hgher at 10.15%. However for the arcraft engne whch the ORC s gong to be appled to n the followng secton the nlet of the organc flud used wll be gven by the engne data and the value s fxed for the desgn pont. As a whole the numercal solutons obtaned n ths study are consstent wth those reported n Jan Song et al. [15]. 4

5 Table 1. Desgn parameters of ORC system wth R13 as the workng flud. Descrpton Unt Value Mass flow rate of R13 Heat source nlet Evaporaton Turbne nlet Turbne outlet Turbne effcency kg/s 1. K 453 K 350 MPa 1.1 MPa ORC ntegrated to an arcraft exhaust engne Ths part presents the advantages of ORC system ntegrated to a CFM56-7B7 turbofan engne on an arcraft sze of The workng flud chosen s the R45fa. The specfc heat of R45fa n the range of the desgn value s 1.36 kj/kg.k. Desgn parameters are lsted n Table below [10]. In order to solve ths system the heat transfer flowng out from the exhaust engne s vared untl t matches the heat transfer calculated usng the heat transfer coeffcent and evaporator area from the detaled desgn. By dong ths t ensures that the evaporator ext set by the thermodynamc lmts of R45fa s mantaned. The smulaton s then executed as before and the results attaned are presented n fgure below. Table. Man parameter of the ORC system for the turbofan engne Pump effcency Net power output System thermal effcency kw 59 % 1.7 Descrpton Unt Value Mass flow rate of R45fa Estmated reured surface area of evaporator Reured heat transfer of evaporator kg/s 3.84 m² 3.7 kw 1105 Exhaust heat K 843 Fgure 5. Varatons of the net power output of the ORC system wth heat source nlet. Inlet of R45fa Outlet of R45fa Turbne nlet Turbne nlet Turbne outlet Turbne effcency Pump effcency K 8 K 393 K 39 MPa 1.1 MPa Fgure 6. Varatons of the system thermal effcences of the ORC system wth heat source nlet. The turbne wthn the ORC was connected to a smulaton of an external compressor n order to evaluate 5

6 MATEC Web of Conferences the mpact on the engne when engne bleed ar was reduced. The result shown below n Fg. 7 s detal common cruse condtons wth Mach at alttude of feet. Even wth the small loss n core thrust ths reduces the TSFC below the base cycle by an average of 3.1% defned n Fg. 7 by Regen + Bleed Savngs. Once the feasblty of the dea of mplementng the ORC on an engne bass was confrmed t was reured to estmate the advantages on the combned engne and arcraft system. Fg. 8 descrbed the fuel burn relatve to the base cycle and takes nto account the vehcle weght throughout the flght. The Regen + Bleed Savngs scenaro show the fuel burn effect only from the TSFC change whereas the Add 950 lbm per engne scenaro assumes a nomnal weght mpact of 950 lbm on the detaled ORC system. When the system nstallaton weght s appled the system results n a.0% reducton n fuel burn. power and thermal effcency although some dscrepances could be found at hgher nlet heat source of the evaporator. These dscrepances can partly be attrbuted to the assumpton of lower organc flud nlet of the evaporator whch results n lower mass flow rate of the flud. At the same tme the smulaton model needs more parameters to be defned whch ncreases the varabltes n the smulaton estmatons. The ntal results for the ORC system ntegrated to a turbofan s exhaust engne provde an dea of the mpact of ths system on the engne s thrust and ts fuel consumpton. Parametrc optmzaton shall be conducted n future n order to maxmze the total heat transfer rate through the evaporator. Such results wll be very useful n future to determne the rreversblty losses through calculaton of exergy of each ORC component n order to acheve an optmze system performance and eventually hgher thrust power. The presented research proved that an ORC cycle wth regenerator gves potental advantages when ntegrated to an arcraft engne and used to provde electrcal power to the arcraft. References Fgure 7. ORC engne wth TSFC effects and ORC mpact on msson fuel burn Fgure 8. ORC engne wth TSFC effects and ORC mpact on msson fuel burn 5 Conclusons Ths paper presents a detaled study of a one-dmensonal analyss method of ORC system mplemented n an arcraft engne. A numercal model for the system has been modfed from prevous authors work. The model features nclude an algorthm of calculaton of the total heat transfer rate drven by the NTU method. Valdaton wth other researchers work for an ndustral waste heat recovery shows a reasonable agreement wth net output 1. M. Abdolzadeh M. Sadekhan A. Ahmad Energy ). M. Hasanuzzaman N.A. Rahm R. Sadur and S.N. Kaz ) 3. D. Zvan A. Beyene and M. Venturn Appled Energy ) 4. L.Y. Bronck and D.N. Schochet Proceedngs ASME Turbo Expo GT ) 5. B.F. Tchanche G. Lambrno A. Frangoudaks and G. Papadaks Renewable & Sustanable Energy Revews ) 6. J. Song Y. Song and C.W. Gu Energy ) 7. M.H. Yang and R.H. Yeh Appled Energy ) 8. J. Sun and W. L Appled Thermal Energy ) 9. W. Pu C. Yue D. Han W. He X. Lu Q. Zhang and Y. Chen Appled Thermal Engneerng ) 10. C.A. Perullo D.N. Mavrs and E. Fonseca Proceedngs of ASME Turbo Expo. Turbne Techncal Conference and Exposton 013) 11. M. Krby and D. Mavrs 6 th Internatonal Congress of the Aeronautcal Scences 008) 1. J. Schutte J. Ta and D. Mavrs 48 th AIAA/ASME/SAE/ASEE Jont Propulson Conference & Exhbtton 01) 13. S. Saadon and A.R. Abu Talb IOP Conference Seres: Materals Scence and Engneerng 15 Number 1 016) 14. J.P. Holman Heat Transfer sxth ed. McGraw-Hll Book Company New York 1986) 15. J. Song C. Gu and X. Ren Energy Converson and Management ) 6