Thermodynamics Analysis of Cascade Reserviors Filling Process of Natural Gas Vehicle Cylinders

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1 World Appled Scences Journal 5 (): , 008 ISSN IDOSI Publcatons, 008 Thermodynamcs Analyss of Cascade Reservors Fllng Process of Natural Gas Vehcle Cylnders 1 Mahmood Farzaneh-Gord, 1 Shahram Hashem and Ahmad Farzaneh-Kord 1 The Faculty of Mechancal Engneerng, Shahrood Unversty of Technology, Shahrood, Iran Department of Physcs, Tarbat Moallem Unversty of Sabzevar, Sabzeva, Iran Abstract: The accurate modelng of the fast-fll dynamcs occurrng n Compressed Natural Gas (CNG) fueled vehcle storage cylnders s a complex process and to date those dynamcs have not been thoroughly studed. Here, based on frst law of thermodynamcs, conversaton of mass and thermodynamcs propertes table a numercal method has been developed to study fast fllng process of natural gas vehcle s (NGV) cylnder. The model has been appled for commonly cascade group of three reservor tanks. The results ndcated that there was a temperature rse n order 40 K or more durng chargng process. The results also ndcated that ambent temperature has bg effect on fllng process and fnal NGV cylnder condtons. Key words: Compressed natural gas CNG cylnder Fast fllng process INTRODUCTION Compressed natural gas s used as a clean alternatve to other automoble fuels such as gasolne (petrol) and desel. The natural gas (CNG) fueled vehcle storage cylnders encountered a rse n storage gas cylnder temperature (n the range of 40 K or more) durng the fast fllng due to complex compresson and mxng processes. Ths temperature rse reduces the densty of the gas n the cylnder, resultng n an under flled cylnder, relatve to ts rated specfcaton. If ths temperature rse s not compensated for n the fuelng staton dspenser, by transently overpressurzng the tank, the vehcle user wll experence a reduced drvng range. The on-board storage capacty of natural gas vehcles (NGVs) s a crtcal ssue to the wde spread marketng of these alternate fuelled vehcles. CNG s dspensed to an NGV through a process known as the fast fll process, snce t s completed n less than fve mnutes. Underfllng of NGV cylnders, durng fast fll chargng operatons, can occur at fuellng statons, at ambent temperatures greater than 30 C. The resultng reduced drvng range of the vehcle s a serous obstacle whch the gas ndustry s strvng to overcome, wthout resortng to unnecessarly hgh fuellng staton pressures, or by applyng extensve over-pressurzaton of the cylnder durng the fuellng operaton. Undercharged storage cylnders are a result of the elevated temperature whch occurs n the NGV storage cylnder, due to compresson and other processes whch have not, to the author's knowledge, been studed, analyzed and documented to date. There have been lmted researches n the fled of current study n lteratures. Kountz et al. [1] were frst who modeled fast fllng process of natural gas storage cylnder based on frst law of thermodynamcs. They develop a computer program to model fast fllng process for a sngle reservor. Kountz et al. [3-5], have also developed a natural gas dspenser control algorthm that nsures complete fllng of NGV cylnders under a fast fll scenaro. The researches are also under way to model fast fllng of hydrogen-based fuellng nfrastructure ncludng [6, 7]. Newhouse et al. [8] have studed fast fllng of hydrogen cylnder usng number of experments. They reported a hgh temperature ncrease n the cylnder durng the process. A few expermental studes were also carred out to study fast fllng of natural gas cylnder ncludng [9, 10]. Shply [10] concluded that ambent temperature change can have an affect on the fast fll process. He also concluded that, the test cylnder was under-flled every tme t was rapdly recharged. Correspondng Author: Mahmood Farzaneh-Gord, The Faculty of Mechancal Engneerng, Shahrood Unversty of Technology, Shahrood, Iran 143

2 Fg. 1: NGV fllng staton-schematc Farzaneh et al. [11] have also modelled fast fllng process. They developed a computer programme based on Peng-Robnson state equaton for sngle reservor. They nvestgated effects of ambent temperature and ntal cylnder pressure on fnal cylnder condtons. The number of natural gas fuel based vehcles n Iran growng rapdly recently due to government polcy. Most of owners of those vehcles have reported under-fllng charge compared to rated condtons. To understand fast fllng process and study effect of ambent temperature, a computer programme has been developed based on frst law of thermodynamcs, conversaton of mass and thermodynamcs propertes table of the gas n ths study. Ths enables us to study effects of varous parameters on dynamcs change n the NGV cylnder. The fast fll process was assumed to be quas-statc process and the natural gas presumed to be purely Methane. CNG FILLING STATION Fgure 1 shows a typcal CNG fllng staton. Gas from the dstrbuton ppelne, usually low pressure at <4bar (0.4MPa) or possbly medum pressure (16 bar), s compressed usng a large mult-stage compressor nto a cascade storage system. Ths system s mantaned at a pressure hgher than that n the vehcle s on-board storage so that gas flows to the vehcle under 144 dfferental pressure. Typcally, the cascade storage wll operate n range of 05 bar to 50 bar, whle the vehcle s maxmum onboard cylnder pressure s 00bar. In order to make the utlzaton of the compressor and buffer storage more effcent, fast fll CNG statons usually operate usng a three-stage cascade storage system. COMPRESSED NATURAL GAS CYLINDERS The natural gas cylnders have varous desgn types based on materals of constructon used. Desgn types nclude Type 1, whch are all-metal, Type, whch have a metal lner and hoop wrapped composte renforcement, Type 3, whch have a metal lner and a full wrapped composte renforcement and Type 4, whch have a non-metallc lner and a full wrapped composte renforcement. Metal contaners and lners are typcally steel or alumnum. Composte renforcements are typcally carbon or glass fbbers n an epoxy resn matrx. CNG cylnders are desgned for a specfed nomnal servce pressure at a specfed temperature essentally a specfed densty (kg/m 3 ) of fuel. Ths wll result n a gven mass of natural gas stored n the fuel contaner. The actual pressure n the fuel contaner wll vary from the nomnal servce pressure as the temperature of the fuel n the contaner vares. Fuellng statons normally fll the cylnder up

3 Table 1: Expermental analyss of natural gas composton the Khangran refnery (the Khangran refnery offcal webste [1]) Chemcal Expermental analyss Component formula (mole fracton %) Carbon doxde CO Ntrogen N 0.48 Methane CH Ethane C H Propane C 3 H Iso butane C 4 H n-butane C 4 H Iso-Pentane C 5 H C 6 +C Total = 100% Fg. : Dmensons of a typcal CNG cylnder 15% of nomnal servce pressure to avod under chargng but ths hghly depends to ambent temperature. The common CNG cylnder type n Iran s Type 1. They have same nsde dameter wth varous heghts dependng to ther volume. Fgure 1 shows dmensons of a typcal CNG cylnder. The contnuty and frst law of thermodynamcs has been appled to the cylnder to fnd thermodynamcs propertes. Consderng the onboard NGV cylnder as a control volume and knowng t has only 1 nlet, the contnuty (conservaton of mass) equaton may be wrtten as follow: dm dt c = m (1) CHEMICAL COMPOSITIONS OF NATURAL GAS Natural gas composton (mxture) vares wth locaton, clmate and other factors. The gas s refned before flowng nto the ppe lnes. Table 1 shows an expermental analyss of typcal natural gas composton whch flows n Iran ppe lnes accordng to the Khangran refnery offcal webste [1]. It can be realsed that the most of compostons occuped very low percentage. By knowng that Methane s occuped about 99% of the gas, For the sake of smplcty t s assumed that Methane s the only substance n the Natural gas. In equaton 1, m s nlet mass flow rate and can be calculated by consderng an sentropc expanson through an orfce. Applyng gas dynamcs laws: 1 1 γ 1 c γ γ r c γ p p p m = CdρrA orfce( ) ( )( ) 1 ( ) pr γ 1 ρr pr γ pc γ 1 f ( ) pr 1 γ+ () m = Cd γp rρra orfce γ+ 1 MATHEMATICAL MODELING γ pc γ 1 f > ( ) Governng equatons: In ths study to model the fast (3) pr γ+ 1 fllng process and develop a mathematcal method, the NGV cylnder s consdered as a thermodynamcs open In equatons, 3 C d s dscharge coeffcent of system whch goes through a quas-steady process. the orfce. 145 γ+ 1 ( γ 1)

4 The Frst laws of thermodynamcs for a control volume n general form can be wrtten as follow: Q + m(h + V / + gz) cv = m( h + V / + gz) + d/dt[m(u + V /+ gz)] + W e e e e cv cv The work term s zero n the fllng process and the change n potental and knetc energy can be neglected. For smplcty heat transfer through the cylnder walls nto envronment s not consdered. By applyng the above assumptons, the equaton 4 can be rewrtten as follow: cv (4) d(mu) /dt = m ( h + V /) (5) Consderng stagnaton enthalpy as h r = h + V / whch s actually equal to enthalpy of the reservor tanks, the equaton 5 s now as follow dmc duc uc + mc = mh r (6) dt dt Combnng equaton 6 & 1, the followng equaton can be easly drven: c um c + m du c = mh r (7) dt NUMERICAL PROCEDURE In theory, t should be possble to calculate all thermodynamcs propertes by knowng two ndependent propertes. The numercal procedure starts usng equaton &3 to calculate nlet mass flow rate. The dfferental equatons 1 and 6 are solved usng Rung-Kuta forth order method to obtan nternal energy (u) and mass (m c ) n the next tme step. Knowng total mass and volume of the cylnder, specfc volume (v) can be calculated. Now by knowng two ndependent thermodynamcs propertes (v,u), other thermodynamc propertes can be calculated by usng Methane propertes table provded by [13]. Solutons end when the NGV cylnder pressure reaches a user-nput pressure (00 bar) level. RESULTS AND DISCUSSTION Here, the NGV cylnder has been consdered adabatc as a result the characterstcs of the orfce, wll not affect the end temperature state n the cylnder. The orfce dameter and the cylnder volume were consdered to be 1 mm and 67 Lters as n [1] respectvely. To study the effect of ambent temperature, ntal temperature of NGV cylnder and reservor tanks are set to the ambent temperature. The results have been presented for commonly used cascade group of three as shown n Fg. 3. It worth also ponted out that no prevous work has been detected n lterature to model cascade system. Cascade reservors: In ths secton, the results are presented for commonly used cascade group of three as shown n Fg. 3. The results presented here are unque and no smlar results have been presented n lterature. Fgure 4 and 5 show effects of ntal cylnder and reservor tank temperature on dynamc pressure and temperature profles of NGV cylnder respectvely, durng fllng process. Note from Fg. 4, there s no dp n temperature profle n early fllng tme however there are two dps n temperature profle whch occurs when supply tank change. Ths behavour could be dscussed as: the Joule-Thompson coolng effect s not hgh enough n early stage of the fllng process and wouldn't be able to overcome converson of supply enthalpy energy to cylnder nternal energy. When the supply Fg. 3: A schematc dagram of cascade reservors 146

5 T=80 T=90 K T= Tme (seconds) Fg. 4: Effect of ntal (ambent) temperature on dynamc temperature profle (cascade system).5e+07.0e E+07 T=80 T=90 K T= E E E Tme (seconds) Fg. 5: Effect of ntal (ambent) temperature on dynamc pressure profle (cascade system) 9.0E E E E E+00 T=80 T=90 K T= E E+00.0E E E Tme (seconds) Fg. 6: Effect of ntal (ambent) temperature on the amount of charged gas (cascade system) 147

6 Chargng tme Fnal cylder Temperature (K) Intal temperature (K) Fg. 7: Effect of ntal (ambent) temperature on chargng tme and the fnal cylnder temperature (cascade system) Rated mass value =10.16 Kg Fll rato Fnal cylder mass (kg) Intal temperature (K) Fg. 8: Effect of ntal (ambent) temperature on fll rato and the amount of charged gas (cascade system) system swtches to hgher pressure reservor tank, the hgher Joule-Thompson coolng effect wth help of low temperature nlet gas, cause small dps n temperature profle. Note from Fg. 4, rse n temperature only occur when the NGV cylnder s connected to lowest pressure reservor tank and temperature profle s nearly monotonc otherwse. Note, dscontnuty n pressure profle n Fg. 5 s due to swtchng to another reservor tank. Fgure 6 shows effects of ntal cylnder and reservor tank temperature on amount of nlet gas. It can be seen that at early tme of fllng, ntal temperature has lttle effect on dynamcs pressure profle. It can be also realzed that low and medum reservor tanks supply about 5% of charged mass and hgh pressure reservor tank supply the rest (50%). 148 The effect of ntal (ambent) temperature on chargng tme and fnal onboard NGV cylnder temperature s shown n Fg. 7. Note from ths fgure, as ntal temperature ncreases, chargng tme decrease and fnal temperature ncreases. The effect of ntal (ambent) temp erature on fll rato and fnal mass of charged gas s shown n Fg. 8. Note form ths fgure, as ntal temperature ncreases, fll rato and the fnal mass of charged gas decreases. CONCLUSION In ths study a numercal method has been developed based on frst law of thermodynamcs, conservaton of mass and thermodynamc propertes of methane to smulate fast fllng process of NGV cylnder. Based on the method a computer program

7 has been bult to study effect of ambent temperature and ntal NGV cylnder pressure. The model has been appled for sngle reservor tank and commonly cascade group of three. The results ndcated that there s a temperature rse n order 40 K or more durng chargng process. Ths would cause under-flled the NGV cylnder and reduce drvng range of the NGV. The results also ndcated that ambent temperature has bg effect on fllng process and fnal NGV cylnder condtons. As ambent temperature rse, the fll rato and amount of charged gas drop whch cause low drvng range as a result, fllng the NGV durng nght probably more effcent than durng the day especally durng summer. ACKNOWLEDGEMENT Ths study has been supported by Shahrood Unversty of Technology funds. Nomenclature A Area (m ) C d Orfce dscharge coeffcent c p, c v Constant pressure &volume specfc heats (kj/kg K) g Gravtatonal acceleraton (m/s ) h Specfc enthalpy (kj/kg) m Mass flow rate (kg/s) M Molecular weght (kg/kmol) P Pressure (bar or Pa) Q Heat transfer rate (KW) T Temperature (K or o C) U Internal energy (kj/kg) v Specfc volume (m 3 /kg) V Velocty (m/s) W Actual work (kj/kg) W Actual work rate (KW or MW) z Heght (m) + C 6 All hydrocarbon compounds wth more than 5 carbon n ther chemcal formula ρ Densty (kg/m 3 ) Subscrpt c NGV cylnder r Reservor tank Intal or nlet condton REFERENCES 1. Kountz, K., 1994, Modelng The Fast Fll Process n Natural Gas Vehcle Storage Cylnders. Amercan Chemcal Socety Paper at 07th Natonal ACS Meetng, March Kountz, K., W. Lss and C. Blazek, A New Natural Gas Dspenser Control System. Paper at 1998 Internatonal Gas Research Conference, San Dego, November 3, Kountz, K., W. Lss and C. Blazek, Method and Apparatus For Dspensng Compressed Natural Gas. U.S. Patent 5: 75, Kountz, K., W. Lss and C. Blazek, Automated Process and System For Dspensng Compressed Natural Gas. U.S. Patent 5: 810, Kountz, Kenneth J. and F. Blazek Chrstopher, NGV Fuelng Staton and Dspenser Control Systems. report GRI-97/0398. Gas Research Insttute, Chcago, Illnos. 6. Lss, W.E. and M. Rchards, 00. Development of a Natural Gas to Hydrogen Fuelng Staton. Topcal Report for U.S. DOE, GTI-0/ Lss, W.E., M.E. Rchards, K. Kountz and K. Krha, 003. Modelng and Testng of Fast-Fll Control Algorthms for Hydrogen Fuelng. Natonal Hydrogen Assocaton Meetng. 8. Newhouse, N.L. and W.E. Lss, Fast Fllng of NGV Fuel Contaners. SAE paper Thomas, G., J. Gouldng and C. Munteam, Measurement, Approval and Verfcaton of CNG Dspensers. NWML KT11 Report. 10. Shpley, E., 00. Study of Natural Gas Vehcles (NGV) durng the fast flls process. Thess for Master of Scence, 00. College of Engneerng and Mneral Resources at West Vrgna Unversty. 11. Farzaneh-Gord, M., H. Eftekhar, S. Hashem, M. Magreb and M. Dorafshan, The Effect of Intal Condtons on Fllng Process of CNG Cylnders. The second Internatonal conference on Modelng, Smulaton and Appled optmzaton, Abu Dhab, UAE. 1. Khangran refnery offcal webste, Natonal Insttute of Standards and Technology webste, avalable at stry/flud/. 149