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Available at journal homepage: /locate/ijhydeneModel-based determination of hydrogen system emissionsof motor vehicles using climate-chamber test facilitiesMartin Weilenmanna,?, Christian Bacha, Philippe Novaka, Andrea Fischerb, Matthias HillbaEMPA, Swiss Federal Laboratories for Materials Testing and Research, I.C. Engines Laboratory, CH-8600 Duebendorf, SwitzerlandbEMPA, Swiss Federal Laboratories for Materials Testing and Research, Air Pollution/Environmental Technology Laboratory,CH-8600 Duebendorf, Switzerlanda r t i c l e i n f oArticle history:Received 21 March 2007Received in revised form28 September 2007Accepted 28 September 2007Available online 26 November 2007Keywords:Fuel cellGaseous fuelsHydrogenSystem emissionsTest cellConcentration measurementa b s t r a c tBecause of air quality problems, the problem of CO2related greenhouse gas emissionsand shortage of fossil fuels, many vehicles with gaseous fuels (CNG, biogas, hydrogen, etc.)are under research and development. Such vehicles have to prove that both their exhaustemissions and their overall system emissions (including running loss) remain belowcertain safety limits before they can be used in practice. This paper presents a cost-effective way of monitoring such system emissions from hydrogen or other gaseousfuel powered vehicles within an air-conditioned chassis dynamometer test cell, ascommonly used for low ambient emission tests on gasoline vehicles. The only additionalequipment needed is a low-concentration sensor for the gas of interest (e.g. hydrogen).The method is based on concentration measurements and a dynamic mass balance model.It is shown by a real experiment that very low emissions can be recorded. Additionally,error bounds and sensitivities on different parameters such as air exchange ratio arequantified.& 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rightsreserved.1.IntroductionVehicles with gaseous fuels are becoming more and morecommon as they show many advantages compared to gaso-line or diesel fuelled cars 1. In some countries LPG issignificantly cheaper than liquid fuels since it is left behind aswaste in the refinery process. Natural gas as fuel offersnotable CO2and exhaust emission benefits over gasoline anddiesel. In addition, the production of biogenic methane thatcan be used as natural gas shows one of the highest field-to-wheel efficiencies and the best CO2balance among biofuelsand additionally can be produced from waste 1. Hydrogen asfuel for fuel cells as well as for I.C. engines is likely to play animportant role in future vehicle propulsion technology. Thedevelopment of hydrogen-powered vehicles is also driven byair quality factors, the problem of CO2and other greenhousegases and fossil fuel supply dependency 2,3.For all these gaseous fuels there are different fuel storagesystems such as high-pressure gas bottles, low temperatureliquidation, metal hydrides and others, operating at a certainoverpressure. Thus, it is of great interest to developers,manufacturers and legislators to be able to monitor theoverall system emissions of these gaseous fuels underrealistic conditions. This holds for both the case of a parkedcar subject to changing ambient conditions and emissions atsystem start, while running and at system stop 4,5.All these situations can be simulated on chassis dynam-ometers embedded in climatic chambers. However, it is onlypossible with enormous effort to keep such climatic cham-bers so airtight that the evaporative emissions can beARTICLE IN PRESS0360-3199/$-see front matter & 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2007.09.041?Corresponding author: Tel.: +41448234679; fax: +41448234044.E-mail address: martin.weilenmannempa.ch (M. Weilenmann).I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y33 (2008) 863 869measured by monitoring the increasing gas concentrationwithinthechamberdirectly.Themethodstomonitorevaporative hydrocarbon emissions from gasoline vehiclesdescribed in 6,7 allow either the measurement in an airtightchamber (SHED sealed housing for evaporative determina-tion) or the measurement with the so-called point sourcemethod. While the SHED-method works well for test on thestanding car, severe sealing problems occurred for running-loss tests, where a chassis dynamometer is needed within theSHED. For hydrogen in particular, the sealing of the housingposes even more of a challenge than for evaporative emissionmeasurement of hydrocarbons. In the alternative pointsource method all potential points of leakage as fuel tankcap or vent opening need to be equipped with funnels leadingto an analyser with an appropriate air flow, strong enough tocollect all emissions but small enough to keep concentrationsmeasurable. Thus, this method results in rather extendedequipment that needs to be adapted to each vehicle, and onecannot be sure that the total of evaporative emissions is reallymonitored.Alternatively this paper presents a method to monitorrunning loss and system emissions of gas-fuelled cars byapplying the mass balance method to a climatic test cell withventilation. It is shown what sensor equipment is needed andhow the source emissions are calculated. The method isvalidated by tests. A sensitivity analysis is also presentedshowing what limiting conditions have to be fulfilled to reacha certain quality of measurement.2.Methodology2.1.Mass balanceThe basic idea of this approach is the fact that atoms cannotvanish. Based on Fig. 1 showing a chassis dynamometer in aclimatic chamber, this leads to the mass balance of Eq. (1).The change in mass of a gaseous matter (subsequently calledgas G) inside the chamber is the difference between all massflowing into the chamber and all mass flowing out of thechamber. This assumes that no chemical reactions betweenthe gas concerned and other matter take place. This istypically true of hydrogen, methane or propane at ambientconditions and concentrations below 1ppm 8.qmGqtXm?G;in?Xm?G;out,qmGqt m?G;vent_in m?G;car?Xm?G;out.1qmGqtdenotes the change in mass of gas G within the cell,Pm?G;inthe sum of all mass flows of gas G into the chamberandPm?G;outthe sum of all mass flows of gas G out of thechamber, m?G;vent_inthe mass flow into the chamber fromventilation and m?G;carthe source flow of interest. All variablesare functions of time.A mass flow of gas G into the chamber occurs if this matteris found in the ambient air. Thus, the mass flow of ventilationair and the concentration of gas G in the intake air need to beARTICLE IN PRESSChassis dynamometerDrivers aidAirstream ventilatorMax. flow: 100000 m3/hInternal ventilation, airconditionning heat exchangerVintakeVair,vent_in*VleakageVexhaustVair,vent_outFig. 1 Sketch of climatic chamber with ventilation flows.I N T E R NAT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y33 (2008) 863 869864measured. The second mass flow into the chamber is theevaporation from the vehicle, which is of interest.There are different possibilities for flows of gas G out of thechamber:?Intended ventilation.?Leakage. The doors of the chamber as well as channels forcables and pipes are not airtight, so some air leaks. Mostclimatic chambers operate with a slight overpressure toensure that air flows out at all openings, since inflowinghumid air, when operating at low temperatures, wouldcause dangerous ice formation and additionally disturbthe humidity control of the chamber (Fig. 1).?If the vehicle is running and is propelled by a system thatconsumes air (engine or fuel cell system), either thecorresponding air supply can be from outside the chamberor air from the room is used. Since the exhaust gases aretypically led outside the chamber and measured there, thelatter case is also an outflow for the mass balance of gas G.It is obviously not possible to measure the mass flow andconcentrations of gas G at all the outflow locations, but thisproblem can be bypassed by the following approach.The chassis dynamometers for exhaust emission measure-ments are equipped with fans for the cooling of the vehicle.Together with the ventilation of the air conditioning of thecell, this can cause such high turbulence that the concentra-tionofgasGintheroomcanbeconsideredtobehomogeneously distributed. In other words the mixing timeconstant in the room must be significantly lower than the airexchange rate. It must be ensured that no dead zones whereventilation is poor exist inside the climatic chamber. In otherwords, in most cases where chassis dynamometers areinstalled in climatic cells, the rolls of the dynamometer aswell as the breaking electric motor are in an under-floorcompartment that is contained in the cell. It must thereforebe possible for this compartment to be ventilated intention-ally by opening covers and adding additional ventilators.If the concentration of gas G inside the chamber is indeedhomogenous and measured, this concentration also holds forall the outflows of the cell. As long as the pressure remainsstable within the cell, which is controlled by the ventilation,the total mass flow of air out of the cell is equal to the flowinto the cell. It is thus sufficient to measure the air inflow.In addition, since the concentration inside the climaticchamber is homogenous, it needs to be measured at just onelocation.Naturally, a flow of gas G as the inflow m?G;vent_incannotbe measured directly. By assuming ideal gases, it may bedetermined as follows. Any mass flow of air m?airis theproduct of air density rairand volume flow V?air.m?air rairV?air.(2)The contained mass flow of gas G then ism?G cGrGV?air,(3)where cGis the concentration of gas G and rGits density. Sincethe tests take place in a climatic chamber and do not last fordays, it may be assumed that both temperature and pressureremain stable, and thus that densities are constant. It is thussufficientto measure the volume flow of air and theconcentration of gas G to determine its mass flow. For thechamber it correspondingly holds thatmG cGrGVch.(4)The index ch stands for chamber. Assuming that the volumeflow of air out of the chamber is equal to the inflow and thatthe distribution of gas G in the chamber is homogenous,Eqs. (1)(4) giveqcG;chqtrGVch cG;vent_inrGV?air;vent_in m?G;car? cG;chrGV?air;vent_in.5And this can be solved for the mass flow of the source, thusthe carm?G;carqcG;chqtrGVch? cG;vent_inrGV?air;vent_in cG;chrGV?air;vent_in.6So, the system emissions as mass per time unit can becalculated by knowing the chamber volume, the density ofgas G (thus temperature and pressure) and measuring thevolume flow of air into the chamber as well as the gasconcentration of G inside the chamber and in the air intake.As pressure and temperature in inflow and outflow are alike,densities for both flows can be considered to be equal.2.2.Measurement equipmentA commercial gas chromatograph (Reduction Gas Analyzer(RGA3), Trace Analytical Inc., California, USA) was used tomeasure H2inside the climatic chamber. The RGA3 is anultra-trace level gas detection system capable of monitoringlow ppb concentrations of reducing gases such as H2.The instrument consists of a microprocessor-controlled gaschromatograph which utilises method of reduction gasdetection.Synthetic air preconditioned by molecular sieve 5AandSOFNOCAT to remove H2O and reaction impurities (CO andH2) is used as carrier gas. Aliquots of air samples are flushedwith a rate of 20ml/min over a 1ml sample loop. Afterequilibration,thesamplevolumeisinjectedontothecolumns. Sample components of interest are separatedchromatographically in an isothermal mandrel-heating col-umn oven. The chromatographic precolumn (Unibeads 1S,60/80 mesh; 1=800? 3000) is mainly used to remove CO2, H2Oand hydrocarbons. Subsequently H2and CO are separated bythe analytical column (molecular sieve 5A, 60/80 mesh;1=800? 3000) and pass into the detector which contains aheated bed of mercuric oxide. Within the bed a reactionbetween mercuric oxide (solid) and H2occurs and theresultant mercury vapour in the reaction is quantitativelydetermined by means of an ultraviolet photometer locatedimmediately downstream of the reaction bed. The columnsare kept at 751C; the detector is heated to 2701C. The amountof H2in the air sample is proportional to the amount ofmercury that is determined.During the quasi-continuous observations of the H2con-centration in the test chamber, measurements were takenARTICLE IN PRESSI N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y33 (2008) 863 869865every 2min. At the beginning and end of each test cyclethe ambient air concentration (concentration of the ventila-tion inflow) was measured for 30min. Typically the concen-trations were very constant over the short time of one testcycle and in the range of the mean of 576 ? 94ppb atDuebendorf 9.Two high concentration reference gases (50 and 100.2ppmH2; Messer Schweiz, Switzerland) were dynamically dilutedwith zero air to the range of interest by means of a dilutionunit (MKAL diluter, Breitfuss Messtechnik GmbH, Harpstedt,Germany). The dilution unit was indirectly referenced againstthe primary gas flow standard of the Swiss Federal Office ofMetrology. The different mixtures of the two high concentra-tion standards showed excellent agreement with each otherandtheNOAA/GDMscale10.DetectionlimitforH2was ?10ppb and the standard uncertainty of the measure-ment 5%.2.3.Analysis methodologyAs described in the previous section the low concentrations ofthe gases of interest cannot be measured with high timeresolution, i.e. within seconds. The equipment describedabove allows a sampling rate of 2min. Thus Eq. (6) needs to besolved discretely.The most direct and simple approach of discretisation isreplacing the derivative of the chamber concentration by thedifference of the last two measured values. For time step kthis results inqcG;chqtt kT ?cG;ch;k? cG;ch;k?1T,(7)where T is the sampling interval 11. Since both ambientconcentration of gas G and ventilation air flow typicallychange very little over one time interval, it does not matter ifthe values at the beginning or the end of the samplinginterval are used. The chamber concentration, however, maychange substantially; thus average concentration during onesampling step is approximated by the mean of the valuesmeasured at either end of it. The mass balance results inm?G;car;kcG;ch;k? cG;ch;k?1TrGVch? cG;vent_in;krGV?air;vent_in;kcG;ch;k cG;ch;k?12rGV?air;vent_in;k.8So the mass emitted during the sampling interval k ismG;car;k rGcG;ch;k? cG;ch;k?1Vch V?air;vent_in;kT?cG;ch;k cG;ch;k?12? cG;vent_in;k?.9Mathematically more complex but also more accurate is thediscretisation by solving the differential equation (5) analyti-cally for one time step, what needs certain assumptions.Here there is freedom to assume all input signals (i.e.V?air;vent_int, cG;vent_int, m?G;cart) as arbitrary functions of time.Hence, if necessary it might be possible to measure theventilation air flow at high time resolution and use that timefunction for the calculus, but usually this flow is reasonablyconstant. The ambient concentration of the gas G typically isconstant too if not working downwind of a huge non-uniformgas source. Of course the time function m?G;cart of how thevehicle emits the gas G is unknown. If the total mass emittedduring one time step mG;car;kis given, the most extreme casefor the calculus is if all of it is released immediately after thetime interval starts or immediately before the time intervalends (peak functions, Fig. 2). The average case happens ifthe vehicle is constantly emitting gas G. For benchmarkingthe quality of this methodology in Section 3.2, Eq. (5) is solvedsubsequently for all three assumptions.In the case of the early peak, the solution of Eq. (5) for thetime t kT iscG;ch;k cG;vent_incG;ch;k?1mG;carrV? cG;vent_in?e?V?=VT,(10)and thus, the mass of gas G emitted in one time period ismG;car;k rVcG;ch;k? cG;vent_ine?V?=VT cG;vent_in? cG;ch;k?1 !.(11)For the late peak case we obtaincG;ch;k cG;vent_in cG;ch;k?1? cG;vent_ine?V?=VTmG;carrV,(12)mG;car;k rVcG;ch;k? cG;vent_in cG;vent_in? cG;ch;k?1e?V?=VT.(13)ARTICLE IN PRESS*mG,car (t)t(k-1)TttCase: early peakCase: constantCase: late peakkT(k-1)TkT(k-1)TkT*mG,car (t)*mG,car (t)Fig. 2 Most extreme possibilities of time functions of gas release for benchmarking.I N T E R NAT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y33 (2008) 863 869866And for the average case of a constant emitting sourcem?G;cart mG;car;k=T:cG;ch;k cG;vent_inmG;carrTV?cG;ch;k?1? cG;vent_in?mG;carrTV?01Ae?V?=VT,(14)mG;car;krTV?VcG;ch;k? cG;ch;k?1e?V?=VT1 ? e?V?=VT? cG;vent_in01A.(15)Even though Eqs. (11), (13) and (15) look rather different,their outputs remain similar as long as the sampling intervalT is small compared to the ventilations time constant V=V?. Sothe quality of this method rises if both the sampling intervaland ventilation are small. Realistic examples are given in thenext section, where both the different methods (Eqs.(11), (13)and (15) and different sampling intervals are applied tothe same test data to highlight how accuracy depends on thedifferent parameters of the system.3.Example and sensitivity analysisThe test examples described here were conducted in theclimatic cell chassis dynamometer of Empa. All numericvalues belong to this test equipment.3.1.Determination of chamber volumeEstimating the air volume in the chamber by geometricalmeans is quite difficult, since the volumes of car, ventilators,heat exchange units, etc., are difficult to describe. Thus a testwhere a well-defined volume of helium was released im-mediately and its concentration was measured subsequently,while external ventilation was closed but internal circulationwas on, allowed the chamber volume to be estimated fromthe dilution ratio. The value was found to be 256m3with astandard deviation of 8m3.3.2.Identification of volume flow and validationIf the volume flow of the ventilation is not possible to bemeasured directly, but is constant over time, it is possible todetermine it by the following test.As above, a certain volume of a measurable gas such ashelium is injected into the cell (with ventilation running).After the mixing phase in the cell the helium concentrationwill follow Eq. (5) or one of its solutions (10), (12) or (14) with azero source activity of the car m?G;cart 0. Measurements areshown in Fig. 3. Subtracting the background concentrationand building the logarithm of the He concentration result in astraight line for the first 2000s where concentrations arereasonably above the detection limit.The gradient of this straight line is directly the air exchangerate, thus V?=V. Its inverse is the above discussed air exchangetime constant V?=V, and if one of the chamber volume orventilation volume flow is known the other can be deter-mined. Here, with the given chamber volume the volume flowis found to be 0:5605m3=s with a standard deviation of0:005m3=s. The volume flow was found to depend on ambientpressure, and this identification should thus be repeated onthe day of the evaporation tests.In addition, if the volume flow of the ventilation is knownby measurement, similar tests can be used to validate theoverall model. A known amount of helium (or hydrogen) canbe released either in one moment as above or repeatedly ifequipment allows and the calculus (Eqs. (11), (13) or (15)using measured values must give the amount released. Thiswas repeatedly checked.3.3.Evaporation test and accuracy analysisReal hydrogen system emission tests were conducted with ahydrogen vehicle. The test shown here included a parkingphase from 1 to 2523s, then a test ride up to 3842s, whereanother parking phase is monitored up to 7100s (Fig. 4).The left plot in Fig. 4 shows the hydrogen concentrationmeasured with a 2min interval. On the right side theemissions of the car for each time interval are displayed.ARTICLE IN PRESS05001000150020002500300001020304050Helium concentration in air exchange testHe ppmtime s050010001500200025003000-4-2024log(He - Heambient)He ppmtime sFig. 3 Determination of air exchange rate by helium injection experiment.I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y33 (2008) 863 869867They are calculated applying the different methods andassumptions, i.e. Eqs. (9), (11), (13) and (15).For the given situation with a chamber volume of 256m3, aventilation volume flow of 0:5605m3=s (giving an air exchangetime constant of 463s or 7.72min) and a sampling rate of2min, the accuracy results are as follows: The approximateformula (9) and the accurate formula (15), both assuming thatthe vehicle emissions are constant over one sampling intervaldiffer less than 0.5% from each other. The values calculatedby the worst case equations (11) and (13), assuming shortemission peaks at the beginning or end of the samplingintervals, produce errors of 14% and ?12%. As can be seenfrom the overall characteristic of the mass emission curve(Fig. 4, right), however, it is very implausible that theemissions of the vehicles are peak-like and those peaksexactly synchronised with the sampling. Thus, the realaccuracy locally, when emissions start or stop, may be asuncertain as ?12% to 14%. The overall or aggregated emis-sions, however (as displayed in Fig. 5), will show a muchhigher accuracy in all practical cases.From Figs. 4 and 5 it can be readily seen that this vehicleshows rather small system emissions while running, i.e.0.0046g after a 21min ride (3842s). Conversely they riseremarkably after system stop. The maximal gas flow reaches4.32mg/min some 20min (1200s) after engine stop anddecreases slightly afterwards. Obviously some parts of thehydrogen system leak after system stop until they areexhausted.Note that all variables such as ventilation flow and ambientconcentrationsareconsideredtobeconstantwithineach time step. If they vary slowly and their values aremeasured, this methodology is also applicable with the sameaccuracy.3.4.Sensitivity analysisThe sensitivity to measurement errors of this method can beanalysed by standard error propagation methods 12. Itshows that random errors in the measurement of thechamberconcentrationhaveaconsiderableimpactonthe quality of step by step results, caused by building thedifference of two measured values. These errors are, however,compensated when the integral emission is built.A systematic error of the concentration values, i.e. a biasbetween the chamber values and the ambient (or inflow)values would result in an incorrect linear trend underlyingthe integral signal. Such a trend can be detected easily, if thetested vehicle shows phases with assured zero emissions,such as after being stationary overnight. Alternatively, suchbias can be reduced by using the same sensor for ambient(intake) and chamber concentration measurements, what isrecommended.In addition this method is sensitive to the ratio of samplingrate and air exchange rate.This sensitivity is highlighted by just neglecting intermedi-ate data points in the above example. In this way, thesampling rate can easily be simulated to be a multiple ofthe original sampling of 2min. It can be seen in Table 1 thatwithincreasedsamplingtimetherangeoftheoreticaluncertainty increases. When the sampling time reachessimilar values to the air exchange time constant of 7.72min,i.e. 6 or 8min, then the maximal uncertainty rises above 50%,and the values of single steps thus become somewhatARTICLE IN PRESS02000400060008000600800100012001400160018002000Hydrogen concentrationConc (H2) ppbtime s02000400060008000-202468x 10-5Massflow of hydrogen form carMassflow H2 g/stime sequ. (9)equ. (15)equ. (11)equ. (13)Fig. 4 Evaporation test: left: chamber concentration, right: calculated emission mass flow. Note: Although the shape of theleft and right curve appears similar, the right one is sharper because it includes the derivative of the left curve.02000400060008000-0.0500.0Cumulated hydrogen emissionsMass H2 gtime sFig. 5 Cumulative hydrogen emissions.I N T E R NAT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y33 (2008) 863 869868unreliable. In parallel, the error of the simplified approach ofEq. (9) also rises when sampling time is increased.This finding coincides with Shannon information theorempostulating that the sampling frequency should be higherthan twice of the highest system frequency, thus, here,sampling should be clearly faster than half the air exchangerate. Thus the sampling rate of 2min fulfils Shannon theoremas the system time constant (air exchange rate) is 7.72min,what results in the above-mentioned accuracy of ?12 to 14%.Again, since in practice the emissions of the vehicle will notoccur in a peak-like manner with multiple peaks synchro-nised with the measurements sampling, the integral accuracywill be much better than the maximal local errors suggest.This can also be seen in Fig. 6, where the cumulativehydrogen emission curves are almost equal for four differentsampling rates. The final error is below 1% for the 8minsampling compared to the 2min sampling.4.ConclusionsA method to measure system emissions for vehicles withgaseous fuels has been introduced. This method is based onconcentration measurements in the test cell and dynamicmass balance calculus. Emissions as small as 2g per hour areeasily detectable.This method is applicable if the following conditions hold:?Internal ventilation of test cell is high, so that the chamberconcentration can be considered as homogenously dis-tributed.?Air exchange rate is at least two times slower than thesampling rate of concentration measurements. Accuracyrises with faster sampling and slower air exchange rate.?The air exchange rate and the inflow (ambient) concentra-tion of the gas in question must be measured.The latter is easily feasible if the test cell is air-conditioned byan overpressure system, where all