Hydrogen and its competitors, 2004

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42Risø Energy Report 3Hydrogen for transport5.4Hydrogen for transportANTONIO FUGANTI AND EMANUELE BELLERATE, CENTRO RICERCHE FIAT; ALLAN SCHRØDER PEDERSEN, RISØ NATIONAL LABORATORYIntroductionOne reason to look seriously at hydrogen as an energycarrier is that it can readily be used to power road vehicles,locomotives, ships and aircrafts. The most advanceduse of hydrogen in the transport sector is for motor vehicles,especially cars. This is why automotive manufacturersall over the world are putting a lot of money intohydrogen R&D.Hydrogen can be used to power vehicles by means ofinternal combustion engines (ICEs), fuel cells or gasturbines. Since fuel cells have higher useful energyconversion efficiencies than simple ICEs, they are veryattractive in automotive applications (see below and alsoChapter 5.3).ICEs, however, are a well-established technology that isfairly easy to convert from conventional liquid fuels tohydrogen. For these reasons, some car manufacturers arealso working on ICEs specifically for hydrogen.Most gas turbines today are much too large to be used inroad vehicles. However, R&D in countries includingGermany and the USA aims to develop small hydrogenfuelledgas turbines suitable for road vehicles. When usedwith other energy conversion technologies in hybridcycles, gas turbines should be able to match the efficiencyof fuel cells. For this reason, some people believethat gas turbines may be the winning technology inhydrogen for transport applications.Hydrogen has been applied for transport purposes inaviation (Russian and American aeroplanes) as well as inseveral areas of surface transport (buses, cars and trucks).As mentioned, however, the most intense and advanceddevelopments have been done in the automobile sectorand therefore a detailed description hereof is given in thefollowing section.Hydrogen from the car manufacturers' pointof viewDrivers and technologyEnergy consumption associated with transport isexpected to grow around 2% per year in Europe over thenext few years, and much faster in developing countriessuch as China and India. Oil is the source of 96% of theenergy currently used in transport.Within this context, automakers face increasingly tightlegal restrictions on polluting emissions. There is alsogrowing public pressure to decrease emissions of carbondioxide and other greenhouse gases (GHGs), which areparticularly associated with transport. In Europe, avoluntary agreement by all the car manufacturers has setcarbon dioxide emissions targets of 140 g CO 2 /km for2008 and 120 g CO 2 /km for 2012.CO 2 g/km190Refinements in conventional engine technology willprobably be able to keep pace with legislative requirementsfor pollutants such as NOx and hydrocarbons. Thetargets for CO 2 , however, require improvements in fueleconomy that conventional engines will probably not beable to meet (because with conventional fuels, a givenamount of energy is always associated with a fixed quantityof CO 2 ).The answer may be to use hydrogen-fuelled internalcombustion engines (ICEs) or fuel cells. ICEs running onhydrogen emit NOx, but since their fuel contains nocarbon, their exhaust contains no CO 2 or hydrocarbons.Fuel cells are more efficient than ICEs, and have nomoving parts. When fuelled by hydrogen, they produceonly water. For these reasons, most of the hydrogen R&Dcarried out by automotive manufacturers in recent yearshas focused on fuel cells, particularly proton exchangemembrane fuel cells (PEMFCs). Among the favourablecharacteristics of PEMFCs for transport applications arelow operating temperatures and short start-up times.Figure 18 shows the power train of a simple fuel cellvehicle. The fuel cell generates electricity, which drivesan electric motor. Although the fuel cell vehicle has twoseparate energy conversion steps, compared to one in aconventional ICE vehicle, its overall efficiency can behigher because the fuel cell and especially the electricmotor have very high individual conversion efficiencies.Fuel cells can overcome the problems of range, performanceand refuelling time associated with traditional electricvehicles, which use accumulators to store electricity.Because chemical energy storage media such as hydrogenhave a higher energy density than accumulators, fuel cellvehicles have better range and performance than tradi-165-1701401201995Figure 17: Targets for CO 2 emissions.CEC – ACEA Agreement -26%CEC Review-14%2003 2008 2012 Year
Risø Energy Report 3Hydrogen for transport 435.4Number of prototypes16HydrogenFuel CellDC powerInverterMech power1412AUX F.C.AirElectricMotor108Fuel CellSystem64Figure 18: The power train of a simple fuel cell vehicle.20199719981999200020012002Figure 20: Figures for prototype fuel cell vehicles show that althoughmethanol was a popular choice some years ago, the consensus is now touse pure hydrogen.■ Methanol ■ Hydrogen ■ Gasolinetional electric vehicles. They can also be “recharged” in afew minutes, compared to the hours needed to charge abattery-powered vehicle.Fuels for fuel cell vehiclesThe range of a fuel cell vehicle depends on the amountof hydrogen it can carry. Weight for weight, hydrogencontains more energy than any other chemical energystorage medium, but its low density is a problem in boththe liquid and gas phases. Gasoline, for example, has avolumetric energy content of around 32 MJ/l, but liquidhydrogen is only around 8.4 MJ/l.The low volumetric energy content of hydrogen, and thelack of infrastructure for hydrogen refuelling, hasencouraged car manufacturers to study the use of moreconventional liquid fuels that can be converted tohydrogen-rich gas mixtures by a “fuel processor” in theFigure 19: Car manufacturers have in the past had differing ideas aboutthe use of liquid hydrogen and alternative liquid fuels that can be convertedto hydrogen on board the vehicle.Compressed Liquid Gasoline Methanolhydrogen hydrogen reformer reformerBMW■DC ■ ■ ■Ford■GM ■ ■ ■Hyundai ■Honda ■ ■Nissan■Opel■Renault ■ ■Toyota ■ ■VW ■ ■Fiat■vehicle. Gasoline and methanol are two obvious choices(Figure 19).An example of parallel development in different fueltechnologies is that of DaimlerChrysler. Different prototypesof NECAR (a prototype fuel cell vehicle based on aMercedes-Benz A-Class) used both methanol reforming(NECAR III and V) and pure hydrogen (NECAR II and IV).Today, however, there is a clear consensus among the carmanufacturers on the use of direct hydrogen fuel cells(Figure 20). In most cases, high-pressure gaseoushydrogen is preferred over liquid hydrogen.The reason is that on-board reforming has not lived upto its original promise. It implies additional complexityand cost, and has seriously hindered vehicle performancesbecause of time lags in the reformer. It reducesoverall efficiency and eliminates the “zero emission”benefit of fuel cell vehicles, to the extent that a fuel cellvehicle with an onboard fuel processor has no environmentaladvantages over ICE vehicles. Figure 21 shows,for example, that overall (“well to wheel”) CO 2 emissionsfor a fuel cell vehicle with a gasoline fuel processorare the same as those for an ordinary diesel vehicle.Technology barriers to the development of fuel cells intransportPEMFCs and related components have shown stunningprogress in the last decade, notably in power density andspecific power (Figure 22).Nevertheless, serious technology barriers remain at thecomponent level. Laboratory tests cannot prove thatPEMFCs will be commercially viable in mass-producedvehicles. The main challenges now are to:• reduce the cost of the fuel cell stack itself;• reduce cost of other devices required by the fuel cellsystem;• improve performance at temperatures below freezing;• increase tolerance to carbon monoxide and othercontaminants in the hydrogen;
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Hydrogen and its competitors, 2004

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