US transportation fuel use is projected to reach nearly 4.6 trillion kWh, even if the Partnership for the Next-Generation of Vehicles (PNGV) succeeds in tripling automobile fleet fuel economy by 2020. Light-duty passenger vehicles will then account for approximately 25% of fuel use, aircraft for 35%, and heavy trucks for 40%. Fueling this demand with natural gas will produce direct carbon emissions of 248 mmtC/yr.
It will be easiest for hydrogen to displace natural gas in the light-duty vehicle fleet first. Passenger vehicles are idle 90%+ of the time, with fuel costs accounting for 5-10% of ownership costs. The development of hybrid electric cars and trucks and later fuel cell vehicles makes the prospect of achieving 80 mpg equivalent fuel economy over the entire vehicle fleet quite likely by 2020. This improved fuel economy is the single most important step in making hydrogen fueled vehicles viable, dramatically reducing refueling cost and the size, weight, and cost of onboard fuel storage.
Hydrogen vehicles are expected to require 5 gallons of gasoline equivalent (5 kg hydrogen) for a cruising range of about 400 miles. Hydrogen fuel can be stored onboard using lightweight composite pressure vessels, similar to natural gas vehicles. Other onboard storage approaches include hydrogen absorption and release from high surface area metal powders at moderate pressures, as well as cryogenic liquid hydrogen tanks operating near ambient pressures. Both approaches have been demonstrated in Germany for over a decade. As com posite materials have improved in strength and cost, pressure vessel hydrogen storage is becoming more attractive. Another option is to insulate pressure vessels for cryogenic hydrogen service, using compressed hydrogen for routine refueling (perhaps at home or work), and cryogenic liquid hydrogen refueling for occasional long trips. Multi-layer insulation would be sufficient to store liquid hydrogen onboard vehicles even if parked for weeks at a time. Such a hydrogen storage vessel (Aceves et al., 1998) is expected to be relatively compact (100 L), lightweight (100 kg), and low cost ($500-1000).
Hydrogen fueled freight-transportation differs substantially from light-duty passenger vehicles. A typical long-haul tractor trailer truck travels about 100000 miles/yr, so fuel costs are a much higher proportion of total costs. This high sensitivity to fuel cost is the chief reason truck engines are so efficient today. In the future, fuel cost sensitivity will provide the incentive for compressed hydrogen storage onboard trucks. This can reduce hydrogen fuel cost substantially, as compressed hydrogen is less energy intensive to produce than cryogenic liquid hydrogen. 25-50 kg of hydrogen fuel onboard an 18 wheel tractor trailer would provide a range of 300 miles for a fuel economy equivalent to 6-12 mpg. Onboard storage of this hydrogen (at 5000 psi) will require a volume of about 250-500 gallons. A distinct advantage of hydrogen fueled trucks is the benefit to urban air pollution. Particulate, hydrocarbon, and carbon monoxide emissions would be eliminated, reducing, perhaps obviating, the need for onboard pollution control equipment. Hydrogen fueled tractor trailer trucks have been demonstrated in Japan.
Energy density considerations dictate that carbonless aircraft will have to be fueled with liquid hydrogen. From an emissions and energy perspective, cryogenic liquefaction of hydrogen fuel is not advantageous. Typically, liquefaction is very energy intensive, requiring up to 40% of the fuel energy in the hydrogen. It is also reasonably expensive, liquefaction plants are estimated to cost about $500/kW, while hydrogen compressors can be one-fifth this cost. The two principal advantages of liquid hydrogen are its low weight and low capital cost of large scale storage. The liquid hydrogen infrastructure at Kennedy Space Center is roughly one-tenth the scale required for a large civilian airport.
Liquid hydrogen has one-third the mass of jet fuel for equivalent energy, but nearly four times the volume. These characteristics can substantially reduce takeoff weight for cryogenically fueled aircraft, with the attendant advantages, at the cost of substantial changes in aircraft design. Calculations indicate (Winter and Nitsch, 1988) that hydrogen aircraft would likely use 10% less fuel for subsonic flight and 50% less fuel for supersonic flight than fossil-fueled counterparts. Given the uncertainties surrounding future air travel and to be conservative in comparisons, these potential advantages were neglected in the scenarios involving liquid hydrogen aircraft.
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