Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
the energy density of jet fuel, fuselages would have to be considerably larger than current
designs-increasing drag and fuel consumption. For long-range flights, this penalty would be
offset by a reduction in take-off weight because hydrogen and, to a small extent, methane have
higher specific energies than kerosene. Design studies for hydrogen-fueled, long-range (10,000
km) aircraft have shown that the lighter fuel weight results in almost a 20% reduction in energy
consumption compared to kerosene-fueled aircraft even accounting for losses (Momenthy,
1996). The same study showed, however, that for medium- and short-range (5500 km and 3200
km) aircraft, there is an energy penalty of 17-38%. For methane, there was only a small benefit
for long-range aircraft and penalties of 10-28% for medium- and short-range aircraft.
Of these two cryogenic fuels, hydrogen may be more attractive from an emissions standpoint.
CO 2 and SO x O emissions would be eliminated. However, water vapor would increase
significantly despite the reduction in energy consumption. For the same energy consumption,
burning methane would yield about 25% less CO 2 and about 60% more H2O than burning jet fuel. Burning hydrogen would result in 2.6 times
as much water vapor as burning jet fuel, but no CO 2 . Table 7-11 compares the energy-specific
emissions indices of CO 2 and H 2 O at constant payload. (Effects of weight savings/penalties are
not included in Table 7-11 because they depend heavily on the range of the aircraft.)
This basis of comparison could yield a quantitative "greenhouse" comparison of these three fuels
if the greenhouse equivalency were known. Figure 7-37 presents the results from such a study
for a hydrogen-fueled aircraft derived from the Airbus A310; the analysis takes into account the
relative greenhouse effects of H 2 O, CO 2 , and NO x at different altitudes and shows that hydrogen
offers a significant reduction in greenhouse effect over kerosene at all altitudes for this aircraft
(Klug et al, 1996). Inefficiencies of production would alter these results somewhat in a
comprehensive energy comparison, although water emissions are of environmental concern only
at cruise altitudes. Figure 7-38 compares relative CO 2 emissions from the manufacture and use
Figure 7-40: Engine pressure ratio versus EI(NO x )
for large and small engines (*Lipfert, 1972; EPA,
1976).
of alternative aviation fuels from different resources (Hadaller et al., 1993). The potential benefits
of hydrogen can be realized only if hydrogen can be obtained from water without the use of fossil
fuels to provide the energy. Nuclear power is the best method identified in Figure 7-38. The Kvaemer process is being developed as a method for converting
hydrocarbons into hydrogen, with carbon as a byproduct (as opposed to CO 2 with the steam reforming process); if the energy requirements are sufficient, this fuel
would appear on Figure 7-38 with a value less than 1.0. Kerosene from biomass [via a Fischer-Tropsch (F-T) synthesis process] would also have relative CO2 emissions less than 1.0 if included in Figure 7-38.
Liquid hydrogen offers an environmental advantage only if this fuel were produced on a renewable energy basis, as explained above. The necessary technology exists,
but such liquid hydrogen is not economically competitive with kerosene at current price levels. On the other hand, liquid hydrogen based on renewable energy is the
only candidate aviation fuel known today that would completely eliminate CO 2 emission by aviation. Safety issues in the siting of storage and handling systems at
airports pose significant challenges, however.
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