Environmental Engineering Reference
In-Depth Information
fuel tanks placed on top of the fuselage, increasing drag. Other options are available
for smaller aircraft for short-haul flights.
The engine will also need to be modified. One potential problem is that the sto-
ichiometric temperature
3
of hydrogen is about 100
°
C hotter than kerosene. This has
implications not only for the materials in the combustion and turbine area, but also
for the formation of NO
x
. However, it is believed that the low flame-out limit of hydro-
gen will enable lean burn, allowing lower temperatures in the combustor, thus limit-
ing NO
x
production and keeping engine efficiency at its current level.
There are a number of environmental concerns with using hydrogen as a fuel for
aircraft. The consequent water-vapour emissions in the upper atmosphere could exac-
erbate contrail formation, which would tend to warm the Earth's surface.
Although
it has been indicated that hydrogen offers a significant reduction in greenhouse poten-
tial at all altitudes (IPCC, 1999, p257), studies on the upper atmosphere are continu-
ing and uncertainty still remains. In addition, the energy requirement to mass produce
sufficient quantities of liquid hydrogen is considerable, but could be overcome should
renewable energy be used. At the current time, however, such renewable energy sources
are not widely available. The final impediments to the introduction of hydrogen are
the costs and logistics of developing the required infrastructure to support switching
to the new fuel.
It is believed that liquid hydrogen can only offer an environmental benefit if it is
produced using renewable energy. The technology currently exists to do this; but,
currently, it is not economically competitive with kerosene. It is, however, the only
alternative fuel that would completely eliminate the direct production of CO
2
from
aircraft.
Innovative configurations
A huge range of alternatives to the traditional fuselage and podded engines exists.
These have included multi-hull layouts, span loaders, tandem wing layouts, flatbed
layouts and the use of canards (Jenkinson et al, 1999). Indeed, one Russian design
has incorporated some aspects of all the potential new designs. At the current time,
the blended wing body concept (BWB) is receiving most attention.
The BWB concept could offer a significant improvement in the operational and
environmental costs of air transport. It offers a potential reduction in fuel burn of
up to 27 per cent (Daggett, 2001) through improved lift/drag ratios and reduced
weight achieved through incorporating more composites in the aircraft structure.
The novel design also provides more opportunities to integrate novel propulsion sys-
tems. For example, locating the engines above the wing would have two benefits:
sufficient ground clearance to allow the use of large fan engines, and reduced noise
as the aircraft itself would act as a shield reflecting noise upwards. Ducted and unducted
prop-fan concepts, which were first developed during the early 1980s, may provide a
suitable propulsion system for this type of aircraft. These options have substantially
better fuel burn than modern engines, but cause the aircraft to fly more slowly. An
additional consideration is the potential for BWB to use hydrogen for fuel as the air-
craft shape lends itself to storing the cylindrical tanks that are necessary. Some con-
cerns exist with respect to passenger transport of this kind, including the time
