Environmental Engineering Reference
In-Depth Information
powered aircraft could reduce greenhouse gas and nitrous oxide pollution
from jet engines while being more efficient than present jet fuels.
Fuel cells have to compete with the turbine, in installed cost as an
aircraft power plant. A study by the U.K.'s Cranfield University conclud-
ed that fuel cells are still too heavy for propulsion. A large aircraft requires
many megawatts, generated by at least two turbine engines weighing
about 3,900-kg (8,600lb) each. Today's fuel cells that generate 1,000-kW
weight over 3,200-kg each.
A major question for autos has been if the user generates hydrogen
on board or obtains it from a hydrogen refinery. This question also applies
to aviation. Do you have a reformer on the aircraft, or do you generate the
hydrogen at a central location? Both have advantages. Reforming on board
allows the hydrogen to be transported in a form that is easy to move, such
as methanol, natural gas and gasoline. The disadvantage is that having re-
formers on vehicles is not as efficient as central generation. But, how would
the hydrogen be produced centrally? A gas- or coal-powered power plant
produces more carbon dioxide, defeating the one object of using hydro-
gen. Nuclear power is a low carbon cost option, but faces political opposi-
tion. Renewable energy sources, such as solar power, wind and wave pow-
er, have been proposed as sources of power for electrolysis. But renewable
technology is not mature enough to supply all the power required.
Hydrogen and oxygen storage is another issue. There are significant
mass impacts for the pressure vessels needed which are insulated to stop
boil off.
Hydrogen aircraft have been studied by NASA. This involved a
fuel-cell-powered aircraft the size of a Boeing 737 in its Revolutionary
Aeropropulsion Concepts program. The hydrogen 737 would use a solid
oxide fuel cell (SOFC) for power.
Boeing will test a SOFC auxiliary power unit (APU) in one of its
737s. The APU is 45% efficient in turning hydrogen into electricity. In con-
trast, a gas turbine is 15% efficient. The APU will use a reformer to pro-
cess jet fuel to obtain the hydrogen needed. Boeing hopes that by 2010, the
technology will be mature enough to offer the APU on future versions of
its 787 Dreamliner.
The 787 Dreamliner will use 20% less fuel than the comparably sized
767. A completely new manufacturing process is used with sections of the
fuselage produced around the world and then flown to the assembly plant
in Everett, Washington in a special 747 large cargo freighter. The body uses
composite fibers of carbon graphite held together by epoxy for 50% of the
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