Aviation accounts for 3% to 6% of all world sources of global warming making it
one of the top ten contributors to the climate crisis. As a percentage of all
sources of climate change, aviation is expected to grow dramatically both due
to projected increases in aviation activity and improvements in other sectors.
It is widely agreed that addressing the climate impact of aviation is both
difficult and important.
Fuel is a critical element of aviation technology. There are precisely seven
fuels in current or proposed commercial aviation use: Jet A, SAF, 100LL,
G100UL, battery electric, hydrogen and aluminum. All of these fuels have
problems. I am aware of no other viable proposals for aviation fuel.
1. Jet A is standard jet fuel. This is a fossil fuel and must be phased out to
prevent climate impact. Nearly all the climate impact of aviation comes from
aircraft that burn jet fuel.
2. SAF (Sustainable Aviation Fuel) is a synthetic form of jet fuel. It is in
limited use and there are questions about its true sustainability and
commercial viability in large production quantities. I posted a longer article
about SAF in Lincolntalk recently and also spoke about it during the May HFAC
meeting.
3. 100LL is 100 octane, low lead AvGas for piston engine aircraft. This is a
fossil fuel and contains lead. It must be phased out both for the lead content
and for the fossil source. I have talked about 100LL and G100UL in several HFAC
meetings.
4. G100UL is GAMI 100 octane, unleaded AvGas for piston engine aircraft. It is
not yet in use but will soon be available starting in California and will
replace 100LL to address the lead problem. This is still a fossil fuel and
should eventually be phased out for climate considerations. There is no fossil
free fuel in production or development that can power small piston engine
aircraft.
5. Battery Electric seems to be on the verge of commercial availability but the
weight of batteries limits its aviation use to short range flights (hundreds of
miles). No one expects battery electric planes to cross continents or oceans
unless there is a revolutionary and unexpected improvement in battery
technology. Senator Barrett has talked about battery electric aircraft. Cape
Air has purchased 75 battery electric aircraft with delivery expected within
two to five years according to various sources. While battery powered flight
may be imminent, companies are encountering many difficulties in the
development of fully practical models.
6. Hydrogen is an excellent energy source but difficult to produce, transport
and store. Currently most hydrogen used for industrial purposes is derived from
fossil fuel sources for cost reasons, but it is possible to produce entirely
“green” hydrogen using renewable electricity. Hydrogen can provide power by
combustion (in an engine) or by chemical process in a fuel cell that produces
electricity. The first successful ground demonstration of a hydrogen powered
jet engine was in November, 2022 (six months ago). Hydrogen fuel cells produce
electricity and water from hydrogen and air using a catalyst on the anode to
separate electrons and form electricity. It is conceivable that electricity
from hydrogen fuel cells and from batteries could be used to implement longer
range electric aircraft but that has not been proven. Hydrogen fuel cells are a
mature technology, widely used to power forklifts inside warehouses (since the
emissions are safe). There are several commercially produced hydrogen fuel cell
automobiles, but the fuel is only available in certain limited areas.
7. Aluminum seems like an unlikely fuel source, but there are industrial
processes using a reversible reaction to generate electricity from aluminum. At
least one company is pursuing technology to use this for flight and plans a
demonstration in a year or two. Theoretically, this source provides a good
compromise of required volume and weight compared with hydrogen, while
potentially being more climate friendly than jet fuel.
Those are the only aviation fuel sources in current or proposed commercial use.
Of those seven sources, only SAF, Battery Electric, Hydrogen and Aluminum can
be obtained without using fossil sources. Some combination of these four
potentially climate friendly fuel sources must be implemented to make aviation
climate friendly. Because development of aviation technology is very slow there
is a strong public need for vigorous, effective and accelerated development of
climate friendly aviation technology.
-Christopher Eliot, Ph.D.
Hanscom Field Advisory Commission, Chair
124 Bedford Road
Lincoln
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