AOPA Pilot Magazine - August 2021 - 87

IN THE NEVER-ENDING quest to eliminate
carbon dioxide emissions from aviation
there is now lots of talk about hydrogenpowered
aircraft. ZeroAvia has converted
a Piper Malibu to run on hydrogen and
Airbus has a proposal for an airliner. Is
hydrogen practical as a fuel for airplanes?
Will it bring any advantages?
There are two options to consider
with using hydrogen for airplane propulsion.
The easiest is to substitute hydrogen
for kerosene in a turbine. Instead of producing
a molecule of carbon dioxide for
each molecule of water, it's all water
vapor, so the contrails remain! This is
what Airbus proposes for its concept
airplanes. But there might be metallurgical
issues because of the possibility of
increased corrosion of the turbine blades.
The other option is using hydrogen
in a fuel cell to generate electricity that
drives an electric motor. The motor then
drives a propeller. To compare this to a
piston or turbine engine driving a propeller
we should look at the fraction of
energy stored in the fuel that is actually
used to move the airplane through the
air. The poor thermal efficiency of piston
engines means that only 25 percent of the
energy in the fuel gets used. For turbines
it's somewhat greater, about 30 to 40 percent.
The efficiency of fuel cells is about
50 to 60 percent, so the overall efficiency
of the fuel-cell-powered electric motor
is about twice that of the piston engine.
That means half the fuel is needed to go
a given distance.
Although the energy for a given
weight of hydrogen is about three times
greater than the energy in the same
weight of avgas or kerosene, the volume
needed to store the fuel is much greater.
The density of hydrogen is very low, even
as a liquid. The energy in a tank filled
with hydrogen at atmospheric pressure
would have 0.03 percent of the energy of
the same tank filled with avgas or kerosene.
Even if the tank were filled with
liquid hydrogen, requiring it to be cooled
to minus 253 degrees Celsius (minus
423 degrees Fahrenheit, only 20 degrees
Celsius or 36 degrees Fahrenheit above
absolute zero) it would only have 24
percent of the energy of the equivalent
volume of kerosene or avgas. Handling
liquids at these extremely low temperatures
is not easy-but it's worth the
effort for rockets. The advantage that liquid
hydrogen has in terms of energy per
unit weight compared to kerosene (RP1)
means that the payload to low Earth
orbit, or beyond, is almost doubled when
used in a second stage.
To find out what might be more practical
we should look at Toyota's hydrogen
fuel cell car, the Mirai. Its hydrogen is
stored at 700 times atmospheric pressure.
The energy stored in the tank
would be about 15 percent of the equivalent
kerosene or avgas-not quite as good
as liquid hydrogen, but somewhat easier
to manage.
There are still considerable practical
problems. It's not possible to use a
metal tank as the hydrogen gets in the
gaps between the atoms in the metal,
causing it to fall apart (hydrogen embrittlement).
Instead, a three-layer tank
of polymer composite is used. The two
cylindrical tanks have a combined volume
of 122 liters, a weight of 87.5 kg, and
a capacity of 5 kg of hydrogen. So, unlike
The real difficulties with
hydrogen are storing it, and
managing refueling.
with gasoline, the weight of the tanks
is much greater than the weight of the
fuel-although interestingly, 122 liters of
gasoline would weigh about the same as
the tank and the hydrogen.
The problem in aircraft is running out
of space for the fuel. On large intercontinental
aircraft the whole wing is used as
a fuel tank. There isn't any extra space. If
hydrogen were used as fuel for the turbofans,
the range would be reduced by a
factor of six.
Only a small part of the wing is used
for the fuel tank in a light plane, so in
principle the extra space is available.
Furthermore, in a piston engine airplane
the combination of fuel cell and electric
motor could be used to drive the propeller,
gaining the factor of two in efficiency
AOPA PILOT / August 2021
87
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AOPA Pilot Magazine - August 2021

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