Airbus aims to put primarily hydrogen fueled gas turbine based commercial aircraft in service by 2035. In comparison with the currently used fuel Kerosene, the energy density (in MJ/m3) of liquid Hydrogen in roughly ¼ of the energy density of Kerosene. If we assume that the efficiency of a jet engine is constant despite of the change of the fuel, how does the volume of fuel tanks in Hydrogen airplanes change compared to current technology if the stored energy content is equal?Image: pixabay.comSingle choice
A
a. Decreases by 4
B
b. Increases by 4
C
c. Decreases by 1/4

Question

Airbus aims to put primarily hydrogen fueled gas turbine based commercial aircraft in service by 2035. In comparison with the currently used fuel Kerosene, the energy density (in MJ/m3) of liquid Hydrogen in roughly ¼ of the energy density of Kerosene. If we assume that the efficiency of a jet engine is constant despite of the change of the fuel, how does the volume of fuel tanks in Hydrogen airplanes change compared to current technology if the stored energy content is equal?Image: pixabay.comSingle choice

A

a. Decreases by 4

B

b. Increases by 4

C

c. Decreases by 1/4

BlackTom AI · Accepted Answer

First, restating the problem in my own words helps set the stage: hydrogen fuel has roughly 1/4 the energy density by volume (MJ per m^3) of kerosene. If the engine efficiency stays the same and we require the same stored energy, how does the required fuel tank volume change?
Option a: Decreases by 4. This option suggests that the volume of hydrogen tanks needed is one quarter of the kerosene tanks, i.e., a 4x decrease. Since the volumetric energy density of liquid hydrogen is about four times lower than that of kerosene, to store the same total energy you would indeed need only about 1/4 of the kerosene volume if comparing volumes directly. Intuitively, if kerosene provides roughly 34,000 MJ/m^3 and liquid hydrogen provides about 8,500 MJ/m^3, the same energy implies hydrogen volume ≈ (34,000 / 8,500) times smaller, i.e., about 4 times smaller in volume. This makes sense as a direct volume scaling factor when energy density differs by ~4:1.
Option b: Increases by 4. This would mean hydrogen tanks must be four times larger in volume to store the same energy. That ignores the fact that hydrogen’s energy density per volume is lower, not higher. If anything, the needed volume should decrease, not increase, because you can pack less energy per unit volume into hydrogen than kerosene.
Option c: Decreases by 1/4. This option says the hydrogen tank volume is one-quarter of the kerosene tank volume. That would require hydrogen to have a volumetric energy density about four times that of kerosene, which contradicts the given statement that hydrogen is roughly 1/4 of kerosene in energy density. If the energy per unit volume is four times lower, you would not reduce the volume to one-quarter; you would actually need a larger energy-carrying volume if the energy per unit volume were lower, but because we’re fixing stored energy, the volume should compensate to reach the same energy.
In summary, the logic hinges on the relationship between energy density per volume and required volume for the same total energy. Since hydrogen’s volumetric energy density is about four times lower than kerosene, the hydrogen tank volume needed to store the same energy is about four times smaller than the kerosene tank volume, corresponding to a fourfold decrease. The options reflect different interpretations of this scaling, and the only consistent one with a fourfold reduction is option a.

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