Alternative Fuels: Vaporware

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.

The real difficulties with hydrogen are storing it, and managing refueling.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 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 as mentioned above, so only three times as much volume is needed. But there is still the problem of designing the tanks to withstand a pressure of 700 atmospheres. Ideally, tanks would be highly symmetric shapes like spheres, or the domed cylinders that are used in the Toyota Mirai.

That means not all the space in the wings can be used, and the volume for hydrogen storage will be even lower, reduced by about 20 percent. Flexible wing structures will only be possible if relatively small tanks are used. Since three times the volume will be needed and the weight of the tank is about the same as a tank of the same volume filled with avgas, there will be a considerable weight penalty—so in practice, not all the available space in the wing can be used.

The real difficulties with hydrogen are storing it, and managing refueling. Hydrogen is very flammable and although the flame shoots upwards, the ease with which it ignites and the fact that the flame is invisible make it a serious hazard. Storing liquid hydrogen for long periods requires carefully constructed tanks with inbuilt refrigeration, like the tank NASA uses at the Kennedy Space Center. Handling hydrogen at pressures of 700 to 800 atmospheres requires special couplings with refueling only being done by trained personnel under close supervision.

It would appear that hydrogen is not practical for longer-range intercontinental aircraft. Although in principle light or short-range aircraft have enough space in the wing, in practice either the range or payload would be reduced because of the weight of the hydrogen tanks. The real problems are the difficulty of storing a liquid at extremely low temperatures or a gas at very high pressures.

“The only problems with hydrogen are how to make it, how to store it and how to transport it—other than that it’s wonderful!” says my colleague, chemistry professor Tom Moore. And professor of materials Peter Crozier likes to say that “The best place for a hydrogen atom is attached to a carbon atom.”

Peter Rez is an Arizona State University physics professor from Scottsdale who flies a Mooney.