Powering airplanes by hydrogen, as reported on by David Kramer in the December 2020 issue of Physics Today (page 27), is a nice theoretical idea that brings little practical benefit.
Apart from the difficulties of handling hydrogen as a cryogenic liquid or a gas at very high pressure, the most serious problem with hydrogen as an aviation fuel isn’t the weight of the tanks containing it but rather its low density, even as a liquid. Consider the Toyota Mirai, an electric car powered by a hydrogen fuel cell: The hydrogen is stored as a gas in polycarbonate tanks at 700 bar, twice the pressure proposed for the hydrogen-powered aircraft. The 2021 Mirai can hold 5.6 kg of hydrogen, but that’s just 6% of the combined mass of the fuel and the fuel tanks. For tanks of conventional aviation fuel—kerosene or aviation gasoline (avgas)—the mass is mostly fuel, not tank structure.
The energy per unit volume of liquid hydrogen is 24% that of avgas or kerosene; that of hydrogen at 350 bar, only about 8%. Light aircraft use only a small part of the wing to store fuel. The combination of fuel cell and electric motor has approximately twice the efficiency of an internal combustion engine, though, so only half as much energy needs to be stored.
The situation is very different for long-range, turbine-powered aircraft used for intercontinental travel. The whole wing serves as a fuel tank, and fuel can account for 45% of a plane’s allowed maximum takeoff mass. Even for the high-bypass-ratio turbofans found on a commercial aircraft, a substantial part of the high-altitude cruise thrust comes from the turbine core, not the fan, so driving the fan with a fuel-cell-powered electric motor effectively makes the aircraft more like a slower turboprop. The low density of hydrogen, even as a liquid, means that the aircraft doesn’t have the space for the fuel needed for an intercontinental journey.
A flight of 500 nautical miles (900 km) takes about 1.25 hours. If the aim is to minimize carbon dioxide emissions from travel, then for flights of less than that distance—for which hydrogen is viable, though not necessarily practical—it would be better to just take the train!
