#todayinsight: Hydrogen-Powered Aviation: The Next Frontier in Green Transportation

Last year was the hottest year on record. The ice cover on the Great Lakes was also dangerously low in the winter. This intuitively shows that global warming is approaching us even faster than we had anticipated. To prevent serious climate change consequences, the world must stop generating greenhouse gases, and currently aviation is a major source of it. According to scientific guidelines, each individual’s carbon budget is approximately two tonnes per year if global warming is to be limited to two degrees Celsius. In contrast, an intercontinental flight would generate tons of CO2 for each passenger. Thus, a single intercontinental journey is sufficient to deplete the passenger’s annual carbon budget. 

Decarbonization efforts are gaining traction; lately, Edler and Martins released a review paper that compiled all of the obstacles and sophistications involved in the design and manufacturing of green commercial airplanes. The primary idea is to use hydrogen instead of kerosene. The hydrogen is created by an electrolysis process powered by renewable energy, making the flight environmentally friendly as well. 

The first and most complex step is to locate Hydrogen in the aircraft. Hydrogen has nearly three times the energy per unit mass as kerosene. However, its per-volume energy is significantly lower. At 350 bar pressure, the per volume energy of hydrogen is one-twelfth that of kerosene, while at 700 bar it is one-eighth. In the most extreme case, hydrogen is held in the form of cryogenic liquid within a super-insulated tank. Liquid hydrogen has one-fourth the energy confinement of kerosene and requires particular treatment because it boils at 20 Kelvin degrees. 

The storage problem is an open debate. Hydrogen cannot be stored in the wings; it must be deposited someplace inside the fuselage. The wing is then “dry” as aircraft designers refer to it. Dry wings do not benefit from “load alleviation” of fuel loads. It indicates that wings most likely require a stronger structure, increasing the aircraft’s weight. On the other hand, hydrogen fuel weighs substantially less than kerosene. Fuel cells may increase the fuel conversion efficiency as well. This means that less tonnes of hydrogen are required for a particular flight path. Scientists are debating whether hydrogen planes use more or less energy to fly. Hydrogen tank technology is important in this context.

Tank efficiency is calculated as the fuel weight divided by the sum of the fuel weight and empty tank weight. Kerosene tanks have a 100% efficiency. This is because the fuel is nested within the wings. However, with hydrogen, this is not the case. Tank efficiency for regular metal gas storage is around 1% to 2%. Universal Hydrogen, a firm producing hydrogen tanks, uses composite materials to create a hydrogen gas storage tank with a tank efficiency of 17%, still very low for aviation. Tank efficiency for cryogenic liquid hydrogen is now at 50%. When the efficiency is low, hydrogen airplanes require significantly more energy than kerosene, thus intercontinental hydrogen flights become almost impossible. Given the low tank efficiency for Hydrogen gas, cryogenic liquid Hydrogen appears to be the only viable option. However, considerable further development is still required to improve tank design and efficiency.

There are numerous open concerns to be addressed regarding the configuration, structure, and propulsion. Regarding propulsion, fuel cells can replace turbofan engines. These cells use the reverse hydrolysis process to convert hydrogen into electricity current. This method has an efficiency of roughly 60%, which is significantly higher than combustion. 

There is considerable ambiguity over flying and maintenance costs. The estimates range from less than typical freight costs to more than double that. With a more advanced design, the maintenance costs appear to be higher. However, green flight costs are mostly determined by hydrogen prices, which are expected to be significantly higher than kerosene prices over the next fifteen years, according to numerous projections. Green flights are among the most difficult stages for decarbonizing the global economy. 

Source:

Adler, Eytan J., and Joaquim RRA Martins. “Hydrogen-powered aircraft: Fundamental concepts, key technologies, and environmental impacts.” Progress in Aerospace Sciences 141 (2023): 100922.