Hydrogen: the zero-carbon energy carrier for aircraft?

Hydrogen as aircraft fuel has been researched extensively for many years, and the concept has been proven in flight. Already in the 1950s the first hydrogen demonstrator aircraft were built in the US. Complicated logistics were the main reason preventing a liquid hydrogen infrastructure set-up at airports around the world, which meant  large-scale implementation of hydrogen as aircraft fuel was not possible. But now we have to re-consider because of an urgent, critical reason: climate change. Hydrogen can be a promising zero-carbon fuel or energy carrier: no CO2 is being emitted during the flight and it also cuts the emissions of soot, sulphate and NOx.

How is it used?
It can be used to propel aircraft in different ways:

  • Using hydrogen as a fuel in the aircraft’s combustion engine
  • Electric power generation through electrochemical reaction of hydrogen in a fuel cell in combination with an electric motor
  • A combination of the two in a hybrid propulsion system. For example, a combination of combustion and fuel cell or a combination of hydrogen with kerosene or SAF.

More research is needed in order to optimise the existing technologies for burning hydrogen in an aircraft engine and using it in a fuel cell, plus to overcome some hurdles. Aspects that will need to be considered are mentioned below.

Hydrogen storage and distribution systems in aircraft
Although hydrogen has a much higher energy-to-mass ratio than kerosene, it is in an extremely impractical state at atmospheric pressure and temperature, being a low-density gas. To reach a manageable volume, it must either be highly compressed (300-850 bar) or cryogenically cooled to liquid state (-253°C). Where kerosene is stored at ambient pressure and temperature in aircraft structures that act as structural elements, hydrogen needs separate strong high-pressure vessels or highly insulated cryogenic tanks. The volume and weight of these tanks are a challenge as aircraft are always as light as possible and airlines will rather transport passengers and freight than just the hydrogen they need to get from A to B.

Flying on hydrogen – whether you do it by burning it as a fuel or using it in a fuel cell – will also require new distribution systems: vaporisation of liquid hydrogen, distribution of liquid or high-pressure hydrogen and pressure regulation. Ducts, pumps, valves and other equipment must be compatible with hydrogen and the extremely low temperatures in case of liquefied hydrogen. Safety requirements will lead to dedicated design solutions, for example double-walled piping. Extensive research is required in order to obtain an optimal solution with respect to weight, volume, performance and cost that satisfies the safety constraints.

Climate effects
Flying on hydrogen halt aircraft from emitting CO2 and reduce the emissions of other damaging particles such as soot and NOx.  Nevertheless it does lead to a considerably larger emission of water or water vapour (about 2½ times the amount compared to kerosene). The climate impact of water vapour from hydrogen combustion needs significant additional research, in particular the induced formation of contrails and cirrus clouds, so that mitigations, such as cruising at lower altitude, can be assessed.

Moreover, it is important to stress that hydrogen is only as clean as the way it’s produced. Zero-carbon hydrogen has to be produced with sustainable electricity. Currently, sustainable electricity is scarce.

Safety & Economics
There is no reason why hydrogen aircraft could not be made as safe as kerosene aircraft, but it requires extensive research and standardisation efforts in order to obtain that level. Moreover, the effect of reduced passenger capacity of hydrogen aircraft (due to the large volume occupied by the hydrogen) requires market research to identify the economic viability of the proposed concepts.

Source: M. Nagelsmit and L.L.M. Veldhuis (2021). Whitepaper TU Delft NLR [link: https://www.nlr.org/news/royal-nlr-and-tu-delft-present-vision-for-route-to-sustainable-air-transport/] : Towards a sustainable air transport system.