Hydrogen powered aircraft
Introduction🔗
Conventional fuels used nowadays for aircraft propulsion release carbon emissions into the atmosphere during combustion – as they are partly composed of carbon molecules, which is a contributor to global warming.
Introducing hydrogen propulsion, a technology that emits zero carbon dioxide (CO2) while flying and could be used to power a fuel cell to generate electricity to power an aircraft.
When the hydrogen protons reach the oxygen, they react to form mostly water. In a hydrogen propulsion system, the electrical power generated is typically used either directly to power an electric motor that rotates a propeller or stored in batteries for later usage.
Engineering Challenges🔗
Storage of Hydrogen🔗
Storage is not only a challenge because of potential leakage, but also because of the tanks required. Current aircraft have integral tanks – meaning they are formed by the cavities of the regular aircraft structure.
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They require equipment such as pumps and valves as well as a special coating, but their mass is minimal in relation to the mass of fuel they carry. This will not be the case for hydrogen tanks. The quality of their designs is typically measured in terms of gravimetric efficiency – the mass of hydrogen stored inside the tank over the total mass of the tank and stored hydrogen. For example, a tank with gravimetric efficiency above 33% means that the combined mass of fuel and its storage will be lower than that of jet fuel. Nevertheless, it will not reach the theoretical 3x improvement of the fuel mass for the same amount of energy in comparison to jet fuel. At 33%, the combined masses will be equal.
Handling liquified Hydrogen🔗
As much as hydrogen’s energy density is high, its volumetric energy density is low. For a given amount of energy, hydrogen would occupy at least three times more volume than jet fuel in ambient conditions.
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To tackle this issue, two main streams are pursued:
- Liquefying: Liquid hydrogen requires very low temperatures (~ -250°C), with the constraints that come with it like - cold management, mechanical differential dilatations, and phase change.
- Compressing: This requires pressures in the order of 700 bar (10,000 psi) to be attractive, which increases the leakage and reduces the lifetime of the mechanical parts. Thus, the hydrogen molecules infiltrate the material, impacting durability. Due to the so-called Joule-Thompson effect, heating with leakage is an additional concern.
The tutorial for this application explores liquified Hydrogen.