Chapter 14: Hydrogen Engines

Hydrogen as an aviation energy source

Hydrogen is being explored as a leading contender to avoid waiting for battery technology to catch up with aircraft flight range.
Hydrogen is the most abundant element and is estimated to make up 75\% of all matter in the universe.
If hydrogen is created with renewable energy, the emissions from a hydrogen engine would be minimal.
The chapter provides a general overview of how hydrogen engines may operate and the challenges to overcome, noting that this is a rapidly evolving area.
Hydrogen can power aircraft in two ways:
- Hydrogen Fuel Cell: uses hydrogen to generate electricity, which is then used for an electric motor.
- Combustion Hydrogen: burns hydrogen inside the engine to move the aircraft, similar to current aviation fuels in piston and gas turbine engines.
An aircraft with a hydrogen fuel cell will have some basic components of a battery-powered aircraft: an electric motor (or multiple motors) powered by electricity, which is produced during flight rather than stored in large batteries.
The goal is to provide a general overview of operation rather than a fixed, unchanging design, recognizing rapid evolution in this field.

In a hydrogen fuel cell, electricity is generated chemically rather than stored in large batteries.
The system has two ingredients:
- Hydrogen, stored in a tank in the aircraft.
- Oxygen, obtained from outside air.
An electrochemical reaction occurs between hydrogen and oxygen inside the fuel cell, producing electricity.
The byproducts of the fuel cell are heat and water only, with no emissions at all from the process itself.
The generated electricity can be directed toward an electric motor or stored in a battery for later use (the battery can be much smaller than in a battery-powered aircraft because the fuel cell continuously recharges it).
A hydrogen fuel-cell aircraft would resemble a battery-powered aircraft in terms of having electric propulsion, but with continuous electricity generation during flight instead of reliance on onboard heavy energy storage.
Figure 14.1 is referenced as showing the basic components that might be required for an aircraft with a hydrogen fuel cell.
Chemical reaction example (representing the main fuel-cell process):
2\,\mathrm{H2} + \mathrm{O2} \rightarrow 2\,\mathrm{H_2O}
Summary benefits: zero emissions from the fuel-cell process itself (water and heat only); potential for lower weight and continuous energy supply during flight if hydrogen is produced from renewable sources.

Hydrogen used for combustion would be similar to fossil fuels in piston and gas turbine engines: it would burn in a combustion chamber to produce high-energy exhaust that propels the aircraft.
Hydrogen has different burning characteristics than conventional fuels, so engine designs would require modifications, but the general principles of combustion in a cylinder or a gas turbine still apply.
Emissions from hydrogen combustion are not as environmentally friendly as the fuel-cell option, since some emissions are still produced through the combustion process.

The main challenge for both fuel-cell and combustion approaches is storing hydrogen on board the aircraft.
Hydrogen storage takes up more volume than the same energy content of jet fuel, even in high-pressure tanks.
To maximize space, hydrogen is typically stored in the fuselage rather than the wings, reducing available space for passengers or cargo.
Storing hydrogen as a liquid reduces storage volume but requires very cold storage conditions and possibly energy-intensive heating or conversion steps during use:
- Liquid hydrogen storage requires about -420^\circ\mathrm{F} / -250^\circ\mathrm{C} temperatures.
- Depending on how hydrogen is used, it may need to be converted back to gas before use.
Regardless of storage form, hydrogen systems are generally more complex and heavier than current aviation fuel systems.
Hydrogen production and its environmental impact depend on the energy source used to dissociate hydrogen from compounds (e.g., water):
- If renewable energy (solar, wind, etc.) is used to produce hydrogen, the life-cycle emissions of hydrogen are very small.
- If fossil-fuel energy (e.g., natural gas) is used, the environmental impact remains high even if engine emissions are small.
Although hydrogen-powered engines have enormous potential, there are many challenges to overcome in making and storing the hydrogen that would power thousands of aircraft.

Regardless of propulsion evolution, pilots must understand how future aircraft may be powered (hydrogen, electricity, or other power sources).
The area is rapidly changing, with aircraft engine technology potentially seeing its biggest change since the advent of jet propulsion.