Hydrogen Fuel Cell Technology
Hydrogen fuel cells combine hydrogen with oxygen to produce water and induce a voltage. That voltage can then be used to charge a battery or power an electric motor or thruster.
Hydrogen Fuel Cell technology has advanced in recent years due to its growing adoption in aerospace and industrial applications. Toyota has now sold over 10,000 Mirai’s a mass-market hydrogen fuel cell car and Airbus has proposed 3 variations of hydrogen fuel cell aircraft that can enter service by 2035.
Despite recent improvements in hydrogen fuel cell technology, modern diesel and heavy fuel engines and generators still beat hydrogen fuel cells in weight efficiency by a factor of up to 4. So why is hydrogen fuel cell technology now of a practical interest to aerospace?
Sustainability: As water is the only emission, hydrogen fuel cells are non-damaging to the environment.
Availability: While hydrocarbon fuels such as gasoline, diesel and jet A are available all around the world, they must at some point be mined from the Earth, and there is a finite supply. Separating water into hydrogen and oxygen by electrolysis is a familiar process widely used to produce hydrogen commercially from seawater. Hydrogen can be produced in-situ almost anywhere in the world with an energy source and the appropriate equipment.
Reversibility: This is the most interesting one from an aeronautical point of view. No practical technology exists today to turn the biproducts of diesel combustion back into diesel and oxygen, to be burned again. If they did, the process would be heavy and energy intensive and likely a net-negative in energy. Comparatively, the hydrogen to water reaction is reversible and many hydrogen fuel cells are designed to be used in this way.
Hydrogen Fuel Cell VS LiPo Batteries
We usually define Fuel as a substance that is consumed irreversibly to produce Energy.
We define a Battery as a tool to store electrical energy for easy use.
A comparison can be drawn between the weight-efficiency of electrical energy produced by hydrogen fuel cells versus by Lithium Polymer batteries.
Take for example a comparable 800W fuel cell system versus an 800W battery system:
In doubling the capacity of the systems from 800 Wh to 1600 Wh, the scaling effects can be realized. For the hydrogen fuel cell system, only the tank size increases, and the tank weight increases accordingly. For the Lithium Polymer system, you must double the number of batteries, doubling the system weight.
If the following 4 conditions are met within the system design, a fuel cell system can effectively replace a battery. We touch upon this cycle and its usage in airships in Part I of this post.
Exhaust water vapor from the fuel cell system can be captured and stored
Reversible fuel cell converts stored exhaust water back to hydrogen
External energy is available to power the conversion (Charge the battery)
Produced hydrogen appropriately pressurized to feed back to the fuel cell
While the minimum battery of this type is larger, more complex and involves more components than a comparable lithium battery, scaling the capacity of such a battery for a fixed power output allows for economies not possible with lithium.
Use in a Solar-Hydrogen Airship Hybrid
For a solar airship to remain aloft for extended periods, sufficient electrical energy must be stored on board for the airship to maintain trajectory and control overnight when the solar panels are not generating electricity.
A reversible hydrogen fuel cell system is a strong candidate to store this energy on board.
Ballast & Lifting Gas Auxiliary Systems
Additional benefits to use of a hydrogen fuel cell battery on Lighter-Than-Air Aircraft is the potential for the system to augment the Ballast and Lifting gas systems.
The fuel cell’s primary process of converting hydrogen to water requires the addition of oxygen, which is obtained from the ambient air. A water molecule is 9 times the weight of a hydrogen molecule, and storing the exhaust water will slowly add to the weight of the craft.
The inverse is true when the process is reversed as electrolyzing water to hydrogen releases the oxygen and lightens the weight of the craft.
Assuming sufficient buffer is available in the aircraft’s stored hydrogen, this feature can be used to effect ballast control.
Additionally hydrogen is a very effective lifting gas. Whether the primary lifting gas of the aircraft is Helium, or Hydrogen, loss of lifting gas is a primary detractor from the possible time aloft. If a subsystem were installed to vent some of the fuel hydrogen into the envelope, producing hydrogen on-board can be used to greatly augment time aloft.