Updated: Mar 23, 2021
Hydrogen Fuel Cells
Hydrogen fuel cells (HFCs) 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.
The fuel cell is an electrochemical device that transfers chemical energy directly into electrical energy. It works through a Proton Exchange Membrane (PEM):
Proton Exchange Membranes
Hydrogen is the simplest elemental gas. The entire atom is just 1 electron orbiting 1 proton. A proton exchange membrane is a membrane designed such that when pressurized hydrogen gas is pushed against it, the hydrogen nuclei (protons) are able to pass through and become ions, but the electrons cannot pass through, and are effectively stripped away the nuclei. They accumulate on the anode.
On the other side of the membrane at the cathode, hydrogen ions combine with oxygen to produce water vapor. This process takes electrons from the cathode, charging it positively.
Now with electrons accumulating at the anode, and a positive cathode, a potential difference (voltage) is created which can be used to power a load.
Hydrogen Fuel Cell Technology
HFC 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.
Plug Power found success in selling HFC powered forklifts to large warehouses. Sustained emissions from frequent use of gas powered forklifts indoors can be harmful to warehouse staff and contaminate products. Battery powered forklifts don't have the endurance to move back and forth within a large warehouse. The HFC forklift niche has been significant enough for Plug power to grow their business and raise significant funding.
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.
Low Heat & Noise: Fuel burning propulsion systems produce noise and heat which increase chances of detection. HFCs produce less noise and heat than a comparable internal combustion engine. Additionally, less heat shielding and cooling infrastructure is needed onboard.
HFCs as an Alternative Power Source to LiPo Batteries in sUAS
Today, fuel cell power modules for UAV are available from several companies in various sizes including Intelligent Energy, Doosan and Ballard. (Note: Since this original writing Ballard's UAV fuel cell division has been acquired by Honeywell)
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: Intelligent Energy currently produces the lightest-weight commercially available Hydrogen FCPM for UAVs weighing in at 850g. Adding all essential components such as regulator, hybrid battery, harness and a minimal hydrogen tank brings the system to 1,500g.
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 to double its power, it must double the number of batteries, which doubles the system weight.
While a mass of hydrogen gas stores more energy than the equivalent mass of batteries, it cannot supply the power without the fuel cell whereas the batteries can both store energy and supply power. Around 2,000g of power system weight, the fuel cell system begins to store more energy than LiPo for a 600W drone.
This property makes Hydrogen Fuel Cells an ideal candidate for powering UAS intended specifically for long endurance flights.
Lithium cells can come in very small sizes and are likely to remain the dominant power source for UAS ranging from 10g - 2,000g. In UAS weighing 2,000g or more, alternative power sources such as fuel burning engines, and fuel cells can be considered and effectively integrated.