Updated: Jan 13, 2020
Hydrogen Fuel Cells
A fuel cell is an electrochemical device that transfers chemical energy into electrical energy. In Proton Exchange Membrane (PEM) fuel cells, that chemical energy comes from Hydrogen combining with Oxygen and producing pure water vapour. The generated electrical energy can be used to power electric motors, UAV systems or charge batteries.
Hydrogen Fuel Cells as a Power Source for sUAS
Lithium cells can come in very small sizes and are 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.
Hydrogen Fuel Cell modules are particularly favourable due to their relative cleanliness and safety and recyclability. Today, fuel cell power modules for UAV are available from several companies in various sizes including Intelligent Energy, Doosan and Ballard.
Considering for example Intelligent Energy's 650W Fuel Cell Power Module (FCPM), currently 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.
At the Hydrogen FCPM's minimum functional system weight, an equivalent weight of LiPo batteries may be comparable in terms of available energy and therefor flight times. However, as you move to increase the available energy of the system by either adding batteries, or by increasing the size of the hydrogen tank, the amount of flight time gained per gram of mass added is much higher for the FCPM. This effect can be seen in the diagram below.
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 indented specifically for long endurance.
Hydrogen Fuel Cell Integration to sUAS Airframes
Among unmanned aircraft types, longer flight times and higher efficiencies can be achieved with airships and LTA than with copters. Airships require less constant power to remain aloft than other airframe types as described in Solar Power for Unmanned Air Vehicles.
Compressed hydrogen for Fuel Cell Powered UAVs is stored in tanks, usually made from carbon fiber and are lined with either aluminium or plastic. Storing all onboard hydrogen fuel in a single tank is necessary to keep the best weight efficiency. While a single hydrogen tank may weigh less than an equivalent energy stack of batteries, it may be volumetrically larger, round and complicated to integrate neatly into most quadcopter airframes.
Unmanned airships, such as eBlimp e2000b1200, are designed to move larger volumes through the air most efficiently and can easily tolerate long or large rounded tanks on the airframe or internally. Much convenient and efficient synergy may be realized by mating unmanned airships with hydrogen fuel cell power for long endurance missions.
Hydrogen and Solar Airships for Multiple Day Flights
The best thing about Solar Airships, such as the Photon is that they can fly all day in Infinity Cruise conditions. But what do you do at night?
One suggestion for multiple day flights, seen also in Airship Design by Nicolai and Carichner, is to add hydrogen fuel cells and an electrolyzer to the airship and create a closed loop solar hydrogen battery system.
The airship, in infinity cruise, flies for the entire daylight period on solar power.
After sunset, the airship switches from solar power to hydrogen power. Water vapour exhausted from the fuel cell is collected and stored.
After sunrise, the solar panels turn back on powering now both the airship's systems and the electrolyzer converting the collected water back into hydrogen.
This closed loop system could unlock the potential for aircraft flights spanning days, weeks or longer depending on the scale at which it is implemented.
One of the challenges facing this system today is small, aircraft-ready electrolysers capable of producing the needed volume of hydrogen in a small form factor and light weight. At Mothership Aeronautics, we would be interested in speaking to any companies working on this.
Efficiently unlocking this cycle in a production aircraft may be one of the key technological improvements enabling sustainable long term aerial persistence.