How Hydrogen Fuel Cells Compare to Propane for UAS Power

Drone Flying Over Runway

One of the most important factors when choosing a power source for Unmanned Aircraft Systems (UAS) is weight. A commercial drone needs lightweight power that will keep it operational for long flight times. Historically, batteries have been the main power source for UAS, but fuel cells are gaining adoption because they can provide more power for the same weight as batteries, enabling longer missions by making the same payload last longer.

Why Fuel Cells Are Ideal for UAS

Every fuel cell requires two components: the main body of the power-generating unit (i.e., the fuel cell) and the fuel tank. The amount of energy stored in a fuel tank is the energy used to move the UAS forward and power payloads.

For a battery-only UAS, the fuel tank is the battery. Advanced batteries store 250 Watt-hours (Wh) per kilogram of battery mass, compared to propane, which can store 13,800Wh per kilogram of mass. Propane can store nearly 55 times more energy than a battery.

The fuel cell is what converts this chemical energy into electrical energy. The improvement in stored energy for a complete power system depends on the mass of the fuel cell, the fuel tank, and the efficiency of the fuel cell. The effect is that fuel cells have a much higher energy-to-mass ratio than traditional battery systems, which equals significantly longer flight times.

Benefits and Limitations of Hydrogen Fuel Cells

But there are many types of fuel cells utilizing a variety of fuels, such as hydrogen and propane, ; each with its own unique benefits. Hydrogen, for example, is lightweight and offers improved endurance over batteries. However, while hydrogen alone is not heavy or a complex fuel, operators will have to contend with extremely heavy tanks to hold the fuel. This is because hydrogen has a high energy content by weight, but not by volume.

The volume density of hydrogen is a limiting factor for a drone that requires minimal added weight. Hydrogen is a gas and must be compressed and stored at high pressures, requiring thicker and denser tanks for storage. For safety, these tanks also have pressure relief devices to prevent ruptures or implosions due to pressure changes or high temperatures. This adds both more weight and an additional maintenance requirement.

Hydrogen tanks can get up to 10,000 PSI (pounds per square inch), whereas propane normally sits at around 100 PSI. This means hydrogen-fueled PEM (polymer electrolyte membrane, a type of fuel cell) systems require a significant weight investment for the tanks alone as compared to propane-based systems.

The delivery infrastructure for hydrogen is also still lacking. While some hydrogen consumers can call a local welding supply company to have hydrogen delivered in industrial tanks at 2,500psi, UAS operators require additional complexity.

One of the most advanced sourcing options for hydrogen is to use an electrolyzer to separate water into hydrogen and oxygen (which requires both water and an energy source), and a compressor to compress the generated hydrogen to tank pressures. This is difficult to achieve on missions in remote areas that lack good infrastructure or basic utilities.

How SOFC Improve UAS Endurance

Meanwhile, our solid oxide fuel cell (SOFC) technology provides lightweight power in the same or smaller footprint as batteries using a readily available fuel source: propane. The advantages of propane are its low storage pressure, high volumetric energy, and worldwide availability. In a drone application, this high volumetric energy translates into the ability to use extremely lightweight carbon fiber fuel tanks.

The benefits of SOFC for UAS don’t just end at weight. Because an SOFC has no moving parts, the system requires no routine maintenance and can power nearly a hundred missions thanks to the built-in cycling capabilities. Plus, our onboard power conditioning makes our SOFC easily integrated into hybrid systems – meaning you can utilize solar while still having a fully dependable backup energy source when needed.

Unmanned Aerial System (UAS) Success Story With SOFC

Recently, our VXE UAS platform broke all previous records for flight time. The power source they chose for this massive test of endurance? Adaptive Energy’s Defender Series 450W SOFC. This establishes a new record in the Group 2 (5 to <25-kilogram) UAS category with a flight time of 39 hours, 17 minutes and 7 seconds. Read more about it here.

When looking towards the future of UAS, integrating a power source that is lightweight, low-maintenance and efficient will be pivotal for platform success.

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