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What Are the Refueling Options for Fuel Cell Powered UAVs?

Oct. 25, 2018
Article by Jim Sisco

There are expected changes to Beyond Visual Line Of Sight (BVLOS) regulations by the FAA.  These expected changes are going to translate into big opportunities for commercial drone operators.

Now that the ability to operate commercially is on the horizon, drone operators need to think about extending the power for potentially longer flight duration.

Many operators are already integrating hydrogen fuel cell power into both fixed-wing and rotary platforms. Why? Because it offers two-to-three times the “in operation capability” when compared to the best batteries available.

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However, when it comes to hydrogen refueling for UAVs, the infrastructure isn’t a “one size fits all” solution. What works well in the training grounds is often completely incompatible with real world requirements.

Download our free resource to learn how operators can find the best hydrogen  solution for their unique UAV application.>>In this blog, we’ll explore several of the hydrogen refueling solutions being deployed today, and discuss the pros and cons of each.

On-Board Hydrogen Storage: How Does It Work?

Before we get into the refueling approaches, it is useful to review how hydrogen is stored on-board the aircraft.

On-board UAV hydrogen storage is close to being standardized. Hydrogen is kept as a high pressure compressed gas in composite overwrapped pressure vessel (COPV) tanks using a plastic liner. This is the same technology that is used to store hydrogen in fuel cell automobiles. Hydrogen storage mass fractions approaching 6% may be achieved by filling these flight tanks to high pressures on the order of 350-400 bar.

Alternatively, hydrogen may be stored as cryogenic liquid. However, this approach is limited by currently available tank designs. Fortunately, lightweight cryogenic liquid hydrogen tank technology that enables more stringent boil-off control is rapidly maturing. This means hydrogen mass fractions of 8-10% may be achievable with liquid hydrogen tank technology in the future.

(See also: Fuel Cell Drones: Giving Battery Drones a Run for the Money)

A Closer Look at Hydrogen Refueling Solutions for Commercial UAVs

Standard portable industrial hydrogen cylinders are commonly available in all areas throughout the world, and are extremely cost effective. However, they come with transportation, capacity, and tank pressure limitations. Pressure boost systems and secondary storage tanks can address these limitations, but they add complexity and cost to the overall system.

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Ballard’s FC-Air 4.7 litre hydrogen fuel tank for fuel cell-powered UAVs

In contrast, some operators are now using portable electrolysis systems. When coupled with a gasoline, diesel, or a JP-8 (military diesel) generator, these systems can produce hydrogen in a variety of locations. Of course the cost, ruggedness of hardware, and availability of fuel are all factors which come into play with this type of solution.  

Now let’s take a look at the four types of refueling solutions, including the technical and business factors associated with each.

1. Blowdown Refueling

Portable compressed hydrogen cylinders sourced from industrial gas suppliers serve as the most readily available hydrogen source for drone operators. These cylinders may be used as a high pressure, high volume source to drive a blowdown fill of the flight tank. This is a very straightforward filling approach. However, it provides limited control of the flight tank pressure and leaves a large residual of hydrogen in the source container. This refueling approach is best suited for short, intermittent flight operations of a day at a time.

2. Boost Compressor Refueling

Boost compressor filling from a high volume source, such as an industrial gas cylinder, enables tight control of the flight tank pressure. This significantly reduces residual hydrogen in the source container, but adds complexity because of the addition of the compressor. A boost compression system can be transported on the bed of a pickup truck, and is best suited for multi-day (consecutive) flight operations.

3. Electrolysis-Based Refueling

With electrolysis, operators can generate hydrogen on site. The process works by feeding water and electricity into an electrolyzer and using a boost compressor to increase pressure for flight tank filling. Manufacturers are currently developing high pressure electrolyzers that may reduce the size of, or completely eliminate, the boost compressor by supplying hydrogen at flight tank storage pressures. The biggest benefit of an electrolysis-based system is that the hydrogen fueling operation is not reliant on outside hydrogen sources. However, it adds complexity due to the array of components required for operation, and it also has a high up-front cost. An electrolysis-based refueling system could be transported on a small two-wheeled trailer or in a box truck, and is best equipped for week-long flight operations or longer.

4. Liquid Hydrogen Refueling

Operators that opt for on-board liquid hydrogen storage may refuel through blowdown filling from a high volume cryogenic dewar sourced from industrial cryogenic liquid suppliers. A drawback of liquid hydrogen filling is that a significant portion of the fuel is lost through the tank chilling process. However, this approach can still be suitable for anything from one-day to multi-day flight operations. To liquefy gaseous hydrogen on-site, operators can use a cryocooler supplied by industrial gas cylinders, or an electrolyzer. Adding liquefaction does increase the complexity of the system and the up-front cost. The system is portable via a trailer or box truck and is best suited for extended (one week or longer) flight operations.

Conclusion

There is no one-size-fits-all hydrogen storage or refueling solution for all fuel cell powered UAV systems. Each approach has distinct benefits and challenges. The choice will be unique for each UAV application depending on the mission and fleet requirements, flight operation duration, portability needs, operating location(s), and budget.  

We hope this blog has helped you understand the different solutions available, so you can choose the one that’s best for your unique situation. Ballard has active development efforts in each of these areas and is happy to work with you to determine the best solution for your application. 

Next Step

For more information on why hydrogen fuel cell power is a feasible solution to the mandate today's UAVs demand, download our white paper below.

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Hydrogen Fuel Cells for UAVs: Refueling Developments

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