What are fuel cells?
Fuel cells are electrochemical devices that convert chemical energy from fuels and oxidisers, without combustion, into useful electrical energy that can be used to power devices and vehicles. The cell itself does not require charging, but does require a steady flow of fuel and oxidiser.
Hydrogen is the most common fuel used in fuel cells, with oxygen from the air being the most common oxidiser. Multiple fuel cell technologies exist, including proton-exchange membrane (PEM), solid acid, and solid oxide. In the case of fuel cells for UAVs, PEM is the most widely-used technology as it has the highest power density of any fuel cell type. PEM cells also run at lower temperatures, making their construction and engineering less complex and costly, and giving them a lower thermal signature.
Hydrogen PEM fuel cells
PEM fuel cells are based around a positive terminal and a negative terminal separated by a solid polymer membrane.
The hydrogen fuel is fed to the negative terminal, where the hydrogen atoms are stripped of their electrons to become protons. The protons permeate through the membrane to the positive terminal. The electrons travel to the positive terminal via an external route, and this flow of current provides the useful electrical output of the fuel cell.
Oxygen from the air is fed to the positive terminal, and combines with the protons and electrons from the hydrogen to form water. Because water is the only by-product of the process, PEM hydrogen fuel cells are highly favourable from an environmental and emissions standpoint.
PEM fuel cells versus batteries and internal combustion engines
Hydrogen PEM fuel cells have a number of advantages over batteries and internal combustion engines for UAV and drone applications. They have a high energy density, meaning that they provide a greater energy output to mass ratio, which is particularly important for aviation and unmanned aircraft applications. Hydrogen PEM fuel cells have provided up to three times the endurance at equivalent aircraft/payload weights compared to drone battery systems.
In addition to having a relatively low energy density, batteries take a long time to recharge, with a charge cycle often being many times the typical half-hour flight endurance that they provide. Hydrogen cylinders for fuel cells can be swapped out within minutes, allowing for greater operational efficiency.
Fuel cells are quiet compared to internal combustion engines, the noise and vibration from which can affect the operation of UAV sensors and payloads. Internal combustion engines also require more maintenance and typically have a much lower MTBF (Mean Time Between Failures) than fuel cells, requiring more spare parts and more manpower.
Fuel cells are more efficient at higher altitudes than internal combustion engines, which suffer more from derating due to the lower air density. Small internal combustion engines function at peak fuel efficiency only within a narrow band of conditions, and are also noisy and polluting with a high thermal signature.
Hydrogen fuel cells for UAVs and drones
Fuel cells can be stacked in a modular fashion depending on the output voltage required. A complete drone hydrogen fuel cell power system will additionally incorporate control systems for fuel and air, as well as hydrogen regulators and liquid or air cooling systems.
Although they feature a high energy density, hydrogen fuel cells exhibit a relatively low power density, which translates to a low thrust to weight ratio for UAVs and other aircraft. This means that currently fuel cell technology is not applicable to the largest UAVs, which require very high peak output power and thus are better served by traditional combustion engine technology.
The smallest UAVs are also currently unable to take advantage of fuel cell technology due to the weight required and the lack of usable space on the aircraft. These drones, which tend to be used for shorter missions, are better served by battery technology.
Small-to-medium sized UAVs are thus the main current target market for PEM hydrogen fuel cell systems. Hybridized systems can also be fitted, which pair fuel cells with a small battery that provides extra power during high-demand phases of flight such as takeoff, rapid climbing and high wind resistance, thus offsetting the aforementioned low power density disadvantage of the fuel cell. The hybrid battery can be recharged during periods of low demand.
PEM hydrogen fuel cell systems can potentially enable increased efficiency for BVLOS (beyond visual line of sight), ISR (intelligence, surveillance and reconnaissance) and long-range mapping and surveying UAS missions.
Fuel Cell Systems: Frequently Asked Questions