Unmanned Systems Technology 038 l Skyeton Raybird-3 l Data storage l Sea-Kit X-Class USV l USVs insight l Spectronik PEM fuel cells l Blue White Robotics UVIO l Antennas l AUVSI Xponential Virtual 2021 report

67 Spectronik PEM fuel cells | Dossier “Smaller fuel cells typically use a dead-ended, non-circulating fuel stream that produces stagnant fuel zones in the anode reaction area. By-product water can also cross over to the anode channels, impeding the reactions. Purging allows that water to be flushed out of the anode streams and restores the rich hydrogen concentration in the anode. “Our FCC monitors and calculates the accumulated energy output between each purge, and accelerates or decelerates purging when a certain threshold for energy output in kWh has been reached.” This function enables more frequent purging during heavy loads to maintain an optimal fuel cell power output, as well as reduced rates of purging during low loads to prevent unnecessary hydrogen fuel losses, as some hydrogen molecules will inevitably be lost along with the by- product water if the valve is opened more often than needed. “Some fuel cells will still keep purging at a fixed rate – say every 20 seconds – even when they’re on standby; a lot of fuel can be wasted that way,” Jap observes. “Or some users will run them at the top of their rated output for a long time, then start to lose power and accrue damage, and they don’t realise it’s because the purging system isn’t being operated intelligently. Our smart accumulator constantly checks for the optimal purge frequency, and controls the valve accordingly.” The FCC is also programmed with an early leak detection function. This works by monitoring the pressure at the stack’s inlets and outlets, calculating the differential between them, and tracking when it deviates from expected values. During such deviations it can immediately alert the user that stack sealing might have been compromised. Other cell designs can take longer to detect leaks, for example by using hydrogen sensors outside the stack to detect gases only when build- ups have already happened. Or, in some instances, the cell supplier merely recommends that the end- user integrates such sensors for leak detection inside their vehicle hull themselves, which some users inevitably ignore. By contrast, Spectronik’s use of software and integrated pressure sensors can detect leaks and notify the end- user as quickly as within a second of them occurring, enabling a UAV’s safe and early landing or a return to base, theoretically preventing emergency landings in cities or over water. An altitude sensor is also integrated so that the FCC can continuously determine and track changes in air density, as that directly affects the fuel cell’s oxidant supply and hence performance. “We’ve pre-programmed and embedded a blower curve algorithm into the FCC, so that at higher altitudes (and lower air densities) than the standard 1500 m, it can compensate by running the blower faster to force more air into the stack,” Jap says. “Also, as our customers will always deliver us a typical load profile of the vehicle they want to run using our cells, we’ve noticed that more and more of them want to engage in heavy transient loading – hard accelerations and braking, for example. That kills the lifetime of fuel cells; specifically, the heavy shifts in output voltage will damage the catalyst layer and material. “So we’ve introduced a power regulator board with modular DC-DC converters and an additional dedicated controller to regulate the fuel cell output during transients, by progressively ramping current up or down in a stepwise manner. That prevents the cell voltage from Unmanned Systems Technology | June/July 2021 A map of the blower’s output curves at different altitudes, oxygen densities and power outputs We’ve noticed that more and more users want to engage in heavy transient loading such as hard braking. That kills the lifetime of cells