Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

70 other manufacturers might; we regulate the hydrogen pressure with a pressure sensor on the stack near the hydrogen inlet. We might move it to the regulator but that would add wiring and impair our simple and safe approach. “We’ve done hours of tests and simulations, in some cases above and beyond certification requirements, to validate the safety of our approach, and we’re sure that adding valves for further gas control would just increase weight and cost, not safety.” Air compressor A custom-designed air compressor is used to draw in and slightly pressurise the ambient air before feeding it into the cathodes of the PEM stack layers. While a high purity of hydrogen is fed directly into the fuel cell, ambient air is run through a filter that sits upstream from the compressor, to prevent particles such as desert sand or marine salt spray from entering and contaminating the stack. “Air being drawn by the suction of the compressor first enters an inlet pipe, where we have a humidifier,” Ratcliffe adds. “Maintaining a minimum level of humidity inside a fuel cell is critical to prevent derating, so we re-engineered a COTS humidifier to make it smaller and lighter. “The humidifier draws in some of the excess exhaust water via Nafion tubes – it’s a passive process so there’s no energy loss – and that gets drawn into the moving air. It sounds simple but it took a huge amount of r&d to manage all these fluid dynamics, to keep the coolant, the fuel and the reactant where they’re supposed to be, and never crossing over into any other spaces throughout the PEMFC or BoP.” The FCCU NWUAV has developed a custom FCCU to control the fuel cell, and wrote the control code in-house. It points out that this is another key benefit of its fuel cell system, as it allows any custom changes to the code requested by customers to be made quickly. As with all fuel cells, the NWFC-1500 will periodically build up excess water in its exhaust. Some of this is removed or reused by the humidifier, while some (particularly water that has seeped into the anode side of the stack) is expelled by a purge valve. The rate at which the purge valve opens and closes is calculated and mapped according to a combination of power, temperature and other data inputs. Efforts have been made to purge at the lowest rate possible while still preventing flooding, as some hydrogen is inevitably lost during excess purging. “We continue producing power as it purges though,” Ratcliffe says. “Some PEMFCs turn off power, or current-pulse to rejuvenate the stack while purging, but as a closed-cathode system with closed- loop cooling, we maintain our internal humidity well enough that we don’t need to current-pulse.” The mapping is embedded in the FCCU. The control software architecture has been written to comply with DO- 178C DAL-A as a minimum of aviation safety validation, and to ensure the fuel cell can react as quickly as changes in throttle demands from an autopilot. The FCCU takes real-time data over a CAN bus from sensors for temperature, voltage, current, humidity and fluid pressures in the stack. Based on these readings, the system will alter (also over CAN) the speed of the liquid-coolant pump, the pressure induced by the air compressor, and the cell voltages and currents. “One of the most important developments was our cell voltage monitoring system,” Ratcliffe says. “If this product is to power an aircraft, the operator needs to know if even one cell is under- or overvoltage, so that troubleshooting or mitigation can be performed and prevent flying on a degraded power output if it can be avoided. “We don’t have voltage sensors on the cell layers. We connect with every PEM cell, and we have a proprietary method for accurately inferring the voltage coming from each one. Devising that system was absolutely key to be able to call this an airworthy system; a lot of rigour went into the architecture of the cells to enable that.” Future plans Extensive efforts have been made to design the NWFC-1500 for serial production. Extensive tooling, test equipment and hydrogen generation systems are now installed at the company’s facilities in Oregon, in the US, to precisely stack, press, seal and validate the system with consistency and safety to aviation standards. December/January 2022 | Unmanned Systems Technology Air (for oxygen input) is drawn through a compressor from Eberspaecher Vairex (grey) and a filter (brown) before entering the fuel cells