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

68 Dossier | Northwest UAV NWFC-1500 fuel cell temperature of the system really quickly, so not only do we have a wide operating environment range but our customers can start up the power unit and change the power delivery really fast, maybe going from an idle loiter to top speed very suddenly,” Ratcliffe says. “They can’t afford to wait 10 minutes because of a slow ramp rate or depend on an oversized battery to make up the shortfall. We can start up from cold to full power in just a few seconds thanks to this cooling system.” He adds that one of the critical factors that hydrogen fuel cells must maximise is consistency, echoing the sentiments of Spectronik whose closed-cathode fuel cell (investigated in UST 38, June/July 2021) was designed specifically for enhanced reliability over open-cathode systems. “You want a consistent electrochemical reaction, response and longevity across every plate, and the best way to do that is to have a consistent temperature across every plate,” Ratcliffe says. “We’re confident that the high thermal density of our cooling medium allows us to do that far better than air-cooling – it might even restart and respond more consistently than IC engines, on top of lasting far longer.” The fuel cell has a cooling circuit quite similar to that of Teledyne Energy Systems’ EDR fuel cell ( UST 35, December 2019/January 2020). Cooling liquid runs through pipes into the upper side of the unit, flowing down through the cell layers through channels oriented in the z axis and exiting through pipes in the bottom side, with a pump mounted externally to the fuel cell. Notably, NWUAV has forgone standard water-glycol coolants, as such liquids have undesirably high electrical conductivities, mandating the use of de-ionising filters that must be routinely replaced, to the detriment of long-run operating costs. Instead, the company has opted for a special fuel cell coolant with corrosion inhibitors. “We keep the circuit as a totally closed loop, so there’s no need for filtering, which saves weight and maintenance costs,” Ratcliffe says. “At the end of the system – the back of the tube in the standard-issue BoP arrangement – we install a custom radiator, so that end- users and integrators don’t have to make or break connections to get the heat out, or worry about bubbles appearing, which would count as an impurity in the cooling medium.” He adds that temperature sensors are installed inside the coolant circuit and form the fuel cell control unit’s (FCCU’s) primary means of gauging the health and thermal stability of the stack when calculating control responses. Hydrogen injection While PEMFCs are largely agnostic to the types of fuel tanks used, NWUAV typically uses ones made from carbon-plastic and carbon-metal for storage at a range of pressure levels. Given the maturity of hydrogen storage and regulation technology, the company has gone for a keep-it-simple approach to hydrogen management, so it uses a COTS pressure regulator that was chosen for being lightweight and easily coupled to the tanks. The hydrogen is regulated from 345 bar (5000 psi) down to 0.7-1.4 bar (roughly atmospheric pressure), for injection into the stack, from where it flows into a channel that distributes it into the anode face of each MEA. It is consumed at a rate of 0.5 g/minute for 500 W cruising power, and up to 1.8 g/minute at 1500 W. Ratcliffe notes, “We didn’t opt for valving on this system as much as December/January 2022 | Unmanned Systems Technology A radiator is included in the balance of plant and can be mounted flexibly to ensure rapid thermal management of the stack Customers can’t afford to wait 10 minutes because of a slow ramp rate. We can start up from cold to full power in just a few seconds thanks to our cooling system