Unmanned Systems Technology 024 | Wingcopter 178 l 5G focus l UUVs insight l CES report l Stromkind KAT l Intelligent Energy fuel cell l Earthsense TerraSentia l Connectors focus l Advanced Engineering report

70 Dossier | Intelligent Energy fuel cell inputs such as humidity and where the stack is in its lifespan, and fixes it there, between 40 and 60 C,” Kelly notes. After flowing through the electronics section, air is drawn through the fuel cell. A large number of airflow channels run widthways through each of the 34 cells, 50% of which are for cooling, and the other half being reaction channels connecting to the cathodes’ diffusion layers. The geometry of the cell channels has been designed and optimised through multiple rounds of CFD simulation, prototyping and testing. “If the air passed through a straight channel, you’d only get cooling at the very edges, because the air in the middle wouldn’t touch any of the surfaces you need to cool, or any of the reaction area,” Kelly explains. “But if you have a non-straight channel that has the flow disturbed, the air moves around and you don’t get that laminar flow. Instead you get flow that is better at contacting the channel walls and thus better at taking heat from the surrounding surfaces.” Notably, the open-cathode design of the module, with the fuel cell stack and the balance of plant exposed to the open air, allows dust and particulates to enter the fuel cell. The team says filtration would have simply led to blocked filters and higher maintenance costs, while a closed-cathode system would have been more expensive and complex than their design envelope permitted. Instead, dust and particles entering the intake vents typically end up being expelled through the fan ducts along with the purged hydrogen and humid air. While very small quantities of dust might end up trapped in the stack’s air channels, IE’s tests have found this has no effect on the fuel cell’s performance over the course of its 1000-hour lifespan. Hydrogen supply The hydrogen gas is typically stored in one of two container types. “Type 3 pressure vessels use an aluminium internal liner with a carbon fibre wrap, while a Type 4 cylinder has a polymer liner, also wrapped in carbon fibre, the latter type being lighter,” Ninan says. “These are widely available COTS pressure cylinders, so there’s no call for anything bespoke there.” The pressure regulator, which weighs 250 g, was developed in partnership with another UK company, Pressure- Tech. The regulator is based on existing technology but made to be as light as possible, as expected of components targeted at UAV. “Pressure-Tech had done some work on UAVs, but never anywhere near this competitive, weight-wise,” says Kelly. “So they changed almost all of the regulator’s structure from stainless steel to aluminium, and removed material wherever possible.” IE also builds in burst discs, which give way in case the temperature, pressure or both become too high in the cylinder. Pressure relief valves (PRVs) are also built in to provide a similar safety function on the low-pressure end. If a fault should occur in the regulator, the PRV allows the gas to escape before the situation becomes dangerous. In use, the regulator lowers the hydrogen’s 300 bar pressure to roughly 0.5 bar, the optimal level for the fuel cell. The pressure system also contains a pressure transducer – which detects how much pressure (and thus fuel) is left, and outputs these readings to the control unit – as well as a solenoid valve from The Lee Compnay for turning the hydrogen supply on and off. The regulator is connected to the electronics module by a signal cable, which runs under the hydrogen tube connected to the inlet valve. The cable delivers a voltage signal from the transducer for readings on remaining pressure/fuel, as well as a proprietary PWM signal for controlling the solenoid valve. February/March 2019 | Unmanned Systems Technology The fuel cell is typically used with either Type 3 pressure vessels or a Type 4, which is lighter In a non-straight channel you get flow that is better at contacting the channel walls and thus better at taking heat from the surroundings