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

62 cooling air through a filter to protect the cathodes, the filter interferes with the air pressure, so about 10-15% more fan power is required for the same amount of cooling,” Jap adds. “So our filtering approach helps avoid fan-related losses, particularly in warmer weather.” Cathode airbox For distributing oxygen into cathodes and hydrogen into anodes, some fuel cell designs have used bipolar plates designed with corner ‘windows’, which when stacked on top of each of other create longitudinal tunnel-like internal manifolds for air and hydrogen gas to flow through. Spectronik uses an alternative, patent-pending configuration for its cell and stack, which retains the hydrogen windows and manifold (respectively), but forgoes the oxygen windows for an external, airbox-like manifold. This manifold is bolted onto the side of the stack. Made from ABS plastic, internally it features a number of channels and valves for dynamically directing and proportioning airflow during operations, with the oxidant blower mounted on top to feed air into it. Meanwhile, the cathode separator plates are designed with an open side facing the external manifold’s output region to receive air directly into their flow fields. “We’ve come up with this approach for a couple of key reasons,” Jap says. “If you use an internal manifold consisting of fixed windows, there must be a limit to how many cells you can modularly stack – at a certain point, the window will be too small volumetrically to deliver enough air at the stoichiometric pressure needed for the load. “That means the cells at the farther end of the stack will produce less power than those nearer the air inlet. If a stack isn’t using all its cells equally, it isn’t optimising its efficiency.” The external manifold uses one large window to deliver air to all the cells at once, meaning there is no drop-off in power at the cell extremities, even in the 2.5 kW Protium-2500 or any larger ones customers may request. Spectronik’s COO Maung Maung Zarli adds, “The reason we don’t use this same airbox-type system for the hydrogen feed is that hydrogen gas is generally always stored at much higher pressure than air. Even with a blower you get only maybe 0.2 bar of air compared with upwards of 300 bar for hydrogen, so it isn’t easy for large PEM cells to get the right air pressure all the way through the stack. We don’t want our end-users to have to use a larger air compressor, as that would add more volume, weight and cost than our external manifold.” Another key advantage of the external manifold comes from its internal structure and valves, which on top of the manifold outlet’s uniform spread of air can enable controlled, non-uniform distribution of it. For instance, as performance levels can vary between different cells in a stack or different regions of cells, a fuel cell controller (FCC) can detect which cells are producing less power, and widen or close valves in the manifold. This will increase the delivery of air to the underperforming cells to ramp up their electricity generation, and optimise the performance and efficiency across the stack. June/July 2021 | Unmanned Systems Technology Extensive research and testing of both open- and closed-cathode fuel cells and ancillary systems has informed Spectronik’s engineers on a variety of engineering choices The fuel cells’ blowers and cathode manifolds are carefully designed to optimise airflow and efficiency across the stack