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

59 Protium-1000, which measures 322 x 290 x 154 mm, weighs 3.03 kg, incorporates 30 cells in total for current generation, and can output up to 1.2 kW, or a continuous 1000 W (hence its name), consuming hydrogen gas at a rate of 12.5 litres/minute (1.12 g/minute). The three larger variants are also named for their continuous rated power outputs. The Protium-1500 weighs 4.22 kg and can produce up to 1.8 kW, the Protium-2000 peaks at 2.4 kW and weighs 5.27 kg, and the Protium-2500 can output up to 3 kW from a system weight of 5.84 kg (with 75 cells in the largest system’s stack). All four systems are air-cooled, producing a rated current output of 55.5 A over an operating temperature range of -10 to +45 C, with a rated operating altitude of 1.5 km above ground level, although they have been tested successfully at higher altitudes. Also, they all have a mean lifetime of 1000 hours – after that, power outputs can start to drop below their rated levels, but the cells can be refurbished by changing the membrane electrode assemblies (MEAs). The cells consist of the MEAs with polymer electrolyte membranes sandwiched by catalyst and gas diffusion layers, and extra layers for sealing and cooling. Each cell stack comes with an external airbox-like manifold, axial cooling fans, radial intake blowers and an electronics module containing control systems, power converters and a radio modem for live performance monitoring at the vehicle operator’s GCS. Airflow management Since open-cathode cells use the same airstream for cooling and the reaction of oxygen and hydrogen to produce electric current, that implies a simpler and less expensive architecture than if separate airstreams are used. Most often, that is manifested by their developers forgoing an extra plate that would otherwise normally be installed to seal the cathode’s air supply. However, this reliance on a single stream of air for thermal management and oxidant supply produces a number of inefficiencies relative to having two dedicated and separate streams. The Protium cells are designed to avoid these inefficiencies – and indeed improve considerably over existing cell architectures – by having a dedicated radial blower installed at the corner of each cell stack. This draws in and compresses air for delivery to the cells’ cathodes through channels running the length of the powerplant, while axial fans pull coolant air through channels that run along its width. Separating the oxidant and cooling channels enables the fuel cell to function safely (with a stable and controlled internal level of humidity) across a wider ambient temperature range. For example, if the stack’s internal and surrounding environments are cold, running the cooling fans is unnecessary. At such times, only the oxidant blower needs to work, and the system can save on parasitic losses from the cooling fans. “In an open-cathode cell, the fans would still need to run to provide reactant airflow, further cooling an already cold fuel cell,” Spectronik’s CEO Jogjaman Jap explains. “As a result, the performance would drop because optimum cell temperature couldn’t be reached. That excess cooling also risks flooding the cells, which would lead to fuel starvation, cell degradation and damage to the cell materials.” Also, in a hot environment, having separate oxidant and cooling channels means the cooling fans can turn at a high speed while the oxidant blower can spin at a far lower speed and power to maintain stoichiometry for the electrochemical reaction. That allows an appropriate level of humidity in the cell to be maintained so that the stack can continue outputting the required power. In an open-cathode cell, running the fans at maximum to intensify air-cooling in hot environments would also remove much of the internal water content, severely risking dehydration of the polymer electrode membrane and Spectronik PEM fuel cells | Dossier Unmanned Systems Technology | June/July 2021 Proton exchange membrane or polymer electrolyte membrane (PEM) fuel cells generate electrical energy via an electrochemical reaction of hydrogen and oxygen taking place inside the membrane electrode assembly (MEA) at the core of each cell structure. The MEA consists of a PEM layer sandwiched between an anode and a cathode that serve to diffuse the reactant gases (and each also embeds a catalyst layer, typically platinum). Hydrogen flows into the PEM’s anode, while oxygen flows into the cathode, each gas typically being distributed via a flow field installed into the two plates that enclose each cell. The catalyst layer splits the hydrogen gas molecules at the anode into protons and electrons. While protons pass through the central, semi-permeable membrane, reacting with the oxygen molecules to form water and leave the cell via the exhaust, the electrons flow along a circuit as an electric current. A number of ancillary sensors and other systems ensure there is the right balance of heat, humidity and reactants inside the stack of cells, with additional control systems managing the power output and hence the rate at which reactants are consumed. Collectively these systems are referred to as the balance of plant. PEM fuel cells