Modelling, simulation and performance evaluation: PEM fuel cells for high altitude UAS

A fuel cell is a device that converts energy in the fuel and reactant into electrical DC power. Fuel cell powered aircraft are generally characterised by a low power to weight ratio (W/kg). The propulsion system of an unmanned aircraft needs a large range of power and fast response to fulfil the requirements of different flight phases and to balance the variations in the load demand. A proton exchange membrane (PEM) fuel cell is considered as a potential power source for high altitude UAS (unmanned aircraft systems) operations. At altitudes in excess of 10 km, very low atmospheric temperatures and pressures, and unexpected variations in the load demand put severe stresses on the operation and performance of PEM fuel cells. A stable and robust controller and fuel supply system that can provide fast and sufficient flow of hydrogen and air/oxygen to the reaction of the fuel cell is one of the critical objectives. In this research, a simplified mathematical model of the PEM fuel cell stack system is developed and validated with the commercially available 1 kW PEM fuel cell stack (H-1000) developed by Horizon Fuel Cell Technologies. Matlab-Simulink is used to implement the necessary design and simulations under various operational conditions. The implications of high altitudes on the operation and performance of a PEM fuel cell stack are investigated, and a PID controller is adopted to efficiently optimise and provide a sufficient flow of hydrogen and air/oxygen to the stack, in particular maintaining the flow rates of the reactants was deemed most critical at high altitudes operation. Also, in order to store the required oxygen and hydrogen, the design of storage vessels is considered. This research presents a design of a PEM fuel cell power system for unmanned aircraft systems with an integrated approach that enables estimation of required power for high altitudes UAS operation which is then used to determine the size and weight of the combined power-plant of fuel cell stack with hydrogen and air/oxygen vessels and the propulsion system of the UAS. This approach takes into the consideration the power capacity of fuel cell stack and the flight endurance as two main factors in designing the size and weight of storage vessels, and hence determining the overall weight of the UAS, which is a key requirement in the preliminary aircraft design phase. One of the research outcomes shows a potential in extending the flying duration and altitude for more than five hours and a half, reaching up to 11 km altitude, for a UAS with an overall weight of 32 kg, including a payload capacity of 2 kg, based on a 1 kW PEM fuel cell propulsion system.