Ultra-Compact Power System for Long-Endurance Small Unmanned Aerial Systems

Air-launched Small Unmanned Aerial Systems (SUASs) provide critical information to warfighters, but are currently limited by the power and energy available from small electric propulsion systems. This paper describes proof-of-concept testing of a novel power system for SUASs. The power system comprises a compact hydrogen generator and a hydrogen PEM fuel cell. The hydrogen generator uses ammonia borane (AB) as a solid chemical hydrogen storage material and heats the AB to produce hydrogen through thermal decomposition. The innovative ignition and control process generates highly pure hydrogen on-demand from a system that is very compact, lightweight, and rugged. We built a proof-of-concept hydrogen generator and used it to supply hydrogen to a small PEM fuel cell. The proof-of-concept generator used prototypical AB, heat source, control scheme, and purification media to absorb trace amounts of ammonia, borazine, and carbon monoxide (CO). The hydrogen generator operated as expected, producing hydrogen as needed to maintain the supply pressure within specified bounds. The fuel cell produced 4 A/8.6 V for 15 minutes, amounting to 8.6 W-hr of electricity. We measured the fuel cell's current/voltage characteristics before and after the test, and found no measurable change due to operation with AB-generated hydrogen. Based on these data, we scaled up the design of the proof-of-concept unit and produced a design for a 720 W-hr hydrogen generator with a very high hydrogen density (5.6 wt% and 36 g H₂/L). The hydrogen generator is extremely simple and well-suited for operation in an air-launched SUAS. Combined with a compact fuel cell, our system should enable a small power system for SUAS with an overall energy density in the range of 350 to 600 W-hr/kg.