UAVs

A combination of fuel cell and battery is the best option to power micro air vehicles (MAV) in the near term, says the US Air Force Research Laboratory (AFRL). The combination is required for different MAV performance requirements during a mission.

After an analysis of various fuel cell and battery types, the AFRL's preferred combination is a high-density battery with a high-power fuel cell. "The fuel cell is for when you really need to go for long flight times," says AFRL propulsion directorate advanced UAV propulsion engineer Lt Daniel Erickson.

The cell type selected uses adisposable fuel cartridge containing gaseous hydrogen in a hydride compound. It can provide at least 240Wh/kg power densityand is expected within two or three years to enable a 6-8h endurance for a MAV.

Battery options include lithium ion with a 150-180Wh/kg power density, lithium sulphur dioxide for 200Wh/kg or bi-polar lithium ion for 250Wh/kg. Within two years, lithium sulphur batteries are expected to deliver better performance, achieving 300Wh/kg. Future work will focus on energy harvesting using solar power and air current thermals for soaring.