Current Technologies and Future Trends of Hydrogen Propulsion Systems in Hybrid Small Unmanned Aerial Vehicles

The use of small unmanned aerial vehicles (UAVs) with electrical propulsion systems in both military and civilian areas has attracted considerable attention in recent years. In such UAVs, batteries, supercapacitors, hydrogen-fuel cell systems, piezo materials, and solar cells can be used to power the electric motors during the flight. These systems have different specifications in terms of cost, energy and power density, dynamic response, efficiency, lifetime, etc. and none of these energy sources alone can hardly meet the power demand of a UAV. For this reason, hybrid systems, which are a combination of these energy components, have been proposed in the literature. Such hybrid systems require appropriate energy management to increase the total efficiency of the system, lifetime of each energy component, and flight time/range of UAV. Energy management techniques applied to hybrid systems are divided into two classes, active or passive. Active energy management strategies are more efficient but require more complex system topologies than passive methods. This study presents different hydrogen-based systems that can be used to feed the electric motors in UAVs and compare them with other technologies in terms of several Key Performance Indexes. Moreover, different hybrid system topologies proposed for fuel cell-based small UAVs and their passive/ active energy management techniques such as rule-based, optimization-based, and learning-based strategies are examined and discussed. Finally, future workopics on the use of fuel cells in UAVs are proposed, which could be important to optimize the flight time/range and the energy consumption of UAVs.