Review on the Hybrid-Electric Propulsion System and Renewables and Energy Storage for Unmanned Aerial Vehicles

The unmanned aerial vehicle (UAV) is an aircraft without a human pilot, and thus the primary applications of UAVs are surveillance without human loss. A low altitude surveillance aircraft is the foundation for the use of light sensor payloads in a small airframe. As surveillance usually needs to be performed covertly, thus the silent flight's capability allows the use of low altitude aircraft. For the UAV propulsion, Photovoltaics are useful to harvest solar energy during the daytime, whose one part is being used directly to power the propulsion unit and onboard instruments while the remaining part is being stored in energy storage system for night-time. In this context, electrochemical energy sources stored in batteries and fuel cells are the two best candidates because of the highest gravimetric energy density. To conclude, this review aims to improve the high-altitude long-endurance of a UAV equipped with the hybrid-electric propulsion system.