Energy harvesting fueling the revival of self-powered unmanned aerial vehicles

Unmanned aerial vehicles (UAVs) have been widely used in various situations, such as life rescue, environmental exploration, and wireless communication, to avoid the safety risk of human operations. Particularly, battery-powered UAVs have gained considerable interest due to their portability, low cost, and manufacturing convenience. However, their potential use greatly relies on limited battery life, and energy harvesting as a sustainable energy strategy is opening a promising way to realize self-powered UAVs. Here, we focus on discussing the existing UAV energy harvesting methods from the perspective of solar and mechanical energy. Based on these energy sources, we also discuss the commonly used energy conversion mechanisms and some representative architectures of the latest UAV energy harvesters. Solar energy harvesting for UAVs mainly relies on photovoltaic cells and can reach watt-scale output power. In contrast, mechanical energy harvesting for UAVs can be further refined to wind-induced vibration and flapping wing motion whose output power is in the milliwatt scale. Dynamic soaring, inspired by the maneuver of albatrosses, is also reviewed as one mechanical energy harvesting method. Furthermore, challenges and future research directions are discussed and proposed.