# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 1 Introduction
Supercapacitors can also contribute to the power supply process due to their high-power density and rapid response to peak power demands during UAV take-off and sudden maneuvers. They excel in providing high power density, enabling electric UAVs to execute swift maneuvers and respond promptly to changing flight conditions. Additionally, supercapacitors boost a longer cycle life, meaning they can endure a higher number of charge and discharge cycles without significant degradation. The combination of batteries and supercapacitors in a hybrid system capitalizes on the strengths of both technologies, optimizing energy storage and delivery for UAVs. Fixed-wing UAVs have the advantage of incorporating solar cells and harnessing solar energy, which can reduce fuel consumption and greatly enhance endurance when combined with an energy storage system [49]. Therefore, in order to achieve longer endurance for UAVs, it becomes necessary to employ hybrid power supply systems that combine multiple power sources. The selection of the power supply system structure is crucial and depends not only on the characteristics of the power sources but also on the specific mission requirements of the UAV. To effectively manage the distribution of power among the onboard power sources and ensure optimal performance and efficiency for the intended mission, the implementation of an energy management system (EMS) is essential. An EMS is a sophisticated control system that plays a pivotal role in efficiently managing the distribution of power among the UAV's onboard power sources. It employs a combination of current and voltage sensors for monitoring power flow, converters for regulating the power source outputs, and a processing unit responsible for executing the adopted power management strategy. The EMS ensures that power is allocated optimally, enabling the UAV to meet its specific mission requirements. In addition to hybridization, there are alternative techniques available to extend the endurance of battery-based UAVs. These techniques include battery swapping [50; 51], in-flight recharging using laser beams [52; 53; 54; 55], and tethered UAVs [56; 57; 58]. Battery swapping involves replenishing depleted batteries of a UAV during its mission by utilizing ground stations located at specific locations. In-flight recharging utilizes a laser beam transmitted from a generator at a ground station to provide light power, enabling recharging without the need for landing. Tethered UAVs have the advantage of unlimited endurance as they receive a continuous power supply through connection lines that link the UAV to a ground-based power supply station. Numerous review papers in the literature have extensively covered various aspects of UAVs. These include topics such as fuel storage and generation in small fixed-wing UAVs [41], the classification, advancements, and research trends in small-scale UAVs [58], the classification and design challenges of UAVs [30], path planning algorithms for fixed-wing UAVs [59], the guidance, navigation, and control of rotorcraft unmanned aircraft systems [60], and the modeling and control of quadrotors [61]. However, the specific focus of this review is centered on the energy aspect of UAVs. Its objective is to provide a comprehensive and critical assessment of the existing power supply structures [62] and their corresponding energy management systems. This review will not only identify any existing gaps but also provide valuable recommendations and prospects to guide future research in this field.