# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 1 Introduction  
An unmanned aerial vehicle (UAV), also referred to as a flying robot or drone, possesses the ability to operate independently or under remote control for specific missions [1]. In recent times, UAVs have garnered considerable attention as a result of advancements in microprocessors [2, 3] and artificial intelligence (AI) [4, 5, 6, 7], sensors [8, 9, 10, 11, 12, 13, 14], and the design [15, 16, 17, 18] and manufacturing process [16, 17, 18, 19, 20], facilitating the creation of intelligent UAVs. The term \"intelligent UAVs\" refers to these drones' capacity to operate autonomously or semi-autonomously, making real-time decisions based on data from their sensors and AI algorithms. These intelligent UAVs can adapt to changing environments, navigate with precision, and perform tasks with a level of autonomy that was once reserved for sciencefiction [21]. Moreover, the UAV wireless charging system represents a significant departure from conventional cable-based charging methods. This innovation has attracted substantial interest in the field [22], as it offers a promising alternative to traditional charging mechanisms, further enhancing the appeal of UAV technology. These technologically advanced UAVs offer a multitude of benefits, including cost efficiency and exceptional maneuverability, leading to their utilization in diverse military and civilian domains such as mine clearance, surveillance, delivery services, wireless connectivity, and agriculture [23; 24]. It is crucial to highlight that UAVs have significant advantages over UGV operational requirements [25], but UGVs are preferred for heavy-load missions due to their ability to carry substantial payloads [26]. Prominent multinational corporations are making substantial investments to enhance UAV capabilities and expand their application spectrum. Nowadays detection techniques improve safe and reliable UAV operations due to fault detection methods [27]. It is estimated that the market value of UAVs will reach a staggering USD 127 billion by 2022 [28; 29]. This surge is propelled by remarkable advancements in microprocessors, artificial intelligence (AI), sensors, design, and manufacturing processes. Recent statistics and trends underscore their growing significance, as UAVs have become integral in numerous real-world applications. For instance, in 2021, the global drone market was valued at USD 22.5 billion, and it is projected to reach a staggering USD 42.8 billion by 2026 [28; 29]. These numbers reflect not only the market's explosive growth but also the increasing adoption of UAVs across various sectors. From precision agriculture to search and rescue missions, surveillance, and even package delivery services, the ubiquity of UAVs is a testament to their versatile and dynamic role in modern society. UAVs exhibit diverse features and configurations that are tailored to meet specific mission demands. Consequently, the literature provides multiple classifications that emphasize various parameters [30; 31]. In terms of their physical structure, UAVs can be classified into distinct categories such as fixed-wing, rotary-wing, flapping-wing, hybrid, or balloon configurations. When considering their dimensions, UAVs come in different sizes, including mini, micro, and nano variations. The North Atlantic Treaty Organization (NATO) has established a classification system for UAVs, grouping them into three classes based on their maximum take-off weight (MTOW). Each class is further divided into subcategories, which take into account factors such as altitude and mission radius [32]. Electric UAVs are favored for their advantageous characteristics, such as their reliability [33], reduced noise, disturbance rejection [34] and thermal signatures [35; 36], high efficiency [5], absence of pollutant emissions, self-starting capabilities, and advanced control mechanisms that enable exceptional maneuverability. For instance, electric UAVs have excelled in urban environments where noise and emissions regulations are stringent.