# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 1 Introduction
They are being utilized for tasks like monitoring traffic, delivering medical supplies, or capturing aerial footage with minimal disruption. On the other hand, internal combustion engine (ICE)-based UAVs offer longer endurance due to the high power and energy densities of ICE. Thus, ICE-based UAVs, with their extended endurance, still have their place in missions requiring long flight durations, such as search and rescue operations in remote areas or the surveillance of large, sparsely populated regions. The choice between electric and ICE-based UAVs often depends on the specific demands of the mission and the environmental considerations [35]. However, they necessitate an auxiliary starting motor, involve more complex control demands [37], and generate high acoustic and thermal signatures. Some researchers [38; 39] have explored hybrid architectures that combine electric motors with ICE to harness the advantages of both engine types. Nevertheless, the depletion of fossil fuels and the growing concerns regarding greenhouse gas (GHG) emissions have diminished the appeal of thermal engines, prompting the adoption of electric propulsion as a sustainable technology across various sectors, including transportation [40]. International policies and market trends that aim to phase out ICE vehicles [41] are expected to extend to UAVs in the foreseeable future, fostering the development of environmentally friendly devices. The shift away from ICE in UAVs, particularly for tasks that do not require extensive endurance, can be attributed to these factors. Batteries serve as the primary source of electric power, but their flight durations in UAVs are limited by their low energy density and long-lasting charging time [41] similar to all unmanned types of unmanned vehicles [42]. These challenges are common to various types of unmanned vehicles. In addition to energy density and charging time concerns, the safety of battery energy storage systems has become a paramount consideration, especially in light of the increasing use of electric vehicles [43]. Monitoring the state of charge (SOC) and battery temperature has proven to be instrumental in enabling comprehensive decision-making within the system, ultimately enhancing safety and performance. Furthermore, batteries may not offer the required power response for rapid maneuvers due to their slow power dynamics. To address these limitations, supercapacitors present a suitable option for balancing the constraints imposed by batteries [44]. It should be noted that flight endurance can be improved through aerodynamic optimization during the UAV design phase [45]. Recent advancements in battery technology have slightly extended endurance to approximately 90 min using lithium polymer (LiPo) batteries [46]. However, increasing the number of batteries becomes impractical due to weight and space limitations. One of the key challenges is the limited energy density of batteries, which constrains the amount of energy that can be stored within a given weight and volume [47]. This limitation impacts the flight duration and payload capacity of electric UAVs, making it a critical consideration in mission planning. Additionally, batteries exhibit slow power dynamics, meaning they may not provide the rapid power response required for sudden maneuvers or emergencies. These challenges are central to the quest for improving the endurance and overall capabilities of electric UAVs [22]. To enhance UAV endurance, additional power sources must be employed to supplement batteries while adhering to weight and space constraints. In this regard, fuel cells offer a promising solution due to their high specific energy and near instantaneous refueling capability. They can typically achieve up to five times higher energy density than LiPo batteries, significantly enhancing the endurance of hybrid UAVs [48]. It is worth mentioning that many existing electric UAVs utilize fuel cells as their primary power source.