# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 2 Unmanned Aerial Vehicle (UAV)
### Hybrid Power Sources (Fuel Cell and Battery)
The storage mass penalty, or the additional weight of the storage system, is relatively low compared to other storage methods. Additionally, high-pressure storage allows for rapid refueling capability, enabling quick turnaround times for UAV operations.} \\ \multicolumn{3}{c}{In summary, high-pressure storage offers several advantages. It is a relatively simple method, requiring a pressure vessel to contain the hydrogen gas. The storage mass penalty, or the additional weight of the storage system, is relatively low compared to other storage methods. Additionally, high-pressure storage offers several advantages. It is a relatively simple method, requiring a pressure vessel to contain the hydrogen gas. The storage mass penalty, or the additional weight of the storage system, is relatively low compared to other storage methods. Additionally, high-pressure storage storage methods, high-which can negatively impact the fuel cell system's lifetime, reliability, and efficiency [94]. To tackle these challenges, researchers have explored the integration of fuel cells with batteries to form hybrid power supply systems, which have emerged as a promising solution. By harnessing the strengths of both power sources and mitigating their weaknesses, hybrid systems can deliver enhanced performance and efficiency for UAV propulsion [48,95,96]. The battery can swiftly provide power for sudden changes in demand, while the fuel cell can supply sustained power for extended durations. This approach enables superior power management, heightened system dependability, and overall improved UAV performance. In a hybrid UAV propulsion system, the battery plays a crucial role in supplying power during high-demand maneuvers, like take-off and climbing. Its higher power density, quicker response time, and greater efficiency make it well-suited for such tasks compared to a fuel cell. On the other hand, the fuel cell takes over as the primary power source during cruise or descent phases, providing sustained power and also recharging the battery to maintain its state of charge (SOC) above a certain threshold. To evaluate the performance of hybrid UAV propulsion systems, researchers have employed hardware-in-the-loop (HIL) simulations. These simulations analyze the behavior of each power source in various test scenarios, considering factors such as endurance and hydrogen consumption. The role of the battery in the hybrid system has been extensively considered in studies, such as [63]. These experiments have specifically focused on analyzing the battery's performance during different flight mission phases and under varying demands. Additionally, ref. [75] provides a comprehensive characterization of the hybrid propulsion system using diverse mission profiles and speeds. However, it should be noted that these studies mainly concentrated on passive power-splitting methods and did not delve into the development of an active energy management strategy. The implementation of an energy management strategy is a crucial aspect of hybrid UAV propulsion systems. It involves actively regulating the power flow between the fuel cell and battery to optimize the overall performance, efficiency, and longevity of the system. This strategy determines the appropriate utilization of each power source based on the prevailing flight conditions, load requirements, and the battery's state of charge. By employing intelligent power management algorithms, the hybrid system can achieve enhanced efficiency and prolong the UAV's mission endurance. Table 5 in this review paper offers a comprehensive comparison between batteries and supercapacitors, providing a detailed analysis of their characteristics and performance. Moving forward, Table 6 provides a comprehensive overview of the advantages and disadvantages associated with batteries and supercapacitors, aiding in the assessment of their suitability for various applications.