# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 2 Unmanned Aerial Vehicle (UAV)
### Fuel Cell Powered UAVs  
Hydrogen-powered UAVs offer significant benefits compared to traditional battery-powered UAVs, particularly in terms of flight duration and refueling time [86; 87]. LiPo batteries, commonly used in UAVs, have a specific energy of up to 250 Wh/kg. In contrast, a fuel cell system with a compressed hydrogen tank can provide a specific energy of up to 1000 Wh/kg, allowing for much longer flight times [46; 48]. This substantial increase in specific energy enables hydrogen-powered UAVs to fly for hours instead of just a few minutes. Furthermore, refueling hydrogen-powered UAVs is nearly instantaneous compared to the time-consuming recharging process required for batteries. While batteries need a significant amount of time to recharge and replenish their energy, refueling a hydrogen-powered UAV involves simply replacing the depleted hydrogen tank with a fully charged one. This enables quick turnaround times and continuous operation. These advantages make hydrogen-powered UAVs highly desirable for applications that require extended flight durations and rapid refueling, such as long-range surveillance, mapping, and monitoring missions. However, it is essential to consider the infrastructure needed for hydrogen storage, transportation, and refueling, as well as the safety considerations associated with handling hydrogen gas [88]. Proper safety protocols and infrastructure development are necessary to ensure the efficient and secure use of hydrogen-powered UAVs. Thus, a comprehensive comparison of battery-based supplying techniques is presented in Table 3 for a better understanding on their advantages and limitations. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Power Supplying Technique** & **Advantages** & **Limitations and Drawbacks** \\ \hline Hydrogen-powered UAVs offer the advantage of unlimited operating time, making them a viable option for long-range missions. Unlike battery-powered UAVs that require multiple battery packs or frequent recharging, hydrogen-powered UAVs rely on a single energy source, namely hydrogen fuel. This significantly reduces the weight and complexity of power management systems. & The use of a ground station (GS) in the context of battery-powered UAVs can introduce certain challenges and considerations. While a GS can provide benefits, such as battery charging or swapping capabilities, it also brings along some limitations that impact operational efficiency. & One of the major concerns is the increased cost associated with deploying and maintaining a GS. As the number of batteries and UAVs in operation increases, the infrastructure and resources required for the GS also need to scale accordingly. This can result in higher costs for equipment, maintenance, and operational management. & Another challenge is the coordination and cooperation between UAVs and the GS. Efficient communication and synchronization are essential for smooth operations. UAVs need to interact with the GS to initiate battery charging or swapping processes, which requires effective communication protocols and coordination algorithms. Failure in communication or synchronization can disrupt operations and lead to inefficiencies. \\ Swapping & Autonomous swapping, which involves landing and battery-changing operations, can introduce additional issues. Precise UAV positioning during landing is crucial to ensure safe and accurate battery swapping. Any errors or discrepancies in landing can affect the efficiency and reliability of the swapping process. Designing and implementing robust algorithms and mechanisms to achieve precise positioning and seamless battery-changing operations is a complex task. \\ \hline \end{tabular}
\end{table} 
Table 3: A comparison between battery-based supplying techniques [54; 58; 78; 79; 80].