# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 2 Unmanned Aerial Vehicle (UAV)
### Hybrid Power Sources (Fuel Cell and Battery)  
Fuel cells face limitations when used as the sole power source for UAVs. Their time constant is relatively long, typically in the range of seconds, due to the requirements of fuel and air supply facilitated by pumps, valves, and compressors. This sluggish response is primarily attributed to the mechanical characteristics of the pumps, flow delay, thermodynamic properties, and the effect of capacitance [93]. Consequently, when there are significant fluctuations in current demand, there is a potential risk of fuel shortage,  
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\begin{tabular}{c c c} \hline \hline  & **Compressed H2** & **Liquid H2** \\ \hline \multicolumn{3}{c}{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 allows for rapid refueling capability, enabling quick turnaround times for UAV operations. & There are several advantages to using hydrogen as a fuel despite its low density. One advantage is its high energy density, which means that a relatively small volume of hydrogen can store a large amount of energy. This high energy density enables longer flight durations and increased operational efficiency for UAVs. & Hydrogen storage also offers improved safety compared to other fuel options. \\ \multicolumn{3}{c}{However, there are certain drawbacks to high-pressure storage. One significant disadvantage is the low storage efficiency. The energy required to compress hydrogen to high pressures results in energy losses, reducing the overall storage efficiency. This can impact the range and endurance of the UAV.} & Hydrogen storage also offers improved safety compared to other fuel options. \\ \multicolumn{3}{c}{Safety risks are also a concern with high-pressure storage. The high pressures involved in the storage process increase the potential for leaks or ruptures, which can be hazardous. Proper safety measures and precautions must be in place to mitigate these risks.} & Hydrogen storage also offers improved safety compared to other fuel options. \\ \multicolumn{3}{c}{Another consideration is the larger volume required for high-pressure storage systems. The pressure vessels need to be sized to accommodate the required amount of hydrogen at the specified pressure. This can increase the overall size and volume of the UAV, affecting its aerodynamics and payload capacity.} & Handling and transporting hydrogen in its liquid form can be difficult due to the low temperatures involved and the need for specialized storage and transportation infrastructure. Additionally, the weight of the storage tanks can be significant, impacting the overall weight of the UAV. \\ \multicolumn{3}{c}{In summary, while hydrogen offers advantages, such as high energy density, improved safety, and the potential for long-duration flights, there are challenges associated with its low density, high liquefaction energy requirements, impractical small-scale production, and the need for careful handling and storage. These factors must be carefully considered in the design and implementation of hydrogen storage systems for UAV applications.} \\ \multicolumn{3}{c}{In summary, high-pressure storage is a commonly used method for hydrogen storage in UAVs. It offers simplicity, low storage mass penalty, and rapid refueling capabilities. However, it has drawbacks such as low storage efficiency, safety risks, and the need for larger storage volumes. These factors should be carefully considered when selecting the appropriate storage method for UAV applications.} \\ \multicolumn{3}{c}{In summary, high-pressure storage is a commonly used method for hydrogen storage in UAVs. It offers simplicity, low storage mass penalty, and rapid refueling capabilities. However, it has drawbacks such as low storage efficiency, safety risks, and the need for larger storage volumes.