# Towards the Unmanned Aerial Vehicles (UAVs): A Comprehensive Review
## 4 UAV Battery Charging
### Wireless Power Tranfer (WPT)
#### 4.1.1 Photovoltaic Cell-Based UAV Charging  
The PV cells are commonly utilized to charge batteries and enhance flight time of UAVs. PV cells make use of sunlight to charge batteries of UAVs. Whenever sunlight is present, the required power is provided by PV cells and in the absence of sunlight batteries are used to deliver required power to UAV. Research works on solar-powered UAV report that several parameters play a key role such as temperature intensity, angle of incidence of sunlight, geometry, orientation and position of PV cells [85]. This technique is not suitable in scenarios with insufficient sunlight. There is a need to adopt alternative strategies whenever sunlight is not present to continue UAV flight. These strategies include additional power supplies, enhanced battery power, PV cells size and automatic position adjustment according to sun position. As PV cells require certain payload capacity and wing length on the UAV, so it is feasible for fixed-wing UAVs. Moreover, environmental conditions such as humidity, temperature, fog, clouds reduce the system efficiency and reliability. Thus, these atmospheric conditions must be kept into consideration.  
Solar energy-based UAVs have gained the attention of several research groups. Solar-powered aircrafts are currently being used for power line inspection, forest fire fighting, border surveillance and high altitude communication. This technology can be used to supersede environmental, scientific and communication satellite to be used for military and civilian applications. Sufficient flight power can be achieved through appropriate selection of PV cells and taking account of efficiency and weight is the key factor to ensure long flight time and high endurance in solar-powered UAVs. To achieve cost-effectiveness and great efficiency, several materials are used in solar cells. Some studies  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Year** & **Research Focus** \\ \hline
[15] & 2017 & Lightweight WPT system for the mid-air charging of drones. \\
[16] & 2018 & WPT techniques for UAVs. A review, extensions and reconceptualization. \\
[18] & 2018 & Designing of drone battery charging system through wireless power transfer (WPT). \\
[70] & 2018 & WPT system to charge autonomous electric UAV using a small secondary coil. \\
[80] & 2018 & WPT technology-based drone charging stations to charge drone over the buildings. \\
[19] & 2019 & Auction-enabled charge scheduling using deep learning framework for a network of multiple drones. \\
[67] & 2019 & WPT based sleep/active method for a drone charging station in smart agriculture. \\
[81] & 2019 & Neural blockchain-empowered assistance for drone swarms. \\
[82] & 2020 & Energy-efficient UAV crowd-sensing through multiple charging stations using deep learning. \\
[83] & 2020 & Novel application of a distributed network of charging stations and UAVs through advanced blockchain. \\
[84] & 2021 & Blockchain-empowered charge scheduling for UAVs in smart cities. \\ \hline \hline \end{tabular}
\end{table}
Table 7: Studies on charge scheduling of UAVs.  
suggest mono-crystalline silicon PV cells due to affordable cost and high efficiency [86]. Furthermore, mono-crystalline silicon PV cells offer high flexibility that supports easy integration in the UAV wing. Table 8 summarizes a comparison of mono-crystalline silicon PV cells from various manufacturing companies. It is still important to investigate PV cells with novel designs, energy management strategies and manufacturing material to ensure high efficiency and high availability [85]. Research fraternity should focus more on material science developments as low photoelectric efficiency is a major limiting factor in this research domain [87]. Some studies propose an effective approach to optimize solar-powered UAV flight trajectories in order to gain more solar radiation along with low consumption of mechanical energy [88].