# Towards the Unmanned Aerial Vehicles (UAVs): A Comprehensive Review
## 4 UAV Battery Charging
### Wireless Power Tranfer (WPT)
#### 4.1.2 Charging with Laser Beaming  
Laser power transfer (LPT) is another method used to charge UAVs and is mostly considered in space and military applications [89]. In this charging technique, a laser beam of specific wavelength and frequency feeds PV cells mounted on UAV. These PV cells are used to harvest energy from laser transmitter to power UAV and charge its batteries. Laser beaming technique is used for rotary wing and fixed-wing UAVs. It has become a viable solution to offer unlimited endurance. It has the capability to deliver high energy to the receiver using narrow beam divergence [90]. Laser power transfer is envisaged to empower several energy-hungry missions of UAVs over a long distance. Some studies [91; 92] have reported the feasibility of laser power transfer for UAVs. In ref. [93], authors presented results pertaining to current, voltage and efficiency based on laser wavelength in a laser PV cell and material type. In ref. [94], authors discussed results pertaining to wavelength and temperature output of PV cells used for charging system through laser beams. In ref. [92], authors proposed a controller design strategy based on laser PV module output characteristics to control the power converter of a laser power transfer system. In LPT, some critical concerns are blockage, mobility and performance in long range flights. This method is restricted in certain areas such as military areas, airports and scenarios where laser beam is hazardous to human health and living environments. Figure 9 illustrates an overview of multi-UAV WPT system using laser beams. Table 9 summarizes efficiencies of different PV materials when illuminated with laser light of different wavelengths. As given in Table 9, GaAs PV cell gives highest efficiencies around 850 nm wavelengths.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Company** & **PV Cell** & **Dimension (mm)** & **Weight (g/m3)** & **Flexibility** & **Efficiency (\%)** \\ \hline Gochermann Solar technology & SunPower C60 & 125 \(\times\) 125 & 950–1000 & Semi-flex & 22.6 \\ Gochermann Solar technology & SunPower E60 & 125 \(\times\) 125 & - & Semi-flex & 23.8 \\ Bsolar & TG18.5 BR & 156 \(\times\) 156 & - & - & 17.5–18.39 \\ Delsolar & D6F & 156 \(\times\) 156 & - & - & 18–20 \\ Gochermann Solar technology & SunPower A300 & 125 \(\times\) 125 & - & - & 20 min \\ IXYS (IXOLAR) & KXOB22-12X1 & 27 \(\times\) 7 & 2645 & Semi-flex & 22 \\ Bosch Solar Energy & M3 BB & 156 \(\times\) 156 & 1027 & No-flex & 18.43 \\ SunOWE & 156 MM & 156 \(\times\) 156 & 1027 & No-flex & 18.2 \\ \hline \hline \end{tabular}
\end{table}
Table 8: PV cells comparison [86].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**PV Material** & **GaAs** & **Si** & **InGaAs** & **InGaP** & **CIS** \\ \hline Laser Wavelength & 810 nm & 950 nm & \textgreater{}1000 nm & \textgreater{}1000 nm & \textgreater{}1000 nm & \textgreater{}1000 nm \\ PV Cell Efficiency (\%) & 60 & 53.4 & 28 & 27.7 & 40.6 & 40 & 19.7 \\ Laser Intensity (\(\text{kW}/\text{m}^{2}\)) & 110 & 430 & 110 & 10 & 2.37 & 2.6 & 10 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Efficiencies of various PV materials.