# Towards the Unmanned Aerial Vehicles (UAVs): A Comprehensive Review
## 4 UAV Battery Charging
### Wireless Power Tranfer (WPT)  
According to [71], it is envisaged that WPT has generated 12,000 million dollars of revenue in 2020. The statistics of drones and WPT market growth can be seen in Figure 7. It is expected that drone market will hit $43 billion in total sales with a Compound Annual Growth Rate (CARG) of 13.8% by 2025, as shown in Figure 7[10]. It is due to diverse applications in the electronic industry with several key benefits in terms of safety, convenience, reliability and a fully automated charging mechanism. These benefits can be attained through different WPT techniques. Another key feature of WPT is that it is very essential in different environments where wired power transfer technique is dangerous, difficult and impossible, such as harsh underwater environments and high voltage power applications. Research fraternity has been exploring trade-offs and evaluated different WPT techniques. These techniques are categorized as: radiative electromagnetic (EM) and non-EM techniques. Figure 8 shows these WPT techniques which are further divided into nine categories. In non-EM, power is transferred using acoustics or optical sources such as laser. EM is further classified into radiative far-field, non-radiative and mid-filed radiative and non-radiative. RF is used for power transmission in radiative, while capacitive coupling; inductive coupling and magnetic resonance coupling are used in non-radiative. Currently, these WPT techniques are commercially available for different applications such as smart phone charging and charging implantable medical devices etc. and still being developed for different applications. Next, we have highlighted some WPT techniques and some research contributions on WPT.  
The aforementioned WPT techniques ensure efficient and reliable wireless power transmission between UAV and the base station. The application of WPT technology to UAVs should take on some critical issues such as misalignment, interference and payload into consideration. UAV-based WPT system must be lightweight to avoid payload reduction. Furthermore, the WPT techniques must ensure misalignment tolerance between coils, high landing precision and efficient power transfer. Among these challenges, misalignment affects are a dominant issue as landing accuracy is low in the case of UAVs. Consequently, it affects coupling factor, transfer efficient and power transfer. Several re  
Figure 8: Different WPT techniques.  
Figure 7: Statistics of Drones and WPT market growth (Data Source: Drone Industry Insight Report [72]).  
search studies have reported these major issues about recharging UAV batteries through WPT techniques [70; 73; 74]. UAV charging can be achieved through inductive coupling WPT to enhance both range and flight time for inspection, monitoring and surveillance tasks. UAV-empowered inspection technique can overcome several limitations of existing inspection techniques, such as expensive task and hazardous operation through manned helicopters. In ref. [75], the authors demonstrated magnetic resonance coupling (MRC) WPT to recharge drone and performed testing of proposed system for different distances and misalignment topologies. Authors reported 90% transfer efficient at 10 cm distance. [PERSON] et al. [76] presented a vision-based, closed-loop target detection through UAV for outdoor applications. Authors designed a charging station that prolongs the flight time and enhances endurance of UAV. In ref. [77], [PERSON] proposed a novel mid-air inductive charging mechanism to charge multiple drones at the same time without any need of landing using global energy transmission (GET). It is worth mentioning that most studies in the literature focused on UAV charge scheduling while taking into account centralized architecture. Only a few studies looked at the peer-to-peer network of UAVs charging through blockchain technology [78; 79]. Table 7 summarizes some reported studies in the literature on the charge scheduling of UAVs.