## 3 State-of-the-Art Review on Energy Optimization Techniques in UIoT
### Auto-Redarge/Battery-Free System for UIoT  
In the past two decades, underwater piezoelectric particles and devices have played an essential role in energy generation and harvesting in UIoT applications, as they provide sophisticated energy density and are flexible for integration when compared to electromagnetic devices [195]. Numerous researchers have used piezoelectric components to harvest electrical energy from humans and animals [196], and this methodology can be utilized for UIoT networks. [PERSON] et al. [197] proposed a new charging mechanism for battery-less UIoT sensor nodes, for ocean fish farm monitoring applications. [PERSON] et al. [24] provide an acoustically powered battery-less platform for UIoT; this is still considered the first battery-less design for UIoT. [PERSON] et al. [198] estimate the battery life of UIoT devices to provide efficient energy optimization for UWASN. Figure 6 shows the battery-free sensor node developed by the Massachusetts Institute of Technology for ocean exploration; this navigation system was powered by an acoustic signal [25]. They have built a battery-less indicative system named underwater backscatter localization (UBL). Rather than producing its sound signals, UBL replicates modulated signals from the UIoT environment. In this case, the developers can find the location information with zero energy. This technology is still under development, in the future, UBL technology will be considered the key technology for environmental monitoring applications in UIoT networks and US Navy.  
[PERSON] et al. [174] proposed magnetic interface energy reconverting mechanism for underwater electric vehicles. In [199], the recharging model for AUVs was proposed based on ambient wave-induced motion. In this approach, a control moment gyroscope (CMG) ideology is considered, it utilizes the gyroscopic response of a flywheel that is equipped within AUVs to generate electric energy based on the rotation speed of AUVs. In [200], a  
Figure 6: (a) Model of battery-free sensor nodes. (b) Design of piezoelectric sensor nodes invented by Massachusetts Institute of Technology (MIT), navigation system powered by sound [25].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Year**} & \multicolumn{1}{c}{**Reference**} & \multicolumn{1}{c}{**Type**} & \multicolumn{1}{c}{**Application**} & \multicolumn{1}{c}{**Frequency [kHz]**} & \multicolumn{1}{c}{**Analysis and Performance Level**} \\ \hline \multirow{3}{*}{2019} & [PERSON] et al. [192] & Docking system for U-WPT & Omnidirectional charging system for AUVs & \(\approx\)90 kHz & Reducing 95\% of eddy’s current loss \\ \hline \multirow{3}{*}{2020} & [PERSON] et al. [193] & Dipole-Coil magnetic coupler & Wireless charging system for AUVs & 50 kHz & Achieved efficiency of 89.7\% \\ \hline \multirow{3}{*}{2020} & [PERSON] et al. [194] & Power efficiency tracking & U-WPT system for the marine vehicle & 85 kHz & Achieved a system efficiency of 88\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._novel method to harvest electrical energy from aquatic species was proposed. It can be denoted as a self-powered acoustic transmitter. In [158; 195], a piezoelectric beam-based energy source was generated from sea species activities such as swimming, noise generation, etc., and stored as the battery backup for rechargeable and self-powered sensor nodes.