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Wireless power transfer (WPT) system in seawater exhibits distinct transmission characteristics from those in air due to the unique properties of seawater medium and the complexity of the seawater environment. In this paper, transmission characteristics of a 2-transmitter (2TX) and 1-receiver (1RX) WPT system in the frequency band of 350 ~ 550 kHz in seawater are investigated. When the transfer distance is at midrange, the maximum load voltage point of the system is shifted from 460 to 430 kHz compared to that in air, and the maximum transfer efficiency is 7 times of that in air, which is very different from the traditional view that the transfer efficiency in water is always worse than in air. The system maintains a stable output voltage when the receiving coil is at different offsets. To grasp the influence mechanism of seawater on wireless power transfer, two new parameters, complex self-inductance and complex mutual inductance, are introduced into the circuit model to explain the shift of the optimal frequency and the increase of transfer efficiency, and the model is verified through experiments. [ABSTRACT FROM AUTHOR]

The eddy current loss caused by the conductivity of seawater results in a relatively low transfer efficiency of underwater wireless power transfer (WPT). And the transfer distance of the current WPT system is relatively short. Considering that most of the wireless power transfer devices in practical applications are asymmetric, few studies have explored the transfer characteristics of asymmetric midrange WPT in seawater. In this study, it is experimentally found that the load voltage and transfer efficiency of an asymmetric midrange WPT system with reduced primary balancing resistance in seawater are nearly twice as high as those of a symmetric one at a 50 cm transfer distance and a 410 kHz operation frequency with a 44.4 Ω load resistance. A new circuit model of the underwater WPT system with complex impedance and complex mutual inductance is then presented, and the load voltages predicted by the model are consistent highly with the experimental values; the model is then utilized for the explanation of the experimental observations. Changing the load resistance also improves the transfer efficiency of the system; however, the eddy current loss results in a relatively low transfer efficiency of 30.9% at an optimal load resistance of 90 Ω. The asymmetric midrange underwater WPT system can be applied in scenarios where the transfer distance is prioritized. [ABSTRACT FROM AUTHOR]

Summary: It is reported generally in previous studies that the transmission efficiency of wireless power transfer (WPT) system in seawater is lower than that in air due to eddy current loss. In this study, it is found experimentally that the transfer efficiency in seawater can reach 17 times of that in air on average at midrange for a WPT system with two 22.5‐cm planar coils and a 460‐kHz resonance frequency. These show that the eddy current assists wireless transfer in seawater, and then the eddy current is treated as a relay coil to explain the experimental observations. Compared with the model from electromagnetic field theory, which is complex and difficult to grasp, the circuit model proposed in this study is concise and easy to understand. Though the underwater WPT system has a relatively low average transfer efficiency of 20% at midrange, it can be used in a transfer distance priority scene. [ABSTRACT FROM AUTHOR]