Design of Capacitive Coupler for Wireless Power Transfer Under Fresh Water Focusing on kQ Product

This paper presents the design of a capacitive coupler for underwater wireless power transfer (U-WPT) focusing on kQ product. Power transfer efficiency hinges on the coupling coefficient $k$ of the coupler and unloaded Q-factor of water. First, $k$ is derived from the equivalent circuit of the coupler and it is elucidated that $k$ is dependent on the frequency. The pivotal elements that improve $k$ are also found. Next, the frequency characteristic of the Q-factor in tap water is calculated from the measured results. Then, the design parameters at maximum kQ product are determined. Finally, it is demonstrated that the efficiency of U-WPT with the designed coupler achieves 80.9% which is as high as a conventional magnetic coupling.