Design of a Capacitive Wireless Power Transfer System for Operation in Fresh Water

This paper presents the design of a capacitive wireless power transfer system for operation in fresh water. This paper clarifies two particular points. One is the behavior of $k$ and kQ product for the frequency; $k$ means coupling coefficient in the coupler and kQ product means the product of $k$ and $Q$ -factor in the coupler. The other is the design method of the general capacitive coupler under fresh water. First, $k$ is derived from the equivalent circuit of the coupler, and it is elucidated that $k$ and kQ product are dependent on the frequency. The pivotal elements that improve kQ product are also found. Next, it is then clarified that the design parameters at maximum kQ product have the maximum available efficiency and we clarify the applicable condition for our design method. We achieved an efficiency of 91.3% at a transfer distance of 20 mm via experimentation. Finally, it is demonstrated that power is wirelessly transferred to the Lux sensor module using our coupler implemented on the charging station and that measured data are also wirelessly transferred to it.