Experimental validation and empirical AC loss modeling of underwater wireless power transfer using Litz wire coils

Autonomous Underwater Vehicles (AUVs) are increasingly utilized for oceanographic observation and resource exploration. However, their operational range is restricted by limited battery capacity and the need for periodic surfacing to recharge. To address this limitation, Wireless Power Transfer (WPT) technology in conductive media such as seawater has attracted significant attention as a means of enabling contactless underwater recharging. This study investigates the electromagnetic behavior and loss characteristics of a Litz wire solenoid coil designed for underwater WPT. The DC and equivalent AC resistance of the coil are experimentally evaluated in saline water across a range of operating frequencies. Magnetic flux density distributions are then analyzed using finite element simulations (JMAG) to visualize field distortion and identify loss regions. Finally, the power transmission efficiency is assessed under realistic operating conditions. The results demonstrate that Litz wire significantly reduces AC resistance and mitigates eddy current losses compared to conventional conductors, thereby improving transmission efficiency in conductive environments. Moreover, rather than treating Dowell’s equation as directly predictive for air-core WPT coils in conductive media, we use it as a non-conductive baseline and introduce an empirical model tailored to saline-water environments. These findings offer practical insight into how conductive media influence frequency-dependent AC loss and provide guidance for understanding and mitigating medium-induced losses in underwater WPT systems, without implying universality across different coil geometries.