Study on Simultaneous Wireless Power Transfer in Air and Underwater Using Magnetic Field Coupling

In recent years, there has been a growing interest in smart aquaculture, which involves activities such as observing tidal currents and fish schools using IoT sensors and drones. This study investigates a method for powering these IoT sensors and drones, both underwater and airborne, using magnetic field-coupled wireless power transmission. Since magnetic fields are generated on both sides of the coil, it is possible to power two terminals simultaneously by placing receiving coils on both sides of a single power supply coil. However, there has been little consideration given to cases where the dielectric constants of the media, such as underwater and airborne environments, differ. In fact, the relative dielectric constant $\varepsilon_{r}$ of water is very high, around 81, compared to the typical dielectric constant $\varepsilon_{r}$ of 2 to 4 for conventional dielectric substrates. As a result, the wavelength becomes much smaller, about 1/10 th of that in air. Therefore, this study conducted a fundamental investigation into a method for simultaneously transmitting wireless power to underwater and airborne environments using a single transmitting coil. First, we examined changes in the magnetic field in the media when the transmitting coil was placed in water, air, and at the boundary surface between water and air. The results revealed that placing the coil on the water surface, i.e., the boundary surface of the media, maximizes the magnetic fields of each medium. Next, we calculated the wireless power transmission efficiency and maximum wireless power transmission efficiency by placing the transmitting coil on the water surface and the receiving coil underwater. The results showed that the maximum wireless power transmission efficiency reaches 80% around the frequency of 2 MHz, compared to the coil resonating around 20 MHz.