Efficiency Enhancement in Mid-Range RWPT Systems by GRIN Metasurface Lenses

The Magnetic Resonant Wireless Power Transfer (RWPT) is one of the best suitable techniques for mid-range applications. Therefore, there are different approaches to enhance RWPT efficiency, many of them based on metamaterial devices. In this work, the GRadient-INdex (GRIN) technique is applied to increase the coupling between transmitter and receiver coils and collimate the magnetic field, reducing its dispersion. This proposed lens topology operates at 28 MHz and is formed by a 5x5 planar and periodic arrangement of Split Ring Resonators (SRR) unit cells. In each one, a lumped capacitor is added to reduce the lens's size and radially create a refractive index gradient. Then, a four-coil RWPT system is designed at the same operating frequency. Finally, the coils and lenses are prototyped, and the RWPT efficiency is measured in three cases: without any lenses, with a conventional (uniform) metasurface, and with the proposed GRIN lens. The experimental results demonstrate that the RWPT efficiency, which is about 8% without any lenses, becomes almost four times higher (33%) after the inclusion of metasurface GRIN lenses. When the same measurement is carried out with a uniform lens, typical topology in literature, the obtained efficiency is 17.59%, nearly half of the obtained with the proposed metasurface.