Double phononic crystal lens-based enhancement of underwater power transfer

Acoustic waves are widely used for underwater communication and also investigated for power transfer owing to their low attenuation compared to electromagnetic waves. We investigate enhancing the performance of an underwater Acoustic Power Transfer (APT) system using Gradient-Index Phononic Crystals (GRIN-PCs). The GRIN-PCs consist of air inclusions in a 3D-printed polymer and are used to construct 3D acoustic lenses capable of collimating and focusing acoustic waves. The lenses are used to improve the directivity of a point source transmitter and focus the acoustic waves at a point receiver to reduce spreading losses in the APT system. The developed lenses allow for using lower frequency transducers to transmit power, hence reducing wave attenuation, and enhancing the efficiency of the system. The enhancement is verified experimentally using hydrophones as point transmitters and receivers in a double-lens system. Additionally, the lenses were used to enhance the performance of 100 kHz piezoelectric transducers with limited directivity, increasing the output power by an order of magnitude. Numerical models based on the finite element method were used to study the effect of varying the separation distance, lens aperture, and frequency on the pressure field. The developed APT concept could find applications for power delivery to wireless devices used in ocean monitoring, underwater navigation, marine life tracking, among others.