The Internet of Underwater Things (IoUT) is a promising approach to future military, scientific, and commercial applications at sea. Vital components of the IoUT are underwater wireless sensor networks (UWSNs) and autonomous underwater vehicles (AUVs). Powering of these systems in deep water still remains one of the main challenges, since UWSN nodes and AUVs are typically powered by batteries that need to be replaced or recharged through expensive and difficult operations. This article presents the design of the first batteryless underwater sensor node that can be wirelessly recharged through ultrasonic waves from longer distances than allowed by current inductive and magnetic technologies. First, the architecture of an underwater platform capable of extracting electrical energy from ultrasonic waves is introduced. We then illustrate how to interface this system with an underwater digital communication unit. We discuss the design of a prototype of the proposed architecture where the storage unit is realized with a batch of supercapacitors. We show through experiments that the harvested energy stored in the supercapacitors is sufficient to provide an underwater sensor node with the power necessary to perform a sensing operation and power an acoustic modem for ultrasonic communications. We evaluate the system performance in terms of wireless power transfer efficiency (PTE). Our system is characterized by a lower electrical-to-radiated power conversion efficiency when compared to other technologies. However, given the reduced attenuation of ultrasonic waves in water, we were able to show that our approach can cover longer distances with less transmission power. Last, we evaluate the operating efficiency that we define as the maximum achievable digital data rate relative to the charging and transmission times.
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