Propeller-based Underwater Piezoelectric Energy Harvesting System for an Autonomous IoT Sensor System

While there is a demand for energy harvesting in environments with water, such as rivers, lakes seas, it has been difficult to use energy harvesting, especially piezoelectricity, because of the waterproof problem and the low output scale. Here, we report a propeller-based underwater piezoelectric energy harvester consisting of a propeller, hitting sticks and a piezoelectric module. The hitting sticks spin with a rotating axis connected to a propeller rotated by water flow and hit a piezoelectric cantilever beam. Unlike previous tip mass method, a newly applied technique makes it possible to match a frequency by easily controlling the bending length of a piezoelectric module using an acrylic plate to find the maximum output power. For reliability, we performed an experiment at 1.2 m/s, which is the actual water flow rate of the Han River in South Korea, and a frequency of 24.5 Hz occurred with four hitting sticks. The frequency-matched bending length was 80 mm (acrylic plate length: 10 mm). Then, impedance matching was carried out: At a resistance of 10.8 kΩ, an output power of 17 mWrms and a power density of 57.4 mW/cm3 were obtained. This result is one of the highest results in a field of similar piezoelectric energy harvesting devices considering challenging conditions that occur in real environments. Finally, the system was applied to an LED light system, one of applications under water, and succeeded in operating a total of 972 LEDs with only one piezoelectric device. This research proves the potential and the feasibility of applying a piezoelectric energy harvesting system to a real underwater environment.