Broadband Vibrational Energy Harvesting with a Spherical Piezoelectric Transducer Devoted to Underwater Wireless Sensor Networks

A new sensing node container based on a spherical piezoelectric transducer is proposed. This device provides broadband vibrational energy harvesting and sensing facilities intended for underwater wireless sensor networks. The transducer is composed of two acrylic glass (PMMA) half-spherical shells and a Pz26 piezoelectric ring clamped between the two shells. A simulation model of vibrational energy harvesting has been developed with electromechanical circuits for thickness and radial vibrational modes. This approach was validated by a finite element simulation. As a result, optimal power harvesting conditions and estimated harvested voltage were defined. A prototype of 2.2 cm in diameter was realized and characterized. Analysis in air environment reveals several structural resonance modes in the 20–80 kHz frequency range. The directivity patterns corresponding to these modes was obtained using laser Doppler vibrometry. The measurements for the underwater environment show that the structural resonance modes shift down in frequency to the 10–60 kHz range, and exhibiting low directivity dependence. Power harvesting performances was measured and quantified relative to acoustical pres- sure measurements using a hydrophone. The average conversion coefficient value was found to be in the order of 3 V/MPa. In broadband excitation mode, and for an acoustic pressure of 10 kPa, the amount of harvested power out of 5 main resonance modes is 3.3 μW.