Vibration and flow energy harvesting using piezoelectric cantilevers

Networks of low power wireless devices are increasingly used in applications ranging from environmental to factory automation monitoring. Most of these devices must be operative 24hrs a day and may be in locations where manual battery replacement is difficult or costly. It would be desirable if there exists a miniaturized device that can convert ambient mechanical energies such as vibrations or flows, which are readily available 24hrs a day, to power wireless devices. Over the past decade, piezoelectric cantilever(PC) energy harvesters have been increasingly investigated for this application. The challenges are two folds: improving the voltage and power output within the constraints of size and weight and effectively converting air flow especially low-speed air flow (<5m/s) into electricity. Traditional PCs use piezoelectric and nonpiezoelectric layers of the same length. We investigated PCs with unequal piezoelectric and nonpiezoelectric lengths namely two-section PCs. For step-wise tip forces the results showed that a longer nonpiezoelectric layer is preferred for generating a higher induced voltage while a longer piezoelectric layer reduces the induced voltage due to charge cancellation. With harmonic base vibrations, the results showed that there exists an optimal nonpiezoelectric-to-piezoelectric length ratio at which output voltage, current, and power can be maximized. Theoretical analysis of two-section PCs was performed within the framework of beam theory. The results were in good agreement with experiments. We attach a hollow bluff extension at the tip of a PC as a piezoelectric flow energy harvester(PFEH). Because the bluff extension is light-weight, it vibrates when the air flows past it, which in turn drives the PC into vibration and generating electricity. Turbulence and vortex shedding forces were identified as the driving mechanisms of the PFEH. The voltage and power output of the PFEH increased with the increasing size of the bluff extension. Also, a rectangular bluff extension helped generate higher voltage and power than a circular one of same dimensions. The PFEH was used to power a LED intermittently with an instant power output more than 12mW by using a charge storage capacitor and a low-power temperature sensor continually with an average power output of 18[mu]W at 4.7m/s wind velocity.