The physical acoustics of energy harvesting

Energy harvesting systems based on the transformation of acoustic vibrations into electrical energy are increasingly being used for niche applications due to the reduction in power consumption of modern day electronic systems. Typically these applications involve extracting energy at remote or isolated locations where local long term power is unavailable or inside sealed or rotating systems where cabling and electrical commutation are problematic. The available acoustic power spectra can be in the form of longitudinal, transverse, bending, hydrostatic or shear waves of frequencies ranging from less than a Hz to 10's of kHz. The input stress/vibration power can be generated by machines, humans or nature. We will present a variety of acoustic energy harvesting modes/devices and look at the commonalities of these devices. The common elements of these systems are: an input mechanical power spectrum, an effective acoustic impedance matching, a conversion of the input mechanical energy into electrical energy using piezoelectric or biased electrostrictive transducers and a matched electrical load. This paper will focus of the physical acoustics of these energy harvesting systems and identify the elements of these devices and look at the current limits of the harvested electrical power from these devices. Recent results on an acoustic electric feed-through device demonstrated acoustic power conversions of the order of 70 W/cm
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and 25 W/cm
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using a pre-stressed stacked PZT ceramics operating at 16 kHz with an efficiency of 84%. These results suggest the piezoelectric is not the limiting element of these devices and we will show that the main impediment to increased power is the vibration source amplitude, frequency, inertia and the size limitations of the energy harvesting systems or in the case of human powered systems the requirement that the device remains unobtrusive. Although the power densities of these devices may be limited there are plenty of applications that are feasible within the available power densities due to the wonders of CMOS.