Effect of acoustic radiation from circular plate installed with piezoelectric element on electricity generation performance

This study examined the electricity generation performance of an energy-harvesting system, in which a circular plate with a piezoelectric element was excited by a harmonic point force. The theoretical electricity generation efficiency, which does not involve acoustic radiation from the plate, is greater than the experimental efficiency in such a power generation system, and the discrepancy between the theoretical and experimental efficiencies increases with the plate thickness. A sound field is formed in the atmosphere surrounding the plate, whose vibration compresses the medium on the plate surface and causes a reaction force to the plate. In such a system, it is difficult to completely avoid the effect of acoustic radiation; therefore, the abovementioned effect must be examined to estimate the electricity generation characteristics of the system. The generation characteristics are estimated not only by the radiation resistance that is involved in the acoustic radiation, but also by the reaction force caused by the sound pressure on the plate. Consequently, the power reduced by the reflection of the reaction force and sound pressure decreased because of the increasing flexural rigidity of the plate when the plate thickness increased while the radius remained constant. However, because the radiation resistance increases with the plate thickness, the acoustic radiation energy, except for the work exerted by the sound pressure, is emitted to the atmosphere. To utilise such acoustic radiation energy, a new electricity generation system comprising a steel cylinder with two plates at both ends was used, where a sound field was formed within the cylinder. The system performance was estimated in terms of the electricity generation efficiency by comparing it with that of the abovementioned plate system. As a result, although the thin plate was preferable to increase the efficiency because of the weakened acoustic radiation, the thick plate was useful to harvest vibration energy, which was unused normally, because of enabling to apply mechanical–acoustic coupling between the plate vibrations and the internal sound field.