Low frequency energy harvesting using a force amplified piezoelectric stack

Abstract

This paper presents a focus on harvesting energy from mechanical sources operating at 20Hz or below. For many practical energy harvesting applications, low frequency mechanical sources present an issue for efficient energy conversion. Often energy harvesters become inefficient when driven outside of a narrow band about their natural frequency, which may be orders of magnitude above the frequency of the target source. In this paper, an energy harvester is presented that combines the use of a piezoelectric stack mounted within a flexure frame, used to amplify the force applied to the piezoelectric elements. This force amplification serves to increase the low frequency, off-resonance performance of the device, achieving force amplification levels up to 7.95 times the input. This style of harvester is particularly suited to underfloor energy harvesting, typically targeting the motion of human or vehicle traffic overhead. In such an application, minimising the impact on the traffic is of high importance. Alternate methods of harvesting from such sources often involve mechanically driven systems, which can be disruptive or increase the required energy input. The proposed harvester addresses these known issues by offering a method of converting energy from very low levels of displacement. The harvester was proven to harvest energy from deflections of 112μm or less. In this work, the force amplification factor and the loaded displacement of the device are modelled using finite element analysis and experimentally proven. An iterative approach was taken using FEA software in order to optimise the force amplification of the harvester frame. The work serves to extend the knowledge of these devices to quantify their performance in the 0 – 20Hz range. Furthermore, the force amplification is explicitly simulated and proven experimentally, which has not previously been achieved in practice.