Modeling and comparison of bimorph power harvesters with piezoelectric elements connected in parallel and series

Power harvesting devices are designed to convert the ambient energy surrounding a system to usable electric energy. The strong desire to create self-powered systems, which do not rely on traditional energy sources such as electrochemical batteries has led to the rapid growth of this field. One type of energy harvesting is the use of piezoelectric materials to directly transform ambient vibration to electrical energy. In the majority of applications the piezoelectric is configured as a bimorph bender, where the two patches of piezoceramic material can be electrically connected in either series or parallel. A reduced model will be used here to develop a set of closed form solutions to the power harvesting performance of the system based on the electrical connection of the piezoelectrics. It will be theoretically and experimentally shown that the maximum power output and efficiency is independent of the electrical connection. However, the voltage (and current) outputs between a series and parallel conditions are related by a factor of two with a symmetric system. Additionally, a critical impedance will be derived to serve as a criterion on selecting the appropriate electrical connection between the piezoelectrics to tune the systems performance based on the impedance of external circuitry.