Piezoelectric energy harvesting from low frequency and random excitation using frequency up-conversion

Abstract

The field of energy harvesting comprises all methods to produce energy locally and from surrounding sources, e.g. solar illumination, thermal gradients, vibration, radio frequency, etc. The focus of this thesis is on inertial power generation from host motion, in particular for low frequency and random excitation sources such as the human body. Under such excitation, the kinetic energy available to be converted into electrical energy is small and conversion efficiency is of utmost importance. Broadband harvesting based on frequency tuning or on non-linear vibrations is a possible strategy to overcome this challenge. The technique of frequency up-conversion, where the low frequency excitation is converted to a higher frequency that is optimal for the operation of the transducer is especially promising. Regardless of the source excitation, energy is converted more efficiently. After a general introduction to the research area, two different prototypes based on this latter principle and using piezoelectric bending beams as transducers are presented, one linear design and one rotational. Especially for human motion, the advantages of rotational designs are discussed. Furthermore, magnetic coupling is used to prevent impact on the brittle piezoceramic material when actuating. A mathematical model, combining the magnetic interaction forces and the constitutive mechanical and electrical equations for the piezoelectric bending beam is introduced and the results are provided. Theoretical findings are supported by experimental measurements and the calculation model is validated. The outcome is the successful demonstration of a rotational energy harvester, tested on a custom made shaking set-up and in the real world when worn on the upper arm during running.