• Energy Research

In this contribution, we discuss the implementation of a novel microelectromechanical-systems (MEMS)-based energy harvester (EH) concept within the technology platform available at the ISAS Institute (TU Vienna, Austria). The device, already presented by the authors, exploits the piezoelectric effect to convert environmental vibrations energy into electricity, and presents multiple resonant modes in the frequency range of interest (i.e. below 10 kHz). The experimental characterisation of a sputter deposited aluminium nitride piezoelectric thin-film layer is reported, leading to the extraction of material properties parameters. Such values are then incorporated in the finite element method model of the EH, implemented in Ansys Workbench (TM), in order to get reasonable estimates of the converted power levels achievable by the proposed device solution. Multiphysics simulations indicate that extracted power values in the range of several mu W can be addressed by the EH-MEMS concept when subjected to mechanical vibrations up to 10 kHz, operating in closed-loop conditions (i.e. piezoelectric generator connected to a 100 k Omega resistive load). This represents an encouraging result, opening up the floor to exploitations of the proposed EH-MEMS device in the field of wireless sensor networks and zero-power sensing nodes.

Silicon resonators are widely used in a large class of applications including sensing and actuation, signal processing and energy harvesting. Very often, their application as sensors requires the deposition of metallic thin films or dielectric coatings, to set the electrical conductivity, the optical coupling, or other physical-chemical properties of the device. Invariably coatings degrade the quality factor (Q) of resonance by increasing the amount of energy dissipated during vibration. In this paper, we show a class of resonators used for the investigation of thermal noise statistical properties in non-thermodynamic equilibrium. Design strategies to preserve the silicon Q-factor are discussed.

In this work we discuss a novel design concept of energy harvester (EH), based on Microsystem (MEMS) technology, meant to convert mechanical energy, available in the form of vibrations scattered in the surrounding environment, into electrical energy by means of the piezoelectric conversion principle. The resonant structure, named four-leaf clover (FLC), is circular and based on four petal-like double mass-spring systems, kept suspended through four straight beams anchored to the surrounding Silicon frame. Differently from standard cantilever-type EHs that typically convert energy uniquely in correspondence with the fundamental vibration frequency, this particular shape is aimed to exploit multiple resonant modes and, thereby, to increase the performance and the operation bandwidth of the MEMS device. A preliminary non-optimized design of the FLC is discussed and physical samples of the sole mechanical resonator, fabricated at the DIMES Technology Center (Delft University of Technology, the Netherlands), are experimentally characterized. Their behaviour is compared against simulations performed in ANSYS Workbench(TM), confirming good accuracy of the predictive method. Furthermore, the electromechanical multiphysical behaviour of the FLC EH is also analysed in Workbench, by adding a layer with piezoelectric conversion properties in the simulation. The measured and simulated data reported in this paper confirm that the MEMS converter exhibits multiple resonant modes in the frequency range below 1 kHz, where most of the environmental vibration energy is scattered, and extracted power levels of 0.2 ?W can be achieved as well, in closed-loop conditions. Further developments of this work are expected to fully prove the high-performance of the FLC concept, and are going to be addressed by the authors of this work in the on-going activities. © 2013 Springer-Verlag Berlin Heidelberg.

The aim of this contribution is to report and discuss a preliminary study and rough optimization of a novel concept of MEMS device for vibration energy harvesting, based on a multi-modal dynamic behavior. The circular-shaped device features Four-Leaf Clover-like (FLC) double spring-mass cascaded systems, kept constrained to the surrounding frame by means of four straight beams. The combination of flexural bending behavior of the slender beams plus deformable parts of the petals enable to populate the desired vibration frequency range with a number of resonant modes, and improve the energy conversion capability of the micro-transducer. The harvester device, conceived for piezoelectric mechanical into electric energy conversion, is intended to sense environmental vibrations and, thereby, its geometry is optimized to have a large concentration of resonant modes in a frequency range below 5-10 kHz. The results of FEM (Finite Element Method) based analysis performed in ANSYS TM Workbench are reported, both concerning modal and harmonic response, providing important indications related to the device geometry optimization. The analysis reported in this work is limited to the sole mechanical modeling of the proposed MEMS harvester device concept. Future developments of the study will encompass the inclusion of piezoelectric conversion in the FEM simulations, in order to have indications of the actual power levels achievable with the proposed harvester concept. Furthermore, the results of the FEM studies here discussed, will be validated against experimental data, as soon as the MEMS resonator specimens, currently under fabrication, are ready for testing.