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Energy harvesting at low frequency is a challenge for microelectromechanical systems. In this work we present a piezoelectric vibration energy harvester based on freestanding molybdenum (Mo) and aluminum nitride (AlN) ring-microelectromechanical-system (RMEMS) resonators. The freestanding ring layout has high energy efficiency due to the additional torsional modes which are absent in planar cantilevers systems. The realized RMEMS prototypes show very low resonance frequencies without adding proof masses, providing the record high power density of 30.20 μW mm-3 at 64 Hz with an acceleration of 2g. The power density refers to the volume of the vibrating RMEMS layout

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Photovoltaic scavenging circuits have been presented to reduce installation and maintenance costs of wireless sensor networks. When small-size photovoltaic modules are adopted, optimizing the efficiency of the harvesting process and tracking the Maximum Power Point (MPP) becomes very difficult, and the development of a photovoltaic harvester has to be preceded by extensive simulations. The paper focuses on the definition of the model for a small PV cell allowing the simulation of harvester systems. The model is validated on a case study of MPPT circuit for sensor networks.

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