In this paper, we propose a methodology for optimizing a solar harvester with maximum power point tracking for self-powered wireless sensor network (WSN) nodes. We focus on maximizing the harvester's efficiency in transferring energy from the solar panel to the energy storing device. A photovoltaic panel analytical model, based on a simplified parameter extraction procedure, is adopted. This model predicts the instantaneous power collected by the panel helping the harvester design and optimization procedure. Moreover, a detailed modeling of the harvester is proposed to understand basic harvester behavior and optimize the circuit. Experimental results based on the presented design guidelines demonstrate the effectiveness of the adopted methodology. This design procedure helps in boosting efficiency, allowing to reach a maximum efficiency of 85% with discrete components. The application field of this circuit is not limited to self-powered WSN nodes; it can easily be extended in embedded portable applications to extend the battery life.

In this paper we present a solar energy harvesting circuit for low-power applications describing circuit architecture and guidelines for an optimal design. We evaluate the performance of two implemented prototypes intended to power a wireless embedded system under different light intensities and different switching frequencies. Measurements show that higher switching frequencies allow reaching the maximum efficiency (~90%) at higher light intensities, whereas lower operating frequencies perform better under lower irradiance. Experimental results show that circuit optimization depends on light conditions and the proposed solar energy harvester can autonomously supply the nodes of a wireless sensor network WSN.

A differential RF-DC CMOS converter for RF energy scavenging based on a reconfigurable voltage rectifier topology is presented. The converter efficiency and sensitivity are optimized thanks to the proposed reconfigurable architecture. Prototypes, realized in 130 nm, provide a regulated output voltage of ~2 V when working at 868 MHz, with a -21 dBm sensitivity. The circuit efficiency peaks at 60%, remaining above the 40% for a 18 dB input power range.

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.

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.