This paper reports on a method to optimize an electromagnetic energy harvester converting the low-frequency body motion and aimed at powering wireless body area sensors. This method is based on recorded accelerations, and mechanical and transduction models that enable an efficient joint optimization of the structural parameters. An optimized prototype of 14.8 mmØ×52 mm, weighting 20 g, has generated up to 4.95 mW in a resistive load when worn at the arm during a run, and 6.57 mW when hand-shaken. Among the inertial electromagnetic energy harvesters reported so far, this one exhibits one of the highest power densities (up to 730 μW cm −3 ). The energy harvester was finally used to power a bluetooth low energy wireless sensor node with accelerations measurements at 25 Hz.

This paper presents a small-scale airflow energy harvester built on an axial turbine architecture and exploiting an electret-based electrostatic converter. When the airflow velocity is high enough, the windmill starts rotating and creates a periodic relative motion between a stator and a rotor which induces variations of capacitance. These ones are directly converted into electricity thanks to the use of Teflon electrets charged at −1400 V which polarize the variable capacitors. We focus our study on a 4-blade axial turbine with a diameter of D=40 mm, a depth of W=10 mm, for a total volume of 12.6 cm 3 . This windmill has been tested with various blade angles and different types of electrostatic converters and output powers up to 90 μW at 1.5 m s −1 (7.5 μW cm −3 ) and 1.8 mW at 10 m s −1 (111 μW cm −3 ) have been obtained so far. The coefficient of power reaches C p =5.8% and among the small-scale airflow energy harvesters previously reported, this one has the lowest cut-in speed (1.5 m s −1 ).