Energy management optimization of a gravitational energy harvester powering wireless sensor nodes for freight trains monitoring

Energy harvesting is a promising solution for the realization of self-powered wireless sensor nodes (WSNs), minimizing battery waste and environmental impact. The harvesting devices studied in this paper are gravitational vibration-based energy harvesters (GVEHs), converting the ultra-low-frequency ambient vibrations of structures or vehicles into electric power. The main efficiency losses are related to the AC/DC rectification and battery storage processes. Experimental tests confirm the optimized layout of negative voltage converter (NVC) using MOSFETs and a schottky diode with a 1000 $\mu$F smoothing capacitor, achieving power rectification efficiency of 65%. The rectified and smoothed power is stored in a Li-Ion 3.6 V 40 mAh coin cell and supplied to the load, consisting of a 3.3 V micro-controller unit, temperature sensor and sub-GHz wireless communication module. A nano power boost charger with buck converter manages power between the battery and the load. Experimental battery charge tests are performed for charging power evaluation at different external excitation amplitudes and frequencies. WSN average power consumption is analyzed with master–slave communication tests at different signal strengths and relative distances between the nodes. Finally, a duty cycle between active and sleep phase is defined to guarantee continuous activity of the WSN and net positive charge to the battery.