A high-power, robust piezoelectric energy harvester for wireless sensor networks in railway applications

Piezoelectric energy harvesting techniques are increasingly seen as promising power sources for wireless sensor networks that monitor railway infrastructure. However, the piezoelectric generators currently available for railway applications suffer from low power output, as well as inadequate durability and robustness. To tackle these issues, this study introduces a novel, high-power, sturdy piezo stack energy harvester's design, optimization, and testing for powering wireless sensor networks in rail systems. The aim is to improve both the power output and the durability and robustness of the device. The proposed harvester's high-power generation is facilitated by a frequency up-conversion mechanism, mechanical transformer design and optimization, and the application of the piezo stack's compression mode (d₃₃ mode). The frequency up-conversion mechanism allows the harvester to function at low-frequency track vibrations with high power. The mechanical transformer significantly magnifies the force exerted on the piezo stack. The compression mode boots the energy conversion efficiency due to its higher coupling factor. To enhance durability and robustness, innovative approaches are employed. The mechanical transformer is optimized for maximum energy transmission efficiency without exceeding the material's fatigue limit. Moreover, the piezo stack is designed to operate under pre-compression, preventing tensile stress and taking advantage of the piezoelectric ceramics' remarkable compressive strength. Plate springs are also integrated into the mechanical transformer to maintain motion along the vibration direction. Experimental results from prototype testing provide strong evidence for the high-power output of the proposed harvester and its ability to power a wireless sensor. A maximum power of 511 mW and an average power of 24.5 mW are achieved at a harmonic excitation with 21 Hz and 0.7 RMS (Root Mean Square) g, while a maximum power of 568 mW and an average power of 7.3 mW are generated under a measured railway track vibration signal.