Mechanical property identification and performance evaluation of a power take-off combined with a mechanical motion rectifier and a magnetic bistable device

The bistable mechanism has attracted lots of attention due to its potential in improving the efficiency and broadening the bandwidth of wave energy converters. However, most of existing researches are based on simplified numerical models that ignore nonlinear features of power take-off (PTO) and bistable device. Therefore, in this study, a mechanical PTO with both a mechanical motion rectifier (MMR) and a gyration magnetic bistable device (MBD) was designed and integrated into a self-reacting point absorber (SRPA). First, the gyration dynamic and bench tests were carried out to identify the mechanical properties and to explore the working principle of the PTO. Besides, the mechanical parameters of the PTO are obtained for the further analysis of the SRPA coupling dynamics. The disengagement of the MMR and the snap-through feature of the MBD were observed. As for the PTO force, the increment of the inertia moment in backside of the MMR and the electromagnetic damping increases the PTO force, while the MBD turns it more fluctuant due to its bi-stability. With regard to the MMR disengagement, the rise of the inertia moment and the drop of the electromagnetic damping expand the disengagement duration of the MMR. The result indicates the equivalent electromagnetic damping is a critical factor of efficiency improvement, and the oscillation velocity is a subordinary factor beneficial to the efficiency promotion, while the disengagement of the MMR contributes to the efficiency improvement to a limited extent. The PTO efficiency is determined by the proportion of the work done by the nonconservative force in nature. The overall efficiency of the equipment in the bench test achieves 62.9%. Then, the holistic perspective of the self-reacting point absorber integrated with the proposed PTO is given through the numerical analysis. The relatively weak bi-stability of the MBD degrades the potential barrier of the restoring force, and it is favorable to the snap-through oscillation and to promoting the energy harvesting performance of the SRPA. The proposed PTO sustains the relatively high efficiency level of the energy conversion between 50% and 70%, which manifests a good performance for the application of the wave energy utilization.