Study application of an unmoored ocean wave energy harvester with harmonic and random excitation

Abstract

Vibration-based Energy Harvesting offers a promising technique for generating sustainable energy for electronic devices by converting ambient kinetic energy into predictable electrical energy. On the other hand, these systems can be considered as a replacement for the battery, which of course should be portable. Ocean wave energy is a rich source of ambient energy that is often underutilized for vibration energy generation due to the low frequency of ocean waves. Although ocean wave energy harvesters are considered by many researchers, unmoored vibration-based ocean wave energy harvesters with low frequency and random excitation have not been studied. This study proposes and examines a new energy harvesting system that unlike usual ocean wave energy harvesters is not anchored to the seabed and can use both roll and pitch motions of the ocean waves. The dynamic behavior of the energy harvester, which consists of an inverted pendulum and a mechanical rectifier, is formulated, and experimentally validated. In order to do this, the damping of the rectifier is experimentally determined and considered as part of the equivalent damping of the whole system. Then the effect of changing the nondimensional mass and geometry of the floater on the oscillatory behavior of the system under harmonic and random excitation is investigated. To do so, using the strip theory method, equivalent dynamic parameters of the different floater are calculated. It is shown that optimizing the non-dimensional mass of the energy harvester can lead to an energy harvester that produces relatively equal power under both random and harmonic excitation. To study the random behavior of ocean waves, Pierson-Moskowitz Spectrum is used. The relevant design recommendations for the presented energy harvester are summarized in a table. Finally, it is shown that the presented system is flexible in the choice of design parameters and can generate large amounts of energy.