Variable-Stiffness Power Take-Off System for Broadband Ocean Wave Energy Point Absorbers

Abstract This research presents a biologically-inspired method for harvesting energy from ocean waves using a variable-stiffness power take-off mechanism with a point absorber. A key limitation in current approaches to wave energy converter (WEC) design is that most WECs can only be tuned to perform optimally at one chosen period. In practice, ocean wave conditions are continually changing, causing the WECs to perform suboptimally throughout much of their operating time. As a result, WEC designs with variable, controllable resonant periods should be considered. The power take-off mechanism explored in this work is a fiber-reinforced elastic hose that acts as a pump: when stretched axially, the internal volume of the pump decreases, jetting fluid out, and when allowed to contract, the internal volume increases, drawing fluid back into the pump. This pumping action spins a turbine which drives a generator to produce electricity. The design of the pump itself draws inspiration from the helical arrangement of collagen fibers in the mantles of squids, which allows squids to be one of the fastest aquatic invertebrates when jetting. This type of elastic fiber-reinforced pump has a pressure-dependent variable stiffness property that, within this WEC system, can be controlled by a variable gearing device between the turbine and generator. The ability to control the pump stiffness opens the opportunity to adapt the WEC natural frequency to match the frequency of the instantaneous driving waves, allowing the WEC to operate optimally at a broader range of wave states. This research investigates the effects of controlled variable stiffness on the dynamics and performance of this pumping wave energy point absorber by developing dynamic system modeling and fluid flow modeling, as well as experimentally validating these models and demonstrating controllable variable stiffness on a lab-scale pump prototype.