Embedding parametric resonance in a 2:1 wave energy converter to get a broader bandwidth

Abstract

The effort to increase the converted power is a common challenge to players in the field of wave energy conversion, both academic and industrial. In the case devices are found to be prone to parametric resonance, it typically has a negative impact on power harvesting and may jeopardize the reliability of the device. This paper makes the case that parametric resonance is not a danger that should be avoided, but rather a chance to achieve a broader system response bandwidth and ultimately increase the amount of power available at the power take-off. Since a time-varying wetted surface causes the highly nonlinear phenomenon of parametric resonance, linear models are unable to fully capture this instability. As a result, nonlinear Froude–Krylov forces are herein implemented via a computationally effective method for prismatic floaters that is compatible with both exhaustive simulation methods and real-time computing, as the whole simulations runs up to 50 times faster than real-time. A novel pendulum-based device is intentionally defined to exhibit a 2:1 ratio between heave and pitch natural frequencies, causing parametric instability. Results demonstrate that linear models predict a single zone of meaningful potential power extraction around the pitch natural frequency, as expected; however, by using the designed attitude to develop parametric instability, a second additional region develops near the heave natural period. As a result, the free response bandwidth is in fact increased, making more energy available at the power take-off axis thanks to the nonlinear instability embedded in the wave energy converter.