Some Fundamental Results for Cyclorotor Wave Energy Converters for Optimum Power Capture

Abstract

Cyclorotor-based wave energy converters (WECs) present a relatively new and innovative paradigm for wave energy harvesting. Their operational principle is based on the generation of lift forces on the rotating hydrofoils due to their interaction with wave-induced circulation of water particles. As a result, relatively little is known about their optimal operation. To date, cyclorotor device performance has been measured by the power of waves radiated by the WEC, while a constant rotational velocity, consistent with the wave frequency, is employed. In this note we show (a) that variation of the cyclorotor velocity within the incoming monochromatic wave period significantly increases the generated mechanical power, while (b) optimising wave cancellation is at odds with the maximisation of shaft power. To optimise shaft power, the letter adopts a multi-harmonic solution for variable cyclorotor rotation rate, inspired by methods developed in other WEC domains.

Optimal Control of Pitch and Rotational Velocity for a Cyclorotor Wave Energy Device

“…The obtained results have confirmed the preliminary results presented in [20], that the control of rotational rate in monochromatic waves can significantly increase increase the generated shaft power P Shaf t . In contrast, active pitch control in monochromatic waves does not significantly increase the shaft power P Shaf t , suggesting the use of an optimal constant pitch angle, for monochromatic waves, especially considering the additional complexity of underwater pitch actuators.…”

“…As members of the LiftWEC consortium, the authors derive new optimal pitch and rotational rate control strategies for a cyclorotor with two hydrofoils [17]- [19]. The presented strategies can potentially significantly increase the cyclorotor performance, in terms of the generated shaft power, consistent with some preliminary fundamental results presented in [20].…”

“…Our goal is to directly maximise mechanical shaft power rather than maximisation of the difference between the incoming and transmitted wave. The difference in the shaft power and wave cancellation metric is primarily due to the fact that, when maximising shaft power, tangential force needs to be maximised while, for wave cancellation circulation Γ needs to be optimised [6], [20], [22].…”

Energy-Maximising Control Philosophy for a Cyclorotor Wave Energy Device

“…where the additional phase offset terms (c i , ĉi ) and (d i , di ) are included beyond (14) and the monochromatic wave frequency ω is replaced with a nominal rotational speed θ0 , which is the optimum (constant) rotational rate for the fundamental period [T 0 , T ], again with the intention of easing the global optimisation problem. T ext , in (15), is an extended time interval selected as twice the interval on which we consider the energy generation [T 0 , T ].…”

“…These pioneering studies focus on wave cancellation as a control performance metric and used a constant cyclorotor rotational speed. More recently, some studies have shown the benefit of a variable rotational velocity, and the use of shaft power as a primary performance metric [14]. Specific studies [15] examined the use of both relatively fast acting (generator torque, foil pitch angles), and slow acting (submergence depth, rotor radius) control actuators have recently emerged, showing potential improvement in power capture of over 100%, Given the disparity of time scales over which control actuators can operate, to date there has been no analysis of the interaction between these different actuator sets on the system performance.…”