Rotors for wave energy conversion—Practice and possibilities

J. Ocean Eng. Mar. Energy

2021

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We present a new analytical model from which a model-based controller can be derived for a cyclorotor-based wave energy converter (WEC). Few cyclorotor-based WEC concepts and models have previously been studied and only one control strategy for the entire wave cancellation has been tested. Our model is derived for a horizontal cyclorotor with N hydrofoils and is suitable for the application of various control algorithms and the calculation of various performance metrics. The mechanical model is based on Newton’s second law for rotation. The cyclorotor operates in two dimensional potential flow. This paper modeled the velocity field in detail around the turbine with N hydrofoils by explaining each velocity term and estimated the generated torque using two methods (point source method and thin-chord method). The developed model is very convenient for control design, using the power take off torque and hydrofoil pitch angles as control inputs. The authors of this work have derived new, exact analytic functions for the free surface perturbation and induced fluid velocity field caused by hydrofoil rotation. These new formulae significantly decrease the model calculation time and increase the accuracy of the results. The new equations also provide useful insight into the nature of the associated variables, and are successfully validated against the results of physical experiments and numerical calculations previously published by two independent research groups. Representation of hydrofoils as both a point source and a thin chord were analysed, with both models cross-validated for the case of free rotation in monochromatic waves.

Section: Overview Of the Existing Prototypes And Control Strategiesmentioning

confidence: 99%

A control-orientated analytical model for a cyclorotor wave energy device with N hydrofoils

J. Ocean Eng. Mar. Energy

2021

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“…The cyclorotor-based WEC is simulated with the use of a bespoke mathematical model which has an integro-differential equation form (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), simulated in Python [32]. The control problem is non-convex due to the high non-linearity of the model equations.…”

Section: Mathematical Formulation Of the Optimal Control Problemmentioning

confidence: 99%

“…However, as it has been shown in [3]- [6], the performance of the cyclorotor requires real-time control (especially in panchromatic seas). The reviews of cyclorotor-based WECs [6]- [8] have identified the variation of the rotational rate and hydrofoil pitch angle as the most advanced actuators. However, the optimal control strategies, the best real-time actuators, and appropriate performance metrics, need to be identified.…”

Section: Introductionmentioning

confidence: 99%

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

Marie

IEEE Trans. Sustain. Energy

2022

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The development of optimal control strategies for cyclorotor wave energy converters (WECs) is at an early stage. In this paper, we present new methods and solutions for optimal pitch and/or rotational velocity control strategies for different configurations of cyclorotor-based WECs, in both monochromatic and panchromatic waves. The cyclorotor is modelled with the use of an approximate two-dimensional mathematical model, where hydrofoils are approximated as point source vortices in potential waves. The goal of the developed control strategy is to determine the optimal velocity profile, and/or pitch angle variations, for maximum energy conversion, in terms of generated mechanical shaft power. The solutions, obtained with the use of spectral methods, show a clear benefit in using a variable velocity for cyclorotors with either one or two hydrofoils, in monochromatic waves, with a typical increase in energy capture of 30-40%, while the optimal pitching did not significantly increase the value of the absorbed wave energy. We also present control results for panchromatic waves, assuming all the properties of incoming wave packages, within a 10 second time interval, are known. The obtained solutions have shown significant benefit in joint optimal pitch and velocity control, especially in the case of panchromatic waves. It has been also shown that successful implementation of cyclorotor control strategies requires optimal configuration of the static characteristics of the cyclorotor. In conclusion, we discuss the optimal control benefits and highlight problems which must be solved for further successful development of these control strategies.

“…T HOUGH research into lift-based cyclorotor-based wave energy converters (WECs) began over 40 years ago, development in the interim has been sporadic and slow [1]. Much of the recent progress has been documented in the PhD thesis of Scharmann [2] and the work of Hermans [3] and Siegel [4], [5].…”

Section: Introductionmentioning

confidence: 99%

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

Marie

IEEE Trans. Sustain. Energy

2022

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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.