Adaptive Control of a Wave Energy Converter

Abstract

Energy maximising controllers (EMCs), for wave energy converters (WECs), based on linear models, are attractive in terms of simplicity and computation. However, such (Cummins equation) models are normally built around the still water level as an equilibrium point and assume small movement, leading to poor model validity for realistic WEC motions, especially for the large amplitude motions obtained by a well controlled WEC. The method proposed here is to use an adaptive algorithm to estimate the control model online, whereby system identification techniques are employed to identify a linear model that is most representative of the actual controlled WEC behaviour. Using exponential forgetting, the linear model can be regularly adapted to remain representative in changing operational conditions. To that end, this paper presents a novel adaptive controller based on a receding horizon pseudospectral formulation. A case study is presented, providing an preliminary evaluation of the adaptive receding horizon pseudospectral control (ARHPC), using simulations based on both linear hydrodynamic modelling and computational fluid dynamics (CFD). The parameter adaptation is shown to behave as expected in both different simulation environments. The ARHPC is observed to perform well in irregular sea states, absorbing more power than its nonadaptive counterpart. The optimal trajectory calculated by the adaptive model is seen to have a smaller motion and power takeoff (PTO) forces, compared to those calculated by the nonadaptive linear control model, due to the increased amount of hydrodynamic resistance estimated by the adaptive model, as identified from the nonlinear viscous CFD simulation.

A high-fidelity wave-to-wire simulation platform for wave energy converters: Coupled numerical wave tank and power take-off models

“…The software communication between the OpenFOAM and MATLAB® environments, enabling the coupling of the CNWT and PTO models, is achieved following similar procedures reported in the literature [38,61]. The method was first demonstrated in [61], where mooring forces on a floating WEC are calculated in MATLAB® at each time-step of an OpenFOAM CNWT simulation.…”

“…The method was first demonstrated in [61], where mooring forces on a floating WEC are calculated in MATLAB® at each time-step of an OpenFOAM CNWT simulation. Similarly, OpenFOAM and MATLAB® are coupled in [38] for the evaluation of energy maximising control strategies, where an adaptive receding horizon pseudospectral control algorithm is implemented in MATLAB® to calculate the optimal PTO force applied to the WEC at each CNWT time-step. The present paper follows these pioneering couplings of CNWT simulations with mooring models and control algorithms, and presents a first coupling of a full PTO model with a CFD model.…”

Nonlinear Constrained Optimal Control of Wave Energy Converters With Adaptive Dynamic Programming

“…This problem can become numerically intractable if the dynamics of a WEC are complicated and need to be described accurately by a highorder model with nonlinearities to retain the modeling fidelity and constraints for safe operations. To address this issue, some alternative methods have been proposed, such as control based on convex optimization with a modified objective function [9], adaptive control [10], nonlinear MPC with pseudospectral control [11], and nonlinear MPC based on a combination of the pseudospectral method and the differential flatness [12], where the plant model is assumed to be accurate.…”

Experimental Analysis of a Novel Adaptively Counter-Rotating Wave Energy Converter for Powering Drifters

“…For more than two centuries, inventors have proposed many different devices to utilize wave power for human purposes [14][15][16], and several research papers relating to ocean wave energy conversion have been emerged in an endless stream [17][18][19][20]. Control strategies for wave energy conversion systems also have been studied in several papers [21][22][23][24]. António F. de O. Falcão [25] categorized ocean wave energy converters (WECs) into three main categories: oscillating water column, overtopping device, and point absorber.…”