A Broadband Time-Varying Energy Maximising Control for Wave Energy Systems (LiTe-Con+): Framework and Experimental Assessment

Motion of wave energy converters (WECs) is usually exaggerated as a consequence of the application of control strategies for energy absorption maximisation. With the aim of preserving the physical integrity of the devices, constraint handling mechanisms, as part of the underlying control strategies, are considered a key component. Recent developments in wave energy control include a linear time-invariant-based controller presented in the literature as LiTe-Con, which provides a simple constraint handling mechanism. However, this handling method can lead to conservative performance in certain scenarios. To overcome such limitations, this study presents a time-varying methodology for an online adaptation of the constraint handling mechanism in LiTe-Con, while preserving its original simplicity and efficiency. Experimental assessment of the presented control methodology is provided in this study, using a broad range of operating conditions. Results show that the presented control strategy (LiTe-Con+) exceeds the performance achievable with the original LiTe-Con. Additionally, the benefits of LiTe-Con+, such as low computational demand, technical versatility, and impressive performance level are highlighted.Fil: García Violini, Diego Demián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; Argentina. Maynooth University; IrlandaFil: Pena Sanchez, Yerai. Universidad del País Vasco; EspañaFil: Faedo, Nicolás Ezequiel. Politecnico di Torino; ItaliaFil: Ferri, Francesco. Aalborg University; DinamarcaFil: Ringwood, John V.. Maynooth University; Irland

A Critical Comparison of Excitation Force Estimators for Wave-Energy Devices

The implementation of energy-maximizing control systems (EMCSs) can significantly increase the efficiency and economic viability of resonant wave-energy converters (WECs). To achieve optimal control and drive the WEC into resonance with the incoming wave field, knowledge of the wave excitation force is required. In operational conditions, this quantity is immeasurable and, thus, has to be estimated. This article presents a critical comparison of the available excitation force estimators found in the literature. A reference measurement of the excitation force is determined using computational fluid dynamics (CFD) simulation, allowing an absolute comparison of the different estimation strategies. The estimators are compared based on the required input data, achieved accuracy, computational delay, and estimation time. In total, 11 estimation strategies are compared, with three, in particular, emerging with relatively superior performance

An Energy-Maximising Linear Time Invariant Controller (LiTe-Con) for Wave Energy Devices

A Linear Time Invariant (LTI) energy-maximising control strategy for Wave Energy Converters (WECs) is proposed in this paper. Using the fundamental requirement of impedance matching, the controller is tuned to maximise the energy obtained under polychromatic wave excitation. Given the LTI nature of the proposed controller, the design and implementation procedure is significantly simpler than well-established energy-maximising controllers, including state-of-the-art numerical optimisation routines, which are predominant in this field. Additionally, a LTI constraint handling mechanism is provided. The effectiveness of both the LTI control strategy and the constraint handling mechanism are assessed using regular and irregular waves in unconstrained and constrained cases. The resulting performance is compared to those obtained using existing WEC optimal control strategies. Finally, the benefits, in terms of power production, for both the controller and the constraint handling mechanism are explicitly highlighted by means of an application cas

A Simple and Effective Excitation Force Estimator for Wave Energy Systems

Wave energy converters (WECs) need to be optimally controlled to be commercially viable. These controllers often require an estimate of the (unmeasurable) wave excitation force. To date, observers for WECs are often based upon ‘complex’ techniques, which are counter-intuitive in their design, additionally requiring an explicit model to describe the excitation as part of an (augmented) system. The latter imposes strong assumptions on the design of each observer, while also implying an additional computational burden associated with the necessity of augmenting the WEC model to include the dynamics of the input.We propose a simple and effective excitation force estimator based on linear time-invariant (LTI) theory, without the need for an explicit model of the input. In particular, we re-formulate the unknown-input estimation problem as a tracking control-loop, so that a wide-variety of LTI design techniques (arising from either classical or modern control theory) can be used to compute an estimate of the excitation force. We demonstrate performance, simplicity, and intuitive appeal of the proposed observer, by means of a case study based on a realistic computational fluid dynamics simulation, comparing the technique against a large set of WEC observers, showing that the approach is able to outperform available estimators

Experimental Implementation and Validation of a Broadband LTI Energy-Maximizing Control Strategy for the Wavestar Device

This study addresses the experimental validation of a linear time-invariant (LTI) energy-maximizing control strategy for wave energy converters (WECs), applied to a 1/20 scale Wavestar WEC. To fulfill this objective, system identification routines are utilized to compute a mathematical (parametric) model of the input–output dynamics of the device, suitable for control design and implementation. With this parametric model, the so-called LiTe-Con energy-maximizing strategy, recently published in the literature, is designed, synthesized, and tested under irregular wave excitation in the wave basin at Aalborg University. Given that the LiTe-Con requires instantaneous knowledge of the wave excitation effects, estimates are provided by means of an unknown-input Kalman filter, designed in close synergy with the so-called internal model principle. For the experimental assessment, both controller and estimator are directly implemented in a real-time architecture. The performance of the LiTe-Con is evaluated in terms of energy-absorption, showing consistent results with respect to those obtained in numerical simulation, hence validating the LiTe-Con controller in a realistic real-time scenari