A class of globally stabilising controllers for the control of wave energy devices for potable water production

This paper provides a stability analysis for a system that captures wave energy in order to produce potable water. The system, introduced in [1], is a Wave Energy Converter (WEC) of the point-absorber type coupled to a hydraulic Power Take-Off (PTO) that converts wave energy into pressure. Previous work has used a partial state-feedback controller with integral action and feed-forward to provide good nominal control behaviour. Although open-loop stability was proven in [1], no guarantees of closed-loop stability were given; in this paper we provide such guarantees for a class of controllers, of which the controller proposed in [1] is a special case

A class of globally stabilising controllers for the control of wave energy devices for potable water production

This paper provides a stability analysis for a system that captures wave energy in order to produce potable water. The system, introduced in [1], is a Wave Energy Converter (WEC) of the point-absorber type coupled to a hydraulic Power Take-Off (PTO) that converts wave energy into pressure. Previous work has used a partial state-feedback controller with integral action and feed-forward to provide good nominal control behaviour. Although open-loop stability was proven in [1], no guarantees of closed-loop stability were given; in this paper we provide such guarantees for a class of controllers, of which the controller proposed in [1] is a special case

A Study of the Prediction Requirements in Real-Time Control of Wave Energy Converters

It is widely acknowledged that real-time control of wave energy converters (WECs) can benefit from prediction of the excitation force. The prediction requirements (how far ahead into the future do we need to predict?) and the achievable predictions (how far ahead can we predict?) are quantified when unconstrained reactive control is implemented. The fundamental properties of the floating system that influence the length of the required forecasting horizon, as well as the achievable prediction, are characterized. The possibility of manipulating the control, based on prior knowledge of the wave spectral distribution, is also proposed for the reduction of the prediction requirements, such that they are within the range of predictability offered by simple stochastic predictors. The proposed methodology is validated on real wave data and heaving buoys with different geometries

A Simple and Effective Real-Time Controller for Wave Energy Converters

A novel strategy for the real-time control of oscillating wave energy converters (WECs) is proposed. The controller tunes the oscillation of the system such that it is always in phase with the wave excitation force and the amplitude of the oscillation is within given constraints. Based on a nonstationary, harmonic approximation of the wave excitation force, the controller is easily tuned in real-time for performance and constraints handling, through one single parameter of direct physical meaning. The effectiveness of the proposed solution is assessed for a heaving system in one degree of freedom, in a variety of irregular (simulated and real) wave conditions. A performance close to reactive control and to model predictive control is achieved. Additional benefits in terms of simplicity and robustness are obtained

Suboptimal Causal Reactive Control of Wave Energy Converters Using a Second Order System Model

Wave Energy Converters (WECs) based on oscillating bodies can achieve optimal energy absorption under certain conditions associated with reactive control. These conditions, in general, are not realisable in practice because non-causal and future values of the excitation force need to be known. In this paper, an alternative approach is presented, where the relationship between the optimal velocity and the excitation force is realised through a simple coefficient of proportionality, thus removing the problem of non-causality. From theoretical considerations and numerical simulations over a range of heaving WECs in different sea conditions, it is shown that such suboptimal and causal approximation, while significantly reducing the complexity and improving the robustness of reactive control, allows the achievement of values of energy capture very close to the ideal optimum

Wave energy control: status and perspectives 2020

Wave energy has a significant part to play in providing a carbon-free solution to the world’s increasing appetite for energy. In many countries, there is sufficient wave energy to cater for the entire national demand, and wave energy also has some attractive features in being relatively uncorrelated with wind, solar and tidal energy, easing the renewable energy dispatch problem. However, wave energy has not yet reached commercial viability, despite the first device designs being proposed in 1898. Control technology can play a major part in the drive for economic viability of wave energy and this paper charts the progress made since the first wave energy control systems were suggested in the 1970s, and examines current outstanding challenges for the control community

Quantification of the Prediction Requirements in Reactive Control of Wave Energy Converters

Optimal reactive control for maximum ocean wave power absorption from Wave Energy Converters (WECs) consisting of oscillating systems, is based on the principle of tuning their oscillation so that it is in resonance with the excitation force produced by the incident waves. Reactive control, however, is non-causal and cannot be implemented in real time. This paper analyses the prediction requirements of one possible solution, where predictions of the excitation force are utilised to resolve the non-causality. The study is focused on the analysis of the required forecasting horizon against the achievable prediction. Also, through the aid of numerical simulations of a number of specific systems over several wave conditions, a link is found between some fundamental properties of the system and the prediction requirements

Quantification of the Prediction Requirements in Reactive Control of Wave Energy Converters

Optimal reactive control for maximum ocean wave power absorption from Wave Energy Converters (WECs) consisting of oscillating systems, is based on the principle of tuning their oscillation so that it is in resonance with the excitation force produced by the incident waves. Reactive control, however, is non-causal and cannot be implemented in real time. This paper analyses the prediction requirements of one possible solution, where predictions of the excitation force are utilised to resolve the non-causality. The study is focused on the analysis of the required forecasting horizon against the achievable prediction. Also, through the aid of numerical simulations of a number of specific systems over several wave conditions, a link is found between some fundamental properties of the system and the prediction requirements

A study on Prediction Requirements in time-domain Control of Wave Energy Converters

Wave energy converters (WECs) based on oscillating bodies or oscillating water columns would earn huge benefits from a time-domain control on a wave by wave basis. Such a control would allow efficient energy extraction over a wider range of frequencies than what could possibly be achieved when no real-time control is adopted, thus increasing the economical attractiveness of the WECs. Almost every control strategy that showed some potential, however, sffers from the problem that future knowledge of the incident wave elevation, or wave excitation force, is required. In this paper a general control framework for oscillating WECs is presented and a methodology to understand and quantify the wave excitation force prediction requirements, along with the achievable prediction accuracy, is discussed. The two features are compared against each other and linked to the dynamic characteristics of a device. Along with the qualitative discussion, the methodology is applied to some heaving cylinders when reactive control and linear passive control are applied, under different sea conditions

Real-time Forecasting and Control for Oscillating Wave Energy Devices

Ocean wave energy represents a signicant resource of renewable energy and can make an important contribution to the development of a more sustainable solution in support of the contemporary society, which is becoming more and more energy intensive. A perspective is given on the benefits that wave energy can introduce, in terms of variability of the power supply, when combined with oshore wind. Despite its potential, however, the technology for the generation of electricity from ocean waves is not mature yet. In order to raise the economic performance of Wave energy converters (WECs), still far from being competitive, a large scope exists for the improvement of their capacity factor through more intelligent control systems. Most control solutions proposed in the literature, for the enhancement of the power absorption of WECs, are not implemented in practise because they require future knowledge of the wave elevation or wave excitation force. The non-causality of the unconstrained optimal conditions, termed complex-conjugate control, for the maximum wave energy absorption of WECs consisting of oscillating systems, is analysed. A link between fundamental properties of the radiation of the floating body and the prediction horizon required for an effective implementation of complex-conjugate control is identified. An extensive investigation of the problem of wave elevation and wave excitation force forecasting is then presented. The prediction is treated as a purely stochastic problem, where future values of the wave elevation or wave excitation force are estimated from past measurements at the device location only. The correlation of ocean waves, in fact, allows the achievement of accurate predictions for 1 or 2 wave periods into the future, with linear Autoregressive (AR) models. A relationship between predictability of the excitation force and excitation properties of the floating body is also identified. Finally, a controller for an oscillating wave energy device is developed. Based on the assumption that the excitation force is a narrow-banded harmonic process, the controller is effectively tuned through a single parameter of immediate physical meaning, for performance and motion constraint handling. The non-causality is removed by the parametrisation, the only input of the controller being an on-line estimate of the frequency and amplitude of the excitation force. Simulations in (synthetic and real) irregular waves demonstrate that the solution allows the achievement of levels of power capture that are very close to non-causal complex-conjugate control, in the unconstrained case, and Model predictive control (MPC), in the constrained case. In addition, the hierarchical structure of the proposed controller allows the treatment of the issue of robustness to model uncertainties in quite a straightforward and effective way