Empowering wave energy with control technology: Possibilities and pitfalls

With an increasing focus on climate action and energy security, an appropriate mix of renewable energy technologies is imperative. Despite having considerable global potential, wave energy has still not reached a state of maturity or economic competitiveness to have made an impact. Challenges include the high capital and operational costs associated with deployment in the harsh ocean environment, so it is imperative that the full energy harnessing capacity of wave energy devices, and arrays of devices in farms, is realised. To this end, control technology has an important role to play in maximising power capture, while ensuring that physical system constraints are respected, and control actions do not adversely affect device lifetime. Within the gamut of control technology, a variety of tools can be brought to bear on the wave energy control problem, including various control strategies (optimal, robust, nonlinear, etc.), data-based model identification, estimation, and forecasting. However, the wave energy problem displays a number of unique features which challenge the traditional application of these techniques, while also presenting a number of control ‘paradoxes’. This review articulates the important control-related characteristics of the wave energy control problem, provides a survey of currently applied control and control-related techniques, and gives some perspectives on the outstanding challenges and future possibilities. The emerging area of control co-design, which is especially relevant to the relatively immature area of wave energy system design, is also covered

Energy-maximising model predictive control for a multi degree-of-freedom pendulum-based wave energy system

Renewable energy sources can be a solution for the recent pollution increasing scenario and the need for diversification of the energy market. Among such alternative sources,wave energy represents a viable solution, due to the its high power density and accessibility.Nonetheless, wave energy is still in phase of development, and a key stepping stone towards commercialisation is strongly linked to the availability of optimal control strategies for maximum energy harvesting. With its ability to handle system constraints and optimise power absorption directly, model predictive control (MPC) has gained popularity within the WEC community as a potential solution for the corresponding energy-maximising problem. In this study, an MPC strategy is developed for real-time control of the so-called PeWEC energy harvesting system,providing also a solution for the wave excitation estimation and forecasting problem, inherently required by the MPC controller to achieve optimal performance. Improved computational requirements are obtained via definition of a reduced control-oriented model, describing the dynamics of the system in a compact form. The performance of the proposed strategy is illustrated via a comprehensive numerical appraisal

Control, forecasting and optimisation for wave energy conversion

This paper presents an overview of the motivation, background to and state-of- the-art in energy maximising control of wave energy devices. The underpinning mathematical modelling is described and the control fundamentals established. Two example control schemes are presented, along with some algorithms for wave forecasting, which can be a necessary requirement, due to the non-causal nature of some optimal control strategies. One of the control schemes is extended to show how cooperative control of devices in a wave farm can be beneficial. The paper also includes perspectives on the interaction between control and the broader objectives of optimal wave energy device geometry and full techno-economic optimisation of wave energy converters

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

Wave to wire power maximisation from a wave energy converter

In this paper a back-to-back voltage source converter controlled linear permanent magnet generator (LPMG) is utilised as the power take off (PTO) for a point absorber wave energy converter system (WEC). It is shown that reactive control which seems promising when an ideal PTO is assumed, is actually infeasible with a real PTO as the electrical losses of the LPMG are excessive when the wave frequency is lower than the natural frequency. A Zero Order Hold (ZOH) and First Order Hold (FOH) Model Predictive Control (MPC) which maximises the mechanical power is first utilised. The two MPC systems show that more electrical power is extracted for a lower horizon when the MPC is optimised for mechanical power. The electrical losses from the LPMG and voltage source converter (VSC) are then incorporated in the cost function of the MPC systems and demonstrates significant improvements in the electrical power extracted when compared to the electrical power extracted via mechanical power optimisation. PTO force and heave displacement constraints are then incorporated into the optimisation, to further demonstrate the limitations of performance when a realistic PTO is utilised. It is shown here that the electrical power can be maximised, whilst the PTO force and heave displacement are shown to be within limits. The power quality from the ZOH MPC is then compared to the power quality from the FOH MPC

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

Study of scale modelling, verification and control of a heaving point absorber wave energy converter

This study focuses on scale modelling of a heaving Point Absorber Wave Energy Converter (PAWEC), model verification via wave tank tests and power maximisation control development. Starting from the boundary element method simulation of the wave-PAWEC interaction, linear and non-linear modelling approaches of Wave-To-Excitation-Force (W2EF), Force-To-Motion (F2M), Wave-To-Motion (W2M) are studied. To verify the proposed models, a 1/50 scale PAWEC has been designed, simulated, constructed and tested in a wave tank under a variety of regular and irregular wave conditions. To study the coupling between the PAWEC hydrodynamics and the Power Take-Off (PTO) mechanism, a Finite Element Method (FEM) is applied to simulate and optimise a Tubular Permanent Magnet Linear Generator (TPMLG) as the PTO system and control actuator. Thus linear and non-linear Wave-To-Wire (W2W) models are proposed via combining the W2M and PTO models for the study and development of power maximisation control.The main contributions of this study are summarised as follows:Linear and non-linear F2M models are derived with the radiation force approximated by a finite order state-space model. The non-linear friction is modelled as the Tustin model, a summation of the Stribeck, Coloumb and damping friction forces, whilst the non-linear viscous force is simulated as the drag term in the Morison equation. Thus a non-linear F2M model is derived considering the non-linear friction and viscous forces as a correction or calibration to the linear F2M model. A wide variety of free-decay tests are conducted in the wave tank and the experimental data fit the non-linear F2M modelling results to a high degree. Further, the mechanism how these non-linear factors influence the PAWEC dynamics and energy dissipations is discussed with numerical and experimental results.Three approaches are proposed in this thesis to approximate the wave excitation force:(i) identifying the excitation force from wave elevation, referred to as the W2EF method, (ii) estimating the excitation force from the measurements of pressure, acceleration and displacement, referred to as the Pressure-Acceleration-Displacement-To-Excitation-Force (PAD2EF) approach and (iii) observing the excitation force via an unknown input observer, referred to as the Unknown-Input-Observation-of-Excitation-Force (UIOEF) technique. The W2EF model is integrated with the linear/non-linear F2M models to deduce linear/non-linear W2M models. A series of excitation tests are conducted under regular and irregular wave conditions to verify the W2EF model in both the time- and frequency-domains. The numerical results of the proposed W2EF model show a high accordance to the excitation test data and hence the W2EF method is valid for the 1/50 scale PAWEC. Meanwhile, a wide range of forced-motion tests are conducted to compare the excitation force approximation results between the W2EF, PAD2EF and UIOEF approaches and to verify the linear and non-linear W2M models. Comparison of the PAWEC displacement responses between the linear/non-linear W2M models and forced-motion tests indicates that the non-linear modelling approach considering the friction and viscous forces can give more accurate PAWEC dynamic representation than the linear modelling approach.Based on the 1/50 scale PAWEC dimension and wave-maker conditions, a three-phase TPMLG is designed, simulated and optimised via FEM simulation with special focus on cogging force reduction. The cogging force reduction is achieved by optimise the TPMLG geometric design of the permanent magnets, slots, pole-shoe and back iron. The TPMLG is acting as the PTO mechanism and control actuator. The TPMLG is connected with the buoy rigidly and hence the coupling is achieved by the PTO force. Linear and non-linear W2W models are derived for the study of power maximisation control. To investigate the control performance on the linear and non-linear W2W models, reactive control and phase control by latching are developed numerically with electrical implementation on the TPMLG. Further, a W2W tracking control structure is proposed to achieve power maximisation and displacement constriction under both regular and irregular wave conditions

Robust control of wave energy converters

Energy-maximising controllers for wave energy devices are normally based on linear hydrodynamic device models. Such models ignore nonlinear effects which typically manifest themselves for large device motion (typical in this application) and may also include other modelling errors. In this paper, we present a methodology for reducing the sensitivity to modelling errors and nonlinear effects by the use of a hierarchical robust controller, which also allows good energy maximisation to be recovered through a passivity-based control approach