Operation Of Wave Energy Converter Research Articles

Hardware in the Loop Wave Energy Converter Control Under Control Faults and Model Mismatch

Ocean wave energy conversion is operated in a challenging environment featuring massive and impulsive stresses and is subjected to large amounts of biofouling and marine growth. As most wave energy converter technologies are off-shore, operation, maintenance, and repair costs may be significant. Therefore, any cost-effective control and operation of Wave Energy Converters (WECs) must be robust to faults and changes in the operational environment. This paper presents a Bayesian policy gradient-based WEC control approach that is responsive and adaptive to faults in controller feedback, controller actuation, and changes in the plant model. The controller is tested via Hardware in the Loop real-time testbed. The results show that the proposed control is capable of adapting its policy to the loss of information from one or two sensors and recovering nearly full power operation. In the case where sensor loss occurs, after the loss of plate excitation force estimation and displacement sensors, the power decreases to 215.2 kW from 252.7 kW before recovering to 238.4 kW mean power over 20 minutes. Moreover, the Bayesian RL-based control model proves its adaptability by compensating a large portion of power loss due to PTO failures and rapid changes to the dynamic system because of non-destructive damage to the float structure.

A controllable-valve buoy for high-efficiency wave energy conversion under a wide range of wave conditions

Heaving buoys are common devices for wave energy conversion. In recent years, many researchers have been devoted to improve their energy-absorbing capability by configuration optimization. Traditionally, the operation of wave energy converter (WEC) has to be interrupted for safety when waves become excessively strong. In this paper, in order to render the WEC able to work under a wide range of wave conditions so as to enhance the energy-harvesting capability and increase the total operation hours and total power production, a new type of buoy, controllable-valve buoy (CVB) is proposed. It is a hemisphere-cylinder combined buoy with a hollow channel covered by a controllable valve that can operate in two modes. In gentle sea states the valve is closed and the buoy absorbs wave energy to a large extent, while in rough sea states the valve is opened and the buoy is able to work and produce an acceptable power. The geometrical parameters of CVB are optimized via a systematic method combined with wave statistics of the target installation site. The radius and the draft of the optimized buoy are 4.8 m and 8 m, and it has a hollow channel with a radius of 2 m. Simulation results show that the optimized buoy demonstrates a good performance in both closed and opened mode.

Representative linearised models for a wave energy converter using various levels of force excitation

In guiding the progression, development, and operation of wave energy converters (WECs) in a more efficient way, mathematical analysis and understanding of the dynamic process is essential. Mathematical WEC models, obtained either by numerical analysis or physical modelling, form the basis of most (model-based) energy maximising control strategies available in the literature, where experimental design and system identification methodology directly impact the resulting model. This study, using an experimental-based WEC model (which can be used for linear control design), investigates the dynamic behaviour of a WEC by analysing the dominant poles of the system, generated using fully nonlinear computational fluid dynamics (CFD)-based numerical wave tank (NWT) experiments. The aim is to effectively track the dominant dynamics of the WEC, using different force-input amplitude levels in the NWT setup, and perform a comparison with the classical linear boundary-element-methods (BEM) equivalent methodology. Thus, the presented case studies are shown to agree with previously proposed model assessment of linear WEC models, based on a free-decay NWT setup. In addition, the representative WEC models determined as part of this study can be used for WEC controller design, either singly, or using a form of model/controller gain scheduling.

Experimental Assessment of Generalised Super-Twisting Control for Optimal Reference Tracking in Wave Energy Systems

Maximising wave energy extraction is essential for the successful operation of wave energy converters, and appropriate control strategies play a vital role in achieving this objective. This paper presents a control scheme for an experimental wave energy converter setup. The strategy combines an optimal moment-based reference generation technique with a higher-order sliding mode tracking controller, which provides robust tracking of the associated reference, regardless of the uncertainty present in the experimental operation. The device is tested with two different irregular sea states, and results show that the control structure effectively deals with the system's underlying uncertainty, achieving excellent tracking performance while guiding the system to the corresponding surface following the theoretical phase plot convergence. Furthermore, the comparison with a PID tracking control structure indicates that a proper modification of the tracking element of the loop can improve energy extraction. Finally, consistent tracking error and phase plot evolution behaviour are observed for both sea states.

Phase-resolved wave prediction in highly spread seas using optimised arrays of buoys

In many offshore operations, including the operation of wave energy converters, phase-resolved wave prediction is essential for safety and efficiency. However, for sea-states with large directional spreading angles, wave prediction becomes increasingly difficult. This study investigates the benefit of exploiting concurrent surface-displacement time histories measured by a buoy in three degrees of freedom (DoF) to facilitate prediction in such highly spread sea-states. A direct comparison is made with arrays of sensors measuring in a single DoF using the same formulation. The arrays considered are extensively optimised along with multiple sets of representative angles used to describe wave propagation, which is assumed to be normally distributed. The performance of the scheme is tested on a range of synthetically generated, realistic sea-states comprising single and crossing swells prevalent along the south coast of Australia. Additional tests involve wind-sea and swell waves described using Ewans models. Accurate predictions are obtained for times up to three wave periods into the future, in typical cases. With the multiple sets of optimal angles obtained for each optimal array, we further enhance the capability of the scheme by aggregating and averaging the different predictions from these sets. We observe a marked increase in both performance with large spreading and tolerance to possible variation in mean wave directions.

Multi-Agent Reinforcement Learning Controller to Maximize Energy Efficiency for Multi-Generator Industrial Wave Energy Converter

Waves in the oceans are one of the most significant renewable energy sources and are an excellent resource to tackle climate challenges through decarbonizing energy generation. Lowering the Levelized Cost of Energy (LCOE) for energy generation from ocean waves is critical for competitiveness with other forms of clean energy like wind and solar. It requires complex controllers to maximize efficiency for state-of-the-art multi-generator industrial Wave Energy Converters (WEC), which optimizes the reactive forces of the generators on multiple legs of WEC. This paper introduces Multi-Agent Reinforcement Learning controller (MARL) architectures that can handle these various objectives for LCOE. MARL can help increase energy capture efficiency to boost revenue, reduce structural stress to limit maintenance cost, and adaptively and proactively protect the wave energy converter from catastrophic weather events preserving investments and lowering effective capital cost. These architectures include 2-agent and 3-agent MARL implementing proximal policy optimization (PPO) with various optimizations to help sustain the training convergence in the complex hyperplane without falling off the cliff. Also, the design for trust assures the operation of WEC within a safe zone of mechanical compliance. As a part of this design, reward shaping for multiple objectives of energy capture and penalty for harmful motions minimizes stress and lowers the cost of maintenance. We achieved double-digit gains in energy capture efficiency across the waves of different principal frequencies over the baseline Spring Damper controller with the proposed MARL controllers.

Reactive control of wave energy devices – the modelling paradox

The implementation of energy maximising control systems (EMCSs) in wave energy converter (WEC) devices is an important step towards commercially viable operation of WECs. During the design stage of such EMCSs, linear hydrodynamic models are commonly used and are, in fact, the most viable option due to the real–time computational requirements of optimisation routines associated with energy-maximising optimal control techniques. However, the objective function of EMCSs, i.e. maximising the generated power by exaggerating WEC motion, inherently violates the underlying assumption of the linear hydrodynamic control design models, i.e. small amplitude device motion (compared to the device dimensions). Consequently, the linear models, used as a basis for EMCSs, in fact conspire to violate the very assumption upon which they were built - hence leading to a modelling paradox. It is important to evaluate WEC controllers in realistic physical or numerical environments, to gain knowledge of the disparity between the performance prediction from the EMCS design and performance evaluation models. This paper presents a comprehensive assessment of the performance prediction by a linear and non–linear hydrodynamic model of three different EMCSs, implemented in two different WEC structures, in an attempt to quantify the severity of this modelling disparity, or paradox.

Mathematical Modelling of Mooring Systems for Wave Energy Converters—A Review

Mathematical analysis is an essential tool for the successful development and operation of wave energy converters (WECs). Mathematical models of moorings systems are therefore a requisite in the overall techno-economic design and operation of floating WECs. Mooring models (MMs) can be applied to a range of areas, such as WEC simulation, performance evaluation and optimisation, control design and implementation, extreme load calculation, mooring line fatigue life evaluation, mooring design, and array layout optimisation. The mathematical modelling of mooring systems is a venture from physics to numerics, and as such, there are a broad range of details to consider when applying MMs to WEC analysis. A large body of work exists on MMs, developed within other related ocean engineering fields, due to the common requirement of mooring floating bodies, such as vessels and offshore oil and gas platforms. This paper reviews the mathematical modelling of the mooring systems for WECs, detailing the relevant material developed in other offshore industries and presenting the published usage of MMs for WEC analysis.

Experimental study of the functionality of a semisubmersible wind turbine combined with flap-type Wave Energy Converters

In the present paper the functionality of the Semisubmersible wind energy and Flap-type wave energy Converter (SFC) is examined experimentally. In order to study the functionality of the SFC, the focus is on operational environmental conditions. SFC is a combined concept that utilizes offshore wind energy and ocean wave energy for power production. Details are presented as far as the physical modelling of the wind turbine with the use of a redesigned small-scale rotor and of the Power Take-Off mechanism of the Wave Energy Converters (WECs) with the use of a configuration that is based on a mechanical rotary damper. Tests with quasi-static excitation, motion decay, regular and irregular waves without and with wind that is uniform are conducted on an 1:50 scale physical model. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model using Simo/Riflex tool. A good agreement is observed between experimental and numerical predictions. The combined operation of WECs doesn't affect the tension of mooring lines nor the acceleration of nacelle and the bending moment in tower's base. The produced power of the WECs of the SFC and consequently the functionality of the SFC is estimated.

A hybrid genetic algorithm—extreme learning machine approach for accurate significant wave height reconstruction

Wave parameters computed from time series measured by buoys (significant wave height Hs, mean wave period, etc.) play a key role in coastal engineering and in the design and operation of wave energy converters. Storms or navigation accidents can make measuring buoys break down, leading to missing data gaps. In this paper we tackle the problem of locally reconstructing Hs at out-of-operation buoys by using wave parameters from nearby buoys, based on the spatial correlation among values at neighboring buoy locations. The novelty of our approach for its potential application to problems in coastal engineering is twofold. On one hand, we propose a genetic algorithm hybridized with an extreme learning machine that selects, among the available wave parameters from the nearby buoys, a subset FnSP with nSP parameters that minimizes the Hs reconstruction error. On the other hand, we evaluate to what extent the selected parameters in subset FnSP are good enough in assisting other machine learning (ML) regressors (extreme learning machines, support vector machines and gaussian process regression) to reconstruct Hs. The results show that all the ML method explored achieve a good Hs reconstruction in the two different locations studied (Caribbean Sea and West Atlantic).

Power optimisation of a point absorber wave energy converter by means of linear model predictive control

This study presents a model predictive control (MPC) scheme for a wave energy converter (WEC); in particular, for a buoy-type point absorber. The WEC is a two-body system which is taut-moored to the sea floor with three cables. Much research has been done recently to achieve optimal operation of WECs. The goal is to maximise the power conversion without violating system limits. In practice, there are physical constraints on position, velocity and the power take-off (PTO) force. MPC is a promising and beneficial approach to achieve this goal. It poses a control formulation including constraints in a natural way. Furthermore, MPC can exploit predictions for the sea motion a standard MPC approach always needs a reference trajectory. For one-body point absorber, an optimal velocity trajectory can be calculated. However, an optimal trajectory is not easily available for the two-body case. The proposed formulation in the presented work does not require an optimal trajectory. For this reason it is possible to apply this MPC scheme to a two-body model as well. This work demonstrates that the proposed control algorithm optimises the power extraction without violating the system constraints. Finally, the performance of MPC is compared to linear passive load control through simulation.

Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter

The question of optimal operation of wave-energy converters has been a key issue since modern research on the topic emerged in the early 1970s, and criteria for maximum wave-energy absorption soon emerged from frequency domain analysis. However, constraints on motions and forces give the need for time-domain modeling, where numerical optimization must be used to exploit the full absorption potential of an installed converter. A heaving, semisubmerged sphere is used to study optimal constrained motion of wave-energy converters. Based on a linear model of the wave-body interactions, a procedure for the optimization of the machinery force is developed and demonstrated. Moreover, a model-predictive controller is defined and tested for irregular sea. It repeatedly solves the optimization problem online in order to compute the optimal constrained machinery force on a receding horizon. The wave excitation force is predicted by use of an augmented Kalman filter based on a damped harmonic oscillator model of the wave process. It is shown how constraints influence the optimal motion of the heaving wave-energy converter, and also how close it is possible to approach previously published theoretical upper bounds. The model-predictive controller is found to perform close to optimum in irregular waves, depending on the quality of the wave force predictions. An absorbed power equal to or larger than 90% of the ideal constrained optimum is achieved for a chosen range of realistic sea states. Under certain circumstances, the optimal wave-energy absorption may be better in irregular waves than for a corresponding regular wave having the same energy period and wave-power level. An argument is presented to explain this observation.

Certification of Marine Energy Converters: The Experience so Far

In the past few years, DNV has been involved in a variety of projects related to marine energy converters. All projects have been characterized for the handling of technical uncertainties due to the application of new technology or proven technology in different areas of application. A systematic approach based on the DNV RP-A203 Qualification of New Technology was applied combined with the Guidelines for Design and Operation of Wave Energy Converters (May 2005—work carried out by DNV under Commission of Carbon Trust as part of Marine Energy Challenge) to steer the third party activity, but, more importantly, to allow developers to systematically identify and deal with the risks in a rational manner with traceability of decisions throughout the development of the energy converter. From the very start of our engagement, it was clear that the handling of technical uncertainties was affected, not only by the technical barriers, but also by financial and time constraints. The establishment of the safety and functional targets to be achieved by the energy converter are to be based, not only on the safety and asset integrity aspects, but also on the financial∕business model. The experience of using the qualification process and the guidelines on these projects, achieving the right balance between the constraints, handling of uncertainties, financial targets, and safety and functional requirements, are briefly described in this paper as well as the future steps to be taken to improve the process and consolidate the experience so far. In this paper, the use of the DNV OSS-312 (Certification of Wave and Tidal Energy Converters) on the certification process of marine energy converters is also addressed.