Model Of Wave Energy Converter Research Articles

High-fidelity numerical modelling of a two-WEC array with accurate implementation of the PTO system and control strategy using DualSPHysics

In order for Wave Energy Converters (WECs) to become economically viable they will have to be installed closely together in so-called WEC arrays. When WECs are placed closely together they influence each other’s motion, which is challenging to implement in a numerical model. The accurate modelling of WECs in WEC arrays is crucial for their further development into cost-effective renewable energy converters. This paper presents the study of numerical modelling of an array of two heaving WECs (a two-WEC array) placed in close proximity of each other in order to increase their interaction effect. The numerical model uses DualSPHysics, a Smoothed Particle Hydrodynamics (SPH) based software to model wave-WEC interaction, coupled to Project Chrono for the modelling of the Power Take-Off (PTO) system and the friction. First, a series of numerical system identification tests are simulated: a free-decay test of an isolated WEC, radiation tests and an excitation test. During the radiation test one WEC is moved by an external force with a chirp-up noise signal, while the other WEC is kept still. This allows the computation of the impedance matrix, and thereby also the hydrodynamic coefficients of the two-WEC array, which has not been done before in SPH. Results were validated with experimental data from the WECfarm test campaign. Secondly, a series of numerical dynamic tests are simulated of the two-WEC array in regular and irregular waves. In the experimental test campaign a friction force caused by the motor and gearbox was counteracting the WEC’s motion, so a friction compensation (FC) force was added to cancel out this friction. This friction and FC force was numerically modelled by adding a novel implementation in Project Chrono. Furthermore, a novel global control strategy was implemented using Project Chrono, in which the PTO force on each WEC depends on the velocity of both WECs. Overall, good to very good agreement was found in terms of heave and free surface elevation when comparing numerical to experimental results.

M4 WEC development and wave basin Froude testing

The M4 wave energy converter (WEC) is a moored, multi-float, multi-PTO (power takeoff), multi-mode, attenuator-type system designed to be capable of order MW capacity. The strategy is first described with technical requirements. Wave basin testing was undertaken to demonstrate the system and validate linear diffraction-radiation modelling, used for initial assessment and to generalise the configurations. Froude-scale modelling including power takeoff has been demonstrated for the first, possibly only, time. Further wave basin testing was undertaken to validate a multi-float generalisation. Moorings are a critical component with the elasticity of (synthetic) cables reducing snap loads markedly but with complex dynamics requiring wave basin testing and further model development. Preliminary results are presented. Limitations of wave basins with regard to the control of wave (and current) conditions and associated measurement are discussed along with suggested improvements. Computational modelling ranges from efficient linear diffraction-radiation modelling to massively resource-intensive computational fluid dynamics (CFD). The former extended to second order is of main practical importance and is well suited to generalise WEC configurations and model complexity, such as nonlinear power takeoff control and moorings, but are limited in accurately representing physics such wave nonlinearity, slamming and viscous effects. However, CFD has yet to become a practical tool for realistic irregular wave conditions. Wave basin modelling based on Froude scaling is of vital importance for both concept exploration and model validation.

Reliability modeling of wave energy converters based on pelamis technology

Wave generators, as renewable sources, are increasingly applied for power generation in developed electric networks. With the development of wave generator technologies, large-scale devices such as oscillating water columns, pelamis, wave-dragon, oyster, buoy, sea wave slot cone converter, and tapchan have been invented to extract electrical power from wave energies. The output of wave generators relies on the height and period of waves, and because of the wide variation in these quantities, the produced power from them changes widely. Therefore, in electric networks incorporating large wave generators, reliability is influenced and should be analyzed. In this paper, for the first time, the reliability modeling of pelamis, as a kind of commercial-scale wave generator, is executed. In the proposed multi-state reliability model of pelamis, failures of composed components and changes in the production of power are addressed. To obtain the optimal number of power states in the model, the XB factor is computed. Then, to reduce the number of power states in the model, the fuzzy c-means clustering methodology is applied. The obtained model is used to examine the adequacy assessment of an electric network containing a pelamis farm. Numerical results show that wave converters can improve the reliability performance of electric systems. However, wave period and height affect the reliability indices of the power system. The effectiveness of the proposed method is validated by comparing the outcomes obtained by the analytical and Sequential Monte Carlo Simulation methods.

Spectral-Domain Modelling of Wave Energy Converters as an Efficient Tool for Adjustment of PTO Model Parameters

The power take-off (PTO) system is a core component in WECs as it plays a critical role in power production. In numerical models the PTO systems are commonly represented and simplified through a combination of linear stiffness and damping terms in the equations of motion. These parameters are influential to the dynamic response and thus affect the power performance of WECs. In the preliminary design and optimization of WECs, proper tuning of the PTO damping and stiffness could reflect better the potential of the concept. In practice, the PTO damping and stiffness are tuned to maximize the absorbed power by achieving the desired velocity amplitude or phase of the velocity with respect to the excitation force. However, recent literature has indicated that the selection of PTO parameters for maximum mechanical power absorption is not necessarily optimal for the maximum production of electrical power when the conversion efficiency of the electrical machine is included. To obtain these parameters which maximize the delivered electrical power, wave-to-wire models are widely used. Nevertheless, wave-to-wire models are predominately established by using time-domain models which can be associated with large computational efforts from the perspective of early-stage design and concept evaluation. To tackle this challenge, a spectral-domain-based wave-to-wire model is proposed to cover both hydrodynamic and electrical responses. In this paper, a spherical heaving point absorber integrated with a linear permanent-magnet generator is used as reference. The relevant nonlinear effects are incorporated by statistical linearization using spectral-domain modelling. In particular, the nonlinear effects considered in this work include the viscous drag force, the electrical current saturation and the partial overlap between the translator and stator components of the linear generator. The model results are then verified against a nonlinear time-domain-based wave-to-wire model. Subsequently, the proposed model is applied to identify the PTO parameters for maximizing the electrical power in various wave states. The computational efficiency and accuracy of the proposed spectral-domain model are compared with the time-domain model, with regard to the identification of the proper PTO damping and stiffness. Based on the results, the advantage of using the spectral-domain-based wave-to-wire modeling in PTO tuning is demonstrated.

Comparing Numerical Models of the TALOS Wave Energy Converter

In this study, we present the development of a numerical model of the TALOS Wave Energy Converter (WEC) in WEC-Sim and compare it with existing numerical in-house models from Lancaster and results from DNV's Sesam code. The objective of this work is to validate the performance of the TALOS WEC using WEC-Sim and compare the results with those obtained from other numerical models. The TALOS WEC is a promising technology for the generation of clean, renewable energy from ocean waves. The development of a reliable numerical model for the TALOS WEC is crucial for its design and optimization. To achieve this, we have implemented the TALOS WEC in WEC-Sim and compared the results to in-house models from Lancaster and to results from DNV's Sesam code. Our results show good agreement with the results from Lancaster and DNV, demonstrating the accuracy and reliability of the numerical model developed in WEC-Sim. This work advances the field of wave energy conversion by providing a verified numerical model of the TALOS WEC, which can be used for further optimization and design studies.

Wave Excitation Tests on a Fixed Sphere

Wave excitation tests on a fixed sphere with the center at the still water level were carried out with three different physical wave basin setups. The tests were completed as a continued effort of the working group OES Wave Energy Converters Modelling Verification and Validation to increase confidence in numerical models of wave energy converters by generation of accurate benchmarks datasets for numerical model validation. An idealized testcase with wave excitation of a fixed sphere to be used with the benchmarks was formulated. The three investigated physical wave basin setups included: 1) a six degree-of-freedom load cell mounted to the top of the sphere, 2) a bending beam force transducer mounted to the top of the sphere, and 3) a system of six pretensioned wires mounted to the top and bottom of the sphere with force transducers attached to each wire. The aim of the present paper is to identify the best representation of the idealized testcase. To this end, the three experimental setups are inter-compared in terms of dynamic properties, sensitivity, and disturbances of the water phase from the presence of measurement equipment. Low inter-experiment variability was disclosed, i.e., 5-8% depending on wave-nonlinearity, indicating accurate representations of the idealized testcase across all setups. Setup 3 was found to be the more accurate representation and further work with this setup to release a public benchmark dataset was planned.

Dynamic Simulation of Wave Point Absorbers Connected to a Central Floating Platform

AmAmong the challenges currently being faced by the wave energy industry, there are the ones related to the mathematical and numerical modelling of Wave Energy Converters. Because various levels of physical complexity are reflected in the dynamics of wave converters, the mathematical modelling of such systems usually comes up with nonlinear dynamic equations to be solved. The nonlinearities, however, may appear in many ways. In this paper, the nonlinear geometric constraints that arise naturally in hinged structures are investigated for floating multi-body systems including wave point absorbers. To achieve that, a method of constraint linearization is proposed and applied to a realistic case study. The method is based on generalized coordinates and generates a robust first-order dynamic matrix to characterize the multi-degrees of freedom hydrodynamic system. The simulation outputs the motion response for all floating bodies, as well as the constraining forces responses, among other parameters. The method requires knowledge of the geometries of the system but rather few assumptions, namely, to perform the linearization of constraints. The method is illustrated with a case study, where three wave point absorbers are concentrically attached to a Floating Offshore Wind Turbine platform with an onboard hydraulic Power-Take Off system.

Quantification of uncertainty in linear wave energy hydrodynamic models from experimental data

Considerable testing and modeling are required in order to fully realise, efficiently develop, and successfully industrialise the wave energy converters (WECs). Numerical modeling, full-scale measurements, and scaled prototype testing are the various methodologies that can be applied to model WECs and predict the dynamic response. Mathematical WEC models form the basis of model-based energy maximising control and directly affect the ability of model-based controllers to maximise energy capture. Linear WEC models are attractive in leading to simpler control designs, but may not cover the complete operational space. One solution is to identify a range of linear models at different operating points, which give a measure of the underlying nonlinear behaviour [1], [2]. This model set can then be used to extract a nominal model, and an associated uncertainty region, which can be used as a basis for a robust WEC controller synthesis process, such as articulated in [3].
 Recently, such an approach has been adopted using data generated from a high-fidelity numerical computational fluid dynamics (CFD) model [4]. However, numerical wave tanks (NWTs) and physical wave tanks differ significantly in terms of the range of tests which can be performed, and the contamination which can affect the measurements used to determine the data-based models e.g. measurement noise, numerical effects, wave reflections, etc [5]. As a result, the determination of nominal models and uncertainty regions in a physical wave tank may provide some advantages (and disadvantages) which need to be examined carefully. In addition, the range of post-processing techniques which could, or should, be applied to the different experimental/numerical domains, to improve the fidelity of the identified models, may differ between domains.
 In this paper, experimental testing of a WEC, by recreating a wave field similar to real-life conditions and a small-scale version of the device, is used to understand the hydrodynamic behaviour and to obtain an accurate dynamic model for WECs, which are considered to be essential towards optimal WEC design. Physical wave tank experiments, even though having their own disadvantages, overcome some difficulties of CFD-based NWT experiments, most notably huge computation time, problems in accurate representation of viscous fluids, uncertainty in the specification of an appropriate turbulence model, and propagation of incident waves [6]. 
 In this study, representative linear models of a point-absorber type WEC from a physical wave tank in the wave basin at Aalborg University are determined which give insight into the system dynamics and provide a basis for robust control of WECs. Among different stimulation techniques to excite the system dynamics in physical wave tank tests, the particular types of excitation signals covering the complete range of frequencies and amplitudes of the system dynamics, while considering limitations on the range of excitation signals or the wave tank reflections, are adopted for the determination of representative linear models. Moreover, a full investigation is carried out to ascertain the appropriate post-processing needed to optimise the signals as the basis for model identification. The model identification includes the non-parametric frequency response by means of empirical transfer function estimate (ETFE). 
 

Ensuring Resilience in Ocean Energy Power Plants: A Survey of Cybersecurity Measures

Offshore Renewable Energy (ORE) is a promising solution to address the challenges of climate change and the depletion of fossil fuels [1]. Wave power, a form of ORE, is considered one of the purest energy sources with significant growth potential [2]. In addition to investing in these energy sources, nations are also working to enhance the protection of Critical Infrastructure (CI). CI encompasses all services crucial to the functioning of society and the economy, including electric power systems and their various forms of generation, such as renewable energy sources. Hence, in addition to exploring various forms of power generation, the cybersecurity of the networks connecting the devices in these systems is a crucial aspect to consider to prevent attacks and minimize the risk of cyber threats to suppliers and customers [3]. For instance, the European Commission states that reducing CI vulnerability and increasing its resilience is one of the main objectives of the European Union.
 However, to date, a comprehensive review that synthesizes the various approaches to cybersecurity in ocean energy is yet to be published. The objective of this study is to present a comprehensive survey of the application of cybersecurity measures to renewable energy sources, with a specific focus on ocean energy. A systematic review of the literature was carried out, following the steps outlined by Kitchenham [4]. The methodology steps are illustrated in the flowchart (see Figure 1). Of the 49 articles selected, three main study topics emerged: i) smart ocean, ii) cybersecurity for renewable energy systems, and iii) marine data security. These three topics are interrelated as a smart ocean can be considered as an integrated sensing, communication, and computing ecosystem that connects marine objects in surface and underwater environments [5]. Once the wave energy converters (WECs) are installed, it is also essential to develop safety systems for these devices, as demonstrated in the first report on cybersecurity guidance for MRE (Marine Renewable Energy) systems [6] prepared by the Pacific Northwest National Laboratory (PNNL). In preparation for this report, researchers reviewed the cyber threats and vulnerabilities of information technology (IT) and operational technology (OT) equipment used in various WEC models. Figure 2 presents an example of the possible threats and attacks on WEC devices.
 In conclusion, this article provides a comprehensive survey of the application of cybersecurity measures in ocean energy, highlighting the importance of reducing vulnerability in the cybersecurity of power plants in this sector. Through a systematic review of the literature, three main study topics were identified and analysed, providing a valuable resource for future research in this area. The findings of this study can inform and guide the development of more secure and resilient systems, contributing to the overall improvement of critical infrastructure in the field of ocean energy. As such, this article offers a significant contribution to the ongoing effort to address the challenges posed by the changing energy landscape and the need to protect critical infrastructure from cyber threats.
 Please refer to uploaded PDF to see references and figures.
 

A CFD-FEM analysis for Anaconda WEC with mooring lines

Anaconda wave energy converter (WEC) [1] is a typical flexible tube WEC design. In the past decade, the hydro-elastic performance of Anaconda model has been studied through experiments [2-3] and reduced-order numerical simulations [4-5]. However, to date, most of the undertaken research assumes that both ends of the tube are fixed, neglecting mooring lines that exist at designed conditions. The tube is allowed to move freely head to waves when mooring lines are considered. This is evident by an experiment conducted by Checkmate Flexible Engineering Ltd., where they observed a significant heave motion of Anaconda WEC, which may impact the dynamic response of WEC and further the energy conversion [6]. In our previous work [7], supported by the EPSRC project BASM-WEC (No. EP/V040553/1), a coupled numerical analysis tool using computational fluid dynamics (CFD) and finite element method (FEM) has been proposed to perform a numerical study for an Anaconda WEC model without considering mooring lines.In this paper, the coupled fluid-structure interaction (FSI) analysis tool [7] based on CFD-FEM method will be used to investigate Anaconda WEC with mooring lines. To account for the effects of free motion of WEC, we develop a sub-module to be embedded into the existing tool. The fluid and structure are solved by a two-phase CFD solver developed based on OpenFOAM and a three-dimensional FEM code CalculiX, respectively. The strong coupling between OpenFOAM and CalculiX is achieved by a multi-physics coupling tool preCICE.For the flexible tube, a commercial Natural Rubber material is studied. The hyper-elastic model YEOH is utilized to describe the nonlinear behaviour of material, and the model constants are calculated from the bi-axial tests. With the tool developed, numerical simulations are performed for different given regular waves. The flow details, velocity and pressure variation, structure deformation and stress distribution will be fully examined to better understand energy conversion performance of Anaconda WEC with mooring lines.
 References[1] Farley F J M D, Rainey R C T. Distensible tube wave energy converter: U.S. Patent 7,980,071[P]. 2011-7-19.[2] Chaplin J R, Heller V, Farley F J M, et al. Laboratory testing the Anaconda[J]. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012, 370(1959): 403-424.[3] Heller V, Chaplin J R, Farley F J M, et al. Physical model tests of the anaconda wave energy converter[C]//Proc. 1st IAHR European Congress. 2000.[4] Farley F J M, Rainey R C T, Chaplin J R. Rubber tubes in the sea[J]. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012, 370(1959): 381-402.[5] Babarit A, Singh J, Mélis C, et al. A linear numerical model for analysing the hydroelastic response of a flexible electroactive wave energy converter[J]. Journal of Fluids and Structures, 2017, 74: 356-384.[6] Checkmate Flexible Engineering, https://www.checkmateukseaenergy.com/, accessed 15th December 2022.[7] Huang Y, Xiao Q, Idarraga G, et al. Numerical analysis of flexible tube wave energy converter using CFD-FEA method[C]//International Conference on Offshore Mechanics and Arctic Engineering. American Society of Mechanical Engineers, 2023 (submitted).

A spectral-domain wave-to-wire model of wave energy converters

Wave-to-Wire models play an important role in the development of wave energy converters. They could provide insight into the complete operating process of wave energy converters, from the power absorption stage to the power conversion stage. In order to cover a set of relevant nonlinear effects, wave-to-wire models are predominately established in the time domain. However, the low computational efficiency of time-domain modeling is hindering the extensive application of wave-to-wire models, especially in early-stage design and optimization where a large number of iterations are required. To address this issue, a spectral-domain wave-to-wire model is proposed, and the nonlinear effects are incorporated by stochastic linearization. This model can significantly reduce the computational load and maintain good accuracy. The reference concept studied in this paper is defined as a heaving point absorber coupled with a linear permanent-magnet generator. Four representative nonlinear effects involved in both the hydrodynamic stage and the electrical stage of the concept are considered. The proposed model is verified against a corresponding nonlinear time-domain wave-to-wire model, and a good agreement is observed. The relative error of the proposed spectral-domain wave-to-wire model is around 2 % in typical operational regions and is still within 7 % for wave states with large significant wave heights, regarding the estimate of the power conversion efficiency. Meanwhile, the computational load of the spectral-domain wave-to-wire model is reduced by 2 to 3 orders of magnitudes compared with the conventional time-domain approach. Finally, a case study of tuning the PTO damping to maximize power production is conducted to demonstrate the performance of the proposed spectral-domain wave-to-wire model.

Machine learning and particle swarm inspired success history based adaptive multi-objective differential evolution for optimization of heaving buoy point absorber

Success-History-Based Parameter Adaptation Multi-Objective Differential Evolution (SHAMODE) has shown excellent performance recently compared to other multi-objective evolutionary algorithms (MOEAs) and therefore caught the interest of researchers lately. The SHAMODE framework employs historical memory of control parameters, which stores set values of control parameters that have performed well in the past generation and produces new set values of the parameter by directly sampling the parameter space close to one of these stored pairs. Since the algorithm's performance depends on the problem to be optimized, this paper proposes an enhanced version of SHAMODE that is compatible with optimizing the multi-objective wave energy converter problem. The improved version of SHAMODE is achieved by incorporating competing machine learning algorithms, Particle Swarm Optimization, and a modified bubble-net attacking method formula of the Whale optimization algorithm from its original form. Simulations and comparisons are used to verify the proposed method's performance based on some famous multi-objective problem (MOP) benchmark functions. The results reveal that the proposed algorithm performs better than other algorithms that have been recently developed. In addition, the suggested technique is used to optimize the multi-objective wave energy converter model in order to maximize the annual production of electric energy while minimizing the per-unit energy cost.

New Strategy on Power Absorption of a Concentric Two-Body Wave Energy Converter

The wave energy converter (WEC) system, which extracts electricity through the relative heave motion of two concentric cylinders, comprises an inner cylinder and a torus-type outer cylinder sliding along the inner cylinder. To maximize the relative heave motion between two cylinders, the natural frequencies of the two cylinders must be precisely tuned to resonate and be situated on each side of the peak frequency of the wave spectrum. However, the demerit of this strategy is that it demands a deep draft of each cylinder for tuning, and a large-scale PTO damping device is necessary for mechanical power amplified by resonance. As an alternative to efficient and stable WECs, we adopt a new strategy in which the outer cylinder follows the incoming waves and the motion of the inner one is restricted to be minimal using a heave disk. The viscous damping due to formation of vortices at the disk edge is realized by the drag force in the Morison equation. The developed hydrodynamic model of two-body WEC based on a matched eigenfunction expansion method (MEEM) is applied to irregular waves characterized by significant wave height and peak period. It is found that the present two-body WEC with heave disk produces wave energy stably across a wide range of wave frequencies compared to the previous two-body WECs using resonance.

Modeling of a hinged-raft wave energy converter via deep operator learning and wave tank experiments

Model identification for a hinged-raft wave energy converter (WEC) is investigated in this paper, based on wave tank experiments and deep operator learning. Different from previous works which all formulated this issue as a function approximation task, this work, for the first time, formulates it as an operator approximation task (which learns the mapping from a function space to another function space). As such, a continuous-time WEC model is identified from data, greatly expanding the horizon of data-based WEC modeling because previous works were limited to discrete-time model identification. The error accumulation for multi-step predictions in the discrete-time formulation is thus also addressed. The model is developed by first carrying out a set of wave tank experiments to generate the training data, and then the deep operator learning model, i.e. the DeepONet, is constructed and trained based on the experimental data. The validation study shows that the model captures the WEC dynamics accurately. A new set of experimental runs are further carried out and the results show that after training, the model can be used as a digital wave tank, an alternative to the expensive numerical and physical wave tanks, for accurate and real-time simulations of the WEC dynamics.

Validation of a control-oriented point vortex model for a cyclorotor-based wave energy device

Recently conducted analytical assessment of the potential performance of cyclorotor wave energy converters (WECs) have shown that such devices offer the best wave absorption behaviour, if energy capture can be optimised through suitable control. Such claims require additional investigation. This article is dedicated to validation of the control-oriented point vortex model of cyclorotor WECs against numerical and experimental assessments conducted by various research groups. The validation is conducted in terms of the traditional metrics for cyclorotor WECs: (a) cancellation of incoming waves; (b) generation of lift and drag forces (c) mechanical power generation.It is shown that the point vortex model generally confirms the previously conducted analytical assessment of device performance. However, accounting for the influence of the hydrofoil induced wakes decreases performance estimates to some extent. It is also shown that, overall, wave cancellation metrics are more optimistic than actual shaft power generation.Analysis of the lift and drag coefficients, which were derived from experimental data, reveal a range of hydrodynamic and mechanic effects which could influence actual device performance. It has been shown that, due to the complexity of hydrodynamic effects, lift and drag coefficients for the control-oriented model should be considered not only as functions of the Reynolds number and angle of attack, but also related to submergence of the foils and direction of their rotation with respect to the free surface. This method allows us to achieve the best validation against experimental results in terms of generation of tangential and radial forces.

Research on Laboratory Test Method of Wave Energy Converter Wave-Wire Conversion Ratio in Irregular Waves

The laboratory test of the wave energy converter model is an important means to evaluate the performance of the device. At present, there are few performance tests for complete specifications under the irregular wave. Referring to the test methods and standards at home and abroad, combined with the actual test work experience in the laboratory, using the irregular wave power calculation formula with the effective wave height and the spectral peak period as parameters, then the wave-wire conversion ratio test method of the wave energy converter physical model under irregular waves in the laboratory is proposed. The method is applied to the basin test experiment of the physical model of the horn-shaped backward bent duct buoy (BBDB) wave energy converter. The research results show that the established test method and process of wave-wire conversion performance have achieved good application results in the irregular waves laboratory test, and can better reflect the device operating characteristics in real sea conditions. The test results provide data support for the model design of the wave energy converter in the next test stage, the demonstration test of the prototype, and the prediction of power generation in real sea conditions.

Offshore Measurements and Numerical Validation of the Mooring Forces on a 1:5 Scale Buoy

Wave energy conversion is a renewable energy technology with a promising potential. Although it has been developed for more than 200 years, the technology is still far from mature. The survivability in extreme weather conditions is a key parameter halting its development. We present here results from two weeks of measurement with a force measurement buoy deployed at Uppsala University’s test site for wave energy research at the west coast of Sweden. The collected data have been used to investigate the reliability for two typical numerical wave energy converter models: one low fidelity model based on linear wave theory and one high fidelity Reynolds-Averaged Navier–Stokes model. The line force data is also analysed by extreme value theory using the peak-over-threshold method to study the statistical distribution of extreme forces and to predict the return period. The high fidelity model shows rather good agreement for the smaller waves, but overestimates the forces for larger waves, which can be attributed to uncertainties related to field measurements and numerical modelling uncertainties. The peak-over-threshold method gives a rather satisfying result for this data set. A significant deviation is observed in the measured force for sea states with the same significant wave height. This indicates that it will be difficult to calculate the force based on the significant wave height only, which points out the importance of more offshore experiments.

Numerical study on the hydrodynamic performance of a symmetrical dual-chamber oscillating water column wave energy converter

In this study, a novel floating oscillating water column (OWC) wave energy converter (WEC) with dual chambers is proposed, and its hydrodynamic performance and primary energy conversion characteristics are investigated by numerical calculation. It consists of a floating body and two long vertical pipes opening downward at the bottom, forming dual chambers at the top. These two rectangular pipes are fixed to the front and back ends of the buoyancy tank with the same width, and it can be regarded as an oscillating single floating body as a whole. Under the action of the incident wave, the WEC captures wave power by the heave motion and relative motion of the pipe and the water column in it to form an oscillating water column, outputting pneumatic power. The geometry size of the vertical pipes is optimized by comparing the hydrodynamic performance and capture width ratio (CWR) of the WEC models with several rectangular pipes of different sizes. The calculation results show that increasing the draft, which is positively correlated with the total mass of the WEC model, increases its optimal response period. By comparing the numerical calculation results of the hydrodynamic performance and CWR of the WEC models with three kinds of floater bottom shapes, semi-cylindrical, sharp-bottomed, and flat-bottomed, it is found that the flat-bottomed model has the best capture performance.

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.

Bayesian Actor-Critic Wave Energy Converter Control With Modeling Errors

This paper presents a comparison of a Reinforcement-Learning (RL) based wave energy conversion controller against standard reactive damping and model predictive control (MPC) approaches, in the presence of modeling errors. Wave energy converters (WECs) are under the influence of many non-linear hydrodynamic forces, yet for ease and expediency, it is common to formulate linear WEC models and control laws. Therefore it is expected that significant modeling errors may be present, which may degrade model-based control performance. Model-free RL approaches to control may offer a significant advantage in robustness to modeling errors, in that the model is learned by the controller by experience. It is shown that, for an annual average sea state, RL-based controllers can outperform model-based control – reactive control and MPC – by 19% and 16%, respectively, when significant modeling error is present. Furthermore, compared to similar studies of RL-based control, the proposed model can reduce the training time from 8.4 hr to 1.5 hr.