Wave Energy Conversion System Research Articles

Modeling and Analysis of an Inertia Wave Energy Converter and Its Optimal Design

A novel structural design of a wave energy converter (WEC) is proposed, utilizing a gyroscope as the main component for energy absorption. A vacuum-sealed, high-speed flywheel is implemented within the gyroscope to convert energy and reduce energy loss from air resistance. A dynamic model that considers the response of the floater, gyroscope, and power take-off (PTO) system with multi-degree-of-freedom (DoF) wave excitations is created. The primary parameters such as flywheel speed, damping, and stiffness of the WEC system that affect the gyroscope and PTO on energy absorption are analyzed through numerical simulations. The simulation results demonstrate that changes to these parameters can significantly impact the energy-absorption peak value. Furthermore, an improved multi-objective evolutionary algorithm based on the decomposition (MOEA/D) algorithm is applied to optimize key parameters, such as the power conversion and gyro precession of the WEC system, using a hybrid constraint handling strategy for enhanced diversity and the HV value as the evaluation criterion. The optimal design solution is selected from the Pareto solution set using a technique for order preference by similarity to the ideal solution (TOPSIS) method based on entropy weighting so that technical guidance can be provided for the design and control of the WEC system.

Adaptive Fault-Tolerant Control for a 2-Body Point Absorber Wave Energy Converter Against Actuator Faults: An Iterative Learning Control Approach

In this paper, the design issue of adaptive fault-tolerant control (FTC) is investigated for a class of continuous-time 2-body point absorber wave energy converter (WEC) systems against actuator faults based on the iterative learning approach. The actuator faults considered in this paper contain both the lock-in-place and the loss of effectiveness faults, simultaneously. The WEC dynamic equations, including two moving parts (i.e., the float and the spar), are firstly transformed into a state-space model. Then, a group of novel iterative learning based adaptive multiple controllers are developed to decrease the tracking error between the measurement output and the desired output, and two novel adaptive laws are designed to cope with two types of actuator faults. Based on the theories mentioned above, a novel algorithm is provided to present the operation flows of both adaptive laws and iterative learning. Furthermore, a sufficient condition is obtained with the aid of proper Lyapunov function, such that the related closed-loop faulty-WEC system is asymptotically stable with a guaranteed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H_{\infty }$</tex-math></inline-formula> performance index. Finally, an example with a set of physical parameters of a WEC dynamic model is worked out to verify the applicability and effectiveness of the proposed iterative learning based adaptive FTC strategy.

An Investigation on the Integration of a Hybrid Offshore Wind-Wave Energy Conversion System With the Distribution Network

This paper introduces the hybridization of multiple linear permanent magnet generator (LPMG)-based wave energy conversion systems with a doubly-fed induction generator (DFIG)-based wind energy conversion system. A detailed investigation of the impact of the integration of the wind-wave hybrid system with the distribution network is presented in this study. The control scheme for the multiple LPMGs ensures the optimum extraction of the wave power and the ability to maintain voltage balance at the outputs of the LPMGs. Similarly, the back-to-back converters of the DFIG are controlled for the maximum power point tracking of the wind turbine and the dc-link voltage regulation. The turbine rotor blade pitch angle is controlled to minimize the power fluctuations caused by the integration of LPMGs at the common DC bus. The time-domain simulation results confirm the effectiveness of the proposed system by showing a stable operation when integrated with the distribution test feeder. Finally, the steady-state analysis shows that the voltage profile of the distribution network is also improved, and the step voltage regulator of the distribution feeder can be removed in the presence of the proposed system without violating the voltage limits at any node of the selected distribution network. Further, the response of the distribution network is also evaluated under an electrical fault condition in the presence of the wind-wave hybrid system.

Comprehensive wave-to-wire model and control strategy design for wave energy conversion system

Considering the practical application of wave energy generation, a comprehensive wave-to-wire (W2W) model is proposed in this paper. A novel wave energy conversion (WEC) system with a center of gravity adjustment mechanism is proposed. The proposed W2W model involves the coupling modeling between the absorber and the center of gravity adjustment mechanism as well as the coupling modeling between the absorber and the power take-off (PTO) system. Based on the coupling relationship between the absorber and the center of gravity adjustment mechanism, a resonance control strategy is proposed. In addition, based on the coupling relationship between the absorber and the PTO system, an impedance-matching control strategy is proposed. Combined with the above two control strategies, the two conditions of optimal power capture can be met at the same time, so the WEC system can realize the maximum power point tracking (MPPT). In addition, in order to ensure the safe operation of the WEC system under extreme wave conditions, a power constraint control strategy is proposed to constrain the absorbed power. Finally, the feasibility of the W2W model and the accuracy of the proposed control strategies are verified under the regular and irregular waves.

Maximum power point tracking control strategy based on frequency and amplitude control for the wave energy conversion system

This paper presents a pendulum-type wave energy conversion (WEC) system, which can change its natural frequency by a center of gravity adjustment mechanism. Based on this characteristic, a frequency and amplitude control (FAC) strategy is proposed to realize the maximum power point tracking (MPPT) of the WEC system. Firstly, the natural frequency of the WEC system is adjusted according to the peak frequency of the wave spectrum to make the device resonate with the waves, which is the frequency control (FC) method. The essence of the FC method is to change the position of the mass block inside the pendulum by controlling the angular displacement of the motor, so as to change the moment of inertia and recovery stiffness of the device. Secondly, the load of the mass block is very large, which brings a great challenge to the motor starting, so this paper proposes a soft-start control strategy to solve this problem. Thirdly, a weighted average method is proposed to calculate the intrinsic impedance of the WEC system under irregular waves, and then make the mechanical resistance of the power take-off (PTO) system equal to the intrinsic impedance of the WEC system by controlling the damping torque of the generator, which is the amplitude control (AC) method. By combining the FC method and AC method, the amplitude and phase conditions can be satisfied simultaneously to realize the MPPT of the WEC system. Finally, the feasibility and accuracy of the proposed control strategies are verified by simulation results.

A nonlinear generalized predictive control scheme for the oscillating water column plants

Under unknown waves, the efficiency of the wave energy conversion systems can be improved by applying a novel control algorithm that is able to anticipate the behavior of the plant. In this paper, a generalized predictive control scheme for wave power generation systems that includes a Wells turbine and an induction generator is proposed. The main idea is to maintain the flow coefficient at its optimal point for generated power maximization and, at the same time, improve the efficiency of the Wells turbine and avoid the stalling phenomenon by means of regulating the turbine rotational speed. Moreover, by using the capabilities of the predictive controller, the limitations of the stator current and the effect of the mechanical dead time of the system are integrated into the regulator design process, as well as the flux weakening technique. Simulation and experimental results, through a test bench that has been designed and constructed, show the validity and capability of the proposed scheme in both the rated and flux weakening regimes.

Performance of a hybrid TLP floating wind turbine combined with arrays of heaving point absorbers

In the present study, the hydrodynamic performance of circular and concentric arrangements of cone-cylinder-type heaving point absorbers around a Submerged Tension-Leg Platform (STLP) is analysed using the numerical model in the frequency domain based on the potential flow theory. The presence of the Wave Energy Converters (WECs) around the STLP floating wind turbine platform affects the hydrodynamic performance of the hybrid floating platform. So to illustrate the effects of WECs on the platform, the ratio of hydrodynamic coefficients for a single WEC system to that for a hybrid system is analysed. An array of heaving point absorbers is placed in circular and concentric patterns to understand the performance of heaving point absorbers in the absorption of wave energy. The cone-cylinder type heaving point absorber is selected for the present study as they yield more power as compared to other shaped point absorbers. The study compares the wave power absorption of each point absorber around the platform for irregular wave conditions of the North Sea. The effect of incoming waves is illustrated by analysing four different wave heading angles. To quantify the performance of the WECs in an array, the q-factor and coefficient of variation are studied for each array at different sea states. The study suggested the best possible arrangement pattern for wave power absorption and power uniformity among the floaters in the array. The study performed will be helpful in the design and analysis of the possible arrangement of point absorbers around the floating wind turbine platform for wave power absorption.

STUDY OF WAVE HEIGHT POTENTIAL FOR DETERMINING THE LOCATION OF WECS IN THE WATERS OF KETAPANG REGENCY BY USING CEDAS SOFTWARE

Ketapang Regency's increasing demand for electrical energy could lead to an energy crisis. According to these issues, the Sungai Kinjil and Kinjil Pesisir coastal communities in the Ketapang Regency have wave potential that can be used as energy for marine wave power plants. As a result, The expertise required to evaluate ocean wave resources and determine the output power of electrical energy produced by ocean waves. CEDAS and ArcGIS software is used for wave modeling, with CEDAS input data in the form of wave forecasting from BMKG wind data, bathymetry, and coastline data obtained from the official Indonesia Geospatial website, and primary data in the form of wave electric power measurement data obtained from Wave energy conversion system prototypes. CEDAS processing results show the highest wave height of 0,998 meters at a depth of 1,3 meters. The potential for electrical energy at the research site is 1,908–4,512 kw/m2.

Hydroelastic analysis of surface gravity wave interaction with multiple flexible porous structures

A solution to the problem of wave interaction with multiple flexible porous barriers is presented using the Galerkin approximation technique in deep water. The study has analyzed the reflection coefficient, dissipated energy, and wave forces on the barriers and presented results for different inclination angles, separation lengths, porosity, and flexural rigidity of the barriers. The comparison of the results with those available in the literature adds credibility to the study. The finding that the flexural rigidity of the porous barriers has a significant effect on wave reflection, dissipated energy, and wave forces is an important contribution to the field. The observation that an increase in the number of barriers reduces the wave load significantly is also noteworthy. Overall, this study provides valuable insights into the behavior of wave interaction with multiple flexible porous barriers, which could have practical implications for coastal protection and wave energy conversion systems.

SWELL: An open-access experimental dataset for arrays of wave energy conversion systems

Achieving large-scale commercial exploitation of ocean wave energy inherently encompasses the design and deployment of arrays of wave energy converters (WECs), in an effort to reduce the associated levelised cost of energy. In this context, understanding the interactions between devices in a controlled WEC array is hence essential to achieve optimal layout configurations, as well as to provide guidance on the area required for array installation, reliability, life-time, and overall cost of the farm. Successful achievement of these vital objectives for the wave energy industry has been constantly aided by the use of appropriate numerical models. Regardless of the specific modelling approach adopted, model reliability is always a major concern: Numerical models need to able be to represent reality to be useful in supporting the different stages of development, hence providing significant results for decision making. To test reliability of a model, experimental results are an invaluable asset for validation.Recognising the striking absence of real-world data concerning arrays of WEC systems, and its inherent value for model validation and data-based modelling purposes, we present, in this paper, an experimental campaign fully conducted with the sole objective of generating and providing an open-access dataset on WEC farms: SWELL (Standardised Wave Energy converter array Learning Library). The generated dataset, included alongside this manuscript, comprises an approximate total of ∼3000 variables and more than ∼108 datapoints, for up to 5 devices in 9 diverse WEC array layouts with different levels of interaction, and 19 carefully selected operating conditions. Four different categories of tests are considered, providing measures of key variables required for model validation and data-based modelling tasks. As such, SWELL provides a crucial resource to achieve confidence in numerical modelling, helping towards creating reliable tools for decision making in the WEC field, hence effectively supporting the pathway towards effective commercialisation of ocean wave energy.

Predication of Ocean Wave Height for Ocean Wave Energy Conversion System

Ocean wave height is one of the critical factors to decide the efficiency of the ocean wave energy conversion system. Usually, only when the resonate occurs between the ocean wave height (ocean wave speed in the vertical direction) and ocean wave energy conversion system, can the conversion efficiency from ocean wave energy into electric energy be maximized. Therefore, this paper proposes two predication methods to predict the future ocean wave height in 1.5–2.5 s. Firstly, the data fitting of real ocean wave height is achieved by the polynomial method, which is beneficial to the predication of ocean wave height. Secondly, the models of the moving average (MA) predication method and auto regressive (AR) predication method are presented by the time series analysis process. Lastly, after the predication of ocean wave height by the MA method and AR method, and compared with the data fitting result of real ocean wave height, it can be found that the AR method is more accurate for the predication of ocean wave height. In addition, the predication results also indicated that the error between the predication value and true value in the future 2.5 s is considered acceptable, which provides enough time to optimize the operation process of the ocean wave energy conversion system by a suitable control method.

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.

A wave-to-wire analysis of the adjustable draft point absorber wave energy converter coupled with a linear permanent-magnet generator

An adjustable draft point absorber was recently proposed as a novel approach to improve power absorption with constrained power take-off (PTO) capacities. The key feature of the novel wave energy converter (WEC) concept is to adjust the buoy draft by regulating the ballast water inside the buoy, which aims to enable variation of the natural frequency of the WEC. Although previous research has shown benefits for the energy absorption stage, the impact of the draft adjustment on the power conversion efficiency and overall performance has not been examined yet. Therefore, a wave-to-wire model is established to provide an in-depth insight into the systematic performance of the adjustable draft point absorber integrated with a linear permanent magnet generator. Both a nonlinear hydrodynamic model and an analytical generator model are derived, thus the complete process from the wave power input through the whole WEC system to the usable electricity is covered. Based on the established model, wave-to-wire responses of the novel concept are obtained and analyzed. The negative effects of the draft adjustment on the stroke and overlap between the stator and translator are demonstrated. Moreover, a comparison is made between this novel WEC and conventional fixed draft WEC, and both regular and irregular wave states are considered. The results show that the adjustable draft system could increase not only the absorbed power but also the generator conversion efficiency. In specific conditions, the delivered electrical power of the adjustable draft WEC was over 20 % and 10 % higher than a traditional fixed draft system for regular and irregular waves respectively.

Design and Optimization of a Rack and Pinion Type WEC Using an Auxiliary Vibrating System

Research on wave energy converters with Rack and pinion type Power Take-Off (PTO) has been increasing over the last few years. A few control methods are used to optimize the performance of the said Wave Energy Converters (WECs). This paper presents a novel auxiliary vibrating system that can be implemented to improve the power input to a wave energy converter with a rack and pinion type PTO in regular waves. The design of the WEC system includes a floater, a double rack and pinion arrangement, a vibrating system, and a Mechanical Motion Rectifier (MMR) consisting of two one-way bearings that can convert the bidirectional wave motion to a unidirectional rotation of the output shaft. Once the waves move the floater upwards, this compresses the vibrating system which absorbs some of the energy and then the vibrating system helps the floater return to its original position by releasing the stored energy. The vibrating system also serves as a control method for limiting rack movement, so the impact of the waves is not detrimental to the system. This article aims to approximate the optimized power input to the system and investigate whether the implementation of a novel vibrating system improves the system power input. Allowing the WEC’s natural frequency to reach the wave’s natural frequency is important as it allows for maximum power absorption. The use of vibration systems to tune the WEC’s natural frequency close to the waves’ is novel and serves as the main factor in choosing this research. The WEC was modeled as 2 spring mass damper systems. Then the characteristic equations of the systems were extracted from the equations of motion and solved analytically to obtain the responses. One-factor-at-a-time (OFAT) method together with two different algorithms (Genetic and Multi-Start algorithms) from MATLAB code were used to optimize the response. The optimized power input to the system was then approximated. For system one, the maximum amplitude of the response was seen at a system mass of 500 kg and stiffness in the range of 100&lt;k&lt;240 N/m. The same was achieved for system two at a system mass of 500 kg and stiffness in the range of 100&lt;k&lt;138. The effect of the stiffness and mass on the response and input power has also been discussed.

Experimental assessment and validation of energy-maximising moment-based optimal control for a prototype wave energy converter

In recent years, a framework for control of wave energy converters (WECs), termed moment-based control, has been developed within the wave energy conversion literature, exhibiting a remarkable performance in terms of energy absorption, while retaining real-time computational capabilities. Nonetheless, to date, practical results regarding moment-based control for WEC systems have been exclusively provided in numerical simulation, i.e. there is currently a lack of ‘real-world’ assessment of this promising control framework. Motivated by this, we present, in this paper, an experimental assessment and validation of moment-based WEC control for a prototype Wavestar wave energy converter, at the tank-testing facilities of Aalborg University, Denmark. In particular, we address the control design and synthesis procedure in an integrated fashion, covering experimental (physically consistent) system identification, unknown-input estimation and forecasting of wave excitation force, and subsequent control law implementation. We demonstrate that the moment-based controller is able to effectively maximise energy absorption from the wave resource in real-time, with a significant and consistent improvement with respect to the defined benchmark case. The results presented and discussed in this paper demonstrate the feasibility of moment-based WEC control, filling the gap between the attractive theoretical aspects associated with such a strategy, and the practical WEC energy conversion application.

Finite Element Modeling and Simulation of a Submerged Wave Energy Converter System for Application to Oceanic Islands in Tropical Atlantic

The development of efficient and sustainable marine energy converter systems is a great challenge, especially in remote areas such as oceanic islands. This work proposes a numerical modeling methodology to assess the mechanical behavior of a wave energy converter (WEC) to be applied outside Fernando de Noronha Island (Pernambuco, Brazil). First, oceanographic data collected in situ were analyzed to determine different sea state scenarios in the region. The Airy theory and second-order Stokes’ theory were used to obtain the velocity profiles for the maximum and operational swells. These profiles were then implemented in a flow model developed in COMSOL Multiphysics software (Burlington, MA, USA) to calculate the wave distributions of pressure on the WEC structure. Finally, wave pressure distributions obtained from simulations were implemented in a static analysis of the system by the finite element method using SolidWorks (France). The results highlighted the most critical system inclination and the parts of the WEC structure more likely to be damaged under extreme swell conditions. The 0° inclination was the most critical situation, leading to the exceeding of the elastic limits of some parts of the WEC structure. The methodology developed in this work showed to be efficient to study and propose project improvement for the strength of the WEC system.

Optimization on Hydrodynamic Performance for First Level Energy-Capturing Enhancement of a Floating Wave Energy Converter System with Flapping-Panel-Slope

Based on the wave reflection principle, a floating flapping-panel wave energy converter was developed. The feasibility study and optimization study of the new WEC was carried out by laboratory research and computational fluid dynamics techniques. A numerical model was developed for an in-depth study to establish the relationship between slope tilt angle and power. The results for different wave periods show that the power take-off damping coefficient has a significant effect on the power. Meanwhile, the effects of flap length and wave height on converter resonance and power are investigated. Finally, a preliminary laboratory physical model test of the device is conducted. The flapping-panel-slope structure is very feasible and effective with good hydrodynamic performance.

Principle and control strategy of a novel wave-to-wire system embedded ocean energy storage optimization

Fluctuation and unpredictability of wave power output affect the safe operation of the power grid, which greatly restricts the development of wave power generation. This paper firstly introduces the principle and control strategy of wave to wire (W2W) model, whose input is from wave energy conversion (WEC) system and output to the electrical power injected into grid for stable power. Then, an adapted particle swarm optimization (APSO) algorithm-based maximum power point tracking technology (APSO-MPPT) considering adaptive inertia weighting variables is presented for quick dynamic reaction. Comparisons are made with the traditional constant voltage method, INC method, P&Q method, and PSO algorithm, the APSO-MPPT method can reduce the tracking time by 9.3% less than the quickest traditional. Secondly, a K–SOC curve algorithm-based hybrid energy storage system (KSC-HESS) optimization is conducted for smoothing the wave power output, with real time control performance. Simulations under output power fluctuation, load pulsation and load unbalancing situations are occupied for demonstrating high performance for the characteristics of the proposed KSC-HESS, in both reducing high-frequency burden for super-capacitors and protecting batteries from over voltage. Furthermore, results of prototype experiment verified the effectiveness with better efficiency by adapting the proposed APSO-MPPT and KSC-HESS for the proposed W2W system. Analysis shows the proposed W2W approach has comparative advantages of control complexity and high reliability for renewable power output.

Hydrodynamic performance analysis of a new hybrid wave energy converter system using OpenFOAM

In this research study, a newly proposed hybrid device of Wave Energy Converters (WEC) is investigated by considering computational fluid dynamic (CFD)-based numerical wave tanks (NWTs). The open-source CFD code solver, OpenFOAM (Open Field Operation and Manipulation) is implemented, which is numerically solved the Reynolds-averaged Navier–Stokes (RANS) equations to simulate the two-phase flow. The hybrid system consists of an Oscillating Water Column (OWC) and a point absorber (Wavestar) device installed in a shared platform. The main goal is to recognize how wave diffractions caused by the adjacent floating body could affect the rate of power absorption by the Fixed-OWC. This aim is followed by a 2D numerical analysis of three different installation configurations, variable intervals between the Wavestars' buoy and the Fixed-OWCs' front wall, in four different wavelengths with and without Power Take-Off (PTO). Finally, the efficiency characteristics of the integrated system such as free surface velocity and air pressure within the chamber, besides floating body motions are investigated and compared for the hybrid system. Although the overall assessment for 28 different case studies reveals an efficiency reduction in some cases, the superiority of this hybrid plan is recording several incremental efficiency rates.

Fault Diagnosis For Wave Energy Converters With Model Uncertainties

Unexpected sensor and actuator faults degrade the control performance and even introduce damage breaking down the wave energy converter (WEC) system. Fault detection for wave energy converters is of great importance in enhancing the reliability and robustness of WECs. This paper investigates a robust fault diagnosis method for single-point absorbers. An unknown input observer is designed to estimate the fault in real-time, which is robust against model uncertainties. This method can also be straightforwardly applied to other types of WEC. The parameters of the proposed observer can be calculated offline, which enhances the real-time implementation with a low computational burden.