Oscillating Wave Energy Converter Research Articles

The Energy Conversion and Coupling Technologies of Hybrid Wind–Wave Power Generation Systems: A Technological Review

Based on the mutual compensation of offshore wind energy and wave energy, a hybrid wind–wave power generation system can provide a highly cost-effective solution to the increasing demands for offshore power. To provide comprehensive guidance for future research, this study reviews the energy conversion and coupling technologies of existing hybrid Wind–wave power generation systems which have not been reported in previous publications. The working principles of various wind and wave energy conversion technologies are summarised in detail. In addition, existing energy coupling technologies are specifically classified and described. All aforementioned technologies are comprehensively compared and discussed. Technological gaps are highlighted, and future development forecasts are proposed. It is found that the integration of hydraulic wind turbines and oscillating wave energy converters is the most promising choice for hybrid wind–wave power extraction. DC and hydraulic coupling are expected to become mainstream energy coupling schemes in the future. Currently, the main technological gaps include short their operating life, low energy production, limited economic viability, and the scarcity of theoretical research and experimental tests. The field offers significant opportunities for expansion and innovation.

A double-objective prediction and optimization method for buoys performance based on the artificial neural network

Determining the optimal geometry of the buoy is an important step in the design of oscillating wave energy converters, its main problem of which is the complexity and computational cost of the buoy shape parameterization process. To solve the above problems, the Genetic Algorism Back Propagation (GABP) neural network is used to optimize the longitudinal section of the buoy, and the Bessel equation is used to simplify the coordinates. 280 buoys with regular shapes are used as the training set, and 5% of them are selected as the test set. The trained neural network can accurately predict both the maximum heave displacement and additional mass of the buoy under a uniform wave condition, with a prediction accuracy of 93%. An optimal buoy geometry shape is constructed by randomly varying Bessel control points which enables the trained neural network to perform double-objective optimization of the buoy longitudinal section. The method proposed in this study can provide a new idea for the optimization of buoy geometry shape and performance prediction.

Exergy and environmental analysis of oscillating wave energy converter hybrid with other renewable energy resources: A case study

ABSTRACT In order to evaluate the hybrid renewable configurations in diverse applications, this inquiry tries to propose a novel decision-making procedure. The open-source WEC dynamics simulator, exergetic assessment, and Hybrid Optimization Model for Electric Renewables Pro software are integrated. In this innovative method, the exergy of the component is calculated using MATLAB software. Technical analysis for the goal of rural electrification is examined in this work. Thus, several integrated energy systems are technically researched. This study deals with the exergy, environmental, and exergoenvironmental analyses of the hybrid renewable energy system, to assess its performance and environmental impact when operating on Siri Island. In the photovoltaic solar panels, wind turbines, wave energy converters, diesel generators, and batteries set up, the total exergy efficiency and exergy destruction rate were 18.4% and 7.24 106 kWh/year, respectively. Additionally, it was discovered that the wave energy converter was responsible for 5% of the total energy wasted. Photovoltaic solar panels, wind turbines, and wave energy converters all have mean annual exergy efficiencies calculated to be 11.7%, 13.1%, and 20.8%, respectively. Environmental and exergoenvironmental analyses illustrate that the off-grid scenario decreases CO2 emission by 95% compared to the on-grid scenario and utilizing diesel generators increases the exergy stability factor of the energy system. The suggested decision-making procedure can be a potent and reliable instrument for analyzing any hybrid renewable arrangement from a technological standpoint.

Influence on Structural Loading of a Wave Energy Converter by Controlling Variable-Geometry Components and the Power Take-Off

AbstractOceans are harsh environments and can impose significant loads on deployed structures. A wave energy converter (WEC) should be designed to maximize the energy absorbed while ensuring the operating wave condition does not exceed the failure limits of the device itself. Therefore, the loads endured by the support structure are a design constraint for the system. Furthermore, the WEC should be adaptable to different sea states. This work uses a WEC-Sim model of a variable-geometry oscillating wave energy converter (VGOSWEC) mounted on a support structure simulated under different wave scenarios. A VGOSWEC resembles a paddle pitching about a fixed hinge perpendicular to the incoming wave fronts. The geometry of the VGOSWEC is varied by opening a series of controllable flaps on the pitching paddle when the structure experiences threshold loads. It is hypothesized that opening the flaps should result in load shedding at the base of the support structure by reducing the moments about the hinge axis. This work compares the hydrodynamic coefficients, natural periods, and response amplitude operators from completely closed to completely open configurations of the controllable flaps. This work shows that the completely open configuration can reduce the pitch and surge loads on the base of the support structure by as much as 80%. Increased loads at the structure’s natural period can be mitigated by an axial power take-off damping acting as an additional design parameter to control the loads at the WEC’s support structure.

Is the Velocity Always in Phase with the Wave Excitation Force in Constrained Optimal Control of Wave Energy Converters?

The resonance condition for maximum absorption of the energy carried by ocean waves incident to an oscillating wave energy converter (WEC) is one of the most well known results of the WEC control research. It was shown that the maximum occurs when the WEC velocity is in phase with the wave excitation force. The condition was obtained with the assumption that the WEC frees to oscillate with whatever amplitude was necessary, and that the power take-off (PTO) system can deliver the unlimited force. In practical WEC implementations, this assumption does not hold. The purpose of this paper is twofold. First, to obtain the frequency response of the closed-loop WEC motion, and of the optimal control law in terms of the hydrodynamic coefficients with limited PTO force and limited motion amplitudes. Secondly, the obtained results are used to investigate the resonance condition of the WEC velocity with the incoming wave in the presence of constraints.

An efficient fully Lagrangian solver for modeling wave interaction with oscillating wave surge converter

In this paper, we present an efficient, accurate and fully Lagrangian numerical solver for modeling wave interaction with oscillating wave surge converter (OWSC). The key idea is to couple SPHinXsys, an open-source multi-physics library in unified smoothed particle hydrodynamic (SPH) framework, with Simbody which presents an object-oriented application programming interface for multi-body dynamics. More precisely, the wave dynamics and its interaction with OWSC is resolved by a Riemann-based weakly-compressible SPH method in SPHinXsys, and the solid-body kinematics is computed by Simbody library. Numerical experiments demonstrate that the proposed solver can accurately predict the wave elevations, flap rotation and wave loading on the flap in comparison with laboratory experiment and numerical investigation. In particularly, the new solver shows optimized computational performance through CPU cost analysis and comparison with commercial software package ANSYS FLUENT and other SPH-based solvers in literature. Furthermore, a linear damper is applied for imitating the power take-off system to study its effects on the hydrodynamics properties of OWSC and efficiency of energy harvesting. In addition, the present solver is used to model extreme wave condition using the focused wave approach to investigate the extreme loads and motions of OWSC under such extreme wave conditions. It is also worth noting that though the model validation used herein is a bottom hinged oscillating wave energy converter (WEC), the obtained numerical results show promising potential of the proposed solver to future applications in the design of high-performance WECs.

Flap-type wave energy converter arrays: Nonlinear dynamic analysis

The wave-body interaction of resonant oscillating wave energy converters (WECs) is complex and nonlinear. Rapid development of WECs needs a reliable simulation tool for analysis and design procedure with the ability to capture the instantaneous nonlinear behavior. This paper presents a methodology to study the behavior of a flap-type WEC utilizing hybrid physical and numerical simulation. Dynamic characteristics of the device as isolated and in an array are estimated and the time and oscillation amplitude-dependency of these parameters are explained. This methodology is developed for a device composed of a series of flaps placed on a vertical breakwater. However, the developed algorithm can be updated for any other WEC concept with special consideration on the consequent changes in the load transfer, surrounding hydrodynamic, and wave-structure interaction. This paper increases our understanding and knowledge of all involved phenomena and their direct effects on the natural period and the response and can be used for tunning dynamic characteristics of WEC device to incoming waves. Finally, the developed numerical model is used for the simulation of the WEC to different wave loads and the results are validated with the experimental tests.

Experimental Study on Interaction Between Degrees of Freedom in a Wave Buoy

Increasing degrees of freedom (DOFs) is a useful way to raise the power capture efficiency of oscillating wave energy converters. Thus, this study proposes a buoy with three DOFs, which are surge, heave, and pitch. The hydrodynamic performance and power capture efficiency of the buoy is physically modeled. Amplitudes of unidirectional and coupled motions are compared to analyze the interaction effect between freedoms under conditions with and without power take-off damping. The capture width ratio and corresponding growth rates are also calculated. Results show that the buoy makes a periodic sinusoidal (or approximate) movement in every DOF. Coupling effect can cause an increase in the amplitude in one DOF and a decrease in the amplitudes of the others. This phenomenon shows that the kinematic energy of the buoy redistributes to all DOFs compared with the unidirectional conditions. Adding DOFs can improve the power absorption of the buoy in most cases, but the number of DOFs is not the more the better.

The pressure impulse of wave slamming on an oscillating wave energy converter

Recent wave tank experiments on a flap-type wave energy converter showed the occurrence of extreme wave loads, corresponding to slamming events in highly energetic seas. In this paper, we analyse pressure-impulse values from available pressure measurements, for a series of experimental slamming tests. Then, we devise a pressure-impulse model of the slamming of a flapping plate, including the effects caused by air entrapment near the plate. Using a double conformal-mapping technique, we map the original domain into a semi-infinite channel, by means of Gauss’ hypergeometric functions. This allows us to express the pressure impulse as a superimposition of orthogonal eigenfunctions in the transformed space. The mathematical model is validated against the experimental data. Parametric analysis shows that the system is much more sensitive to the impact angle than to the initial wetted portion of the flap. Furthermore, the presence of an aerated region determines the pressure-impulse values to increase significantly at all points on the flap surface.

Modeling and Emulation of a Rotating Paddle Type Wave Energy Converter

Torque-speed characteristics of an oscillating wave energy converter (WEC) is necessary to determine its optimal power point and to extract the corresponding design parameters for the generator and power converters used to generate power from the WEC. In this paper, analytical expressions for the mechanical torque on the WEC due to the oscillating surge of seawater are derived, and the dynamic model of the WEC is developed. A method for hardware emulation of the WEC is proposed by controlling the torque of an electric motor. The reference torque of the motor is derived by solving the mechanical dynamics of the WEC system in a real-time computing device considering a typical sea wave input. The emulated WEC dynamics is validated through variable loading on a generator coupled with the emulator. Experimental results support the existence of an optimal power point of the derived WEC model.

Hydrodynamic Characteristics and Strength Analysis of a Novel Dot-matrix Oscillating Wave Energy Converter

The paper analyzes hydrodynamic characteristics and the strength of a novel dot-matrix oscillating wave energy converter, which is in accordance with nowadays’ research tendency: high power, high efficiency, high reliability and low cost. Based on three-dimensional potential flow theory, the paper establishes motion control equations of the wave energy converter unit and calculates wave loads and motions. On this basis, a three-dimensional finite element model of the device is built to check its strength. Through the analysis, it can be confirmed that the WEC is feasible and the research results could be a reference for wave energy’s exploration and utilization.

Numerical analysis for a proposed hybrid system with single HAWT, double HATCT and vertical oscillating wave energy converters on a single tower

South Korea has huge renewable energy potential that has been to some extent and can be further extracted to reduce its heavy reliance on fossil fuels. This project focuses on ocean and coastal wind resources available in the south-western regions of Korea that are currently being monitored and found to have huge potential. The project will propose a new concept of wind, tidal current and wave energy converting systems combined together that can boost output power capacity at a single location also making it easier for grid connection. Wind turbines nowadays have been quite efficiently developed and implemented throughout the world both onshore and offshore. However, tidal current and wave energy have yet more room for improvement. The current study will combine the offshore wind turbine (HAWT) system with two tidal current turbines (HATCT) both in horizontal axis classification on a single tower. It will further discuss on adding a wave energy converter in between the wind and tidal current turbines. Optimization of the tower is also a minor part of this study. The structural analysis of the main tower, which is the main focus of the study, is investigated by dynamic response analysis to check its feasibility for combining the three types of renewable energy converters on one support. Actual extreme conditions of the wind, ocean currents and waves are simulated using ANSYS CFX fluid flow analysis. The results are used for conducting a one way FSI analysis assuming a rigid structure to verify the safety of the system.

An oscillating wave energy converter with nonlinear snap-through Power-Take-Off systems in regular waves

Floating oscillating bodies constitute a large class of wave energy converters, especially for offshore deployment. Usually the Power-Take-Off (PTO) system is a directly linear electric generator or a hydraulic motor that drives an electric generator. The PTO system is simplified as a linear spring and a linear damper. However the conversion is less powerful with wave periods off resonance. Thus, a nonlinear snap-through mechanism with two symmetrically oblique springs and a linear damper is applied in the PTO system. The nonlinear snap-through mechanism is characteristics of negative stiffness and double-well potential. An important nonlinear parameter γ is defined as the ratio of half of the horizontal distance between the two springs to the original length of both springs. Time domain method is applied to the dynamics of wave energy converter in regular waves. And the state space model is used to replace the convolution terms in the time domain equation. The results show that the energy harvested by the nonlinear PTO system is larger than that by linear system for low frequency input. While the power captured by nonlinear converters is slightly smaller than that by linear converters for high frequency input. The wave amplitude, damping coefficient of PTO systems and the nonlinear parameter γ affect power capture performance of nonlinear converters. The oscillation of nonlinear wave energy converters may be local or periodically inter well for certain values of the incident wave frequency and the nonlinear parameter γ, which is different from linear converters characteristics of sinusoidal response in regular waves.

Effect of a straight coast on the hydrodynamics and performance of the Oscillating Wave Surge Converter

This paper describes the behaviour of a flap-type oscillating wave energy converter near a straight coast. The mathematical formulation is based on a linear potential flow model. Application of Green׳s theorem to a semi-infinite fluid domain yields a hypersingular integral equation for the velocity potential which is solved using a series expansion of Chebyshev polynomials. Extremes in the hydrodynamic characteristics of the system are shown to occur at certain wave periods when the device is located at specific distances from the coast. This dynamics can have either detrimental or favourable effects on the performance of the converter, depending on the system parameters. Surprisingly, when the device is located very close to the coast, the qualitative behaviour of the system resembles that of a single device in the open ocean. In addition, the analysis shows that under such circumstances, the device consistently achieves much higher levels of efficiency than it would in an open ocean.

Effects of wave excitation force prediction deviations on the discrete control performance of an oscillating wave energy converter

Oscillating-body converters are widely used in offshore engineering, to capture wave energies. In such a case, discrete control including both latching and declutching controls is adopted to improve the power capture performance of oscillating wave energy converters (WECs). A reliable prediction of wave excitation forces on the WEC is essential for the discrete control strategy. In this study, a time domain model is developed to calculate the hydrodynamic responses of a hemispherical oscillating WEC with discrete control in regular waves. A state space model is used to deal with the convolution term in the time domain equation, taking into account the memory effects of wave surface. Based on the developed numerical model, the effects of prediction deviations, such as the amplitude and phase of wave excitation force, have been studied. It is observed that the amplitude prediction deviation exhibits very few effects on the control performance. However, the phase prediction deviation performances significantly influence on the control performance. In some conditions, the phase prediction deviation will reduce the efficiency of the discrete control.

Study on a New Array of Oscillating Wave Energy Converter

The paper presents a new array of oscillating wave energy converter, which is in accordance with nowadays’ research tendency of China: high power, high efficiency, high reliability and low cost. The paper gives the design scheme and design parameters of the new wave energy converter. It's composed of the wave energy converter unit, which can be assembled to arrays based on demand. Besides, the paper analyzes the power generation principle and advantages of the device. The research results could be a reference for wave energy’s exploration and utilization.

Declutching control of a point absorber with direct linear electric PTO systems

Declutching control is applied to a hemispherical wave energy converter with direct linear electric Power-Take-Off systems oscillating in heave direction in both regular and irregular waves. The direct linear Power-Take-Off system can be simplified as a mechanical spring and damper system. Time domain model is applied to dynamics of the hemispherical wave energy converter in both regular and irregular waves. And state space model is used to replace the convolution term in time domain equation of the heave oscillation of the converter due to its inconvenience in analyzing the controlled motion of the converters. The declutching control strategy is conducted by optimal command theory based on Pontryagin's maximum principle to gain the controlled optimum sequence of Power-Take-Off forces. The results show that the wave energy converter with declutching control captures more energy than that without control and the former's amplitude and velocity is relatively larger. However, the amplification ratio of the absorbed power by declutching control is only slightly larger than 1. This may indicate that declutching control method may be inapplicable for oscillating wave energy converters with direct linear Power-Take-Off systems in real random sea state, considering the error of prediction of the wave excitation force.

Impact of a Medium-Size Wave Farm on Grids of Different Strength Levels

Power fluctuations generated by most oscillating wave energy converters may have a negative impact on the power quality of the local grid to which the wave farms will be connected. Hence, assessing their impact is an important step in the selection process of a suitable deployment location. However, site-specific grid impact assessment studies are relatively time-consuming and require a high level of detail on the local network. Both of these constraints mean that grid impact studies are usually not performed in the preliminary stages of the site selection process, despite the extremely negative consequences resulting from poor power quality. This paper details a comprehensive study based on a relatively typical wave farm design connected to networks of different strength levels. The study was performed using experimental electrical power time series of an oscillating water column (OWC) device generated under the framework of the European FP7 project “CORES”. Simulations were performed using DIgSILENT power system simulator “PowerFactory”.

Hydrodynamics of the oscillating wave surge converter in the open ocean

A potential flow model is derived for a large flap-type oscillating wave energy converter in the open ocean. Application of Green’s integral theorem in the fluid domain yields a hypersingular integral equation for the jump in potential across the flap. The solution is found via a series expansion in terms of the Chebyshev polynomials of the second kind and even order. Several relationships are then derived between the hydrodynamic parameters of the system. Comparison is made between the behaviour of the converter in the open ocean and in a channel. The degree of accuracy of wave tank experiments aiming at reproducing the performance of the device in the open ocean is quantified. A parametric analysis of the system is then undertaken. In particular, it is shown that increasing the flap width has the beneficial effect of broadening the bandwidth of the capture factor curve. This phenomenon can be exploited in random seas to achieve high levels of efficiency.

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

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