Wave Energy Conversion Efficiency Research Articles

Performance analysis of a novel hybrid device with floating breakwater and wave energy converter integrated

As a kind of clean energy, wave energy has attracted increasing attention in recent years. However, due to its high cost and low efficiency, wave energy has not been widely commercialized worldwide. According to previous research works, the combination of wave energy devices and floating breakwaters can reduce the cost of wave power generation devices efficiently. In this paper, a hybrid device with wave energy converter (WEC) and flexible porous floating breakwater integrated is proposed. The hydrodynamic performance of this device is studied by numerical simulation based on Reynolds-Averaged Navier–Stokes (RANS) method which has been validated through experimental results. Based on this, a series of numerical simulations are performed under different power take-off stiffness (KPTO) and different power take-off damping coefficient (CPTO) to calculate the motion and transmission coefficient of the device. By comparing the numerical results, the influence of KPTO and CPTO of the hybrid device on its wave attenuation performance and energy conversion efficiency are analyzed. The results indicate that under the incident waves of period 0.56 s and 0.89 s, the recommended value of KPTO is 3.0 N/m and 0.5 N/m respectively. While for incident waves of period 0.56s, in order to achieve better wave attenuation performance and power generation efficiency, the recommended value of CPTO should be between 10.55 Ns/m and 14.23 Ns/m. On this basis, the influence of the size of breakwater on the wave attenuation performance and power generation efficiency of the hybrid device is also discussed. Moreover, through a cost analysis, it can be inferred that the utilization of the baseline model is more economically viable.

One Meter Triboelectric Nanogenerator for Efficient Harvesting of Meter‐Scale Wave Energy

AbstractTriboelectric nanogenerators (TENGs) are effective in harvesting high‐entropy low‐frequency wave energy, yet traditional TENGs struggle to align with the meter‐scale wavelengths prevalent in marine environments, significantly impairing their wave energy conversion efficiency. Addressing this, the One Meter TENG (OM‐TENG) is introduced, a pioneering advancement in TENG design that extends the shell size to 1 m. This innovation features a novel lantern‐shaped stacked silicon‐manganese steel sheet configuration, optimizing internal space usage, and achieving a single‐cycle transfer charge of 60.82 µC and a friction layer area density of 1.76 cm−1. Furthermore, by integrating a segmented limiter device for rational control of different sliders, effective contact separation among numerous TENG units is realized. Comprehensive evaluations through theoretical analysis, multiphysics field numerical simulations, sensor data analytics, and comparative experimental measurements have unequivocally demonstrated OM‐TENG's superior capability in capturing meter‐scale wave energy under actual environmental conditions, alongside its remarkable anti‐capsize performance. Achieving peak outputs of 584 V and 444 µA, OM‐TENG can power 96 2 W LEDs and drive wireless communication modules, marking a significant step forward in the efficient utilization of wave energy and the design of large‐scale TENGs.

Experimental study on the hydrodynamic performance of a multi-DOF WEC-type floating breakwater

A hybrid system consisting of wave energy converters and breakwaters can be applied for both wave energy conversion and shoreline protection. Besides, the cost of wave energy conversion is believed to be reduced by such a combination. In this paper, a two-dimensional asymmetric wave energy converter-type breakwater with a triangle-baffle bottom that can convert wave energy through its heave motion is proposed and investigated experimentally in regular waves. A supporting system is designed and established to allow the model to move in a single degree of freedom or prescribed combination of multiple-degree-of-freedoms. The effects of the geometric asymmetry, the bottom slope, the width-to-draft ratio, and the multiple-degree-of-freedom motion on the wave energy conversion and attenuation performances are analyzed. The results show that the triangle-baffle wave energy converter-type breakwater has much higher wave energy conversion efficiency than its rectangular counterpart with the same displacement. Increasing the bottom slope and allowing pitch motion can improve efficiency, allowing surge motion will reduce efficiency, and the effect of increasing the width is uncertain due to two conflicting effects. The asymmetry, bottom slope, and width only have a limited influence on the wave attenuation, whereas surge and pitch motions reduce the wave attenuation in short waves, analogizing a wavemaker.

Research on Maximum Power Control of Direct-Drive Wave Power Generation Device Based on BP Neural Network PID Method

Ocean wave energy is a new type of clean energy. To improve the power generation and wave energy conversion efficiency of the direct-drive wave power generation system, by addressing the issue of large output errors and poor system stability commonly associated with the currently used PID (proportional, integral, and derivative) control methods, this paper proposes a maximum power control method based on BP (back propagation) neural network PID control. Combined with Kalman filtering, this method not only achieves maximum power tracking but also reduces output ripple and tracking error, thereby enhancing the system’s control quality. This study uses a permanent magnet linear generator as the power generation device, establishes a system dynamics model, and predicts the main frequency of irregular waves through the Fast Fourier Transform method. It designs a desired current tracking curve that meets the maximum power strategy. On this basis, a comparative analysis of the control accuracy and stability of three control methods is conducted. The simulation results show that the BP neural network PID control method improves power generation and exhibits better accuracy and stability.

Optimization of power take-off system settings and regional site selection procedure for a wave energy converter

Ocean wave energy stands as a crucial component in the quest for sustainable and renewable energy sources, essential in the global effort to mitigate climate change. However, a significant challenge in this field is optimizing the efficiency of Wave Energy Converters (WECs) on a regional scale, particularly Oscillating Surge Wave Energy Converters (OSWECs). This challenge stems from the complex, nonlinear interactions between ocean waves and these devices, necessitating precise tuning of Power Take-Off (PTO) system settings and optimal placement for the highest possible performance and stability. To address this challenge, our study introduces the Hill Climb - Explorative Grey Wolf Optimizer (HC-EGWO), a novel algorithm combining local search and swarm-based global optimization strategies. This hybrid approach effectively balances exploration and exploitation in the solution space, leading to more optimal PTO settings for OSWECs. Alongside this algorithmic development, we conduct a thorough feasibility analysis based on the constraints of the flap’s maximum angular motion. This ensures the optimized OSWEC operates within safe and efficient limits. In a comparative analysis with the Genetic Algorithm (GA), the Particle Swarm Optimization (PSO), the artificial Gorilla Troops Optimizer (GTO), and different implementations of the GWO, our results show an improvement in power output, with the HC-EGWO method achieving up to a 3.31% increase over other variations of the GWO and 45% increase compared to all the methods. The findings of this study not only demonstrate the effectiveness of the HC-EGWO method but also provide strategic insights for the deployment of OSWECs in areas like the South Caspian Sea, where unique environmental factors imply careful consideration in the site selection process.

Enhancing Wave Energy Conversion Efficiency through Supervised Regression Machine Learning Models

The incorporation of machine learning (ML) has yielded substantial benefits in detecting nonlinear patterns across a wide range of applications, including offshore engineering. Existing ML works, specifically supervised regression models, have not undergone exhaustive scrutiny, and there are no potential or concurrent models for improving the performance of wave energy converter (WEC) devices. This study employs supervised regression ML models, including multi-layer perceptron, support vector regression, and XGBoost, to optimize the geometric aspects of an asymmetric WEC inspired by Salter’s duck, based on key parameters. These important parameters, the ballast weight and its position, vary along a guided line within the available geometric resilience of the asymmetric WEC. Each supervised regression ML model was fine-tuned through hyperparameter optimization using Grid cross-validation. When evaluating the performance of each ML model, it became evident that the tuned hyperparameters of XGBoost led to predictions that strongly aligned with the actual values compared to other models. Furthermore, the study extended to assess the performance of the optimized WEC at the designated deployment test site location.

Wave energy converter efficiency based on wave transmission and relative capture width performance

A series of quantitative analyses were performed to identify the wave properties and potential, by means of records collected from an offshore buoy in North-West Ireland. Based on the data collected, a series of quantitative analyses was conducted to determine the dominant wind directions and wave properties on an annual basis. In addition, the wave power is computed based on relevant wave heights and periods, and the Pierson-Moskowitz spectral model was used to generate the maximum wave energy spectra for each year. The results show that waves with wave powers of around 100 kW/m were mostly approaching eastward at a rather narrow frequency. In order to compare the relative capture width and the power absorption capacity of three floating structures, the Wave Dragon, Board Net Breakwater, and Cylindrical Floating Breakwater are analyzed. Also outlined is the impact of the transmission coefficient on the effectiveness of wave energy converters (WEC) throughout the energy harvesting process. This was accomplished by fusing information from three distinct field investigations and experimental research with four different wave transmission coefficient models. The results show that as the wave steepness increases, the transmission coefficient decreases and the hydrodynamic performance of wave energy converters increases. Also, it is found that the hydrodynamic efficiency of wave energy converters is higher in summer than in winter, and the Wave Dragon is the most efficient wave energy converter in regard to relative capture width and power absorption.

Hydrodynamic performance of a three-unit heave wave energy converter array under different arrangement

A pile-restrained floating wave energy converter (WEC) array is proposed as an alternative to a single floater of the size of the array for use as a floating breakwater. The hydrodynamics of the WEC are modelled based on the Navier-Stokes equations and the model is verified by comparing its results with existing experimental data. The model then is used to characterize the array composed by a line of three WECs in terms of floater heaving, wave energy conversion, wave reflection, transmission and dissipation, for different layouts.In the examined array configuration, the aligned arrays exhibit superior performance compared to the staggered arrays, comprehensively considering both wave energy conversion and wave transmission. Specifically, when khi > 1.73, the wave energy conversion efficiency of the aligned array with a spacing of 0.1 times the WEC width ranges from 0.141 to 0.330, while the wave transmission coefficient ranges from 0.187 to 0.472, indicating the effectiveness of the arrays in simultaneously reducing wave transmission and converting wave energy under shorter-wavelength conditions. Compared to a single WEC of the same dimensions, the array exhibits a remarkable increase in wave energy conversion efficiency and effectively reduce wave reflection.

Experimental validation of rollout-based model predictive control for wave energy converters on a two-body, taut-moored point absorber prototype

Model predictive control (MPC) has proven its effectiveness in improving the energy capture efficiency of wave energy converters (WECs) under physical constraints. Further application of MPC requires to speed up its online computation for an industrial controller. To this end, the rollout-based MPC (RMPC) for WECs has been developed. The idea of rollout is to decouple the optimization horizon and the prediction horizon, so as to achieve long-horizon performance with short-horizon optimization. In this paper, the RMPC that has previously only been validated by simulation is further put through wave tank testing. The experimental device is a realistic two-body, taut-moored point absorber prototype. The RMPC is based on a simplified model of the device and implemented in real time with wave force estimation and prediction. Experiment results confirm RMPC’s energy efficiency as well as constraint satisfaction, so its computational advantage against conventional MPC is highlighted.

NUMERICAL METHOD FOR ASSESSING EFFICIENCY OF WAVE ENERGY CONVERTOR INTEGRATED INTO COASTAL PROTECTION STRUCTURES

The use of wave energy by converting it into electricity depends on the efficiency of wave energy converters, which are negatively affected by marine factors. Many design flaws can be eliminated at the stage of design and testing of devices by using its numerical models based on the application of the hydrodynamic theory of propagation and interaction of waves with marine structures. The use of the modern computational methods of hydrodynamics to determine the influence of some design factors of wave energy converters incorporated into a submerged breakwater is considered in the paper. The results of numerical studies showed that if a submerged breakwater is located in the wave stream and with a transducer installed on its crest, there is a change in flow velocities and pressure in the transducer chamber, which affects the efficiency of energy extraction. Additional advantages arise as a result of the introduction of a complex structure with coastal protection and power supply functions. The obtained calculation results visualize the qualitative indicator of the influence of the design factors of the converter. Additional conclusions can be obtained by processing the numerical results. As a result of numerical experiments conducted with using REEF3D CFD computer system, the measures for selecting the parameters of the converters were proposed based on mathematical modeling of specific structures. The research related to the determination of the mechanisms of wave energy transformation when interacting with coastal protection elements focused on increasing the efficiency of the convertor by locating in deeper water, which includes detailed testing, investigations and improvement of various types of constructions, is carried out on the computing and laboratory base of Institute of Hydromechanics.

Novel computational fluid dynamics-finite element analysis solution for the study of flexible material wave energy converters

The use of flexible materials for primary mover and power takeoff of wave energy converters (WECs) has attracted considerable attention in recent years, owing to their potential to enhance the reliability, survivability, and wave energy conversion efficiency. Although some reduced order models have been used to study the fluid–structure interaction (FSI) responses of flexible wave energy converters (fWECs), they are somehow inappropriate due to their limited accuracy and applicability span. To gain a deeper understanding of the physical mechanisms in fWECs, a high-fidelity approach is required. In this work, we build up a fluid–structure interaction analysis framework based on computational fluid dynamics and a finite element analysis method. The incompressible viscous flow is resolved by solving three-dimensional unsteady Navier–Stokes equations with a finite volume approach. The structure dynamics are solved by a finite element method, taking the nonlinear behavior of flexible material into consideration. A strong coupling strategy is utilized to enhance the numerical stability and convergence of the iterative process. We demonstrate the present FSI tool is able to provide rich flow field information and structural response details, such as the velocity, pressure, and structural stress distribution. This is illustrated through several case studies, including two types of fWECs. The unsteady wave–structure-interaction and the associated nonlinear phenomena are also accurately captured by this tool.

Double-layer model predictive control for wave energy converters with model mismatch

To reduce the impact of the model mismatch on the conversion efficiency of wave energy converters (WECs), a double-layer model predictive control (DMPC) method is proposed. Two closely related control layers are included in the DMPC method: (1) The first control layer is compensation control, which deals with the model mismatch by using the prediction data from the second layer. (2) The second layer aims to maximize energy output while taking the compensation value from the first layer into consideration. The sum of the compensation value obtained at the first layer and the control value obtained at the second layer refers to the final control input for the real system. The simulation results demonstrate that the proposed method effectively reduces the impact of the model mismatch and complies with control input and float’s position limitations.

Viscous Damping Performance Analysis and Prediction of Axisymmetric Cylindrical Oscillating Buoys With Different Geometrical Configurations

Abstract With the intensification of the world's energy demand, clean and efficient marine energy has received more attention, and energy conversion devices are particularly important. As an important part of the oscillating wave energy conversion device, the hydrodynamic characteristics of cylindrical absorbers have a great impacton wave energy conversion efficiency. When cylindrical buoys with different bottom configurations heave under the action of waves, the oscillating motion and flow field become more complex. At present, potential flow theory is usually used to predict the motion response of cylindrical buoys in waves. Although the calculation speed is fast, there will be large errors due to the lack of consideration of the effect of fluid viscosity. To explore this error range, a three-dimensional numerical wave pool is established based on the viscous fluid theory and STARCCM+ software to study the response of buoy and wave coupling action considering the viscous effect. At the same time, based upon the potential flow theory, the analytical solution of the cylindrical buoy oscillating in waves is established. Combined with the computational fluid dynamics numerical simulation results, the oscillation laws of the buoys with different bottom configurations in waves with different periods are compared, and the effects of viscosity on the buoys' motion are also explored. According to the calculation results of floating buoy movement under viscous and nonviscous fluid, the statistical correction algorithmis adopted to obtain the viscous hydrodynamic correction algorithm of floating buoy movement based on the potential flow theory, and the feasibility is verified by viscous numerical simulation under other wave periods.

Research on Size Optimization of Wave Energy Converters Based on a Floating Wind-Wave Combined Power Generation Platform

Wind energy and wave energy often co-exist in offshore waters, which have the potential and development advantages of combined utilization. Therefore, the combined utilization of wind and waves has become a research hotspot in the field of marine renewable energy. Against this background, this study analyses a novel integrated wind-wave power generation platform combining a semi-submersible floating wind turbine foundation and a point absorber wave energy converter (WEC), with emphasis on the size optimization of the WEC. Based on the engineering toolset software ANSYS-AQWA, numerical simulation is carried out to study the influence of different point absorber sizes on the hydrodynamic characteristics and wave energy conversion efficiency of the integrated power generation platform. The well-proven CFD software STAR CCM+ is used to modify the heaving viscosity damping of the point absorber to study the influence of fluid viscosity on the response of the point absorber. Based on this, the multi-body coupled time-domain model of the integrated power generation platform is established, and the performance of the integrated power generation platform is evaluated from two aspects, including the motion characteristics and wave energy conversion efficiency, which provides an important reference for the design and optimization of the floating wind-wave power generation platform.

Energy assessment of potential locations for OWC instalation at the Portuguese coast

This work aims to determine the exploitable wave energy resource at five potential sites close to harbour protection facilities at the Portuguese coast, namely at the Azores archipelago, at Madeira Island and at Sines, on the coast of mainland Portugal. For that purpose, a third-generation wave model SWAN is used to transfer the offshore estimates of sea wave conditions to those points over the last 40 years. Sea states and wind fields are provided by the climate reanalysis datasets ERA5. Using sea states as boundary conditions and wind fields as forcings in the numerical domains of the SWAN model, the sea states were propagated shoreward, in order to estimate and analyse the wave conditions in the regions of interest. By combining the average energy flux per unit length of wave front and the probability of occurrence of each sea state, the average exploitable annual energy per unit length of wave crest can be computed. The variability of this energy flux is analysed since it is of paramount importance for the efficiency of Wave Energy Converters (WEC). This assessment showed that the best location for the installation of dual-chamber OWC devices is at the Azores archipelago.

Numerical and experimental research on a resonance-based wave energy converter

Inefficiency and intermittency caused by high variabilities of wave properties are common problems in existing wave energy technology. Resonant wave energy conversion technology can effectively solve these problems. However, the nonlinearity induced by the wave energy converter’s radiation and operation inhibits the resonance. After weakening the nonlinearity by hydrodynamic optimization and operation manner design, a novel wave energy converter that can realize resonance was proposed. In this paper, it is verified that this wave energy conversion system is close to linear, even in the case of large amplitude oscillation. For the novel wave energy converter, a radiation hydrodynamic parameter testing method is proposed, which provides parameters to accurately control the wave energy conversion system to achieve resonance; a wave force testing method is proposed, which can provide in-situ wave information to control resonance and judge resonance state; the effects of resonance, damping and nonlinearity on wave energy conversion are revealed, which provides a theoretical basis for the design of wave energy converter. Experimental studies showed that the laws of wave energy conversion are basically the same in regular and irregular waves, the methods and laws investigated in the regular waves also apply to irregular waves. And experiments also showed that the novel wave energy conversion system could be controlled to achieve resonance truly, and the novel wave energy converter can achieve resonance and highly efficient wave energy conversion in different wave conditions. The novel wave energy converter can self-perceive the wave changes and actively control its natural period to achieve resonance with incident waves, thus significantly increasing the wave energy conversion efficiency and improving intermittency.

Advanced efficiency improvement of a sloping wall oscillating water column via a novel streamlined chamber design

Despite the huge potential of ocean waves as an alternative renewable energy resource, very low wave to wire efficiency of wave energy converters (WECs) hampers its commercialization. In this study, a novel streamlined chamber design has been proposed for performance improvement of an oscillating water column (OWC) with a sloping wall. Physical experiments are carried out for the calculation of capture width ratio (CWR) as a measure of hydrodynamic performance for various levels of PTO damping and sea conditions. The results are compared with those of a classical OWC chamber design. It is found that streamlined chamber design has an important bearing on OWC efficiency. As opposed to classical chamber shape, proposed streamlined chamber geometry increases the OWC performance as much as 31% for the incident waves under which the OWC operates efficiently. Further, results show that optimal level of PTO damping that for utmost energy extraction depends on the chamber design. Streamlined chamber geometry even further improves the OWC efficiency even when dominant sloshing water column motion is present in the chamber with a maximum and average value of 54% and 44%, respectively, however, chamber design has no influence on the mechanism that generates sloshing phenomenon in the chamber.

Towards Robust and High-performance Operations of Wave Energy Converters: an Adaptive Tube-based Model Predictive Control Approach

Model predictive control (MPC) is an effective control method to improve the energy conversion efficiency of wave energy converters (WECs). However, the current developed WEC MPC has not reached commercial viability since the control performance is significantly dependent on the WEC model fidelity. To overcome the plant-model mismatch issue in the WEC MPC control problem, this paper proposes a robust tube-based MPC method to bound plant states within disturbance invariant sets centered around the noise-free model trajectory. The invariant sets are also utilized for tightening the nominal model's constraints that robustly enable constraint satisfaction. Yet overly conservative invariant sets can narrow the feasible region of the states and control inputs, and hence a data-driven quantile recurrent neural network (QRNN) is proposed in this work to form a learning-based adaptive tube with reduced conservatism by quantifying WEC model uncertainties. The theoretical root is that time-dependent historical data can offer valuable insight into the future behaviour of uncertainties. Numerical simulations have validated that the proposed method can improve the energy capture rate compared to the TMPC approach, by synthesizing the QRNN-based tube with MPC.

Sliding Mode Control of a Nonlinear Wave Energy Converter Model

The most accurate wave energy converter models for heaving point absorbers include nonlinearities, which increase as resonance is achieved to maximize the energy capture. Over the power production spectrum and within the physical limits of the devices, the efficiency of wave energy converters can be enhanced by employing a control scheme that accounts for these nonlinearities. This paper proposes a sliding mode control for a heaving point absorber that includes the nonlinear effects of the dynamic and static Froude-Krylov forces. The sliding mode controller tracks a reference velocity that matches the phase of the excitation force to ensure higher energy absorption. This control algorithm is tested in regular linear waves and is compared to a complex-conjugate control and a nonlinear variation of the complex-conjugate control. The results show that the sliding mode control successfully tracks the reference and keeps the device displacement bounded while absorbing more energy than the other control strategies. Furthermore, due to the robustness of the control law, it can also accommodate disturbances and uncertainties in the dynamic model of the wave energy converter.

Hydrodynamic Performance of an Asymmetry OWC Device Mounted on a Box-Type Breakwater

To share the construction and maintenance cost, an asymmetric oscillating water column (OWC) device integrated with a pile-fixed box-typed offshore breakwater is considered experimentally and numerically. A fully nonlinear numerical wave tank is established and validated with the open source solver OpenFOAM. The effects of the width and draft of rear box, and the incident wave height on the wave energy conversion efficiency, reflection and transmission coefficients, and energy dissipation coefficient are examined. In addition, the superiority of the present coupling system, compared to the traditional box-type breakwater, is discussed. With well comparisons, the results show that the existence of the rear breakwater is beneficial for the formation of partial standing waves and further wave energy conversion. In the range of wave heights tested, the higher the incident wave height, the larger the energy absorption efficiency except for the short-wave regimes. Moreover, the OWC-breakwater coupling system can obtain a similar wave blocking ability to the traditional one, and simultaneously extract wave energy and decrease wave reflection.