Performance Of Wave Energy Converter Research Articles

Design of hydraulic power take-off systems unit parameters for multi-point absorbers wave energy converter

Ocean waves represent a substantial renewable energy source, prompting this study's exploration of harnessing this potential via multi-point floating absorbers equipped with hydraulic Power Take-Off (PTO) systems. The study's primary motivation is to comprehensively model a Wave Energy Converter (WEC), including the hydraulic PTO system's control mechanism and protection system. To optimize the WEC design, we first investigate the hydrodynamic parameters of the floating absorber, employing potential flow theories and assessing the effects of varying the diameter-to-draft ratio. Our research involves analyzing Response Amplitude Operators (RAOs) and hydrodynamic forces for different ratios to identify the most effective design, followed by using computed wave forces as inputs for PTO simulations in MATLAB/SimScape to determine the unit parameters. The analysis of RAOs and wave-exciting forces reveals that diameter-to-draft ratios between 0.67 and 1.0 yield the best WEC performance, offering promising recommendations. In the final step, we compare the performance of single and multi-point absorber system simulations, assessing factors such as pressure drop, flow rate, and power output at the hydraulic motor. This comparison concludes that multi-point absorbers generate a more stable power output than their single-absorber counterparts.

Hydrodynamic performance and energy redistribution characteristics of wind–wave hybrid system based on different WEC microarrays

Wave energy converters (WECs) are often arranged in the form of microarrays to maximize the energy capture and synergistic effects in a wind–wave hybrid system. However, the hydrodynamic coupling in hybrid systems is complicated and tough to directly evaluate. Although there have been numerous studies on wind–wave hybrid systems, the influence of the structural form of the microarray on the interactions and performance of WECs has not yet been thoroughly investigated. Hence, the hydrodynamic coupled modelling of a hybrid system considering different WECs microarrays was developed. Based on this system, the relationship between the microarray arrangement and hydrodynamic performance of the hybrid system was studied, coupled with the energy redistribution characteristics of the microarray. The results show that the gain of WECs microarray to the floating offshore wind turbine in pitch mode decreases with increasing wave direction. Further, Upstream WECs alter the wave field experienced by downstream ones through radiation and diffraction waves, thereby affecting the energy capture of downstream WECs. This study presents insights into the effects of WECs microarrays on hydrodynamic coupling and energy redistribution characteristics in hybrid systems, which will help guide the design of WEC layout in wind–wave hybrid systems.

Optimal strategy of the asymmetric wave energy converter survival in extreme waves

Enhancing the survival performance of wave energy converters (WECs) in extreme wave conditions is crucial, and reducing wave loads is a key aspect of this. Placing the device underwater has been recognized as a beneficial strategy, yet the determination of the optimal submerged depth and the effects of varying wave conditions remain ambiguous. To address this, the study numerically analyzes the total forces in both horizontal and vertical directions, along with their harmonic components, across different wave configurations. A computational fluid dynamics method is employed to investigate a triangular-baffle bottom-shaped oscillating floater, which is known for its high energy conversion efficiency. The findings indicate that submerging the device to a depth equivalent to half the actual focused amplitude (1/2Ab) is the most effective strategy in the given sea state, offering superior wave force reduction vertically and robust performance horizontally. The analysis of harmonics reveals the significant contribution of high-order components to the total wave forces. Additionally, the study examines the impact of focused wave amplitudes and peak frequencies, showing that although force reductions are lessened in more extreme conditions, the optimal submerged depth of 1/2Ab still yields near 30% reduction in total vertical force and 22% in total horizontal force. This research provides theoretical insight that can guide the enhancement of WECs' survival capabilities in practical engineering applications.

Hydrodynamic performance of vertical cylindrical wave energy absorbers in front of a vertical wall

This paper studies the hydrodynamic performance of vertical cylindrical absorbers in front of a vertical wall. All the absorbers are independent of each other and restricted to only the heave motion. Based on a linear potential flow theory, an analytical solution is developed for the problems of wave diffraction and radiation by absorbers. In the solving procedure, the hydrodynamic problem is first transformed into an equivalent problem in an open water domain using the image principle. The number of absorbers in the equivalent problem is twice that in the real problem, and the plane layout is symmetric about the original vertical wall. The velocity potential of the fluid domain is obtained using the method of variable separation, and the unknown expansion coefficients in the velocity potential are determined by the matched boundary conditions. The heave excitation force, added mass, radiation damping, motion response, and energy capture width of the absorbers are calculated. Case studies are presented to show the effects of the wall reflection and hydrodynamic interaction on the energy extraction performance of the wave energy converter (WEC) system. Subsequently, the WEC performance under the action of irregular waves is analyzed by considering an incident wave spectrum, and the mean annual absorbed power of the device is estimated by considering the wave data statistics at the actual sites. The results indicate that when the wave motion resonates with the absorber motion, the energy extraction performance of the absorbers is significantly improved. The performance of the absorbers can be effectively improved when the structures are close to the antinodes of a standing wave field. By designing a reasonable plane layout, the hydrodynamic interaction can play a constructive role in the performance of the WEC system.

Economic feasibility study for wave energy conversion device deployment in Faroese waters

As the world continues to battle with increasing energy demand along with the serious effects of climate change, there is a need for more reliable renewable energy sources. The objective of the present work is to study the economic feasibility for deployment of wave energy conversion devices in the Faroese coastal region. We analyze eight different wave energy conversion concepts under development at nine different nearshore locations in the Faroese coastal region. The nine different locations are classified into three coastal locations — three on the west coast, three on the east coast and three on the north coast. The eight wave energy conversion devices mostly rely on different working principles, though some of these are similar. Results show that there is quite a significant difference in the performance of the devices from coast to coast. There is also a difference between the different conversion devices as they rely on different principles and the suitability of each device varies from location to location. We also show that the Faroe Islands are a suitable location for wave energy converter deployment compared to other places. Furthermore, results show that the performance of wave energy converters in the Faroe Islands can prove to be a direct competitor to offshore floating wind energy.

A Numerical Investigation of the Hydrodynamic Performance of a Pitch-Type Wave Energy Converter Using Weakly and Fully Nonlinear Models

In this study, the performance of a wave energy converter (WEC) rotor under regular and irregular wave conditions was investigated using 3D nonlinear numerical models. Factors such as the power take-off (PTO) load torque, wave periods, spacing of multiple WEC rotors, and wave steepness were analyzed. Two models were employed: a weakly nonlinear model formulated by incorporating the nonlinear restoring moment and Coulomb-type PTO load torque based on the potential flow theory, and a fully nonlinear model based on computational fluid dynamics. The results show that the average power estimated by both numerical models is consistent, with a wave steepness of 0.03 for the range of one-way and two-way PTO load torques, except for the deviations observed in the long range of the one-way PTO load torque. Furthermore, the average power of the WEC rotor under the applied PTO load torque exhibits a quadratic dependency, regardless of the wave steepness. In addition, adopting a one-way PTO load torque was more efficient than adopting a two-way PTO load torque. Therefore, the fully nonlinear model demonstrated its ability to handle a high degree of nonlinearity, surpassing the limitations of the weakly nonlinear model, which was limited to moderate wave steepness.

Performance analysis of the combination of a pitching wave surge energy converter and floating breakwaters

ABSTRACT Wave Energy Converters (WEC) captured the wave energy while Floating Breakwaters (FB) attenuated the wave energy to make a calm area for harbour activities. This study investigates the combination of a pitching Wave Energy Converter (WEC) and Floating Breakwater (FB) for simultaneous wave energy capture and wave attenuation in harbour areas. Three different configurations were analyzed in nine different hydrodynamic conditions to optimise the performance of both FB and WEC. The results show that the optimised combined device with rectangular draft has the most promising performance, with an average wave transmission operation of 38.8% and a captured power of 533.2 kW across all conditions. Longer arms of the device also lead to better performance. Additionally, wave steepness was found to be a significant factor in the performance of the combined device.

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.

Study on the power performance of wave energy converters mounted around an offshore wind turbine jacket platform

With the deepening of renewable energy research, the economic-friendly multi-energy coupling development method is gradually favored. This paper proposes a jacket-mounted hybrid wind-wave energy converter, which includes a bottom-fixed offshore wind turbine (OWT) and a microarray composed of three point-absorbing wave energy converters (WECs). Numerical simulations were conducted to study the power performance of the wave energy converter array, and a model test with a scale of 1:8 was carried out to calibrate and validate the numerical model. The array converts wave energy through a hydraulic system, and a dynamic-link library was created to simulate the hydraulic power take-off (PTO) system that can take into account the real-time pressure changes of the accumulator. The time domain simulation results show that the maximum capture width ratio (CWR) of the WEC array exceeds 0.5 when the wave period is nearly to the natural period, and the optimal CWR of a single WEC exceeds 0.8. When the wave period is less than 2 times the natural period, the energy capture behavior of WEC is sensitive to the change in wave direction, and this sensitivity is weakened when the device forms an array. Specifically, a single WEC prefers waves to be incident from the back side, while the WEC array prefers waves to be incident obliquely. This work will provide design guidance for the engineering application of stationary hybrid wind-wave energy concepts.

Numerical and Experimental Investigation of the Dynamics of a U-Shaped Sloshing Tank to Increase the Performance of Wave Energy Converters

In this investigation, a comprehensive study was conducted on a U-shaped sloshing tank, based on reversing the classical treatment of such devices as motion stabilizers and using them instead to improve the performance of wave energy converters. The modeling encompasses a comparative analysis between a linear model and Computational Fluid Dynamics (CFD) simulations. The validation of the CFD methodology was rigorously executed via a series of experimental tests, subsequently enhancing the linear model. The refined linear model demonstrates a notable alignment with rigorously verified results, thus establishing itself as a reliable tool for advanced research, indicating promise for various applications. Furthermore, this novelty is addressed by simulating the integration of a U-tank device with a pitch-based wave energy converter, displaying a broadening of the operational bandwidth and a substantial performance improvement, raising the pitch motion of the floater to about 850% in correspondence with the new secondary peak over extended periods, effectively addressing previously identified limitations. This achievement contributes to the system’s practical relevance in marine energy conversion.

Adaptive systematic optimization of a multi-axis ocean wave energy converter

Due to the growing global energy needs, renewable energy systems, particularly wave energy converters (WECs), are a feasible solution to satisfy current energy demand. Recently, wave farms with diverse technologies, geometries, and layouts have been developed; however, evaluating the performance of these devices is complicated and requires precise hydrodynamic modeling to efficiently deploy wave farms. This study proposes a multi-scenario model using boundary element method (BEM) solver, NEMOH, integrated with evolutionary many-objective algorithms to evaluate the performance of a multi-axis point absorber WEC with respect to cylindrical, triangular, quadrilateral, and octagonal geometries and varying dimensions, that is, radius, draft, and height. To this end, six objective functions were considered to maximize the energy absorption and significant velocity and to minimize the separation distance, levelized cost of energy, net present value, and q-factor. Accordingly, three EMnO frameworks were utilized: the non-dominated sorings genetic algorithm (NSGA-III), reference point-based NSGA-III (R-NSGA-III), and multi-objective evolutionary algorithm by decomposition (MOEA/D). The results of the three optimization algorithms indicate that R-NSGA-III converges faster than the other two and also found that the cylindrical and octagonal geometries produce more annual energy among other forms. Comparing the performances of the three different layouts for cylindrical and octagonal geometries reveals that the arrow layout with thirty buoys produced more energy and had a lower levelized cost of energy and net present value.

On the Effect of Wave Direction on Control and Performance of a Moored Pitching Wave Energy Conversion System

In the pathways towards the commercialisation of wave energy systems, the need for reliable mathematical models is of paramount importance for the design and synthesis of model-based control techniques to maximise the performance of wave energy converters (WECs). Furthermore, these offshore marine systems are held in position by the use of mooring systems, which have recently been analysed beyond survivability conditions to investigate their influence on control synthesis and device performance. In this study, we delve into the complex challenge of incorporating relevant mooring dynamics in defining a representative control action while also examining the influence of wave directionality on the overall procedure. For the specific case of a spread mooring system, where the hull cannot weathervane and operates based on directionality, control synthesis must be performed taking into account this characteristic of the resource. In this context, because it is able to harvest energy from only the bow-directed waves, the PeWEC is considered as a representative case study. The control synthesis is realised using a tailored data-based model, and device performance is evaluated across different site conditions while accounting for wave direction. Among our overall conclusions, we show that neglecting the directionality of the wave resource for the PeWEC case study can lead to an overestimation of device performance of up to 50%, even though a prevalent wave direction exists at the site.

Control co-design and uncertainty analysis of the LUPA’s PTO using WecOptTool

Control co-design has been shown to significantly improve the performance of wave energy converters (WEC). By considering the control and WEC design concurrently, the space searched by the optimization routine is greatly expanded which results in better performing devices. Recently, an open-source WEC co-design code, WecOptTool, was released to perform control co-design research and facilitate its adoption in the community. In this study, we use WecOptTool to perform control co-optimization and uncertainty analysis of the LUPA device. The Laboratory Upgrade Point Absorber (LUPA) is a new open-source laboratory-scale WEC that provides a platform for testing new concepts, innovating control schemes, and validating numerical models. The LUPA can be adjusted to different configurations, including changing the number of bodies, the degrees of freedom (DOF), the float and spar geometry, and the diameter of the drive sprocket pulley in the power take off (PTO) system, as well as providing different control algorithms and input waves. The drive sprocket diameter influences the torque vs speed of the generator, which allows for more flexibility in operating under different wave conditions or with different control schemes. In this study we optimize the drive sprocket diameter, while considering the optimal control algorithm for each potential design, to identify the optimal diameter for electric power production at the PacWave South WEC test site. This case study demonstrates several new capabilities of WecOptTool including a multi-body multi-DOF system and multi-directional irregular waves. The PTO dynamics are modeled using first principle methods for a parametrized model of the mechanical subcomponents in combination with generator model obtained using a power-invariant Park transform. The case-study will be made available to serve as a design tool along the LUPA hardware. Users can readily use this model to perform their own design optimization prior to testing with the physical LUPA device. Finally, we use the automatic differentiation capability of WecOptTool to perform a sensitivity and uncertainty analysis of the LUPA device.

Effects of control strategies on the performance of floating WEC point absorbers operating attached to a breakwater by time-domain simulator

The estimation of the performance of Wave Energy Converter (WEC) arrays of the type of simple floaters operating in nearshore and coastal areas, characterized by variable bottom topography, is important for the estimation of the wave power absorption and determination of the operational characteristics of the system and could significantly contribute to the efficient design and layout of WEC farms. For this purpose, full 3D models based on Boundary Element Method have been developed, supporting the systematic use for optimization studies. Apart from WEC arrays in nearshore and coastal regions a promising possibility is the installation at the exposed side of port breakwaters with significant energy potential. In this work, the analysis of the above system is extended to the prediction of WEC performance in multichromatic incident waves under latching control strategy using a time-domain simulator, showing improvement of performance in particular conditions.

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.

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).

Hybrid linear potential flow - machine learning model for enhanced prediction of WEC performance

Numerical models based on the linear potential flow equations are of paramount importance in the design of wave energy converters (WECs). Over the years methods such as wave stretching, nonlinear Froude-Krylov and Morrison drag have been developed to overcome the short-comings of the underlying assumptions of small amplitude wave, small motion and inviscous flow. In this work we present a different approach to enhance the performance of the linear method: a hybrid linear potential flow – machine learning (LPF-ML) model. A hierarchy of high-fidelity models – Reynolds-Averaged Navier-Stokes, Euler and fully nonlinear potential flow – is used to create training data for correction factors targeting nonlinear hydrodynamics, pressure drag and skin friction, respectively. Long short-term memory (LSTM) networks are then trained and added to the LPF model. LSTM networks are heavy to train but fast to evaluate so the computational efficiency of the LPF model is kept high. Simple decay tests of generic bodies (sphere, box, etc) are used to validate the LPF-ML model. Finally, the LPF-ML is applied to a model-scale point-absorber WEC to assess the power production.

Multi-PTO Wave Energy Converter for Low Energetic Seas: Ensenada Bay Case.

The paper presents a wave energy converter concept to harvest energy at Ensenada Bay, Mexico. The study area can be classified as a low energy sea due to the mean wave power being around 10 kW/m; the wave conditions are significant wave height of 0.5 to 2.5 and peak period of 5 to 20 s. The wave energy converter is formed by a torus buoy and anchored to the sea bottom by four connection structures distributed every 90°. The connected structures play a piston role, and their response is leveraged to run a power take-off system, for example, a linear or hydraulic system. This work does not focus on designing the power take-off system; therefore, it is simplified as linear damping, and an iterative process calculates its value. The hydrodynamic buoy study is made on the frequency domain with Nemoh BEM solver and the power absorption on the time domain with the WEC-Sim code. The capture width ratio is used to evaluate the wave energy converter performance and is a key factor in choosing the optimal size. The wave energy converter captures an average of 20% of the available energy and is well-fixed between periods 8 and 16 s. The present study achieves its target: providing a wave energy converter system to operate under site wave conditions.

Hydrodynamic investigation of a parabolic breakwater for wave energy focusing

The integration of wave energy converters (WECs) with a breakwater can reduce construction and maintenance costs and enhance the energy conversion performance of WECs. Intelligent use of a parabolic breakwater to focus water waves to a specific region can significantly increase the available wave energy. However, the theoretically calculated focal point of a parabolic breakwater may not always coincide with the observed wave focusing position. To determine the actual position of the focal point for installing the WEC, both numerical simulations and experimental research are conducted to investigate hydrodynamic characteristics of the parabolic breakwater. The realized focal point is found to be located at a position separated from the theoretical position, and its location varies with the incident wavelength. The underlying cause of this behavior is identified and investigated. The effects of chord length, parabolic wall aspect ratio, and incident wave directions on the wave amplitude at the focal point are examined. It is found that the shift in the actual focusing position exhibits a cyclic configuration as the incident wavelength changes, with the observed focal point revolving around the theoretical focal point. The position of the recorded focal point is determined by the interaction between the incident and reflected wave crests. The wave amplitudes at the theoretical focal point reaches their maxima when the focal lengths are nearly equal to integer multiples of half incident wavelength. The wave amplitudes at both the theoretical and observed focal points are shown to magnify with increasing chord length and with decreasing focal length. The parabolic wall is demonstrated to consistently maintain a substantial focusing effect for incident wave angles up to 15°.

Hydrodynamic performance of wave energy converter integrated with pile restrained floating structure near a partially reflecting seawall

The integration of a Wave Energy Converter (WEC) with a Pile-Restrained Rectangular Floating Breakwater (PRFB) in the presence of a partially reflecting vertical seawall is analysed to enhance the hydrodynamic performance and WEC efficiency of the integrated breakwater-WEC device based on small amplitude wave theory using the Boundary Element Method (BEM). The rectangular floating breakwater is designed to have heave motion with a pile-restrained floating structure placed in a position to attenuate the incoming wave in the transmitted region and the linear power take-off (PTO) damping is employed to calculate the absorbed power. The study is performed to understand the effectiveness of wave energy conversion and its hydrodynamic performance due to changes in the seawall's porosity, relative structural width, relative structural draft, wave energy conversion power take-off damping coefficients, and the relative gap of the WEC integrated with PRFB from the seawall. The study demonstrated that in the presence of a fully reflecting seawall, the wave energy extraction is enhanced for the integrated WEC system without compromising the defined threshold wave reflection coefficient but at the expense of a constrained range of wavenumbers that correspond near the system's fundamental natural frequency. Moreover, the capture width ratio is noted to be higher for relatively smaller structural drafts, while the wave reflection coefficient shows precisely the reverse trend. However, under such circumstances, the integrated WEC system operates as a motion-trapping structure, especially when the reflection coefficient of the seawall, CR≥0.75. Thus, the present study will assist the designer in determining the appropriate degrees of efficiency of the WEC device without sacrificing hydrodynamic performance by fine-tuning the hybrid floating breakwater system's geometrical parameters.