Simulation Of Wave Energy Converter Research Articles

Review of TEAMER Awards for WEC-Sim Support

Testing Expertise and Access for Marine Energy Research (TEAMER) is a U.S. Department of Energy Water Power Technologies Office sponsored program, overseen by the Pacific Ocean Energy Trust, which aims to advance the state of marine energy technologies. The program connects technology developers with experts at U.S. facilities, including numerical modeling and analysis facilities, like WEC-Sim. The WEC-Sim facility is supported by the WEC-Sim development team at Sandia National Laboratories and the National Renewable Energy Laboratory. WEC-Sim (Wave Energy Converter SIMulator) is an open-source software for simulating wave energy converters. WEC-Sim can model the multi-body dynamics of devices comprised of bodies, joints, power take-off systems, and mooring systems. Since TEAMER’s first round of support in 2020, there have been eighteen TEAMER awards focused on numerical model development in WEC-Sim. TEAMER awards for WEC-Sim support have modeled a wide range of wave energy converter archetypes, including point absorbers, attenuators, oscillating water columns, and many other novel architectures. A wide variety of studies have been conducted, leading to important insights for TEAMER partners and software improvements for WECSim. This article highlights several successful WEC-Sim TEAMER awards. The awards described herein include TEAMER recipients Ocean Motion Technologies, AquaHarmonics, iProTech, East Carolina University, Virginia Tech, Maiden Wave Energy, and the University of Massachusetts Dartmouth. The awards of these seven partners contain a wide range of investigations and cover the creation of baseline hydrodynamic models, PTO modeling, geometry optimization in both boundary element methods and WECSim, and model tuning and validation.

Spectral approach to evaluate multi-body floating wave energy converters in complex sea states

This study presents a novel approach for evaluating Multi-Body Floating Wave Energy Converters (MBFWEC) operating in complex sea states. To this end, a Transfer Function (TF) over a desired range of wave frequencies is developed by investigating power absorption capacities for four MBFWEC configurations with different floating shapes, diameters, and drafts. To examine the performance of MBFWEC systems in a practical case, this approach is applied to the wave data measured at the northern coast of the Oman Sea where complex sea states dominate. Hydrodynamic coefficients are obtained in the frequency domain using AQWA at first. Then, time domain analyses are carried out utilizing an open-source tool called WEC-Sim (Wave Energy Converter Simulator) and MATLAB®-Simulink®. Finally, the developed TF was multiplied by the region's Mean Wave Power Spectrum (MWPS) to determine the total extracted power for each configuration. The proposed spectral approach was compared to a direct method based on time history analysis, and the results demonstrated strong compatibility, thus ensuring the method's accuracy. Since the proposed TF is independent of sea states and its application is simple, it can be implemented efficiently in any region with complex sea states to evaluate or tune any FWEC device. Based on the results, it was found that a deeper draft of floating members enables the capturing of power from waves with larger periods, which is desirable in the study region. Additionally, the introduced method allows for the adjustment of the TF of an MBFWEC to be consistent with the MWPS to achieve the highest performance.

Review of WEC-Sim development and applications

WEC-Sim (Wave Energy Converter Simulator) is an open-source code for simulating wave energy converters, which has been actively developed and applied to simulate a wide variety of device archetypes, and has become a popular tool since its release. This paper reviewed the development efforts and the usage of WEC-Sim. The publications considered in this study have been broken down into six topic areas, namely feature development, experimental validation, device modeling, control modeling, PTO and grid modeling, and novel applications, which even includes some non-wave energy applications. This review paper has also attempted to recognize the contributions of the broader WEC-Sim development effort, meaning not only the internal WEC-Sim development team but also the external efforts from the academia researchers and technology developers around the world. The growing trend of external applications of WEC-Sim has demonstrated the broader acceptance of the open-source code, and how WEC-Sim has been used in a certain topic area also highlights the potential future development needs.

イナーター機構による洋上浮体向け外設型制振発電装置の数値的検証

In recent years, ocean energy such as wind and waves has been attracting attention as promising renewable energy sources. Especially, offshore wind turbines (OWTs) show great potential, and various kinds of OWTs have been proposed so far. Among them, floating OWTs are considered more suitable for Japan due to the deep waters in the nearshore area. However, floating type is vulnerable to wind- and wave-induced vibration to compared to bottom-fixed OWTs. On the other hand, a wave energy converter (WEC) with an inerter mechanism consisting of a rotational mass was proposed. Thus, this study proposes novel externally installed devices employing inerters for vibration mitigation and wave energy conversion of floating OWTs. A simple analytical model including the effects of additional mass, radiation damping, and wave excitation force on the floating body is developed for the heave motion behavior of the proposed floating OWT. Then, appropriate design parameters are examined, and the vibration mitigation and power generation performance are investigated through numerical simulation studies to irregular waves generated by the JONSWAP spectrum using WEC-Sim (Wave Energy Converter SIMulator). The result shows that the proposed device works well to control the floating OWT and generate energy.

Comparative Study of Oscillating Surge Wave Energy Converter Performance: A Case Study for Southern Coasts of the Caspian Sea

The search for renewable energy supplies for today’s global energy demand, particularly ocean wave energy for coastal areas, has become undeniably widespread in the last two decades. The Caspian Sea represents an immense opportunity for using ocean renewable energy, especially considering its long shoreline. In this study, the locations with maximum potential wave energy were chosen in the central, eastern, and western zones of the Caspian Sea’s southern coasts. Accordingly, the wave and bathymetric data were used as the input to calculate the oscillating surge wave energy converter’s flap geometric dimensions based on previous studies. Then, the geometric model was designed, and then the wave energy converters were modeled in the Wave Energy Converter Simulator (WEC-Sim) module in the MATLAB software. Furthermore, eight models in each sea state were simulated to find the best value of the PTO damping coefficient, which led to the highest capture factor. Finally, all the external forces on the WEC’s flap and the converter’s power output results were compared, taking into account the effects of the flap height on the total power output. It was found that Nowshahr port has more potential than the Anzali and Amirabad ports, as the converter’s absorbed power proved to be 16.7 kW/m (Capture factor = 63%) at this site. Consequently, by conducting a comparative analysis between the selected sites, the excitation, radiation damping, and power take-off forces were scrutinized. The results show that the highest applied forces to the converter’s flap occurred at Nowshahr port, followed by the Anzali and Amirabad ports, due to the directional characteristics of the waves at the central coasts of the Caspian Sea.

Numerical study of bean-float wave energy converter with float number parametrization using WEC-Sim in regular waves with the Levelized Cost of Electricity assessment for Indian sea states

Present study deliberates on the numerical analysis of a new, bean-shaped, multi-body floating wave energy converter (BFWEC) using an open-source time-domain modeling tool called WEC-Sim (Wave Energy Converter SIMulator). The proposed device is a directionally-insensitive, floating-point absorber with a set of bean-shaped floats attached circumferentially around a central cylinder or buoy (CB or CS), extracting the wave energy under heave degree due to the relative motion of the floats with CB. The shape and arrangement of the floats around the CB make the BFWEC different from other existing devices. Authors propose three different layouts of BFWEC with multiple floats, namely: 4, 6, and 8, to study the influence of float's number on the device's overall performance. All layouts are incorporated with a linear damping power take-off (PTO) system to estimate absorbed power where the influence of damping on the performance is studied. The frequency-dependent parameters required to initiate time-domain analysis are obtained from the boundary element method (BEM). The influence of multi-float interactions on performance is assessed over an isolated single float arrangement. The Levelized Cost of assessment (LCoE) of BFWEC is calculated for various sea states of the Arabian Sea and the Bay of Bengal.

Hardware-in-the-loop simulation of wave energy converters based on dielectric elastomer generators

Dielectric elastomer generators (DEGs) are soft electrostatic generators based on low-cost electroactive polymer materials. These devices have attracted the attention of the marine energy community as a promising solution to implement economically viable wave energy converters (WECs). This paper introduces a hardware-in-the-loop (HIL) simulation framework for a class of WECs that combines the concept of the oscillating water columns (OWCs) with the DEGs. The proposed HIL system replicates in a laboratory environment the realistic operating conditions of an OWC/DEG plant, while drastically reducing the experimental burden compared to wave tank or sea tests. The HIL simulator is driven by a closed-loop real-time hydrodynamic model that is based on a novel coupling criterion which allows rendering a realistic dynamic response for a diversity of scenarios, including large scale DEG plants, whose dimensions and topologies are largely different from those available in the HIL setup. A case study is also introduced, which simulates the application of DEGs on an OWC plant installed in a mild real sea laboratory test-site. Comparisons with available real sea-test data demonstrated the ability of the HIL setup to effectively replicate a realistic operating scenario. The insights gathered on the promising performance of the analysed OWC/DEG systems pave the way to pursue further sea trials in the future.

Time-domain analysis of a bean-shaped multi-body floating wave energy converter with a hydraulic power take-off using WEC-Sim

A novel floating wave energy converter (BFWEC) with multiple ‘bean shape’ floats and a hydraulic power-take-off system (HPTO) is presented. The overall setup consists of a central buoy (C·B.) connected to a set of floats around it using a lever arm and is position restrained using a taut-mooring system. Three configurations of BFWEC are proposed with four, six, and eight number of floats. A detailed numerical analysis is carried out by modeling the device in a time-domain using an open-source code: WEC-Sim (Wave Energy Converter Simulator), assuming potential wave theory (PWT). By incorporating HPTO to all the three configurations, simulations are performed in monochromatic waves. Appropriate parameters of HPTO components for each configuration are estimated for the maximum capture width of BFWEC. The effect of the HPTO damping coefficient in maximizing the output of BFWEC is also assessed. Despite the dimensions of BFWEC tailored to the shallow-intermediate Indian waters, the device is simulated with extreme wave characteristics to test its conversion characteristics under the wave conditions away from the resonance.

Numerical analysis of a new multi-body floating wave energy converter with a linear power take-off system

Abstract A floating wave energy converter (FWEC) with central buoy (CB), circumferentially connected by a set of four, smoothly-finished tubular floats is proposed. The central buoy and floats are connected rigidly at one end while the other end is hinged, enabling a relative motion. The rotation of floats about the CB represents the energy extraction mode of the device. Float shapes of five different configurations, including a newly-proposed ‘bean float,’ are investigated to estimate their power outputs with linear PTO. FWEC remains afloat and re-centered using taut-moored tethers. The frequency-domain model is utilized to determine the hydrodynamic coefficients of the device, assuming a Potential Wave Theory (PWT). The research evaluates the impact of the obtained hydrodynamic coefficients on the average power absorbed by the FWEC. The influence of a linear power take-off (PTO) system on the average power output of FWEC is studied by optimization of the damping coefficients for the encountered regular waves. Further, time-domain analysis is carried out on the ‘bean float’ - FWEC configuration using an open-source computer-aided engineering tool called WEC-Sim (Wave Energy Converter Simulator), and MATLAB® - Simulink® implementing nonlinear buoyancy and Froude-Krylov forces.

CFD Simulation of Wave Energy Converters in Focused Wave Groups Using Overset Mesh

This paper presents the numerical modelling of two point absorber wave energy converters (WECs) with and without a moonpool under focused wave conditions. The numerical model applies the overset mesh technique in order for the mesh to conform with the large-amplitude WEC motion induced by the focused wave groups. The incident wave group is first examined by a mesh convergence test and by comparing with the experimental data. The simulations are then carried out with the presence of the WEC. In total, three wave conditions are considered, each with the same wave period but with different wave heights. Nonlinear effects on the WEC motion are clearly exhibited when the wave steepness increases. The accuracy of the numerical results is carefully assessed against experimental data. Furthermore, the effects of the moonpool on the dynamics of the WEC are also discussed, where the WEC motion is compared for the case with and without a moonpool under the same wave conditions.

Efficient Nonlinear Hydrodynamic Models for Wave Energy Converter Design—A Scoping Study

This review focuses on the most suitable form of hydrodynamic modeling for the next generation wave energy converter (WEC) design tools. To design and optimize a WEC, it is estimated that several million hours of operation must be simulated, perhaps one million hours of WEC simulation per year of the R&D program. This level of coverage is possible with linear potential flow (LPF) models, but the fidelity of the physics included is not adequate. Conversely, while Reynolds averaged Navier–Stokes (RANS) type computational fluid dynamics (CFD) solvers provide a high fidelity representation of the physics, the increased computational burden of these models renders the required amount of simulations infeasible. To scope the fast, high fidelity options, the present literature review aims to focus on what CFD theories exist intermediate to LPF and RANS as well as other modeling options that are computationally fast while retaining higher fidelity than LPF.

LTI energy-maximising control for the Wave Star wave energy converter: identification, design, and implementation

Considering the Wave Star wave energy converter (WEC), which is a standard point absorber prototype, this study addresses the complete energy maximising control design procedure. The WEC model is obtained using system identification routines. Then, using the identified model, a LTI energy maximising control strategy, recently presented in the literature, is designed. Additionally, for the wave excitation force estimation, a standard Kalman filter with a harmonic oscillator is considered. Finally, for the assessment of the performance of the complete system, the controller and estimator are implemented in the numerical WEC simulation environment WEC-Sim. The system is tested under realistic conditions and satisfying performance of the LTI controller is shown. Thus, through a non-conventional approach, and considering a realistic software environment, a novel energy maximising controller is implemented, obtaining results which indicate the feasibility of the approach.

Evaluation of the overset grid method for control studies of wave energy converters in OpenFOAM numerical wave tanks

Computational fluid dynamics (CFD)-based numerical wave tanks are valuable tools for the development and evaluation of energy maximising control systems for wave energy converters (WECs). However, the exaggerated body motion amplitude, which can be induced by the energy maximising control system, challenges the commonly applied mesh morphing method in CFD, due to the resulting mesh distortion and subsequent numerical instability. A more advanced mesh motion method is the overset grid method, which can inherently handle large-amplitude body motions and has recently become freely available in the open-source CFD software OpenFOAM. The overset grid method can, therefore, potentially eliminate the mesh distortion problem, hindering the simulation of WECs under controlled conditions. To evaluate the capability of the overset grid method for control studies of WECs in an OpenFOAM numerical wave tank, this paper presents a detailed comparison of the overset grid and mesh morphing methods, considering five test cases of increasing complexity. The test cases range from a static equilibrium test to the modelling of a controlled WEC, and good agreement is demonstrated between the two mesh motion methods, except for the case of the controlled WEC, when the device motion becomes large, and the mesh morphing simulation crashes. The runtimes for overset grid simulations are observed to be approximately double the time required for the mesh morphing simulations.

Modeling of a Wave Energy Oscillating Water Column as a Point Absorber Using WEC-Sim

This study evaluates the accuracy of using the open-source wave energy converter modeling Wave Energy Converter-SIMulator (WEC-Sim) code to model an oscillating water column (OWC) as a virtual point absorber. WEC-Sim is a free open-source hydrodynamic modeling extension of MATLAB/Simulink and is designed primarily for ease of use and the modeling of oscillating body wave energy converters (e.g., point absorbers). OWCs are a different class of wave energy converter and can be difficult to model. If OWCs can be accurately modeled as oscillating bodies, it may more easily enable the use of well-established hydrodynamic theory and control theory that exists for oscillating bodies for better simulated estimation of performance. A comparison of simulation and experiment was conducted on OWCs in the OH Hinsdale Wave Research Laboratory, Oregon State University. Results show good agreement between the experimental OWCs and the simulated virtual point absorber over most ranges of operation, with the exception of near-resonant frequencies, in which case the virtual point absorber over-predicts performance. For regular waves, the accuracy of modeling an OWC as a point absorber was generally within 5%, except near the device resonant period, in which case the heave amplitude modeling error was 11.4% and the power modeling error was 25.0%. For an irregular sea state of dominant period 2.6 s, the modeling error in power was 2.0%.

Experimental and numerical comparisons of a dual-flap floating oscillating surge wave energy converter in regular waves

The floating oscillating surge wave energy converter (FOSWEC) is a wave energy converter that was designed, built, and tested to develop an open-access data set for the purpose of numerical model validation. This paper details the experimental testing of the 1:33-scale FOSWEC in a directional wave basin, and compares experimental data to numerical simulations using the wave energy converter simulator (WEC-Sim) open-source code. The FOSWEC consists of a floating platform moving in heave, pitch, and surge, and two pitching flaps. Power is extracted through relative motion between each of the flaps and the platform. The device was designed to constrain different degrees of freedom so that it could be configured into a variety of operating conditions with varying dynamics. The FOSWEC was tested in a range of different conditions including: static offset, free decay, forced oscillation, wave excitation, and dynamic response to regular waves. In this paper, results from the range of experimental tests are presented and compared to numerical simulations using the WEC-Sim code.

Analysing the Near-Field Effects and the Power Production of Near-Shore WEC Array Using a New Wave-to-Wire Model

In this study, a series of modules is integrated into a wave-to-wire (W2W) model that links a Boundary Element Method (BEM) solver to a Wave Energy Converter (WEC) motion solver which are in turn coupled to a wave propagation model. The hydrodynamics of the WECs are resolved in the wave structure interaction solver NEMOH, the Power Take-off (PTO) is simulated in the WEC simulation tool WEC-Sim, and the resulting perturbed wave field is coupled to the mild-slope propagation model MILDwave. The W2W model is run for verified for a realistic wave energy project consisting of a WEC farm composed of 10 5-WEC arrays of Oscillating Surging Wave Energy Converters (OSWECs). The investigated WEC farm is modelled for a real wave climate and a sloping bathymetry based on a proposed OSWEC array project off the coast of Bretagne, France. Each WEC array is arranged in a power-maximizing 2-row configuration that also minimizes the inter-array separation distance d x and d y and the arrays are located in a staggered energy maximizing configuration that also decreases the along-shore WEC farm extent. The WEC farm power output and the near and far-field effects are simulated for irregular waves with various significant wave heights wave peak periods and mean wave incidence directions β based on the modelled site wave climatology. The PTO system of each WEC in each farm is modelled as a closed-circuit hydraulic PTO system optimized for each set of incident wave conditions, mimicking the proposed site technology, namely the WaveRoller® OSWEC developed by AW Energy Ltd. The investigation in this study provides a proof of concept of the proposed W2W model in investigating potential commercial WEC projects.

Simulation of wave energy converter with designed pendulor-slope combination

Abstract Nowadays the global demand for energy is growing rapidly in China, which will be the world’s largest country of energy consumption, it is thus highly urgent to conduct the research and development of efficient renewable wave energy equipment, e.g. wave energy converter (WEC). So, this paper aims to numerically develop a new WEC suitable for wave conditions in Zhoushan sea area. The specific research contents include the following aspects: (1) theoretic design by using the floating pendulum to capture the wave energy; (2) Computational Fluid Dynamics (CFD) simulation which is based on 3-Dimensional unsteady Reynolds Average Navier-Storkes (RANS) equations to analyze the hydrodynamic performance of floating pendulor capturing wave energy. The initial design has been optimized to find the parameters of swing device floating in different output loads of energy capture, and to recognize the conditions of the device for subsequent energy conversion system design.

A Nonlinear Frequency-Domain Approach for Numerical Simulation of Wave Energy Converters

Nonlinear, analytical wave-energy converter (WEC) models are generally simulated through time-domain (TD) numerical integration. However, the relatively high computational requirements of TD integration are not compatible with applications where a large number of simulations are needed. Spectral domain (SD) linearization has also been proposed to take into account some nonlinear effects, while being much faster than TD integration. However, as explained in this paper, such SD models have limited accuracy, and cannot extend to the static nonlinear forces. In this paper, a nonlinear frequency-domain (NLFD) method is investigated for WEC simulation, using a projection of the dynamical equations onto a basis of trigonometric functions. A comparison with TD integration (second-order Runge$-$Kutta-RK2) and SD methods is provided, through theoretical considerations, and by means of numerical simulations based on two case studies. In the cases considered, the proposed NLFD method allows for significant computational savings compared to RK2 integration, without requiring any approximation for the radiation forces. NLFD thus shows promising potential for applications involving extensive WEC simulation, such as power production assessment, while preserving the WEC model accuracy. Although slower than SD, NLFD has significant benefits in terms of accuracy and range of applicability.

Ocean power technology design optimization

The National Renewable Energy Laboratory and Ocean Power Technologies (OPT) conducted a collaborative code validation and design optimization study for OPT’s PowerBuoy wave energy converter (WEC). NREL utilized WEC-Sim, an open-source WEC simulator, to compare four design variations of OPT’s PowerBuoy. As an input to the WEC-Sim models, viscous drag coefficients for the PowerBuoy floats were first evaluated using computational fluid dynamics. The resulting WEC-Sim PowerBuoy models were then validated with experimental power output and fatigue load data provided by OPT. The validated WEC-Sim models were then used to simulate the power performance and loads for operational conditions, extreme conditions, and directional waves, for each of the four PowerBuoy design variations, assuming the wave environment of Humboldt Bay, California. And finally, ratios of power-to-weight, power-to-fatigue-load, power-to-maximum-extreme-load, power-to-water-plane-area, and power-to-wetted-surface-area were used to make a final comparison of the potential PowerBuoy WEC designs. The design comparison methodologies developed and presented in this study are applicable to other WEC devices and may be useful as a framework for future WEC design development projects.

Statistical Analysis of a 1:7 Scale Field Test Wave Energy Converter Using WEC-Sim

This study uses the open-source wave energy converter simulator (WEC-Sim) code to model the Columbia Power Technologies SeaRay 1:7 scale WEC. WEC-Sim is intended to run quickly on standard desktop equipment and provide a very gentle learning curve for WEC modeling. This paper focuses on the linear implementation of WEC-Sim as that requires the least simulation time and is often the starting point for basic system design. WECSim results are compared against the SeaRay experimental data. Two studies were conducted: A comparison of WEC-Sim predications versus experimental data across 285 trials of varying sea states to determine the overall average power and energy production; and a determination of WEC-Sim's accuracy in predicting the experimental ranges of position, speed, torque, and power. The study of average power production across many sea states shows that the WEC-Sim predicts the average power of the aft float well, within 15%, but the error in the fore float is larger at 34%. The error in total predicted power is 24%. The detailed analysis of range of motion shows WEC-Sim predicted 95th percentile outliers (which dominate the design considerations) in position, speed, and torque by +15%, +14%, and +17%, respectively, for the fore float and -1%, -9%, and -6%, respectively, for the aft float.