Wave Energy Converter Research Articles

A Novel Approach to Wave Energy Conversion Using CFD Technique

Abstract This article details the development and evaluation of a novel wave energy converter (WEC) aimed at efficiently capturing wave energy for electricity production. The study employs Computational Fluid Dynamics (CFD) techniques, specifically the URANS method and the k-ω SST turbulence model, to solve the Navier-Stokes equations and capture the free surface using the Volume of Fluid (VOF) model. The CFD results are validated against experimental data to ensure accuracy. Various design parameters of the proposed device were tested, revealing that the arms and bottom angle significantly affect its performance. Unlike the floating Wave Dragon (WD) device, which utilises potential energy and is set in deep water, the new fixed-seabed device is positioned in the transitional wave region near the shore, where waves retain 80% of their energy. It can be constructed from environmentally friendly cement, making it resistant to hurricanes and suitable for any wave turbine in the open sea. The MP687 turbine was used to capture the wave energy in the proposed device, testing its performance in three positions: in the open sea, in the middle of the device, and at the device’s outlet. The results show that the device significantly enhances wave energy concentration, especially when the turbine is placed at the outlet. The proposed device offers numerous advantages, including its fixed position in a high-energy wave zone, the efficient use of turbulent kinetic energy, and robust construction that can withstand storms.

Real-time Wells turbine simulation on an oscillating-water-column wave energy converter physical model

Due to its simplicity and low cost, the Wells turbine is the most common choice for driving oscillating-water-column (OWC) wave energy converters (WECs) power take-off system. This turbine is characterized by a flow rate that is a linear function of the pressure head and inversely proportional to the rotational speed before the onset of hard stall conditions. The Wells turbine has been simulated in wave tanks using porous plugs, where the flow rate exhibits linear behaviour. However, numerical and experimental results have shown that rotational speed variations significantly influence the performance of OWC WECs. This work aims to develop a novel real-time simulator of a Wells turbine for use in physical models of OWC power plants. The proposed simulator consists of a diaphragm whose diameter is adjusted in real-time as a function of the pressure head measured in the laboratory and the rotational speed calculated by a hardware-in-the-loop model. In this way, it is possible to reproduce the full behaviour of the Wells turbine before and after a hard stall. Experimental results demonstrate the effectiveness of the simulator. A performance analysis was conducted to understand the device’s potential and compare the damping that Wells and impulse turbines introduce.

Numerical Analysis of the Submerged Horizontal Plate Device Subjected to Representative Regular and Realistic Irregular Waves of a Sea State

This study numerically analyzes a submerged horizontal plate (SHP) device subjected to both regular and irregular waves. This device can be used either as a breakwater or a wave energy converter (WEC). The WaveMIMO methodology was applied for the numerical generation and wave propagation of the sea state of the Rio Grande coast in southern Brazil. The finite volume method was employed to solve conservation equations for mass, momentum, and volume fraction transport. The volume of fluid model was employed to handle the water-air mixture. The SHP length (Lp) effects were carried out in five cases. Results indicate that relying solely on regular waves in numerical studies is insufficient for accurately determining the real hydrodynamic behavior. The efficiency of the SHP as a breakwater and WEC varied depending on the wave approach. Specifically, the SHP demonstrates its highest breakwater efficiency in reducing wave height at 2.5Lp for regular waves and 3Lp for irregular waves. As a WEC, it achieves its highest axial velocity at 3Lp for regular waves and 2Lp for irregular waves. Since the literature lacks studies on SHP devices under the incidence of realistic irregular waves, this study significantly contributes to the state of the art.

Characteristics of wave loading and viscous energy loss of a bottom-sitting U-OWC wave energy device: A validated numerical perspective

This study involves numerical simulations of regular waves interacting with a bottom-sitting circular OWC wave energy converter with a U-shaped duct (circular U-OWC). A numerical wave tank is built based on the RANS equation and is validated against experiments. The effect of water depth and the height of the outer tube of the U-shaped duct on capture efficiency, wave loading, and viscous energy loss is investigated numerically. Results show that the height of the outer tube of the U-shaped duct has a significant effect on the capture efficiency of the device. Wave loading analysis reveals a critical mode of stability that may threaten the operation safety of the device. It is found that increasing the height of the U-shaped duct opening increases the viscous energy loss inside the U-shaped duct. A semi-theoretical model between work done by waves to the device and the viscous energy loss in the U-shaped duct is established, numerical data indicates that a significant amount of work done by waves to the device is lost in viscous effects. This suggests that certain optimization for reducing wave loading could possibly also reduce viscous energy loss, thus improving the overall efficiency of the device.

Hydrogen Production from Wave Power Farms to Refuel Hydrogen-Powered Ships in the Mediterranean Sea

The maritime industry is a major source of greenhouse gas (GHG) emissions, largely due to ships running on fossil fuels. Transitioning to hydrogen-powered marine transportation in the Mediterranean Sea requires the development of a network of hydrogen refueling stations across the region to ensure a steady supply of green hydrogen. This paper explores the technoeconomic viability of harnessing wave energy from the Mediterranean Sea to produce green hydrogen for hydrogen-powered ships. Four promising island locations—near Sardegna, Galite, Western Crete, and Eastern Crete—were selected based on their favorable wave potential for green hydrogen production. A thorough analysis of the costs associated with wave power facilities and hydrogen production was conducted to accurately model economic viability. The techno-economic results suggest that, with anticipated cost reductions in wave energy converters, the levelized cost of hydrogen could decrease to as low as 3.6 €/kg, 4.3 €/kg, 5.5 €/kg, and 3.9 €/kg for Sardegna, Galite, Western Crete, and Eastern Crete, respectively. Furthermore, the study estimates that, in order for the hydrogen-fueled ships to compete effectively with their oil-fueled counterparts, the levelized cost of hydrogen must drop below 3.5 €/kg. Thus, despite the competitive costs, further measures are necessary to make hydrogen-fueled ships a viable alternative to conventional diesel-fueled ships.

Comparative Study on the Performances of a Hinged Flap-Type Wave Energy Converter Considering Both Fixed and Floating Bases

The dynamical modeling and power optimization of floating wind–wave platforms, especially in regard to configurations based on constrained floating multi-body systems, lack in-depth systematic investigation. In this study, a floating wind-flap platform consisting of a flap-type wave energy converter and a floating offshore wind turbine is solved in the frequency domain considering the mechanical and hydrodynamic couplings of floating multi-body geometries and a model that suits the constraints of the hinge connection, which can accurately calculate the frequency domain dynamic response of the flap-type WEC. The results are compared with bottom-fixed flap-type wave energy converters in the absence of coupling with a floating wind platform. Moreover, combined with traditional optimization methods of power take-off systems for wave energy conversion, an optimization method is developed to suit the requirements of floating wind-flap platform configurations. The results are drawn for a specific operation site in the South China Sea, whereas a sensitivity analysis of the parameters is performed. It is found that the floating wind-flap platform has better wave energy absorption performance in the low-frequency range than the bottom-fixed flap-type wave energy converter; the average power generation in the low-frequency range can increase by up to 150 kW, mainly due to constructive hydrodynamic interactions, though it significantly fluctuates from the sea waves’ frequency range to the high-frequency range. Based on spectral analysis, operational results are drawn for irregular sea states, and the expected power for both types of flap-type WECs is around 30 kW, which points to a similar wave energy absorption performance when comparing the bottom-fixed flap with the flap within the hybrid configuration.

Wave energy converter with multiple degrees of freedom for sustainable repurposing of decommissioned offshore platforms: An experimental study

Targeting at the sustainable repurposing of decommissioned offshore platforms, a novel wave energy converter with a distinct mechanical-motion-rectifier power take-off system is developed to provide renewable energy. A scaled prototype is constructed and experimentally investigated in a large wave tank, as an essential step before sea trials. A new phenomenon of parametric oscillation is observed and explored, as physical evidence of chaotic instabilities of wave energy devices. Several advantages of the multi-degree-of-freedom device over its single-degree-of-freedom counterpart are disclosed physically for the first time. Releasing the pitch and roll degrees of freedom helps enlarge the heave amplitude of the energy-absorbing buoy, receive a larger hydrodynamic force from water waves to the power take-off system, reduce structural deflection and friction of the transmission machinery, and eliminate spiky and noisy vibrations of the internal force. Accordingly, the maximum efficiencies in the energy absorption and transmission stages increase from 45% and 11% to 90% and 30%, respectively; the overall efficiency of energy conversion is increased by up to three times in a broad spectrum of wave conditions.

Comparative evaluation of floating OWC and BBDB wave energy converters by regular and irregular waves using numerical approach

ABSTRACT Point absorber wave energy converters (WECs) need to be predicted in order to estimate their durability in high sea states. Although little is known about hydrodynamics and there are few data available. Models of the wave conditions have been developed at the laboratory size using smooth particle hydrodynamics. To investigate how focused waves interact with a point absorber WEC, fixed OWC and BBDB type WEC are coupled. A validation research is evaluated by taking into account the wave amplitude and frequency. The hydrodynamic force and the motion response of the point absorber WEC have been measured throughout time. According to the numerical results, the hybrid configuration with the fixed OWC and BBDB has a significant impact on the motion response and hydrodynamic force. It is demonstrated that the position of hybrid OWCs affects the hydrodynamic force and intense motion response significantly.

Experimental Study on the Spring-like Effect on the Hydrodynamic Performance of an Oscillating Water Column Wave Energy Converter

The oscillating water column (OWC) wave energy converter has demonstrated significant potential for converting ocean wave energy. The spring-like effect of air compressibility can significantly affect the hydrodynamic behavior of the device, but it has rarely been investigated through experimental studies. In this study, an experimental test on a model-scaled OWC device was carried out in a wave flume using a series of regular and irregular waves. The spring-like effect was taken into account by the combination of the air chamber with an additional air reservoir of appropriate volume, where the total volume was scaled according to the square of the Froude scale. The hydrodynamic performance was compared with the results obtained without considering the spring-like effect. A phase difference between the air pressure and airflow rate was observed when employing the additional air reservoir. The amplitudes of free surface elevation and airflow rate increased, while the air pressure was reduced when the spring-like effect was considered. The results demonstrate that failure to consider the spring-like effect can lead to overestimation of the hydrodynamic efficiencies, and the errors were mainly affected by the incident wave frequency.

Non-linear turbine selection for an OWC wave energy converter

Turbine-induced damping is a critical parameter affecting the performance of oscillating water column (OWC) wave energy converters. Therefore, selecting the appropriate turbine-chamber combination is an essential step in their design. In this work, a methodology is developed to determine the optimum turbine diameter for a given chamber, i.e., the diameter which maximizes the pneumatic energy capture of the chamber under an extensive set of wave conditions—covering virtually the entire range of wave conditions relevant for wave energy exploitation. This novel approach combines physical and numerical modelling with dimensional analysis. Importantly, it results in a turbine diameter that enables the turbine to operate at maximum efficiency. Through the different modelling techniques applied, the methodology accounts for air compressibility effects and other non-linear effects. It is applicable to non-linear turbines, with the study focusing on the promising biradial turbine. The results indicate that using the proposed methodology to select the turbine diameter significantly improves the capture-width ratio of the OWC, with increases of up to 100% for individual sea states. Two turbine diameters were identified as appropriate for the proposed OWC chamber design, 1.1 m for low-energy sites, and 1.4 m for mid- and high-energy sites.

An Attention-Based Deep Learning Model for Phase-Resolved Wave Prediction

Abstract Phase-resolved wave prediction capability, even if only over two wave periods in advance, is of value for optimal control of wave energy converters, resulting in a dramatic increase in power generation efficiency. Previous studies on wave-by-wave predictions have shown that an artificial neural network (ANN) model can outperform the traditional linear wave theory-based model in terms of both prediction accuracy and prediction horizon when using synthetic wave data. However, the prediction performance of ANN models is significantly reduced by the varying wave conditions and buoy positions that occur in the field. To overcome these limitations, a novel wave prediction method is developed based on the neural network with an attention mechanism. This study validates the new model using wave data measured at sea. The model utilizes past time histories of three Sofar Spotter wave buoys at upwave locations to predict the vertical motion of a Datawell Waverider-4 at a downwave location. The results show that the attention-based neural network model is capable of capturing the slow variation in the displacement of the buoys, which reduces the prediction error compared to a standard ANN and long short-term memory model.

1D -FNO - Residual convolutional neural network autoregressive prediction model for deep water wave train evolution based on high-order spectral (HOS) data

Accurate prediction of deep-water wave dynamics is essential for ocean engineering, meteorology, and maritime safety. Traditional physical models, though effective, face significant computational and time challenges with complex nonlinear dynamics. This study introduces an autoregressive prediction model using a one-dimensional adaptive Fourier Neural Operator-Residual Convolutional Neural Network (1D-FNO-ResNet) to enhance prediction accuracy. We developed a high-precision single-step prediction model capable of capturing wave characteristics such as height and fluid particle velocity. A fine-tuning training strategy based on multi-step prediction errors was proposed for autoregressive evolution. Performance comparisons between the ResNet and FNO-ResNet models were made under various fine-tuning steps. A correction model was also introduced to control error accumulation, improving autoregressive prediction steps. The model's generalization performance was validated with modulated and random wave trains under different conditions, optimizing predictive stability and accuracy. This approach demonstrates efficient 1D deep-water wave evolution prediction, offering a novel deep learning solution for wave dynamics prediction in ocean engineering, supporting wave energy conversion device design and optimization.

Experimental Validation of a Modular All-Electric Power Take-Off Topology for Wave Energy Converter Enabling Marine Renewable Energy Interconnection

Power electronic converters are an enabling technology for the emerging marine energy applications, such as using ocean waves to produce electricity. This paper outlines the power take-off system and its key components used in a wave energy converter offering modularity and scalability to generate power efficiently. The proposed power take-off system was implemented based on a modular multilevel converter and could be deployed to convert any alternating current electrical energy to a different alternating current for interconnection to grid or non-grid applications. Examples of widespread deployment are supplying electricity to coastal communities or producing clean drinking water. The analysis using both the simulation tests and laboratory experiments verified the design objectives and basic functionality of the developed power take-off system. An acceptable response using a field programmable gate array-based controlled laboratory testbench was achieved, complying with guidelines specified in the prevalent industry standards. Seamless operation during steady-state and transients for the studied wave energy converter was achieved as supported by the obtained results. The key findings of this work were experimentally examined under different load conditions, direct current bus voltage fluctuations, and generator speed–torque regulation. The ability of the power take-off system to generate high-power quality of the waveforms, e.g., against adhering to the IEEE 519-2022 standard for total harmonic distortion limits, is also confirmed.

Lowering the levelized cost of energy (LCOE) for mass-adjustable-buoy-based wave energy converters

This study is aimed at optimized control strategies for ocean wave energy converters (WECs) with mass-adjustable buoys. The conventional buoys are usually with fixed parameters and cannot perform well when wave periods are varying. In this study, mass-adjustable buoys are utilized to deal with the variations of wave periods. In order to coordinate with mass-adjustable buoys, four strategies, maximum power (MP) strategy, power factor priority (PFP) strategy, unit power factor (UPF) strategy and maximum capacity factor (MCF) strategy, are proposed and analyzed. The concept of levelized cost of energy (LCOE) is adopted to evaluate the economic efficiencies of different strategies. Hill-climbing method and reverse-hill-climbing method are adopted to approximate the target point because other kinds of methods usually require too much information and cannot perform well in practical engineering. The simulation and experimental results indicate that the LCOEs for these four strategies are respectively $0.7596/kWh, $0.7836/kWh, $0.3936/kWh, and $0.3807/kWh. Additionally, when the wave period and elevation changes from case 3 to case 7, the average active power is improved by 17.80% through adjusting the mass of the buoy. Experiments are carried out as well to verify the hill-climbing method and reverse-hill-climbing method.

A Dual-Function Design of an Oscillating Water Column Integrated with a Slotted Breakwater: A Wave Flume Study

Wave energy conversion holds promise for renewable energy, but challenges like high initial costs hinder commercialization. Integrating wave-energy converters (WECs) into shore-protection structures creates dual-function structures for both electricity generation and coastal protection. Oscillating water columns (OWCs) have been well studied in the past with their simple generation mechanism and their out-of-water power take-off (PTO) system, which can minimize bio-fouling effects and maintenance costs compared to other submerged WECs. In addition, a slotted barrier allows for better circulation behind the breakwater while dissipating incoming wave energy through viscous damping. This study examines the performance of a new design which combines an OWC with a slotted breakwater. Small-scale (1:49) laboratory tests were performed with a piston-type wave generator. The performance is evaluated in terms of wave transmission, wave energy extraction, and wave loading under various wave conditions while focusing on the effects of the porosity of the slotted barrier and tide level changes. Results show that under larger waves, a decreasing wave transmission, increasing power extraction from the OWC, and energy dissipation from the slotted barrier are observed. On the other hand, under increasing wavelengths, wave transmission is observed to be constant; this is important for harbor design, which means that the breakwater is effective under a wider range of wavelengths. Porosity allows for more transmission while inducing less horizontal force on the structure.

Research on data analysis method of oscillating water column wave energy converters

Abstract Taking the floating oscillating water column central tube wave energy converters (OWC WECs) as the research goal, the Fourier transform (FFT), and the ensemble empirical mode decomposition method (EEMD) were used to statistically analyze the wave height, motion acceleration, velocity and displacement of the device in the physical model test from the perspective of a frequency domain. Through the Fourier transform, the energy spectrum of the incident wave and the motion of the WEC is obtained, and it is found that the peak frequency of the two is the same, that is, the period is 1.138s, but the peak energy of the former is slightly greater than that of the latter. Through the ensemble empirical mode decomposition method, the 1-8 intrinsic mode functions of the incident wave and the displacement of the device are obtained, which shows that the device efficiently captures the dominant frequency wave in the incident wave, that is, the wave height is 0.0628m, the period is 1.1s, and the motion displacement of the vertical direction obtained by the device is 0.0585mm, and the period is 1.1119s. At the same time, it absorbs low-frequency, long-period reflected waves in the pool. The analysis results of the two methods are highly consistent, and the OWC WEC shows good hydrodynamic performance, with efficient wave trapping ability and fast wave responsability. This paper’s research methods and results provide a basis for the later optimization of the device and a new idea for the data processing method of the WECs.

Fully spectral approach to evaluate the performance of floating wave energy converters in directional complex sea states

Accurate assessment of Wave Energy Converters (WECs) performance requires a deep understanding of wave systems and energy distributions within both frequency and directional domains. This study introduces an innovative approach to evaluate the energy output of Multi-Body Floating Wave Energy Converters (MBFWECs) in complex sea states with multipeak wave spectra. The methodology involves developing a 2D Transfer Function (2D TF) to explore the power absorption capabilities of MBFWECs across specified wave frequency and directional ranges. To extend the approach to a real case, the waves measured at the northern coast of the Oman Sea are employed. Initially, hydrodynamic coefficients are determined using AQWA, and then, time-domain analyses are conducted using WEC-Sim, integrated with MATLAB-Simulink. Finally, to calculate the total extracted power, the derived 2D TF is multiplied by the Frequency-Directional Power Spectrum (FDPS) and the effective length of the device. A comparison between the results and those obtained through time history analysis confirms the accuracy of the introduced method. Furthermore, it is observed that neglecting the direction effect and assessing WEC power solely based on conventional approaches can lead to misleading information, particularly for sea states with multi-directional peaks (more than 20% error). Results reveal the importance of considering wave directionality in calculating the generated power. In addition, the wave energy of each direction can be absorbed efficiently by adjusting the draft in that direction separately. This innovative approach has significant implications for advancing wave energy conversion technologies and offers a practical approach to evaluating the real behavior of FWECs. In addition, it allows the optimal alignment of MBFWEC direction and geometries based on the FDPS of any region.

Hydrodynamic Performance of An Integrated System of Breakwater and A Multi-Chamber OWC Wave Energy Converter

Hydrodynamic Performance of An Integrated System of Breakwater and A Multi-Chamber OWC Wave Energy Converter

Harvesting the power of the oceans: Advancements, challenges, and the promising future of wave energy

This dissertation explores the pivotal domain of wave energy, delving into its historical evolution, technological advancements, and economic prospects. In response to escalating environmental challenges and the imperative to address climate change, the pursuit of sustainable and ecologically benign energy sources has gained prominence. Wave energy emerges as a compelling frontier within the realm of renewable energy, tapping into the perpetual undulations of oceans. The study traces the conceptual antecedents of wave energy from the 19th century to its current status as a burgeoning renewable energy source. Significant strides in technology during the 1990s and 2010s marked a shift towards optimizing energy conversion efficiencies, exploring novel materials, and refining wave energy converter designs. The dissertation analyzes diverse wave energy converter types, their operational principles, and their potential applications. Despite challenges such as environmental impact, operational reliability, and high capital costs, the unwavering commitment of researchers positions Wave Energy as a sanguine contender in the renewable energy landscape. The economic outlook for wave energy is promising, with global potential estimated at 2 billion kilowatts, fostering socio-economic benefits and contributing to carbon emission reduction. By presenting a comprehensive examination of wave energy, this dissertation underscores its significance in the pursuit of sustainable, environmentally friendly, and reliable energy solutions.

Enhancing Wave Energy Converters: Dynamic Inertia Strategies for Efficiency Improvement

Wave energy conversion is a promising field of renewable energy, but it still faces several technological and economic challenges. One of these challenges is to improve the energy efficiency and adaptability of Wave Energy Converters to varying wave conditions. A technological approach to solve this efficiency challenge is the negative spring mechanisms illustrated in recent studies. This paper proposes and analyzes a novel negative spring technological concept that dynamically modifies the mass and inertia of a Wave Energy Converter by transferring seawater between its compartments. The added value of the presented technology relies on interoperability, ease of manufacturing and operating, and increased energy efficiency for heterogeneous sea states. The concept is presented in two analyzed alternatives: a passive one, which requires no electrical consumption and is purely based on the relative motion of the bodies, and an active one, which uses a controlled pump system to force the water transfer. The system is evaluated numerically using widely accepted simulation tools, such as WECSIM, and validated by physical testing in a wave flume using decay and regular test scenarios. Key findings include a relevant discussion about system limitations and a demonstrated increase in the extracted energy efficiency up to 12.7% while limiting the maximum power extraction for a singular wave frequency to 3.41%, indicating an increased adaptability to different wave frequencies because of the amplified range of near-resonance operation of the WEC up to 0.21 rad/s.