Wave Energy Converter Research Articles

An inerter-integrated wave energy converter for vibration mitigation of offshore floating buoys and its experimental validation

This paper presents an inerter-integrated wave energy converter (WEC) for vibration mitigation of offshore floating structures. The proposed WEC complies a floating buoy and an upper structure placed over the floating buoy connected by a spring and a ball screw with a rotational mass, which converts linear motion to rotational one, and the ball screw is connected to a rotating generator. As the two bodies move separately, the inertial force proportional to the relative acceleration of the two bodies called inerter is produced. This mechanism, similar to passive vibration reduction systems in automobiles and civil structures, not only produces electric power but also mitigates the vibration induced by the wave disturbances acting on the floating buoy. In this research, its analytical model was developed based on the two-degree-of-freedom dynamic model and the circuit equation of the generator. Numerical studies considering only the heave direction showed that the proposed WEC can achieve electric power generation and vibration mitigation of the upper structure at the same time. Moreover, to verify the efficacy experimentally, a small-scaled prototype was designed, and wave flume testing was conducted, which demonstrated a response reduction of more than 40% compared to the device without the proposed mechanism.

Modeling dynamic power generation from ocean waves in the Kingdom of Tonga: A comprehensive analysis using integrated mechanical and electrical framework

Globally, ocean wave energy can significantly reduce carbon dioxide emissions from the electricity generation sector. Wave energy converters (WECs) could be installed in the Kingdom of Tonga, which is surrounded by ocean, where the average wave energy flux is greater than 7 kW/m. There is a lack of feasibility studies in this area due to the unavailability of measured data on wave parameters and properties; hence, the potential of WEC is yet to be discovered or exploited. This study proposes an integrated mechanical and electrical modeling framework to analyze the impact of wave characteristics on dynamic power generation in three main islands of Tonga, including ‘Eua, Tongatapu, and Niuafo’ou. The mechanical simulation involves using a 2D Fourier wave model to identify the important factors that affect the power potential of ocean waves, such as wave height and period. The wave model is linked to a comprehensive electrical simulation model developed in Matlab Simulink, which includes a permanent magnet linear synchronous generator (PMLSG), rectifier, buck converter, inverter, and load to estimate the electrical power potential per 1 m2 of ocean area. According to the findings, in ‘Eua, the maximum annual generation intensity output is 90.45 kWh/m2, whereas in Niuafo'ou, the minimum is 48.5 kWh/m2. Applying this to a region (12.57 m2) where a WEC has been deployed in Australia, the highest yearly electricity generation in ‘Eua ranges between 926.41 and 1136.96 kWh/y, but in Tongatapu and Niuafo'ou, it ranges between around 610 and 899 kWh/y.

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.

Experimental and computational analysis of elastomer membranes used in oscillating water column WECs

The study investigates the structural characterisation of flexible membranes used in oscillating water column (OWC) wave energy converters (WECs). Four commonly utilised elastomers – natural rubber, nitrile rubber, silicone, and latex – were subjected to a novel hyperelastic model selection process. A custom bulge test setup enabled the selection of second-order Mooney-Rivlin (SOMR) and Yeoh models for relevant accuracy (RMSE<0.018 MPa), stability and numerical validation. A 1:20 scale OWC model with latex was tested in a water tank to examine the effects of waves with a frequency range of 0.25–1.4 Hz and up to 0.24m amplitude. Water tank experiments demonstrated smooth frequency responses for OWC with membrane, beneficial for consistent power generation. A dry test rig was designed and built to replicate OWC inflation conditions and apply cyclic loadings up to 1.5 Hz, overcoming pressure limitations of the water tank, exploring wider material options, and validating numerical simulation. An optical motion capture system, Qualisys, supported the validation process by providing precise data on membrane deformation during experiments. Furthermore, finite element analysis (FEA) was utilised to conduct stress analysis and parametric studies, assessing the suitability of these materials for flexible OWC application.

Effects of the second-order hydrodynamics on the dynamic behavior of the platform among the wind-wave hybrid systems

This paper presents three different wind-wave hybrid systems (HSs) with multiple point-absorption wave energy converters (WECs) integrated onto a floating semi-submersible wind turbine (WT). The focus of the present study is to explore the effects of the second-order wave forces on the dynamic behavior of the platform in the HS, including the platform motion and the mooring line tension, under both operational and extreme sea states. ANSYS/AQWA based on 3D diffraction/radiation potential theory is used to conduct numerical investigation on the second-order hydrodynamics of the HS. To ensure the reliability of the investigation, the validations of the semi-submersible platform and point-absorption WEC are done based on the available experimental data. Additionally, two models, "1st-order" and "1st&2nd-order", are set up for comparative analysis, to highlight the significance of the second-order forces. The simulation results show that the second-order hydrodynamics have a significant influence on the surge and pitch responses of the platform, but almost no effects on the heave response. In addition to this, ignoring the second-order hydrodynamics will significantly underestimate the mooring line tension response. Overall, this study suggests that the second-order hydrodynamics should be considered in the design of wind-wave HSs, in order to ensure their performance and safety, especially under the severe sea states.

Numerical investigations on the hydrodynamic performances of an isolated OWC and its integration with a semi-circular breakwater

The exploration of multifunctional systems to harvest wave energy has become increasingly prominent in recent research. This paper presents a comprehensive investigation into the effectiveness of utilizing a wave energy converter (WEC) not only for energy generation but also as a coastal protection measure. Among the various types of WECs, the Oscillating Water Column (OWC) stands out as particularly promising due to its inherent advantages and notable efficiency. The versatility of the OWC allows for standalone installation or integration into hybrid systems, such as combining it with breakwaters. While numerous studies have examined the integration of OWCs with Caisson-type breakwaters, research on their integration with Semi-circular breakwaters (SCBW) is scarce. This study fills this gap by utilizing the open-source computational fluid dynamics (CFD) tool, OpenFOAM, to analyze the hydrodynamic performance of an OWC embedded within an SCBW. The numerical findings are validated against existing experimental data, enabling a thorough comparative analysis between the performance of an isolated OWC and a hybrid OWC integrated with SCBW. The investigation delves into various parameters, including wave amplification factor, chamber air and water pressure, external water pressure, energy transfer efficiency from chamber water to air, and wave run-up over the SCBW face. The results conclusively demonstrate that an OWC integrated with SCBW exhibits superior hydrodynamic performance compared to standalone configurations.

Hydrodynamic and power conversion performance for linear and triangular dual-buoy wave energy converter array

Hydrodynamic and power conversion performance for linear and triangular dual-buoy wave energy converter array

Adaptive PI Control of Wave Energy Converters With Force and Motion Constraints

Adaptive PI Control of Wave Energy Converters With Force and Motion Constraints

Wave Energy Converter Power Take-Off Modeling and Validation From Experimental Bench Tests

Wave Energy Converter Power Take-Off Modeling and Validation From Experimental Bench Tests

An Anti-Windup Mechanism for State Constrained Linear Control of Wave Energy Conversion Systems: Design, Synthesis, and Experimental Assessment

An Anti-Windup Mechanism for State Constrained Linear Control of Wave Energy Conversion Systems: Design, Synthesis, and Experimental Assessment

Multiple moonpools within a vessel free to heave and pitch: An experimental study

Moonpools in marine vessels and offshore platforms are vertical openings through the structure used to support underwater operations. Experimental investigations of multiple moonpool resonant responses coupled with vessel heave and pitch motions are presented in this study. For three moonpools at forward, central, and aft locations of a rectangular vessel, the simultaneous location-dependent free surface response amplitudes and oscillation phases are compared for a range of incident wave frequencies in the piston mode range. For the vessel free to heave and pitch, the central moonpool has the highest response amplitude; whereas, a previous study with fixed vessel (Garad et al. 2023) showed that the largest free surface responses are observed in the forward moonpool. Due to coupled hydrodynamic interactions between the moonpool oscillations and the vessel motions, the moonpool resonance frequency is linked with the vessel heave response. Multiple response amplitudes peaks are observed in the forward and aft moonpools due to body motions. The non-dimensional response amplitude increases nonlinearly with decreasing wave steepness close to the resonance frequency. The multiple moonpool oscillations show distinct phase differences from one another and with the outer wave, the individual values depending on the incident wave frequency. Considering the vessel motions, comparison with experiments without moonpools shows that the pitch response is increased due to multiple moonpool oscillations. The hydrodynamic interactions between multiple moonpools and a floating vessel have prospective applications in the case of marine platforms, where the resonant motions can be used for wave energy conversion through oscillating water column devices.

A wind semi-sub platform with hinged floats for omnidirectional swell wave energy conversion

AbstractThe capacity of wind turbines on offshore wind platforms is presently much greater than that for wave energy conversion. However, wind availability with speed greater than 5 m/s, just above cut in, is typically 30–40% requiring storage to provide uniformity of supply, but this may be improved by adding swell wave energy conversion with typical availability of 90%. A hybrid platform is considered with three effectively rigid cylindrical floats connected by beams at right angles to support a wind turbine with its base on the central float, and two wave energy floats, opposite the wind floats, with beams at 90° and hinges with dampers for mechanical energy absorption on the central float. With swell periods over 10 s, pitch resonance may be achieved with the fore and aft floats about half a wavelength apart with anti-phase forcing causing a moment on hinges above water level. The NREL 5 MW wind turbine is incorporated and average swell wave power absorption in a typical significant wave height of 2 m is over 200 kW. The analysis is by time domain linear diffraction–radiation modelling validated for other multi-float configurations. Significant wave energy conversion is omnidirectional over a wide range of heading angles. An added benefit is that in larger waves associated with strong winds, when the wave energy conversion would be disengaged, the wave float rotation on free hinges reduces the hub accelerations below that for rigid floats, enabling a longer time for wind power generation.

Research on the Accounting and Prediction of Carbon Emission from Wave Energy Convertor Based on the Whole Lifecycle

Wave energy, as a significant renewable and clean energy source with vast global reserves, exhibits no greenhouse gas or other pollution during real-sea operational conditions. However, throughout the entire lifecycle, wave energy convertors can produce additional CO2 emissions due to the use of raw materials and emissions during transportation. Based on laboratory test data from a wave energy convertor model, this study ensures consistency between the model and the actual sea-deployed wave energy convertors in terms of performance, materials, and geometric shapes using similarity criteria. Carbon emission factors from China, the European Union, Brazil, and Japan are selected to predict the carbon emissions of wave energy convertors in real-sea conditions. The research indicates: (1) The predicted carbon emission coefficient for unit electricity generation (EFco2) of wave energy is 0.008–0.057 kg CO2/kWh; when the traditional steel production mode is adopted, the EFco2 in this paper is 0.014–0.059 kg CO2/kWh, similar to existing research conclusions for the emission factor of CO2 for wave energy convertor (0.012–0.050 kg CO2/kWh). The predicted data on carbon emissions in the lifecycle of wave energy convertors aligns closely with actual operational data. (2) The main source of carbon emissions in the life cycle of a wave energy converter, excluding the recycling of manufacturing metal materials, is the manufacturing stage, which accounts for 90% of the total carbon emissions. When the recycling of manufacturing metal materials is considered, the carbon emissions in the manufacturing stage are reduced, and the carbon emissions in the transport stage are increased, from about 7% to about 20%. (3) Under the most ideal conditions, the carbon payback period for a wave energy convertor ranges from 0.28 to 2.06 years, and the carbon reduction during the design lifespan (20 years) varies from 238.33 t CO2 (minimum) to 261.80 t CO2 (maximum).

Bead-like Co/C@MoS2Composites connected by Multi-Walled carbon nanotubes for high-efficiency electromagnetic wave absorption

This study presents a novel approach to synthesize a composite material composed of bead-like Co/C nanoparticles anchored on MWCNTs, encapsulated by MoS2 nanosheets through a combination of carbonization and hydrothermal treatment using ZIF-67 as the precursor. The resulting composite demonstrates exceptional electromagnetic wave (EMW) absorption performance, with a minimum reflection loss (RL) of −63 dB at a thickness of 2.44 mm and an effective absorption bandwidth (EAB) of 7.2 GHz when RL is below 10 dB together with an expanded EAB of 12.3 GHz (d = 5 mm).The superior absorption capabilities arise from a well-thought-out design of material composition and structure. The synergy of Co/C and MWCNTs enhances ferromagnetic resonance, conductivity, and polarization loss, while sulfur vacancies in MoS2 act as polarization centers, facilitating dipole polarization and efficient conversion of electromagnetic wave energy. The core–shell structure and heterogeneous interfaces introduce various energy loss mechanisms, including multiple polarizations, scattering effects, and dielectric loss. This work introduces a facile method for synthesizing a composite material with outstanding EMW absorption performance. The synergistic combination of unique material components and the carefully engineered structure make this composite a promising candidate for advanced applications in electromagnetic wave absorption technology.

Research on Wave Energy Converters

With the acceleration of the global warming process, the clean energy crisis is becoming serious; conventional energy is unlikely to solve the current crisis, so people pay attention to new energy. As wave energy is widely distributed, renewable, and clean, hundreds of wave energy converters emerge. In order to understand the research progress of wave energy converters better, this paper divides wave energy converters into overtopping type, oscillating water column type, and oscillating body type according to the working principle and divides the oscillating body type into oscillating float type and oscillating pendulum type by different ways of energy capture. Based on the classification, various types of engineering cases, physical tests and digital simulation, and other academic research results are summarized, especially the generation power and energy conversion efficiency of various devices, and some shortcomings and suggestions are put forward, hoping to provide help for readers to study wave energy generation converters.

Numerical Study of an Innovative Concept for a Multibody Anti-Pitching Semi-Submersible Floating Wind Turbine

The floating offshore wind turbine provides a feasible solution for the development of renewable ocean energy. However, the sizeable rotor diameter of the wind turbine results in large wind heeling moments and pitch amplitude. It will increase the structural loads and cause safety problems. Additionally, the contradictory nature between the stability and the sea-keeping of the floating structure requires that the more flexible method should be adopted to reduce the motion response of the floating offshore wind turbine. Therefore, an innovative concept of a multibody anti-pitching semi-submersible floating offshore wind turbine, named the MBAPSF, is proposed in this paper. The MBAPSF consists of a 5 MW braceless semi-submersible wind turbine and three wave energy converters. The multibody coupled numerical model is established by using an F2A tool, and the dynamic performance of the MBAPSF is compared with that of the traditional semi-submersible wind turbine named the TSSF. The results show that the innovative concept proposed in this paper can reduce pitch motion up to approximately 27% under different load cases, and the maximum bending moment and shearing force at the tower base are also reduced by more than 10%. Meanwhile, WECs are beneficial for increases in the total power generation capacity.

Hydrodynamic response analysis of a hybrid TLP and heaving-buoy wave energy converter with PTO damping

In the present study, the numerical investigation is performed to analyse the hydrodynamic performance of circular and concentric arrangements of cone-cylinder-type heaving point absorber wave energy converter (WEC) around a Frustum Tension-Leg Platform (FTLP) based on potential flow theory. The responses of the single FTLP and the FTLP-WEC hybrid system are analysed for the rated wind speed of a 5 MW wind turbine to observe the influence of the WECs on wind power absorption of wind turbines supported on FTLP. The presence of the FTLP floating wind turbine platform and other WECs affects the hydrodynamic coefficients of the WEC. The influence of the hybrid system on the hydrodynamic coefficients is analysed on determining the ratio of the hydrodynamic coefficients for a single WEC system to those for a hybrid system. Further, the study analyses the instantaneous wave power absorption for the WECs arranged around the FTLP in a circular and concentric pattern. The hydraulic power take-off for the hybrid system with two different control strategies is then discussed to improve the wave power absorption of the WECs. The study observed higher wave power absorption of the WECs with the influence of the PTO system. The mean interaction factor and the capture width ratio of the hybrid system are further studied to understand the influence of array arrangement for the WECs. The hybrid system is noted to have favourable dynamic responses for different environmental factors and contributes positively in increasing power output.

Improving wave energy conversion performance of a floating BBDB-OWC system by using dual chambers and a novel enhancement plate

In this study, a novel floating dual-chamber backward bent duct buoy oscillating water column (BBDB-OWC) wave energy converter (WEC) is introduced, featuring a horizontal plate at the bottom of the front chamber to act as an enhancement plate. A three-dimensional computational fluid dynamics (CFD) model is developed and validated by comparing its results with existing experimental measurements. The validated model is employed to investigate the hydrodynamic performance and power generation characteristics of the dual-chamber BBDB-OWC WEC under various conditions, including variations in the length of the horizontal plate (lp/lf) and different regular wave conditions. Key performance metrics, including peak to average ratio of power (PTARP), wave energy capture width ratio (ξtotal), and its wave period respond bandwidth (indicated by Pξtotal > 0.5 and Pξtotal > 0.7), are analyzed and compared with those of a traditional single-chamber BBDB-OWC WEC. The results reveal that, compared to the single-chamber WEC, the dual-chamber WEC with a specific horizontal plate length reduces the average PTARP from 2.88 to a minimum value of 1.82 for lp/lf = 0.5, improves the average ξtotal from 0.55 to a maximum value of 0.64 for lp/lf = 2.5, and increases Pξtotal > 0.5 and Pξtotal > 0.7 from 71 % and 14 % to maximum values of 86 % and 43 % for lp/lf = 2.5, respectively. An explanation for these observations is also provided in the context of structure motion and flow fields.

Optimization of wind‐wave hybrid system based on wind‐wave coupling model

AbstractThe integration of wave energy converters (WECs) into floating offshore wind turbine (FOWT) can effectively reduce costs and increase power generation. When WECs are integrated into FOWT, the hydrodynamic interference, motion coupling, and other factors contribute to the high spatial dimension of the coupled optimization, making it difficult to find the globally optimal solution. Therefore, this study proposes an optimization method based on a wind‐wave coupling model, and takes a new wind‐wave hybrid system as an example for verification and analysis. First, the experimental design is completed through random sampling, and the corresponding WECs and wind turbine power of each sample point are calculated using full coupling simulation. And then according to the design input and simulation results, the wind‐wave coupling model is obtained by training the elliptical basis functions neural network (EBFNN). At last, based on this model, the non‐dominated sorting genetic algorithm II (NSGA‐II) is used to optimize the WECs microarray. The results show that the prediction model established in this paper has high accuracy and is used with the NSGA‐II to effectively improve the wind‐wave coupling energy harvesting. This method can effectively solve the problem of high coupling dimensions in the process of hybrid system design.

Hydrodynamic performance optimization of a cost-effective WEC-type floating breakwater with half-airfoil bottom

Aiming to design and develop an affordable integrated floating breakwater and wave energy converter (WEC) system that has high performance in power absorption and wave attenuation, a novel integrated floater with a half-airfoil bottom is proposed. The genetic algorithm combined with the boundary element method is employed to optimize the half-airfoil bottom surface represented by the class-shape function transformation parameterization method, for maximizing the power density over the operating bandwidth. Then the operating performance of the optimal half-airfoil bottom floater is further studied in a validated two-dimensional viscous numerical wave tank. The result shows that the optimal half-airfoil bottom floater is a more effective and affordable solution than square and triangular bottom ones. The effective ratio of the floater breadth to the wavelength can be reduced to about a tenth, far below that of the conventional floating breakwater, denoting its excellent wave attenuation capability. The increase in floater breadth induces more intense vortex dynamic behaviors and thus higher dissipation coefficient. A shallow front-wall draft design can improve the maximum conversion efficiency to more than 74%, slightly broaden the resonance bandwidth, and reduce energy dissipation while minimizing costs. The nonlinear PTO damping force contributes to a more stable response. The research results denote the feasibility of bio-inspired WEC-type floating breakwaters and show their superiority in energy extraction, wave attenuation, as well as cost reduction.