Search

Based on linear potential flow theory, this study investigates the hydrodynamic performance of a co-located farm with an array of floating offshore wind turbines (FOWTs) and floating photovoltaics (FPVs). In this process, to evaluate the wave–structure interaction, domain decomposition and matched eigenfunction method are applied to address the boundary value problem for a complex-shaped co-located farm, and the velocity potential can be decomposed into radiation and diffraction problems. Under the framework of linearized theory, we establish the coupled motion equations by modeling rigid and articulated constraints to evaluate the kinematic response of the FOWTs and FPVs in the co-located farm. For such a system, a co-located farm consisting of an array of OC4-DeepCwind FOWTs and FPVs is proposed and investigated in this study. After running convergence analysis and model validation, the present model is employed to perform a multiparameter effect analysis. Case studies are presented to clarify the effects of solar platform geometric parameters (including column depth, thickness, radius, and total draft), articulated system, and shadow effect on the hydrodynamic behavior of wind and solar platforms. The findings elucidated in this work provide guidance for the optimized design of FPVs and indicate the potential for synergies between wind and solar energy utilization on floating platforms. [ABSTRACT FROM AUTHOR]

The clinch-adhesive joint has good mechanical properties and the curing degree of the adhesive during the clinching process shows significant impacts on the joint forming and its mechanical performance. During the clinching process, the interface of the cured adhesive layer restricts the flow of the metal thus shows a negative influence on the forming quality of the clinch-adhesive joint. This paper focuses on the interface layers change during the clinching process and the infulence of curing degree on the forming of clinch-adhesive joint by the finite element simulation model. Firstly, the constitutive model and interfacial failure criterion of Araldite 2015 epoxy adhesive considering curing degree were fitted through tensile tests for adhesive dumbbell specimens and bonding joints. Then, the forming process of the clinch-adhesive joints with fully cured adhesive were simulated. Finally, the effects of curing degree on the joint shape and interface failure were investigated. The results show that interface damage occurs at the location of the self-locking, and the curing degree mainly shows influences on the sizes of adhesive pocket and undercut, the damaged width of the interface layers, and the maximum damage of the interface layers. [ABSTRACT FROM AUTHOR]

The effects of the curing degree of the adhesive during clinching on the forming process and the mechanical performance of the clinch-adhesive joints made of AA5754 Aluminum Alloy, Q235 steel, and Araldite 2015 epoxy adhesive are investigated. First, a differential scanning calorimeter is employed in the curing experiment on the epoxy adhesive to collect the temperature heating rates in order to implement a curing reaction kinetic model that adopts the cure time as the only parameter. Second, the curing effects on the forming, mechanical performance, and failure mode of the clinch-adhesive joints are evaluated under five different curing degrees of the adhesive, including uncured, half-cured, and completely cured states. Finally, the dynamic failure processes of the hybrid joints under tensile-shear and peeling loads are investigated using a digital image correlation system. The results show that a clinch-adhesive joint, which is manufactured when the adhesive is uncured, achieves the highest level of mechanical performance, the hybrid joints that are produced using adhesives in half-cured states present low performance due to defects in the neck and "S" shape locking; specially, hybrid joints are manufactured when curing degree is 0.57, experience neck fractures and thus have the lowest mechanical performance. [ABSTRACT FROM AUTHOR]

• A novel fully coupled numerical framework for hybrid wind-wave systems is proposed. • The effectiveness of the proposed framework is demonstrated under various conditions. • The dynamic and power absorption characteristics of the hybrid system are investigated. • The impact of the key design variables on the hybrid system performance is evaluated. The need for comprehensively understanding the complex behaviour of floating hybrid wind-wave energy systems has attracted interest from both academia and industry. Due to the complexity of hybrid systems, existing studies often employ either an aerodynamic or hydrodynamic approximation and focus only on sparse wind and wave conditions at specific sites. Limitations in model accuracy and environmental scope severely restrict the applicability of the findings. To address these gaps, this paper presents a novel fully coupled aero-hydro-servo-elastic-mooring-multibody numerical simulation framework (OFPA) for the dynamic analysis of hybrid systems. OFPA integrates the wind turbine (WT) modules from OpenFAST into ANSYS-AQWA and simulates the multibody constraint system and the Power Take-Off (PTO) system through a Python external force module. The fidelity and accuracy of OFPA are then thoroughly validated under various conditions. Based on this framework, the dynamic and power absorption characteristics of a hybrid system consisting of a semi-submersible floating offshore wind turbine and a series of heaving Wave Energy Converters (WECs) are analysed under diverse wind and wave conditions. A parametric investigation is also carried out to discuss the effect of the PTO damping, geometry and number of WECs on the performance of the hybrid system. Results show that the proposed OFPA accurately predicts the complicated dynamic behaviour of the hybrid system and offers advantages in simulating the multibody constraint system and the PTO system compared to traditional methods. WECs do not have a significant negative effect on the mooring tension, WT motion and WT power absorption. In turn, the aerodynamic load has a positive effect on the WEC performance. It is also advisable to adopt a geometry with a draft-to-radius ratio of 0.75 and to integrate more WECs. [ABSTRACT FROM AUTHOR]

Modal decomposition techniques, flow field, and spectral analysis are employed to investigate the wake dynamics and destabilization mechanisms of a four-bladed marine propeller with or without a nozzle. Numerical simulations are conducted using the delayed detached eddy simulation model for the wake and the arbitrary mesh interface method for the blade rotation. The presence of the nozzle significantly reduces the wake's streamwise velocity, delays the wake destabilization, increases the wake length, and changes the morphologies of wake vortices. In particular, the hub vortex in the ducted propeller wake is broken down into chaotic turbulence by the perturbation of the backflow. Two modal decomposition methods, namely, proper orthogonal decomposition and dynamic mode decomposition, are used to decompose the vorticity magnitude in the rotor wake field. From modal analysis, the spatial scale of flow phenomena decreases with the increase in modal frequency. Underlying destabilization mechanisms in the wake correspond to some characteristic frequencies. The interaction of each sheet vortex with the previously shed tip (leakage) vortices occurs at blade passing frequency (BPF). The pairing of adjacent tip (leakage) vortices occurs at half-BPF. The long-wave instability of the hub vortex and the wake meandering are stochastic processes, each of which occurs at a frequency lower or equal to shaft frequency. These four destabilization mechanisms can approximately reconstruct the large-scale flow phenomena in the wake. Moreover, each sheet vortex's alternating connection and disconnection with the previously shed tip (leakage) vortices cause the short-wave instability of the tip (leakage) vortices and generate the secondary vortices. The radial expansion motion of large-scale helical vortices in the outer slipstream dominates the wake meandering phenomenon. [ABSTRACT FROM AUTHOR]

A mathematical model is developed to investigate the performance of a hybrid system comprising semi-submersible floating offshore wind turbines (FOWT) coupled to an array of point-absorbing wave energy converters (WECs) under irregular waves and dynamic winds. In this study, the hydrodynamic interaction of the hybrid system is conducted by applying the matching-method of eigenfunctions to solve the diffraction and radiation velocity potentials. Non-linearities of the hybrid system are taken into account in the Cummins framework, covering aerodynamic, catenary mooring, PTO system, fluid viscosity modeling, and frequency–time domain transformation of hydrodynamic coefficients. After running the convergence analysis and model validation, the present model is employed to perform a multiparameter effect analysis. Case studies are presented to clarify the effects of WEC parameters (including radius, draft, PTO damping, and layout) on the performance of a hybrid system. Nevertheless, a significant aspect of this work revolves around the development of a precise and efficient mathematical model, capable of accurately computing hydrodynamic coefficients for specific marine structures and evaluating the motion response of the interconnected floaters. Additionally, the study offers valuable insights into the preliminary design of hybrid systems and lays a mathematical foundation along with corresponding code for hybrid system projects. • An accurate and efficient model is presented to calculate the hydrodynamic coefficients of hybrid systems. • A practical mathematical model has been developed to evaluate the dynamic performance of coupled marine structures. • Integrating WECs significantly reduce the power spectrum density in pitch mode of the wind platform. • Linear PTO contributes more to platform stability than quadratic and constant PTO systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Wave energy has significant worldwide exploitable resource and its exploitation has attracted renewable energy investigator' attention. Great progress on calculating device performance has been made by means of theoretical, numerical and model tests. This paper presents a method of calculating the energy extraction of a wave energy converter (WEC) based on Wave Height Take-off (WHTO). The method provides a means to improve the capture efficiency of designs, including demonstrating how well different kinds of WEC are optimized for certain wave conditions. Numerical simulations of a heaving buoy and a bottom-hinged pendulum in a 2D wave flume with different damping types (linear and nonlinear) are presented. The results show that the difference between the calculated energy extraction from the wave height reduction and from the model power take-off (PTO) was not significant in a 2D flume. Physical model tests were conducted using a simplified PTO consisting of a system of lifting weights, used to measure the energy extraction directly. Based on both numerical and physical model analyses, the article defines WHTO, which is equivalent to energy extracted by PTO, but determined without taking direct measurements. This paper aims to promote and validate the concept of the WHTO. • A methodology of calculating the energy extraction of a WEC by WHTO was promoted. • A numerical model was used to validate the concept of WHTO. • A heaving buoy and a pendulum were applied to find the differences PTO and WHTO. [ABSTRACT FROM AUTHOR]

The global pursuit of clean energy resources to cope with escalating energy demands has underscored the pivotal roles of wind and wave power as two prominent renewable energy sources for the power industry. In order to promote the comprehensive harnessing of ocean energy, research efforts have increasingly focused on hybrid approaches, aiming to concurrently capture wind and wave energies. This paper proposes two types of wind-wave hybrid device, which combines wind turbine with a wave energy converter (WEC), based on jacket-frame foundation. The numerical models of the WECs were created using SOLIDWORKS, and the subsequent simulations were conducted using ANSYS Workbench. The comparative analysis revealed that the heave-type WEC had less added mass and radiation damping, coupled with larger response amplitude operator (RAO) and shorter natural period compared to the swinging counterpart. Meanwhile, the swing-type WEC exhibited higher motion speed and greater support reaction force and connecting force, thereby exerting more pronounced influence on the foundation. However, the average hourly power generation of the swinging WEC was significantly better in terms of power capture. In practice, the choice between these WEC configurations should consider aligning the power matrix with the probability distribution of prevailing sea state conditions. • Two types of wind-wave hybrid device integrated into a fixed jacket-frame foundation were proposed. • The numerical models were created using SOLIDWORKS, and simulations were conducted using ANSYS Workbench. • The numerical simulation and generation estimation in specific sea states were comparatively analyzed. • The advantages and disadvantages of the two devices have been analyzed. [ABSTRACT FROM AUTHOR]

A practical model is developed to investigate the performance of a hybrid system with semi-submersible floating offshore wind turbines (FOWT) coupled to an array of point-absorbing wave energy converters (WECs). In this study, the boundary value problem is solved by applying the matching-method of eigenfunctions to solve a complex-shaped hybrid system and the velocity potential can be decomposed into radiation and diffraction problems. For each component in the structure, we consider it consists of three coaxial cylinders of different dimensions, making our mathematical model applicable to many marine structures. Within the framework of a linearized theory, We develop the coupled equations of motion to model the stiffness and damping constraints and to evaluate the effect of coupled motion between the floating platform and vertical truncated cylinders, taking into account wind forces, mooring lines, power take-off (PTO) systems and viscous effects on the hybrid system. For such a system, the combination of the OC4-DeepCwind platform with an array of point-absorbing WECs is investigated in this study. After running the convergence analysis and model validation, the present model is employed to perform a multiparameter effect analysis. Case studies are presented to clarify the effects of WEC parameters (i.e. radius, draft, PTO damping and layout), base column submergence, wave heading and frequency on the motion response of wind platform and mean capture width ratio of the WECs array. Our results provide insights into the relationship between the variables analyzed and the performance of the hybrid system. Moreover, the theoretical model developed in this study accurately calculates the hydrodynamic coefficients and motion performance of some marine structures. • A practical model was presented to accurately calculate the hydrodynamic coefficients. • A coupled motion equation was developed to model the rigidity and damping constraints in a hybrid system. • The study examined the synergistic effects between a wind platform and PTO damping. • Added WECs significantly reduce the pitch motion response of the wind platform. • Increasing the submergence depth of the base column has a negative impact on the hybrid system. [ABSTRACT FROM AUTHOR]

Abstract Heaving buoy convertor is one of the most common wave energy convertors (WECs) for its brief structure form and adaptability to wave directions. In reality, some heaving buoy convertors are designed to combine with other marine structures like floating platform, seawall and offshore wind turbine. In such cases, the budget or the installation space may be a limitation for buoy's characteristics. Therefore, the choices of dimension and mass are of great importance to capturing as much as possible energy under the limited condition. However, in the optimization of the buoys, this study, about the effects on the hydrodynamic performance and energy output by the relationship between section surface, mass and draft, is not sufficient. This paper studies the optimization design of a conical-bottomed heaving buoy converter with consideration of Power Take-Offs (PTO) which are linear, constant, and quadratic non-linear. A 3D numerical wave tank (NWT) based on the RANS equations and two-phase VOF model is established using CFD software Flow-3D. The numerical model is applied to deal with the buoy's movement under different PTOs, and is validated by the experimental data. The movements of a conical-bottomed heaving buoy with different mass and section surface are studied numerically, which provide more information of the effects on the energy capture performance. The conclusion of this paper gives a guidance to the buoy prototype optimization design under the certain wave conditions. Highlights • A numerical wave tank based on the RANS equations and VOF model was established and validated with the experimental data. • The influences of mass, section surface and their joint effect were studied numerically. • The influence of different Power Take-Offs which are linear, constant, and quadratic non-linear were also considered. • A choose strategy of heaving buoy's dimension and mass to guide the buoy prototype optimization design was given. [ABSTRACT FROM AUTHOR]

Abstract Hydrodynamic performance and power absorption of an oscillating buoy wave energy converter (WEC) with 3 degrees of freedom (DOFs) are studied. The DOFs are surge, heave and pitch. Linear potential theory is utilized. Optimal PTO damping coefficient is selected. Optimal PTO power output and capture width ratios are calculated and compared. Influences of the buoy's radius, draft and geometry are investigated. The geometries are cylinder, hemisphere-bottom cylinder, cone-bottom cylinder, linear-chamfered cylinder and circular-chamfered cylinder. Results show that only surge and pitch interact with each other. The interaction is intensified on the resonant frequency of pitch, and is significant with special radius and draft. Power absorption of 3-DOF WEC increases significantly (at least 46.6% here) than unidirectional WEC and the power absorption band is wider. Heave is the main power capture DOF. Pitch gets the biggest power absorption at the resonant wave frequency. Increasing radius benefits the power capture efficiency at low wave frequencies and is harmful at high wave frequencies, except for pitch motion. Increasing draft benefits the power capture efficiency in surge and heave, but is harmful in pitch. For surge, heave and 3-DOF motion, cylinder is the best shape. For pitch, the hemisphere is the best shape. [ABSTRACT FROM AUTHOR]

The work presented in this paper focuses on evaluating the rotational characteristics and capture efficiency of a novel variable guide vane wave energy converter (WEC). The rotation of the guide vanes under the action of waves drives the rotation of the vane wheel, generating electricity. The vane wheel always rotates in one direction, regardless of the rotation direction of the guide vanes. The optimal maximum turning angle of the guide vane is determined using a simplified model run with commercial CFD software [25], which is then validated by experimentation. Experiments were conducted on a 1:4 scale model using guide vanes set at the optimal maximum turning angle under conditions both with and without a generator. It is observed in these tests that the device intermittently pauses when the velocity of the water particles at the height of the vane wheel is less than 0.211 m/s under no-generator conditions. Capture efficiency is approximately 7% because of the short time, small area, and small range of the device for harnessing the wave energy. An electronic load with a resistance varying in the range of 1–500 Ω is then connected to the generator, and the working performance of the WEC is investigated. Finally, two alterations intended to improve the capture efficiency of the variable guide vane WEC are proposed and evaluated, and the capture efficiency of a scaled-up model is discussed. [ABSTRACT FROM AUTHOR]

Global economic development and environmental protection requirements are increasing the demand for renewable energy rapidly. Wave energy converters are a hotspot in research, and many designs have been proposed. Pendulum-based wave energy converters, because of their sealed structure and high reliability, are drawing researchers' attention. This article considers the concept of a Vertical Axis Parametric Pendulum Wave Energy Converter (VAPPWEC) and focuses mainly on the characteristics of the pendulum system. The dynamic function of the pendulum is proposed. A new evaluation indicator, Cumulative Frequency curves of Angular Velocity (CFAV) is introduced to analyze pendulum motion accurately and evaluate the harvesting performance conveniently. The model test shows the parametric resonance zone of the pendulum with a notable boundary and the pendulum's stability under power take-off (PTO) damping. Additionally, a large-range analysis method based on the Prescribed Excitation Model (PEM) is proposed, and the output power is assessed. Linear damping performs better than constant damping, with a maximum capture width ratio of 39.05%. • The theoretical model of the vertical axis pendulum is established. • The characteristics of the pendulum are investigated by experiment campaign. • A new indicator for evaluating the motion of the pendulum is proposed. • A large-range analysis method is introduced to predict the pendulum's performance. [ABSTRACT FROM AUTHOR]

Circular ramp overtopping wave energy converter (CROWN) is an offshore overtopping wave energy converter that consists of a reservoir surrounded by a circular ramp and an outflow duct installed with a low water head turbine. Physical model experiments were conducted in a wave tank as a preliminary study at Ocean University of China. The wave overtopping behaviors demonstrated that guide vanes can effectively lead the wave water motion on the ramp for overtopping into the reservoir. An analysis of the overtopping discharge shows that the resonant zone is located in the smaller relative wave length domain. Within the scope of shape parameters used in this study, guide vanes and milder ramp slopes were found to significantly enhance the overtopping discharge. The orientation of the guide vane angle also had a large influence on the overtopping flow rates for smaller relative wave heights and lengths. The experimental results are presented with the aim of designing and optimizing the shape parameters of CROWN for a future prototype device. [ABSTRACT FROM AUTHOR]

Heaving buoy wave energy converter is applicable to small wave height and short wave period, which are the main characteristics of the China Sea. The device captures the wave power by buoy's heaving motion, transfers it by hydraulic system, and converts it into the electric power by generator. Power Take-Off (PTO) of the device is related to the resistance of the hydraulic system. However, effects of the hydraulic damping on the motion of buoy are often neglected and the study on the optimal hydraulic system is rare. In this paper, a new theoretical analysis has been developed, which is more applicable to the buoy considering hydraulic system. The paper establishes the governing equation considering buoyancy, wave dynamic force and hydraulic damping. The displacement and the velocity of the buoy are both obtained. The time-averaged power captured is calculated by integrating the square-velocity expression derived, and the existence of the maximum PTO is proved. Theoretical result is compared with the experimental result. The paper demonstrates the displacements and average output power of the buoy with different values of damping and inertia coefficients. The results of the study could be a guidance for the PTO design of the heaving buoy sets. [ABSTRACT FROM AUTHOR]

In this paper, a new type of combined wind-wave system is proposed, that is, composite bucket foundation-oscillating buoy (CBF-OB) combined device. A three-dimensional numerical wave flume is established by using the renormalization group k–ε model. The hydrodynamic characteristics around the wave and the combined power generation device are studied. The relationship between wave parameters and wave run-up and wave pressure is analyzed. The absorption efficiency performance of the combined power generation device is evaluated. The results show that in the combined power generation system, the wave run-up and wave pressure at 0°–135° around the CBF are smaller than those in the presence of only CBF, but near 180° the ones are larger than those in the presence of only CBF. In the rear side of the combined power generation system, the smaller the scattering parameters, the more obvious the phenomenon of the second wave peak, and the stronger the nonlinearity of the wave and CBF-OB combined power generation system. The proposed CBF-OB combined power generation device can significantly improve the absorption efficiency of the buoy, which can be increased by about 1.5–4.0 times compared with the absorption efficiency under the action of only the buoy alone. There is an optimal power takeoff parameter, that is, when the damping parameter is 500 N·s/m, to maximize the absorption efficiency of the combined power generation device. [ABSTRACT FROM AUTHOR]

In this paper, the influence of the spatial configuration of a wave energy converter (WEC) array upon total power output is investigated. A power prediction model has been proposed to improve the efficiency of calculating the total power from the array with guaranteed accuracy. The prediction model consists of two main components. One is the automatic generation model for training samples by establishing an interface between MATLAB and AQWA. The other is the artificial neural network (ANN) adaptive training model improved by the adaptive genetic algorithm (AGA), which helps to improve the prediction accuracy by 80% according to experimental validation and simulation comparison. The traditional genetic algorithm (GA) has been widely used in WEC arrays optimization. The method proposed in this paper has improved the GA optimization efficiency by about 50%. Arrays with different numbers of devices are studied in irregular waves measured in the previous sea trials, and similar optimal layouts appear in all cases, which allows extrapolation of the results to even larger arrays. The solution space calculated by the optimization algorithm is further analyzed based on separate considerations of different spacing between devices and wave directions. The resulting behaviour is expected to guide engineering applications. • A new power prediction method for WEC array, including an automatic generation model and an adaptive training model, is proposed. • A new adaptive genetic algorithm is proposed to optimize WEC array. • The influence of the spatial configuration of a WEC array upon total power output is investigated. • The linear arrangement of WEC has important properties and the resulting behaviour is expected to guide engineering applications. [ABSTRACT FROM AUTHOR]

In this paper, an active optimization method of a wave energy converter (WEC) array that focuses on the characteristics of the random sea condition is proposed. Physical model tests were conducted in both regular and irregular wave conditions. Three representative regular wave cases were determined from the target wave spectrum using the method. A rigid cylindrical heaving buoy model with different constant power take-off (PTO) damping forces and drafts was applied to find the optimal isolated WEC parameter under each regular wave condition. Three equal buoys with respective optimized parameters were arranged rectilinearly into a WEC array for irregular wave tests. The power output of arrays with different settings are compared, indicating the advantages of the different optimized array and the significance of unit order. To amplify the optimization effect, a supplementary numerical investigation of a 5-body WEC array was carried out using the boundary element method (BEM). The results demonstrate the method and show the influence of the number of different units. • An optimization method of WEC array that based on the target wave spectrum is proposed. • Model tests for isolated buoy and array were conducted in regular and irregular waves. • Supplementary simulations of a 5-body array were executed using BEM to validate the method. [ABSTRACT FROM AUTHOR]

As the two-body heaving wave energy converter (TBH-WEC) moves toward more commercial and large-scale applications, it usually has to be deployed in the form of arrays to reduce costs and achieve better performance. However, hydrodynamic interactions in TBH-WEC arrays are highly complex and difficult to evaluate. The effect of array configuration on the performance and interactions of TBH-WECs within the array has not been fully investigated. To address this gap, the power output time-domain modeling of TBH-WEC arrays is first established using the enhanced MATLAB-APDL-AQWA unified simulation system. Based on this system, this work investigates the internal relationship between incident wave frequency, array size, array layout, and Power Take-Off (PTO) system, and array interaction factors in both unidirectional regular and irregular waves, as well as multidirectional real-wave scenarios. The energy redistribution properties of the array wavefield are then analyzed, as is the correlation between array layout and the disturbance wavefield of the devices. Results show that array layout optimisation and PTO configuration optimisation are relatively independent from each other, and that better array layouts can be quickly obtained by iteratively adding clusters or devices in the amplified region of the existing array disturbance wavefield. [ABSTRACT FROM AUTHOR]

Following the development of offshore wind turbine (OWT) systems and wave energy converters (WECs), there is an increasing demand for the development of hybrid systems that combine OWTs with WECs, for the purpose of reducing the Levelized Cost of Electricity (LCOE) of WECs by sharing foundations, increasing overall power output, and optimizing the utilization of marine space. One of the hybrid systems integrates WECs with the most widely adopted foundation used by existing wind farms, i.e. the fixed monopile foundation. While existing research mainly focuses on the characteristics of loads and power performance of WECs, the corresponding characteristics related to wave load on the monopile foundation and the hybrid system play a critical role in securing the safety and stability of the monopile foundation but is limited in the available literature. This paper presents an analysis of wave load characteristics on a fixed monopile foundation of a 15 MW OWT integrated with a torus-shaped heaving WEC using the ANASYS-Aqwa adopting the linear potential theory. The structure is subjected to regular and irregular waves in the operational sea conditions. The power take-off (PTO) system of the WEC is modelled by applying a linear damping in the present numerical work. The results suggest that the PTO damping has an insignificant impact on the wave load statistics. It is further confirmed that, the integrated WEC effectively reduces the wave load and overturning moment acting on the monopile foundation within a wide range of wave conditions. Moreover, the presence of the monopile also reduces the horizontal wave load on the WEC. However, the overall load on the hybrid system, which is the resultant force of horizontal wave force acting on buoy and monopile, is higher than that experienced by the monopile foundation of the OWT prototype. To the best of our knowledge, this is the first work investigating into the effect of integrated torus-shaped WECs on wave loads affecting fixed foundations. The findings offer valuable insights for the design and deployment of such hybrid systems in the near future. • Wave load characteristics on the monopile foundation and hybrid system are systematically studied. • Wave load characteristics in the operational sea are analyzed considering the effects of the PTO system on the buoy. • The coupled buoy mainly provides a shielding effect and reduces the diffraction wave force on the monopile. • The overall load on system is higher with considering the introduced buoy within the range of consideration of this paper. [ABSTRACT FROM AUTHOR]

Hybrid wind-wave systems combining the wave energy converters (WECs) with offshore wind turbines (OWTs) is a promising way to enhance the power production and improve the sea space utilization. In this paper, a novel hybrid wind-wave conceptual system, in which a multi-buoy WEC is integrated with a fixed monopile OWT, is proposed. This is the first concept utilizing multi-buoy WECs and is distinguished from existing hybrid wind-wave systems with a fixed monopile OWT, which integrate a single oscillating water column or a heaving point absorber. To characterize the hydrodynamics associated with the proposed system in operational wave conditions with different directionalities, a potential flow solver with an appropriate power take-off (PTO) model is applied. The results demonstrate a significant buoy-buoy and buoy-monopile hydrodynamic interaction, suggesting that the existing hydrodynamic characteristics for the wind-wave system with a single buoy WEC may not be applicable to the new system. More importantly, the power performance of the present system is proven to be better than the corresponding single-buoy wind-wave system, as being quantitatively assessed by the newly-defined evaluation index within the range of the consideration of this paper. [ABSTRACT FROM AUTHOR]

Abstract The pitching hydrofoil is a promising device for generating thrust or harvesting tidal stream energy in shallow water. A two-dimensional numerical model, based on the computational fluid dynamics software ANSYS-Fluent, is established to investigate a fully passive pitching hydrofoil restricted to one degree of freedom. In the model, the fluid-structure interaction is realized by updating the flow field and hydrofoil position alternately, and this is validated by various pitching-foil cases. In freely pitching calculations, the effects of the pivot location, moment of inertia, and Reynolds number on the pitching angular amplitude, hydrodynamic torque and passive reduced frequency are studied. More complicated flow fields and dynamic behaviors of the pitching hydrofoil are observed under a reaction linear load acting as a substitution for a synchronous generator. The energy conversion efficiency contributed by the pure pitching motion alone is up to 0.13. The hydrodynamic performance of the passive pitching hydrofoil reported in this paper could provide valuable information for the design and optimization of this type of device. Highlights • A numerical model for the 2D pitching hydrofoil is established. • The fully passive pitching motions of a hydrofoil are simulated numerically. • The freely-pitching amplitude is mainly determined by the profile of the hydrofoil. • The hydrodynamic characteristics under a linear load condition are more complex. • The passive foil-pitching can make contribution to the current energy harvesting. [ABSTRACT FROM AUTHOR]

Based on the Wave Energy Capture Spectrum (ECS) theory, the characteristics of the ECS of a cylindrical heaving buoy WEC with consideration of linear PTO damping are studied in this paper. A 3D numerical wave tank (NWT) based on the RANS equations and the two-phase VOF model was established using the CFD software Flow-3D and validated by the corresponding experimental data. The ECSs of the heaving buoy WEC with different settings were calculated by the energy capture fluctuation signal, which was based on the incident wave elevation and the power output of the heaving buoy. The procedures of ECS solution using numerical method were proposed, and the effects of irregular wave, linear PTO damping, and relative draught on the ECS of the heaving buoy were studied, respectively. The results showed that the average period of incident wave is the key control factor of the peak frequency of the ECS. Compared to linear PTO damping, the relatively draught has little influence on the peak frequency of ECS. With the evaluation of the ECS method, the cylindrical heaving buoy WEC shows good adaptability to variable wave conditions and could be further improved by adjusting its ECS. • A wave energy converter (WEC) evaluation method based on spectrum method was presented numerically. • A numerical wave tank based on the RANS equations and VOF model was established and validated with the experimental data. • The effects of irregular wave, linear PTO damping and relative draught on the ECS were studied numerically. • The performance evaluation of a cylindrical heaving buoy under the Energy Capture Spectrum (ECS) was given. • The feasibility of the proposed approach is verified with a cylindrical heaving buoy case study. [ABSTRACT FROM AUTHOR]

A new wave energy converter is proposed in this paper, consisting of three split heave point absorbers, combined with a taut-moored floating turbine. It is adapted to the waves in the China Sea area, which are characterized by short periods and small amplitudes. Based on a series of physical model tests in regular, irregular, and extreme waves, the hydrodynamic performance of the integrated device is systematically investigated under different damping forces and incident wave directions. The experimental results reveal that the split point absorber presents new hydrodynamic characteristics and that the wave energy capture efficiency of the new device is greatly improved for the short-period waves in low sea states. Moreover, due to the out-of-phase heave motion, as well as the induced shallow water effect, the submerged platform makes a contribution to improving the energy capture efficiency of the split floater, particularly pronounced in the case of a high damping force in the power takeoff system. Under the condition of incident wave direction being coincident with the horizontal projection of mooring lines, the energies of pitch motion and mooring force of the integrated system are increased as a result of the high-frequency oscillation, which needs to be solved by further optimizing the taut mooring system. [ABSTRACT FROM AUTHOR]

Overtopping wave energy converter (OWEC) is capable of converting the unstable wave energy into the relatively stable potential energy in its reservoir. The small wave height and the large tidal range in China will cause a lower efficiency of the traditional single-level overtopping device especially at the lower tidal level. The design of the multi-level reservoirs may be a solution to improve the overtopping performance, which can be integrated in the sloping-ramp breakwater. In this study, a multi-level breakwater for wave energy conversion, consisting of two reservoirs with sloping walls at different levels, is proposed. A numerical wave tank based on the VOF model is established in ANSYS-Fluent ® 12.0 for the OWEC's performance investigation. The numerical model on predicting the overtopping process and time-averaged discharges is validated by corresponding experimental data. Effects of the opening width of the lower reservoir, sloping angles of two reservoirs and the gap height between the reservoirs on the dimensionless overtopping discharges are studied numerically. It is found that a smaller opening width, larger height ratio, and two sloping angles of 30° have positive effects to improve the overtopping performance of the device. [ABSTRACT FROM AUTHOR]

Many marine structures are composed of vertical axisymmetric floats. In this paper, a semi-analytical model based on the linear potential flow theory and an eigenfunction expansion method is developed to study wave diffraction and radiation by an array of cylindrical structures. Each structure can be formed by three coaxial cylinders of different dimensions. Based on the semi-analytical model, a constrained matrix equation of motion is presented and solved to evaluate the performance of multiple interconnected cylindrical structures. In order to verify the accuracy of the semi-analytical model, a typical OC4-DeepCwind floating offshore wind turbine (FOWT) is selected for validation. The validated model is, then, applied to study the effect of base column submergence depth, radius, and thickness on the motion response of a semi-submersible platform. Although the results of this study are of significance for the selection of the optimum semi-submersible FOWT for specific locations (with specific prevailing wave directions), the focus of this work was placed on the semi-analytical model itself, which is efficient in modeling the interaction of the wave field and can be used in future FOWT projects. [ABSTRACT FROM AUTHOR]

A compact buoy-array-type wave energy converter called multi-heaving-buoys (MHB) is introduced in this study. The hydrodynamic performance of MHB under regular wave conditions was first investigated experimentally in a wave tank located in Ocean University of China. It was found that a limited number of heaving buoys had little effect on the wave fields around the device. The small period of the incident waves caused an intense interaction between the waves and the buoys. The phase difference between the buoys in different rows was determined by the distance between the buoys. It was found that the response amplitude operator of the buoys varied from 0.6 to 1.2. Correspondingly, the range of the averaged relative velocity of the heaving buoys was 0.6-1.3. The upper limit of the acceleration of the buoys' motion was 0.2 times that of gravity. All of the experimental results provide valuable information for the future design of the hydraulic pressure power take-off systems. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Based on the developed high-fidelity time-domain simulation framework, this paper presents a parametric study on arrays of point absorber wave energy converters under irregular waves. Its purpose is to assess the effects of coupling among array configuration parameters, array interactions, and array layouts, and to provide guidance for array design. Parameters considered include the incident wavelength, draft, slenderness ratio, bottom shape, Power Take-Off damping, and mooring stiffness. It is shown that the device parameter configuration has a negligible effect on the optimization of the array layout and the performance improvement obtained by tuning the configuration of each device in the array is not significant compared to using the optimal value of the isolated device for the whole array. Therefore, optimizing the parameter configuration of the isolated devices first, and then the layout of the array composed of these devices, is proved to be a more recommended optimization procedure. Finally, the obtained findings and recommended optimization procedures effectively guide the design of a 10-PA-WEC array in a realistic scenario. And the array optimization results support the findings of the parameter study. • A high-fidelity simulation framework for wave energy arrays is developed. • The effects of different parameters on array interactions are assessed. • Better layouts can be quickly derived based on the motion response field. • Parametric study results effectively guide the design of arrays in realistic. [ABSTRACT FROM AUTHOR]

The interaction between a wave and an oscillating buoy is complex, and it directly affects the stability, working state, and efficiency of the buoy. In this study, an experimental investigation of the interaction between a wave and an oscillating buoy (including wave breaking, wave run-up, and the relative motion between waves and buoys) is conducted under constant power take-off (PTO) values. The wave conditions selected in the experiment are all obtained from measured data on the Qingdao coast, China. The experimental results show that when the wave steepness is greater than 0.0597, there is an obvious wave breaking in front of the oscillating buoy and that the steepness of the critical wave decreases with an increase in PTO. When the relative damping force is 0.101. In addition, an empirical formula based on the wave steepness, the PTO value, and the wave run-up is proposed in accordance with the M5' model tree. The results obtained in this study contribute to an increased understanding of the development of wave energy from buoys. In turn, this improvement in understanding will inform the design and deployment of buoys in real-life scenarios. • Wave run-up in front of the buoy is investigated under wave conditions. • Maximum wave run-up increases with wave steepness. • The critical wave steepness of wave breaking on the front of the oscillating buoy decreases with the increase of PTO. • M5' model tree algorithm was implemented to develop formulae in run-up prediction. [ABSTRACT FROM AUTHOR]

The dual-chamber oscillating water column (OWC) is a modified wave energy converter that has received considerable attention owing to its high capture efficiency and adaptability to wave conditions. In this study, the hydrodynamic performance of a land-based dual-chamber OWC was numerically investigated, focusing on the effect of damping combinations and chamber breadth on hydrodynamic efficiency. The commercial software ANSYS Fluent was employed to solve the Reynolds-averaged Navier–Stokes equation, and the solution was verified through a comparison with experimental data. The variations in water surface elevation, pressure, and efficiency of a dual-chamber OWC with different orifices and chamber breadth ratios were investigated. The results indicate that an OWC with different opening ratios is more efficient and offers advantages in adapting to variable wave environments compared with an OWC comprising two chambers with the same orifice dimensions. Furthermore, it was found that incident waves with a high frequency result in a higher absorption efficiency for the combination of smaller damping of the front chamber and larger damping of the rear chamber; by contrast, waves at lower frequencies have higher absorption efficiencies with the opposite combination. The breadth ratio has different effects on the hydrodynamic efficiency for different damping conditions. The findings can serve as a reference for the practical application of dual-chamber OWCs. • Dual-chamber OWC with different damping in both chambers are more efficient than OWC with the same damping. • The optimal damping combination for fixed sea conditions was explored. • The damping significantly affects the resonance band position. • The chamber breadth ratio exhibits different effects on hydrodynamic efficiency under different damping conditions. [ABSTRACT FROM AUTHOR]

Three focused waves, which have almost identical temporal profiles, are successfully generated in a numerical wave tank with both linear and nonlinear methods, and they are utilized to act upon an LNG carrier to study the induced differences of the ship peak responses for the first time. The influences of focal location and incident wave direction are taken into account and the fundamental reason for the response difference is revealed from the perspective of energy variation. It can be concluded that when the LNG carrier encounters the same temporal focused waves, the peak motion responses can be different due to the different hydrodynamic characteristics under the focused crests, which can be further attributed to the distinct physical mechanisms involved in the energy concentration of these focused waves. The focal location and incident wave angle can further enlarge the corresponding discrepancy under some circumstances. From a spectral point of view, the motion discrepancy is mainly caused by the actions of the distinct fundamental wave components. Specifically speaking, the relatively high-frequency components of the nonlinear focused waves tend to induce a smaller heave response and the quasi-monochromatic wave component cannot trigger a larger pitch motion than the linear focused waves. • Linear focused wave can induce larger ship motion than nonlinear focused wave. • Response discrepancy can be enlarged by focal location and incident wave direction. • Motion difference arises from the action of distinct fundamental wave components. • Heave is more sensitive to hydrodynamic characteristics of focused waves than pitch. [ABSTRACT FROM AUTHOR]

Wave energy converters (WECs) of the hinged-raft type offer flexible arrangements and can be further applied in arrays. This study employs a numerical model to investigate the energy capture spectrum (ECS) of a two-body hinged-raft WEC under random waves utilizing the open-source simulation software WEC-Sim. The model explores in detail the combined effects of random wave parameters and WEC parameters, including the power take-off (PTO) damping coefficient, PTO stiffness coefficient, and horizontal hinge position, on the ECS. The findings revealed that the hinged-raft WEC is highly sensitive to the average wave period and adaptable to varying wave heights. Optimal energy capture under conditions of varying wave heights can be achieved by adjusting a specific PTO coefficient. These findings underscore the complex response of the two-body hinged-raft WEC to wave parameters and emphasize the importance of understanding these sensitivities and adaptabilities to optimize the WEC system, enhance efficiency, and promote its application in arrays. • The energy capture spectrum (ECS) of a two-body hinged-raft WEC is investigated. • The optimal PTO damping coefficient is mainly affected by the average wave period as the wave condition changes. • The increase in the stiffness coefficient significantly reduces the total energy capture. • Positioning the hinge at the front-middle part of the WEC is advantageous for overall energy capture. [ABSTRACT FROM AUTHOR]

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. • A constrained and hydrodynamic coupled model of WECs microarrays was developed. • The energy redistribution characteristics within the WECs microarrays were analyzed. • The gain of microarray to FOWT in pitch decreases with increasing wave direction. • Wave asymmetric incidence can increase the hydrodynamic response of microarrays. [ABSTRACT FROM AUTHOR]

This research delves into the effects of multiple degrees of freedom on a raft-type wave energy converter (WEC), using both experimental and computational methods. The WEC features two interconnected floating bodies linked by a specialized parallelogram joint, allowing for distinct heave, pitch, and surge motions. The power take-off (PTO) system, integrating Coulomb damping, is analysed for its impact on energy absorption. Despite varying damping amplitudes, peak energy absorption is consistent across different wave periods, as shown in scaled model tests. Computational simulations further investigate the multi-freedom effects by imposing constraints on various degrees of freedom, thus elucidating their individual and combined influences on energy absorption. Preliminary results indicate that the addition of degrees of freedom can enhance the WEC's efficiency, especially through coordinated release of the bodies' relative pitch and surge motions. The study also finds that the energy absorption efficiency from pitch motion depends on the release of surge motion. Moreover, the energy distribution among each degree of freedom is significantly affected by the device's multi-freedom characteristics. This research provides valuable insights and practical guidelines for the development of advanced multi-freedom WEC technologies. • Study on a WEC with multiple freedoms and a mechanical PTO system. • Tested its movement and energy efficiency in a water basin. • Nonlinear model used to analyze multi-freedom and PTO impacts. [ABSTRACT FROM AUTHOR]

Wave energy, one of the safest and currently growing forms of renewable energy, requires the development of the appropriate wave energy converters (WECs). This study presents a recently patented WEC; and proves the feasibility of this concept using a series of experiments. To achieve this, two rectangular chambers of an oscillating water column (OWC) system were arranged in series, and the wave with a propagation direction that forms a relatively small angle of 30° with the front walls was referred to as obliquely incident wave. The effects of the wave amplitude, front wall draughts, and opening ratios were investigated in relation to the overall hydrodynamic performance and the respective performances of the two sub-chambers. It was found that the overall efficiency of the system was greatly reduced as the front wall draught in the high-frequency zone increased. Furthermore, the opening ratios of the individual chambers significantly affected the overall hydrodynamic performance for all frequency bandwidths under obliquely incident waves, and the optimal efficiency achieved at the opening ratio of ε = 1%. The rear chamber is inferior to the front chamber for most of the wave frequencies. • A patented multi-chamber OWC wave energy converter was investigated. • Scaled model experiments are performed under obliquely incident waves. • The opening ratios significantly affect the overall hydrodynamic performance. • The rear chamber is inferior to the front chamber for most of the wave frequencies. [ABSTRACT FROM AUTHOR]

In order to investigate the influence of initial condition uncertainty on the profile of maximum wave, two typical short-term sea states are reconsidered in this paper. The associated uncertainty is investigated with the Monte Carlo Simulation method and the maximum wave is extracted from the numerical simulation performed with the fully nonlinear Chalikov-Sheinin (CS) model which has been validated before. Six parameters are defined in this work to describe the property of the profile of maximum wave and analysed with statistical method. To catch the maximum wave in each realization, it is proved that both zero-up-crossing and zero-down-crossing methods have to be utilized in the wave-by-wave analysis. After analysing the sensitivity of different variables to the initial random-phase/amplitude model, it reveals that the maximum crest, trough and crest-to-trough heights in a sea state are only sensitive to the amplitudes of a certain range of free waves (around the peak wavenumber). Briefly speaking, three types of uncertainties can be identified in the initial conditions involving the natural, knowledge and mixed uncertainties and they contribute a lot to the versatility of the profile of maximum wave. The conclusions drawn in this paper can cast some light on the research of real-time prediction of maximum wave in ocean engineering. • Influence of initial condition uncertainty on the profile of maximum wave is considered in two typical short-term sea states. • Three types of uncertainties can be identified and they contribute a lot to the versatility of the profile of maximum wave. • The height of maximum wave is sensitive to a few free waves but its asymmetric property is determined by all wave components. [ABSTRACT FROM AUTHOR]

The optimisation of WECs is a focus current research. With the increase in optimisation variables, traditional optimisation methods are insufficient to find the best design in a wide and complex solution space, and more advanced algorithms are required. In addition, the iterative optimisation of the WEC requires a reliable and accurate WEC simulation method to ensure the effectiveness of optimisation. Therefore, a MATLAB-APDL-AQWA united simulation system (MAA-system) is developed to calculate the WEC generation power accurately. Then, a memory mechanism is designed to avoid calculating the fitness of duplicate individuals in the search process by creating a memory base to store individual coding information and fitness information. Seven metaheuristic algorithms — the genetic algorithm, differential evolution, immune algorithm (IA), covariance matrix adaptation evolution strategy (CMA-ES), particle swarm optimisation, ant colony optimisation, and simulated annealing — with memory mechanisms are utilised to optimise the draft, power take-off parameter, and layout of WEC globally in random wave conditions. The performances of these algorithms in different search spaces are analysed. The results show CMA-ES is the best choice for solving the unimodal optimisation problem of WEC, and IA is more suitable for multimodal optimisation such as the WEC array layout. · The MATLAB-APDL-AQWA united simulation system (MAA-system) for accurately simulating WEC is developed. · The performances of seven metaheuristic algorithms with memory mechanism are compared in different search spaces based on MAA-system. · CMA-ES is the best choice to solve the unimodal optimisation problem of WEC. · IA performs better in solving the WEC multimodal optimisation problem. [ABSTRACT FROM AUTHOR]

A numerical model of two-buoy and single-buoy floating wave energy converters (WECs) is developed based on a modified nonlinear version of WEC-Sim. The model is validated against experimental data from a 1:5 geometric scale model of a two-buoy floating WEC. Nonlinear behavior is primarily attributed to viscous damping and mechanical friction effects. It is found that a modified version of WEC-Sim obtained by correcting the phase of the wave excitation force in the source code provides an accurate representation of a two-buoy WEC operating in either linear or nonlinear mode. An equivalent single-buoy WEC is modeled by locking the two-buoys together. The two-buoy WEC exhibits more than double the predicted maximum energy capture efficiency than its single-buoy counterpart within the range of numerical tests considered. Simulations of idealized floating WECs in conditions representative of Zhaitang Island, China indicate that the maximum energy capture efficiency may be enhanced by 13 % when using a two-buoy system instead of a single-buoy WEC. The findings should be useful to practitioners involved in the design of floating wave energy converters. • Nonlinear dynamic numerical modeling method is proposed. • Correction to phase of wave excitation force implemented in WEC-Sim model. • Parameter study compares CWRs of corresponding two-buoy and single-buoy systems. • Useful data for the design and development of prototype two-buoy devices. [ABSTRACT FROM AUTHOR]

To study the difference between wave run-up and wave forces on a fixed column under the action of focused waves formed with different physical mechanisms, a 3D two-phase numerical wave tank is developed based on the Navier-Stokes equation and the Finite Volume Method. The influences of wave nonlinearity, column radius and water depth on the run-up and force discrepancies are systematically investigated in four sets of numerical experiments. For focused waves with identical crest height and trough-to-trough period, the run-up height and dynamic pressure around the circumference of a cylinder are mostly larger in linear focused waves than in nonlinear focused waves, due to their different hydrodynamic characteristics. With the increased wave nonlinearity either by changing the period or crest height of the focused waves, the discrepancies become greater for run-up height while smaller for dynamic pressure. Moreover, both of them, less affected by water depth, are more sensitive to the variation of column radius in linear focused waves, showing a steeper gradient in space. Therefore, in practical engineering, the physical generation mechanism must be considered in evaluating the safety of columns of wind turbines hit by freak waves. • Focused waves with identical profile can be formed by linear and nonlinear methods. • Hydrodynamic characteristic is different between linear and nonlinear focused waves. • Run-up height and dynamic pressure on cylinder are larger in linear focused waves. • Nonlinearities enlarge difference of run-up but suppress disparity of pressure. • Wave generation mechanism must be considered in evaluating the safety of column. [ABSTRACT FROM AUTHOR]

This study proposes a coupling strategy between OpenFAST and WEC-Sim (OWS) to provide a general solution for the design of offshore floating wind turbines (FOWTs). The fully-coupled calculation is implemented by transferring the tower-base loads from OpenFAST to WEC-Sim, which in turn transfers the platform displacements, velocities and accelerations back to the former. The cross-platform data interface is achieved via S-function in Simulink, and a parallel route in OWS ensures a comparable calculation speed compared with OpenFAST. The model validation is conducted based on the IEA-15-MW reference wind turbine (RWT) and UMaine-VolturnUS-S semisubmersible platform with the reference site located at the northern North Sea. Through code-to-code comparisons against OpenFAST, the newly developed OWS model is tested in various environmental conditions to ensure its correctness and reliability. Furthermore, the dynamic responses of the FOWT are comprehensively investigated using spectral analysis. The time lag of the platform displacements with respect to the incident wave is quantitatively evaluated based on correlation analysis. Generally, there is little time lag between the heave displacement and the wave, but the time lag in the pitch direction (6–9 s) is approximately twice that (3–4 s) in the surge direction. [ABSTRACT FROM AUTHOR]

An analytical model is applied to investigate the dynamic performance of a type of hybrid system (PAWT) with a semi-submersible floating offshore wind turbine (FOWT) coupled to an array of point-absorbing wave energy converters (PA). In this process, the linear potential flow theory and eigenfunction matching method are employed to study the wave–structure interactions, and a coupled set of equations is proposed to evaluate the motion response in frequency-domain. To account for the pronounced impact of nonlinear factors inherent in the hybrid system, a constellation of mathematical models has been integrated into Cummins equation framework, including aerodynamic, mooring lines, power take-off (PTO), and viscosity modeling. After running the convergence analysis and model validation, the present model is employed to clarify the effects of PTO damping, WEC radius, layout, and real sea states on the motion response and wave energy capture efficiency. Results demonstrate that integrating Wavestar-type WECs is more beneficial for improving the pitch stability of FOWT than heaving-type. Although augmenting the number of integrated WECs cannot enhance FOWT stability, it does contribute to broadening the bandwidth of the capture width ratio (CWR). Notably, the power take-off (PTO) is simplified as a linear damping to model a linear permanent magnet generator (PMG) in this study. Furthermore, the interest of this work lies in the full-process analytical model itself, which is found to be efficient in modeling the interaction of the wind-wave field and can be used in future hybrid system projects. • A full-process analytical model has been developed for hybrid systems. • Multiple types of engineering-practical hybrid systems are introduced. • The effect of PTO damping, WEC radius, layout, and sea states are discussed. • Revealing the effect factors and interconnections on the dynamic characteristic. [ABSTRACT FROM AUTHOR]

The effect of non-linear phenomenon such as wave run-up and wave impact on the heaving buoy wave energy converter may lead to its efficiency decrease and overtopping. In this study, a hydrodynamic simulation of wave run-up heights and maximum wave loads on a cylindrical heaving buoy is constructed and preliminarily analysed using the Reynolds-averaged Navier–Stokes equations. The numerical results show that kinetic energy is mainly concentrated in the wave-ward and leeward sides of the buoy. Further, the maximum pressure at the bottom of the buoy occurs on the leeward side when the wave hits the buoy, increasing by 47% over the initial pressure and 32% over the maximum pressure at the centre of the buoy. For a heaving buoy with the power take-off mechanism (PTO), the damping coefficient is the main parameter that affects the wave run-up and wave loads. By contrast, for a heaving buoy without PTO, the wave height and the radius of the buoy are the main parameter. The accuracy of the numerical results is verified through model tests of the motion response of the buoy and the wave surface characteristics around it. The computational method can guide future buoy designs in terms of safety and reliability. • Investigated run-up and wave field distributions around the heaving buoy under regular wave conditions. • Design of heaving buoy with power take-off is influenced by damping coefficient. • Kinetic energy is concentrated in the wave-ward and leeward sides of the buoy. • Maximum wave load on the heaving buoy increases with wave steepness. [ABSTRACT FROM AUTHOR]

A geometric optimization methodology of a floating two-buoy wave energy converter is proposed based on a combination of the design of experiment method for sensitivity analysis of geometric parameters and hydrodynamic calculation of optimal performance in the time-domain. Viscosity-corrected fitting curves obtained from physical test data are incorporated in the time-domain simulation to improve model accuracy. Constant, linear, and quadratic power take-off models are applied to the two-buoy system, and the optimal power take-off model identified and applied to the optimization process. The configuration of geometric parameters is selected by means of the central composite design method within a prescribed parameter space. AQWA-WEC-Sim co-simulations in the time domain are used to determine the optimal capture width ratio for different geometric parameters. A response surface methodology is employed in conjunction with central composite design to establish empirical equations that predict the maximum capture width ratio (as an objective function) based on three geometric parameters, without need for multiple boundary element simulations. Parameter sensitivity and interaction effects on energy absorption are examined, and optimal dimensions determined according to maximum absorbed energy. The proposed optimization method is computationally efficient and could facilitate parametric optimization of devices of arbitrary geometry. • A geometric optimization methodology of a floating two-buoy WEC is proposed using response surface methodology. • Fitted quadratic viscous damping and an optimal PTO model are applied in the optimization process. • Parameter sensitivity and optimal dimensions are determined. • The optimization method is computationally efficient and could facilitate parametric optimization of WECs. [ABSTRACT FROM AUTHOR]

Combined floating offshore wind turbines (FOWTs), wave energy converters (WECs), and floating solar photovoltaics (FPVs) systems have the potential to provide cost-effective solutions for offshore multi-energy complementation and structure protection. In this study, a theoretical model based on the potential flow theory and eigenfunction matching method is utilized to study wave diffraction and radiation by a co-located system, in which the main components of the wind platform and WECs are made of vertical cylindrical floats. Based on the displacement constraint matrix, coupled equations of motion are developed to calculate the kinematic response of the co-located systems. After running the convergence analysis and model validation, the present model is employed to perform a multiparameter impact analysis. Case studies are presented to clarify the effects of the WEC radius, draft, layout, power take-off (PTO) system, and incident wave heading and frequency on the hydrodynamic coefficient, wave energy capture width, and motion response of the wind platform. Our findings highlight that several factors play a crucial role in the performance of the co-located system, more importantly, that the theoretical model developed in this study is capable of effectively predicting the wave-structure interactions in wave fields, making it applicable to future wave farm projects. • A theoretical model is presented to calculate the hydrodynamic coefficients. • A dynamic motion equation is developed to evaluate the motion response. • A multiparameter impact analysis is conducted on the co-located system. • Our study demonstrates the synergistic benefits of combining wind-wave system. [ABSTRACT FROM AUTHOR]

The energy capture spectrum (ECS) is a valuable tool for evaluating and optimizing the performance of wave energy converters (WECs) under random waves. However, current methods used to measure the ECS can be time-consuming and costly, which hinders its practical application. This paper proposes a novel method for calculating the ECS of WECs with complex power take-off (PTO) systems by defining the periodic energy capture matrix as an inherent parameter of the WECs. This parameter can be measured or calculated in pre-tests, thereby reducing subsequent cost and time. As a case study, a multi-DOF buoy with a mechanical motion rectifier (MMR) PTO system is investigated using this method. A numerical model is built with the open-source WEC-Sim software and is validated by an experimental test. The working mechanism of the MMR-PTO system is investigated, and the proposed ECS calculation method is compared with a previous method, demonstrating the accuracy of the proposed method. The energy capture characteristics of the multi-DOF buoy in the heave and free motion modes are also investigated and compared. The results show that the buoy in the free motion mode performs better in the high-frequency domain with a large PTO damping. The proposed method has the potential to promote the engineering application of the ECS and facilitate the performance optimization of WECs under random waves. • A novel method for calculating the energy capture spectrum of WECs with complex PTO systems is promoted. • A numerical model of a multi-DOF buoy "Rolling" with an MMR-PTO system is established and validated. • The energy capture characteristics of a multi-DOF buoy "Rolling" in heave and free motion modes are investigated. [ABSTRACT FROM AUTHOR]

Aerodynamic load is one of the important loads that must be considered for wind-wave energy hybrid devices. In this study, the effects of blade angle and foundation structure vibration on aerodynamic loads are considered, and the traditional blade element momentum (BEM) is dynamically modified by the method of coordinate matrix transformation and vibration function analysis, starting from the relative inflow angle and the instantaneous wind speed of the wind turbine, and the aerodynamic loads of the wind turbine were simulated by using the improved BEM theory. The correctness of the proposed method in this study is verified by comparing the data of a large number of researchers and the computational results of OpenFast. A simulation study has been carried out using AQWA, combining the modified aerodynamic load of the wind turbine and the hydrodynamic load of the wave energy device, to analyze the variation trend of the base structure response of the wind-wave hybrid device under general and extreme conditions. The results show that the method developed in this paper can provide theoretical reference for the development and stable operation of wind-wave hybrid device. • In this study, the classical BEM theory was modified based on the blade installation angle and foundation structure vibration. • The load fluctuation rules of the structure under the combined effect of blade angle and foundation vibration were revealed. • The distribution rules of the axial thrust of the blades were obtained by considered the effects of wind shear and fluctuating wind on the aerodynamic loads. • The response change rules of the foundation structure of hybrid devices under general and extreme conditions were analyzed. [ABSTRACT FROM AUTHOR]

• A combined numerical model is designed with the aim of reducing numerical errors caused by liquid viscosity. • A basin test including the decay and oscillation cases is conducted to apply the physical validation. • The dimension, mass and PTO system of heaving buoy WEC are systemically optimized. This optimisation study focused on the mass, dimension, hydrodynamic response, and power take-off (PTO) damping for heaving buoy wave energy converters (WEC). A numerical model, consisting of a potential flow (PF) model and a computational fluid dynamics (CFD) model, is designed and applied to address numerical errors caused by liquid viscosity after physical validations. Comparing results from the PF and CFD models, it is evident that the liquid viscosity will produce additional radiation damping for the oscillator near its resonance frequency, leading the decrease of the device's energy absorption. Optimisations based on the combined model were conducted, including non-PTO and PTO cases with both regular and irregular incident waves. The results of non-PTO cases indicate that a buoy with a relatively larger mass is more sensitive to the liquid viscosity, but it still can obtain a better optimal hydrodynamic performance with a higher wavelength-to-diameter ratio. The PTO cases compare the energy absorption caused by the linear PTO and the Coulomb PTO. The comparison results demonstrate that the Coulomb PTO can reduce the effects of viscosity and absorb more energy under identical wave conditions. This paper presents the methods and considerations for working towards the overall optimisations of the heaving buoy WEC. The work will be useful for practitioners and researcher working on wave energy utilization. [ABSTRACT FROM AUTHOR]

• A coupled-pitching hydrofoil was tested under the semi-passive mode. • The magnetic damper and the electric generator were used as the power take-off. • The increase in the activated pitching amplitude enhanced the leading-edge vortex. • The optimal pivot location and moment of inertia were obtained. • The peak power coefficient and efficiency were achieved at 0.75 and 0.33. The semi-passive flapping hydrofoil is a promising device for harvesting tidal stream energy in shallow waters with significant practical advantages compared to traditional rotor turbines. The energy-harvesting performance of a coupled-pitching hydrofoil under the semi-passive mode was studied experimentally and numerically. In the water channel test, a magnetic damper and an electric generator was employed to provide the constant and varying loads, respectively. The hydrofoil model performed better in energy-harvesting for the reduced frequency of 0.10 and the activated pitching amplitude of 70–90°. The two-dimensional and three-dimensional numerical models were established and the dual-sliding-overlapped-mesh technology was used to realize the complicated coupled-pitching motions and to predict the fluid-structure interaction of the hydrofoil. The vortex structures and corresponding pressure distribution characteristics over the hydrofoil surface were presented for analysis of the operating performance of the semi-passive system. The optimal moment of inertia and location of the activated pitching pivot were obtained. In the numerical study, the peak power coefficient and efficiency were achieved at 0.75 and 0.33, respectively. [ABSTRACT FROM AUTHOR]

• A model-scaled OWC was tested under irregular wave conditions experimentally. • An impulse turbine model was connected to the OWC chamber directly. • Time histories and spectral analysis results of the OWC performance were presented. • The primary-stage, secondary-stage, and overall efficiencies were presented. • The results can be used for further validations of the numerical models. The rapid development of the oscillating water column (OWC) wave energy converter proposes a demand to predict its overall performance accurately. Traditional experimental and numerical studies only employed the orifice or porous media as the power take-off substitutes, causing an inaccurate evaluation of the OWC performance. In this study, an impulse turbine model was used directly in a generic OWC model for the secondary-stage energy conversion. The wave-flume tests were conducted under irregular wave conditions and the rotation speeds of the turbine were fixed. Typical time histories of some performance parameters were presented. The primary-stage and secondary-stage efficiencies were analyzed under various test conditions. It was found that the distribution characteristics of the overall efficiency of the chamber-turbine system is significantly determined by the secondary-stage efficiency. The different distribution characteristics of the efficiencies for the regular and irregular wave conditions indicated that the irregular wave tests are preferred for the shape parameter and operating mode design of the OWC plant. [ABSTRACT FROM AUTHOR]