Search

Ocean energy systems (OESs) convert the kinetic, potential, and thermal energy from oceans and seas to electricity. These systems are broadly classified into tidal, wave, thermal, and current marine systems. If fully utilized, the OESs can supply the planet with the required electricity demand as they are capable of generating approximately 2 TW of energy. The wave energy converter (WEC) systems capture the kinetic and potential energy in the waves using suitable mechanical energy capturers such as turbines and paddles. The energy density in the ocean waves is in the range of tens of kilowatts per square meter, which makes them a very attractive energy source due to the high predictability and low variability when compared with other renewable sources. Because the final objective of any renewable energy source (RES), including the WECs, is to produce electricity, the energy capturer of the WEC systems is coupled with an electrical generator, which is controlled then by power electronic converters to generate the electrical power and inject the output current into the utility AC grid. The power electronic converters used in other RESs such as photovoltaics and wind systems have been progressing significantly in the last decade, which improved the energy harvesting process, which can benefit the WECs. In this context, this paper reviews the main power converter architectures used in the present WEC systems to aid in the development of these systems and provide a useful background for researchers in this area. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The combined offshore wind and wave energy on an integrated platform is an economical solution for the offshore energy industry as they share the infrastructure and ocean space. The study presents the dynamic analysis of the Submerged Tension-Leg Platform (STLP) combined with a heaving-type point absorber wave energy converter (WEC). The feasibility study of the hybrid concept is performed using the aero-servo-hydro-elastic simulation tool FAST. The study analyzes the responses of the combined system to understand the influence of the WECs on the STLP platform for various operating conditions of the wind turbine under regular and irregular waves. Positive synergy is observed between the platform and the WECs, and the study also focuses on the forces and moments developed at the interface of the tower and platform to understand the effect of wind energy on the turbine tower and the importance of motion amplitudes on the performance of the combined platform system. The mean and standard deviation for the translation and rotational motions of combined wind and wave energy converters are determined for different sea states under both regular and irregular waves to analyze the change in responses of the structure. The study observed a reduction in motion amplitudes of the hybrid floating system with the addition of the wave energy converters around the STLP floater to improve the energy efficiency of the hybrid system. The study helps in understanding the best possible arrangement of point absorber-type wave energy converters at the conceptual stage of the design process. [ABSTRACT FROM AUTHOR]

The Oscillating Water Column (OWC) is leading the Wave Energy Converters in terms of efforts and research devoted. However, the efficiency achieved by these devices is still low enough to become a real renewable energy source. The main objective of this paper is to study the influence of the turbine characteristics in the thermodynamic process inside the OWC chamber, which is defined by the polytropic exponent associated with the polytropic process. Wind tunnel tests have been carried out to obtain experimental data under stationary flow, applying a real gas model where moisture plays a fundamental role. The results show the importance of the turbine characteristics and its effects on the thermodynamic compression process, affecting the power output of the OWC device. In conclusion, this study provides a better understanding of the OWC performance to improve its efficiency. [ABSTRACT FROM AUTHOR]

• Wave device/farm-location most relevant, complementary, and representative pairing. • Wave-resource potential mapping is necessary but not sufficient to select locations. • The improved generator limitation optimisation is found necessary for this pairing. • Farm selection potential surpasses the standard selection based on individual devices. • Alternating wave converters allows most Canary archipelago's energy independence. Wave energy is a promising actor for the renewable energy-mix that needs to be increasingly prominent worldwide. However, wave energy often remains too neglected in energy-mix studies. To avoid pre-selecting locations with potential underestimation of Wave Energy Converter performances, indexes and methods have been developed to pair wave energy converter(s) and locations, but (a) resource potential maps have not been compared to Wave Energy Converter performance maps; (b) often farm potentials are obtained based on individual Wave Energy Converter selection; and (c) there is a lack of assessment in the complementarity and relevancy of these indexes. This work aims to address these gaps by (i) investigating wave and converter pairing indexes to select the most relevant, complementary, and representative ones; (ii) merging and improving main pieces of these methods into a novel framework based on the selected indexes and related parameters; and (iii) mapping the most appropriate wave farm for each location according to this framework and compare with resource potential maps. This study is the first to integrate electricity demand and production from wave energy combined with other renewables for an energy-independent archipelago. Results lead to conclude that the annual energy production, energy demand-response index, and multi-criteria approach are the most compliant indexes for the framework developed in this research. Additionally, the wave resource was proven necessary but insufficient to select potential locations because the selected indexes provided a more restrictive converter-location pair potential map following this wave farm framework. The individual-converter-selection using these selected indexes showed farm energy production about twice below the one of selecting the farms directly, which reaches up to 136 GWh/year. The improved framework developed in this research also demonstrated the efficiency of a revised generator-limitation optimisation that increases the capacity factor by up to an improved average of 50% with limited reductions in annual energy production. The high technology readiness level converters highlighted the possibility for the Canary archipelago to become electrically independent (some islands could lose their thermal power stations). This sustainable solution would require an alternation of one and two Wave Dragon farms, and a mix of Wave Dragon, Undigen, Weptos, and Wavepiston farms alongshore. [ABSTRACT FROM AUTHOR]

Latching control is a phase optimization control mechanism for improving the energy conversion efficiency of a wave energy converter (WEC). This paper, for the first time, extends the latching mechanism to vibration control to improve energy dissipation efficiency. An innovative semi-active latched mass damper (LMD) is proposed in this paper by integrating conventional tuned mass dampers (TMD) and the latching mechanism. First, the theoretical models of WEC and TMD are compared. Second, the mathematical model of an LMD installed on a single-degree-of-freedom structure is established, and three latching control strategies with different latching time instants, latching durations, unlatching time instants, and required information feedback are tested. Subsequently, the performance of the LMD is evaluated through the comparison with the detuned and optimally tuned TMDs. The characteristics of the latching control force are analyzed in terms of phase, component, energy dissipation, and damping enhancement effect. Lastly, the feasibility and effectiveness of the proposed LMD with three different latching strategies are proven. The latching control can reduce the stiffness component and increase the damping component of the control force, thereby optimizing the phase lag between the control force and the structural response. The proposed latching mechanism and strategies for vibration control can provide an innovative solution to improve the effectiveness of high-frequency detuned dampers, facilitate the design of long-period dampers, and develop adaptive semi-active dampers in the future. [ABSTRACT FROM AUTHOR]

A pile-restrained floating wave energy converter (WEC) array is proposed as an alternative to a single floater of the size of the array for use as a floating breakwater. The hydrodynamics of the WEC are modelled based on the Navier-Stokes equations and the model is verified by comparing its results with existing experimental data. The model then is used to characterize the array composed by a line of three WECs in terms of floater heaving, wave energy conversion, wave reflection, transmission and dissipation, for different layouts. In the examined array configuration, the aligned arrays exhibit superior performance compared to the staggered arrays, comprehensively considering both wave energy conversion and wave transmission. Specifically, when kh i > 1.73, the wave energy conversion efficiency of the aligned array with a spacing of 0.1 times the WEC width ranges from 0.141 to 0.330, while the wave transmission coefficient ranges from 0.187 to 0.472, indicating the effectiveness of the arrays in simultaneously reducing wave transmission and converting wave energy under shorter-wavelength conditions. Compared to a single WEC of the same dimensions, the array exhibits a remarkable increase in wave energy conversion efficiency and effectively reduce wave reflection. [ABSTRACT FROM AUTHOR]

This paper deals with the thermodynamic processes governing an Oscillating Water Column (OWC) device, focusing on the entropy variation and the energy and heat budgets over the expansion–compression cycles. The influence of the thermodynamic performance on the Levelized Cost of Energy (LCOE) and the carbon footprint is analysed. The research deals with an experimental research on a simple OWC off-shore model for the purpose, with the open gas system inside the chamber formulated by a real gas model, and conceptually represented by an equivalent closed one in order to apply the First Principle of Thermodynamics appropriately. Analysed results show that the compression process is an active process, while the expansion process is a passive one. In addition, the observed non-adiabatic performance of the complete cycle implies a efficiency reduction, with consequences on the LCOE. Furthermore, an approach to emergy (embodied energy) analysis is considered, providing with concluding remarks on OWC renewability and possible impacts on the biosphere and GHG emissions. An utter approach on OWC energy and emergy assessment will be develop on future researches. • The air expansion/compression process can be considered as cyclic since Δ U = Δ S = 0. • The cyclic process can be divided in an active compression and a passive expansion. • Enthalpy and exergy variation indicates the useful and wasted energy obtained. • The heat exchange reduces the efficiency and increases the LCOE. • Emergy analysis shows a similar renewability than other renewable sources. [ABSTRACT FROM AUTHOR]

Direct-drive wave energy converter (DD-WEC) simplifies the structure and improves the efficiency of wave energy conversion. Permanent magnet transverse flux linear generator (PMTFLG) combines the merits of transverse flux linear generator (TFLG) and permanent magnet linear generator (PMLG), and is suitable for low-speed high thrust density applications, like DD-WEC. However, it suffers from a significant demerit which is the high cost due to the rare earth permanent magnets (PMs). A stator-PM spiral translator PMTFLG (ST-PMTFLG) is proposed in this article. Armature winding and PMs are arranged in outer stator and inner stator, respectively while the translator is made of spiral steel blocks and nonmagnetic blocks. Similar to the magnetically geared machine, the proposed topology is suitable to be used in low-speed long-stroke application. Results show that ST-PMTFLG has advantages of low PM consumption and high thrust density. The thrust per PM volume of 12/11 ST-PMTFLG is 4.18 MN/m3. [ABSTRACT FROM AUTHOR]

The utilization of the existing breakwater constructions into wave energy conversion has been often adopted to rendering a revenue of the capital cost of investment. The paper has contributed to viable concept of a new integrated design through more effectively capturing wave-overtopping which finally converts into electrical energy. This design is hereafter called Overtopping Breakwater for Energy Conversion (OBREC). The development of an experimental test of the current OBREC has been conducted to obtain a proper ramp shape through evaluating the amounts of the wave-overtopping discharges into the reservoir incorporated with wave-reflection coefficients. To achieve the objective, ramps of several geometries such as linear, convex, concave and cubic shapes have been experimentally investigated at the National Research Institute Malaysia (NAHRIM) laboratory. The experimental study showed that the cubic-ramp shape has resulted in more significant amount of the wave-overtopping discharge into the reservoir associated with the low wave-reflection coefficient than the other ramp shapes. In general, it is merely concluded that this investigation provides very promising concept of the new proposed OBREC design to harness the larger wave energy. [ABSTRACT FROM AUTHOR]

Marine equipment generally lacks efficient power supply methods. The high energy density of wave energy can facilitate the converter miniaturization and make them easily integrated into marine equipment. This paper presents a novel point energy absorber-type wave energy converter, which is based on the counter-rotating self-adaptable movement mechanism. Comprehensive theoretical analysis, numerical optimization and verification experiments were conducted on its performance characteristics. It was found that the hydrodynamic performances of the WEC are greatly affected by the rotational speed and diameter, and the interaction between the double absorbers is exacerbated by the stabilizing fins. [ABSTRACT FROM AUTHOR]

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

Under unknown waves, the efficiency of the wave energy conversion systems can be improved by applying a novel control algorithm that is able to anticipate the behavior of the plant. In this paper, a generalized predictive control scheme for wave power generation systems that includes a Wells turbine and an induction generator is proposed. The main idea is to maintain the flow coefficient at its optimal point for generated power maximization and, at the same time, improve the efficiency of the Wells turbine and avoid the stalling phenomenon by means of regulating the turbine rotational speed. Moreover, by using the capabilities of the predictive controller, the limitations of the stator current and the effect of the mechanical dead time of the system are integrated into the regulator design process, as well as the flux weakening technique. Simulation and experimental results, through a test bench that has been designed and constructed, show the validity and capability of the proposed scheme in both the rated and flux weakening regimes. • Designing a generalized predictive control scheme for wave power generation systems that includes a Wells turbine and an induction generator. • Maximizing the generated power and avoiding the stalling phenomenon. • Applying the constraints on the stator windings in the controller conceptual design process. • Measuring and compensating the mechanical dead time of the generator. • Validating the proposal by means of the Matlab/Simulink and experimental platform. • Demonstrating the validity and capability of the proposed scheme in both the rated and flux weakening regimes. [ABSTRACT FROM AUTHOR]

In the present study, the hydrodynamic performance of circular and concentric arrangements of cone-cylinder-type heaving point absorbers around a Submerged Tension-Leg Platform (STLP) is analysed using the numerical model in the frequency domain based on the potential flow theory. The presence of the Wave Energy Converters (WECs) around the STLP floating wind turbine platform affects the hydrodynamic performance of the hybrid floating platform. So to illustrate the effects of WECs on the platform, the ratio of hydrodynamic coefficients for a single WEC system to that for a hybrid system is analysed. An array of heaving point absorbers is placed in circular and concentric patterns to understand the performance of heaving point absorbers in the absorption of wave energy. The cone-cylinder type heaving point absorber is selected for the present study as they yield more power as compared to other shaped point absorbers. The study compares the wave power absorption of each point absorber around the platform for irregular wave conditions of the North Sea. The effect of incoming waves is illustrated by analysing four different wave heading angles. To quantify the performance of the WECs in an array, the q-factor and coefficient of variation are studied for each array at different sea states. The study suggested the best possible arrangement pattern for wave power absorption and power uniformity among the floaters in the array. The study performed will be helpful in the design and analysis of the possible arrangement of point absorbers around the floating wind turbine platform for wave power absorption. • The cone-cylinder heaving point absorber wave energy converter (WEC) is analysed for circular and concentric array around the Submerged tension leg platform to observe the influence of WEC in the hybrid wind-wave system. • The influence of the STLP and other WECs on the performance of the WEC is examined by analyzing the hydrodynamic coefficients. • The total wave power absorbed and the impact of waves on the WEC is studied for different hubrid floater configurations. [ABSTRACT FROM AUTHOR]

Highlights • Detailed design of model scale WEC, with focus on actuator (power take-off; PTO), sensors, data acquisition, and real time control. • System identification (SID) of actuator system confirming desired performance. • Dynamics matching to perform dry bench test experiments on actuator system. • Wave tank testing and dry bench tests show proportional and proportional-integral controllers capable of greatly increasing WEC energy absorption. Abstract An increasing number of experiments are being conducted to study the design and performance of wave energy converters. Often in these tests, a real-time realization of prospective control algorithms is applied in order to assess and optimize energy absorption as well as other factors. This paper details the design and execution of an experiment for evaluating the capability of a model-scale WEC to execute basic control algorithms. Model-scale hardware, system, and experimental design are considered, with a focus on providing an experimental setup capable of meeting the dynamic requirements of a control system. To more efficiently execute such tests, a dry bench testing method is proposed and utilized to allow for controller tuning and to give an initial assessment of controller performance; this is followed by wave tank testing. The trends from the dry bench test and wave tank test results show good agreement with theory and confirm the ability of a relatively simple feedback controller to substantially improve energy absorption. Additionally, the dry bench testing approach is shown to be an effective and efficient means of designing and testing both controllers and actuator systems for wave energy converters. [ABSTRACT FROM AUTHOR]

Abstract In this work, we develop a frequency-domain model to study the hydrodynamic behaviour of a floater blanket (FB), i.e., an array of floater elements individually connected to power take-off (PTO) systems, which constitutes the core technology of the novel Ocean Grazer (OG) wave energy converter (WEC). The boundary element method open-source code NEMOH is used to solve the scattering and radiation problem. The aforementioned floater elements that comprise the FB are mechanically interconnected via (cylindrical, revolutional or spring) joints, which add extra constraint equations to the multibody problem. Various scenarios are investigated to understand the hydrodynamic response of the FB. The variation of the capture factor, PTO damping coefficients, q -factor and response amplitude operator (RAO) of each scenario are analysed, in order to quantify the device performance. A new concept based on a negative-stiffness spring joint is proposed to increase the energy output of the FB. Attention is also paid to the anti-resonance that is found in the numerical simulations. This study provides further insight into the hydrodynamic behaviour of dense or sparse interconnected arrays of WECs, which is fundamental for the design and optimisation of the OG-WEC. Highlights • Hydrodynamics of a floater blanket are investigated using a frequency-domain model. • A floater blanket linked with negative-stiffness springs can increase the energy output. • A floater blanket should be calibrated to avoid anti-resonant behaviour. [ABSTRACT FROM AUTHOR]

Highlights • A modular raft WEC-type attachment comprising multiple independent pontoons is proposed for extracting energy while reducing hydroelastic responses of VLFS. • The hybrid Finite Element-Boundary Element (FE-BE) is extended for hydroelastic analysis of VLFS with WEC-type attachment. • Effects of pontoon length, PTO damping coefficient, and incident wave angle are investigated. • The proposed modular attachment outperforms the rigid attachment consisting of a single wide pontoon in extracting energy and reducing deflections of VLFS in oblique waves. • It is possible to have a considerable gap between pontoons without significantly compromising the effectiveness of the modular attachment. Abstract This paper presents the use of a modular raft Wave Energy Converter (WEC)-type attachment at the fore edge of a rectangular Very Large Floating Structure (VLFS) for extracting wave energy while reducing hydroelastic responses of the VLFS under wave action. The proposed modular attachment comprises multiple independent auxiliary pontoons (i.e. modules) that are connected to the fore edge of the VLFS with hinges and linear Power Take-Off (PTO) systems. For the hydroelastic analysis, the auxiliary pontoons and the VLFS are modelled by using the Mindlin plate theory while the linear wave theory is used for modelling the fluid motion. The analysis is performed in the frequency domain using the hybrid Finite Element-Boundary Element (FE-BE) method. Parametric studies are carried out to investigate the effects of pontoon length, PTO damping coefficient, gap between auxiliary pontoons, and incident wave angle on the power capture factor as well as reductions in the hydroelastic responses of the VLFS with the modular attachment. It is found that in oblique waves, the modular attachment comprising multiple narrow pontoons outperforms the corresponding rigid attachment that consists of a single wide pontoon with respect to the power capture factor and the reduction in the deflection of the VLFS. In addition, it is possible to have a considerable gap between pontoons without significantly compromising the effectiveness of the modular attachment. [ABSTRACT FROM AUTHOR]

Abstract This paper presents optimization of modular raft WEC-type attachment to VLFS and connections between floating modules of the VLFS for regular and irregular waves. The modular raft WEC-type attachment consists of multiple independent auxiliary pontoons connected to the fore edge of the VLFS with hinges and linear Power Take-Off (PTO) systems. The two objectives of the optimization are to maximize the reduction in hydroelastic response of VLFS and the power capture factor. The optimization process is proposed by using differential evolution (DE) algorithm. A numerical analysis framework based on finite element-boundary element (FE-BE) method is presented for determining the power capture factor as well as the hydroelastic response reduction for regular and irregular waves. The proposed optimization process is used to investigate the effects of the design variables, including PTO damping coefficient, pontoon length, the number of connections and connection stiffness, on the trade-off curve (i.e. Pareto front). In addition, the maximum power capture factor and hydroelastic response reduction are examined for various regular and irregular waves. Highlights • Optimization of modular raft WEC-type attachment to VLFS and connections between floating modules of VLFS. • Bi-objective optimization process is proposed based on the differential evolution algorithm. • Effects of design variables on the Pareto front are investigated. • The maximum power capture factor and hydroelastic response reduction are examined for various regular and irregular waves. [ABSTRACT FROM AUTHOR]

Abstract Of all the wave energy converter (WEC) categories, the single-tether point absorber (PA) is one of the most widely used in the ocean renewable energy industry. In most published research, only the heave motion of the buoy is considered in the motion equation for the analysis. This is because the heave motion of the buoy strongly couples to the power take-off device (PTO), whereas the surge and pitch motions barely couple to the PTO. As a result, only the power arising from heave motion of the buoy can be efficiently absorbed when a single-tether PTO is used, leading to deficiency of the design in absorbing the power arising from its surge and pitch motion. In this paper, the deficiencies of single-tether PAs are addressed by simply shifting the center of gravity of the buoy away from its geometric centre. A spherical buoy with asymmetric mass is used in this paper for its simplicity. The asymmetric mass distribution of the buoy causes motion coupling across surge, heave and pitch motions, which enables strong coupling between the buoy's surge motion and the PTO movement. The operation principle and power generation of the spherical point absorber with asymmetric mass distribution (SPAMD) are investigated via a modal analysis conducted on a validated frequency-domain model. The results show that the SPAMD can be up to 3 times more efficient than the generic PAs when subjected to regular waves in the frequency range from 0.34 rad/sec to 1.4 rad/sec. Highlights • Significant efficiency improvement of a single-tethered point absorber by utilizing motion-coupling. • 3DOF frequency domain model with linearised viscous drag forces. • Using model analysis to understand the dynamic of system. • Identification of key factors for efficiency improvement. • Identification of operation regimes of system throughout wave frequencies. [ABSTRACT FROM AUTHOR]

The primary objectives of this study are to evaluate the levelized cost of energy (LCOE) for different wave energy conversion strategies and to examine cost reduction pathways such that wave energy conversion projects are competitive, relative to alternative energy industries. The energy production of six different WEC devices was estimated for four sites along the U.S. Pacific coast. The LCOE of pilot-scale wave energy conversion projects was estimated to range between $0.07/kWh and $0.92/kWh higher than the target LCOE of those for early-market offshore wind energy projects. Device capacity factors were generally below the commonly assumed value of 30%. Methods of cost reduction to the target LCOE of $0.30/kWh were explored, including decreasing capital and operational expenditures (CAPEX and OPEX) and increasing annual energy production (AEP) through improvements in the wave energy resource and WEC and WEC array performance, and advanced controls. Results indicate that CAPEX and OPEX should be reduced by at least 45% and AEP should be increased by 200%. A reduction of CAPEX and OPEX by 75%, combined with array evaluation and control strategies capable of increasing AEP by 12%–55% could also result in LCOE for wave energy conversion projects of less than $0.30/kWh. [ABSTRACT FROM AUTHOR]

Sigma Energy has performed its foremost prototype tests of scaled wave energy converter in a real sea environment. The prototype was a point absorber with a cylindrical buoy, a mechanical power take-off system with a counterweight, moored to the seabed as a tension leg platform (TLP) with three equal tendons. In these extensive experiments, numerous device characteristics were measured and analysed. The present paper focuses on the dynamic forces in the mooring lines, and some unexpected and rare data obtained. It is well known that TLP tendons are prone to a brief loss of tension (the slacking) and that, after such events, high snapping forces of short duration can arise. Partly by intention, and partly due to underestimation of the dynamical forces, several such slacking-snapping incidents were recorded during the experiments. In some severe storms, the snapping forces were up to six times higher than the tendon pretension. The paper presents several recordings of dynamic forces and platform motion during these critical events. It analysis them, and gives a typical scenario under which they occur. It gives also some theoretical explanations, and numerical predictions of dynamical tendon forces, with their comparison to the experimental results. • Sea trials of novel wave energy converter. • Dynamic forces in tension leg platform (TLP) mooring lines. • Slacking and snapping of TLP tendons. • Recordings of dynamical forces and platform motion in slacking-snapping incidents. • A typical scenario of critical slacking-snapping events. [ABSTRACT FROM AUTHOR]

To meet the growing global demand for carbon neutral electricity, wave energy is increasingly being identified for its global prevalence and the emerging clean technologies that can harness it. To provide the scale of electricity needed, Wave Energy Converters (WECs) will need to be deployed in large arrays. However, the effects of a WEC array's layout on the surrounding wave conditions in both the near and far fields is poorly understood. The published literature describes numerous methods to model interactions of incident waves and WEC arrays. Yet, none of the published methods provide a methodology which spectrally resolves the individual WEC's energy conversion characteristics, and has the flexibility to be applied to any emerging WEC design. This study develops and implements two novel candidate high-fidelity representations of a Backwards Bent Duct Buoy Oscillating Water Column (BBDB) WEC within a Simulating WAves Nearshore (SWAN) coastal model. Each candidate representation is developed by post-processing time series data generated from a time domain numerical simulation to form a meta-model, referred to as an obstacle case, consistent with the SWAN governing equations and discretization scheme. The study is executed within the framework of SNL-SWAN – a modified version of SWAN that includes a WEC obstacle case that is built on the established concept of using SWAN's transmission coefficient to emulate the wave-WEC interactions. The two new WEC obstacle cases build on SNL-SWAN by considering the WEC's intercepted power , the rate of energy extraction from the waves, instead of the captured power , the rate of energy conversion by the power-take-off. The two new candidates differ in their level of spectral resolution: one homogenizes the intercepted power across the frequency domain while the other attempts to spectrally resolve the energy transfer mechanisms. The two new candidate obstacle cases were compared by considering up to 5 BBDB devices subject to irregular wave conditions. For an individual device, a region of significantly perturbed wave heights, referred to as a wake, stretched 200 m in the lee of the WEC, with the maximum impact on wave heights seen 50 m behind the device. The wake behind five devices is largely dependent on the device spacing. The five unit WEC array only presented decreases of wave height between 0.2 and 0.7 m 200 m behind the last device in the array. A five unit WEC array model showed that estimates of individual WEC power performance was reduced by 2.3% and 6.0% when implementing the two new obstacle cases. [ABSTRACT FROM AUTHOR]

This study proposes a damping control strategy for high-speed operation of a wave energy converter (WEC). The control strategy of WEC is based on the heave-motion (up/down motion) mechanism of buoy. The applied WEC system extracts power through damping control of the permanent magnet linear generator, which is directly connected to the buoy. This study first analyzed the mechanical dynamics of a WEC system. Further, a damping control method that could improve the extracted power for a wide speed range is examined. The results showed that the operating region could be expanded and the power extraction could be maximized using only the proposed software implementation, without requiring additional hardware. The validity and benefits of the proposed control algorithm were experimentally verified using a down-scaled WEC model. [ABSTRACT FROM AUTHOR]

This paper presents the development of a Computational Fluid Dynamics (CFD) model for a free heaving Oscillating Water Column (OWC) spar buoy with non-linear Power Take Off (PTO). Firstly, a freely heaving barge was applied to a 2D Numerical Wave Tank (NWT), used to validate a 1 Degree Of Freedom (DOF) modelling methodology. Multiple sets of regular waves were used to assess the heave response compared to previous experimental and numerical studies. In parallel, the NWT was extended to 3D where analyses of incident waves have been conducted to ensure accurate waves are portrayed. A PTO boundary condition was created to replicate a non-linear impulse turbine, typically simulated by an orifice plate in scaled models. The PTO boundary was compared and validated using experimental data. Finally, a comprehensive system comprising of the 3D NWT, 1DOF set-up and non-linear PTO allowed the development of a heave-only OWC spar buoy model with a non-linear PTO. Experiments completed by UCC MaREI centre in LIR-NOTF ocean wave basin under FP7 MARINET project is detailed and used to validate the comprehensive model. A range of regular waves were applied and responses of heave and chamber pressures were compared to experimental data, which showed excellent correlation. [ABSTRACT FROM AUTHOR]

In this article, reinforcement learning is used to obtain optimal reactive control of a two-body point absorber. In particular, the Q-learning algorithm is adopted for the maximization of the energy extraction in each sea state. The controller damping and stiffness coefficients are varied in steps, observing the associated reward, which corresponds to an increase in the absorbed power, or penalty, owing to large displacements. The generated power is averaged over a time horizon spanning several wave cycles due to the periodicity of ocean waves, discarding the transient effects at the start of each new episode. The model of a two-body point absorber is developed in order to validate the control strategy in both regular and irregular waves. In all analysed sea states, the controller learns the optimal damping and stiffness coefficients. Furthermore, the scheme is independent of internal models of the device response, which means that it can adapt to variations in the unit dynamics with time and does not suffer from modelling errors. [ABSTRACT FROM AUTHOR]

A wave energy converter must be designed to survive and function efficiently, often in highly energetic ocean environments. This represents a challenging engineering problem, comprising systematic failure mode analysis, environmental characterization, modeling, experimental testing, fatigue and extreme response analysis. While, when compared with other ocean systems such as ships and offshore platforms, there is relatively little experience in wave energy converter design, a great deal of recent work has been done within these various areas. This paper summarizes the general stages and workflow for wave energy converter design, relying on supporting articles to provide insight. By surveying published work on wave energy converter survival and design response analyses, this paper seeks to provide the reader with an understanding of the different components of this process and the range of methodologies that can be brought to bear. In this way, the reader is provided with a large set of tools to perform design response analyses on wave energy converters. [ABSTRACT FROM AUTHOR]

The wave energy potential in the Mediterranean has been estimated using numerical models. However, the veracity of these estimates in the nearshore, where wave energy harnessing is likely to take place, remains largely unknown. This paper compares measured wave data (AWAC/ADCP) with estimates of the third-generation spectral WAve Model (WAM), in Varkiza, Athens, a typical south facing nearshore Greek locale. Overall, good agreement in the mean wave power ( P mean ) was found, but WAM overestimated the peak P max and had an overall error of ∼19° when estimating the mean wave direction (M dir ). The results are suggestive that WAM could act as a screening tool to identify nearshore wave energy hotspots and avoid areas with extreme sea states. Nonetheless, in-situ wave measurements are required to determine the wave energy power potential and the direction of wave propagation so as to select the appropriate conversion technology that better fits local conditions. Finally, it is suggested that existing wave energy technology should be, redesigned, downscaled and optimized to suit the milder Mediterranean wave regime, thus enacting wave energy development in Greece. [ABSTRACT FROM AUTHOR]

The wave energy, as a clean and non-pollution renewable energy sources, has become a hot research topic at home and abroad and is likely to become a new industry in the future. In this article, to effectively extract and maximize the energy from ocean waves, a vertical axisymmetric wave energy converter (WEC) was presented according to investigating of the advantages and disadvantages of the current WEC. The linear and quadratic equations in frequency-domain for the reactive controlled single-point converter property under regular waves condition are proposed for an efficient power take-off (PTO). A method of damping coefficients, theoretical added mass and exciting force are calculated with the analytical method which is in use of the series expansion of eigen functions. The loads of optimal reactive and resistive, the amplitudes of corresponding oscillation, and the width ratios of energy capture are determined approximately and discussed in numerical results. [ABSTRACT FROM AUTHOR]

Wave energy converters (WECs) have received much attention with the increasing environmental concerns. And the direct-drive WECs are more attractive due to the elimination of intermediate mechanical conversion parts. We have proposed a concept of a WEC device, in which a tubular linear generator (TLG) is directly driven by a heaving buoy. We design a high-temperature superconducting TLG (HTS-TLG), a permanent magnet (PM) TLG, and a copper TLG for the WEC. In this paper, we give a brief description on the conceptual structures and the electrical design method of these three types of TLGs, then show the way of determining main machine parameters considering the basic generator performance (weight, efficiency, and the amount of field conductors/PMs). Design results of these TLGs are finally presented with machine performances and costs compared. We found that the HTS-TLG is the most promising one compared to the other counterparts. [ABSTRACT FROM AUTHOR]

Numerical modelling has become commonplace in the development of offshore structures, however for dynamic systems such as Wave Energy Convertors (WECs) conventional numerical tools may not be able to capture the full range of behaviours required for engineering analysis. This is particularly problematic in extreme seas where the nonlinearities and coupled nature of these systems become important. In this work a fully nonlinear, coupled tool for simulation of WECs is produced and compare with physical measurements. Regular wave interactions with both a fixed and freely-pitching, 1:10 scale model of the Wavestar are reproduced numerically. The numerical model is shown to be capable of predicting the pressure on the float's surface and the fully coupled motion of the device. However, the results indicate that the higher-order free surface behaviour in the vicinity of the device is not being captured correctly. Finally, the numerical model is shown to cope with an extreme regular wave including large amplitude motion, full submersion and high levels of free surface distortion. The results presented suggest that the quality of the numerical reproduction does not decrease with wave steepness; however the execution time of simulations increases significantly with increased float oscillation. [ABSTRACT FROM AUTHOR]

Ocean waves are a more predictable resource with a higher energy density compared to solar and wind. In addition, and specifically for the United States, resource locations with high wave power are close to major load centers being located along coastlines. These features have sparked a surge of attention in the United States on trying to economically harness ocean wave power. The aim of this article is to provide a concise review of the current state of ocean wave energy conversion technologies and industry status in the United States including research and development as well as commercial activities and governmental support, concluding with a discussion of future industry perspectives. Existing facilities, softwares as well as laboratory and open-water test facilities and resources, active research groups and commercial activities have been identified. Over one third of commercially active wave energy developers worldwide are located within the United States, but only a few have reached a high Technology Readiness Level. These findings, together with a relevant practical resource located within the U.S. and the advantageous nature of the resource compared to other renewable resources, indicate that the United States is well positioned to advance the wave energy industry in the near future. [ABSTRACT FROM AUTHOR]

In guiding the progression, development, and operation of wave energy converters (WECs) in a more efficient way, mathematical analysis and understanding of the dynamic process is essential. Mathematical WEC models, obtained either by numerical analysis or physical modelling, form the basis of most (model-based) energy maximising control strategies available in the literature, where experimental design and system identification methodology directly impact the resulting model. This study, using an experimental-based WEC model (which can be used for linear control design), investigates the dynamic behaviour of a WEC by analysing the dominant poles of the system, generated using fully nonlinear computational fluid dynamics (CFD)-based numerical wave tank (NWT) experiments. The aim is to effectively track the dominant dynamics of the WEC, using different force-input amplitude levels in the NWT setup, and perform a comparison with the classical linear boundary-element-methods (BEM) equivalent methodology. Thus, the presented case studies are shown to agree with previously proposed model assessment of linear WEC models, based on a free-decay NWT setup. In addition, the representative WEC models determined as part of this study can be used for WEC controller design, either singly, or using a form of model/controller gain scheduling. • Representative linear wave energy converter models for model-based control design. • Assessment of wave energy device dominant dynamics. • Dominant pole trends for wave energy converters using force-input experiments. • Model identification of wave energy converters from numerical wave tank data. [ABSTRACT FROM AUTHOR]

As a result of international agreements on the reduction of CO2 emissions, new technologies using hydrogen are being developed. Hydrogen, despite being the most abundant element in Nature, cannot be found in its pure state. Water is one of the most abundant sources of hydrogen on the planet. The proposal here is to use energy from the sea in order to obtain hydrogen from water. If plants to obtain hydrogen were to be placed in the ocean, the impact of long submarines piping to the coast will be reduced. Further, this will open the way for the development of ships propelled by hydrogen. This paper discusses the feasibility of an offshore installation to obtain hydrogen from the sea, using ocean wave energy. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to analyze the energy production in the Maltese islands, focusing on the employment of renewable energies in order to increase their energy independence. The main renewable source here proposed is wave energy: thanks to a strategic position, Malta will be able to produce electrical energy using an innovative type of Wave Energy Converter (WEC) based on the prototype of a linear generator realized by University of Palermo. The use of this new technology will be able to cut down the electrical energy production from traditional power plants and, consequently, the greenhouse gas emissions (GHG).Wave energy source and off-shore photovoltaic (PV) technology are here proposed. Particularly, the installation of 12 wave farms, for a total installed capacity of 86 MW, will generate about 9.5% of Malta's energy requirement in 2025, while the installation of 9.6 MW of off-shore PV will generate about 0.73%. [ABSTRACT FROM AUTHOR]

Marine energy sources represent an attractive and inexhaustible reservoir able to contribute to the fulfillment of the world energy demand in accordance with climate/energy regulatory frameworks. Wave energy converter (WEC) integration into the main grid requires both the maximization of the harvested energy and the proper management of the generation variability. The present paper focuses on both these mentioned issues. More specifically, it presents an embedded point pivoted absorber (PPA) and its related control strategy aimed at maximizing the harvested energy. Experimental and numerical investigations have been carried out in a wave/towing tank facility in order to derive the design characteristics of the full-scale model and demonstrate the validity and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

The energy absorption of the wave energy converters (WEC) characterized by a limited stroke length --like the point absorbers developed at Uppsala University--depends on the sea level variation at the deployment site. In coastal areas characterized by high tidal ranges, the daily energy production of the generators is not optimal. The study presented in this paper quantifies the effects of the changing sea level at theWave Hub test site, located at the south-west coast of England. This area is strongly affected by tides: the tidal height calculated as the difference between the Mean High Water Spring and the Mean LowWater Spring in 2014 was about 6.6 m. The results are obtained from a hydro-mechanic model that analyzes the behaviour of the point absorber at the Wave Hub, taking into account the sea state occurrence scatter diagram and the tidal time series at the site. It turns out that the impact of the tide decreases the energy absorption by 53%. For this reason, the need for a tidal compensation system to be included in the design of the WEC becomes compelling. The economic advantages are evaluated for different scenarios: the economic analysis proposed within the paper allows an educated guess to be made on the profits. The alternative of extending the stroke length of the WEC is investigated, and the gain in energy absorption is estimated. [ABSTRACT FROM AUTHOR]

The Lysekil wave energy converter (WEC), developed by the wave energy research group of Uppsala University, has evolved through a variety of mechanical designs since the first prototype was installed in 2006. The hundreds of engineering decisions made throughout the design processes have been based on a combination of theory, know-how from previous experiments, and educated guesses. One key parameter in the design of the WECs linear generator is the stroke length. A long stroke requires a taller WEC with associated economical and mechanical challenges, but a short stroke limits the power production. The 2-m stroke of the current WECs has been an educated guess for the Swedish wave climate, though the consequences of this choice on energy absorption have not been studied. When the WEC technology is considered for international waters, with larger waves and challenges of energy absorption and survivability, the subject of stroke length becomes even more relevant. This paper studies the impact of generator stroke length on energy absorption for three sites off the coasts of Sweden, Chile and Scotland. 2-m, 4-m, and unlimited stroke are considered. Power matrices for the studied WEC prototype are presented for each of the studied stroke lengths. Presented results quantify the losses incurred by a limited stroke. The results indicate that a 2-m stroke length is likely to be a good choice for Sweden, but 4-m is likely to be necessary in more energetic international waters. [ABSTRACT FROM AUTHOR]

Despite the huge potential of ocean waves as an alternative renewable energy resource, very low wave to wire efficiency of wave energy converters (WECs) hampers its commercialization. In this study, a novel streamlined chamber design has been proposed for performance improvement of an oscillating water column (OWC) with a sloping wall. Physical experiments are carried out for the calculation of capture width ratio (CWR) as a measure of hydrodynamic performance for various levels of PTO damping and sea conditions. The results are compared with those of a classical OWC chamber design. It is found that streamlined chamber design has an important bearing on OWC efficiency. As opposed to classical chamber shape, proposed streamlined chamber geometry increases the OWC performance as much as 31% for the incident waves under which the OWC operates efficiently. Further, results show that optimal level of PTO damping that for utmost energy extraction depends on the chamber design. Streamlined chamber geometry even further improves the OWC efficiency even when dominant sloshing water column motion is present in the chamber with a maximum and average value of 54% and 44%, respectively, however, chamber design has no influence on the mechanism that generates sloshing phenomenon in the chamber. • A new novel streamlined OWC chamber geometry is proposed for improved efficiency. • Efficiency of streamlined and a classical OWC experimentally obtained and compared. • A maximum of 31% efficiency improvement is found due to streamlined chamber design. • Optimum level of PTO damping that has to be applied is influenced by chamber design. • Design of chamber geometry has no bearing on the sloshing generating dynamics. [ABSTRACT FROM AUTHOR]

With a world needing to progress to a more renewable-based energy mix, wave renewable is indispensable to achieve this goal. Studies showed that it is necessary to cartography wave energy converter and location pairs as varying wave climates require different devices. However, there is not one but many designs of the converters. Hence, pre-matching converter classes with wave climates would enable such computational-demanding global cartography. Then, devices would be matched to these classes to find where they suit most. Power production potentials display features to be naturally pre-classified: (1) presence of a plateau of same maximal value, and (2) associated capture width and possible linear reduction to a 1-dimensional curve; and different power-representations are considered. Then, the principal component analyses are employed to determine the new power production potential classification. This method enables to cluster into a simple 3-dimensional space distributions of data characterised by many dimensions. After calculating the Euclidian distance between the projected data to determine the clusters, different technics are assessed to obtain the representative classes. Results highlighted the non-correlation between previous classifications and the power production potential. The 46 considered power production potentials formed 16 classes. Finally, guidelines are provided to use this new classification. • New wave energy converters' classifications arise from their power potential. • The uncorrelation between previous classifications and the power is investigated. • The power linearity is assessed to obtain possible values limited by a generator. • This new classification is pre-classified between linearity and power limitations. • Methods are provided to develop such a classification to other fields, and its use. [ABSTRACT FROM AUTHOR]

The development of new wave energy devices (WECs) has continued unabated over the past decades. For large-scale applications, integrating individual WEC into an array system (WEC net) requires considerable expertise and research due to its highly complex and interrelated nature. Often for a WEC net, it contains main structures and multiple sub-structures. The WEC net response is defined as the total responses from all sub-structures, which is highly complex and closely interconnected with each other. This paper aims to develop a fully-coupled numerical modeling tool that can cope with the wave-structure interaction as well as the mechanical interaction among each sub-structure in a WEC net. The fluid field is solved by a Computational Fluid Dynamic (CFD) solver coupled with a Multi-body Dynamic structural solver. The hydrodynamic and power take-off performance of Albatern 12S Squid WEC net is studied and the results are validated against available laboratory testing data, and commercial mooring and hydrodynamics analysis software. It is found that the motion response of the CFD and experimental approach is in close agreement with each other. The interaction force among sub-structures can be well captured, and the results indicate that the mode response of individual float is strongly affected by the mechanical linking-arms as well as the incident wave conditions, which is hard to achieve without such integrated CFD tool. The power take-off (PTO) is modeled using a damping system. The predicted peak output power is found to increase with the decreasing of wave period and an optimal device's damping to reach a maximum power capture exists, which is dependent on the incoming wave period and height. • A numerical modeling tool coupling Computational Fluid Dynamic (CFD) and Multi-body Dynamic (MBD) is developed that can cope with the wave-structure interaction as well as the mechanical interaction among each sub-structure in a WEC net. • The interaction force among sub-structures can be well captured, the dynamic mode response of individual float is studied, which is strongly affected by the mechanical linking-arms as well as the incident wave conditions. • This work serves as a solid foundation for the optimizing of WECs in a design stage. It can also predict the weakest mechanical component, where the peak mechanical force may occur and thus become the most vulnerable part in the system. [ABSTRACT FROM AUTHOR]

Modeled nearshore wave propagation was investigated downstream of simulated wave energy converters (WECs) to evaluate overall near- and far-field effects of WEC arrays. Model sensitivity to WEC characteristics and WEC array deployment scenarios was evaluated using a modified version of an industry standard wave model, Simulating WAves Nearshore (SWAN), which allows the incorporation of device-specific WEC characteristics to specify obstacle transmission. The sensitivity study illustrated that WEC device type and subsequently its size directly resulted in wave height variations in the lee of the WEC array. Wave heights decreased up to 30% between modeled scenarios with and without WECs for large arrays (100 devices) of relatively sizable devices (26 m in diameter) with peak power generation near to the modeled incident wave height. Other WEC types resulted in less than 15% differences in modeled wave height with and without WECs, with lesser influence for WECs less than 10 m in diameter. Wave directions and periods were largely insensitive to changes in parameters. However, additional model parameterization and analysis are required to fully explore the model sensitivity of peak wave period and mean wave direction to the varying of the parameters. [ABSTRACT FROM AUTHOR]

This paper presents a new concept of multi-axis wave energy converter (WEC), which can capture wave energy from multiple directions of motion. It describes the modelling of the multi-axis WEC and presents the results of laboratory experiments on the power capture of the multi-axis WEC in a wave tank subjected to synthesized scaled sea waves. The experiments investigate the influence of parameters such as buoy structure, damping and inertia modification (ballasting) on the hydrodynamic performance. The experimental results indicate that the multi-axis WEC yields very good performance under a variety of wave conditions. It is also demonstrated that, by utilizing more than one direction of motion, the multi-axis WEC proves capable of supplying more power than a single-axis WEC. This work constitutes a preliminary step towards the development of an advanced concept of WEC. [ABSTRACT FROM AUTHOR]

Renewable energies are mostly compared using the Levelised Cost of Energy (LCoE). Two major components of LCoE are of interest in this study, the others being still experimental: the Annual Energy Production (AEP) and the Capital Expenditure (CapEx). Since many Wave Energy Converters (WECs), including Wavepiston, are wave direction-dependent, this research investigates the wave direction impact on the relationship between the site-configurations (site characteristics and wave climates), WEC-configurations and power, alongside AEP and CapEx. Wave climate and WEC power are expressed in 3-dimension by adding the wave direction to the common 2-dimensional space of wave height and period. Wavepiston's WEF costs are calculated using the CapEx method. Assessing the relationship between the diverse sources of AEP/CapEx variations, enabled the development of the Cost-AEP Threshold Criterion (CAEPTC) to determine the threshold where an increase in AEP becomes negligible compared to CapEx increase. CAEPTC is perhaps a better alternative to LCoE for enhanced WEC/site-configuration pairing. Based on this Criterion and the limited dataset (a challenge to most wave companies), it was apparent that the best WEC/site-configuration pairs came from configurations with the highest yearly energy production, which is not necessarily the most expensive, and for sites with lower wave energy. • Wave direction-dependent active-surge Wave Energy Converter (WEC) power analysis. • Wavepiston WEC farm Capital Expenditure (CapEx) and Annual Energy Production (AEP). • WEC-configuration, site characteristic, wave climate, CapEx/AEP interrelationship. • Development of the Cost-AEP Threshold Criterion for WEC/site-configuration pairing. • Unidirectional or high-energy wave climates appeal less WEC/site-configuration pair. [ABSTRACT FROM AUTHOR]

• A new and simple method is provided to verify the applicability of Froude's law. • Wave energy converter optimisations over the generator increase the capacity factor. • Analysis of the impact of scatter diagram discretisation size on energy estimation. • Wave spectra effect on energy estimations is assessed for multi-collector devices. • Energy capture evaluation using wave directions, scatter diagrams, and time series. In the struggle against increasing global warming, wave renewable energy could help progress to the increasingly necessary renewable-based energy-mix. Wave energy has many applications, one of which involves its capture and conversion into usable energy. To date, there has been no agreement in the scientific community with respect to a single wave energy converter design, and given the numerous concepts involved, many approaches have been developed to investigate the performance of these different devices. This work summarises these approaches in addition to completing and extending them regarding wave spectrum, wave direction, generator capacity limitations, and use of the Froude similarity law for wave energy converter scaling. Ultimately, this study aims to improve the selection process of the most appropriate converter configuration for a given resource (i.e. wave distribution climate). The main results showed that neglecting the wave direction can lead to a 10 % energy production overestimation when the method is applied to the Wavepiston device. Furthermore, for Weptos and Wavepiston wave energy converters, different standard wave spectra were found to estimate the energy production with less than 8 % difference, below the 16.5 % estimated in previous studies. Additionally, the Reverse Froude Law method is introduced to help assess the applicability of the Froude similarity law for converter configuration scaling. Finally, a threshold in the decrease of generator capacity is determined such that, despite limiting the device power production potential, annual energy production remained almost as high as without generator limitations for an approximately 40 % capacity factor increase. [ABSTRACT FROM AUTHOR]

This study is on the vertical wave-induced velocity through an opening in a fixed, submerged and impervious horizontal disk, that is positioned near the free surface. Piercing this body is a tubular section, having an opening flush with the top surface and extending completely through the body. Experiments are conducted in a wave flume, where monochromatic waves propagate over the submerged surface of a free-standing disk model. Wave elevation is recorded using a capacitive-type wave gauge. The oscillating wave-induced flow through the submerged surface is measured by a particle image velocimetry (PIV) laser. Numerically, the flow within the opening of the submerged surface is modeled by use of the Green function method within the confines of linear potential theory. Both the experimental and numerical results of this phenomenon are compared. The outcome is pertinent to the development of a new wave energy converter. [ABSTRACT FROM AUTHOR]

In order to reduce the cost of electricity produced by wave energy converters (WECs), the benefit of selling electricity as well as the investment costs of the structure has to be considered. This paper presents a methodology for assessing the control strategy for a WEC with respect to both energy output and structural fatigue loads. Different active and passive control strategies are implemented (proportional (P) controller, proportional-integral (PI) controller, proportional-integral-derivative with memory compensation (PID) controller, model predictive control (MPC) and maximum energy controller (MEC)), and load time-series resulting from numerical simulations are used to design structural parts based on fatigue analysis using rain-flow counting, Stress-Number (SN) curves and Miner's rule. The objective of the methodology is to obtain a cost-effective WEC with a more comprehensive analysis of a WEC based on a combination of well known control strategies and standardised fatigue methods. The presented method is then applied to a particular case study, the Wavestar WEC, for a specific location in the North Sea. Results, which are based on numerical simulations, show the importance of balancing the gained power against structural fatigue. Based on a simple cost model, the PI controller is shown as a viable solution. [ABSTRACT FROM AUTHOR]

Abstract: The application of renewable energies has increased rapidly in the previous decade to solve some problems such as growing energy demand and environmental issues. Wave power as a high potential renewable energy, is more predictable compared to other renewable sources. Although there are many research works about wave energy, however a few of them considered a suitable wave energy converter (WEC) as a power system for remote islands. Wave energy potential for remote islands is discussed in this review by regarding environmental impacts, various types of wave energy converters and applied wave power projects for various islands. The results show that wave energy plays a key role for sustainable development of offshore islands by considering the traditional looks and environmental protection. [Copyright &y& Elsevier]

Abstract: Offshore wind is an important source of renewable energy and is steadier and stronger than onshore wind. Offshore areas not only have strong winds but also contain other potential renewable energy sources, such as ocean waves and tidal currents. Therefore, it is interesting to investigate the possibility to utilise these energy potentials simultaneously, particularly the combination of wind and ocean wave energy due to their natural correlation. For this reason, previous researchers have examined the use of a floating wind turbine (FWT) and a wave energy converter (WEC) on a single platform (Aubault et al., 2011; Peiffer et al., 2011; Soulard and Babarit, 2012). In this paper, a combined concept involving a spar-type FWT and an axi-symmetric two-body WEC is considered and denoted as STC. With respect to operational conditions, a previous study (Muliawan et al., 2013) indicates that the STC not only reduces the total capital cost but also increases the total power production compared to the use of segregated FWT and WEC concepts. As with other floating systems, the STC must be designed to ensure serviceability and survivability during its entire service life. One of the design criteria is the ultimate limit state (ULS), which ensures that the entire STC system will have adequate strength to withstand the load effects imposed by extreme environmental actions. Therefore, in the present study, coupled (wave- and wind-induced response mooring) analysis is performed using SIMO/TDHMILL in the time domain to investigate such responses of the STC system as mooring tension, spar-tower interface bending moment, end stop force, and contact force at the Spar-Torus interface under extreme conditions. Environmental conditions that pertain to the northern North Sea metocean data are selected and include operational, survival and 50-year conditions. Finally, the ULS level responses that are capital cost indicators for both FWT alone and for the STC system are estimated and compared. [Copyright &y& Elsevier]

This paper presents a novel permanent-magnet tubular linear generator (PMTLG) buoy system designed to convert the linear motion of ocean waves into electrical energy. The design incorporates no working seals and a saltwater air-gap bearing surface integration between the PMTLG buoy components. The internal generator design will be discussed, in addition to the system integration with the buoy structure. The simulation and hardware results of the generator are presented. [ABSTRACT FROM AUTHOR]