Flap-type Wave Energy Converter Research Articles

Wave power extraction of flap-type wave energy converter array mounted at the stepped bottom topography

Wave power extraction of flap-type wave energy converter array mounted at the stepped bottom topography

Design considerations for a hybrid wind-wave platform under energy-maximising control

The environmental impact of emissions of fossil fuels and their rising prices, together with countries' commitment to mitigate the effect of the alarming and rapid climate changes, have been a crucial thrust to investigate new solutions to have an energy supply depending on renewable energies. In this scenario, offshore wind-wave hybrid platforms have been recently promoted: sharing facilities, infrastructure, and grid connections, give these systems the potential to increase energy production at a lower cost. However, an efficient realisation of these two combined technologies requires two potentially conflicting control objectives: On the one hand, for the wind turbine, a reduced movement of the platform is required, which essentially translates to enhanced stability of the structure, so that its behaviour resembles standard onshore wind technologies. On the other hand, to maximise the energy produced, wave energy converters (WECs) require optimal control technology, which often leads to large amplitude motion, potentially conflicting with the stability requirement for the wind turbine.
 The aim of this study is to investigate the effects of design changes on the dynamics of the hybrid wind-wave platform under energy-maximising control, which can be analysed in terms of the principle of impedance-matching. A semi-submersible platform with an incorporated flap-type WEC will be analysed both from a closed-loop and open-loop perspective, and the control system will be designed to maximise the energy produced by the WEC. Design changes on the wind-wave conversion platform will be in terms of flap dimensions, starting from a nominal geometry based on the so-called Oyster system. Analyses will be conducted by both increasing and decreasing the flap depth from the nominal case, to investigate the effect of the different geometries on the interactions between the WEC and the platform. A frequency-domain analysis of the overall input/output (velocity) system will be presented, highlighting the situations that can enhance the potential of both devices and exploit their synergies.

Power Performance and Response Analysis of a Semi-Submersible Wind Turbine Combined With Flap-Type and Torus Wave Energy Converters

Abstract An integrated offshore wind and wave energy system is an attractive concept in areas with abundant wind and wave energy resources. The sharing of supporting platform and facilities, e.g., mooring systems, offers significant cost savings. This will effectively lower the levelized cost of energy (LCOE). In the present study, a conceptual design consisting of a braceless semi-submersible floating horizontal axis wind turbine (FHAWT), three flap-type wave energy converters (WECs), as well as a torus (donut-shaped) point absorber-type WEC is proposed. The flap-type WECs harvest wave energy through the flap motion caused by oscillating wave surge, while the torus WEC absorbs wave energy generated from its heaving motion. The absorbed mechanical power of the power take-off (PTO) systems is calculated based on linear damping forces and the motions of the WECs relative to the supporting platform. Hydrodynamic interaction between the WECs and the supporting platform is considered by including the coupling terms in the added mass and potential damping coefficient matrices. A fully coupled aero-servo-hydro-elastic numerical model of the concept is constructed. The feasibility study of the concept is carried out using time-domain simulations. Only operational environmental conditions are simulated based on simultaneous wind and wave hindcast data of a selected offshore site. The effects of the WECs on the wind turbine, platform motions, and WEC power take-off are examined. Based on the power performance of WECs, recommendations are also provided for optimum power absorption.

Experimental study of bed level changes in the vicinity of flap-type wave energy converters

ABSTRACT Details are given herein of the bed level changes in vicinity of the oscillating flap by means of physical modelling. Experiments were performed in a wave flume with a movable bed. Based on the results, changing the flap rotation angle (α) from a constant to an oscillating state caused less bed level changes (reduced range and length). Increasing the (h=water depth, L=wave length) decreases the range of bed level change profile and was effective on bed form. The (H=wave height) only affects the range of bed level changes profile in such a way that increasing this parameter first raise and then decreases the bed level changes. Intraction of above three parameters with the velocity components show that, by increasing the distance from the oscillating flap, more regular trend of velocity changes was observed. While complex behaviour was recognized in close distances to the structure.

Flap-type wave energy converter arrays: Nonlinear dynamic analysis

The wave-body interaction of resonant oscillating wave energy converters (WECs) is complex and nonlinear. Rapid development of WECs needs a reliable simulation tool for analysis and design procedure with the ability to capture the instantaneous nonlinear behavior. This paper presents a methodology to study the behavior of a flap-type WEC utilizing hybrid physical and numerical simulation. Dynamic characteristics of the device as isolated and in an array are estimated and the time and oscillation amplitude-dependency of these parameters are explained. This methodology is developed for a device composed of a series of flaps placed on a vertical breakwater. However, the developed algorithm can be updated for any other WEC concept with special consideration on the consequent changes in the load transfer, surrounding hydrodynamic, and wave-structure interaction. This paper increases our understanding and knowledge of all involved phenomena and their direct effects on the natural period and the response and can be used for tunning dynamic characteristics of WEC device to incoming waves. Finally, the developed numerical model is used for the simulation of the WEC to different wave loads and the results are validated with the experimental tests.

Multipurpose breakwater: Hydrodynamic analysis of flap-type wave energy converter array integrated to a breakwater

Wave Energy Converter (WEC) development needs a thorough dynamic characterization of the device and tuning the design properties to harness the maximum power. This paper addresses this need by using experimentally validated numerical simulation for an array of flap-type WEC mounted on a surface of a breakwater as a coherent approach in sustainable coastal protection. The developed numerical model is combined with a parametric iteration procedure to find the optimized values of power take-off (PTO) coefficients, and the flap's distance from the breakwater. It is shown that tuning the design properties of flap-type WEC integrated into a coastal structure leads to high energy capture around 85 percent of the available power. It was found that the amplitude of oscillation is significantly affected by the presence of different frequencies resultant from standing waves, and the confined water between the flap and the breakwater. It turns out that by tuning the distance between the flap and the breakwater, the standing waves can be used for increasing the amplitude of oscillation and consequently power enhancement.

An Experimental Study of a Bottom-Hinged Wave Energy Converter with a Reflection Wall in Regular Waves—Focusing on Behavioral Characteristics

The hybrid system of wave energy converters (WECs) using coastal structures is an attractive issue in terms of a decrease in construction costs and an improvement of the ability to capture wave energy. Most studies on the utilization of reflected waves from structures, which is one of the hybrid systems, are limited to mathematical analysis based on linear theories. Therefore, this paper presents fundamental experimental results in the presence of a reflection wall simplified as a coastal structure behind a bottom-hinged flap-type WEC under unidirectional regular waves. The behavioral characteristics and the power generation efficiency ke of the flap were investigated, focusing on wave steepness, initial water depth, and distance from the reflection wall. The results show that the condition of the initial water depth being smaller than the flap height is more effective in terms of avoiding unstable rotating of the flap. The maximum ke appeared slightly far from the node position of the standing waves because the flap shape and the power take-off (PTO) damping induce the phase difference between the reciprocating behavior of the flap and the period of the standing wave. The results imply that the optimum position of a WEC is dependent on WEC shape, PTO damping, and installation water depth.

A study on the design and performance of ModuleRaft wave energy converter

This paper presents a novel wave energy converter (WEC), referred to as the ModuleRaft WEC. The WEC consists of a floating modular flap and four rafts hinged at the main floating structure. ModuleRaft WEC is unique due to its ability to convert both wave potential energy and wave kinetic energy by utilizing the pitch motion of rafts and floating modular flap. The motion characteristic, performance and optimization of the ModuleRaft wave energy converter are investigated under regular wave conditions using ANSYS-AQWA. The effect of wave frequency, power take-off (PTO) damping coefficient and floating/fixed main structure on capture factor is analyzed. Results indicate that using a single point mooring (SPM) system, the ModuleRaft WEC can operate optimally by utilizing all directions wave energy. In addition, comparing WEC with and without rafts, it was found that the capture factor of the modular flap with rafts is much better than the conventional floating modular flap-type WEC. Moreover, the three most influential parameters; wave frequency, PTO damping coefficient and fixed main structure, have a significant effect on the capture factor of the ModuleRaft WEC.

A second-order theory for an array of curved wave energy converters in open sea

We present a second-order nonlinear theory for an array of curved flap-type wave energy converters (WECs) in open sea, excited by oblique incident waves. The flap model, when at rest, has a weak displacement of its wetted surface about the vertical plane. Using a perturbation-harmonic expansion technique, we decompose the set of nonlinear governing equations in a sequence of linearized boundary-value problems that can be investigated through analytical methods. The velocity potential at the leading order is solved in terms of elliptical coordinates and related Mathieu functions. Due to quadratic interactions of the first-order potential, a drift flow, a static angular displacement and a bound wave appear at the second order. At the same order the effect of the flap shape forces the first harmonic solution. We then compare an array of flat devices with an array of curved flaps, assuming several shape functions that could be of practical engineering interest. We show that hydrodynamic interactions between the curved flaps and incident waves can have constructive effects in terms of power absorption at large frequencies.

Enhancing power extraction in bottom-hinged flap-type wave energy converters through advanced power take-off techniques

The linear and non-linear dynamics of a bottom-hinged, flap-type wave energy converter in response to regular waves were studied through computational simulations to assess the performance of power take-off techniques and enhance the power extraction. The computational model was developed in Comsol Multiphysics using its Multibody Dynamics Module and was carefully validated. The hydrodynamic coefficients are from the linear wave theory. To avoid damages to the device, especially in extreme sea conditions, a brake mechanism is used to limit the amplitude of flap oscillations. With that limit imposed, we show that the optimum damping coefficient proposed in the literature for power take-off does not actually lead to an optimum power extraction for a range of wave frequencies. Over that range, the brake mechanism becomes engaged, leading to a significant energy loss. We propose new power take-off techniques that avoid the engagement of the brake, yet keep the amplitude within the specified range. They are proposed for both the linear and non-linear flap dynamics, and their efficacy is demonstrated for several flap geometries. The proposed techniques enhance the power extraction by as high as 600% (linear) and 19% (non-linear), in the latter case by minimizing the energy loss due to brake.

Analysis of the Hydrodynamic Performance of an Oyster Wave Energy Converter Using Star-CCM+

A two-dimensional numerical Computational Fluid Dynamics (CFD) model is established on the basis of viscous CFD theory to investigate the motion response and power absorption performance of a bottom-hinged flap-type wave energy converter (WEC) under regular wave conditions. The convergence study of mesh size and time step is performed to ensure that wave height and motion response are sufficiently accurate. Wave height results reveal that the attenuation of wave height along the wave tank is less than 5% only if the suitable mesh size and time step are selected. The model proposed in this work is verified against published experimental and numerical models. The effects of mechanical damping, wave height, wave frequency, and water depth on the motion response, power generation, and energy conversion efficiency of the flap-type WEC are investigated. The selection of the appropriate mechanical damping of the WEC is crucial for the optimal extraction of wave power. The optimal mechanical damping can be readily predicted by using potential flow theory. It can then be verified by applying CFD numerical results. In addition, the motion response and the energy conversion efficiency of the WEC decrease as the incident wave height increases because the strengthened nonlinear effect of waves intensifies energy loss. Moreover, the energy conversion efficiency of the WEC decreases with increasing water depth and remains constant as the water depth reaches a critical value. Therefore, the selection of the optimal parameters during the design process is necessary to ensure that the WEC exhibits the maximum energy conversion efficiency.

Extreme wave impacts on a wave energy converter: load prediction through a SPH model

ABSTRACTThe present work addresses the evaluation by numerical simulation of the extreme loads acting on a flap-type wave energy converter. To this aim, a realistic situation is considered: an extreme wave impacting a bottom-mounted pitching device, consisting of a partly submerged flap, placed in front of a dike on the coast of Bayonne, south-west Atlantic coast, France. The SPH model can be an optimal candidate for this kind of study: its Lagrangian character allows for an accurate description of the extreme breaking wave acting on the structure, while its meshless feature permits an easy interaction of the fluid with moving rigid bodies such as the rotating flap converter. An analysis of the influence of different environmental conditions (tide level, wave breaking occurrence) on the load intensity is performed. Several impact dynamics are studied and it is found that, in some specific conditions, the process develops in a sort of “flip through” impact, similarly to what observed for wave impact on rigid walls. The most critical cases have been also reproduced with a fully 3D SPH model, highlighting the greater complexity of the flow and the limits of the 2D modeling.

Wake effect assessment of a flap type wave energy converter farm under realistic environmental conditions by using a numerical coupling methodology

Ocean Energy Europe has estimated that 100 GW of ocean energy capacity (wave and tidal) could be deployed in Europe by 2050. Along with the European targets it is expected that large farms of Wave Energy Converters (WECs) will be installed in the sea and, as part of the consenting process for their installation, it will be necessary to quantify their impact on the local environment. The objective of this study is to improve the assessment of WEC farms impact on the surrounding wave field (wake effect) through the use of a numerical coupling methodology. The methodology consists of a Boundary Element Method (BEM) solver to obtain the wave perturbation generated by the WEC farm for the near-field accounting for the wave-body interactions within the farm whilst a Wave Propagation Model (WPM) based on the mild-slope equations determines the wave transformation in the far-field. The near-field solution obtained from the BEM solver is described as an internal boundary condition in the WPM and then it is propagated throughout the WPM numerical domain. The internal boundary is described by imposing the solution of the surface elevation and velocity potential at the free-surface at each instant of time along a line surrounding the WEC farm.As a case study the methodology was applied to flap type WECs that are deployed in shallow water conditions. The validation of the technique was done first for a single flap and then for a farm of 5 flaps. Once validated, a realistic scenario was assessed by quantifying the impact of irregular sea states composed of long crested waves on a large WEC farm composed of 18 flaps and located on a real bathymetry. The irregular waves were obtained by superposing the regular wave field solutions for all wave frequencies represented in the considered sea state based on the linear water wave theory. Within the limits of this theory these simulations demonstrate the versatility of the methodology to accurately represent the impact of a WEC farm on the surrounding wave climate. The influence of the peak period and the spacing between flaps on the WEC farm wake effect was assessed as well.

The pressure impulse of wave slamming on an oscillating wave energy converter

Recent wave tank experiments on a flap-type wave energy converter showed the occurrence of extreme wave loads, corresponding to slamming events in highly energetic seas. In this paper, we analyse pressure-impulse values from available pressure measurements, for a series of experimental slamming tests. Then, we devise a pressure-impulse model of the slamming of a flapping plate, including the effects caused by air entrapment near the plate. Using a double conformal-mapping technique, we map the original domain into a semi-infinite channel, by means of Gauss’ hypergeometric functions. This allows us to express the pressure impulse as a superimposition of orthogonal eigenfunctions in the transformed space. The mathematical model is validated against the experimental data. Parametric analysis shows that the system is much more sensitive to the impact angle than to the initial wetted portion of the flap. Furthermore, the presence of an aerated region determines the pressure-impulse values to increase significantly at all points on the flap surface.

Improving the Computational Performance of Nonlinear Pseudospectral Control of Wave Energy Converters

The optimal control problem for a one-degree of freedom wave energy converter (WEC) with dynamical nonlinearities and constraints is formulated in the frequency-domain. The formulation adopted corresponds to a Fourier pseudospectral framework but, in contrast to previous similar approaches found in WEC control literature, it is shown that control force and velocity variables can be eliminated, using a frequency-domain transcription of the nonlinear dynamical equations, thus, resulting in fewer variables and elimination of the equality constraints. Furthermore, it is shown how the gradient and Hessian of the cost function and constraints can be explicitly calculated, which can be used to improve convergence within gradient-based optimisation techniques. The benefits of the proposed developments are illustrated by means of numerical experiments, for a flap-type WEC with viscous drag, under various constraint configurations. The techniques presented are formulated in a generic way, allowing for easy result transposition to a variety of nonlinear WEC models.

Parametric design methodology for maximizing energy capture of a bottom-hinged flap-type WEC with medium wave resources

Abstract This paper describes a parametric design methodology for maximizing the capture factor (CF) of a bottom-hinged flap-type wave energy converter (BHF-WEC). The general equation for CF is first derived using the damped-harmonic-oscillator model. Second, correspondences between the general and the 2-D ideal CF equations are established. Then, a scheme is proposed to account for any effects apart from the 2-D ideal modeling with three parameters, which constitute the basis for fitting any data series stemming from either numerical simulations or experiments. Once these three parameters are evaluated from data fitting, the maximum CF and its occurring conditions can be found. In the present study, WEC-Sim simulations are conducted for a series of finite rectangular BHF-WECs with effects of PTO and varying width (B) for two thicknesses (d) under two characteristic wave lengths (L) of the medium wave resources that Taiwan possesses. It is found that for B/L smaller than about 0.30, the maximum CF in resonance mode, CFres, is greater than 1.0 and much higher than that not in resonance mode, CFopt, which is always below 1.0. The captured power index in resonance mode, CFres × (B/L), is almost invariant in B/L = 0.11–0.30. Several BHF-WEC design guidelines can be deduced from these results.

A comparative study of different methods for predicting the long-term extreme structural responses of the combined wind and wave energy concept semisubmersible wind energy and flap-type wave energy converter

The combined wind and wave concept semisubmersible wind energy and flap-type wave energy converter was developed in the EU FP7 project MARINA Platform. It consists of a four-column semisubmersible ...

A conceptual model of the hydrodynamics of an oscillating wave surge converter

Abstract This paper investigates the hydrodynamics of a seabed-mounted, bottom-hinged, flap-type wave energy converter in shallow water. A conceptual model of the hydrodynamics of the device has been formulated and shows that, as the motion of the flap is highly constrained, the magnitude of the wave force on the flap is the key determinant of power capture. The results from a physical modelling program have been used in conjunction with numerical data from WAMIT to validate the conceptual model. The work finds that designing the device to increase the wave force is more profitable than designing it to be tuned to the incident wave climate. As wave force is the primary driver of device performance it is shown that the flap should fill the water column and pierce the water surface to reduce decoupling due to wave overtopping. It is concluded that, in order to maximize capture factor at a typical North Atlantic site, the flap should be approximately 20–30 m wide, with large diameter rounded side edges, having its pivot close to the seabed and its top edge piercing the water surface.

Modelling of a flap‐type wave energy converter farm in a mild‐slope equation model for a wake effect assessment

It is expected that large farms of wave energy converters (WECs) will be installed and as part of the consenting process it will be necessary to quantify their impact on the local environment. The objective of this study is to assess the impact a WEC farm has on the incoming wave field through the use of a novel methodology. This methodology assesses the changes of the significant wave height surrounding a flap-type WEC farm with a special focus on the lee of the farm. A time-dependent mild-slope equation model is employed to solve the propagation of surface waves and their interaction with the devices. The model represents the devices as obstacle cells with attributed absorption coefficients tuned against near-fields obtained from a boundary element method (BEM) solver. The wake effect of the farm is determined by using a step-by-step approach starting first with an assessment of one device and progressively incrementing to a larger number of flaps. The effect of incident sea states, device separations and water depth changes on the wake effect of the farm is also investigated. This work shows the potential of a WEC farm to reduce significant wave heights on the leeside.

The power-capture of a nearshore, modular, flap-type wave energy converter in regular waves

Bottom-hinged, nearshore flap-type wave energy converters (WECs), have several advantages, such as high power conversion efficiency and survivability. They typically comprise a single flap spanning their full width. However, a potentially beneficial design change would be to split the flap into multiple modules, to make a ‘Modular Flap’. This could provide improvements, such as increased power-capture, reduced foundation loads and lower manufacturing and installation costs. Assessed in this work is the hydrodynamic power-capture of this device, based on physical modelling. Comparisons are made to an equivalent ‘Rigid Flap’. Tests are conducted in regular, head-on and off-angle waves. The simplest control strategy, of damping each module equally, is employed.The results show that, for head-on waves, the power increases towards the centre of the device, with the central modules generating 68% of the total power. Phase differences are also present. Consequently, the total power produced by the Modular Flap is, on average, 23% more smooth than that generated by the Rigid Flap.The Modular Flap has 3% and 1% lower average power-capture than the Rigid Flap in head-on and off-angle waves, respectively. The advantages of the modular concept may therefore be exploited without significantly compromising the power-capture of the flap-type WEC.