Flap-type Wave Energy Converter Research Articles

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.

Optimization of bottom-hinged flap-type wave energy converter for a specific wave rose

In this paper, we conducted a numerical analysis on the bottom-hinged flap-type Wave Energy Convertor (WEC). The basic model, implemented through the study using ANSYS-AQWA, has been validated by a three-dimensional physical model of a pitching vertical cylinder. Then, a systematic parametric assessment has been performed on stiffness, damping, and WEC direction against an incoming wave rose, resulting in an optimized flap-type WEC for a specific spot in the Persian Gulf. Here, stiffness is tuned to have a near-resonance condition considering the wave rose, while damping is modified to capture the highest energy for each device direction. Moreover, such sets of specifications have been checked at different directions to present the best combination of stiffness, damping, and device heading. It has been shown that for a real condition, including different wave heights, periods, and directions, it is very important to implement the methodology introduced here to guarantee device performance.

Wave- and Wind-induced Responses of the Semisubmersible Wind Energy and Flap-type Wave Energy Converter Based on Experiments

Wave- and Wind-induced Responses of the Semisubmersible Wind Energy and Flap-type Wave Energy Converter Based on Experiments

Wave protection effect of periodic row of bottom-hinged flap-type wave energy converters

A flap-type wave energy converter (WEC) is combined with a nearshore breakwater to expand the application of WECs both economically and environmentally. Based on the linear potential theory, an eigenfunction expansion solution is developed for a periodic row of bottom-hinged flap-type WECs exposed to normal waves. Additionally, the viscous effect is considered using the ship rolling solution method with a viscous damping term included in the equation of motion, and the viscous damping expression is also described. The proposed solution is verified by comparison with published literatures. The results including the wave energy conversion efficiency, the reflected and transmitted proportion of the incident wave energy are presented for a range of wave periods and geometric ratios. It is demonstrated that better wave protection effects can be attained with smaller gaps between the WECs, where the transmitted proportion of the incident wave energy is lower. An optimal geometric ratio thus exists for a given wave power absorption and a specific wave period.

Experimental and numerical study of the response of the offshore combined wind/wave energy concept SFC in extreme environmental conditions

This paper deals with an experimental study of the survivability of the offshore combined concept Semisubmersible wind energy and Flap-type wave energy Converter (SFC) and with comparisons of the experimental data with numerical predictions. The SFC is a combined energy concept consisting of a braceless semisubmersible type floating wind turbine and three fully submerged rotating flap-type Wave Energy Converters (WECs). In order to study the survivability of the concept the focus is on extreme environmental conditions. In these conditions the SFC will not produce wind or wave power; the wind turbine is parked with the blades feathered into the wind and the WECs are released to freely rotate about their axis of rotation. Firstly the development and set-up of the physical model are presented. Static, quasi-static, decay, regular waves and irregular waves with wind loading tests are conducted on an 1:50 scale physical model. Aligned and oblique wave with wind loading conditions are considered. Measured variables that are presented include motions of the semisubmersible platform in six rigid body degrees of freedom, rotation of the flap-type WECs, tension of mooring lines, internal loads of the arms that connect the flap with the pontoon of the platform and tower base bending moment. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model. The comparison is made at model scale. A good agreement between experimental data and numerical predictions is observed confirming the accuracy of the numerical models and tools that are used. The discrepancy between numerical and experimental results is smaller for regular than irregular waves. Compared to oblique conditions a better agreement between experimental and numerical results is obtained for the case of aligned wave and wind loadings. The results obtained demonstrate the good performance of the SFC concept in extreme environmental conditions. No strong nonlinear hydrodynamic phenomena are observed in the tests.

Optimal causal control of wave energy converters in stochastic waves – Accommodating nonlinear dynamic and loss models

Recent research has shown that when constrained to causality, the optimal feedback controller for an ocean wave energy converter (WEC) subjected to stochastic waves can be solved as a non-standard Linear Quadratic-Gaussian (LQG) optimal control problem. In this paper, we present a relaxation to the modeling assumptions that must be made to apply this theory. Specifically, we propose a technique that uses the principle of Gaussian Closure to accommodate nonlinear WEC dynamics in the synthesis of the optimal feedback law. The technique is approximate, in the sense that it arrives at a computationally efficient control synthesis technique through a Gaussian approximation of the stationary stochastic response of the system. This approach allows for a wide range of nonlinear dynamical models to be considered, and also accommodates many complex loss mechanisms in the power transmission system. The technique is demonstrated through simulation examples pertaining to a flap-type WEC with a hydraulic power train.

Experimental study of the functionality of a semisubmersible wind turbine combined with flap-type Wave Energy Converters

In the present paper the functionality of the Semisubmersible wind energy and Flap-type wave energy Converter (SFC) is examined experimentally. In order to study the functionality of the SFC, the focus is on operational environmental conditions. SFC is a combined concept that utilizes offshore wind energy and ocean wave energy for power production. Details are presented as far as the physical modelling of the wind turbine with the use of a redesigned small-scale rotor and of the Power Take-Off mechanism of the Wave Energy Converters (WECs) with the use of a configuration that is based on a mechanical rotary damper. Tests with quasi-static excitation, motion decay, regular and irregular waves without and with wind that is uniform are conducted on an 1:50 scale physical model. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model using Simo/Riflex tool. A good agreement is observed between experimental and numerical predictions. The combined operation of WECs doesn't affect the tension of mooring lines nor the acceleration of nacelle and the bending moment in tower's base. The produced power of the WECs of the SFC and consequently the functionality of the SFC is estimated.

Optimisation of arrays of flap-type oscillating wave surge converters

In this paper a finite array of hinged flap-type wave energy converters are modelled using a mathematical approach. These are illustrative of the Oyster device of Aquamarine Power Ltd.11http://www.aquamarinepower.com/. A novel semi-analytic solution method is presented for a set of boundary-value problems involving the scattering and radiation of waves by thin barriers used to model the device hydrodynamics. The approach makes use of the geometry to apply Fourier transforms, deriving non-singular integral equations in terms of the jumps in pressure across the flaps. These are then solved numerically using a highly efficient Galerkin expansion method. The focus of the results is on optimisation. We suggest optimal parameters for a single device, identifying flap length as crucial to device performance. This optimisation is then carried through to arrays with optimal arrangements and spacings being determined for a model sea state. Here, the lateral displacement of the devices emerges as a critical factor in optimal array configuration.

Motion-resonant modes of large articulated damped oscillators in waves

Using a semi-analytical approach, we show that an articulated system of large damped oscillators in the open ocean can be resonated by incoming waves at multiple frequencies. As an application, energy extraction from the system is modelled when the oscillators are used as flap-type wave energy converters. A new parameter – the absorption efficiency – is introduced to analyse the performance of the system at resonance. This allows us to identify the occurrence of detrimental processes near the resonant frequencies, which reduce the sustainability of the energy conversion process. This result challenges the diffused belief that large flap-type wave energy converters must be designed to resonate, which is based on the use of inappropriate performance descriptors.

Wave-power absorption from a finite array of oscillating wave surge converters

Semi-analytical and fully numerical modelling is developed in the framework of the inviscid potential flow theory to investigate the dynamics of a wave farm made by flap-type wave energy converters in the nearshore. The hydrodynamic parameters and the efficiency of the system in typical layouts are calculated with both models. Good agreement is shown between the two approaches. Parametric analysis undertaken with the semi-analytical model allows to identify a near-resonant phenomenon which is responsible for increasing the absorbed power by the single elements of the array. Such result could be used as a preliminary design criterion. The numerical model is then applied to analyse a configuration of practical engineering interest, i.e. an array of two staggered converters. The dynamics arising in this more complex system is explained, showing that non-symmetric layouts can be less effective.

Relations for a periodic array of flap-type wave energy converters

This paper investigates the interaction of plane incident waves with a wave farm in the open ocean. The farm consists of a periodic array of large flap-type wave energy converters. A linear inviscid potential-flow model, already developed by the authors for a single flap in a channel, is considered. Asymptotic analysis of the wave field allows to obtain new expressions of the reflection, transmission and radiation coefficients of the system. It is shown that, unlike a line of heaving buoys, an array of flap-type converters is able to exploit resonance of the system transverse modes in order to attain high capture factor levels. Relations between the hydrodynamic coefficients are derived and applied for optimising the power output of the wave farm.

Spectral modelling of wave energy converters

A spectral model suitable for the representation of wave energy converters is developed. A spectral model is an extension of a frequency-domain model that allows inclusion of non-linear forces and thereby provides improved estimates of wave energy converter performance, without the high computational cost of a time-domain model. The suitability and accuracy of a spectral model representation is demonstrated for a flap-type wave energy converter, by modelling the effect of vortex shedding and large amplitudes of motion. The development of a spectral model of wave energy converters also means that they can be represented in spectral wave models and included explicitly in software tools such as SWAN or Mike21 SW. This means that tools familiar to the industry could be used to determine the environmental impact and energy yield of wave farms efficiently.

The effect of water depth on the performance of a small surging wave energy converter

The effect of water depth on the performance of a small surging wave energy converter (WEC) is investigated analytically, numerically and experimentally. It is shown that although the average annual incident wave power is significantly reduced by water depth, a large proportion of this reduction is due to the dissipation of highly energetic, but largely unexploitable seas. It is also shown that the power capture is related more closely to incident wave force than incident wave power. Experimental results demonstrate that both the surge wave force and power capture of a flap-type WEC increase in shallow water.

Blending ten additives via onepack granules

This paper investigates the interaction of plane incident waves with a wave farm in the open ocean. The farm consists of a periodic array of large flap-type wave energy converters. A linear inviscid potential-flow model, already developed by the authors for a single flap in a channel, is considered. Asymptotic analysis of the wave field allows to obtain new expressions of the reflection, transmission and radiation coefficients of the system. It is shown that, unlike a line of heaving buoys, an array of flap-type converters is able to exploit resonance of the system transverse modes in order to attain high capture factor levels. Relations between the hydrodynamic coefficients are derived and applied for optimising the power output of the wave farm.