Oscillating Wave Surge Converter Forced Oscillation Tests

This paper presents a novel power capture platform concept consisting of a semi-submersible platform and multiple point-absorber wave energy converters (WECs). The primary objective of the current study is to investigate the dynamic behavior of the power capture platform under short-crested irregular wave conditions. A spreading function is used to modify the JONSWAP spectrum into a wave spectrum that accounts for both frequency and direction. To ensure the reliability of the numerical analysis, three-dimensional potential flow theory and boundary element method (BEM) are utilized to perform mesh convergence analysis and hydrodynamic validation of the platform and WEC models. Time-domain simulations are conducted to evaluate the effects of wave directionality on the platform motion, mooring line tension, and power absorption of the WEC array. In addition, three models, “single WEC”, “WEC array” and “fixed power capture platform” are defined. The influence of hydrodynamic interactions and platform motion are predicted by comparing the power absorption of individual WEC, WEC row, and WEC array among these models. The results show that the effects of wave directionality on the performance of the floating power capture platform cannot be ignored. The findings of this study may provide some insights into the design of power capture platforms. Meanwhile, it is recommended to determine the wave characteristics of the target operational area in advance, in order to find out the optimal design for system stability and power absorption.

This paper presents a numerical investigation on the interaction between focused waves and wave energy converter (WEC) models using a hybrid solver, qaleFOAM, which couples a two-phase incompressible Navier–Stokes (NS) solver OpenFOAM/InterDyMFoam with the quasi Lagrangian–Eulerian finite element method (QALE-FEM) based on the fully non-linear potential theory (FNPT) using the domain-decomposition approach. In the qaleFOAM, the NS solver deals with a small region near the structures (NS domain), where the viscous effect may be significant; the QALE-FEM covers the remaining computational domain (FNPT domain); an overlap (transitional) zone is applied between two domains. The WEC models, mooring system and the wave conditions are given by the Collaborative Computational Project in Wave–Structure Interaction (CCP-WSI) Blind Test Series 2. In the numerical simulation, the incident wave is generated in the FNPT domain using a self-correction wavemaker and propagates into the NS domain through the coupling boundaries and attached transitional zones. An improved passive wave absorber is imposed at the outlet of the NS domain for wave absorption. The practical performance of the qaleFOAM is demonstrated by comparing its prediction with the experimental data, including the wave elevation, motion responses (surge, heave and pitch) and mooring load.

Dynamic Wave Energy Converter (WEC) models utilize a wide variety of fundamental hydrodynamic theories. When incorporating novel hydrodynamic theories into numerical models, there are distinct impacts on WEC rigid body motions, cable dynamics, and final power production. This paper focuses on developing an understanding of the influence several refined hydrodynamic theories have on WEC dynamics, including weakly nonlinear Froude-Krylov and hydrostatic forces, body-to-body interactions, and dynamic cable modelling. All theories have evolved from simpler approaches and are of importance to a wide array of WEC archetypes. This study quantifies the impact these theories have on modelling accuracy through a WEC case study. Theoretical differences are first explored in a regular sea state. Subsequently, numerical validation efforts are performed against field data following wave reconstruction techniques. Comparisons of significance are WEC motion and cable tension. It is shown that weakly nonlinear Froude-Krylov and hydrostatic force calculations and dynamic cable modelling both significantly improve simulated WEC dynamics. However, body-to-body interactions are not found to impact simulated WEC dynamics.

Considerable testing and modeling are required in order to fully realise, efficiently develop, and successfully industrialise the wave energy converters (WECs). Numerical modeling, full-scale measurements, and scaled prototype testing are the various methodologies that can be applied to model WECs and predict the dynamic response. Mathematical WEC models form the basis of model-based energy maximising control and directly affect the ability of model-based controllers to maximise energy capture. Linear WEC models are attractive in leading to simpler control designs, but may not cover the complete operational space. One solution is to identify a range of linear models at different operating points, which give a measure of the underlying nonlinear behaviour [1], [2]. This model set can then be used to extract a nominal model, and an associated uncertainty region, which can be used as a basis for a robust WEC controller synthesis process, such as articulated in [3]. Recently, such an approach has been adopted using data generated from a high-fidelity numerical computational fluid dynamics (CFD) model [4]. However, numerical wave tanks (NWTs) and physical wave tanks differ significantly in terms of the range of tests which can be performed, and the contamination which can affect the measurements used to determine the data-based models e.g. measurement noise, numerical effects, wave reflections, etc [5]. As a result, the determination of nominal models and uncertainty regions in a physical wave tank may provide some advantages (and disadvantages) which need to be examined carefully. In addition, the range of post-processing techniques which could, or should, be applied to the different experimental/numerical domains, to improve the fidelity of the identified models, may differ between domains. In this paper, experimental testing of a WEC, by recreating a wave field similar to real-life conditions and a small-scale version of the device, is used to understand the hydrodynamic behaviour and to obtain an accurate dynamic model for WECs, which are considered to be essential towards optimal WEC design. Physical wave tank experiments, even though having their own disadvantages, overcome some difficulties of CFD-based NWT experiments, most notably huge computation time, problems in accurate representation of viscous fluids, uncertainty in the specification of an appropriate turbulence model, and propagation of incident waves [6]. In this study, representative linear models of a point-absorber type WEC from a physical wave tank in the wave basin at Aalborg University are determined which give insight into the system dynamics and provide a basis for robust control of WECs. Among different stimulation techniques to excite the system dynamics in physical wave tank tests, the particular types of excitation signals covering the complete range of frequencies and amplitudes of the system dynamics, while considering limitations on the range of excitation signals or the wave tank reflections, are adopted for the determination of representative linear models. Moreover, a full investigation is carried out to ascertain the appropriate post-processing needed to optimise the signals as the basis for model identification. The model identification includes the non-parametric frequency response by means of empirical transfer function estimate (ETFE).

The output power fluctuations from the wave energy converters (WECs) with a high peak-to-average ratio need to be smoothed out before supplying power to electric loads or power grids. Usually, short-term energy storage is used to smooth out the output power fluctuations of WECs. In this article, a novel multi-filter-based dynamic power-sharing control is devised for a hybrid energy storage system (HESS) integrated with a grid-connected WEC. An advanced model of HESS-integrated WEC consisting of a direct-drive permanent magnet linear generator, a supercapacitor, and a battery system is developed to test the proposed multi-filter-based dynamic control strategy under realistic conditions. The distinguishing features of this study are as follows; 1) the power-sharing strategy is based on a dynamic filter that is designed to change its corner frequency depending upon the incoming ocean wave, 2) the low and the high-frequency power components are allocated based on ocean wave conditions, 3) the proposed strategy is suitable for the real-time control of the proposed HESS integrated with the WEC, and 4) a detailed model of the WEC is used instead of using a pre-saved power profile. The results suggest that with the implementation of the proposed multi-filter-based dynamic power-sharing scheme in the HESS integrated WECs, the output power can be effectively smoothed out before supplying the power to the grid.

Gimbal Joint (GJ) is a device designed to connect the upper and lower parts of a steel riser in a free-hanging system. The mechanism consists of a flexible conduit, inside a structure that is made of several hinged connection elements, making the whole system work as an articulation. Hydrodynamic properties of GJ are recognized as an uncertainty source due to its novelty and very specific geometry. Recently, an experimental campaign of a model scale size — 1:9 — has been performed as a part of technology qualification programme. The forced oscillation tests focused on obtaining drag and hydrodynamic inertia coefficients of the GJ, as a recommended practice to estimate hydrodynamic coefficients of complex bodies. As an oscillating porous body, the hydrodynamic coefficients shall be assessed in terms of the Keulegan Carpenter number, at different flow regimes corresponding to varying Reynolds number. The focus of this paper is to present and discuss the hydrodynamic coefficients obtained in the forced oscillation tests. A discussion about the correlated effect of the model porosity and KC (Keulegan-Carpenter number) compared to Sarpkaya’s experiment from the literature is also presented. Results indicate that porosity changes significantly the hydrodynamic coefficients, providing that standard industry coefficients, normally used in dynamic analysis, shall not be used when GJ is part of the system.

The motion of some geometries of a buoy in a two-body wave energy converter (WEC) system is studied under regular wave excitations. The buoy is coupled to a heave damping plate where the coupling system models a simplified power takeoff (PTO) system. A frequency-domain analysis is conducted on the two-body model to predict the wave-induced motion behavior of the bodies considering the mutual effects of two bodies and the PTO system. The designed buoys are axisymmetric and have an identical water-plane area and submerged volume. The model was analyzed in identical conditions for different buoys so that the effect of the buoy shape on its dynamic properties and the hydrodynamic coefficients, as well as the absorption efficiency of the two-body WEC model, could be evaluated. The impact of the buoy shape and its hydrodynamic coefficients on the optimal and suboptimal design of the PTO system is studied. The results indicate that a suitable geometry for the buoy can have beneficial effects on the improved efficiency of the two-body WEC model, increasing the relative displacement between the two bodies and, also, allowing an optimal design of the PTO system. Furthermore, the buoy shape can reduce the difference between the damping coefficients of the PTO system and the buoy in order to create the resonance condition in the WEC system.

Current global concerns on environmental protection and sustainable development for clean energy have led to the creation of new engineering devices. These technologies reduce harmful greenhouse gas emissions in the atmosphere, global warming and reduce dependence on fossil fuels to produce energy. Many countries are now moving towards strategies to achieve maximum power available from the factory to reduce greenhouse gas emissions are concentrated. Wave energy is an emerging technology that is currently under research and development is considerable. Wave energy is essentially an indirect form of solar energy. There is a long maritime border in the south and north of the country, Iran's potential to absorb the energy of the sea talk show. In this thesis, the energy in waves of Iranian ports to use wave energy absorber is investigated, and an experimental model of wave energy converter designed, manufactured and tested in regular waves and it has been analyzed.

Numerical modelling of wave energy converters (WECs) of the wave activated body type (WAB, see Chap. 2) is based on Newton’s second law, which states that the inertial force is balanced by all forces acting on the WEC’s captor.

Extracting energy from ocean waves has become a heated topic since the energy crisis of the 2000s. Among all the different concepts and designs of Wave Energy Converter (WEC), point absorber is a widely adopted type with great potential, and various configurations and constraints are applicable to it. Here, the point absorber WECs with four different set‐up configurations are explored: single body heaving WEC, two‐body heaving WEC, two‐body WEC with a flat plate (Reference Model 3), and a two‐body WEC with a cylinder‐shaped second body. Dynamic models are established for each case and wave tank tests are conducted for verification. The results show that the power capture of a point absorber can benefit from several aspects: the two‐body WEC with a streamlined shape can double the wave capture width ratio (up to 66.5%) over the single‐body WEC or Reference Model 3, while coupling other motion or mooring dynamics can further improve the capture width ratio by 12% by increasing the relative motion stroke.

A novel point absorber wave energy converter (WEC) is being developed by Columbia Power Technologies, LLC (CPT). Numerical and physical experiments have been performed by Columbia Power, Garrad Hassan and Partners (GH) and Oregon State University (OSU). Three hydrodynamic modeling tools including WAMIT, GH WaveFarmer, and OrcaFlex are used to evaluate the performance of the WEC. GH WaveFarmer is a specialized numerical code being developed specifically for the wave energy industry. Performance and mooring estimates at full scale were initially evaluated and optimized, which were then followed by the development of a 1/33rd scale physical model to obtain comparable datasets, aiming to validate the predictions and reduce the uncertainty associated with other numerical model results. The tests of the 1/33rd scale model of the CPT WEC were recently carried out at the multi-directional wave basin of the O.H. Hinsdale Wave Research Laboratory (HWRL), in conjunction with the Northwest National Marine Renewable Energy Center (NNMREC) at OSU. This paper presents details of the modeling program and progress to date. Emphasis is given to the coupling of WAMIT with GH WaveFarmer for performance estimates and the coupling of WAMIT with the OrcaFlex model for mooring load estimates. An overview of the novel 3-body WEC, including operation and mooring system, is also presented. The 1/33rd scale model functionality is described including an overview of the experimental setup at the basin. Comparisons between the numerical and experimental results are shown for both regular and irregular waves and for several wave headings and dominant directions using a number of spreading functions. The paper concludes with an overview of the next steps for the modeling program and future experimental test plans.

Wave Energy Converters (WECs) are expected to significantly contribute to the energy transition; however, this depends on their interactions with the resource. Calculating the power generated by WECs depends heavily on the accurate modelling of wave-structure interactions. The Boundary Element Method (BEM) based on the potential flow theory has yielded accurate results at low computational costs when compared to complex Computational Fluid Dynamics (CFD) methods. Hydrodynamic Analysis of Marine Structures (HAMS), a recently developed open-source BEM frequency domain solver, originally was created for large marine structures. To date it has only been applied to single WECs with spherical/cylindrical geometries. HAMS offers unique advantages through its efficient removal of irregular frequencies and lower computational costs. This paper aims to compare hydrodynamic coefficients, exciting forces, Response Amplitude Operators (RAOs) and computational costs between HAMS, WAMIT, and NEMOH for a cylindrical point absorber and an oscillating surge WEC, extending the currently limited WECs application in HAMS.

Disruption to benthic habitats by moorings of wave energy installations: A modelling case study and implications for overall ecosystem functioning

In the present study, the hydrodynamic efficiency of a cylindrical wave energy converter (WEC) of vertical symmetry axis and arranged in front of a reflecting orthogonal breakwater is explored. The idea is based on exploiting the anticipated amplification of the scattered and the reflected wave fields originating from the presence of the vertical walls, towards increasing the WEC’s wave power absorption due to the walls’ wave reflections. Two types of converters are examined, namely the heaving device and the oscillating water column (OWC) device, assuming linear potential theory. The associated diffraction-, motion-, and pressure-radiation problems are solved using axisymmetric eigenfunction expansions for the velocity potential around the WECs by properly accounting for the wave field’s modification due to the walls’ presence. To this end, a theoretical formulation dealing with the evaluation of the converter’s performance is presented accounting for the coupling between the WEC and the reflecting vertical walls. The results depict that the amount of the harvested wave power by the WEC in front of an orthogonal wall is amplified compared to the absorbed wave power by the same WEC in the open sea.

The contribution of wave energy to the renewable energy supply is rising. To extract a considerable amount of wave power, Wave Energy Converters (WECs) are arranged in several rows or in a ‘farm’. WECs in a farm are interacting (e.g. The presence of other WECs influence the operational behaviour of a single WEC) and the overall power absorption is affected. In this paper wake effects in the lee of a single WEC and multiple WECs of the overtopping type, where the water volume of overtopped waves is first captured in a basin above mean sea level and then drains back to the sea through hydro turbines, are studied in a time-dependent mild-slope equation model. The wake behind a single WEC is investigated for uni- and multi-directional incident waves. The wake becomes wider for larger wave peak periods. An increasing directional spreading results in a faster wave regeneration and a shorter wake behind the WEC. The wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, with varying lateral and longitudinal spacing. In general, the staggered grid results in the highest overall wave power absorption.