Wave Energy Converter Arrays Research Articles

Storage Minimization of Marine Energy Grids Using Polyphase Power

Multiple wave energy converter (WEC) buoys can be used to establish a WEC array-powered microgrid collectively forming a Marine Energy Grid (MEG). An oceanic domain with gravity waves will have significant spatial variability in phase, causing the power produced by a WEC array to have high peak-to-average ratios. Minimizing these power fluctuations reduces the demand for large energy storage by WEC array-powered DC microgrids while also reducing losses in the undersea cable to the shore. Designs that reduce energy storage requirements are desirable to reduce deployment and maintenance costs. This work demonstrates that polyphase power in conjunction with an energy storage system can be used to maintain constant power. This work shows that an N WEC array geometry can be designed to reduce the energy storage requirements needed to mitigate the power fluctuations if the WEC array produces constant, polyphase power. Additionally, the conditions that identify the wave frequencies and control the effort needed to produce polyphase power are developed. This paper also shows that increasing the number of WECs in an array reduces aggregate power fluctuations. Finally, WEC array power profiles are investigated using simulation results to verify the mathematical conditions developed for the three and six WEC cases.

Experimental study of wave energy converter arrays adapted to a semi-submersible wind platform

The design of an offshore wind turbine floating platform seeks to minimize platform pitching motions that otherwise generate large accelerations and loads on the turbine, nacelle and blades, and decrease their lifetime. This objective is harder to achieve as the upscaling to more powerful wind turbines amplify the effects of pitching motions since the wind turbines’ height and mass are higher. Thus, innovative solutions to control the pitching motions should be developed. This study presents a solution based on a concentric array of wave energy converters which was originally designed to be attached on a floating platform to generate wave energy. Meanwhile, it was found that the concept may provide bigger restoring moments for platforms, amplified by its lever type arms, which are useful to control the pitching motions. Thus, a wind thrust simulator is adapted to the original experimental model and tests performed to understand the advantage of using these converters in the control of the pitching motions of a wind turbine platform. The experimental results indicate that platform motions are controllable by the wave energy converters however must be optimized to achieve better performance. These results also provide some design knowledge to test the concept at bigger rated power wind turbines.

WEC Array Optimization with Multi-Resonance and Phase Control of Electrical Power Take-Off

This new research provides transformative marine energy technology to effectively power the blue economy. Harmonizing the energy capture and power from Wave Energy Converter (WEC) arrays require innovative designs for the buoy, electric machines, energy storage systems (ESS), and coordinated onshore electric power grid (EPG) integration. This paper introduces two innovative elements that are co-designed to extract the maximum power from; i) individual WEC buoys with a multi-resonance controller design and ii) synchronized with power packet network phase control through the physical placement of the WEC arrays reducing ESS requirements. MATLAB/Simulink models were created for the WEC array dynamics and control systems with Bretschneider irregular wave spectrum as inputs. The numerical simulation results show that for ideal physical WEC buoy array phasing of 60 degrees the ESS peak power and energy capacity requirements are minimized while the multi-resonant controllers optimize EPG power output for each WEC buoy.

Analysing the Influence of Power Take-Off Adaptability on the Power Extraction of Dense Wave Energy Converter Arrays

Analysing the Influence of Power Take-Off Adaptability on the Power Extraction of Dense Wave Energy Converter Arrays

Predicting coastal impacts by wave farms: A comparison of wave-averaged and wave-resolving models

Wave energy converters (WECs) will have to be arranged into arrays of many devices to extract commercially viable amounts of energy. To understand the potential coastal impacts of WEC arrays, most research to date has relied on wave-averaged models given their computational efficiency. However, it is unknown how accurate wave-averaged model predictions are given a lack of validation data and their inherent simplifications of various hydrodynamic processes (e.g., diffraction). This paper compares the predictions of coastal wave farm impacts from a coupled wave-averaged and flow model (Delft3D-SNL-SWAN), to a wave-resolving wave-flow model (SWASH) that intrinsically accounts for more of the relevant physics. Model predictions were compared using an idealized coastal bathymetry over a range of wave conditions and wave farm geometries. Both models predicted the largest impacts (changes to the nearshore hydrodynamics) for large and dense wave farms located close to the shore (1 km) and the smallest impacts for the small and widely spaced farm at a greater offshore distance (3 km). However, the wave-resolving model generally predicted somewhat larger impacts (i.e., changes to the nearshore wave heights, mean velocities and mean water levels). We also found that coupling the wave-averaged model to a flow model resulted in more realistic downstream predictions than the stand-alone wave-averaged model.

Effect of resonance and wave reflection in semi-enclosed moonpool on performance enhancement of point absorber arrays

This paper presents a novel means to enhance the energy generation of the wave energy converters (WECs) array by deploying it in a semi-enclosed moonpool. The resonance phenomenon in a moonpool and the wave reflection from the wall of the moonpool result in an amplification of wave amplitude thus increase the energy generation of the wave farm. The wave diffraction and radiation analyses, based on the boundary element method are used to assess the performance of the array. An array of five-point absorbers in a semi-enclosed moonpool is considered and the energy generation, interaction effect and wave climate surrounding the wave farm are investigated. The results show that the width of the moonpool opening affects the performance of the WEC array. The wave climate surrounding the moonpool platform is studied to assess the effectiveness of the moonpool platform as a floating breakwater. The results show that the proposed integrated wave farm and floating breakwater system (moonpool platform) could generate a substantial amount of wave energy and at the same time is effective in attenuating the wave forces. The characteristics of the WECs reveal that the energy generation bandwidth could be increased by deploying the WECs in the semi-enclosed moonpool.

Layout optimisation of offshore wave energy converters using a novel multi-swarm cooperative algorithm with backtracking strategy: A case study from coasts of Australia

Wave energy technologies have the potential to play a significant role in the supply of renewable energy worldwide. One of the most promising designs for wave energy converters (WECs) are fully submerged buoys. In this paper, we explore the optimisation of WEC arrays consisting of three-tether buoys.Such arrays can be optimised for total energy output by adjusting the relative positions of buoys in a wave farm. As there are complex hydrodynamic interactions among WECs, the evaluation of each parameter setting is computationally expensive and thus limits the feasible number of full model evaluations that can be made. Furthermore, these WEC interactions make up a non-convex, multi-modal (with multiple local-optima), continuous and constrained optimisation problem. This problem is challenging to solve using optimisation methods. To tackle the challenge of optimising the positions of WECs in a wave farm, we propose a novel multi-swarm cooperative co-evolution algorithm which consists of three meta-heuristics: the multi verse optimiser (MVO) algorithm, the equilibrium optimisation (EO) method, and the moth flame optimisation (MFO) approach with a backtracking strategy, we introduce a fast, effective new surrogate model to speed up the process of optimisation.To assess the effectiveness of our proposed approach, 11 state-of-the-art bio-inspired algorithms and three recent hybrid heuristic techniques were compared in six real wave situations located on the coasts of Australia, with two wave farm sizes (four and nine WECs).The experimental study presented in this paper shows that our hybrid cooperative framework exhibited the best performance in terms of the quality of obtained solutions, computational efficiency, and convergence speed compared with other 14 state-of-the-art meta-heuristics. Furthermore, we found that the power output of the best-found 9-buoy arrangements were higher than that of perpendicular layouts at at 4.15%, 3.29%, 3.62%, 9.2%, 5.74%, and 2.43% for the Perth, Adelaide, Sydney, Tasmania, Brisbane, and Darwin wave sites, respectively. Our investigations reveal that the best-found arrangement at the Tasmania wave site was able to absorb the highest level of wave power relative to the other locations.

Optimization of a three-dimensional hybrid system combining a floating breakwater and a wave energy converter array

Combining wave energy converters (WECs) with a floating breakwater provides a potential approach to help develop commercial-scale wave power operations. This paper aims to design and optimize a three-dimensional floating breakwater integrated with a WEC array by developing a numerical model of a multi-floating-body coupled system based on potential flow theory with viscous correction in the frequency domain. By analyzing the wave surface elevations around the breakwater, the size of the breakwater was optimized and the optimal installation locations of the WECs were determined. Under the condition of coupled constraints and six degrees-of-freedom motion, the interactions between the WEC array and the breakwater were analyzed. Subsequently, the number of WECs and the distance between the WECs and the breakwater were optimized to maximize the wave energy conversion performance of the hybrid system. Results show that the wave focusing areas appear more frequently near the breakwater. These focusing areas significantly improve WEC power, and thus better wave energy conversion performance can be achieved when the WECs are placed close to the breakwater. The vertical forces on the breakwater significantly increase due to the presence of the WECs, however the horizontal forces are decreased. The findings of this paper provide guidance to design and optimize a hybrid WEC-breakwater system in practical engineering applications.

Energy‐maximising moment‐based constrained optimal control of ocean wave energy farms

Successful commercialisation of wave energy technology inherently incorporates the concept of an array of wave energy converters (WECs). These devices, which constantly interact via hydrodynamic effects, require optimised control that can guarantee maximum energy extraction from incoming ocean waves while ensuring, at the same time, that any physical limitations associated with device and actuator systems are being consistently respected. This paper presents a moment-based energy-maximising optimal control framework for WECs arrays subject to state and input constraints. The authors develop a framework under which the objective function (and system variables) can be mapped to a finite-dimensional tractable quadratic program (QP), which can be efficiently solved using state-of-the-art solvers. Moreover, the authors show that this QP is always concave, i.e. existence and uniqueness of a globally optimal solution is guaranteed under this moment-based framework. The performance of the proposed strategy is demonstrated through a case study, where (state and input constrained) energy-maximisation for a WEC farm composed of CorPower-like WEC devices is considered.

A new framework and tool for ecological risk assessment of wave energy converters projects

Marine renewable energy has considerable potential for enhancing the diversity of renewable sources, to reduce our reliance on fossil fuels and combat climate change. While the technological development of wave energy converters is progressing rapidly, their environmental impacts are still largely unknown, which is a barrier that could hinder their deployment. This research contributes to the state-of-the-art by introducing a framework for quantifying and analysing the ecological risks of three technologies (oscillating water columns, oscillating wave surge converters, and wave turbines). Based on a literature review, expert consultation process, and the development of a web tool, the potential pressures and the ecosystem elements that might be affected during the life cycle of a generic wave farm (an array of wave energy converters) are investigated. The main pressures are found to be physical disturbance, physical loss, hydrological change, and noise. The ecosystem elements sustaining the largest number of pressures and, therefore, at higher ecological risk are fish and cephalopods species, and benthic and pelagic habitats. The ecological risk assessment framework is operationalized into a free-access web tool (https://aztidata.es/wec-era/) for the interactive assessment and visualisation of the pressures and ecological risks. The tool is intended to be used by managers, decision makers, scientists or promoters during the Strategic Environmental Assessment and Environmental Impact Assessment of wave energy projects. The novel approach presented in this work is more sophisticated than previous risk assessment matrices, enabling to better capture the complexity of the interactions between a wave farm and the environment.

Compact floating wave energy converter arrays: Inter-device mooring connectivity and performance

Achieving cost reduction in wave energy conversion is seen as essential to enabling the progress of the sector. At utility scale in a wave farm, multiple devices are likely to be deployed in array configurations. Closely spaced, compact wave energy converter (WEC) arrays are a promising option for cost reduction, realising synergies in operation and maintenance tasks and auxiliary installations, whilst achieving economies of scale. Mooring and anchorage systems are known to be a major component of the structural costs, and the use of interconnecting lines between neighbouring devices can reduce the number of anchors and minimise total line length. In this paper, we present the experimental study of different configurations of a five-device array of spar-buoy oscillating-water-column wave energy converters in a wave basin, focusing on the analysis of the power production performance. The study compares the performance of a single isolated device, an array with independently-moored devices and three arrays with inter-body connections, with different levels of connectivity in the mooring arrangement. Results show considerable performance implications linked to the interconnecting of devices, with the interconnected array configurations yielding a 75% increase in the annual energy extracted compared to the baseline (non-interconnected) arrangement. The performance enhancements are primarily attributed to the interconnecting moorings resulting in greater heave motion at higher frequencies for which the phase relationship between the water column and heave motion is more beneficial. Evidence is also presented that positive intra-array effects occur within interconnected arrays when the wavelength is equal to the array spacing.

Hydrodynamics of large wave energy converter arrays with random configuration variations

Abstract

Hydrodynamic performances of wave energy converter arrays in front of a vertical wall

Wave power absorption with Wave Energy Converters (WECs) arrays in front of a vertical wall is predicted with an in-house transient wave-multibody numerical tool of ITU-WAVE which uses time marching scheme to solve a boundary integral equation for the analyses of hydrodynamic radiation and exciting forces. The perfect reflection of incident waves from a vertical wall is considered with method of images. Mean interaction factor, which can have constructive or destructive effect and determines the performances of WECs, is approximated with different array configurations. The vertical wall effect plays significant role over hydrodynamic parameters as the radiation and exciting forces show quite different behaviour in the case of WECs with and without vertical wall in an array system. The numerical results show that the performance and wave power absorption with WECs arrays in front of vertical wall are much greater compared to WECs arrays without vertical wall effect. This is mainly due to standing and nearly trapped waves between a vertical wall and WECs arrays in addition to strong interactions between WECs. The satisfactory agreements are obtained when the present ITU-WAVE numerical results for different hydrodynamic parameters in an array system are compared with other published analytical and numerical results.

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.

Alternative approaches of evaluating Diffraction Transfer Matrix and Radiation Characteristics using the hybrid source-dipole formulation

The interaction theory presented by Kagemoto and Yue (1986) significantly reduces the computational burden in the wave interaction problem of multiple surface-piercing bodies, particularly arrays of wave energy converters in recent years. Two essential operators of the theory are the so-called Diffraction Transfer Matrix and Radiation Characteristics. Many subsequent researchers (Goo and Yoshida, 1990; Flavià et al., 2018) have implemented the theory using the source distribution method in evaluating the two linear operators of a single unique geometry. However, nowadays, a great majority of boundary element method codes have been written by virtue of the hybrid source-dipole distribution method on account of its high accuracy. In this regard, the present work aims to introduce a full set of mathematical formulations, as well as a complete derivation process of evaluating the two operators based on the hybrid source-dipole distribution method. The proposed formulations are then applied to two benchmark geometries, as given by McNatt et al. (2015) and Flavià et al. (2018). Good agreement is found between the present results and those from the literature. Moreover, two alternative approaches to solve the diffraction problem have been compared to assess both their accuracy and efficiency. It is found that the two methods present similar levels of accuracy but very different computational burden.

Modelling of a wave energy converter array with non‐linear power take‐off using a mixed time‐domain/frequency‐domain method

A mixed time-domain/frequency-domain method is proposed for modelling dense wave energy converter (WEC) arrays with non-linear power take-off (PTO). The model is based on a harmonic balance method which describes the system response in the frequency domain, while evaluating the non-linear PTO force and solving the system equations of motion in the time domain. The non-linear PTO force is computed with Lagrange multipliers. In order to apply the proposed method for WEC array responses in real sea states, the time series is split into time windows and the simulation is carried out individually per window. The method is demonstrated by investigating the dynamics of the Ocean Grazer WEC array (OG-WEC) with an adaptable piston pumping system. The key parameters thought to possibly influence model accuracy, including the number of harmonic components, the length of the time window and overlay, are discussed. It is shown that the proposed model can significantly reduce the computational cost with an acceptable accuracy penalty.

Performance characteristics and parametric analysis of a novel multi-purpose platform combining a moonpool-type floating breakwater and an array of wave energy converters

Integration of Wave Energy Converters (WECs) with floating breakwater system provides a multi-function solution to wave energy extraction and offshore infrastructural protection. The contribution of this work is to guide the optimal size and configuration of a multi-purpose platform including a moonpool-type floating breakwater and an array of heaving oscillating-buoy (OB) WECs. The investigation is performed using a developed time-domain numerical wave tank (NWT) based on the three-dimensional (3D) potential flow theory with fully nonlinear boundary conditions on transient wetted body surfaces and free surfaces. The comparison of the hydrodynamic performance among the multi-purpose platform, the isolated array WECs, and the isolated floating breakwater are examined. The internal fluid motion in the moonpools has a positive effect on the wave energy absorption of WECs, which in turn enhances the wave attenuation capacity of the floating breakwater. WECs with larger diameter have a larger water-plane area, which leads to more extracted wave energy. The wave nonlinearity reduces the optimal PTO damping value and has an adverse effect on the wave energy extraction. However, when wave nonlinearity becomes prominent, the wave attenuation capacity is improved with increasing PTO damping. For an unequal layout of moonpools, the thinner moonpools are the major contributor to wave energy extraction, especially in the short wave region. As a result of mass exchange of fluid from the moonpool to the outer domain, the multi-purpose platform indicates favorable performance of wave energy absorption. The novel floating system makes the utilization of wave energy over a wider frequency range.

Optimisation of wave-power arrays without prescribed geometry over incident wave angle

This paper describes the optimisation of arrays of wave energy converters (WECs) of point absorber type. The WECs are spherical in shape and operate in heave only. Previous work is extended to an optimisation of array layouts without a prescribed geometry. The objective function is chosen as the mean of the array interaction factor over a prescribed range of incident wave angles. This formulation forces the array to perform optimally over a specified range of wave angle, without direct concern for wavelength variations. Both constrained and unconstrained WEC motions are considered, with constrained optimisations limiting device displacements to two or three times the incident wave amplitude. The increased freedom in this more general optimisation results in a 70% to 140% increase in objective function values compared to the analogous linear array optimisations. As in previous studies of this nature, unconstrained arrays tend to contain closely spaced WECs and larger displacement amplitudes, whereas constrained optimal arrays are more widely spaced. It is shown that the prescribed range of incident wave angle has a great effect on the optimal array layout, with better performance achieved for smaller ranges of wave angle due to better tuning of the array members. A previously identified trade-off in linear arrays, between performance stability to different incident wave parameters, is shown not to apply to general array layouts.

Effects of a Vertical Wall on Wave Power Absorptions with Wave Energy Converters Arrays

Effects of a Vertical Wall on Wave Power Absorptions with Wave Energy Converters Arrays

Frequency-Based Performance Analysis of an Array of Wave Energy Converters around a Hybrid Wind–Wave Monopile Support Structure

In this paper, we investigate, in the frequency domain, the performance (hydrodynamic behavior and power absorption) of a circular array of four semi-immersed heaving Wave Energy Converters (WECs) around a hybrid wind–wave monopile (circular cylinder). The diffraction/radiation problem is solved by deploying the conventional boundary integral equation method. Oblate-spheroidal and hemispherical-shaped WECs are considered. For each geometry, we assess the effect of the array’s net radial distance from the monopile and of the incident wave direction on the array’s performance under regular waves. The results illustrate that by placing the oblate spheroidal WECs close to the monopile, the array’s power absorption ability is enhanced in the low frequency range, while the opposite occurs for higher wave frequencies. For hemispherical-shaped WECs, the array’s power absorption ability is improved when the devices are situated close to the monopile. The action of oblique waves, with respect to the WECs’ arrangement, increases the absorbed power in the case of oblate spheroidal WECs, while these WECs show the best power absorption ability among the two examined geometries. Finally, for the most efficient array configuration, consisting of oblate spheroidal WECs situated close to the monopile, we utilize an “active” Power Take-Off (PTO) mechanism, facilitating the consideration of a variable with frequency PTO damping coefficient. By deploying this mechanism, the power absorption ability of the array is significantly enhanced under both regular and irregular waves.