Tag Archives: Array

On the park effect in arrays of oscillating wave energy converters


A. Babarit – Renewable Energy, October, 2013

Abstract

This paper aims to provide guidelines for designing the layout of arrays of oscillating Wave Energy Converters (WECs) based on a review of the literature of wave interactions and park effect in WEC arrays that has been published over the past 30 years.

First, the fundamentals of wave energy absorption by oscillating bodies are summarised, and the principal differences between the park effect in arrays of wave energy converters and wind turbines are highlighted. Then, the numerical approaches commonly used to deal with WEC arrays are outlined briefly and their limitations are discussed. It is argued that, at present, only Boundary Element Methods (BEM) are capable of the appropriate analysis. Finally, previous work on wave interactions and park effect in WEC arrays is reviewed. Similar trends are found in these studies, which allow conclusions to be drawn regarding the significance of the park effect as a function of the number of WECs in the array and their spacing. Based on these conclusions, the following tentative guidelines are proposed:

For small arrays of conventional devices (fewer than 10 devices of typical dimension 10–20 m) with usual layouts (regular or shifted grids with separating distance of order 100–200 m), the park effect appears to be negligible. For larger arrays (more than 10 devices), a negative park effect seems to be increasingly important with increasing number of rows (the lines of WECs perpendicular to the incident wave direction). Therefore, the number of rows should remain as small as possible, with a separating distance as large as possible. For arrays of non-conventional WECs (WECs of typical dimensions much larger than 10–20 m), no information has been found. However, trends similar to the previous cases could be expected, provided that aspect ratios are maintained.

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Development of a point absorber wave energy converter: realisation of power take-off, optimisation of geometry and installation techniques


A. Van de Sijpe – PhD Thesis, Ghent University, 2012

Abstract

The development of renewable energy resources is strongly required due to the increasing energy demand, the shrinking reserves of fossil fuels and the effect of greenhouse gas emissions on the change of the wave climate. At Ghent University, study around the extraction of energy from ocean waves is being performed, more specifically with the aid of point absorber wave energy converters (WECs). To deliver a considerable amount of energy output at one location, large numbers of such devices need to be arranged in arrays or farms at sea. Several performed numerical and experimental studies around point absorbers and WEC-arrays are mentioned, indicating the knowledge gap of large scale physical model tests on WEC-farms, which are necessary to study the near- and far-field effects and to verify and improve numerical models. Within the HYDRALAB IV European programme in the frame of the project WEC wakes, large farms of point absorbers will be tested in the Shallow Water Wave Basin of DHI (Denmark). Continue reading

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Wave farm impact based on realistic wave-WEC interaction


R. Carballo and G. Iglesias – Energy – February, 2013

Abstract

The objective of this work is to investigate the impact of a wave farm on the nearshore wave climate quantifying, for the first time, the interaction of the WECs (Wave Energy Converters) with the waves using ad hoc laboratory tests. To accomplish this objective, a procedure consisting of three main steps is implemented and illustrated with a case study: a wave farm of WaveCats (a lateral overtopping WEC) proposed for the Death Coast (NW Spain). First, the wave climate in the wave farm area is characterised and reference wave conditions are established. Second, wave-WEC interaction and, more specifically, wave energy transmission is determined by means of 3D physical model tests. Third, on the basis of the results of the laboratory tests, the impact of different layouts of the wave farm (single-row and two-row arrays) on the nearshore wave climate is computed using a high-resolution spectral wave model. The results indicate that the difference between the two layouts is negligible at a distance of 5000 m or greater past the farm. Although the case study concerns a specific WEC and area of deployment, the procedure is entirely general in that it can be applied to other WECs and areas of interest.

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Numerical investigations of the effects of different arrays on power extractions of horizontal axis tidal current turbines


Guanghui Bai, Jun Li, Pengfei Fan, Guojun Li – Renewable Energy – May, 2013

Abstract

As the tidal current industry grows, power extraction from tidal sites has received widespread attention. In this paper, a blade element actuator disk model that is coupled with the blade element method and a three-dimensional Navier–Stokes code is developed to analyse the relationship between power extraction and the layout of turbine arrays. First, a numerical model is constructed to simulate an isolated turbine and the model is validated using experimental data. Then, using this validated model, the power extraction of horizontal axis tidal current turbines using different tidal turbine arrays and rotation directions is predicted. The results of this study demonstrate that staggered grid array turbines can absorb more power from tidal flows than can rectilinear grid array turbines and that staggered grid array turbines are less affected by the rotation of upstream turbines. Continue reading

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Energy potential of a tidal fence deployed near a coastal headland


S. Draper, A. Borthwick and G. Houlsby – Phil. Trans. R. Soc. A, 2013

Abstract

Enhanced tidal streams close to coastal headlands appear to present ideal locations for the deployment of tidal energy devices. In this paper, the power potential of tidal streams near an idealized coastal headland with a sloping seabed is investigated using a near-field approximation to represent a tidal fence, i.e. a row of tidal devices, in a two-dimensional depth-averaged numerical model. Simulations indicate that the power extracted by the tidal fence is limited because the flow will bypass the fence, predominantly on the ocean side, as the thrust applied by the devices increases. For the dynamic conditions, fence placements and headland aspect ratios considered, the maximum power extracted at the fence is not related in any obvious way to the local undisturbed kinetic flux or the natural rate of energy dissipation due to bed friction (although both of these have been used in the past to predict the amount of power that may be extracted). The available power (equal to the extracted power net of vertical mixing losses in the immediate wake of devices) is optimized for devices with large area and small centre-to-centre spacing within the fence. The influence of energy extraction on the natural flow field is assessed relative to changes in the M2 component of elevation and velocity, and residual bed shear stress and tidal dispersion.

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Optimization of multiple turbine arrays in a channel with tidally reversing flow by numerical modelling with adaptive mesh


T. Divett, R. Vennell and C. Stevens – Phil. Trans. R. Soc. A, 2013

Abstract

At tidal energy sites, large arrays of hundreds of turbines will be required to generate economically significant amounts of energy. Owing to wake effects within the array, the placement of turbines within will be vital to capturing the maximum energy from the resource. This study presents preliminary results using Gerris, an adaptive mesh flow solver, to investigate the flow through four different arrays of 15 turbines each. The goal is to optimize the position of turbines within an array in an idealized channel. The turbines are represented as areas of increased bottom friction in an adaptive mesh model so that the flow and power capture in tidally reversing flow through large arrays can be studied. The effect of oscillating tides is studied, with interesting dynamics generated as the tidal current reverses direction, forcing turbulent flow through the array. The energy removed from the flow by each of the four arrays is compared over a tidal cycle. A staggered array is found to extract 54 per cent more energy than a non-staggered array. Furthermore, an array positioned to one side of the channel is found to remove a similar amount of energy compared with an array in the centre of the channel.

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Numerical investigations of the effects of different arrays on power extractions of horizontal axis tidal current turbines


G. Bai, J. Li, P. Fan, and G. Li – Renewable Energy, May 2013

Abstract

As the tidal current industry grows, power extraction from tidal sites has received widespread attention. In this paper, a blade element actuator disk model that is coupled with the blade element method and a three-dimensional Navier–Stokes code is developed to analyse the relationship between power extraction and the layout of turbine arrays. First, a numerical model is constructed to simulate an isolated turbine and the model is validated using experimental data. Then, using this validated model, the power extraction of horizontal axis tidal current turbines using different tidal turbine arrays and rotation directions is predicted. The results of this study demonstrate that staggered grid array turbines can absorb more power from tidal flows than can rectilinear grid array turbines and that staggered grid array turbines are less affected by the rotation of upstream turbines. In addition, for staggered gird arrays, the relationships between power coefficients, lateral distance and longitudinal distance are discussed. The appropriate lateral distance is approximately 2.5 turbine diameters, whereas for the longitudinal distance, the largest value possible should be used. The relative power coefficient can achieve 3.74 when the longitudinal distance is 6 times the turbine diameter. To further increase the power extraction, this study suggests an improved staggered grid array layout. The relative power coefficient of the improved four-row turbine arrays is approximately 3–4% higher than that of the original arrays and will increase as the distance between the second-row and third-row increases. Considering only the first two rows of turbines, the total power extraction can be 11% higher than for an equivalent number of isolated turbines.

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Seawave power farm design: A case study


V. Franzitta, A. Viola, and M. Trapanese – AASRI Procedia, 2012

Abstract

Current energy crisis and general concerns on environmental issues have triggered an ever increasing effort on harvesting renewable energies. Amongst renewable energy sources, sea wave energy is definitely the one that has been less utilised. This is probably caused by two reasons: i)the strong technical novelties that are required to extract thee energy that is contained in a highly and stochastically variable motion; ii)the difficulty of find a suitable electrical system able to convert the energy contained in the motion of the sea into electrical energy. In this paper we propose an approach to set up a general methodology toward the design of a sea wave farm. The first step is explained how to choose the conversion system, in the second step is discussed the connection scheme among the converters, in the third step is reported a case study of a whole sea wave farm.

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Methodology for tidal turbine representation in ocean circulation model


T. Roc, D.C. Conley, and D. Greaves – Renewable Energy, March, 2013

Abstract

The present method proposes the use and adaptation of ocean circulation models as an assessment tool framework for tidal current turbine (TCT) array layout optimization. By adapting both momentum and turbulence transport equations of an existing model, the present TCT representation method is proposed to extend the actuator disc concept to 3-D large-scale ocean circulation models. Through the reproduction of experimental flume tests and grid dependency tests, this method has shown its numerical coherence as well as its ability to simulate accurately both momentum and turbulent turbine-induced perturbations in both near and far wakes in a relatively short period of computation time. Consequently the present TCT representation method is a very promising basis for the development of a TCT array layout optimization tool.

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Numerical study on coupling effects among multiple Savonius turbines


X Sun, D Luo, D Huang, G Wu – Journal of Renewable and Sustainable Energy, October, 2012

Abstract

A Savonius rotor can be used as a wind or water current energy conversion device that produces electricity. In spite of their simple structure and assembly, Savonius turbines have less commercial appeal than other types of turbines due to their relatively low energy conversion efficiency. In order to increase the output power of a Savonius turbine, most studies have only focused on optimization of the rotor configuration or installation of ancillary equipment around the rotor. However, previous research has found that a beneficial interaction that existed between two parallel Savonius turbines can also augment the power output of each rotor if they are rotating side by side. This paper numerically examines the interactions among multiple Savonius turbines with the help of the commercial computational fluid dynamics software fluent and finds that these coupling effects can effectively increase the overall power output of a Savonius turbine farm, especially when Savonius turbines are arranged relatively close together. Numerical results also indicate that the separation distance and relative phase angle between the two adjacent Savonius rotors can greatly influence this positive interaction between them.

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