Monthly Archives: January 2013

Tidal Current Energy Resources off the South and West Coasts of Korea: Preliminary Observation-Derived Estimates


D-S Byun, D.E. Hart, and W-J Jeong – Energies – January, 2013

Abstract

In this study we estimate the prospective tidal current energy resources off the south and west coasts of Korea and explore the influence of modeling tidal current energies based on 15-day versus month-long data records for regimes with pronounced perigean/apogean influences. The tidal current energy resources off southern and western Korea were calculated using 29-day in situ observation data from 264 stations. Continue reading

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A Study on Performance of Savonius Tidal Current Turbine


K-W. Ko, C-B. Park, Y. Ryu, K-H. Jung, C.H. Jo – Advanced Science Letters, June, 2013

Abstract

The present study carries out numerical and experimental investigations on the performance of a vertical axis tidal current turbine of drag force type (Savonius turbine). For the performance test, firstly, various numbers of turbine blades are tested. Secondly, the turbine with double layered blades were tested and compared with the one with a single layer. The vertical axis turbine in this study has arch-shaped blades. While the numerical simulations were conducted with a commercial computational fluid dynamics (CFD) program, the experiments were conducted in a circulating water channel. From both approaches, parameters representing the performance of the turbine such as power coefficient and torque are examined. The number of blades and layers which is closely related to the drag force seems important to the Savonius turbine. Based on the validation between the experiments and the numerical simulations, the numerical tests were extended to examine the performance from various cases. The influence of the layer height was also discussed from the experimental results.

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Effect of the generator sizing on a wave energy converter considering different control strategies


L. Alberti, E. Tedeschi, N. Bianchi, M. Santos, A. Fasolo – COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering, 2013

Abstract

The purpose of this paper is to investigate the impact of control strategy selection on the power performance of wave energy converters for different ratings of the Power Take-Off (PTO) system. The case of a point absorber equipped with an all-electric PTO is considered. The effect of control techniques and electrical generator design is analyzed from a theoretical standpoint and then verified through integrated hydrodynamic-electric simulations. It has been proved that control parameters that maximize the power extraction from the waves can be derived based on the power and torque constraints imposed by the electrical machine. An optimized and integrated approach to the control strategy selection and generator design for point absorbers has been presented, which maximizes the electric power generation from sea waves under real conditions and represents a good trade-off for the PTO from both the technical and the economic standpoint.

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Optimizing the shape of rotor blades for maximum power extraction in marine current turbines


H.R. Karbasian and J.A. Esfahani – International Journal of Automotive and Mechanical Engineering, December, 2012

Abstract

In this paper the shape of rotor blades in Marine Current Turbines (MCTs) are investigated. The evaluation of hydrodynamic loads on blades is performed base on the Blade Element Momentum (BEM) theory. According to main parameters in configurations and operations of these devices the shape of blades are optimized. The optimization is conducted based on the ability of blades to harness the maximum energy during operation conditions. The main parameters investigated here are tip speed ratio and angle of attack. Furthermore, the influence of these considered parameters on the maximum energy extraction from fluid flow over hydrofoil are evaluated. It is shown the effect of angle of attack on power extraction is more than that of tip speed ratio, while both of them are found to be noticeable. Additionally, the proper angle of attack is the angle at which the lift to drag ratio is maximum value. However, if a proper angle of attack is chosen, the variations of power coefficient would not be effectively changed with small variations on tip speed ratio.

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Studying the Effect of Class Imbalance in Ocean Turbine Fault Data on Reliable State Detection


J. Duhaney, T. Koshgoftaar, and A. Napolitano – 11th International Conference on Machine Learning and Applications (ICMLA), December, 2012

Abstract

Class imbalance is prevalent in many real world datasets. It occurs when there are significantly fewer examples in one or more classes in a dataset compared to the number of instances in the remaining classes. When trained on highly imbalanced datasets, traditional machine learning techniques can often simply ignore the minority class(es) and label all instances as being of the majority class to maximize accuracy. This problem has been studied in many domains but there is little or no research related to the effect of class imbalance in fault data for condition monitoring of an ocean turbine. This study makes the first efforts in bridging that gap by providing insight into how class imbalance in vibration data can impact a learner’s ability to reliably identify changes in the ocean turbine’s operational state. To do so, we empirically evaluate the performances of three popular, but very different, machine learning algorithms when trained on four datasets with varying class distributions (one balanced and three imbalanced) to distinguish between a normal and an abnormal state. All data used in this study were collected from the testbed for an ocean turbine and were under sampled to simulate the different levels of imbalance. We find here, as in other domains, that the three learners seemed to suffer overall when trained on data with a highly skewed class distribution (with 0.1% examples in a faulty/abnormal state while the remaining 99.9% were captured in a normal operational state). It was noted, however, that the Logistic Regression and Decision Tree classifiers performed better when only 5% of the total number of examples were representative of an abnormal state (the remaining 95% therefore indicating normal operation) than they did when there was no imbalance present.

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Assessment of arrays of in-stream tidal turbines in the Bay of Fundy


R. Karsten, A. Swan and J. Culina – Phil. Trans. R. Soc. A, 2013

Abstract

Theories of in-stream turbines are adapted to analyse the potential electricity generation and impact of turbine arrays deployed in Minas Passage, Bay of Fundy. Linear momentum actuator disc theory (LMADT) is combined with a theory that calculates the flux through the passage to determine both the turbine power and the impact of rows of turbine fences. For realistically small blockage ratios, the theory predicts that extracting 2000–2500 MW of turbine power will result in a reduction in the flow of less than 5 per cent. The theory also suggests that there is little reason to tune the turbines if the blockage ratio remains small. A turbine array model is derived that extends LMADT by using the velocity field from a numerical simulation of the flow through Minas Passage and modelling the turbine wakes. The model calculates the resulting speed of the flow through and around a turbine array, allowing for the sequential positioning of turbines in regions of strongest flow. The model estimates that over 2000 MW of power is possible with only a 2.5 per cent reduction in the flow. If turbines are restricted to depths less than 50 m, the potential power generation is reduced substantially, down to 300 MW. For large turbine arrays, the blockage ratios remain small and the turbines can produce maximum power with a drag coefficient equal to the Betz-limit value.

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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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Modelling of the flow field surrounding tidal turbine arrays for varying positions in a channel


T. Daly, L.Myers and A. Bahaj – Phil. Trans. R. Soc. A, 2013

Abstract

The modelling of tidal turbines and the hydrodynamic effects of tidal power extraction represents a relatively new challenge in the field of computational fluid dynamics. Many different methods of defining flow and boundary conditions have been postulated and examined to determine how accurately they replicate the many parameters associated with tidal power extraction. This paper outlines the results of numerical modelling analysis carried out to investigate different methods of defining the inflow velocity boundary condition. This work is part of a wider research programme investigating flow effects in tidal turbine arrays. Results of this numerical analysis were benchmarked against previous experimental work conducted at the University of Southampton Chilworth hydraulics laboratory. Results show significant differences between certain methods of defining inflow velocities. However, certain methods do show good correlation with experimental results. This correlation would appear to justify the use of these velocity inflow definition methods in future numerical modelling of the far-field flow effects of tidal turbine arrays.

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A large-eddy simulation study of wake propagation and power production in an array of tidal-current turbines


M. Churchfield, Y. Li and P. Moriarty – Phil. Trans. R. Soc. A, 2013

Abstract

This paper presents our initial work in performing large-eddy simulations of tidal turbine array flows. First, a horizontally periodic precursor simulation is performed to create turbulent flow data. Then those data are used as inflow into a tidal turbine array two rows deep and infinitely wide. The turbines are modelled using rotating actuator lines, and the finite-volume method is used to solve the governing equations. In studying the wakes created by the turbines, we observed that the vertical shear of the inflow combined with wake rotation causes lateral wake asymmetry. Also, various turbine configurations are simulated, and the total power production relative to isolated turbines is examined. We found that staggering consecutive rows of turbines in the simulated configurations allows the greatest efficiency using the least downstream row spacing. Counter-rotating consecutive downstream turbines in a non-staggered array shows a small benefit. This work has identified areas for improvement. For example, using a larger precursor domain would better capture elongated turbulent structures, and including salinity and temperature equations would account for density stratification and its effect on turbulence. Additionally, the wall shear stress modelling could be improved, and more array configurations could be examined.

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