Tag Archives: Tidal

The role of tidal asymmetry in characterizing the tidal energy resource of Orkney


Simon P. Neill, M. Reza Hashemi, and Matt J. Lewis – Renewable Energy, August 2014

Abstract

When selecting sites for marine renewable energy projects, there are a wide range of economical and practical constraints to be considered, from the magnitude of the resource through to proximity of grid connections. One factor that is not routinely considered in tidal energy site selection, yet which has an important role in quantifying the resource, is tidal asymmetry, i.e. variations between the flood and ebb phases of the tidal cycle. Here, we present theory and develop a high-resolution three-dimensional ROMS tidal model of Orkney to examine net power output for a range of sites along an energetic channel with varying degrees of tidal asymmetry. Since power output is related to velocity cubed, even small asymmetries in velocity lead to substantial asymmetries in power output. We also use the 3D model to assess how tidal asymmetry changes with height above the bed, i.e. representing different device hub heights, how asymmetry affects turbulence properties, and how asymmetry is influenced by wind-driven currents. Finally, although there is minimal potential for tidal phasing over our study site, we demonstrate that regions of opposing flood- versus ebb-dominant asymmetry occurring over short spatial scales can be aggregated to provide balanced power generation over the tidal cycle.

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A review on flow energy harvesters based on flapping foils


Qing Xiao and Qiang Zhu – Journal of Fluids and Structures, April 2014

Abstract

This article presents an overview of the state of the art investigations on the recently developed oscillating foil energy converters. A summary of available knowledge and up-to-date progress in the application of such bio-inspired systems for renewable energy devices is provided. Starting from concepts and achieved results in three distinguishable categories, various parametric studies are reviewed, along with an in-depth discussion on the potential device performance enhancement via flow control mechanisms. Finally, potential future research directions are discussed.

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Power extraction using flow-induced vibration of a circular cylinder placed near another fixed cylinder


Yoshiki Nishi, Yuta Ueno, Masachika Nishio, Luis Antonio Rodrigues Quadrante, and Kentaroh Kokubun – Journal of Sound and Vibration, May 2014

Abstract

We conducted an experiment in a towing tank to investigate the performance of an energy extraction system using the flow-induced vibration of a circular cylinder. This experiment tested three different cases involving the following arrangements of cylinder(s) of identical diameter: the upstream fixed–downstream movable arrangement (case F); the upstream movable–downstream fixed arrangement (case R); and a movable isolated cylinder (case I). In cases F and R, the separation distance (ratio of the distance between the centers of the two cylinders to their diameters) is fixed at 1.30. Measurement results show that while cases F and I generate vortex-induced vibration (VIV) resonance responses, case R yields wake-induced vibration (WIV) at reduced velocity over 9.0, which is significantly larger than that of the VIV response, leading to the induction of higher electronic power in a generator. Accordingly, primary energy conversion efficiency is higher in the case involving WIV.

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Experimental study of the turbulence intensity effects on marine current turbines behaviour – Part I: One single turbine


Paul Mycek, Benoît Gaurier, Grégory Germain, Grégory Pinon, and Elie Rivoalen – Renewable Energy, February 2014

Abstract

The ambient turbulence intensity in the upstream flow plays a decisive role in the behaviour of horizontal axis marine current turbines. Experimental trials, run in the IFREMER flume tank in Boulogne-Sur-Mer (France) for two different turbulence intensity rates, namely 3% and 15%, are presented. They show, for the studied turbine configuration, that while the wake of the turbine is deeply influenced by the ambient turbulence conditions, its mean performances turn out to be slightly modified. The presented conclusions are crucial in the view of implanting second generation turbines arrays. In addition, complete and detailed data sets (wake profiles and performance graphs) are made available to the scientific community in order to encourage further comparisons.

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Experimental study of the turbulence intensity effects on marine current turbines behaviour – Part II: Two interacting turbines


Paul Mycek, Benoît Gaurier, Grégory Germain, Grégory Pinon, and Elie Rivoalen – Renewable Energy, February 2014

Abstract

The future implantation of second generation marine current turbine arrays depends on the understanding of the negative interaction effects that exist between turbines in close proximity. This is especially the case when the turbines are axially aligned one behind another in the flow. In order to highlight these interaction effects, experiments were performed in a flume tank on 3-bladed 1/30th scale prototypes of horizontal axis turbines. Continue reading

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Energy Extraction from Shallow Tidal Flows


Jack William Giles – University of Southampton, PhD Dissertation, December 2013

Abstract

Over the past decade within the renewable energy sector a strong research and development focus has resulted in the growth of an embryonic tidal stream energy industry. Previous assessments of the tidal stream resource appear to have neglected shallow tidal flows. This resource located in water depths of 10-30m is significant because it is generally more accessible for energy extraction than deeper offshore tidal sites and hence a good location for first generation tidal stream arrays or fences. The close proximity to shore may lead to improvements in construction feasibility and economic prospects. The objective of this project is to investigate several aspects concerning the exploitation of shallow tidal flows for energy extraction. Fundamental to this project is the importance of developing research alongside and in conjunction with industrial shallow water
prototype projects. Continue reading

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Tidal Turbine Representation in an Ocean Circulation Model: Towards Realistic Applications


Thomas Roc, Deborah Greaves, Kristen M. Thyng, and Daniel C. Conley – Ocean Engineering, March 2014

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, this method has shown its ability to simulate accurately both momentum and turbulent wake interactions. In addition, through an up-scaling test, this method has shown to be applicable at any scale. Thanks to its short computational time, the present TCT representation method is a very promising basis for the development of a TCT array layout optimization tool. Furthermore, on the basis of the simulations performed for the present publication, a reflection on the quantification of the array layout effects on power assessment and device deployment strategy has been initiated.

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An Adaptive Nonlinear MPPT Controller for Stand Alone Marine Current Energy Conversion Systems


Khan, N.; Rabbi, S.F. ; Hinchey, M.J. ; Rahman, M.A. – 39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013, November 2013

Abstract

This paper presents an online estimation based adaptive nonlinear maximum power point tracking (MPPT) controller for a stand-alone permanent magnet synchronous generator (PMSG) based marine current energy conversion system. The proposed control strategy requires no flow sensor and also does not need the parameters of a PMSG. A nonlinear control algorithm for the load side converter to extract maximum power has been proposed to adequately deal with the inherent nonlinearities in the energy conversion system. A Lyapunov based online estimation approach is used to continually estimate the time varying input voltage and the output load resistance of the converter. Detailed simulation results of the proposed nonlinear controller namely adaptive backstepping are presented and fully analyzed. Simulation results demonstrate that the proposed nonlinear controller can incessantly extract maximum power from the ocean current at various flow speeds.

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Preliminary Design of Axial Flux Permanent Magnet Machine for Marine Current Turbine


Ju Hyung Kim and B. Sarlioglu – 39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013, November 2013

Abstract

This paper presents an analytical analysis and application of axial flux permanent magnet machine design to marine current turbine. Marine current is a prominent renewable energy source because of its predictable current profile. Rim-driven, direct-drive type is favorable for the marine current turbine application in terms of maintenance and reliability. The axial flux permanent magnet machine is a candidate machine type that not only reduces machine manufacturing costs, but also achieves high performance. The general sizing equation technique is implemented to produce the machine size. The magnetic flux relationship among air gap, stator yoke, rotor yoke, and permanent magnet is taken into account to calculate core material size of the machine. FEA simulation results are compared with analytical calculations. The proposed machine and the reference machine design results are briefly compared. Moreover, machine cost is estimated based on the volume of the proposed machine.

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Experimental study of abrasion characteristics for critical sliding components for use in hydrokinetic devices


A. Bromaghin, M. Ali, T. Ravens, T. Petersen, and J. Hoffman – Renewable Energy, June 2014

Abstract

Hydrokinetic devices have lately reemerged as a promising solution for harnessing energy from renewable sources such as rivers, tidal currents, or artificial channels. This paper describes a customized test flume that is capable of conducting tribological related experiments on sliding components (bearing, shaft, and generator seals) commonly used in hydrokinetic devices. Often while deployed, wear on bearing, shaft, and seal assemblies introduces undesirable clearances between contacting surfaces, which potentially can affect the performance of hydrokinetic devices. Continue reading

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