Monthly Archives: July 2013

The assessment of extreme wave analysis methods applied to potential marine energy sites using numerical model data


Atul Agarwal, Vengatesan Venugopal, and Gareth P. Harrison – Renewable and Sustainable Energy Reviews, November 2013

Abstract

The accurate estimation of extreme conditions, such as 100-yr return levels of significant wave height is an important aspect in the design of marine energy converters, offshore and coastal structures. This study investigates the different approaches for the estimation of extreme waves that have been applied in the past, and determines the 100-yr return levels using the high resolution ERA-Interim dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). It is demonstrated in the paper that fitting a Generalized Pareto Distribution to all exceedances over a high threshold is the most suitable approach. The estimates thus obtained are compared with previously computed estimates for buoys and offshore platforms. The effect of duration of data on the estimates is also investigated. Finally, a 100-yr return level map for the North Atlantic region is presented.

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The influence of flow acceleration on tidal stream turbine wake dynamics: A numerical study using a coupled BEM–CFD model


Ian Masters, Rami Malki, Alison J. Williams, and T. Nicholas Croft – Applied Mathematical Modelling, June 2013

Abstract

Studies of tidal stream turbine performance and of wake development are often conducted in tow-tanks or in regulated flumes with uniform flows across the turbine. Whilst such studies can be very useful, it is questionable as to what extent the results would differ if the flows were more complex in nature, for instance if the flows were unsteady or non-uniform or even both. This study aims to explore whether the results would be affected once we move away from the uniform flow scenario. A numerical modelling study is presented in which tidal stream turbine performance and wake development in non-uniform flow conditions are assessed. The model implements the Blade Element Momentum method for characterising turbine rotor source terms which are used within a computational fluid dynamics model for predicting the interaction between the turbines and the surrounding flow. The model is applied to a rectangular domain and a range of slopes are implemented for the water surface to instigate an increase in flow velocity along the domain. Within an accelerated flow domain wake recovery occurred more rapidly although rotor performance was not affected.

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Lift and drag characteristics of a cascade of flat plates in a configuration of interest for a tidal current energy converter: Numerical simulations analysis


D. Cebrián, J. Ortega-Casanova, and R. Fernandez-Feria – J. Renewable Sustainable Energy, 2013

Abstract

Numerical simulations of the three-dimensional flow through a cascade of flat plates are conducted to analyze its lift and drag characteristics in a configuration of interest for a particular type of tidal hydrokinetic energy converter. To that end, the turbulent model parameters in the computational fluid dynamics code are validated against experimental data for the flow around an isolated plate at different angles of incidence and the same Reynolds number used in the cascade. The lift and drag coefficients of a plate in the cascade, as well as the effective nondimensional power extracted from the tidal current, are compared to the corresponding values for an isolated plate. These results are used as a guide for the design of the optimum configuration of the cascade (angle of attack, blade speed, and solidity) which extracts the maximum power from a tidal current for a given reference value of the Reynolds number.

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On wave energy focusing and conversion in open water


Umesh A. Korde and R. Cengiz Ertekin – Renewable Energy, February 2014

Abstract

This paper investigates wave energy conversion in open water where the goal is to utilize the wave-field focusing effect of a stationary disc submerged a short depth beneath the water surface. Dynamic interaction of the disc with additional coupled, submerged inertias is used to minimize its oscillation. The method used to enable and extend this favorable dynamic coupling is discussed here. An oscillating water column in a submerged duct attached under a small circular opening in the disc and driven by the wave-field over the disc is used for wave energy conversion. Non-real-time reactive control of the water column response to enhance energy absorption is examined. Added mass, radiation damping, and exciting force values for the submerged disc are computed, and the focusing effect of a submerged stationary disc is confirmed with numerical calculations of surface elevation over the disc. Calculations of hydrodynamic performance suggest that energy absorption from the oscillating water column is significantly greater under control holding the disc stationary, and can be improved further by applying reactive loads tuned to the optimal susceptance and conductance associated with the oscillating water column. Although the control forces involved in holding the disc stationary may be large at lower wave numbers, the maximum deflection amplitudes of the compensation system are found to be within reasonable limits.

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Time domain model for a two-body heave converter: Model and applications


A.D. de Andrés, R. Guanche, J.A. Armesto, F. del Jesus, C. Vidal, and I.J. Losada – Ocean Engineering, November 2013

Abstract

This study presents a methodology to obtain the power performance of a two-body heave converter. First, the methodology relies on a time domain model which represents the motion of the two bodies throughout the time. This time model was built substituting the entire equation system with a state-space system, thereby avoiding the convolution integral of the radiation force. This technique is demonstrated to be a reliable and very efficient method in terms of speed. Then, based on this model the instantaneous power of the device can be obtained. The performance of the device is shown through the power production matrix and the 60 year series power production statistics. This long series is obtained by means of using the MaxDiss selection technique in order to compute only the power of the most representative sea states and the Radial Basis Function (RBF) interpolation technique which interpolates in order to obtain the complete power series.

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Modeling, Simulation, and Analysis of Two Hydraulic Power Take-off Systems for Wave Energy Conversion


S. Casey – Masters Thesis, Oregon State University, June 2013

Abstract

Hydraulic power take-off (PTO) systems have been implemented in several wave energy converter (WEC) designs in recent years. Two popular hydraulic PTO configurations coupled to a point absorber hydrodynamic model are simulated in waves representative of an energetic sea state likely to be found in deep waters off the coast of Oregon. The first hydraulic PTO configuration is a passive sys­tem tuned to the dominant forcing period of the sea state. The second system is an actively controlled hydraulic PTO topology tracking an optimal power absorption trajectory. A linear quadratic tracking controller is developed to follow the reference trajectory and performs well despite the nonlinear elements of the system dynamics. Simulation results of the system dynamics are presented for both models. A loading analysis is conducted and loading distributions of the two systems in irregular waves are compared. The distributions show the active system to have a larger variability in loading, while the distributions of the passive sys­tem indicate more frequent mean loading components. Parameters of the passive system components are varied in order to understand the effect on power output and loading.

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Marine current energy resource assessment and design of a marine current turbine for Fiji


Jai N. Goundar and M. Rafiuddin Ahmed – Renewable Energy, July 2013

Abstract

Pacific Island Countries (PICs) have a huge potential for renewable energy to cater for their energy needs. Marine current energy is a reliable and clean energy source. Many marine current streams are available in Fiji’s waters and large amount of marine current energy can be extracted using turbines. Horizontal axis marine current turbine (HAMCT) can be used to extract marine current energy to electrical energy for commercial use. For designing a HAMCT, marine current resource assessment needs to done. A potential site was identified and resource assessment was done for 3 months. Continue reading

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Turbulent inflow characteristics for hydrokinetic energy conversion in rivers


V.S. Neary, B. Gunawan, and D.C. Sale – Renewable and Sustainable Energy Reviews, October 2013

Abstract

Marine and hydrokinetic technologies, which convert kinetic energy from currents in open-channel flows to electricity, require inflow characteristics (e.g. mean velocity and turbulence intensity profiles) for their siting, design, and evaluation. The present study reviews mean velocity and turbulence intensity profiles reported in the literature for open-channel flows to gain a better understanding of the range of current magnitudes and longitudinal turbulence intensities that these technologies may be exposed to. We compare 47 measured vertical profiles of mean current velocity and longitudinal turbulence intensity (normalized by the shear velocity) that have been reported for medium-large rivers, a large canal, and laboratory flumes with classical models developed for turbulent flat plate boundary layer flows. The comparison suggests that a power law (with exponent, 1/a=1/61/a=1/6) and a semi-theoretical exponential decay model can be used to provide first-order approximations of the mean velocity and turbulence intensity profiles in rivers suitable for current energy conversion. Over the design life of a current energy converter, these models can be applied to examine the effects of large spatiotemporal variations of river flow depth on inflow conditions acting over the energy capture area. Significant engineering implications on current energy converter structural loads, annual energy production, and cost of energy arise due to these spatiotemporal variations in the mean velocity, turbulence intensity, hydrodynamic force, and available power over the energy capture area.

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Numerical Simulation of Fully Passive Flapping Foil Power Generation


John Young, Muhammad A. Ashraf, Joseph C. S. Lai, and Max F. Platzer – AIAA Journal, 2013

A fully passive flapping foil turbine was simulated using a two-dimensional Navier–Stokes solver with two-way fluid-structure interaction at a Reynolds number based on freestream flow Re=1100 and 1.1×106 with a NACA 0012 foil. Both pitch angle and angle-of-attack control methodologies were investigated. Efficiencies of up to 30% based on the Betz criterion were found using pitch control, which is commensurate with values reported in the literature for prescribed motion studies. Nonsinusoidal foil pitching motions were found to be superior to sinusoidal motions. Efficiencies exceeding 41% were found using angle-of-attack control, and nonsinusoidal angle-of-attack profiles were found to be superior. The key to improving the efficiency of energy extraction from the flow is to control the timing of the formation and location of the leading-edge vortex at crucial times during the flapping cycle and the interaction of the vortex with the trailing edge. Simulations using Reynolds-averaged Navier–Stokes turbulence modeling suggest that the performance is maintained or only slightly reduced at Re=1.1×106.

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The Influence of Flow Acceleration on Tidal Stream Turbine Wake Dynamics: A Numerical Study Using a Coupled BEM-CFD model


Ian Masters, Rami Malki, Alison J. Williams, and T.N. Croft – Applied Mathematical Modeling, June 2013

Abstract

Studies of tidal stream turbine performance and of wake development are often conducted in tow-tanks or in regulated flumes with uniform flows across the turbine. Whilst such studies can be very useful, it is questionable as to what extent the results would differ if the flows were more complex in nature, for instance if the flows were unsteady or non-uniform or even both. This study aims to explore whether the results would be affected once we move away from the uniform flow scenario. A numerical modelling study is presented in which tidal stream turbine performance and wake development in non-uniform flow conditions are assessed. The model implements the Blade Element Momentum method for characterising turbine rotor source terms which are used within a Computational Fluid Dynamics model for predicting the interaction between the turbines and the surrounding flow. The model is applied to a rectangular domain and a range of slopes are implemented for the water surface to instigate an increase in flow velocity along the domain. Within an accelerated flow domain wake recovery occurred more rapidly although rotor performance was not affected.

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