Monthly Archives: July 2012

Analytical study of the interaction between waves and cylindrical wave energy converters oscillating in two modes


H Heikkinen, MJ Lampinen, J Boling – Renewable Energy, February 2013

Abstract
Ocean wave energy may be recovered by oscillating wave energy converters. The energy converter studied in this work is a horizontally orientated cylinder which can be placed at different depths in the sea. The cylinder can oscillate in horizontal and vertical directions and transfer mechanical energy forward by hydraulic cylinders. To study the interaction between the waves and the converter, we have used potential flow theory separately for both the waves and the oscillating cylinder, and then combined these potential functions by using the principle of superposition. Combined potential flow fields, together with Euler’s equations, enable us to obtain the pressure distribution around the cylinder. When knowing the pressure distribution, both the force upon the cylinder, and the net mechanical power transferred from the waves to the moving cylinder, can be calculated. With this model we have analyzed several interesting topics which affect the efficiency of the wave energy converter. The phase shift is the most important parameter – with the phase shift -π/2 the best efficiency 0.5 was achieved. To achieve the right phase shift for different waves is essential due to the power capture. Furthermore, it is shown that feedback control is necessary for keeping the phase shift constant. Also the cylinder radius has a great effect on the efficiency. The other important parameters studied in this work were the wave height and the wave period.

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Dynamics and optimization of the OWC spar buoy wave energy converter


AFO Falcão, JCC Henriques, JJ Cândido – Renewable Energy, December 2012

Abstract
The paper concerns the hydrodynamic analysis and optimization of an OWC spar buoy, possibly the simplest concept for a floating oscillating water column (OWC) wave energy converter. It is an axisymmetric device consisting basically of a submerged vertical tail-tube-fixed to an axisymmetric floater that oscillates essentially in heave. The air flow displaced by the water motion inside the tube drives a self-rectifying air turbine. The possible advantages of using a tube of non-uniform inner cross section are investigated theoretically and numerically, especially as a way of reducing the draught of the device without significantly impairing its power performance. The unsteady water flow in the tube is modelled as one-dimensional. The frequency-dependent hydrodynamic coefficients of the tube-floater pair were computed with a boundary-element code. A linear air turbine is assumed. The hydrodynamics of the wave energy absorption is analysed in the frequency domain, including the effect of air compressibility in the chamber; special attention is devoted to optimization. Numerical results are presented for device’s performance in regular and irregular waves, including especially optimization of the tube geometry and of the turbine characteristic. Practical implications of these results are discussed.

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The TSE index – A new tool for selecting tidal stream sites in depth-limited regions


G Iglesias, M Sánchez, R Carballo, H Fernández – Renewable Energy, December 2012

Abstract
In many regions strong tidal flow occurs only in areas with restricted water depths, typically within estuaries or rias. Although in some of these areas the depth constraint may preclude the exploitation of this resource, in others it is exploitable – and substantial. The objective of this work is twofold: (i) to develop a tool, the Tidal Stream Exploitability (TSE) index, to facilitate the selection of tidal power sites in depth-limited zones, and (ii) to demonstrate it with a case study. The TSE index combines the flow and water depth information so that the areas with potential as prospective tidal power sites present large TSE values. On the contrary, areas of lesser interest – of weak flow, too shallow, or both – have small TSE values. In the case study (Ria de Ortigueira, a large estuary in NW Spain) a numerical model of the hydrodynamics is implemented. Once validated based on field data, the model is used to compute the flow velocity and power density in the estuary at different moments of the tide. Two areas present high values of power density. One is unsuitable for a tidal stream power plant due to its shallowness; the other, which does have sufficient water depth, clearly stands out in the TSE map. Thus, the TSE index is shown to facilitate the selection of tidal stream sites in depth-limited regions.

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Reliability analysis for hydrokinetic turbine blades


Z Hu, X Du – Renewable Energy, December 2012

Abstract
Reliability is an important element in the performance of hydrokinetic turbines. It is also a driving factor of the system lifetime cost. In this paper, we perform time-dependent reliability analysis for the blades of a river-based horizontal-axis hydrokinetic turbine. Based on the stochastic representation of the monthly river velocity and material strength, a limit-state function is established with the classical blade element momentum method. In the limit-state function, a failure is defined as the event when the flapwise bending moment exceeds the allowable moment that corresponds to the ultimate strength of the material. The upcrossing rate method is employed to calculate the time-dependent reliability of the hydrokinetic turbine blade over its design life period. The results indicate that setting a proper cut-out river velocity is important for the reliability of the hydrokinetic turbine blade.

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Economic modelling of the potential of wave energy


J Hayward, S Behrens, S McGarry, P Osman – Renewable Energy, December 2012

Highlights
► Globally up to 500 GW of wave energy capacity is projected to be installed by 2050. ► In Australia the majority of wave farms are projected to be deployed in Victoria. ► A dispatchable power scenario increases wave energy generation by 22% in Australia. ► Wave energy could supply approx 11% of Australia’s electricity generation by 2050.

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Productivity and economic assessment of wave energy projects through operational simulations


B Teillant, R Costello, J Weber, J Ringwood – Renewable Energy, December 2012

Highlights
► We present a methodology for the assessment of the energy productivity and economic value of ocean wave energy schemes. ► Use of detailed operational simulations developed to generate estimates of operational costs and device availability. ► We perform and illustrate a sample example which tests the assessment for a variety of maintenance activities. ► We show it is possible to identify the areas that are of most influence on the economic performance.

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Keywords Wave energy; Operational simulations; Productivity; Costs; Economic value

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The energetics of large tidal turbine arrays


R Vennell – Renewable Energy, December 2012

Highlights
► Turbines in large tidal straits may exceed the Betz limit on efficiency as they fill the channel’s cross-section. ► The efficiency of turbines in shallow channels falls below the Betz limit as they fill the cross-section. ► Maximizing the power production of large farms is not the same as maximizing the conversion efficiency of the turbines. ► The power produced by each turbine decreases as rows are added to a farm. ► The fraction of total power lost by the flow which is lost to bottom friction deceases as a farm expands.

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Investigation of macro-turbulent flow structures interaction with a vertical hydrokinetic river turbine


AH Birjandi, J Woods, EL Bibeau – Renewable Energy, December 2012

Highlights
► Velocity measurements upstream of a 25-kW vertical turbine in a river are obtained. ► Solutions for ADV signal quality and signal despiking provided. ► Inflow is non-uniform and contains large eddies in the river. ► Operating turbine increases turbulence intensity and breaks large eddies in upstream. ► Higher turbulence intensity induces higher levels of fatigue damage.

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Analysis and optimization of a tethered wave energy converter in irregular waves


EE Bachynski, YL Young, RW Yeung- Renewable Energy, December 2012

Highlights
► Fundamental dynamic response of moored wave energy converters in 3 degrees of freedom. ► Semi-analytical method to maximize energy extraction and ensure structural safety. ► Important to tune resonance frequencies to site-specific probabilistic wave climate. ► Mooring lines introduce additional low-frequency resonance. ► Low center of gravity and low radius of gyration minimize coupled pitch-surge motion.

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Neural control for voltage dips ride-through of oscillating water column-based wave energy converter equipped with doubly-fed induction generator


M Alberdi, M Amundarain, A Garrido, I Garrido – Renewable Energy, December 2012

Highlights
The most demanded skill during voltage drops is a fault-ride-through capability. ► The innovative aspect exploited in this paper is the application of a neural control. ► The controller changes the references according to the pressure drop and voltage dip. ► The neural controller achieves the uninterrupted operation of the wave energy plant.

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