Monthly Archives: February 2013

Offshore wind turbine risk quantification/evaluation under extreme environmental conditions


A.A. Taflanidis, E. Loukogeorgaki, and D.C. Angelides – Reliability Engineering & System Safety, February, 2013

Abstract

A simulation-based framework is discussed in this paper for quantification/evaluation of risk and development of automated risk assessment tools, focusing on applications to offshore wind turbines under extreme environmental conditions. The framework is founded on a probabilistic characterization of the uncertainty in the models for the excitation, the turbine and its performance. Risk is then quantified as the expected value of the risk consequence measure over the probability distributions considered for the uncertain model parameters. Stochastic simulation is proposed for the risk assessment, corresponding to the evaluation of the associated probabilistic integral quantifying risk, as it allows for the adoption of comprehensive computational models for describing the dynamic turbine behavior. For improvement of the computational efficiency, a surrogate modeling approach is introduced based on moving least squares response surface approximations. The assessment is also extended to a probabilistic sensitivity analysis that identifies the importance of each of the uncertain model parameters, i.e. risk factors, towards the total risk as well as towards each of the failure modes contributing to this risk. The versatility and computational efficiency of the advocated approaches is finally exploited to support the development of standalone risk assessment applets for automated implementation of the probabilistic risk quantification/assessment.

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Evaluation of tidal stream resource in a potential array area via direct measurements


I. Fairley, P. Evans, C. Wooldridge, M. Willis, and I. Masters – Renewable Energy, September, 2013

Abstract

ADCP transects have used to characterise tidal stream resources in the Ramsey Sound area of Pembrokeshire, UK. Previous resource assessments have previously suggested that this area is one of the most promising for tidal stream deployments in the UK and this contribution confirms the commercial viability of the area. In this study three channels were considered: Ramsey Sound itself and two channels to the west formed by small offshore islands. Current velocities were used to compute the tidal energy flux through the channels. Maximum instantaneous peak flux through the three channels ranges from 180 MW to 70 MW. Flux cross-sections are presented and the impact of meso-scale bathymetric features on flux and on the cross-transect variation of maximum flux over the tidal cycle is described and discussed. Theoretical values of extractable power potential are calculated and range between 7.2 MW and 21.8 MW. These values are approximately ¼ of the average flux through the measured cross-section. One channel is identified as being preferable for the first stage of array deployments given greatest homogeneity of flux through the channel cross-section and it having the highest power potential.

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Filed under Field Measurements, Resource Assessment, Resource Characterization

Multicylinder flow-induced motions: Enhancement by passive turbulence control at 28,000<Re<120,000


ES Kim, MM Bernitsas and RA Kumar – J. Offshore Mech. Arct. Eng., February, 2013

Abstract

The VIVACE converter was introduced at OMAE 2006 as a single, smooth, circular-cylinder module. The hydrodynamics of VIVACE is being improved continuously to achieve higher density in harnessed hydrokinetic power. Intercylinder spacing and passive turbulence control (PTC) through selectively located roughness are effective tools in enhancement of flow induced motions (FIMs) under high damping for power harnessing. Single cylinders harness energy at high density even in 1 knot currents. For downstream cylinders, questions were raised on energy availability and sustainability of high-amplitude FIM. Through PTC and intercylinder spacing, strongly synergetic FIMs of 2/3/4 cylinders are achieved. Two-cylinder smooth/PTC, and three/four-cylinder PTC systems are tested experimentally. Using the “PTC-to-FIM” map developed in previous work at the Marine Renewable Energy Laboratory (MRELab), PTC is applied and cylinder response is measured for inflow center-to-center distance 2D-5D (D = diameter), transverse center-to-center distance 0.5–1.5 D, Re ε [28,000–120,000], m* ε [1.677–1.690], U ε [0.36–1.45 m/s], aspect ratio l/D = 10.29, and m*ζ ε [0.0283–0.0346]. All experiments are conducted in the low turbulence free surface water (LTFSW) channel of MRELab. Amplitude spectra and broad field-of-view (FOV) visualization help reveal complex flow structures and cylinder interference undergoing VIV, interference/ proximity/wake/soft/hard galloping. FIM amplitudes of 2.2–2.8D are achieved for all cylinders in steady flow for all parameter ranges tested.

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Numerical modeling on hydrodynamic performance of a bottom-hinged flap wave energy converter


Hai-tao Zhao, Zhi-lin Sun, Chun-ling Hao, Jia-fa Shen – China Ocean Engineering, March 2013

Abstract

The hydrodynamic performance of a bottom-hinged flap wave energy converter (WEC) is investigated through a frequency domain numerical model. The numerical model is verified through a two-dimensional analytic solution, as well as the qualitative analysis on the dynamic response of avibrating system. The concept of “optimum density” of the bottom-hinged flap is proposed, and its analytic expression is derived as well. The frequency interval in which the optimum density exists is also obtained. The analytic expression of the optimum linear damping coefficient is obtained by a bottom-hinged WEC. Some basic dynamic properties involving natural period, excitation moment, pitch amplitude, and optimum damping coefficient are analyzed and discussed in detail. In addition, this paper highlights the analysis of effects on the conversion performance of the device exerted by some important parameters. The results indicate that “the optimum linear damping period of 5.0 s” is the most ideal option in the short wave sea states with the wave period below 6.0 s. Shallow water depth, large flap thickness and low flap density are advised in the practical design of the device in short wave sea states in order to maximize power capture. In the sea state with water depth of 5.0 m and wave period of 5.0 s, the results of parametric optimization suggest a flap with the width of 8.0 m, thickness of 1.6 m, and with the density as little as possible when the optimum power take-off (PTO) damping coefficient is adopted.

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Wave power variability over the northwest European shelf seas


S.P. Neill and M.R. Hashemi – Applied Energy, June 2013

Abstract

Regional assessments of the wave energy resource tend to focus on averaged quantities, and so provide potential developers with no sense of temporal variability beyond seasonal means. In particular, such assessments give no indication of inter-annual variability – something that is critical for determining the potential of a region for wave energy convertor (WEC) technology. Here, we apply the third-generation wave model SWAN (Simulating Waves Nearshore) at high resolution to assess the wave resource of the northwest European shelf seas, an area where many wave energy test sites exist, and where many wave energy projects are under development. Continue reading

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Filed under Resource Assessment, Resource Characterization

Wave energy resource assessment in Lanzarote (Spain)


J.P. Sierra, D. González-Marco, J. Sospedra, X. Gironella, C. Mösso, and A. Sánchez-Arcilla – Renewable Energy, July, 2014

Abstract

Lanzarote (Canary Islands, Spain) is a UNESCO Biosphere Reserve (since 1993) located in the Atlantic Ocean. The island is aiming to change its energy production model in order to reduce its dependence on external, fossil-fuel-based energy sources. The local authorities hope to develop an energy production model based on clean, renewable sources, such as wave energy converters (WECs). This study analyses the island’s wave energy resources using a 51-year series of data obtained from numerical modelling (hindcast and forecast). The spatial distribution of wave power is analysed using data from nine points around the island. Significant resources (average wave power exceeding 30 kW/m and average annual wave energy of more than 270 MW h/m) are found to the north of the island, as well as to the west and the east (average wave power 25–30 kW/m). Considerable seasonal variability is found, with winters being rather high-energetic and summers quite mild. Variability coefficients are computed in order to select the best locations for WECs; the composition of the resource at each location is examined in terms of sea states in order to evaluate the suitability of WEC installation. Finally, three sites with similar conditions, all located on the north side of the island, are selected as the best candidates for WEC deployment.

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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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FSI analysis of deformation along offshore pile structure for tidal current power


C-H Jo, D-Y Kim, Y-H Rho, K-H Lee, Cameron Johnstone – Renewable Energy, June, 2013

Abstract

Due to global warming, the need to secure an alternative clean energy resource has become an international issue. Tidal current power is now recognized as one of the clean power resources in Korea, where there are many strong current regions on the west and south coasts. Recently, large scale tidal devices have been deployed with a maximum rotor diameter of 18 m. These devices impose significant loading on supporting structures. In many cases, a pile fixed foundation is used to secure the structure. However, due to the high density of seawater, the drag and lift forces are much larger than in air, causing extensive stress and deflection to the pile tower structure. In this study, a numerical analysis of the hydro-forces from a rotating tidal current turbine to a tower was conducted to determine the deformation distribution along the pile tower.

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Implementing double fed induction generator for converting ocean wave power to electrical


E. Enferad and D. Nazarpour – 4th Power Electronics, Drive Systems and Technologies Conference (PEDSTC), February, 2013

Abstract

Nowadays renewable sources are being considered as potential power source for increasing world power demand and this power sources will have more contribute in future electric grids. Ocean waves are one of these renewable sources that have been investigated in last decades. Ocean waves are highly fluctuating which make generating electrical power a challenging issue from. In this paper a double fed induction machine is implemented by surge Wave Energy Convector (WEC) to generate electrical power from wave power. Meanwhile the interface which connects the WEC and DFIG is a crank and connecting rod. In order to control generator speed and generate smooth electrical power from ocean waves a fly wheel is used in mechanical part for power storage as well as the rotor of electrical generator is controlled by AC/DC/AC power electronic circuit. According to the result, the precise control of back to back invertors makes it possible to generate electrical power by DFIG from ocean waves in an effective manner.

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Taming the Tides


A. Harris – Engineering & Technology, January, 2013

Abstract

According to the British cartographic Society the length of coastline of great Britain plus its principal islands is just shy of 20,000 miles, so it comes as no surprise that marine energy is considered to hold great potential as a future energy source. According to research by the Carbon Trust – Technology Innovation Needs Assessment (TINA) – this has the potential to deliver more than 75TWh a year; over 10 per cent of the UK’s predicted needs, by mid century. Predictions of how much of that energy can be harvested by 2050 vary from 20GW to zero. What is clear, however, is that it will not have any impact before 2020.

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