Energia dos Oceanos - EOC
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- Integrating Offshore Wind and Wave Resource AssessmentPublication . Pontes, M.T.; Sempreviva, A.M.; Barthelmie, R.; Giebel, G.; Costa, Paula Silva; Sood, A.The aim of this paper is to review the sources of wind and wave information, the methodologies to assess offshore wind and wave energy resources, and the more relevant results at the European level as a first step to integration of the evaluation of both resources. In situ and remote sensed wind and wave data (using satellite based sensors) are done generally by distinct systems (except for SAR) but numerical atmospheric models and wind - wave models are closely related. Offshore wind resource studies using various types of data are reviewed especially in northern European seas and in the Mediterranean. The wave energy resource assessment at European and national levels is also reviewed and the various atlases are identified.
- Spectral Bandwidth and WEC Performance AssessmentPublication . Saulnier, J.-B.; Ricci, P.; Pontes, M.T.; Falcão, António F. de O.This paper investigates the dependency of wave energy conversion on the spectral bandwidth of sea-states. To this aim, the performance of an axisymmetrical Wave Energy Converter is assessed in the frequency domain by using a stochastic model in two far different wave climates (Portugal and North Sea) both represented by more than 23000 energy spectral densities obtained from measurements. The correlation between the performance and various bandwidth parameters found in the literature is observed. Then, refined methods for predicting the long-term converted wave energy based on wave statistics including spectral bandwidth are compared to more common procedures and conclusions are drawn.
- Wave energy resource in the North SeaPublication . Beels, C.; Henriques, J.C.C.; De Rouck, J.; Pontes, M.T.; De Backer, G.; Verhaeghe, H.Due to the high potential of wave energy and the goal to raise the share of renewable energy supply in the EU up till 20 % in 2020, the development of wave energy is accelerated. Until now the wave energy resource was highlighted in regions with a high wave energy density. As Wave Energy Converters (WECs) still contend with problems such as structural strength and mooring in a severe and energetic wave climate, the prospects of wave power conversion in a less aggressive wave climate should be investigated. This paper describes the wave power resource in a rather sheltered area i.e., the North Sea. The available wave power is studied on 34 locations. Characteristic sea states are defined for the Belgian, Dutch, German, Danish, Norwegian and UK Continental Shelf. An inverse-ray refraction model, implemented at INETI (Instituto Nacional de Engenharia, Tecnologia e Inovação), is presented to calculate the resource on more convenient locations for wave energy conversion. The wave power potential in the North Sea is compared with the resource of the West European coast. Near shore (< 30 km off the coast) up to a maximum of approximately 11 kW/m is available in the North Sea.
- Frequency-domain and stochastic model for an articulated wave power devicePublication . Cândido, J.; Justino, Paulo AlexandreTo have the first look into device performance, analytical and numerical tools must be used. Assuming that the wave power system hydrodynamics has a linear behaviour, diffraction and radiation coefficients can be computed. If the power take-off equipment may be, for the first approach, regarded as holding a linear behaviour then overall (i.e. hydrodynamic plus mechanical) device performance can be studied for regular waves. In this study a frequency-domain model describes the articulated system behaviour for regular waves. For this paper a stochastic model is found for an articulated wave power device, and probability density functions are defined for the relevant parameters that characterize the wave power system behaviour. For these parameters and for different sea states the probability density functions are found. The articulated system is characterized by these probability density functions. Also, average values for capture width are obtained for these sea state conditions.
- Hydrodynamics of triangular-grid arrays of floating point-absorber wave energy converters with inter-body and bottom slack-mooring connectionsPublication . Vicente, Pedro C.; Falcão, António F. de O.; Gato, L.M.C.; Justino, Paulo AlexandreIt may be convenient that dense arrays of floating point absorbers are spread-moored to the sea bottom through only some of their elements (possibly located in the periphery), while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. An array of identical floating point absorbers located at the grid points of an equilateral triangular grid is considered in the paper. A spread set of slack-mooring lines connect the peripheric floaters to the bottom. A weight is located at the centre of each triangle whose function is to pull the three floaters towards each other and keep the inter-body moorings lines under tension. The whole system – buoys, moorings and power take-off systems – is assumed linear, so that a frequency domain analysis may be employed. Equations are presented for a set of three identical point absorbers. This is then extended to more complex equilateral triangular grid arrays. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of hemispherical converters in arrays of three and seven elements and different mooring and PTO parameters, and wave incidence angles. Comparisons are given with the unmoored and independently-moored buoy situations.
- Stochastics, Time Domain Models and Pontryagin Maximum Priciple for a Two Body Wave Power DevicePublication . Cândido, José J.; Justino, Paulo AlexandreIn this study a stochastic model to describe the behaviour of an articulated system is developed.Optimal mechanical damping and spring coefficients are computed. Probability density functions are defined for the relevant parameters that characterize the device behaviour. For these parameters and for different sea state conditions the probability density functions are found and the articulated system is characterized in terms of these functions. Average values for useful power and capture width are also obtained for these sea state conditions. Time domain models allow the computation of time series for the variables that chracterize the wave power system behaviour. In this study a time domain model is also developed for the articulated wave power device. Results are obtained for regular and irregular waves. Pontryagin Maximum Principle is presented as an algorithm for the control of the device.
- Reactive versus Latching Phase Control of a Two-body Heaving Wave Energy ConverterPublication . Falcão, António F. de O.; Justino, Paulo Alexandre; Henriques, João C.C.; André, José M.C.S.The paper deals with the modelling, optimization and control of the IPS buoy wave energy converter, a two-body point-absorber oscillating in heave, consisting of a floater rigidly connected to a long submerged vertical acceleration tube open at both ends within which a piston can slide, forcing the motion of the inside water below and above it. Energy is converted from the relative motion between the piston and the buoy-tube pair. The modelling is performed in the frequency domain (including reactive phase control), in order to provide some kind of geometry optimization, and also in the timedomain to allow the simulation of a more realistic highpressure- oil power take-off system.
- Assessing the wave energy resource using remote sensed dataPublication . Pontes, M.T.; Bruck, M.; Lehner, S.The use of accurate remote sensed wave data in the coastal area (water depth up to 80m) will enable a high quality characterization of the wave energy resource. Work has been carried out with this objective for a number of years namely assessing the quality of the radar altimeter and SAR sensors data. In this paper a summary of the quality of wave period estimates from the NASA/CNES Jason radar altimeter is presented, showing that the analytical models that have been proposed in recent years provide already accurate results. This paper also includes a verification of ESA ENVISAT SAR data (height, period and direction parameters in addition to the shape of frequency spectra) against NDBC buoy data, which has shown good accuracy for wave energy resource assessment. However, the long Exact-Repeat-Period of NASA (10 days) and of ESA satellites (35 days) poses serious limitation to the usefulness of their wave measurements except for long-term wave climate assessment. These shortcomings are expected to be overcome by the new high spatial-resolution TerraSAR-X satellite that is obtaining reliable data for nearshore areas, being able to provide data at 2 - 3 day interval.
- A time domain analysis of arrays of floating point-absorber wave energy converters including the effect of nonlinear mooring forcesPublication . Vicente, Pedro C.; Falcão, António F. de O.; Justino, Paulo AlexandreThe extensive exploitation of the offshore wave energy resource may require the deployment of dense arrays of point absorbers, the distance between elements being possibly tens of meters. In such cases, it may be more convenient and economical that only elements in the periphery of the array are directly slack-moored to the sea bottom,while the other elements are prevented from drifting and colliding by connections to adjacent elements. Previous work was done in a base configuration of three floating point absorbers located at the grid points of an equilateral triangular, with a solid weight located at the centre of the triangle, which was extended to more complex equilateral triangular grid arrays. The study was based on frequency domain analysis which requires, not only the power take-off system (PTO) to be linear, but also linear mooring forces, which is quite unrealistic as a model of slack moorings. In the present paper those restrictions are removed by using a time-domain, rather than a frequency domain, analysis, which allows nonlinear mooring forces to be considered. The mooring cables are approximately modelled as catenary lines in a quasi-static analysis. The results show very different behaviour for the horizontal and vertical motions of the floating converters, namely the possibility of occurrence of low-frequency horizontal oscillations of large amplitude. Even in the case of incident regular waves, such horizontal motions were found to be non-periodic, a behaviour that is typical of nonlinear systems.
- Non-linear Slack-Mooring Modelling of a Floating Two-Body Wave Energy ConverterPublication . Vicente, Pedro C.; Falcão, António F. de O.; Justino, Paulo AlexandreMost floating oscillating-body wave energy converters that have been proposed and developed so far are two-body systems where the power is extracted from the relative translational motion between the two bodies. As any floating device, floating point absorbers are subject to drift forces due to waves, currents and wind, and therefore need to be kept in place by a proper mooring system. The mooring cables can be approximately modelled as catenary lines in a quasi-static analysis. The use of a time-domain analysis allows for nonlinear mooring forces of slack chain cables to be considered. Numerical results for motion, mooring tensions and absorbed power are presented for a two body system consisting of a hemispherical floater and a submerged body and slack bottom moorings, for regular and irregular waves. Comparisons are given with the unmoored two-body heaving system, the moored heaving twobody system and with the simplified one body linear PTO model. Results show the possibility of occurrence of low-frequency horizontal oscillations of large amplitude, and non linear motions, even for regular waves. Some differences are seen in comparison with the simplified one body model and with the heave two-body system. The moorings were found not to affect very significantly the power absorbed.