Energia dos Oceanos - EOC
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- 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.
- Frequency and Stochastic Domain Models for Two Geometries of the IPS Wave Power BuoyPublication . Cândido, J.; Justino, Paulo Alexandre; Henriques, J.C.C.Frequency-domain analysis is applied to a geometry of the original IPS buoy device concept. The analysis is particularly useful in the early development stages to establish the response of power take-off mechanism characteristic parameters to different frequencies of the wave spectrum. Optimal mechanical damping and spring coefficients are computed for some parameters restrictions. Absorbed power, capture width and other variables, such as relative displacement,are computed for regular waves and these optimal mechanical coefficients. A stochastic model is developed in order to evaluate the IPS buoy behaviour for irregular waves’ conditions. This allows defining probability density functions for parameters that characterize the device’s behaviour. Assuming that the overall system behaviour is linear and that the surface elevation for irregular waves may be regarded as a stochastic process with a Gaussian probability density function, the variables that define the system behaviour, such as bodies’ displacements and velocities, will also hold a Gaussian probability density function. The average power extraction is computed for different sea state conditions.Aiming to enhance the device’s hydrodynamic performance, a new non-axisymmetric IPS geometry is conceived. Using the stochastic modelling approach, the device’s behaviour is studied for several wave directions and compared to the axisymmetric configuration’s behaviour.
- 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.
- 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.
- Modelling of the IPS buoy wave energy converter including the effect of non-uniform tube cross-sectionPublication . Falcão, António F. de O.; Cândido, José J.; Justino, Paulo Alexandre; Henriques, João C. C.An important class of floating wave energy converters (that includes the IPS buoy, the Wavebob and the PowerBuoy) comprehends devices in which the energy is converted from the relative (essentially heaving) motion between two bodies oscillating differently. The paper considers the case of the IPS buoy, consisting of a floater rigidly connected to a fully submerged vertical (acceleration) tube open at both ends. The tube contains a piston whose motion relative to the floater-tube system (motion originated by wave action on the floater and by the inertia of the water enclosed in the tube) drives a power take-off mechanism (PTO) (assumed to be a linear damper). To solve the problem of the end-stops, the central part of the tube, along which the piston slides, bells out at either end to limit the stroke of the piston. The use of a hydraulic turbine inside the tube is examined as an alternative to the piston. A frequency domain analysis of the device in regular waves is developed, combined with a one-dimensional unsteady flow model inside the tube (whose cross-section is in general nonuniform). Numerical results are presented for a cylindrical buoy in regular waves, including the optimization of the acceleration tube geometry and PTO damping coefficient for several wave periods.
- Modelling, control and Pontryagin Maximum Principle for a two-body wave energy devicePublication . Justino, Paulo Alexandre; Cândido, J.Frequency domain analysis is applied to a wave energy device composed by two coaxial axisymmetric bodies. For each frequency optimal damping coefficient values which maximize absorbed power are obtained. Several displacement amplitude restriction scenarios are considered. A stochastic model to describe the device’s behaviour in irregular waves is developed. Optimal mechanical damping and spring coefficients are computed. Considering different sea state conditions, probability density functions are defined for relevant parameters and time averaged absorbed power values are obtained.A time domain model is also developed for the device. A non-linear power take-off mechanism configuration, consisting in a hydraulic circuit with low-pressure and high-pressure gas accumulators, is devised. Time averaged absorbed power is maximized in terms of characteristic mechanism parameter. A sub-optimal method of phase control by latching is applied to the device in order to improve its performance. Analytical development of Pontryagin Maximum Principle is used to establish an algorithm for device’s control.
- 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.
- Nonlinear dynamics of a tightly moored point-absorber wave energy converterPublication . Vicente, Pedro C.; Falcão, António F. de O.; Justino, Paulo AlexandreTightly moored single-body floating devices are an important class of offshore wave energy converters. Examples are the devices under development at the University of Uppsala, Sweden, and Oregon State University, USA, prototypes of which were recently tested. These devices are equipped with a linear electrical generator. The mooring system consists of a cable that is kept tight by a spring or equivalent device. This cable also prevents the buoy from drifting away by providing a horizontal restoring force. The horizontal and (to a lesser extent) the vertical restoring forces are nonlinear functions of the horizontal and vertical displacements of the buoy, which makes the system a nonlinear one (even if the spring and damper are linear), whose modelling requires a time-domain analysis. Such an analysis is presented, preceded, for comparison purposes, by a simpler frequency-domain approach. Numerical results (motions and absorbed power) are shown for a system consisting of a hemispherical buoy in regular and irregular waves and a tight mooring cable. The power take-off is modelled by a simplified system of a linear spring and a linear damper and also by a system formed by a hydraulic piston and spring. Different scenarios are analysed.
- Oceans of energy? The non-linear trajectory of the emerging wave energy technologyPublication . Fontes, Margarida; Sousa, Cristina; Ferreira, J.This paper addresses the construction and structuring of a technological niche – i.e. a protected space where promising but still underperforming technologies are stabilized and articulated with societal needs – and discusses the processes that influence niche development and may enable niche breakout. In theoretical terms the paper is grounded on the multi-level approach to sustainability transitions, and particularly on the niche literature. But it also attempts to address the limitations of this literature in what concerns the spatial dimension of niche development. It is argued that technological niches can transcend the narrow territorial boundaries to which they are often confined, and encompass communities and actions that span several spatial levels, without losing some territorial embeddedness. It is further proposed that these features shape the niche trajectory and, therefore, need to be explicitly considered by the niche theoretical framework. To address this problem the paper builds on and extends the socio-cognitive perspective to technology development, introducing a further dimension – space – which broadens the concept of technological niche and permits to better capture the complexity of niche behaviour. This extended framework is applied to the case of an emerging renewable energy technology – wave energy - which exhibits a particularly slow and non-linear development trajectory. The empirical analysis starts by examining how an “overall niche space” in wave energy was spatially constructed over time. Then it investigates in greater detail the niche development processes that took place in Portugal, a country that was among the pioneers in the field, and whose actors have been, from very early stages, engaged in the activities conducted at various spatial levels. Through this combined analysis, the paper seeks to understand whether and how niche development is shaped by processes taking place at different spatial levels. More specifically it investigates the interplay between territorial and relational elements in niche development, and how these different dynamics influence the performance of the niche processes and impact on the overall niche trajectory. The results confirm the niche multi-spatial dynamics, showing that it is shaped by the interplay between a niche relational space constructed by actors’ actions and interactions on/across levels, and the territorial effects introduced by these actors’ embeddedness in particular geographical and institutional settings. They contribute to a more precise understanding of the processes that can accelerate or slow down the trajectory of a technological niche. In addition, the results shed some light into the niche activities conducted in/originating from a specific territorial setting - Portugal - offering some insights into the behaviour of key actors and its implications for the positioning of the country in the emerging field, which can be relevant for the formulation of strategies and policies for this area.