Frequency-domain and stochastic model for an articulated wave power device

To 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

Modelling, control and Pontryagin Maximum Principle for a two-body wave energy device

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

Stochastics, Time Domain Models and Pontryagin Maximum Priciple for a Two Body Wave Power Device

In 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

Frequency and Stochastic Domain Models for Two Geometries of the IPS Wave Power Buoy

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

A influência da flexibilização do trabalho no comprometimento organizacional e na intenção de abandono: um estudo com os trabalhadores estudantes do ensino superior português

Este estudo aborda um dos desafios centrais da Gestão de Recursos Humanos: as práticas de recursos humanos (RH) que promovem uma redução na intenção de abandono e estimulam a permanência dos trabalhadores nas organizações. Na era atual do conhecimento e da globalização, em que a economia funciona num ritmo acelerado e numa dinâmica de mudanças contínuas, sendo cada vez mais difícil prever o futuro próximo, as pessoas são o fator diferenciador e o pilar da sustentabilidade das organizações a médio e a longo-prazo, pelo que a implementação de estratégias de RH que promovam a retenção dos trabalhadores é algo imperativo (Mazur, 2015; Savanevičienė & Stankevičiūtė, 2017). Por outro lado, é uma realidade que, nos dias de hoje, homens e mulheres integram de igual forma a força de trabalho, colocando novos problemas de conflitualidade trabalho-família, inerentes à sobreposição de papéis (Guerreiro & Pereira, 2006), o que é particularmente evidente no caso dos trabalhadores estudantes do ensino superior, que, regra geral, assumem um papel tripartido (trabalho-família-escola), tal como referido no estudo de Costa (2012). Tendo em conta as considerações anteriores, o objetivo deste estudo é analisar o impacto de medidas específicas de conciliação trabalho-família (WLB – Work Life Balance) e de apoio social (WLS - Work Life Support) no comprometimento organizacional dos trabalhadores-estudantes e, em sequência, o impacto desse comprometimento organizacional na sua intenção de abandono da organização. Os resultados mostram que, em geral, existe uma influência positiva e significativa das medidas WLB e WLS no comprometimento organizacional dos trabalhadores-estudantes do ensino superior em Portugal, sobretudo nas suas dimensões afetiva e calculativa, e mostram também que um maior comprometimento organizacional afetivo dos trabalhadores-estudantes implica uma menor intenção de abandono da organização

Impact of the wave/wind induced oscillations on the power performance of the WindFloat wind turbine

ABSTRACT: The main objective of this work is the characterization of the wave/wind induced oscillations on the power performance of the wind turbine operating on a WindFloat floating system. To assess the potential impact on the wind turbine power performance induced by these oscillations, the nacelle movements of the WindFloat wind turbine were monitored using accelerometer sensors synchronized with : 1) metocean data measured with a buoy; 2) wind turbine power data installed in the WindFloat floating system; and 3) wind speed data gathered from a nacelle-mounted LiDAR. Based on this data, a clustering analysis approach is proposed. No meaningful relationship between the ocean parameters and the nacelle movements or the wind power production could be established. The obtained results suggest that the dynamic adaptation of the drive train (mainly due to wind turbine torque control) to a fast oscillating (primary energy) moving force is the source of the largest oscillations in the nacelle of the WindFloat wind turbine. Nevertheless, results suggest that the wind/wave induced oscillations and their impact on the power performance of the WindFloat wind turbine is low considering its nominal capacity. Outcomes of this work were extremely relevant to demonstrate the stability of the WindFloat system, and, consequently, also important for the development of the floating wind offshore industry (and other technologies).info:eu-repo/semantics/publishedVersio

Non-linear Slack-Mooring Modelling of a Floating Two-Body Wave Energy Converter

Most 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

Optimization of mooring configuration parameters of floating wave energy converts

Floating point absorbers devices are a large class of wave energy converters for deployment offshore, typically in water depths between 40 and 100m. As floating oil and gas platforms,the devices are subject to drift forces due to waves, currents and wind, and therefore have to be kept in place by a proper mooring system. Although similarities can be found between the energy converting systems and floating platforms, the mooring design requirements will have some important differences between them, one of them associated to the fact that, in the case of a wave energy converter, the mooring connections may significantly modify its energy absorption properties by interacting with its oscillations. It is therefore important to examine what might be the more suitable mooring design for wave energy devices, according to the converters specifications. When defining a mooring system for a device, several initial parameters have to be established, such as cable material and thickness, distance to the mooring point on the bottom, and which can influence the device performance in terms of motion,power output and survivability. Different parameters, for which acceptable intervals can be established, will represent different power absorptions,displacements from equilibrium position, load demands on the moorings and of course also different costs. The work presented here analyzes what might be, for wave energy converter floating point absorber, the optimal mooring configuration parameters, respecting certain pre-established acceptable intervals and using a time-domain model that takes into account the non-linearities introduced by the mooring system. Numerical results for the mooring forces demands and also motions and absorbed power, are presented for two different mooring configurations for a system consisting of a hemispherical buoy in regular waves and assuming a liner PTO

A time domain analysis of arrays of floating point-absorber wave energy converters including the effect of nonlinear mooring forces

The 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

The Levelized Cost of Energy (LCOE) of wave energy using GIS based analysis: the case study of Portugal

The main objective of this paper is to establish an economic modelling of wave energy through a Geographical Information System (GIS). Furthermore, this method has been tested for the particular case of the Portuguese coast. It determines the best sea areas to install wave energy converters in this region, using spatial analysis of the Levelized Cost of Energy (LCOE). Several economic parameters, as capital or O&M costs, have been considered. In addition, a sensitivity analysis has been performed by varying the discount rate in three different scenarios. Several types of physical restrictions have been taken into account: bathymetry, submarine electrical cables, seabed geology, environmental conditions, protected areas in terms of heritage, navigation areas, seismic fault lines, etc. Spatial operations have been carried out to complete the procedure, using Model Builder of GIS software. Results indicate the most suitable areas in economic terms in Portugal to install wave energy devices