Control-informed geometric optimisation of wave energy converters

This paper concerns the interplay between the physical geometry of a wave energy converter (WEC) and the control strategy adopted for the converter, with the ultimate aim of optimising the energy output of the device. An energy-based performance function is employed and we attempt to perform numerical optimisation of a heaving buoy employing a latching control strategy. We allow both draught and radius of the axisymmetric buoy to be adjusted using a numerical optimisation. A linear time-domain hydrodynamic program is used in order to simulate the device motion, while the optimization problem is solved by means of a simplex method. Results show the difference in the frequency response of an optimal buoy for a particular sea-state designed with and without knowledge of the control system

State space model of a hydraulic power take off unit for wave energy conversion employing bondgraphs

In this work, the modeling of a Power Take- Off (PTO) unit for a point absorber wave energy converter is described. The PTO influences the energy conversion performance by its efficiency and by the damping force exerted, which affects the motion of the body. The state space model presented gives a description of the damping force and of the internal dynamics of the PTO. The aim of this work is to develop a model for the PTO as a part of a complete wave-to-wire model of a wave energy converter as in Figure 1, used for the design control techniques. Figure 1: Wave-to-wire model structure A bondgraph is employed to model the physical system that provides transparent and methodical means of formulating state space equations and of visualizing energy transfer throughout the system. Bondgraphs have already been shown to be a very useful tool for the modeling of PTO for wave energy converters (2). The dynamic of the mathematical model is then analyzed respect to the variation of parameters; in particular, the non-linear system obtained is linearized and its eigenvalues are calculated as function of the accumulator size and pre-charge pressur

A nonlinear extension for linear boundary element methods in wave energy device modelling

To date, mathematical models for wave energy devices typically follow Cummins equation, with hydrodynamic parameters determined using boundary element methods. The resulting models are, for the vast majority of cases, linear, which has advantages for ease of computation and a basis for control design to maximise energy capture. While these linear models have attractive properties, the assumptions under which linearity is valid are restrictive. In particular, the assumption of small movements about an equilibrium point, so that higher order terms are not significant, needs some scrutiny. While this assumption is reasonable in many applications, in wave energy the main objective is to exaggerate the movement of the device through resonance, so that energy capture can be maximised. This paper examines the value of adding specific nonlinear terms to hydrodynamic models for wave energy devices, to improve the validity of such models across the full operational spectrum

A nonlinear extension for linear boundary element methods in wave energy device modelling

To date, mathematical models for wave energy devices typically follow Cummins equation, with hydrodynamic parameters determined using boundary element methods. The resulting models are, for the vast majority of cases, linear, which has advantages for ease of computation and a basis for control design to maximise energy capture. While these linear models have attractive properties, the assumptions under which linearity is valid are restrictive. In particular, the assumption of small movements about an equilibrium point, so that higher order terms are not significant, needs some scrutiny. While this assumption is reasonable in many applications, in wave energy the main objective is to exaggerate the movement of the device through resonance, so that energy capture can be maximised. This paper examines the value of adding specific nonlinear terms to hydrodynamic models for wave energy devices, to improve the validity of such models across the full operational spectrum

A control system for a self-reacting point absorber wave energy converter subject to constraints

The problem of the maximization of the energy produced by a self reacting point absorber subject to motion restriction is addressed. The main objective is to design a control system suitable for real-time implementation. The method presented for the solution of the optimization problem is based on the approximation of the motion of the device and of the force exerted by the power take off unit by means of a linear combination of basis functions. The result is that the optimal control problem is reformulated as a non linear program where the properties of the cost function and of the constraint are affected by the choice of the basis functions. An example is described where the motion and the force are approximated using Fourier series; an optimization algorithm for the solution of the non linear program is also presented. The control system is implemented and simulated using a real sea profile measured by a waverider buoy

State space model of a hydraulic power take off unit for wave energy conversion employing bondgraphs

In this work, the modeling of a Power Take- Off (PTO) unit for a point absorber wave energy converter is described. The PTO influences the energy conversion performance by its efficiency and by the damping force exerted, which affects the motion of the body. The state space model presented gives a description of the damping force and of the internal dynamics of the PTO. The aim of this work is to develop a model for the PTO as a part of a complete wave-to-wire model of a wave energy converter as in Figure 1, used for the design control techniques. Figure 1: Wave-to-wire model structure A bondgraph is employed to model the physical system that provides transparent and methodical means of formulating state space equations and of visualizing energy transfer throughout the system. Bondgraphs have already been shown to be a very useful tool for the modeling of PTO for wave energy converters (2). The dynamic of the mathematical model is then analyzed respect to the variation of parameters; in particular, the non-linear system obtained is linearized and its eigenvalues are calculated as function of the accumulator size and pre-charge pressur

Comparison of the capacity factor of stationary wind turbines and weather-routed energy ships in the far-offshore

International audienceOffshore wind energy technology has developed rapidly over the last decade. It is expected to significantly contribute to the further increase of renewable energy in the global energy production in the future. However, even with floating wind turbines, only a fraction of the global offshore wind energy potential can be harvested because grid-connection, moorings, installation and maintenance costs increase tremendously as the distance to shore and the water depth increase. Thus, new technologies enabling harvesting the far offshore wind energy resource are required. To tackle this challenge, mobile energy ship concepts have been proposed. In those concepts, electricity is produced by a water turbine attached underneath the hull of a ship propelled by the wind using sails. It includes an on-board energy storage system since energy ships are not grid-connected. Thus, the ships route schedules could be dynamically optimized taking into account weather forecast in order to maximize their capacity factors (CF). The aim of this study is to investigate how high the capacity factors of energy ships could be when using weather-routing and compare them to that of stationary wind turbines that would be deployed in the same areas. To that end, a modified version of the weather-routing software QtVlm was used. Velocity and power production polar plots of an energy ship that was designed at LHEEA were used as input to QtVlm. Results show that capacity factors over 80% can be achieved with energy ships and stationary offshore wind turbines deployed in the North Atlantic Ocean

Simulation numérique d'éolien offshore

Depuis quelques années, la demande en électricité renouvelable a augmenté significativement. Dans ce contexte, les filières de production d'énergies renouvelables se sont rapidement développées. Dans le même temps, l'éolien a atteint un niveau de maturité tel que les parcs éoliens onshore et offshore posés se sont multipliés. Aujourd'hui, la recherche de vents plus forts et plus constants poussent les acteurs du domaine à se tourner vers le développement de parcs éoliens flottants. Les coûts associés à la réalisation de telles machines sont encore élevés et doivent être optimisés. Un des leviers pour la réduction des coûts est la modélisation numérique. Le développement d'outils numériques permettant une prédiction fine du comportement de ces structures en mer va permettre une meilleure prise en compte des différents chargements mécaniques. L'accès à des résultats précis va tendre à réduire les coefficients de sécurité liés au dimensionnement de ces éoliennes, et ainsi contribuer à la réduction des coûts de CAPEX. Ce travail concerne le développement d'une méthodologie pour la simulation directe de plusieurs éoliennes flottantes, avec une modélisation exacte et précise de ses composantes (par exemple, ses pales). La base logicielle utilisée est la bibliothèque ICI-Tech, développée au sein de l'Institut de Calcul Intensif (ICI) de l'Ecole Centrale de Nantes. Une approche monolithique est utilisée, avec un unique maillage dans la simulation, où les différentes interfaces sont définies par des fonctions de phase. La résolution des équations de Navier-Stokes est alors faite à l'aide d'éléments finis stabilisés, en utilisant le formalisme Variational Multi-Scale (VMS). Pour réduire grandement les coûts de calcul usuellement requis pour modéliser précisément des éoliennes, où des phénomènes d'ordres de grandeurs très diverses sont observés, une procédure d'adaptation de maillage anisotrope permet d'obtenir des mailles de taille variable et adaptées aux phénomènes observés partout dans le domaine de calcul. Finalement, les premiers résultats d'immersion de maillage et d'écoulements autour de l'éolienne sont présentés

Objective comparison of particle tracking methods

Particle tracking is of key importance for quantitative analysis of intracellular dynamic processes from time-lapse microscopy image data. Because manually detecting and following large numbers of individual particles is not feasible, automated computational methods have been developed for these tasks by many groups. Aiming to perform an objective comparison of methods, we gathered the community and organized an open competition in which participating teams applied their own methods independently to a commonly defined data set including diverse scenarios. Performance was assessed using commonly defined measures. Although no single method performed best across all scenarios, the results revealed clear differences between the various approaches, leading to notable practical conclusions for users and developers