Optimal damping profiles for a heaving buoy wave-energy converter

This paper explores optimal damping profiles for a heaving buoy wave energy converter (WEC). The approach is mathematical and the model of Eidsmoen (1995) is used as a basis. In order to permit analytical development, the model is initially simplified and an optimal damping profile is determined using numerical optimization. Having found the optimal damping profile, a semi-analytical solution methodology is developed to determine the optimal damping parameters. Finally, the procedure is validated on the original model and some aspects related to the control problem are addressed

Discrete control of resonant wave energy devices

International audienceAiming at amplifying the energy productive motion of wave energy converters (WEC) in response to irregular sea waves, the strategies of discrete control presented here feature some major advantages over continuous control which is known to require, for optimal operation, a bidirectional power take-off (PTO) able to re-inject energy into the WEC system during parts of the oscillation cycles. Three different discrete control strategies are described: latching control, declutching control, and the combination of both, which we term LOD (Latched-Operating-Declutched) control. It is shown that any of these methods can be applied with great benefit, not only to mono-resonant WEC oscillators, but also to bi-resonant and multi-resonant systems. For some of these applications, it is shown how these three discrete control strategies can be optimally defined, either by analytical solution for regular waves, or numerically, by applying the optimal command theory in irregular waves. Applied to a model of a 7 degree-of-freedom system (the SEAREV WEC) to estimate its annual production on several production sites, the most efficient of these discrete control strategies was shown to double the energy production, regardless of the resource level of the site, which may be considered as a real breakthrough, rather than a marginal improvement

Optimizing the Power Take Off of a Wave Energy Converter with Regard to Wave Climate

International audienceConsidered as a source of renewable energy, wave is a resource featuring high variability at all time scales. Furthermore wave climate also changes significantly from place to place. Wave energy converters are very often tuned to suit the more frequent significant wave period at the project site. In this paper we show that optimizing the device necessitates accounting for all possible wave conditions weighted by their annual occurrence frequency, as generally given by the classical wave climate scatter diagrams. A generic and very simple wave energy converter is considered here. It is shown how the optimal parameters can be different considering whether all wave conditions are accounted for or not, whether the device is controlled or not, whether the productive motion is limited or not. We also show how they depend on the area where the device is to be deployed, by applying the same method to three sites with very different wave climate

A Wave to Wire model of the SEAREV Wave Energy Converter

International audienceThis paper describes a numerical wave-to-wire model of the second-generation wave energy converter called SEAREV. Governing equations are given in the time domain for the motion of the masses involved in the device and for the hydraulic power take-off (PTO) used to convert the motion into electricity. The hydrodynamic forces are derived using the standard linear potential theory. The memory term in the radiation force is replaced by additional states using the Prony method in order to change the equation of motion into the ordinary differential equation form. The PTO is composed of hydraulic rams, an accumulator, and a hydraulic generator, which delivers electricity when there is enough energy stored in the accumulator.Using the MATLAB Simulink tool, the equation of motion is solved to simulate the full device (including the power take-off) from the incident wave to the electricity delivered to the grid. Simulation results are presented in the paper and comparisons are made with a simpler PTO: a linear damper. They show that the torque applied to the hydraulic PTO must exceed a threshold to start absorbing energy, unlike the linear damping model. They also show that the power production can be very discontinuous, depending on the level of the incident wave power. This is due to the fact that the generator considered can transform the energy stored in the accumulator faster than the energy transmitted by the rams into the accumulator. It could therefore be interesting to use several generators to adapt the electrical energy production to the level of incident wave power, or a generator that could work efficiently at part load in order to achieve continuous energy production

Declutching control of a wave energy converter

International audienceWhen hydraulic Power Take Off (PTO) is used to convert the mechanical energy of a wave energy converter into a more useful form of energy, the PTO force needs to be controlled. Continuous controlled variation of the PTO force can be approximated by a set of discrete values. This can be implemented using either variable displacement pumps or several hydraulic cylinders or several high pressure accumulators with different pressure levels. This pseudo continuous control could lead to a complex PTO with a lot of components. A simpler way for controlling this hydraulic PTO is declutching control, which consists in switching on and off alternatively the wave energy converter's PTO. This can be achieved practically using a simple bypass valve. In this paper, the control law of the valve is determined by using the optimal command theory. It is shown that, theoretically when considering a wave activated body type of WEC, declutching control can lead to energy absorption performance at least equivalent to that of pseudo-continuous control. The method is then applied to the case of the SEAREV wave energy converter, and it is shown than declutching control can even lead to a higher energy absorption, both in regular and irregular waves

Comparison of latching control strategies for a heaving wave energy device in random sea

International audienceThis paper investigates semi-analytically the latching control applied to a mechanical oscillator; and numerically three strategies of latching control for a point absorber wave energy converter oscillating in the heave mode only. By solving the equation of motion of a mechanical damped oscillator, it is shown that latching control can magnify the amplitude of the motion whatever the frequency of the excitation force, and how it can improve the efficiency of the system, in term of absorbed energy, for excitation frequencies apart from the natural frequency. Assuming that the excitation force is known in the close future and that the body is locked in position at the current time step, equations of motion of the body are solved numerically in the time domain fordifferent initial conditions (i.e. latching durations). For all these simulations, three criteria—one for each strategy—are tested and the latching time leading to the best result is selected. Time domainsimulation results are presented for a heaving buoy in small-amplitude regular and random waves. In regular waves, the same results as for the case of a mechanical oscillator are recovered for the wave energy converter. In random sea, results show that for all the three proposed strategies, efficiency of the wave energy converter is considerably improved in terms of absorbed energy. Numerical study of the period of the controlled system shows that the delay of prediction of the excitation force in the future seems to be bounded by the natural period of the system

Optimisation hydrodynamique et contrôle optimal d'un récupérateur de l'énergie des vagues

The SEAREV project is a new concept of wave energy converter based on a robust principle and a sophisticated control. The wave energy converter is composed of a floating body containing a cylinder of inertia with an off-centered gravity center. The control is known as “latching control” and consists in locking the cylinder to the floating body at some moments of the cycle.In this study, four time domain simulators of the motions have been derived, for four sets of assumptions. Interaction between the fluid and the floating body has been described by the standard linear potential theory. Comparisons are made between the results of the four different simulators to quantify the effects of the non linearities into the mechanics of the cylinder and of the directionality of the incoming wave.Then, optimal control of the system and sub-optimal latching control are considered, the aim being to improve the energy capture of the system. Two methods are introduced in order to assess the benefit that can be brought by latching control. First one is a semi analytical method which allows to solve the problem of the calculation of the latching duration in regular wave. The second one is based on the Pontriaguine’s principle. It is less efficient than the first one in regular wave, but it allows to make simulations in random seas.Energy production depends obviously on the mechanical characteristics of the device, so an optimization has been done on the floating body shape and on the mechanical characteristics of the cylinder. A method using two levels is given when optimization is done without control. The upper level uses genetic algorithm, the lower one is based on a gradient algorithm. Results on different shapes give the key dimensions of the device. Another optimization has been done, using the latching control and gives the order of magnitude of the benefit that can be brought by this kind of control to such systems.Le projet SEAREV est un nouveau concept de récupération de l’énergie des vagues basé sur un principe robuste et un contrôle sophistiqué. Le récupérateur est ainsi composé d’un flotteur clos contenant en son sein un cylindre d’inertie à centre de gravité décentré. Le contrôle envisagé est un contrôle « tout ou rien » (latching control) qui consiste à bloquer le mouvement relatif du cylindre interne en des moments bien choisis de son cycle.Dans le cadre de cette thèse, quatre simulateurs numériques des mouvements du système dans le domaine temporel ont été développés, pour quatre jeux d’hypothèses sur la cinématique du mouvement. La modélisation des efforts d’interaction fluide-structure a elle été abordée dans le cadre d’une théorie potentielle linéarisée classique. Des comparaisons entre les résultats des différents modèles sont présentés et permettent de quantifier des effets tels que les non linéarités dans la mécanique du cylindre et la directionnalité de la houle.Le contrôle optimal du système et le contrôle sub-optimal par latching sont ensuite considérés, dans le but d’améliorer les performances du système. Deux méthodes sont proposées afin d’évaluer le gain que l’on peut attendre d’un contrôle par latching. La première est une méthode semi-analytique originale qui permet de résoudre le problème de la durée de blocage en houle régulière. La seconde est basée sur le principe de Pontriaguine et les lois de la commande optimale. Moins performante en houle régulière, elle permet cependant de traiter le problème en houle aléatoire.Les performances du système dépendant évidemment de ses caractéristiques géométriques, une optimisation a été conduite sur la forme du flotteur et les caractéristiques mécaniques du cylindre. Une méthode faisant intervenir deux niveaux d’optimisation, l’un basé sur les algorithmes génétiques et l’autre sur une méthode de gradient est présentée dans le cas d’une optimisation du système non contrôlé. Les résultats sur plusieurs types de géométrie fournissent un dimensionnement du système. Une optimisation du système contrôlé a également été conduite et présentée. Ces résultats fournissent l’ordre de grandeur du gain que peut apporter le contrôle par latching aux systèmes houlomoteurs

Détection de micro-ARNs in situ : utilisation de sondes LNA

Les micro-ARNs sont de petits ARN non-codants de 21 à 25 nucléotides impliqués dans la régulation de l’expression post-transcriptionnelle de gènes. Ils se fixent par complémentarité de séquence à des ARN messagers (ARNm) cibles afin d’induire leur dégradation ou à l’inhibition de leur traduction en protéine. Récemment, ils ont été montrés comme i) étant dérégulés dans différents contextes de pathologies musculaires et ii) jouant un rôle important dans le processus physiopathologique associé à la myopathie de Duchenne par une implication notamment dans la réponse aux dommages musculaires et à la régénération. Une analyse spatiale de l’expression tissulaire et cellulaire des micro-ARNs s’avère nécessaire pour approfondir le champ des connaissances sur leur degré d’implication dans les mécanismes physiologiques et/ou physiopathologiques. Les techniques conventionnelles d’hybridation in situ − technique de référence en histologie permettant une localisation cellulaire des acides nucléiques − ne sont pas adaptées pour la détection de micro-ARNs en raison de la très petite taille de ces derniers. Par conséquent, les sondes classiques d’ADN ou d’ARN sont remplacées par des sondes spécialement développées pour cet usage, dites sondes LNA (Locked Nucleic Acid). Notre équipe s’est intéressée à l’expression de deux micro-ARNs dans le muscle de chiens myopathes. Par hybridation in situ avec des sondes LNA, nous avons documenté leur expression tissulaire et démontré qu’ils s’expriment plus particulièrement dans les myoblastes et dans les fibres en cours de régénération

The power-capture of a nearshore, modular, flap-type wave energy converter in regular waves

Bottom-hinged, nearshore flap-type wave energy converters (WECs), have several advantages, such as high power conversion efficiency and survivability. They typically comprise a single flap spanning their full width. However, a potentially beneficial design change would be to split the flap into multiple modules, to make a ‘Modular Flap’. This could provide improvements, such as increased power-capture, reduced foundation loads and lower manufacturing and installation costs. Assessed in this work is the hydrodynamic power-capture of this device, based on physical modelling. Comparisons are made to an equivalent ‘Rigid Flap’. Tests are conducted in regular, head-on and off-angle waves. The simplest control strategy, of damping each module equally, is employed. The results show that, for head-on waves, the power increases towards the centre of the device, with the central modules generating 68% of the total power. Phase differences are also present. Consequently, the total power produced by the Modular Flap is, on average, 23% more smooth than that generated by the Rigid Flap. The Modular Flap has 3% and 1% lower average power-capture than the Rigid Flap in head-on and off-angle waves, respectively. The advantages of the modular concept may therefore be exploited without significantly compromising the power-capture of the flap-type WEC

Weakly non linear modeling of submerged wave energy converters

International audienceWave-to-Wire numerical models being developed for the study of wave energy converters usually make use of linear potential flow theory [[1], [2], [3], [4], [5]] to describe wave-structure interaction. This theory is highly efficient from a computational perspective. However, it relies on assumptions of small wave steepness and small amplitude of motion around mean positions. Often, maximization of wave energy converters’ energy performance implies large amplitude motion [[6], [7], [8]], thus contradicting the assumption of small amplitude motion.An alternative approach is to linearize the free surface conditions on the instantaneous incident wave elevation (Weak-Scatterer approach [9]) while the body conditions are evaluated at the exact body position. Studies of wave energy converters’ dynamic response using this method are expected to be more accurate, while maintaining a reasonable computational time. With this aim, a Weak-Scatterer code (CN_WSC) was developed and used to study two submerged wave energy converters. The first is a heaving submerged sphere and the second is a bottom-hinged fully submerged oscillating flap. They are inspired respectively by the Ceto [10] and WaveRoller [11] devices.Initial calculations were performed in linear conditions first to verify the CN_WSC against linear theory. Subsequently, calculations in nonlinear conditions were performed, using large wave steepness and amplitude of body motion. In linear conditions, results of CN_WSC showed good agreement with linear theory, whereas significant deviations from linear theory were observed in nonlinear conditions. As amplitude of body motion increases, linear theory tends to overestimate energy performance in comparison with Weak-Scatterer theory. In contrast, with smaller amplitude of motion but larger wave steepness, the opposite result is obtained: energy performance is underestimated by linear theory compared to Weak-Scatterer theory