Nowadays, renewable energy is in full growth. In particular, offshore wind farms will be at the centre of UK energetic strategy in the coming years. However, other types of marine renewable are still at an early development stage. That is the case for tidal energy. Many projects have been undertaken but there is no candidate for competitive commercial applications yet. Deltastream is one of these numerous pioneering projects. It consists of a set of three marine current turbines mounted on a triangular base put down onto the seabed. The device is not moored and no harm is done to the environment. However, that makes the structure more sensitive to water flows. And it is important to ensure that it will remain at its location and not being carried along with the tidal streams. Using CFD, the present study aims to evaluate the drag on the nacelles of the structure and come up with solutions to reduce it as much as possible.

The performance of a horizontal axis marine current turbine is predicted by three numerical methods, vortex lattice method MPUF-3A, boundary element method PROPCAV and a commercial RANS solver FLUENT. The predictions are compared with the experimental measurements for the same turbine model. A fully unsteady wake alignment is utilized in order to model the realistic wake geometry of the turbine. A lifting line theory based method is developed to produce the optimum circulation distribution for turbines and propellers and a lifting line theory based database searching method is used to achieve the optimum circulation distribution for tidal turbines. A nonlinear optimization method (CAVOPT-3D) and another database-searching design method (CAVOPT-BASE) are utilized to design the blades of marine current turbines and marine propellers. A design procedure for the tidal turbine is proposed by using the developed methods successively. Finally, an interactive viscous/potential flow method is utilized to analyze the effect of nonuniform inflow on the performance of tidal turbines. / text

Ces travaux de thèse concernent l’étude de l’ensemble de la chaîne de puissance d’un système hydrolien utilisant des systèmes de stockage d’énergie pour améliorer la qualité de la puissance produite et la capacité de gestion des échanges d’énergie. Dans un premier temps, les différentes technologies de stockage d’énergie et leurs applications pour lisser les fluctuations de la puissance produite par le système hydrolien sont étudiées et comparées. Ensuite, une stratégie de lissage des fluctuations de la puissance, dues à l’effet de houle (fluctuations de courte durée), est proposée : elle associe une stratégie MPPT avec filtrage (au niveau de la génératrice) à l’utilisation de supercondensateurs pour lisser la puissance injectée au réseau. Puis il est proposé d’utiliser des batteries à circulation d’électrolyte pour la gestion quotidienne de la puissance d’une hydrolienne dans le contexte d’un réseau électrique isolé. Un système hybride hydrolien/batteries/diesel est étudié pour deux configurations simples : le cas d’une alimentation avec une hydrolienne comme sources principales et le cas où les générateurs diesels sont considérés comme sources dominantes. Enfin, des stratégies de limitation de puissance basées sur le défluxage de la génératrice pour contrôler la puissance de l’hydrolienne dans le cas de vitesses de courants marins élevées sont proposées. Dans ce contexte, le contrôle à puissance constante et à puissance maximale en cours de défluxage sont comparés. L’influence des paramètres de la génératrice sur les caractéristiques de fonctionnement commun de la turbine et la génératrice est également étudiée. / This PhD thesis models the whole power chain of a marine current turbine (MCT) system and investigates the use of energy storage devices to improve power quality and energy management capability. First, various energy storage technologies concerning their applications to address the power fluctuation phenomena in tidal current generation system are reviewed and compared. Then, a two-stage power smoothing control strategy for compensating swell-induced short-time fluctuations is proposed. The proposed control strategy uses a modified MPPT with filter strategy on the generator-side and supercapacitors on the grid-side for injecting a smoothed power to the grid. Afterwards, a flow battery system for daily energy management of a hybrid MCT/battery/diesel system is proposed. The MCT dominant power supply case and an island power supply (with diesel generators as the main source) are investigated. Finally, power limitation controls with a robust flux-weakening strategy for a PMSG-based non-pitchable MCT system are proposed for over-rated marine current speed periods. In this context, the constant power control and maximum power control modes at the flux-weakening stage are compared; and the influence of the generator parameters on the joint operating characteristics of the turbine and generator are also discussed.

The Savonius turbine is a self-starting vertical axis turbine that has a few advantages compared to other vertical axis turbines such as lower cost, lower noise and is relatively easy to manufacture. This turbine does however have a lower efficiency and is therefore less used in the electricity production than other turbines.  This thesis is trying to tackle this problem with the use of 2D CFD-simulations of flow directors to modify the flow through the turbine to increase the efficiency. The focus during this project is to use this turbine as a Marine Current Turbine (MCT) in unidirectional flows. The turbine was based on a turbine design from a previous study at Uppsala University. The design of the flow directors was modeled with the intention to increase the available pressure drop from the front to the back of the turbine and therefore increase the velocity through the turbine. The flow directors comprised of two arcs on each side of the turbine so that they resembled a Venturi-tunnel, with a funnel in the front and a diffuser at the back. A validation of the domain dimensions and mesh-size was conducted and after this the different parameters of the flow directors were varied one at a time with the best value of a given parameter being kept for the latter simulations. At the end, the rotational velocity of the turbine was varied to find how sensitive the power output was based on this factor. This study concluded that an increase in the power coefficient of about 3,2 times was achievable compared to the same turbine in free flow. However, this needs to be further investigated and validated in real world tests as this study was conducted using 2D-simulations and other effects may influence the results in the real world. / <p>This project was conducted within Stand up for wind and Stand up for energy.</p>