Large Scale Wind Turbines (LSWTs) have been extensively examined for decades but very few studies have been conducted on the small scale wind turbines (SSWTs) especially for the applications near ground level where wind speed is of order of few meters per second. This study provides the first systematic effort towards design and development of SSWTs (rotor diameter<50 cm) targeted to operate at low wind speeds (<5 m/s). An inverse design and optimization tool based on Blade Element Momentum theory is proposed. The utility and efficacy of the tool was validated by demonstrating a 40 cm diameter small-scale wind energy portable turbine (SWEPT) operating in very low wind speed range of 1 m/s-5 m/s with extremely high power coefficient. In comparison to the published literature, SWEPT is one of the most efficient wind turbines at the small scale and very low wind speeds with the power coefficient of 32% and overall efficiency of 21% at its rated wind speed of 4.0 m/s. It has very low cut-in speed of 1.7 m/s. Wind tunnel experiments revealed that SWEPT has rated power output of 1 W at 4.0 m/s, and it is capable of producing power output up to 9.3 W at wind speed of 10 m/s. The study was further extended to develop a piezoelectric wind turbine which operates below 2.0 m/s wind speed. The piezoelectric wind turbine of overall dimension of 100mm x 78mm x 65mm is capable of producing peak electric power of about 450 microwatt at the rated wind speed of 1.9 m/s. / Master of Science

La thèse résume la recherche sur le fonctionnement et l’écoulement autour d’une éolienne caréné à deux rotors. Le placement d’une turbine à l’entrée d’un canal divergent permet d’augmenter le débit massique à travers le rotor. Afin de mieux tirer parti de l’augmentation de la vitesse du vent à l’entrée du diffuseur, il a été décidé d’examiner la possibilité de placer un deuxième rotor, tournant dans le sens opposé, dans cette zone.L'étude menée combinait plusieurs voies de recherche différentes, y compris les méthodes de la mécanique des fluides numérique (CFD) et des études expérimentales. Cela a permis de mieux comprendre la nature de l'écoulement et du fonctionnement d'une éolienne à deux rotors. Des recherches expérimentales ont été menées dans la soufflerie de l’Institut de Turbomachinerie de l’Ecole Polytechnique de Lodz (Pologne). Une série de mesures de systèmes d'éoliennes divers, avec et sans carénage, à un et deux rotors, a été réalisée. Les résultats recueillis ont permis de confirmer que le carénage pouvait augmenter considérablement (même deux fois) l'efficacité du rotor. Cependant, les forces aérodynamiques et la vitesse de rotation augmentent également. Cet inconvénient peut être partiellement résolu en utilisant un deuxième rotor et en répartissant les charges aérodynamiques sur deux étages de turbine.Une partie importante de l'étude était les simulations numériques. Ils ont permis de préciser la nature et les paramètres de l'écoulement et d'estimer leur impact sur les performances de l'éolienne. Deux modèles numériques différents ont été développés:• Modèle rotor complet (anglais : Fully-resolved Rotor Model, FRM): modèle URANS dans ANSYS CFX, basé sur la discrétisation de la géométrie complète du rotor; ce modèle a été utilisé pour l'analyse de l’écoulement,• Modèle hybride CFD-BET (théorie de l’élément de pâle): modèle RANS dans ANSYS Fluent, dans lequel le rotor est représenté par les termes source dans les équations de Navier-Stokes, déterminés par un code interne; ce modèle a été utilisé pour évaluer les performances de différentes configurations d'éoliennes.Au cours de la recherche, une correction empirique interne de la perte d’extrémité de la pâle (anglais : tip loss correction) a été proposée, en tenant compte de l’influence du diffuseur. L’étude réalisée a permis d’observer, entre autres, que le déplacement du rotor en aval vers la sortie du diffuseur pouvait entraîner une réduction de la vitesse du vent à travers le rotor en amont, placé à l’entrée du diffuseur, et une diminution de la puissance globale du système. / Doctoral dissertation summarizes the research on the functioning and flow around a two-stage, shrouded wind turbine. Placing the turbine at the inlet of a diverging channel allows to increase the mass flow rate of the flow through the rotor. To better take advantage of the increase in wind speed at the diffuser inlet, it was decided to examine the possibility of placing a second rotor in this area, with the opposite direction of rotation.The conducted study combined several different research paths, including Computational Fluid Dynamics (CFD) methods and experimental studies. This allowed for a more refined understanding of the nature of the flow and operation of a wind turbine with two rotors. Experimental research was carried out in the IMP TUL wind tunnel. A series of measurements of various turbine systems with and without shroud, with single- and double-rotor wind turbine were made. The collected results allowed to confirm that the shrouding can significantly (even twice) increase the efficiency of the rotor. However, aerodynamic forces and rotational speed also increase. This disadvantage can be partially addressed by using a second rotor and distributing aerodynamic loads to two turbine stages.An important part of the study were numerical simulations. They allowed to specify in more detail the nature and parameters of the flow and to estimate their impact on the performance of the wind turbine. Two different numerical models were developed:• Fully-resolved Rotor Model: URANS model in ANSYS CFX, based on discretising the entire geometry of the rotor, used for the flow analysis,• Hybrid model CFD-BET (Blade-Element Theory): RANS model in ANSYS Fluent, in which the rotor is represented by source terms in the Navier-Stokes equations, determined by an in-house code; the model was used to evaluate the performance of different wind turbine configurations.In the course of the research an in-house, empirical tip loss correction was proposed, taking into account the influence of the diffuser. The performed study permitted to observe, among others, that moving the rear rotor towards the outlet of the diffuser may result in a reduction of the wind speed through the front rotor, placed at the inlet to the diffuser, and a decrease in the overall system power.

This thesis examines methods for designing and analyzing kinetic turbines based on blade element momentum (BEM) theory and computational fluid dynamics (CFD). The underlying goal of the work was to assess the potential augmentation of power production associated with enclosing the turbine in an expanding duct. Thus, a comparison of the potential performance of ducted and non-ducted turbines was carried out. This required de ning optimal turbine performance for both concepts. BEM is the typical tool used for turbine optimization and is very well established in the context of wind turbine design. BEM was suitable for conventional turbines, but could not account for the influence of ducts, and no established methodology for designing ducted turbines could be found in the literature. Thus, methods were established to design and analyze ducted turbines based on an extended version of BEM (with CFD-derived coe cients), and based on CFD simulation. Additional complications arise in designing tidal turbines because traditional techniques for kinetic turbine design have been established for wind turbines, which are similar in their principle of operation but are driven by flows with inherently different boundary conditions than tidal currents. The major difference is that tidal flows are bounded by the ocean floor, the water surface and channel walls. Thus, analytical and CFD-based methods were established to account for the effects of these boundaries (called blockage effects) on the optimal design and performance of turbines. Additionally, tidal flows are driven by changes in the water surface height in the ocean and their velocity is limited by viscous effects. Turbines introduced into a tidal flow increase the total drag in the system and reduce the total flow in a region (e.g. a tidal channel). An analytical method to account for this was taken from the eld of tidal resource assessment, and along with the methods to account for ducts and blockage effects, was incorporated into a rotor optimization framework. It was found that the non-ducted turbine can produce more power per installed device frontal area and can be operated to induce a lesser reduction to the flow through a given tidal channel for a given level of power production. It was also found that by optimizing turbines for array con gurations that occupy a large portion of the cross sectional area of a given tidal channel (i.e. tidal fences), the per-device power can be improved signi cantly compared to a sparse-array scenario. For turbines occupying 50% of a channel cross section, the predicted power improves is by a factor of three. Thus, it has been recommended that future work focus on analyzing such a strategy in more detail. / Graduate

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>