Wind energy, the fastest growing source of renewable energy, is a promising resource for power generation. Offshore wind energy, in particular,offers favorable conditions for power generation—high winds with low turbulence, minimal visual impacts and high generation capacities. Offshore wind turbines mounted on floating platforms are the most economical and viable solution for deep water sites. A semi-submersible platform is an appropriate floating platform for a deep water site, providing stability through high water-plane area. In the wind energy industry, there has been continuing interest in developing larger turbines. At Sandia National Laboratories (SNL), efforts have led to the development of a 13.2 MW wind turbine model with blades 100 meters in length, significantly larger than commercially available blades at present. Such a large wind turbine needs to be carefully analyzed and studied before it can be considered suitable for commercial purposes. The dynamic analysis of the SNL 13.2 MW wind turbine mounted on a moored semi-submersible platform is the subject of this study. This integrated 13.2 MW wind turbine system has been developed and its various physical properties have been studied in this and another associated study. The semi-submersible platform is developed using various modeling tools. For the wind turbine-platform system model developed, dynamic analyses are performed using simulation tools to understand the coupled behavior of the wind turbine and the platform. A reference site is chosen to define the environmental conditions, based on which the short-term extreme response of the offshore wind turbine is estimated. The system is loaded with selected combinations of winds and waves to assess controlling combinations of wind speeds and wave heights that influence the response. The influence of changes in model parameters on overall response is also studied. / text

The dynamic motion of floating wind turbines is studied using numerical simulations. Floating wind turbines in the deep ocean avoid many of the concerns with land-based wind turbines while allowing access to strong stable winds. The full three-dimensional Navier-Stokes equations are solved on a regular structured grid, using a level set method for the free surface and an immersed boundary method for the turbine platform. The tethers, the tower, the nacelle and the rotor weight are included using reduced order dynamic models, resulting in an efficient numerical approach which can handle nearly all the nonlinear wave forces on the platform, while imposing no limitation on the platform motion. Wind is modeled as a constant thrust force and rotor gyroscopic effects are accounted for. Other aerodynamic loadings and aero-elastic effects are not considered. Several tests, including comparison with other numerical, experimental and grid study tests, have been done to validate and verify the numerical approach. Also for further validation, a 100:1 scale model Tension Leg Platform (TLP) floating wind turbine has been simulated and the results are compared with water flume experiments conducted by our research group. The model has been extended to full scale systems and the response of the tension leg and spar buoy floating wind turbines has been studied. The tension leg platform response to different amplitude waves is examined and for large waves a nonlinear trend is seen. The nonlinearity limits the motion and shows that the linear assumption will lead to over prediction of the TLP response. Studying the flow field behind the TLP for moderate amplitude waves shows vortices during the transient response of the platform but not at the steady state, probably due to the small Keulegan-Carpenter number. The effects of changing the platform shape are considered and finally the nonlinear response of the platform to a large amplitude wave leading to slacking of the tethers is simulated. For the spar buoy floating wind turbine, the response to regular periodic waves is studied first. Then, the model is extended to irregular waves to study the interaction of the buoy with more realistic sea state. The results are presented for a harsh condition, in which waves over 17 m are generated, and linear models might not be accurate enough. The results are studied in both time and frequency domain without relying on any experimental data or linear assumption. Finally a design study has been conducted on the spar buoy platform to study the effects of tethers position, tethers stiffness, and platform aspect ratio, on the response of the floating wind turbine. It is shown that higher aspect ratio platforms generally lead to lower mean pitch and surge responses, but it may also lead to nonlinear trend in standard deviation in pitch and heave, and that the tether attachment points design near the platform center of gravity generally leads to a more stable platform in comparison with attachment points near the tank top or bottom of the platform.

The effects of surface curvature and slope-of-curvature on the performance of aerofoils used in small wind turbines are studied experimentally and numerically. A symmetric aerofoil NACA0012 and an asymmetric aerofoil E387 are judiciously selected as an example of an aerofoil with a surface curvature discontinuity and an example of an aerofoil with slope-of-curvature discontinuities respectively. The prescribed surface curvature distribution blade design (CIRCLE) method is applied to both aerofoils to remove the curvature and slope-ofcurvature discontinuities. The newly designed aerofoils have continuous curvature and slope-of-curvature distributions and have nearly identical geometry compared to the original aerofoils, denoted as QM13F and A7. Low-speed wind tunnel experiments, together with two numerical methods, are conducted to aerofoil E387 and A7 to investigate the effects of slope-of-curvature. The slope-of-curvature discontinuities of E387 result in a larger LSB, which causes higher drag at low angles of attack, and result in premature LSB bursting process at higher angles of attack, causing earlier stall. The impact of the slope-ofcurvature distribution on aerodynamic performance is more profound at higher angles of attack and lower Reynolds number. The aerodynamic improvements are estimated over a 3 kW small HAWT, resulting in up to 10% increase in instantaneous power and 1.6% increase in annual energy production. In terms of the effects of surface curvature, the curvature discontinuity at the leading edge affects aerofoil lift and drag performance near the stalling angle in the steady flow, and it is estimated in a 5 kW small VAWT that the power coefficient can be increased by 9.7% by removing the curvature discontinuity. Acoustic experimental measurements were performed on aerofoil E387 and A7 in an anechoic wind tunnel to investigate effects of slope-of-curvature on aerofoil acoustic performance. The in-house CFD code Cgles was modified to perform large eddy simulation (LES) the 3D aerofoil sections to further investigate the experimental phenomenon. The tonal noise of E387 at different angles of attack is reduced by removing slope-of-curvature discontinuities. It is experimentally and numerically concluded that continuous curvature and slope-of-curvature distributions can result in better aerodynamic performance of the aerofoil used in small wind turbines, leading to lower aerofoil self-noise and higher energy output efficiency.

<p>Power generation using wind turbines is a highly researched control field.</p><p>Many control designs have been proposed based on continuous-time models</p><p>like PI-control, or state observers with state feedback but without special</p><p>regard to robustness to model uncertainties. The aim of this thesis was to</p><p>design a robust digital controller for a wind turbine.</p><p>The design was based on a discrete-time model in the polynomial framework</p><p>that was derived from a continuous-time state-space model based on</p><p>data from a real plant. A digital controller was then designed by interactive</p><p>pole placement to satisfy bounds on sensitivity functions.</p><p>As a result the controller eliminates steady state errors after a step</p><p>response, gives sufficient damping by using dynamical feedback, tolerates</p><p>changes in the dynamics to account for non linear effects, and avoids feedback</p><p>of high frequency un modeled dynamics.</p>

It is preferable that control and bearing condition monitoring are integrated, as the condition of the system should influence control actions. As wind turbines mainly work in remote areas, it becomes necessary to develop a wireless platform for the control system. A fuzzy system with self-tuning mechanism was developed. The input speed error and speed change were selected to control the shaft speed, while the kurtosis and peak-to-peak values were used as another set of inputs to monitor the bearing conditions. To enhance effectiveness, wait-and-see (WAS) logic was used as the pre-processing step for the raw vibration signal. The system was implemented on the LabVIEW platform. Experiments have shown that the system can effectively adjust motor rotating speed in response to bearing conditions. For future studies, more advanced fault detection methods can be integrated with proper tuning mechanisms to enrich the performance and function of the controller.

The project contains a force analysis of the vertical axis wind turbine Lucia's supporting arms and a measuring device to experimentally measure the forces is made. The forces between the supporting arms and the tower are calculated theoretically and then simulated by a computere. A measuring devise is then designed to measure the forces experimentally. The forces acting on the attachment between the supporting arms and the tower is primarily the centripetal force, gravitational force and the aerodynamic forces on the rotor wings. The maximum forces were theoretically calculated and is 13.38 kN along the x-axis, -0.25 kN along the y-axis and then 0.5 kN along the z-axis. The axis are acording to a rotational reference system where the x-axis runs along the supporting arm and the y-axis runs along the axis of rotation. The maximum torque that occurs is 0.53 kNm along the y-axis and 1.29 kNm along the z-axis. The size of the forces have been confirmed with a deviation of up to 1.8 % in the simulation using SolidWorks 2010. For the experimental measurements a measuring device has been developed which consists of S-load cells with wave indicator and transmitter, an attachment for the measuring equipment and distanceplates to stabilize the rotor. S-load cells, wave indicator and transmitter were ordered and drawings for the attachment of the measuring equipment and spacer plates was done. The eigenfrequencies and the stress have been investigated for the parts. The eigenfrequencies for the wind turbine was estimated to decline up to 13 % when the measuring device was mounted and the lowest Factor of Safety was 1.67. Before the attachment of the measuring device and the spacer plates can be ordered the attachment of the supporting arms, how the loadcells should be attached to the device and the safety margins need to be examined.

Vindkraften expanderar allt mer i Sverige och årsproduktionen ökade till 6,1 TWh år 2011, vilket är en ökning med 74 % jämfört med 2010. De nya vindkraftverken som sätts upp idag är oftast av effektstorlekarna 2 000 – 3 000 kW. De som restes tidigare är omkring 200 – 600 kW. I Sverige finns det många vindkraftverk som har varit i drift under en längre tid, och det är flera som passerat sin tekniska livslängd på 20 år. De första vindkraftverken som man reste står ofta på platser med väldigt goda vindförhållanden och har väldigt låga effekter, om man jämför med dagens vindkraftverk. Det börjar därför bli högaktuellt att demontera äldre befintliga verk och resa nya. Detta kallas för vindkraftens generationsskifte, kanske mer känt som repowering. Tyskland, Danmark och Nederländerna är länder som redan har bytt ut stora delar av sina vindkraftsflottor. Danmark är världsledande inom repowering och har bytt ut omkring två tredjedelar av sina äldre verk. I många av de projekt som genomförts har en dubblering och ibland till och med en fyrdubblering av energinmängden skett! Genom att resa nya verk blir det oftast färre verk på platsen, samtidigt som mer energi levereras. På så sätt utnyttjas platsen på ett bättre sätt, vindkraftselen blir billigare. Det blir samtidigt enklare att uppnå politiska riktlinjer med mål inom förnybar energi och många anser att landskapsbilden förbättras. Detta examensarbete genomfördes i syfte att undersöka möjligheterna och lönsamheten för generationsskiften i Hallands län. För att veta vart de olika vindkraftverken är placerade i Halland genomfördes en kartläggning och identifiering initialt. När verken var kartlagda gjordes projekteringar av nya parker. Det visade sig vara ont om plats att resa de nya verken, eftersom de är högre än de gamla och kräver längre avstånd till bostäder. Därför genomfördes inte enbart projektering av parker där befintliga vindkraftverk står utan även på helt nya områden som idag inte har någon vindkraft. I varje projekt utfördes ekonomiska beräkningar som grundar sig på energiberäkningarna från projekteringen i vindatlasprogrammet WindPRO. Arbetet har också behandlat hur och om ersättning till ägare av befintliga verk och markägare kan ske, samt vad alternativen är för de befintliga vindkraftverken vid ett generationsskifte. Projektgruppen har varit i kontakt med vindkraftsägare och ställt frågor om hur de ställer sig till ett generationsskifte. Det främsta resultatet som projektgruppen kommit fram till är att generationsskiften absolut har goda möjligheter till att bli lönsamma. Det kan dock bli problematiskt att få plats med nya vindkraftverk på områden där gamla verk står. Det beror på kommunens regler om avstånd till närboende, och att ett visst antal verk måste resas vid nybyggnation. Ändras dessa regler för generationsskiften, blir möjligheterna väldigt goda. Vad det gäller efterhandsvärde på de gamla verken är slutsatsen, att det går att göra en bra affär genom att sälja sitt verk. Vid skrotning däremot blir intäkterna från försäljning av metallerna för små för att täcka demonteringskostnaden. Generellt sett har vindkraftsägarna en positiv syn till ett generationsskifte. / The installations of wind turbines are expanding rapidly each day in Sweden and 2011 wind energy generated 6,1 TWh. This is an increase with 74 % compared to 2010. The modern wind turbine that becomes integrated in the Swedish electricity grid at the moment often has a power capacity of 2000-3000 kW. Many of those wind turbines which are already integrated in the grid soon need to be replaced. These turbines often have a power capacity of 200-600 kW. Many of these old turbines have been generating electricity for almost 20 year, which is the technical lifetime for a wind turbine. These old turbines are also often located on sites with great energy levels. This makes it interesting to investigate the possibilities and if there is a profit in repowering. Repowering is when you dismantle old turbines and replace them with new ones. Germany, Denmark and the Netherlands are countries in Europe, which have already been doing repowering for some time. Denmark is the country where most repowering has been done. They have exchanged two thirds of their old turbines. In most of the projects which has been done, the energy level has been doubled and in some cases even quadrupled! When the modern larger turbines replace the old smaller ones, fewer turbines are raised on the site although the energy level increases. While doing repowering the sites are used in a better way, it makes the electricity prices cheaper, the political ambitions in renewable energy are easier reached and many people also claim, that the landscape is improved. This bachelor´s thesis has the objective to investigate the possibilities and the profit for future repowering projects in county of Halland, Sweden. To find out where the wind turbines are located in the county, mapping and identification where done. When the mapping was done, projections of new parks where created. It turned out that these old turbines often where located on a very limited area, which makes it hard to establish new larger turbines. This is because they need larger distance to the nearest residences. Because of this, projections also where done on other locations. In every project economical calculations were made, which are based on the energy results from the projections, which were made in WindPRO. The thesis has investigated how and if the owner of the old wind turbines and the land holder can be compensated when repowering. Researching and brainstorming on different scenarios for what will happen to the old turbines has also been done. We have had contact with owners of wind turbines to find out, what their opinion are about repowering. The prime conclusion from this bachelor thesis is that repowering has good possibilities to be profitable. There might be some problems to project new wind turbines on the same site where the old turbines where located, if the municipality don’t change the distance demands to residences and the demand that a group of wind turbines must be raised. If these demands are changed, the future for repowering looks very promising. Another conclusion where that there might be good profit in selling the used wind turbine, but to sell the used material of a wind turbine to recycling is not profitable. Profit from selling the turbine material to recycling does not cover the cost for dismantling it. Owners of the wind turbines have in general a positive attitude to repowering.

With the number of wind power units (WPUs) on the rise worldwide, it is inevitable that some of these would be exposed to natural disasters like tsunamis and it will become a necessity to consider their effects in the design process of WPUs. This study initially attempts to quantify the forces acting on an existing WPU due to a tsunami bore impact. Surge and bore heights of 2m, 5m and 10m are used to compute the forces using the commercially available full 3D Navier Stokes equation solver FLOW3D. The applicability of FLOW3D to solve these types of problems is examined by comparing results obtained from the numerical simulations to those determined by small scale laboratory experiments. The simulated tsunami forces on the WPU are input into a simplified numerical structural model of the WPU to determine its dynamic response. The tsunami force is also used to obtain base excitation which when applied on the WPU would be equivalent dynamically to the tsunami forces acting on it. This base excitation is useful to obtain the response of the WPU experimentally, the setup for which is available at University of California, San Diego's (UCSD) Large High Performance Outdoor Shake Table (LHPOST). The facility allows full scale experimental setup capable of subjecting a 65kW Nordtank wind turbine to random base excitations. A stress analysis of turbine tower cross section is performed in order to assess the structural integrity of the WPU. It has been determined that the WPU is unsafe for bore/surge heights above 5 m. It has also been postulated that the structural responses could be considerable in case of the taller multi megawatt wind power units of present day.

This dissertation addresses the modeling of wind turbine systems (WTS) and wind farms. The WTS reliability model provides the generation state space of a WTS. The generation states are derived from the combinations of the wind states from given wind data and the condition states of each component in a WTS. Wake effect is accounted when there are neighboring WTSs. The results of the reliability model of a WTS are associated with the generation states of the WTS, which include the probability, transition rates to other states, frequency of transitions to other states, and duration. The reliability model of the wind farm is derived by combining the wind states, WTS states and the distribution line states. The results of the reliability model of a wind farm are associated with the generation states of the wind farm, which include the probability, transition rates to other states, frequency to other states, and duration. The reliability model of the wind turbine system and the reliability model of the wind farm presented in this dissertation bring contribution to the planning and operation of bulk power systems with wind farm integration. The developed models can provide the system operator with clear reliability indices in terms of generation states of wind turbine systems and wind farms along with their probability, duration and frequency of transitions.

Power generation using wind turbines is a highly researched control field. Many control designs have been proposed based on continuous-time models like PI-control, or state observers with state feedback but without special regard to robustness to model uncertainties. The aim of this thesis was to design a robust digital controller for a wind turbine. The design was based on a discrete-time model in the polynomial framework that was derived from a continuous-time state-space model based on data from a real plant. A digital controller was then designed by interactive pole placement to satisfy bounds on sensitivity functions. As a result the controller eliminates steady state errors after a step response, gives sufficient damping by using dynamical feedback, tolerates changes in the dynamics to account for non linear effects, and avoids feedback of high frequency un modeled dynamics.