119 research outputs found
Multi-element ducts for ducted wind turbines: A numerical study
No full textMulti-element ducts are used to improve the aerodynamic performance of ducted wind turbines (DWTs). Steady-state, two-dimensional computational fluid dynamics (CFD) simulations are performed for a multi-element duct geometry consisting of a duct and a flap; the goal is to evaluate the effects on the aerodynamic performance of the radial gap length and the deflection angle of the flap. Solutions from inviscid and viscous flow calculations are compared. It is found that increasing the radial gap length results in an augmentation of the total thrust generated by the DWT, whereas a larger deflection angle has an opposite effect. Reasonable to good agreement is seen between the inviscid and viscous flow calculations, except for multi-element duct configurations characterized by large flap deflection angles. The viscous effects become stronger at large flap deflection angles, and the inviscid calculations are incapable of taking this phenomenon into account.</p
TurbyCon: A Modified Sea Container for Vertical Axis Wind Turbine
No full textAerospace EngineeringApplied ScienceSustainable Energy Technolog
Passive and active flow augmentation: From diffusers to multi-rotor machines
No full textFlow augmentation consists in modifying mass flow across the actuation plane of a rotor to enhance energy extraction or propulsive efficiency. The talk sketches the distinction between passive and active rotor augmentation strategies. Power coefficient trends are compared analytically while numerical results illustrate differences in flow topology. Rotors are stylized as actuator disks that exert homogeneous normal forces on the steady flow of inviscid fluids to highlight the distinctive features of each augmentation principle. Passive augmentation principles have been well documented because they guide the design of ducted, shrouded and diffuser-augmented wind turbines1-6. These axisymmetric bodies decrease average static pressures on the rotor plane to increase mass flux and power coefficient. Rotor-body interactions are dominated by conservative forces5,7: the bodies don’t exchange energy with the fluid but act as augmenting devices and affect global energy balance by changing rotor state. Virtual work arguments show that bodies exert streamwise forces4,6 that can be related with the power coefficient through the law of de Vries1,6. Active flow augmentation is a rather recent theoretical concept8. Its simplest energy extraction embodiment consists of an upstream actuator that accelerates flow onto a downstream actuator. This augmentation strategy is coined as active because the upstream actuator injects (spends) energy into the flow for the downstream actuator to extract (produce) energy from a greater mass flux than if it were alone. The interaction mechanism depends on the action of non-conservative forces and actuators interact exclusively through changes in total flow enthalpy when they are sufficiently far apart. No pressure interactions occur in this asymptotic case and a closed solution exists together with an analytical power coefficient law. Parallels can be drawn with wake ingestion propeller setups9 but no practical energy extraction realizations have been attempted yet. Passive and active flow augmentation concepts are different but we hope that parallels between them shed further light on the physics of energy extraction from ideal fluid flows. The communication concludes with a few reflections meant to trigger an open discussion about the implications and applicability of the discussed theories
Suppression of Classical Flutter using a 'Smart' rotor
No full textThe mechanics of aeroelastic instabilities is particularly important in relation to the next generation of wind turbine designs. Sizes approaching the 10MW capacity will have a rotor diameter somewhere in the order of 170m. This could mean much higher loads and more flexible blade designs than current MW wind turbines, most probably resulting in aeroelastic instabilities not commonly seen in the machines of today. Load control on an aerofoil using actuated trailing edge flaps could be a means to mitigate this issue. Load control on future wind turbines should serve three main goals: - Improve the fatigue life - Reduce extreme loads •- Improve aeroelastic stability The two most important are fatigue life and aeroelastic stability; as these are the likely design drivers for future giant wind turbines (10MW and beyond). The content of this report is focused on the effects of trailing edge flaps on the aeroelastic stability of a rotor (‘Smart’ Rotor), in particular, the two degree of freedom Flap-Torsion Flutter instability - Classical Flutter. Current research has shown promising results for load reduction on an aerofoil using trailing edge flaps. The aeroelastic model employed in this study uses Theodorsen’s theory for a flat-plate aerofoil with a trailing edge flap for determination of the lift coefficient. A basic BEM aerodynamic model determines the induced forces on the blades with a blade structural response exhibited according to a modal representation of a blade (eigenmodes and eigenfrequencies). The aeroelastic model of the wind turbine (rigid hub – no tower interaction) is designed with the intention to capture the Classical Flutter instability so that a simple controller for an actuated trailing edge flap can be investigated to show if controllability on the Flutter limits is achievable. The final goal is to show that it could be feasible for giant wind turbines to avoid Flutter regions in their normal operational envelope with implementation of the ‘Smart’ rotor concept.Aerospace EngineeringSustainable Energy Technolog
An improved rotor design for a diffuser augmented wind turbine:improvement of the DonQi Urban Windmill
No full textThe polluting fossil fuels are diminishing and becoming more and more expensive. The trias energetica, Figure 1, states that besides reducing the energy demand and using fossil fuels cleaner, renewable energy technologies are the solution to this problem. One of these renewable energy technologies is small-scale wind energy. The DONQI URBAN WINDMILL is a so-called urban turbine, a small roof mounted wind turbine. Moreover, the DONQI URBAN WINDMILL is a diffuser augmented wind turbine (DAWT), a wind turbine embedded in annular wing shaped diffuser. This diffuser has the function of augmenting the flow through the wind turbine.Aerospace EngineeringSustainable Energy Technolog
The assessment of dynamic wake effects on loading:dynamic wake modeling and comparison of methods for wake loading assessment
No full textThe wake effect from upstream wind turbines is a hot topic recently and many researches on wake modeling have been carried out in the past few years. The wake models have evolved from the simple semi-empirical approach to the dynamic meandering model using the Computational Fluid Dynamics (CFD) codes. However, some of current wake models cannot simulate the dynamic wake meandering process while the other CFD models are too complicated for engineering applications. So the purpose of this thesis is to develop a simple numerical wake model including the detailed wake velocity deficits and dynamic wake meandering process in the atmospheric boundary layer. The Simplified Eddy Viscosity (SEV) model is developed based on the simplified Navier-Stokes equation in Ainslie's work. It describes the wake mixing process in the atmosphere and gives an estimation of quasi-steady two dimensional velocity deficits. Furthermore, the turbulence generator is implemented in the SEV model and provides a three-dimensional turbulence flow field in the wake. An experimental validation study of stationary wake deficits is carried out by using measurements from ECN‘s wind turbine test station. Moreover, the SEV model is modified according to ECN‘s measurements and it shows a fair agreement with GH Bladed eddy viscosity model. Besides the stationary wake deficits, a simple dynamic wake meandering mechanism is introduced in this thesis. According to this mechanism, the Dynamic Wake Meandering (DWM) model is developed, which is an integration of the wake meandering process, the wake deficits from the SEV model, and the aeroelastic model from GH Bladed. The DWM model is validated with Risoe‘s model and GH Bladed and there is good agreement between these models. The DWM model is used for load analysis and then compared with the traditional estimation method MET prescribed by the IEC 61400-1 standard. The flow fields in the DWM model are closer to the physical processes of wake transportation in the atmospheric boundary layer. The results show a significant difference between the methods chosen for the fatigue damage estimation.Aerospace EngineeringWind Energy Research Grou
Wind turbine reliability characteristics and offshore availability assessment
No full textSince the early 90s, wind energy industry has stepped offshore due to a number of interesting characteristics compared to onshore wind harvesting. At least when initially realized, offshore wind farms were not really different than onshore ones, since the same wind turbine technology with slight modifications was directly applied to the new environment. In these early cases, low water depths and small distance to shore may have justified this option. For future large scale offshore wind farms however, it is doubtful whether simply applying the existing onshore wind turbine technology will lead to the desired outcomes, e.g. high energy yield and revenues. Technical reliability of wind turbines affects the economics of an offshore wind farm since failures increase maintenance costs and decrease operational time, thus energy production. In the beginning of this report, the most important aspects that drive modern wind turbine design are investigated, followed by a review of different design choices for the various parts of a wind turbine as well as overall wind turbine topologies. A thorough reliability analysis, focused on comparison of the most prominent architectures, is then carried out using failure data from two German databases, LWK-SH and WMEP. Total annual failure rates of 2 to 5 failures per wind turbine per year were observed but no specific topology seemed to clearly stand out in terms of reliability. Most critical components proved to be blades, electrical and electronic systems, as well as hydraulics. Gearboxes, generators, and shafts and bearings are also important not because they fail very frequently, but due to the high associated downtimes. Using a widely used reliability growth model, there seem to be more failures during the first years of operation for most subassemblies. In order to identify the most important parameters affecting offshore wind farm availability and O&M costs per kWh, several Monte-Carlo simulations were performed using the software tool CONTOFAX. Results showed that availability is not only affected by technical reliability, but also availability of heavy lifting equipment and spare parts, site accessibility, crew and vessel strategy, and distance of the wind farm to shore.Aerospace EngineeringDuWin
Improving the Jensen and Larsen Wake Deficit Models: Using a Free-Wake Vortex Ring Model to Simulate the Near-Wake
No full textThe continued grow of the wind industry has resulted in an increase in turbine cluster density in wind farms. This has made the topic of wind turbine and wind farm wake aerodynamics more relevant than ever. Wind turbine wake models are used to predict the deficit and recovery of the wind velocities in the wake. The currently used wake models in the industry contain a relatively simple approach to model the near-wake of the turbine, assuming an initial hat or Gaussian deficit profile at a fixed location in the wake. This thesis presents an alternative near-wake model based on discrete, inviscid, perfectly circular vortex rings for use in conjuncture with existing far-wake models.Aerospace Engineering | Aerodynamics and Wind Energ
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