The starting and low wind speed behaviour of a small horizontal axis wind turbine

This paper presents an optimization procedure for stall-regulated horizontal-axis wind-turbines. A hybrid approach is used that combines the advantages of a genetic algorithm and an inverse design method. This method is used to determine the optimum blade pitch and blade chord and twist distributions that maximize the annual energy production. To illustrate the method, a family of 25 wind turbines was designed to examine the sensitivity of annual energy production to changes in the rotor blade length and peak rotor power. Trends are revealed that should aid in the design of new rotors for existing turbines. In the second application, a series of five wind turbines was designed to determine the benefits of specifically tailoring wind turbine blades for the average wind speed at a particular site. The results have important practical implications related to rotors designed for the Midwest versus those where the average wind speed may be greater.

Application of a Genetic Algorithm to Wind Turbine Design

Design guidelines for H-Darrieus wind turbines: Optimization of the annual energy yield

An extension of the Blade Element Momentum method applied to Diffuser Augmented Wind Turbines

Equations are developed with which to calculate lift and drag coefficients along the spans of torsionally-stiff rotating airfoils of the type used in wind turbine rotors and wind tunnel fans, at angles of attack in both the unstalled and stalled aerodynamic regimes. Explicit adjustments are made for the effects of aspect ratio (length to chord width) and airfoil thickness ratio. Calculated lift and drag parameters are compared to measured parameters for 55 airfoil data sets including 585 test points. Mean deviation was found to be -0.4 percent and standard deviation was 4.8 percent. When the proposed equations were applied to the calculation of power from a stall-controlled wind turbine tested in a NASA wind tunnel, mean deviation from 54 data points was -1.3 percent and standard deviation was 4.0 percent. Pressure-rise calculations for a large wind tunnel fan deviated by 2.7 percent (mean) and 4.4 percent (standard). The assumption that a single set of lift and drag coefficient equations can represent the stalled aerodynamic behavior of a wide variety of airfoils was found to be satisfactory.

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Models of Lift and Drag Coefficients of Stalled and Unstalled Airfoils in Wind Turbines and Wind Tunnels

Wind Turbine blades operate over a wide angle of attack range. Unlike aircraft, a wind turbine's angle of attack range extends deep into stall where the three dimensional performance characteristics of air foils are not generally known. Peak power predictions upon which wind turbine components are sized, depend on a good understanding of a blade's post stall characteristics. The results of the wind tunnel investigation of untwisted, constant chord blades having four aspect ratios, with an NACA 4415 series airfoil section, at angles of attack ranging from -10 to 100 degrees are discussed. Tests were conducted for aspect ratios of 6, 9, 12 and infinity at four Reynolds numbers ranging from one-quarter million to one million. The results on the same family of airfoil section but with varying thickness ratio are given. Results of force and pitching moment measurements over the angle of attack range for all combinations of Reynolds numbers and aspect ratios, and the effects of boundary layer tripping, are presented. Both initial and secondary stall are presented. The maximum drag coefficient is found to occur an an angle of attack of 90 degrees. The pitching moment is unstable beyond stall. The lift and post-stall drag-coefficients decrease with decreasing aspect ratio. The boundary layer tripping is observed to decrease the lift curve slope and stalling angle of attack. The drag coefficient (with tripping) is significantly affected only at low aspect ratio. Beyond secondary stall, the lift to drag ratio is independent of aspect ratio. The maximum lift to drag ratio for the infinite aspect ratio blade is roughly twice that of the blade with an aspect ratio of six. This effect is independent of Reynolds number in the range studied.

C Ostowari

Papers

Post stall studies of untwisted varying aspect ratio blades with an NACA 4415 airfoil section. Part I