The analysis of aerodynamic performance begins with a disk or area in a stream flow of air. Conservation of energy and momentum are used to determine the limit on the amount of extract-able energy.

Forces of lift and drag on airfoils are measured experimentally in wind tunnels. As previous measurements were for use with airplanes, a lot of airfoil data [1] are available from national labs. Almost any shape can serve as an airfoil, even a flat plate, and the design of airfoils is almost an art. As wind turbine blades operate in different wind speeds than airplane wings, airfoil data with low Reynolds numbers [2] became available. Most of the lift and drag data were limited to attack angles up to stall and a few degrees pass stall, because after the stall point, the airplane loses lift, stalls out, and falls. Lift and drag data for attack angles up to 180° were only available for a few airfoils. Airfoils, which had a large ratio of lift to drag, were developed for sailplanes. Which airfoils are used for wind turbines depends on a number of factors, not just the ratio of lift to drag. As the requirements are different for wind turbines, starting in the late 1980s airfoils were designed specifically for wind turbines. A major change was to design airfoils that were less sensitive to surface roughness.

Different theories (strip theory, circulation, vortex shedding) and experimental data on airfoils are used to predict the rotor performance of wind turbines. This theoretical performance can be checked against the measured output of models in wind tunnels, truck testing for small-diameter units, or field testing (atmospheric) of wind turbines. At one time, a railroad flat car was used for controlled speed testing, as a somewhat larger turbine could be mounted. Overall efficiencies include those of the rotor, drive train, and energy converter (generator, etc.). The complete analysis on design of wind turbines, primarily rotors and structures, can be found in more advanced texts [3-12]; however, beginning physics can be used for a qualitative understanding of rotor performance.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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