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.Power (kw at V), probable time at wind speed (hours at V), and their product (kwh at V), are plotted (for Vavg= 10 mph) vs.wind speed V (mph) in the figure shown at right.Blades begin to feather at wind speeds above 30 mph, causing generated power to remain practically constant at feathered wind speeds.Yield (green area in this figure) is not appreciably lessened (red area) by blade feathering.However, compared to fixed blades (dashed blue curve), peak power that must be handled by the entire system is only about 20% as high.  Also, feathered blades with self-limited spin speed greatly reduce mechanical stress on all windmill parts (compared to fixed blades, with turbine shaft spinning at essentially one speed, or locked during low and high winds).For a windmill that sweeps 300 square feet, with 10 mph average wind speed (5 mph average for locale, doubled by building aerodynamics), and overall efficiency of  40% for windmill, generator, and PWM regulator, we get:Power generated at 10 mph = 588 watts from each windmill Yield from wind power = 729 kwh/month from each windmill Maximum generated power (limited to level at 30 mph wind speed) = 16 kw from each.If the building aerodynamics can result in 20 mph average wind speed at the same windmill, we can get yields of about 5,000 kwh/month, from the same windmill, having the same 16 kw maximum.We can now get interested in the shape, the design of the blades.We saw that they were very important for the capacity of the turbine to adapt to the different types of winds.To calculate the dimensions of those huge blades, some laws have been found from human experience:This blade looks like the shape of an airplane's wing ; if the relative wind is W, it is composed from the natural wind U and the wind created by the turbine itself rΩ.dCx is Uo natural winddCz is r radiusdT Ω angular speeddQ elementary couple of the blade I anglei incidence a axial induction coefficienta' radial induction coefficient alphaThe relative wind has an incidence i with respect to the blade's axis, this axis being oriented alpha with respect to the rotation plane;Angle I is the sum of alpha and incidence angle i.The relative wind W generates an elementary “poussée” dCz and an elementary drag dCx.The force dCz tends to make the blade rotate.The couple given by the forces dCx and dCz is equivalent to the one created by dQ/r and dT with dQ elementary couple produced by the blade and dT elementary drag.The elementary power is deduced:dP = dQ ΩOnce the power stored in the generator, we need to put it in phase with the one used and provided on the network: 50Hz for the Europeans.This part is done by a computer which controls the production with respect to the power of the wind.Wind turbines may be designed with either synchronous or asynchronous generators, and with various forms of direct or of the generator.Direct grid connection mean that the generator is connected directly to the (usually 3-phase) alternating current grid.Indirect grid connection means that the current from the turbine passes through a series of electric devices which adjust the current to match that of the grid.With an asynchronous generator this occurs automatically [ Pobierz caÅ‚ość w formacie PDF ]