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UNDERSTANDING VARIABLE PITCH ADVANTAGES
What is a variable pitch wind turbine?
A variable pitch turbine is one in which the rotor blade pitch angle is continuously adjusted by an internal hub mechanism to provide constant speed output over a wide range of wind speeds. It is a precision solution to overspeed control. [See the ‘Variable Pitch’ curve in the graph below]
Variable pitch wind turbines can start very efficiently in low speed winds, typically found around more populated areas. Upon starting to move, the rotor blades adjust in pitch, enabling the turbine to reach operating speed quicker than fixed pitch blades. Upon reaching the controlled operating speed they continue to adjust and produce power at constant output speed, right up to and including very high wind speeds, regardless of wind speed or electrical load. There is no need to avoid operating in high winds. And, significantly, because constant operating speed is readily reached at lower speed winds, the need for high and costly towers is much reduced.
What is a fixed pitch wind turbine?
A fixed pitch turbine is one in which the turbine rotor blades operate at a fixed pitch angle. They are usually designed for peak performance around 28 mph on a typical operating curve (see the smaller ‘Fixed Pitch’ curve in the graph above).
Above that point (peak output), over-speed control is required for safety reasons. So, although power output is good at peak output, fixed pitch turbines are then forced to limit over-speed operation by aerodynamic blade stall, or furling (swinging out of the wind), or electronic braking, or combinations of these. [Additionally, there is a natural tendency to erect high and expensive towers for these turbines in order to achieve the high rated speed more of the time.]
Why is over-speed control so important for fixed pitch turbines?
If turbine blades rotate too fast, they are subject to high stress levels from centrifugal, bending, torsion, and flapping forces which can ultimately result in dangerous failure. Hence the need for over-speed control in any wind turbine.
Is over-speed control effective in fixed pitch wind turbines?
Over-speed control in fixed pitch turbines is always a challenge, and it comes at a price. Since their blades lack pitch adjustment, fixed pitch turbines must employ stall, furling, or electronic braking methods to achieve over-speed control. These are adequate solutions, but they also give up on virtually all the energy contained in winds above the design speed and a significant amount of energy in winds below it.
Are there additional drawbacks to using fixed pitch angle?
Yes. A less obvious drawback of fixed pitch operation is that you cannot optimize rotor blade pitch angle for both startup and high speed winds. For example, assume you have a 10 degree fixed pitch blade angle. The turbine blades will start turning in a low speed wind. But, at higher wind speeds, with blades now cutting into the oncoming wind at tip speeds up to 6 or so times that of the oncoming wind speed (for good airfoils), the pitch angle requirement diminishes (because of the change in relative wind vector). Now, a fixed pitch angle of only 2 degrees would be much more suitable. That suggests two different settings. This is simply not possible with fixed pitch blades.
Thus, the choice of a single fixed pitch angle forces either good startup and early stall or poor startup and good high speed performance. Either choice translates into a performance compromise, compared to variable pitch blades which adjust to perform well in both cases.
Does fixed pitch operation have a place?
Yes, if sustained wind speeds happen to be close to a typical fixed pitch design speed of ~ 28 mph, fixed pitch performs well. At these speeds, startup concerns are irrelevant. However such special locations are not that plentiful, as local wind maps will show.
Which design has the fundamental advantage?
By definition, a variable pitch wind turbine is more effective over a much larger range of wind speeds, including low speed, which it excels at. Further noting that lower speed winds prevail most of the time in more populated areas, the variable pitch turbine advantage clearly prevails as the logical design choice.
UNDERSTANDING AIRFOILS
Do airfoils matter?
Yes, blade airfoils profoundly affect wind turbine performance, particularly for small wind turbines.
Because aerodynamic performance deteriorates at small scale due to low Reynolds Number effects, small airfoil performance is even more critically dependent on good aerodynamic design. Drag forces for a high performance airfoil can approach a hundredth of the lift force. Our blades (design by Dr. M. Selig, University of Illinois) are in that category.
Achieving and utilizing high blade lift is the only way of extracting good performance out of a small wind turbine.
THE PERMANENT MAGNET ALTERNATOR
Does the stator winding have large effect on performance?
Given the correct choice of rotor magnet, the stator winding is absolutely critical in obtaining maximum turbine generator (permanent magnet alternator) performance at sustained operating speed. A stator winding of the correct wire size, lamination performance, number of turns, and heat dissipation characteristics will deliver the maximum output voltage and current, reliably, for a given rotor magnet field strength. The proper interplay of blade aerodynamic forces, electromagnetic forces, and heat transfer, in terms of generator loading, speed, and output, is like a ballet. When everything is properly orchestrated, performance is outstanding.
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