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.
Variable pitch wind turbines start very efficiently in low speed winds, such as those 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 are set at a fixed pitch angle. They are usually designed for peak performance around 28 mph on a typical operating curve.
Above that point, 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 which they require.]
Why is over-speed control so important for fixed pitch turbines?
Fixed pitch turbines can easily rotate too fast, and are therefore subject to high stress levels from centrifugal, bending, torsion, and flapping forces which can ultimately result in dangerous failure.
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.
What is the major drawback to using fixed pitch blades?
You cannot set the one fixed pitch blade angle for both startup and high speed winds.
The fixed pitch angle delivers 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. Practically, however, such special locations are not 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. Noting that lower speed winds prevail most of the time in more populated areas, the variable pitch turbine is the logical design choice.
Do airfoils matter?
Yes, blade airfoils profoundly affect wind turbine performance, particularly for small wind turbines, where scaling issues become significant.
[Because aerodynamic performance deteriorates for small scale turbines due to low Reynolds Number effects, small airfoil performance is extremely sensitive to good aerodynamic design. Only high performance blades will deliver drag forces less than a hundredth of the lift force. Our blades (developed 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 any small wind turbine. With negligible lift, paddles and cupped sheet metal blades are absolutely the most inefficient choices, at the other extreme.
THE PERMANENT MAGNET ALTERNATOR
Does the stator winding have large effect on performance?
Given the proper choice of rotor magnet, the correct matching stator winding is absolutely critical in achieving maximum turbine permanent magnet alternator performance.
A stator winding of the correct wire size, lamination magnetic properties, number of turns, and heat dissipation characteristics will deliver the maximum output voltage and current, reliably, for a given rotor magnet field strength. In practical terms, the proper interplay of blade aerodynamic forces, electromagnetic forces, and heat transfer, in terms of alternator loading, speed, and output, is like a ballet. When everything is properly orchestrated, performance is outstanding.
Our custom made magnets, rotors, and windings are the result of extensive development and fall into this category.