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Basic Designs and Design Issues of Wind Turbines. Online references from which some of this material was obtained: www.windpower.org/en/tour/design/index.htm www.eere.energy.gov/windandhydro/wind_how.html. Basic Designs of Wind Turbines. One convenient categorization is the following:
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Basic Designs and Design Issues of Wind Turbines • Online references from which some of this material was obtained: • www.windpower.org/en/tour/design/index.htm • www.eere.energy.gov/windandhydro/wind_how.html
Basic Designs of Wind Turbines • One convenient categorization is the following: • “Horizontal” Axis Machines • “Vertical” Axis Machines • The words “horizontal” and “vertical” are normally used, but we are really referring to rotors with axes that are either parallel (horizontal) or perpendicular (vertical) to the local wind velocity. • Each type has certain advantages and disadvantages
Wind Turbine Classification • Horizontal Axis Machines: Machines with rotors that move in a plane perpendicular to the direction of the wind. • A farmers windmill, for example. • Vertical Axis Machines: Machines that have the working surfaces traveling in the direction of the wind. • These machines are sometimes called “panemones.”
Advantages of Vertical Axis Machines • The generator and gearbox can be placed on the ground • The structure is usually simpler. • You do not need a yaw (pointing) mechanism to turn the rotor against the wind. • These are easier for hobbyists to build – little detailed knowledge of aerodynamics is needed for simple designs.
Disadvantages of Vertical Axis Machines • These structures are low to the ground, where wind speeds are lowest. • The overall efficiency is much lower than horizontal axis machines. • Most vertical axis machines are not self starting. • Many vertical axis machines require guy wires which greatly increase the structural footprint. • Maintenance is usually more difficult. • For example, replacement of the generator typically requires disassembly of the entire machine.
Current Status of Vertical Axis Machines • The only commercially available vertical axis machine that was built in large quantities was the Darrieus rotor, built by Flowind Inc. • Flowind declared bankruptcy in 1997. • There are several vertical axis concepts that are being studied. • They all face the same challenges that scuttled previous efforts.
Advantages of Horizontal Axis Machines • The efficiency is higher than that of vertical axis machines. • They are easier to mount high enough to avoid much of the ground effect. • They are self starting. • They are less expensive. • The technology is better developed. • They are available commercially.
Disadvantages of Horizontal Axis Machines • Many of the important parts that require maintenance are high off the ground. • A yaw mechanism must be in place to turn the turbine into the wind.
Current Status of Horizontal Axis Machines • Virtually all grid connected wind turbines in operation today (fall 2005) are propeller type designs mounted on a horizontal axis. • Some of these machines have the ability to pitch up and down to face the wind as directly as possible. • Instructor - make sure everyone knows the aircraft terminology of “pitch,” “yaw,” and “roll” axes! • These designs are being improved on.
Variations of HAWT Designs – Upwind Designs • Upwind Machines: The front rotor faces the wind. • This avoids interference with the wind from the structure. • The rotor must be very stiff and placed far enough from the structure to avoid contact/interference problems. • A yaw control mechanism is necessary. • Most wind turbines in operation are of this design.
Variations of HAWT Designs – Downwind Designs • The rotor is placed on the downwind (lee) side of the structure. • An advantage is that, at least in principle, yaw control is not necessary. • This is not always realized in practice since a mechanism still must exist to prevent the machine from continuously rotating! • In principle, the rotor blades can be much more flexible with these designs. • Both upwind and downwind machines have a power fluctuation as the blades pass in front of or behind the structure. However, this is much more pronounced with downwind designs.
Example of Downwind Design – from The Wind Turbine Companywww.windturbinecompany.com/technology/index.html
Number of Blades and “Solidity” • The property called “solidity” is important in turbine (and compressor) design. • Solidity is defined as the fraction of the swept area that is occupied by a working blade. • A farmers windmill is a high-solidity device, while a single blade rotor, illustrated earlier, is a low solidity device. • An efficient turbine must interact with as much of the wind passing through the swept area as possible. • As the solidity decreases, the rotor speed must increase for this to happen. • Therefore, as the number of blades decrease, the required speed for maximum efficiency must increase.
How Many Blades? • Two-blade designs are problematic because they can lead to structural instability for stiff structures. • This is because as one blade passes the wind shade of the tower, generating its minimum lift, the other blade is above the tower generating its maximum lift. • Three blade designs avoid this problem, and are able to rotate slower. • This is more or less the standard design right now – and is called (at least by the Danes) the “Classical Danish Design.” • This terminology also refers to certain control and power take off schemes.
Windbelt • An alternative to wind turbines • A taut membrane fitted with a pair of magnets that oscillate between wire coils. • Prototypes have generated 40 milliwatts in 10-mph winds • Can be 10 to 30 times as efficient as the best wind microturbines.
How Many Blades? (continued) • One blade concepts have also been tried. • An Illustration of these three types is given by the following hyperlink: • www.windpower.org/en/tour/design/concepts.htm
Noise Issues with Wind Turbine Design • There have been complaints concerning noise generated by wind turbines. • They are really not very loud, but the noise they do put out is pretty much constant! • There are stories of homesteading settlers, driven insane by the constant west Texas wind, running out of there homes and across the plains, never to be seen again! • The noise level doesn’t bother most people, but we should probably avoid instigating insanity among even a small portion of the populace.
Mechanical Noise in Wind Turbines • Mechanical noise emanates from moving parts such as gearboxes and Shafts. • Gearboxes can be designed for quite operation, and insulated to reduce noise. • Certain gear designs (hard surface, softer interior, for example) are quieter than others. • Unstable vibrations of turbine blades and structures can emit sound. • This problem has largely been eliminated through the use of finite element analysis and other modern structural design tools.
Aerodynamic Noise in Wind Turbines • Sources of Aerodynamic Noise • “Singing” and “Whistling” from sharp corners, etc. • Similar phenomena to that used by many musical instruments. • Vibrations induced by Aerodynamic effects may be in the audible range. • A portion of the irreversible losses that occur as the wind transfers energy to the blade results in the “Whishing” sound. • Acoustic principles tell us that this emission goes as generally the fifth power of the rotor blade speed relative to the surrounding wind. • Just as with mechanical noise, these effects can all be minimized through the use of modern design tools.
Structural and Mechanical Design Issues with Wind Turbines • Components of Wind Turbines that are under considerable stress: • Propeller Blades and Hub. • Mechanical Powertrain • Support Tower • Loads are both static and dynamic • Structural vibrations due to both propeller motion and wind loads must be accounted for. • Fatigue failures have been a problem on some designs. • Wind gusts are very common. • Contingencies for extreme winds must be made. • For example, feathering and locking the propeller. • Structural design and analysis will be the subject of a subsequent lecture.