1. Basic Designs and Design Issues of Wind Turbines
Online references from which some of this material was obtained:

2. 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

3. 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.”

4. Examples of Horizontal Axis Machines

5. Examples of Vertical Axis Machines

6. 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.

7. 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.

8. 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.

9. Darrieus Wind Turbine at SNL, Early 1980’s?

10. VAWT that are Under Development

11. Other VAWT Designs

12. Other VAWT Designs (continued)

13. Other VAWT Designs (continued)

14. Early VAWT – The Savonius Rotor. (A very simple design!)

15. 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.

16. 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.

17. 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.

19. 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.

20. Example of Upwind Design – 3.6 MW GE Wind Turbine

21. 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.

22. Example of Downwind Design – from The Wind Turbine Company www
Example of Downwind Design – from The Wind Turbine Company

23. 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.

24. 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.

25. 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.

26. How Many Blades? (continued)
One blade concepts have also been tried.
An Illustration of these three types is given by the following hyperlink:

27. 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.

28. 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.

29. 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.

30. 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.