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Wind Turbine Concepts Defined and Explained

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On this page we attempt to give a brief introduction to the basic concepts of designing and building wind turbines.

| Site | Tower | Anemometers | Generators & Alternators | Cut-In Speed | Alternator Design |
| Rotor Design & Carving | Furling & Shutdown | Regulation | Slip Rings | Futher Reading |

Where do you start???

First, do your homework! Why re-invent the wheel when you can learn from others' successes and failures? There are many useful books, websites and plans available. Check our recommended reading list HERE.

First, figure out how big a wind generator you are willing to tackle, either commercial or home-brewed. There is really only one important measure of windmill size...the swept area. That's how many square feet (or meters, if you are into that sort of thing) of area the windmill's blades cover during a rotation. The formula for swept area is Pi r^2, where Pi is 3.1415 and r is the radius of your prop. The available power from the wind increases dramatically with the swept area...but so do the stresses on your blades, tower, bearings, tail. More stress means stronger engineering and materials are required, and a much larger, more complicated and expensive project.

Site

Tower

Check out our TOWERS page for some home-brewed solutions that are cheap and easy to fabricate, plus lots of details and pictures. There's also lots of tower information, discussion and pictures available by Searching the Otherpower discussion board for 'towers'.

Anemometers

Generators and Alternators

Alternator Design

Rotor

  • A wind generator gets its power from slowing down the wind. The blades slow it down, and the alternator collects the power. BOTH must be correctly designed to work together and do this efficiently. We are not experts at blade design...we sort of started in the middle with a functioning design, and made changes from there. Really, you could make a simple set of blades with a straight 5 degree pitch down the entire length and they would work JUST FINE! But to really tune in the performance of your wind generator, it's important to pay attention to a few factors. ALSO--please forgive us when we slip up and refer to the rotor as a "prop" or "propellor"--it doesn't propel anything! Rotor is the proper term, not to be confused with the rotor of an armature. But we slip up sometimes...
  • Blade Material--Wood is really an ideal material for blades. It is very strong for its weight, easy to carve, inexpensive, and is resistant to fatigue cracking. Choose the best, straightest, most knot-free lumber you can find; pine and spruce are excellent. Hardwoods are generally too heavy. Steel and aluminum blades are much too heavy and prone to fatigue cracking; sheet metal would be a poor choice, and extremely dangerous...check out the photo of fatigue cracks on a sheet metal windmill TAIL in Ward's Prop Gallery and imagine what the vibration would do to sheet metal blades! Cast reinforced Fiberglas® blades are very strong, and are common on commercial windmills--but the moldmaking process would take longer than carving a complete set of blades from wood, and there would be little or no gain in strength.
  • Diameter--Blades that are too short attached to a large alternator will not be able to get it moving fast enough to make good power. Blades that are too large for a small alternator will overpower and burn it up, or overspeed to the point of destruction in high winds--there's not enough of an alternator available to collect the energy coming in from the wind.
  • Number of Blades--The ideal wind generator has an infinite number of infinitely thin blades. In the real world, more blades give more torque, but slower speed, and most alternators need fairly good speed to cut in. 2 bladed designs are very fast (and therefore perform very well) and easy to build, but can suffer from a chattering phenomenon while yawing due to imbalanced forces on the blades. 3 bladed designs are very common and are usually a very good choice, but are harder to build than 2-bladed designs. Going to more than 3 blades results in many complications, such as material strength problems with very thin blades. Even one-bladed designs with a counterweight are possible.
  • Tip Speed Ratio (TSR)--This number defines how much faster than the windspeed the tips of your blades are designed to travel. Your blades will perform best at this speed, but will actually work well over a range of speeds. The ideal tip speed ratio depends on rotor diameter, blade width, blade pitch, RPM needed by the alternator, and wind speed. Higher TSRs are better for alternators and generators that require high rpms--but the windspeed characteristics at your particular site will make a big difference also. If in doubt, start in the middle and change your blade design depending on measured performance.
  • Taper--Generally, wind generator blades are wider at the base and narrower at the tips, since the area swept by the inner portion of blades is relatively small. The taper also adds strength to the blade root where stress is highest, gives an added boost in startup from the wider root, and is slightly more efficient. The ideal taper can be calculated, and it varies depending on the number of blades and the tip speed ratio desired. Hugh Piggott's Windpower Workshop book and his free Blade Design Notes contain the relevant formulas. Honestly, though...if you simply take a look at a picture of a functioning small-scale wind generator's blades and estimate the taper by the eyeball method, you will come very close to meeting the criteria and have a very functional blade. Our Basic Blades page gives a basic introduction to blade design and carving.
  • Pitch and Twist--As we've said before, a simple wind generator blade with a straight 5 degree pitch down the whole length would give adequate performance. There are advantages to having a twist, though--like with taper, having more pitch at the blade root improves startup and efficiency, and less pitch at the tips improves high-speed performance. The wind hits different parts of the moving blade 's leading edge at different angles, hence designing in some twist. One of our common blade designs that's right in the middle for design parameters is to build an even twist of 10 degrees at the root and 5 degrees at the tip--but the ideal solution will also depend on your alternator cut-in speed, efficiency and local wind patterns.
  • Carving--Our layout and carving process is very simple...after marking the cut depth at the trailing edge at both the root and tip, the two depths are connected with a pencil line. DanF likes to use a hand saw to make layout cuts into the blade every couple inches along the length before firing up the electric planer...when the saw kerfs disappear, the pitch is correct. DanB prefers to hack into it with a planer right from the start. In case you are fuzzy about how this all goes together, the drawings below might help.
    Diagram of blade showing area removed

    Blade face diagram showing cuts

  • Airfoil--There are great lengths that you can go to for designing an airfoil...NASA has some great information and calculations out there on the net. But all an airfoil needs to do is maximize lift and minimize drag. You will do fine if you do like we did--find a likely looking airfoil cross section from a working wind generator blade, and copy it. A power planer makes quick work of carving it, and a drawknife is great for carving too, especially with the deep cuts near the blade root.
  • Balancing--The blades must be very well balanced to prevent vibration. This is more easily accomplished with a 2-blade rotor than a 3 bladed one. But generally, we simply use a homemade spring scale to make sure that each blade weighs exactly the same, and that each has the same center of balance. A simple balancing jig for any rotor configuration can be made with an upright spike that sticks into a dimple punched at the exact center of the hub. Excess material from the heavy areas can be removed quickly with a power planer. You'll also need to balance the blade in place on the alternator. It's weight distribution can be adjusted by attaching lead strips to the blade root.
  • Furling and Shutdown Systems

    Regulation

    Slip Rings

    The power produced by the generator must be transferred down the tower to your power system. Since the actual wind generator must yaw to keep pointed into the wind, the main power wires must be able to handle this. There are 2 options...

    Recommended reading list for your 'homework':


    Wind Turbine Seminars and Renewable Energy Fairs
    These can be a very valuable resource for learning about all facets of renewable energy! You'll be able to learn from and network with experts and other interested folks.

    UPCOMING WIND TURBINE CONSTRUCTION SEMINARS


    Wind Turbine Parts and Kits

    We offer for sale a large variety of the 'stuff' you need to build your own wind turbine.
    It's all available at our Online Store.


    Wind Turbine Design and Construction Articles:

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    This page last modified 2/20/2012

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