Q & A: Generating electricity with a model windmill

This is a question for electricity and magnets. Me and some friends of mine are trying to make a working model of a windmill. Are goal is to try and make produce enough electricity to generate something. Like maybe a lightbulb or something smaller. So how are we to approach in doing this? I know it takes a certain amount of wind speed to produce the electricity in the big windmills but will we need that same amount with ours?
- Brittney
St. John’s Lutheran School, Hemlock, MI

Well, you can always generate a small amount of electrical power with a small windmill, and it takes a bigger windmill and lots of wind to generate a lot of power. Power is energy per unit time, and so if you let your small windmill generate electricity for a long time, the total can add up to a large amount, but you'll get it very slowly.

To convert the energy of the moving shaft of the windmill to electrical energy, you'll need a dynamo. For the most part, this consists of a strong magnet with loops of wire that rotate inside the magnetic field. Another possibility is for the loops of wire to be held stationary and a magnet turns around inside. Many permanent magnet motors (with brushes) can be used to generate electricity by turning the shaft. You can also buy a bicycle headlight set that includes a generator that clips on to the frame of the bicycle. Try turning the shaft with your hand first to see how hard it is. A very small windmill may not have the torque to turn a big dynamo's shaft very fast or at all (bicycle generators have a certain bumpiness to their resistance and they get "stuck" unless they are turned with enough torque). You can always get more torque out of even the smallest windmill with gears, but at the price of turning the generator shaft more slowly.

The maximum energy you get out of a windmill is proportional to the area swept out by the blades. Big turbines on hillsides operated for the power grid are sized so as to be economical -- you could do the job with a large number of little ones but then maintenance and synchronizing all the power phases would be expensive. As it is, the cost of maintenance and the reliability of the wind limit the ability of using wind power as a substitute for other sources of electrical energy.

Feel free to demonstrate the usefulness of wind power, though! You may have to size your load appropriately so that the light from your light bulb is visible when the shaft of your dynamo is turned by the windmill. See if you can get the light bulb to light just by turning the dynamo shaft with your fingers. How strong is the windmill going to twist the shaft in a good breeze? (you can also estimate this with your fingers, but be very careful not to get hurt!). A flashlight bulb is a good place to start. Or just a voltmeter.

Tom

(published on 10/22/2007)

i am working on a windmill project. As you said in answer, we can use large numbers of windmill instead of a big one, but relatively it will create small amount of energy. I want to know that if the amount of energy generated by windmill is proportion to the size of the windmill(length or the surface area of blade) or the Rotation(rpm)of the magnetic generator? definitely in small windmill, because of the small size of the generator, energy created will be small. But what directly affect the Created Energy?
- sachin kale (age 27)
Mumbai, India

Dear Sachin,
This is a very complicated and technical problem.  There are many variables that go into the design of an efficient and inexpensive power windmill.  To answer one of your questions, for a given wind speed the power generated is proportional to the surface area of the blades.   There are many web sites that go into the details of building a practical windmill.  I suggest you look into these:
http://otherpower.com/otherpower_wind_tips.html
http://www.otherpower.com/windbasics1.html

LeeH

(published on 10/12/09)