Introduction: Cardboard Savonius Wind Turbine
Goal: build a Savonius wind turbine made out of cardboard to see what
works. This is for the turbine only and not the generator itself. The main photo you see is the goal.
The need for a working model grew out of frustration trying to
jury-rig various designs of a Savonius turbine that in the end would
not turn at all in the wind.
Some Initial Botched Designs
Shown below are several botched designs. All four are attached to the drive shaft of a 24-volt DC battery-operated lawn mower. The vertical bar you see is an allthread bar that is attached to the motor shaft. The galvanized metal is half of a dryer vent tube. The first design would turn half way and then stop because of the resistance of the back side of it coming into the wind. I then added a top disk and attached a number of 2-liter soda bottles and some 1-gallon milk jugs to a disk on top. With a stong wind I actually got the mill to spin if I gave it a start. It has to be the ugliest windmill ever.
The second design is all cardboard and looked really sharp. it didn't budge an inch in a very strong wind.
In the third one I added parts of the dryer vent tube to the cardboard mill and that worked a little bit but there was resistance on the back side coming into the wind.
At this point, I decided that I had to go back to the drawing board with a simple model to see what had a chance of working.
Step 1: Materials Needed
The following project took me one hour from conception to finish and I
was making design decisions as I went along. I was amazed that it
worked perfectly. I guess it should work perfectly because it is the
design described in a number places. Of course, the coffee can and
soda bottle designs were said to work but didn't for me.
a. A large piece of heavy-duty cardboard box
b. 14-inch dowel (or knitting needle, or something similar) to serve as an axis rod
c. 2 small rubber bands
d. Duct tape or electrical tape
e. Hot glue (optional)
What you see in the photograph are the pieces you are going to cut out of the cardboard. You can also see the knitting needle with some rubber bands on it.
Step 2: Make the Vertical Paddles
Cut 4 strips of cardboard 4" x 10". These will be the vertical
paddles that catch the wind. Cut two rectangular notches in each
paddle. Each notch should be 3 inches long and just wide enough that
the disk will fit in snugly. These notches will be located 1 inch from either side of the ends of paddles.
Step 3: Cut the Top and Bottom Disks of the Turbine
You will need to cut two disks 10 inches in diameter. Since I flunked kindergarten, I found this somewhat of a challenge. Trying to draw a true circle without a decent compass is tricky. Then after you have drawn it and cut it, finding the true center is also difficult. Here's an easy way to do it.
1. Cut a 10x10 inch square of cardboard.
2. On all four sides make a mark half way between the edges. That mark will be 5 inches from the corners on each side.
3. Use a ruler to draw lines that connect opposing 5 inch marks through the center of the square. Where these lines intersect, that's will be the center of the circle.
4. Use a compass if you have one and jab it into the center and expand it out to end of one of the radius lines. Now draw a full circle with the compass. I'm not sure school compasses are big enough to do this. An alternative might be to get a ten-inch bowl or something like that and lay it over the square and trace the lines. You can also simply eye-ball it and make an arc from the center of one side the the center of the next. You may not get a perfect circle but I don't think this is too critical for the model we are making. In the real thing it may throw off the ballance.
5. Now cut the circle.
Step 4: Draw Angle Lines on the Disks
Now for a little geometry. Take a look at the first image. Notice that there is an Angle A and sides a, b, and c. Side c is the line that we want the paddle to line up with. Where we draw it depends on angle a. To do a little geometry on this let's cheat and use a web-based right triangle calculator.
Make use of the following url: http://www.csgnetwork.com/righttricalc.html My side b is 5 inches (the radius of the circle) and I chose to make side a 2.25 inches. Accoding to the calculator, this makes angle A 24 degrees. You can play with this calculator by either entering side a or angle A as you experiment with different configurations.
Going with my 24 degrees configuration, make a mark along side a 2.25 inches from the center of the circle and then draw side c. Now do that with the other three radius lines.
After that is done, do the same thing with the other disk.
Notice that there are short cuts in the disk at angle A on each radius line. The pupose of these cuts is dock the paddle to the disks. The cuts should be about a half inch and should follow the path of line c.
Step 5: Attach the Paddles
1. Double or triple up one of the rubber bands and push it up the axis rod to within about 2 inch from the top. The purpose of this and the other rubber band is to anchor the rod to the disk via friction. The also help to keep the turbine from slipping up and down.
2. Push the axis rod down through the center of one of the disks. I suggest that you keep the side of the disk with the angles drawn facing up for easlier alignment of the paddles.
3. Push the rod through the center of the second disk with the side of the disk with the drawn angles facing down. Slid the disk up the rod to within about 8 inches of the top disk.
4. Slide the second rubber band up the rod to within an inch of the lower disk.
5. Push one of the paddles onto upper and lower disks so that the notches on the paddle dock into the angled notches on the upper and lower disks. You might want to temporarily tape the paddle into place from the top and bottom.
6. Mount the other three paddles in same way.
7. Push the rubber band on the bottom up the rod so that it sits firmly up against the lower disk.
8. Now you can tape the paddles more firmly to the lower sides of the disks (the lower sides if you don't want the tape to show. As an alternative, you could use a glue gun to glue the paddles in place. Only do this after you have finished testing the turbine in the wind and are happy with the angles of the paddles.
Step 6: Test the Wind Turbine
Now you can test the wind turbine. Inside the house you can hold up lightly with both hands in front of a fan and watch it turn. You can see that the fan makes the turbine very easily. The outside video, unfortunately, was taken when wind was variable and blowing only about 3 mph.
The two videos that are attached I jury-rigged a setup so that the turbine could stand up by itself without being held. You can use your imagination as to how mount the turbine a little more permanently.
Experiment with different angles for the paddles to see what is the most efficient. I chose 24 degrees. Would 45 degrees work better? How about 10 degrees? You will notice that the smaller the angle, the better the paddle catches the wind on the left side. However, you notices that it also catches it too well on the right side. If you made the paddles 0 degrees, the turbine would sit idle in a strong breeze.
Where to go from here
The turbine, as is, probably could be used to generate a small amount of electricity in a school setting. The trick will be figuring out how to hook a small motor to it. You could also glue magnets to the bottom of the lower disk and then making coils of wire that remain stationary below the turbine.
What I want to do is make this turbine out of plywood and scale it up by double. I will use some of the material shown in the into to this instructable. Specifically, I will use the DC motor, the vertical allthreads bar that will serve as the axis rod, and the clamp I made to mate the motor and axis rod.
It will be interesting what kind of power I get out of the scaled up version. I would like to see if I could light up a series of LED's when the wind is blowing. That would look great at night. I might also try to use the power to pump water up over a waterwheel.
If you build this cardboard turbine, please post lessons learned and additional suggestions.