The purpose of this post is to inform and demonstrate the thinking and problem solving S.T.E.M. skills used by KidWind teams in the design, building and testing of a model wind turbines to produce electric power.
This design will be modeled after the common water pumping, wind mills, used for years on many farms.
With that goal in mind the team must decide on the size and type of blades that will make up the rotor. They must decide on the generator size and drive train that will connect to the rotor. And lastly they need to decide how the rotor and generator will be supported and held in the wind tunnel for testing.
Starting with this 18” bicycle wheel rim. Now the “wheels” of a KidWind team can start turning using their S.T.E.M skills to solve problems they encounter along the way.
What will the blades be made of? I chose galvanized sheet metal because that is what the farm windmills used and I could get this out of the local heating contractor’s scrap bin for nothing and recycle it.
How many blades and what size will the blades be? This design was going to follow that of a water pumper so I wanted a high solidity number. Note:Turbines with high solidity around 0.80 produce high torque at lower speeds. Keeping in mind how I was going to keep the blades from bending. My 20 gauge scrap material was 16” wide so that would be the length of each blade.
What diameter circle could I get with 16” long blades? I would calculate the circumference ( cir = 2 Pi X radius ) to be 100.48 inches. If I choose to make the blade tip 6” wide. Then dividing 6” into 100.48 gave me 16.74 blades. I would round this down to 16 blades and have a slight space between blades (resulting in a very high Solidity).
How would I layout the blades on the sheet metal and then cut them to size? I determined that if I alternated tops and bottoms of the blades on the 16” wide sheet there would be no waste. My 20 gauge sheet metal was to thick to cut by hand with an aviation tin snips so I would need to find a friend in the heating industry with a mechanical squaring shears cut them. Note: for some reason I only cut 8 blades but in hindsight this will prove to be a good thing as it will allow for more testing and comparison when I add in the other 8 blades.
I found a 6” collar in the stove pipe department at Menards as an anchor for the root ends of the blades. The inside diameter of the collar hole was 3.5” so I could calculate the circumference (cir = 2 Pi X radius) to be 11”. Thinking that clipping the point off would provide a wider root end to attach each blade with 1/4” pop rivets to the 6” collar. By chance I tried thought about making them 2” wide but knew they would not fit the 11” circumference. I would divide 11” by 8 blades and come up with a 1.375” wide blade at the root end. Call it “dumb luck”. After clipping off the points of each blade I needed to begin the process of attaching the blades to the bicycle rim. The root ends would be attached with 1/4” steel pop rivets. Using two rivets per blade. The mid-point of each blade would be attached with a 6-32 NFT (National Fine Thread) 3/4” long machine screw on the fixed side and a 8-32 NFT 2” long screws on the pitch adjustable blade edge.
Here you can see the mid-blade attachment screws. The 8-24 NCT 2” long machine screw has a nut that holds it to the bicycle rim. The 2” long one goes into the blade and the nuts can be positioned to set and hold the necessary pitch of each blade. Note: holes for these machine screws could be drilled but a better method is to use a sheet metal punch. This works like a paper punch but is much heavier duty. Any local heating contractor will have this tool.
How will the robot be supported? With the rotor assembled it is now time to think about that. There are several materials and structures that could be considered. Think about what materials and skills you have. Think about the strength, weight and costs. I choose plastic PVC pipe because I had a lot on hand from previous KidWind turbine projects. The idea was to run the 5/16 - 24 NF (National Fine) threaded axles of the bicycle wheel through the centers of the four way fittings.
A problem soon became apparent. The 5/16 - 24 NF (National Fine) threaded axles were to short to go through the 4 - way fittings
This new problem would require some new learning and machine skills. Drilling and threading of a 1/2 - 13 NC (National Course) threaded piece of Ready rod. The drilling could be done on a drill press or with a hand drill but would be best done using an engine lathe. The use of a Tap Drill chart would be needed to determine the correct size of tap drill to use so that enough material would be left for thread cutting. Also a 5/16 - 28 NF tap and tap wrench would be needed. Then the process of cutting the threads would be performed.
How will the turbine support system be built? The idea for the design of the turbine stand would be to place the 1 - 1/2” PVC pipe legs at 45 degree angles as shown. Each leg would have Tee fitting on the end. Two identical front and back frames would be made.
How long will the legs be cut? Critical math calculations would need to be made to determine the correct length for cutting the legs. The depth the PVC pipe goes into each fitting needs to also be considered. You know the height and width of the wind tunnel. You know the location inside of the wind tunnel you want the center of the turbine rotor to be at when it is placed into the wind tunnel. Nice application for the Pythagorean Theorem here I would say.
How far apart should the two frames be spaced? With the front and back halves made and the rotor mounted between them it was a matter of determining the rotor blade clearance I wanted. Then calculating and cutting the PVC pipe spacers to put between the Tee ends.
The design was made to have the rotor blades to be as close as possible to the face of the fan in the wind tunnel. This was done to maximize the amount of air that would strike the blades.
How would the blades be made strong enough to not bend at the tips? This was done by coping what the windmill industry did by lacing a No. 9 fence wire through holes made near the tips of each blade.
How would the No. 9 fence wire be able to go through the blade and not bend the blade? This was done by punching 3 holes to form a slot that the wire could pass through and allow for the 15 degree pitch of each blade and allow for changing the pitch to change the speed and torque of the rotor.
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Rotor supported and placed into the 48” by 48” KidWind wind tunnel as close to the fan as possible.
(Note: I just noticed that in the picture above the rim of the bicycle wheel is between the fan and the blades. This increases the distance between the blades and the fan. A test will be made by reversing this and see if it increases the output of the turbine.)
How will the generator be attached to the frame and rotor? In this design the bicycle wheel will act at a pulley. A belt made out of green 5mm Poly Belt stock bought on Amazon will drive the 2.5 “ V-pulley on the 40 watt 12 volt DC generator. The generator will be attached to a piece of 1/2 plywood with automotive hose clamps and the board is also held to the PVC frame with hose clamps. This arrangement allows for tension adjustment of the drive belt.
How will the pulley be attached to the generator shaft? This problem will be solved by making a bushing with a 1/2” O.D. (Outside Diameter) to fit the 2.5” pulley and a 5/16” I.D. (Inside Diameter) to fit the generator shaft. A cross hole will need to be drilled to allow the set screw in the pulley to pass and seat on the generator shaft. (Note: Hexagon material was used for this bushing to make it easier to drill the cross hole.)
One final touch will be to add a sensor that will be used to measure the RPM of the turbine rotor during testing. This sensor uses the Hall Effect with a magnet taped to a blade. Here you can see the output display, the magnet on a blade and the sensor that is taped to the PVC frame.
Originally the bicycle rim was between the rotor blades and the fan making the distance about 6”. Reversing the set-up so the rotor blades are on the other side of the bicycle rim reducing the distance between the rotor blades and fan to about 3”. Will reversing the set-up so the rotor blades are closer to the fan make any difference? The answer to that is a resounding YES!
The rotor RPM went up from 243 to 293. With the 1”10 ratio the generator RPM went up from 2,430 to 2,930 and with a 45 ohm load the Joule out put went up from 200J to 305J !
Now let’s imagine that for an instant challenge you gave a KidWind team 10 minutes to look and study this turbine, and go over it with this handout sheet, protractor, a ruler, a multimeter and a calculator . The task is to write down as many statements about this turbine they can make.
EXAMPLE
1. It has 8 blades
2. The blades are made out of sheet metal
3.
4.
5…
Below is my list organized by turbine part,
ROTOR
1. 8 Blades
2. Sheet metal blades
3. 16” long blades
4. Area for one blade is ((1.375 + 6) 16 ))/ 2 = 59 sq. In.
5. Total blade area is 59 x 8 = 472 sq. In.
6. Rotor radius is 17.5”
7. Rotor swept area is pi x 17.5 x 17.5 = 961.6 sq. In. (.619 sq m)
8 Solidity is 472 / 961.6 =0.491
9. The blade pitch angle is 15 degrees
10. This is a HAWT (Horizontal Axis Wind Turbine)
11. The rotor circumference is 2 pi x 17.5 =109.956 inches (2.79 m)
12. In a 4 m/s wind at 293 RPM (4.88 RPS) 4.88 x 2.79 = 13.6 m / 4 m = 3.4 TSR
DRIVE TRAIN SYSTEM
13. Pulley and belt
14. 17” diameter drive pulley (bicycle wheel rim
15. 1.5” diameter pulley on generator shaft
16. Ratio 17.5 / 1.5 = 1 to 11.66
17. 1/4” round belt
GENERATOR Name Plate
18. 40 watt
19. 25 volts at 3,000 RPM
20. 25 volts and .186 amps = 4.66 watt
21. Turbine Theoretical power output = ( 1.22 x .619 x 64 ) 2 = 96 watts
22. Power with Betz’s Limit = .58 x 96 = 55 watts
23. System efficiency = 10 w / 55 w = .18 = 18%
STAND or TOWER
24. 8 Tees at $3.59 each = $29
25. 2 Crosses @ $6.11 each = $12
26. 8 - 28” long 1-1/4” PVC pipe = 18.7 feet
27. 4. - 6” long 1-1/4” PVC pipe = 2.0 feet
28. 20.7 feet of 1-1/4” PVC at $1 per foot = $20.70
29. 14” long 1/2” threaded rod
30. I 18” bicycle wheel rim
31. 16 1/4” diameter 1/8” draw steel pop rivets
32. 8 6-32 NF 3/4” long machine screws and nuts
33. 8 8-24 NC 2” long machine screws with 24 nuts
Is this applied S.T.E.M. ?
