Previously I shared with you how I spent three class sessions on Building Wind Turbines from the Windwise Curriculum. In this post I want to share how we tested to see how powerful our turbines were.
Session 4: Increase your Wind Turbine's Ability to Do Work (its Power)
I will admit when I read the power-testing section of the Windwise curriculum I was unsure of how it would work. They had a diagram of 2x4's to hold a power-testing contraption. I didn't feel like making an elaborate contraption for this lab, so I broke it down into what I knew I needed.
Needed to Build a Turbine Testing Apparatus
- Pennies or other unit that has equal weight so numbers may be compared
- Something to hold weight: paper or plastic cup
- String or twine and tape
- Way to attach weight and string to the turbines to be tested
- Elevated location next to the fan that allows the turbine's driveshaft to hold the string & weight
Logistics of Power-Testing Apparatus
For those of you who are natural engineers and love to tinker, you may be able to skip this section. For those of us who are less comfortable with providing engineering experiences for kids, I want to describe the logistics behind the power-testing apparatus. The idea here is that students bring their turbine, which includes a driveshaft attached to a hub that holds the blades (see my first post on Building Wind Turbines if you need more help with this) to the fan for testing. Note that students don't build a vertical pole or a stand. They test their turbines by holding them next to the fan. Therefore, they can't be holding the driveshaft directly, it should slip into some sort of sleeve, like tubing or a straw. In our previous class session, students just held their turbines in front of the fan. However, now we want to know how much power their turbine has so we constructed (assembled is more like it) an apparatus.
To test the power of the wind turbine, you want to hang weight on the end of the driveshaft to see how much weight it can lift. Correctly assembled, when the blades turn from the fan's wind, it turns the hub, which turns the driveshaft, which pulls the string. The string wraps around the driveshaft lifting the weight. The videos below will help you see what I mean.
Our weight holder (see photo above) was two small paper cups that we rigged with string so it would hold our pennies without tipping. Notice how our string makes an X shape to add stability. What you can't see in this photo is how the string is attached to the driveshaft. Remember, this contraption must be able to be removed from one student's driveshaft to be placed on another. The challenge is that the string must not slip when the driveshaft turns or the cup with weight will not be lifted. We overcame this challenge by attaching the string to a two inch piece of straw which then we slipped over the driveshaft and then taped to the driveshaft. This was strong enough to handle our testing yet we could easy remove the tape and slip the straw onto the next turbine being tested.
The photo below shows the overall testing apparatus. Not pretty, I know, but effective. We stacked up boxes and bins, placed the driveshaft of top of the bin, with the blades facing the fan, and measured the string of our measuring device so it rested on the surface without any slack in the string. Here is something you should notice. In the photo below, a red (rubber) tube is sitting on the red bin. This length was perfect for our testing apparatus because the driveshaft never rubbed on the bin. My students who used straws had issues with friction slowing down how fast their driveshafts were turning. I wouldn't suggest "solving" this problem before testing time. Allow students to observe potential issues, think of reasons why they aren't getting the results they expected, and have materials they can use to modify their designs. Make them do the thinking...am I right or am I right?
Conducting the Power-Testing Trials
When a student is ready to test his turbine, he attaches the string/cup apparatus to the driveshaft using masking tape, sets it up on the elevated surface, and turns on the fan. For the first test, we chose to see if the turbine could lift the weight of the empty cup. We deemed a trial "successful" when the knot hit the driveshaft, although this was completely arbitrary. You could darken a spot on the string with a black marker and require a trial to lift the weight to that height. Whatever you do, make it consistent so that students can compare their designs to one another! If their turbine was able to lift the empty cup, we added 5 pennies and see how it did. This is a trial and error process. You don't want them testing in increments of +1, but certainly not in increments of +50 either. Allow them to find a method that works for them, as long as they aren't holding up other students from using the apparatus. My students usually added 5 at a time, and if it the trial was not successful (didn't lift it all the way up) they would remove pennies, one at a time. The number we recorded was the total number of pennies the turbine was able to successfully lift.
Because of the wide variety of blade designs, I had no restrictions on the location of the fan. I allowed the boys to lift the fan up, in order to find the "sweet spot" to get the best results. Here are some videos to help you see the testing in progress.
Having Students Record Turbine Power-Testing Results
Its up to you how much writing students do on testing day. I have a handout that gives students a place to describe (and/or take photos) of each turbine design, collect quantitative measurements (number of pennies successfully lifted) and to write trial notes were they describe things that happened during the testing session. They also must record what modifications they made to Turbine design #1, before testing it again as Turbine design #2.
Tips for Successful Turbine Testing
Provide Adaquate Driveshaft Materials
The power of the turbines will be limited by the strength of the driveshaft. Due to budget issues, I had provided wooden shish-kabob as driveshafts. But the flimsy wood skewers did not hold up well in our testing sessions. Some boys altered their driveshafts by taping several together. The boys' turbines that performed the best were ones who had metal driveshafts, and I'm afraid it was the strength of the driveshaft being tested more than the blade design. Which ended up being OK, only it changed our take-away lesson to be about driveshafts rather than the extensive discussion I thought we would have about the importance of blades. As you know, flexibility is key, particularly when it comes to inquiry learning! Oh, flexibility is key.
Ask Great Questions (and keep your mouth shut)
The key to allow an inquiry lab to take its "full effect" is your ability to hold your tongue. In this particular lab it was easy for me, because I don't know much. I wasn't worried about giving any answers away, because I was as, or more, unsure than the students were. But if turbines are your passion, be careful to not "teach" them too much. Allow them to struggle, make mistakes, and make modifications (like Ms. Frizzle from the Magic School Bus would do!). Your job is to listen to students' thinking and ask good questions. Here are just some examples of the types of questions I was asking during this lab.
- How would you compare that trial to your last one? Why did it (not) improve?
- What can you change to the blades (hub, driveshaft) to improve your design?
- How is friction affecting your results?
- What might be reducing the ability of your turbine to perform at its best?
- What other factors might be affecting the number of pennies you are able to lift?
Conducting Labs with Large Classes
If you have large classes, this sort of lab is a bit more of a challenge. However, you can set up more then one testing apparatus as to not slow down the ability for turbines to be tested. You may also need students to work in groups so there aren't as many designs to be tested.
Encourage Collaboration and Competition
Encourage students to talk to one another. Create an environment where students can talk and think out loud without being chastised (by you or other students). Encourage students to talk to one another about trouble shooting ideas. While they shouldn't copy turbine designs, they can get ideas about how to improve and tweak their own turbines by looking at the ones being built around them.
To promote a sense of competition, I oversaw the testing process, and made a big deal of the "winning" design. I put it on the board as the "number to beat." What I loved about this was that it wasn't the prettiest turbine that was the most powerful!
I hope this description helps you to determine whether or not this activity is one you want to do with your own students. I'd love to hear your feedback on how it went! Also, I know Windwise would love it if you would "like" their Facebook page, and leave them feedback as well!