High school students investigate tabletop wind turbine

Submitted by Joel Rubin
2007-07-02 15:33:24

My 5 9th grade Environmental Science classes at Stoughton High had a great time learning about wind power using the model aerogenerator EnergyTeachers.org lent me last spring.
The tower design (for examples, see http://kidwind.org/) utilizes standard 1" PVC plumbing pipe elbowed and teed to form a slightly angled 3 point base and rise, both 19"; the tail of the base extends 6" from center. This is a variant from the original design that called for a 4-point base using a second cross piece teed to the other end of the 6-incher. I didn't see that being necessary because, with a 3-point base offers a slight back tilt adding stability and needs less material to build. At the business end, 3/32" balsa wood fan blades are spongy enough for a compression fit into slotted dowels attached to a tinker toy hub. This is joined (perhaps with thick epoxy?) to a geared electric motor unit designed for a model airplane (I couldn't determine the maker as the motor was already fitted permanently inside a U-shaped PVC collar).

When Shawn from EnergyTeachers.org lent me the turbine, his initial suggestion was to show electrical generation by taking the motor's leads to a multimeter. My classes seat as many as 24 students in a horseshoe table arrangement that keeps each kid at least 8 feet from the demo. At that range, I didn't think the multimeter needle's movement would sufficiently engage students with the idea that electricity was being generated. To make the demo more dramatic, I asked Shawn to wire in a bright red LED (Light Emitting Diode).
Shawn breadboarded the LED and resistor so as not to blow LEDs which have limited current tolerances. We rubber banded the breadboard to the tower base, wrapping the leads to the resistor on 1 side and the LED on the other (Shawn ran his wires inside the PVC and out a generous hole he drilled through the base elbow). It is important to note that while electricity flows from rotation in either direction, LEDs light only when generators are spun in one direction because diodes, in common practice, are one-way valves for electrons.
As it turns out, household box fans don't provide enough wind to light LEDs, at least not using the materials we had available. However, removing the balsa wood blades (which get in the way here) the LED was easily lit by giving the doweled hub a good flick of the index finger. This shows, if only by analogy, that spinning generates electricity. This point applies regardless of whether the energy source is a human finger, wind, falling water, or steam generated by solar energy or by nuclear, fossil, or biomass fuels. That larger principle, relating light to the spinning generator, was key here as our unit concerned all forms of electrical generation, not wind only.
Lessons on electrical power generation take advantage of opportunities afforded by the apparatus so, in addition to demonstrating that spinning can convert energy into light, and that linear wind striking angled blades can set a rotor spinning, I also showed that a DC electric motor could be a DC electric generator, differing only in the direction of energy flow carried by the charge of the conductive materials. I did this by turning off our wind source, the box fan, and clipping a D-cell to the wire leads. Now the battery lit our LED and spun the balsa wood blades (often fast enough that they went flying off, to no harm given their slight mass--Still, don't allow people to stand in the plane of rotation without goggles). If I reversed leads, the LED no longer lit (that's a diode for you) and the rotor spun in the opposite direction.

Students asked that I turn on the window fan and place our motor/generator in contest between the battery's power and the power of the moving air. With forces in opposition we found that we could bring the blades to complete standstill.
An important additional lesson related to all turbines is that angle of attack determines the ratio of lift to drag generated by rotating blades. Students experimented with adjusting this angle along the range from maximum rotational speed to blades that would not rotate regardless of wind speed, either because feathered to minimize drag (but with no lift either) or with maximum drag, flat-on resisting the wind's direction. Please see the photos included, taken in my smallest class. I will be interested in finding a sufficiently low cost set of LEDs, resistors, motors, hubs, and blade materials to create class sets. If we could clamp these directly to the fronts of our student tables we wouldn't need to build towers, an environmental savings either by reducing rain forest balsa or toxic PVC production.
Two related activities from earlier in our energy unit are worth mentioning. I quickly found that students lacked any concept either of turbines or of electric circuits. To teach about turbines I had students scissor a spiral into a sheet of copy paper and tie a bit of string at the center. Yes, 9th graders enjoyed spinning in circles to make their whirlygigs twirl (example: http://www.thriftyfun.com/tf676654.tip.html). To understand electric circuits I provided paired students with D batteries, flashlight bulbs and strips of aluminum foil (you could use wire if you have any, twist ties work if long enough). An important safety note is to warn students against short circuiting the batteries (placing wires in direct contact with both poles) draining batteries and burning themselves; the increasing heat is sufficient disincentive for most but this is the age group that pours salt on their skin and then sticks ice cubes on it...Within half a period, all students in all classes figured out how to get their bulbs lit in multiple ways (4 in all, based on the 2 poles of the batteries and the 2 electrical contacts on the bulb base -- you might have them draw these). They now have a useful life skill for testing batteries and bulbs. Based on what they had figured out about circuits, we proceeded to diagram their theories about the interior of lightbulbs -- it might have been nice to have had dissected lights available. I think getting them to problem solve and do this theoretical work is central to science learning. Later, you can add vocabulary using lightbulb diagrams such as ones available at http://www.enchantedlearning.com/devices/lightbulb/label/index.shtml.

My Alternative Energy unit further benefited from the generosity of 2 other teaching friends. John Papadonis of Burlington, MA lent several photovoltaic toys plus a 4' satellite dish antenna he'd lined with aluminum foil that set newspapers ablaze if aimed at the sun with the target object held at the focal point, about 18" ahead of the dish. Bart Lee, of St. Francis School in Malden, introduced me to a variety of photovoltaic resources, notably "Gonzo Gizmos" and related books (see: http://astore.amazon.com/contracross-20) and Parker Products (http://www.parkerproducts.net/), an extraordinary electronics and solar cell discounter run by Harold Strand in Reading, MA.

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