Wind is hard. For our midterm in Energy, Jon and I sought to capture and reuse waste wind energy from the subway to power a network appliance to provide a local private message board or neighborhood information service only accessible at the station. We quickly discovered, capturing and using wind is a challenging prospect.
It's important to note that there is risk in causing the train to use more energy by making it more difficult for the train to move through the wind pressure. Jeff used the example of a gigantic wind turbine fan that covered the entire train path, the compression of the air would make it more difficult for the train to continue forward, likely causing a higher energy expenditure than energy generation. In the scale of this project, we don't believe there is a measurable amount of additional friction, at least no more than the people standing on the platform already introduce.
Our initial concept was to to charge a battery by capturing the short bursts of wind that occur when trains enter and leave subway tunnels. Our research began by measuring the wind movement at different subway stations using a handheld anemometer from Amazon.
The results were promising, you can often find bursts of wind up to 25 miles per hour! We assumed that wind would be most powerful at stations that don't have an express track and at the ends of extended tunnels.
Bedford Ave. on the L line seemed like the best place, because it is the first stop after the east river tunnel and the traveling public at this station empirically seems like they would respond well to the concept.
With wind measurements in hand, we explored options for building the turbine. The goal for the midterm was to build a platform that would make it easy to tweak and experiment with different designs. In the setting of a subway platform, a vertical axis turbine makes the most sense, with a smaller footprint and (hopefully) a tolerance for turbulent wind. ITP Professor, Dustyn Roberts posted an awesome instructable for building a vertical axis turbine.
We made modifications to her initial model, making sliding channels that make it easier to adjust the location of the stepper motor and to adapt to different gear ratios.
[gallery columns="2" ids="1177,1180,1179,1178"] The two stepper motor wire sets connected to two bridge rectifiers, converting the AC into DC current.
Lab TestingWith the help of our trusty ITP Box Fan, we got the turbine to turn. And open circuit voltage looked promising. Up to 3V with a 1 to 1 gear set.
Gear Ratio Matters
With a working prototype, we wanted to see how changing the ratio of gears would change the amount of energy generated. Eric helped us understand the concept of diametral pitch and how that value lets you determine what size a gear needs to be to mesh with another gear.
The current prototype is running a 2 to 1 ratio gear set with the larger gear on the aluminum tube and the smaller on the stepper motor. The goal was to make the stepper motor rotate more often per revolution of the blades. We immediately saw an improvement in open circuit voltage, now jumping up to 9.5 v with the box fan.
What about a load?
Things look great when the turbine isn't doing any work. Things get more interesting when you apply a load. In this video, we are connecting a Sparkfun Power Cell - LiPo Charger/Booster to the circuit, and you can quickly see how the turbine rotation speed changes dramatically.
Measuring short circuit current, we found [[ask jon if he remembers]].
We also connected capacitors and calculated the energy stored. I'm not entirely sure this stuff is all correct.
W = 1/2 C V^2Joules = 1/2 2200uF 12V^22200 x 10^-6 F = .0022F0.1584J = 1/2 .0022F (12V)^20.1584JoulesWatt = Joule/Seconds.08W = .158j/2s.08 Watts
In the Subway
Next we went out to the subway. The vimeo video at the top of this post recounts our experience. At times, open circuit voltage jumped above 20 volts! We didn't get a chance to test a operating with a load in the subway yet.
Folks riding the L were interested in knowing what we were doing. Everyone had positive comments, and some riders even shared ideas for what they would do with the subway waste wind, like wind chimes!
There are a number of things that could be improved. Obviously starting with actually capturing and storing the energy generated by the turbine. Additionally, we want to experiment with:
- blade depth & dimensions
- blade pitch & height
- better lubrication of parts
For the midterm, we didn't even get into the Raspberry Pi, additional thought needs to go into exactly how and why the wifi network services would work.
For even more information, here is is the presentation we made in class: