Here's what I used.
- 2-line stunt kite (Mine is the Nexus made by Prism) A simple 1-line kite would potentially work too. Note: After consulting some kite enthousiasts, it turns out that a stunt kite is not the best choice for carrying all the extra weight of the generators, etc. however it is handy for making cool patterns in the sky at night.
- 1/2" square Balsa wood beams (from a hobby shop)
- 8 1/4-20 set screws
- Hot Glue
- Heat-shrink tubing
- 4 Radioshack Motors (I refer to these as motors or generators interchangeably)
- 3D Printed PLA
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Step 1: How Much Power Is Up There?
I started by making a scale drawing of the kite and exploring different dimensions and layouts. (This was before the kite arrived in the mail.) I found that 7" propellers fit nicely with the layout I chose.
Next, I did a rough calculation of how much kinetic energy is in the air that passes through the 7" circle swept by one of the propellers.
- Start with the equation KE = .5 (Mass) * (Velocity).2 We will calculate the kinetic energy in the amount of air that passes by in 1 second.
- (Velocity) A strong kite-flying breeze is about 25 mph. That's close to 10 meters per second.
- The Mass (per second) is (cross sectional Area ) * the Velocity * the Density of air. (The density of air is about 1 kg/m3.)
- That means that KE = .5 * (cross sectional Area) * (Density of air) * (Velocity)3
- This equates to .5 (0.025 m2) * (1kg/m3) * (10 m/s)3 = 12.5 Joules of energy each second, which is 12.5 Watts.
It's important to keep in mind that this is a rough estimate. It's possible that the turbine only extracts 2% of the kinetic energy so we would total 1 Watt. However, on the other hand the kite is actually traveling faster than the windspeed most of the time, so if our average kite speed is 15 m/s we may total 17 Watts.
Step 2: Take Some Measurements
My next step was measuring how much power it takes to light up my LEDs. I cut off a section of 15 LEDs from my long strip and hooked it up to a DC power supply. (A battery and a multimeter would work as well. The power (in Watts) equals the voltage (in Volts) times the current (in Amps). I didn't know how much voltage I could get using my motors yet, so I measured using several voltages from the adjustable DC power supply. The LED strip is optimized for 12V, but works as low as 8V or as high as 24 (however at 24V the LEDs start getting hot after a minute or 2 which can damage them). I found that at 12V a 15 LED section (displaying white) uses 2.64 Watts. At 24.5V it uses 21.1 Watts, and at 8V it uses 0.4 Watts. Great! Those look like about the same ballpark as the numbers I estimated the turbines could produce.
When my generators (actually motors) and propellers arrived, I attached some propellers using hot-glue, grabbed my multimeter, and headed to the top of Twin Peaks, the biggest hill in San Francisco, where there was plenty of wind. I measured the voltage generated by each generator and found that it was about 1.5V to 2V with a strong wind. I was able to power a 5V LED light strip by connecting 2 generators in series. I also strapped the equipment to my kite and found that the kite had plenty of lift to get everything off the ground. I found that the propellers spinned faster with their back side to the wind, (opposite of how they would be mounted on an airplane).
Step 3: Making the Motor (Generator) Mounts
When the generators arrived, I used my calipers to measure them and created a motor mount in CAD using Autodesk's free 123D software. I added a hole for a set screw to hold the mount in place on the 1/2 inch balsa wood beam and I added two holes for machine screws that secure into the motor's face. I started by drawing out my design on paper and writing down all the critical dimensions, then putting it into the computer was really quick and easy. Remember to save often though, since at this point 123D is still in development and has crashed a few times on me. I added about 0.01" of clearance on the critical dimensions to make sure that things weren't overly snug.
I printed these on a 3D printer out of PLA, a biodegradable corn-based plastic on the Series 1 printer made by Type A Machines and the parts came out great. I made them pretty solid, using 4 solid layers and perimeters and 60% fill. The generators fit in snugly and were secured in place with small machine screws on the front. The hole for the set-screw was just the right size for the screw to cut it's own threads without splitting the layers of plastic apart. I used a small file to smooth out the opening for the balsa wood and that fit perfectly.
Step 4: Making the Attachment Clips
The beams that hold the turbines are attached to the kite using 3D printed clips. They make use of a special piece of hardware called a Stubby Retractable Spring Plunger. There are lots of possible ways of attaching, but this one allows for easy clipping-on and unclipping. To attach, simply pull the ring to retract the spring-loaded plunger, slide it onto the kite strut and let go. I printed them with 4 solid layers and perimeters and 60% fill.
Step 5: Wiring the Electronics
Be sure to check out the photo comments and wiring diagram on this step.
Basically, I wired the 4 generators in series, each providing about 2V, so the LED strips receive about 8V. The power goes into the LED controller chip. The chip has 4 outputs. Positive, Red Ground, Blue Ground, and Green Ground. I wired 2 wires to each so that I could have one for each of the 2 LED strips.
Step 6: Flying
So far, I have only flown the kite during the day. It works great with strong winds, like +20 MPH, unfortunately the winds seem to calm down at night so I haven't gotten the chance to do nighttime light-painting photos yet. This type of kite isn't the best for carrying weight so it needs a lot of wind to get enough lift.
When flying, All four propellers get spinning very fast and the LEDs turn on when the capacitors have charged up. It handles pretty well and certainly draws attention at the beach! It doesn't like hitting the ground though. I modified my design slightly to handle impacts better by adding a flexible section to each of the two wooden beams.
The next step is to add a battery charger so that I can use my kite-power in various electronic devices. You never know when you might need some more power!