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
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
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
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
Step 5: Wiring the Electronics
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
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!