HOW TO MAKE a PORTABLE WIND TURBINE

Generally when I want to get away from the city and camp, I take some kind of power source with me. Within my preferences are the Power Banks. With a small and light one, of approximately 40 Watt hours (Wh), I can charge my cell phone about 2 or three times and that is fine if I am only going to spend a day and do not need to power other equipment such as my camera, for example. If I want to spend more time then I usually take a 222Wh “Solar Generator” with a 60W portable panel. For the energy needs that I have in this type of excursion, it is generally enough for me.

About a month ago I made an excursion of several days and to my surprise at the end of the second day my energy sources had been completely exhausted, also no Sun. I have discarded fuel generators because they are very noisy, polluting and can cause fires. There was a small stream near where I camped and there was also wind that I could have taken advantage of, but I really wasn't prepared for it. I promised to find a solution and include it in my new adventures. I am sure that there are several people around the world with this type of need, that is why I carry out this project so that they can also benefit.

In this project I show you how to make a small portable wind turbine, mostly 3D printed, with dimensions similar to a 2L soda bottle when folded. Although its generation capacity is limited, having it can be beneficial during emergencies or when access to more convenient energy sources is zero or very limited.

Although the initial motivation to carry out this project was not linked to teaching at school, I consider that it constitutes an excellent opportunity to link this type of technology with the teaching-learning process. To be able to present a product of this type at School Science Fairs, which harnesses the energy of the wind wherever the conditions exist, small, very portable and easily installed, should be striking and interesting. Students could answer questions and present the results of electrical measurements of their performance at different wind speeds. I think it would be an enriching experience for everyone.

Among the science topics related to this project are:

Renewable Sources of Energy

Conservation of energy: https://en.wikipedia.org/wiki/Conservation_of_energy

Faraday's law of induction: https://en.wikipedia.org/wiki/Faraday%27s_law_of_induction

Drag and Lift Forces: https://en.wikipedia.org/wiki/Lift_(force)

Rotational speed: https://en.wikipedia.org/wiki/Rotational_speed

Inertia: https://en.wikipedia.org/wiki/Inertia

Rotation around a fixed axis: https://en.wikipedia.org/wiki/Rotation_around_a_fixed_axis

Alternating current: https://en.wikipedia.org/wiki/Alternating_current

Voltage: https://en.wikipedia.org/wiki/Voltage

Current: https://en.wikipedia.org/wiki/Current

Impedance: https://en.wikipedia.org/wiki/Impedance

Battery: https://en.wikipedia.org/wiki/Rechargeable_battery

DESIGN CONSIDERATIONS

For some time the idea of designing a portable wind turbine had crossed my mind. From my experience and for it to be useful I knew it had to be small, maybe the size of a soda bottle, able to fit inside a backpack. Its mass should also be as low as possible.

Since the plane of rotation of the propellers in typical wind turbines is perpendicular to the position of the electric generator, they are initially inconvenient to transport and store once assembled, even when small. I also didn't want to have to use tools to mount and dismount the blades every time I was using it. Folding blades, that was the idea, but how to implement it? It took me a while to come up with the answer. Have you seen the way bats sleep? The wings are folded towards the body, that could be the answer. Nature is an excellent source of inspiration, millions of years of evolution support it. Taking the previous idea as inspiration, I designed this wind turbine.

Since on my travels I usually carry some kind of lightweight tripod to take pictures when the light conditions are low or to make time-lapses, perhaps also using it to support the wind turbine would save the need to carry other equipment. There are thousands of possibilities to support a camera and make it compatible with those pre-existing technologies, I think it would be convenient.

Regarding the generator to use, it was another difficult part to decide. Ideally it would have been to design and build a customed one, but unfortunately, I did not have the possibility of building a quality one as I did not have the required manufacturing technology at hand. That is why, a NEMA 17 stepper motor has been selected as generator due to its availability. Another reason to consider when selecting a stepper motor is that you don't need to use multiplication gears to get a useful voltage and it doesn't have brushes.

As creators we may have an amazing digital design, but it should be able to be built with the available resources. 3D printing! Thanks to it, we can now replicate this project as many times as we want. If a part gets damaged, we print it again and voila!

The typical dimensions of a 3D FDM printer are usually between 22 cm wide by 22 cm deep by 25 cm high. All pieces were included within these margins to maximize replicability.

Supplies

Cable Glands https://amzn.to/3KxIsqa

6002-2RS Double Rubber Seal Bearings https://amzn.to/3LRV8sk

Collet Propeller Adapter Update https://amzn.to/3wH3HkC (5mm diameter hole for the stepper motor nema 17 and 6mm diameter threaded shaft) You need to replace the bullet tip with a 11mm outer diameter hex nut

Stepper Motor Nema 17 Bipolar https://amzn.to/35ZAOpm

24 Gauge Electrical Wire 2 Conductor https://amzn.to/3DYYCGC

Capacitors 35V 1000uf Low Impedance https://amzn.to/3v7zkBV

Bridge Rectifier https://amzn.to/3utpRWz

M3 Stainless Steel Bolts Screws and Nuts Assortment Kit https://amzn.to/3Kz4LeU

Gorilla 2 Part Epoxy https://amzn.to/3utDXXV

Socket Head Cap Screw, 1/4-20 x 1" https://amzn.to/3746JWr

1/4" Stainless Flat Washer https://amzn.to/3KpAzmx

Tripod Nut Barrel https://amzn.to/3joq8DQ

Camera Hot Shoe Mount to 1/4"-20 https://amzn.to/3O2b9xy

3D Printer

Step 1: DOWNLOAD AND PRINT ALL THE STL PARTS OF THE ASSEMBLY

You can download the STL files from here. As I make changes to any of them, I will replace the corresponding file so that the most up-to-date ones are the ones that are published.

I've included a STEP file in case you want to take a closer look at the assembly or change something depending on your interests. If you need any special type of file format let me know and I'll see if I can help.

I have printed my prototypes in PLA. I will probably end up printing it in ABS or PETG, although PLA has worked very well for me so far.

In this system, the parts are subjected to relatively high mechanical stress. That is why I have printed all the pieces at 100% infill with the exception of the propellers and tail, that I finally decided to print at 40%. The layer thickness was 0.28mm in all cases. I used a Longer LK4-PRO printer with a 0.4mm nozzle.

The propellers were printed one at a time. They took about 6 hours each. I used supports and the position was as shown in the picture. The fill pattern was CUBIC and I used CURA (Slicer Software).

Update: The versions E1 and F1 are for steppers motors with cable connector

Step 2: INSTALL THE COLLET PROPELLER ADAPTER IN POSITION

To do this you must insert it inside the hole that the Central Hub (Part-A) has. On the other side, place and adjust the nut until the assembly is tight. Since the Blades (Part-C) and Hub will rotate in the opposite direction to the tightening direction of the Collet Propeller adapter, there is less risk of the assembly coming loose.

Step 3: BOLT THE BLADE ENDS TO THE CENTRAL HUB

All the holes in the designed parts are for M3 screws. Place the pieces as shown in the images, but do not overtighten them. The Blade Ends (Part-B) shall be able to articulate freely 90 degrees.

Step 4: ADJUSTMENT OF THE ANGLE OF ATTACK OF THE BLADES

Blade Ends have a scale with a bigger center mark. The Blades also have a mark. Put some epoxy glue inside the holes of the Blade Ends and Insert each Blade into its position. Match the center mark on the scale to the mark on the Blade. Rotating slightly the Blade counterclockwise advance between 2 and 3 marks on the scale. Each mark corresponds to 5 degrees of advance, so you would be giving it an angle of attack of approximately 10-15 degrees.

Note: It's possible to experiment a bit on this, I'd love to hear your results!

Step 5: PLACEMENT OF THE STEPPER MOTOR AND CONNECTIONS BETWEEN THE ELECTRICAL COMPONENTS

The generator (Stepper Motor) should be bolted to the box (Part-E) as shown in the image. For this, 4 M3 screws declared in the list of components are necessary. The generator cables should protrude to the rear.

With the help of a multimeter in the electrical resistance measurement position, the two pairs of cables that have continuity must be identified. Each pair of wires connect to the AC inputs of each Full Wave Rectifier, their order does not matter.

The + and - outputs of each rectifier are connected to each other in parallel and in turn also in parallel with a capacitor of 35V maximum voltage. The value of the capacitor is not critical, although one equal to or greater than 1000uF is suggested. With this, the sum of the currents of each phase coming from the generator is achieved, while the voltage is equalized. The corresponding image shows how the connections were made in Prototype 1. A two-way cable is fixed to the piece (Cover-F) and soldered to the + and – terminals of the capacitor.

To test that everything is as expected, rotate the generator shaft a couple of times and set a multimeter to the position for measuring DC voltage across the capacitor terminals. You should have a constant voltage value. If you short the ends of the wires, it should discharge the capacitor.

In the measurements made and at the RPM range obtained during the field tests, it was possible to conclude that the maximum open circuit voltage obtained does not exceed 25V. The maximum short-circuit current was in the order of 0.7A. The output is not regulated. These voltage and current values make it possible to charge 18650 lithium batteries with a protection circuit. Finally, the cover is placed with the help of 4 M3 screws and their corresponding nuts.

Step 6: ASSEMBLING THE PASSIVE YAW SYSTEM

To the generator box (Part-E) you must glue the Tripod Nut Barrel from the components list with epoxy glue and wait for it to dry. You will also need to glue the nut you removed from the Camera Hot Shoe Mount ¼ to the M-piece. This nut was chosen because it has a larger contact area and lessens the chance of the M-piece breaking during operation. Do not allow the glue to cover the thread of the nut, in case glue gets on the thread, clean it before it dries.

Fix the bearing to the generator box using the screw, washer and part N as shown in the images and in the exploded view. Subsequently, the M piece must be inserted into the previously screwed-in bearing. It should fit tight. In case it is not, use a little glue to fix it. The assembly should rotate freely.

Step 7: FIXING THE VANE SYSTEM

In order for this Wind Turbine to be able to orient itself correctly in the direction of the prevailing wind, it must have an orientation system. Tube (Part-G) must be glued to the generator box lid (Part F) using epoxy glue and allowed to dry. At the other end of the tube, piece H is also fixed. The assembly should be as shown in the image.

This assembly was designed in this way so that the Vane could be better accommodated when the Wind Turbine was fully folded and took up less volume for storage or transport. It can be rotated in 90 degree increments in the plane parallel and perpendicular to the ground.

Step 8: MEASUREMENTS TO THE GENERATOR, RESULTS AND CONCLUSIONS

The following shows how the students could characterize the generation capacities of this Wind Turbine in laboratory practices. The teacher could be guided by this example to carry out research activities of this type.

During my field tests, the maximum voltage value obtained in the internal capacitor was approximately 22V. The wind speed recorded that day was 22km/h, with gusts of 26-28km/h.

In the lab, if we spin the generator rotor until we get that 22V, we could measure the maximum RPM reached during the field tests. Other values of electrical magnitudes could be determined to try to characterize the nema 17 stepper motor in its role as generator in this Wind Turbine.

To carry out the experiments, a three-phase induction motor was used, which could vary the frequency of the supply current with a Variable Frecuency Drive (VFD) and with it, the RPM of its operation. This motor was attached to the chuck of a drill with which the generator shaft would be fixed. Attached to this mandrel was a small magnet that would pass close to a 2000-turn coil of fine gauge wire, when the entire assembly rotate. This coil would be connected to the signal input of an oscilloscope where pulses would be detected. Measuring the time elapsed between pulses with the same phase and determining its reciprocal, the frequency expressed in Hertz (Hz) would be determined. In turn, a multimeter would be placed at the generator output and the voltage and current would be measured based on the RPM. At the end graphs of dependency of these magnitudes based on the RPM would be generated. One area of interest was determining the best series-parallel configuration of 18650 lithium cells to be charged by the generator. Two 1500mAh 18650 cells balanced at a voltage of 3.7V were used in the experiments. Conclusions:

   • The nema 17 stepper motor (generator) apparently is not capable of supplying a short-circuit current greater than 0.65 A. From 500 RPM, a pre-limit value of 0.6A is obtained.There is no need for current limiting circuits to charge 18650 lithium cells directly, just a voltage protection circuit (included in protected cells)

   • The 22V obtained during the field tests correspond to about 870 RPM of the generator. Typical values would be between 500-700 RPM.

   • The open circuit voltage has a linear behavior as a function of RPM. At 600RPM approximately 14.5V is obtained.

   • At typical 700 RPM you get 0.5A charging current on a 1S2P 18650 system, versus approx 0.26A on a 2S1P 18650 system.

   • It makes no sense to use configurations of three or more 18650 cells in series. High RPMs are required that this wind turbine could hardly reach.

   This is all for now, hope you liked !