Sizing a wind turbine to power your Raspberry Pi

How to Fix It?

I added an over-voltage circuit on the wind turbine side of the DC/DC regulator. The wind turbine is rated at 50W, but I am not going to trust that number. I built the over-voltage circuit using TVS diodes and made sure I sized it for about 4A@25V (100W) to cover hurricanes (although in a hurricane, that will be the least of my problems – I would expect the turbine to be halfway to Aruba by then).

I used a 50W, 5-ohm resistor and put 10-5W TVS diodes in parallel (Figure 17). TVS diodes operate by shunting excess current when the induced voltage exceeds the avalanche breakdown potential. This solution acts as a clamping device, suppressing all over-voltages above its breakdown voltage. As with all diodes of this type, it automatically resets when the over-voltage goes away.

Figure 17: The over-voltage circuit serves to protect the system in case of an unexpectedly high voltage.

I set the over-voltage protection at 20V to avoid this problem in the future. You can refer to the Electronic Tutorials site for a good link on a diode-based over-voltage protection circuit [4].

I ran another set of loaded tests for the 50W turbine after the over-voltage was added and got the curve in Figure 18, without generating any smoke.

Figure 18: Voltage versus wind speed for the 50W turbine under load.


Based on the curves and measurements described in this article, I can reach the following conclusions:

  • A wind speed of 20mph is needed to generate any significant power in the system using the two 15W wind turbine system. I am sure there are better low-speed turbines for this system.
  • The average wind speed in Curacao is about 14mph, which means a substantial part of the time the turbines won't be generating power. How can this be fixed? I moved the wind turbine up higher on the tower.
  • Because wind speed is generally greater the higher you are off the ground, I wanted the turbine as high as I could reasonably get it. I gained about 15 percent in wind speed by moving the turbine up 10 meters. I don't have a good number on the spread of wind speeds, but it looks like the bulk of the wind is between 10mph and 25mph, so the setup will get some power, but not much. It's a good thing I have the solar cells.
  • Note how the voltage flattens out as wind speed approaches 50mph at about 16.5V on the open loop 15W turbine test. The system is designed to measure up to 17V (a voltage divider down to 5V for the Arduino) and tolerate up to 18.7V (no more than 5.5V into the Arduino). Again, if wind speeds are more than 50mph, the analog inputs will be the least of my problems.
  • I am going to go with the 50W wind turbine for this project down in the Caribbean. The curve shows some power generation at 15mph, and it gets really good at about 25mph. When I get the box down to Curacao, I will further characterize both the 50W and 15W wind turbines.

This control system takes the weather into account on cloudy days (low luminosity and solar voltage) and turns on the wind turbine. Of course, I will turn on the wind turbine at night all the time.

As the curves show, generating even 10W on an ongoing basis from the wind poses a set of special problems. My next wind-powered system will use better (and more expensive) turbines, but the wind is still a secondary source of energy; the chosen 50W turbine will work for Project Curacao.

For more information about Project Curacao, check out the author's blog [5], and for real-time data from Project Curacao, you can click on the Project Curacao page [6] hosted by MiloCreek.

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