Sizing a wind turbine to power your Raspberry Pi

Loaded Wind Turbine Characterization

I first tested the configuration with two 15W wind turbines. I guessed that I could get at least a trickle of charge out of these turbines at wind speeds of 15mph.

I loaded the 15W turbines with the actual Project Curacao charging circuitry. This circuitry consisted of a DROK 3A/18W DC Buck Converter Car Regulated Power Supplies (9-22V) 12V to 6V, which was wired to a relay that switches between the solar panels and the wind turbines and then to the solar battery charger, the LiPo Rider Pro. (This configuration is the actual design used in Project Curacao.)

This hardware configuration is the "Load." Please understand that your "Load" might not have the same impedance or characteristics of my "Load." An additional way of testing the Loaded characterization would be to discharge the wind turbine into a resistive load and measure the voltage.

I measured the voltage in a car at the wind speeds shown in Figure 14. The graph of current versus wind speed is shown in Figure 15.

Figure 15: Current versus wind speed for the double 15W turbine configuration.

After the test outside, I connected a variable power supply to the Project Curacao box, then read the currents from the power supply current screen and from the Raspberry Pi RasPiConnect screen. My reasoning was that a voltage is a voltage is a voltage when it is DC with little ripple. Thus, I can put the measured voltage into the system and read the currents from the software. This technique allows me to avoid carrying a laptop, wireless connection, iPad, and so forth in the car during a test.

50W Wind Turbine Test

Based on the latest test run, and on some fragmentary data from a previous test run, it is clear that the 50W wind turbine does far better than the smaller turbines at low wind speed. From Figure 15, no significant current is produced with the dual 15W turbines at 15mph. The 50W turbine produces a significant amount of current at 15mph and supplies substantial current at 25mph. No surprise there. Clearly, at about 15mph, I can produce some of the power required. I can generate a loaded regulated voltage of 6.4V at 15mph (based on two measurements). This output gives me plenty of current to charge the battery, even when the Pi is running.

The test shown in Figure 16 blew out the DC/DC voltage regulator with an over-voltage condition. The damage was limited to the regulator.

Figure 16: Testing the 50W wind turbine.

Why did this happen? The open loop voltage measurement on the 50W turbine offers a clue. The regulator is specified as a maximum 19V input. When the turbine hit more than about 25mph, it started to generate more than 22V, even loaded, which eventually fried the regulator.

The 50W wind turbine's very minimal specifications say that, in perfect conditions, I can get 2A at 25V, which is 50W. However, if I'm not taking 2A (which is 2x to 4x the maximum), the voltage goes up. Eventually, it killed the regulator. I did not think that one through.

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