Sizing a wind turbine to power your Raspberry Pi

Lead Image © Edvard Molnar,

This Is Your Pi on Wind

Got a special project for your Raspberry Pi that requires it to operate off the electrical grid? If solar power doesn't provide enough juice, add some wind to the mix with an inexpensive wind turbine.

Project Curacao is a sensor-filled project designed to hang on a radio tower on the island nation of Curacao. Curacao is a desert island 12 degrees north of the equator in the Caribbean, with a harsh environment that includes a strong tropical sun, salt spray from the ocean, and unremitting heat.

Curacao is a beautiful place, but it poses a real challenge for anyone who wants to install a Raspberry-Pi-based environmental monitoring system.

Project Curacao (Figure 1) is designed to monitor the local environment unattended for six months. The system will operate on solar power cells or wind power and will communicate with the designer via an iPad app called RasPiConnect (Figure 2). Most of the electronics fit inside the box shown in Figures 3 and 4.

Figure 1: Tower for the Project Curacao environmental monitoring project.
Figure 2: Block diagram for Project Curacao.
Figure 3: Completed box.
Figure 4: Open Project Curacao box.

Remote operators will monitor the power usage, solar cell degradation, and battery charging/discharging performance. The system will reboot itself via a watchdog timer and will reboot when power becomes available in cases where it shut down because of low power or unfavorable conditions.

The biggest challenge when designing a system that will operate continuously from a remote location is how to provide power. This articles describes the methodology that was used to design the power system for Project Curacao. Of course, your own remote Pi project will have other goals and demands, but you can adapt these techniques as necessary to explore your own power needs when you are operating your Pi off the grid.

I initially designed the system to operate on solar power only. Then, someone pointed out that a wind of about 15mph is almost always blowing in the same direction, especially on the coast. My power calculations showed that, with solar power alone, the system could operate only about 15 hours per day.

Thus, with the solar-only design, the Arduino-based battery watchdog system shuts down the system for the night, only waking up at midnight for an hour to catch the night time environment. I thought wind power might improve the chances for continuous operation.

Solar Only

The first challenge was to design the device for solar power alone. The project required the Raspberry Pi to run all day and for at least three hours before sunrise and three hours after sunset. The goals and budget influenced the hardware choices.

For this project, I could assume:

  • Eight hours of sun running the cells at least at 80 percent of max (12 watts).
  • Current is delivered to the Raspberry Pi at 85 percent efficiency.
  • Raspberry Pi takes 350mA on average with the WiFi running.

Given these assumptions, I could calculate the total Raspberry Pi runtime during a typical day:

It would be nice to have full-time operation from solar power, but 15.8 hours per day is not enough to run the Raspberry Pi from three hours before sunrise until three hours after sunset. Therefore, I decided to explore the possibility of adding some inexpensive wind turbines rated to generate 15W and 50W.

Wind Power

Of course, the wind does not blow steadily at all times, so you need to store the energy in a battery, and then have the battery supply the Raspberry Pi. Almost all inexpensive wind turbines generate a nominal 12 volts – enough to charge a 12V battery (actually, the voltage is more like 13.7V, and the turbine will have to generate more than 13.7V to charge it). I needed a 5V power supply for the Rasp Pi, so I chose a solar cell charging board called the LiPo Rider Pro [1].

The charging board takes a maximum of 6.5V in and will charge a 3.7V LiPo battery and boost the battery voltage to 5V. This is almost perfect, except that the wind turbines generate 12V. I can dial down the voltage by adding a DC-to-DC Buck Converter, which converts the nominal voltage from 12V to 6V DC. This converter is efficient and can handle up to 19V on the input.

The wind power subsystem is shown in Figure 5. Note that I put a relay in between the regulated wind power input and the solar panels to switch from solar to wind under software control.

Figure 5: Wind power subsystem block diagram.

Components include:

  • 15W and 50W wind power turbines
  • 1 SeeedStudio LiPo Rider Pro charger (includes a booster to 5V from 3.7V)
  • 2 3300mAh 3.7V LiPo batteries
  • 1 Raspberry Pi Model A
  • WiFi
  • 3 Adafruit INA219 Current Sensors (I2C)
  • 1 Adafruit 12 bit A/D (I2C)

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