Bill of materials
The bill of materials for the solar fractal plant includes:
- Waveshare 5.65″ ePaper Module ($76)
- Voltaic 2 Watt 6 Volt Solar Panel ($29)
- Arduino Nano 33 IoT ($19)
- Lithium Ion Battery – 3.7v 2000mAh ($12.50)
- Adafruit Universal USB / DC / Solar Lithium Ion/Polymer charger ($10)
- 3.5mm / 1.1mm to 5.5mm / 2.1mm DC Jack Adapter ($1.50)
- Bamboo Plywood (TBD)
- 3D PLA Filament (TBD)
Total BOM: ~$148
E-paper displays are touted for their low power consumption. My understanding for the Waveshare 5.65″ 7-color display is that it draws around 50mW during refresh and can then dip to a sub <0.01uA standby current.
In my tests the Nano 33 IoT drew around 131mW while refreshing the E-paper display, 105mW while the E-paper display was in standby, and 55mW when placed into a low power mode using the Arduino Low Power library. These measurements put the E-paper display closer to 26mW during refresh making the Nano 33 IoT the biggest power hog for this project.
To determine total power requirement I am using the formula (mWh)/(V) =(mAh) I have the equation 55mWh / 3.7V = 14.86mAh. Considering these power requirements, and refreshing the display as little as once a day, I am anticipating drawing ~15mA/hour or 360mAh each day. On a full charge I would expect the 2000 mAh battery to run for a little over 5 days.
Using estimates from the Voltaic website to determine the expected battery charge time, specifically the formula Battery Capacity (in Watt hours) / Panel Power (in Watts) X 2 = Charge Time (in Hours), I anticipate needing ~2 hours of direct sun every 5 days for everything to stay operational. More specifically 7.4 Wh / ( 2.2 W * 2 ) = 1.68 hours under ideal conditions.
This chart from Voltaic also has some helpful expectations for the impact of sun angle on panel output: