Benchmark: Solar Panel Test for the IOT – Raspberry Pi

SwitchDoc Labs is in the process of designing several IOT solar power systems and we decide to compare three sets of solar panels to get a feel for which panel we can use.   The IOT projects are:

• – Solar Powered OurWeather Station
• – Solar Powered Raspberry Pi Weather Board Weather Station
• – Solar Powered Radio Link (Initially designed for remote sensing of Weather Data from WeatherRack and Grove AM2315)

We wanted to get a feel for two items about each of these three panels.   We wanted to know the average current during the sunny part of the day and calculate the efficiency of each panel in terms of area to current.   With a side benefit of being able to calculate Watts/$.   We will set up some tests to look at long term quality and behavior.

The Three Test Solar Panels

We chose three different panels.   All three of these panels are made in China, although the Voltaics System panel is built to very stringent quality standards and is a highly reliable solar panel.   The visual quality of the others seems good, but clearly the Voltaic Systems panel is the better solar panel in terms of quality.

Voltaic Systems 3.5W Solar Panel

Voltaic Systems 6V 3.5 Watt Solar Panel

Waterproof

UV Resistant

Peak Voltage: 6.0V

Peak Current: 615mA

Peak Power: 3.69 Watts

Size of Solar Cell Area: 19.5 x 10.4 cm

List Price:  $39.00 (includes Plug)

Comments

The Voltaic Systems solar panel is a high quality device.   We have run these for years on systems in the tropics with absolutely no warping, peeling or other issues on Project Curacao.   It is expensive but we know it is good.

SwitchDoc Labs Solar Cell 6V 2W Solar Panel

SwitchDoc Labs 6V 2W Solar Cell Link

Peak Voltage: 6.0V

Peak Current: 330mA

Peak Power: 1.98 Watts

Size of Solar Cell Area: 12.6 x 10.3 cm

Comments

This is an inexpensive solar cell, but will work for many applications.   No mounting holes, JST-PH 2 supplied and soldered cable, and it seems to be sealed and well made.

SwitchDoc Labs Solar Cell 6V 0.6W Solar Panel

SwitchDoc Labs 6V 0.6W Solar Cell Link

Peak Voltage: 6.0V

Peak Current: 100mA

Peak Power: 0.6 Watts

Size of Solar Cell Area: 13.2 x 3.7 cm

 

Comments

This is an inexpensive small solar cell, but will work for some IOT applications.   No mounting holes, JST-PH 2 supplied and soldered cable, and it seems to be sealed and well made.

The Test Hardware and Software

We used our recently released DataLogger for the Raspberry Pi to measure all of the three solar cells at the same time.   The block diagram below shows how they are connected.   We have uploaded all the software used in this project to the DataLogger github page.

We used three SunAirPlus boards to measure all the data regarding the three solar cells and put the data in a MySQL database for analysis by MatPlotLib.    All included in the DataLogger Software.

Block Diagram

Parts List

Raspberry Pi 3 (you can use any Raspberry Pi)

SunAirPlus Grove Solar Power Controller/Data Collector (3)

4 Channel I2C Mux Grove

Pi2Grover Grove to Raspberry Pi Interface

Voltaic Systems 6V 3.5 Watt Solar Panel

SwitchDoc Labs 6V 2W Solar Cell

SwitchDoc Labs 6V 0.6W Solar Cell

The Results

The results of the experiment were great right out of the box.   We got all the data we needed for this test in about 4 hours in the sun.   The graphs below show the current and voltage curves for all three of the panels (Panel 1 – Voltaics, Panel 2 -SwitchDoc Labs 2W panel, Panel 3  -SwitchDoc Labs 0.6W panel).     The results are summarized in Table 1.

 

Why are the Average Power numbers less than the rated numbers?    The angle of the panels to the sun might not quite be at the correct angle for maximum sun (about 47 degrees here at SwitchDoc Labs), the panels might be a little dirty and there might be some haze.   You generally will not reach the maximum rated current very often.

Comments

It is also clear that it is not cost effective to use a bunch of 0.6W cells to replace the 3.5W Voltaic Systems panel.   Note that the highest quality panel is the most efficient of the three, but not by as much as we expected.

As far as our projects are concerned?  For the Raspberry Pi and OurWeather units is concerned, we need to do more work on how much power we need to keep those units functioning at our design rates.   Regarding the Wireless Link, the expected average current consumption is about 6mA at 5V.   Any of the solar cells should keep the low power Arduino Pro Mini LP in the wireless unit happy.   That means the wireless system will consume about 0.03W per hour or a total of 0.72 W in a day.   The time of bright sunlight is listed below for each of the panels to generate this much power.

• – Voltaics 3.5W Panel:  ~20 Minutes
• – SwitchDoc Labs 2W Panel:  ~28 Minutes
• – SwitchDoc Labs 0.5W Panel: ~1 Hour 22 Minutes

Even on a cloudy day, any of these panels should work.   Taking into account various inefficiences in the charging and storage system will probably increase the time needed by about 30% by our best guess.