Hi,
Here is a mechanical and temperature related problem I cannot solve:
I am in a process of starting developing a small IoT gadget which will measure some soil parameters (temperature and moisture) and use a Wifi to send the measurement data a few times in a day into the "cloud". The gadget will use solar energy for its operation, and the harvested solar energy will be stored in a rechargeable 4.2V Li-battery. There will be a 5cm x 5cm solar panel used for harvesting the energy.
The solar panel and the rest of the electronics including the Li-battery will be housed in a single plastic enclosure: width=6cm x depth=6cm x height=3cm, the 5cm x 5cm solar panel facing to the sun through the 6cm x 6cm front plastic window. The enclosure needs to be weather proof (winter with snow, summer with rain, temperature -30C ... +45C) and it needs to last for some years, if possible.
So far the design seems to be quite simple as there are quite a few suitable IoT devboards available. However, the temperature effect of the direct sunlight and the generated heat by the sun is my main concern and how it affects the Li-battery. It is now winter here in Finland, so I cannot make any real measurements. I have tried to find some actual data and measurements on how the direct sun exposure affects the internal temperature of a small, closed plastic enclosure.
Selecting a reflective (white) color has a profound effect on the heat load the sun will provide. Unfortunately the black solar panel (5cm x 5cm) needs to be placed inside the enclosure with the electronics and the Li-battery, behind a transparent plastic front panel. The plastic enclosure will not have any holes in order to keep it weather-proof. The electronics inside will not generate any heat in practical terms, but the small 25 cm² solar panel will absorb heat from the sun, and that heat will increase the internal temperature. At this point no heat sinking is possible.
These are the numbers I have right now: During the summer time the sun will provide approximately 1 kW/m^2, the temperature in the sun maybe 30C - 40C, the solar panel size inside the enclosure is 25 cm² facing directly to the sun, the enclosure size is 6cm x 6cm x 3cm and the thickness of the enclosure is 3mm, on a hot summer day the wind is typically non-existent.
Does someone have any links or suggestions for calculating the guesstimates for the temperature rise when the device is in the direct sunlight for a few hours, or would someone have even hands-on experience and/or measurement data for similar situation? I already have a bad feeling that I will be in a trouble with the excessive heat ...
Br,
Kalvin