Keeping a box cool

Projects General Guidance
1

Hey guys,

I am planning on putting a battery, solar charge controller, arduino and some sensors into a sealed plastic box for gathering data in a desert environment for a month. The battery is not supposed to go above 65C, so I am thinking if there are ways to passively reduce the internal temperature as much as possible without a fan.

To prevent heat from entering my box from the bottom, I have filled the bottom of the the box with a layer of soft material like so:

Then I put a plastic plate on top and put my components on it. I am hoping the material can serve like a blanket to prevent the heat from transferring from the bottom of the box to my components.

The box itself is elevated from the ground for around a meter. The top of the box is white, so hopefully some of the sunlight will get reflected. Do you guys have any more recommendation of how to keep the heat from entering the box?

Thank you!!

2

Put it in the shade, keep the sun off of it.
What ambient air temperature are you expecting? Will be hard to cool it less than that. Best bet may to bury it in the ground, that's how the animals avoid the heat.

3

Thanks for the response!
I can't put the box in the shade because I am measuring the Sun :sweat_smile:.
And isn't it elevating the box better than bury it? Because the ground could get heated up by the Sun, and the heat will than transfer to the box from all sides? Whereas heat cannot conduct through air if I am to elevating the box.

I am no physicist but I believe heat transfer involves conduction, convention and radiation. Hot air also goes up. So I guess the white color can deal with radiation and the layer of soft material can deal with hot air rising up from the bottom. I am not sure how to deal with the rest, nor if I am doing things right haha...

4

Put the solar panels above ground, bury the rest.
Air does cool with elevation, about 2 degrees C/1000 ft. How high can you go?

Animals seem to do pretty well by digging in. My basement is always cooler than the 2nd floor of my house when the windows are all closed up.
Draw your own conclusions.

5
  • paint the box white or put aluminium foil on it so it reflects the sunlight?
  • thought of peltier elements ?
  • make the box not square but long and small and put the small side to the south
    ==> so it catches minimal sun during the hottest part
    ==> google images termite hills in Africa
6

Just put your sun sensor and solar panels in the sun, and put the rest in the shade.

That way the box temperature won't rise above the air temperature (unless a significant amount of heat is generated by the components inside).

7

You can check the "heat rise" by measuring the temperature inside and outside of the box and subtracting to find the difference. That number should be fairly constant, so if you know the maximum expected ambient temperature you can predict the maximum internal temperature.

so I am thinking if there are ways to passively reduce the internal temperature as much as possible without a fan.

If you need to move heat out of the box, you can use a heatsink that passes-through the box. Basically a hunk of metal should work. It will work best if it's attached to the heat-generating components on the inside, with fins on the outside. Of course, a heatsink will trasfer temperature both directions... i.e. If there is more heat outside the box, a heatsink will transfer heat into the box.

Or as others have suggested, bury the box. If possible, you might want to make some measurements to see what the underground temperature is.

Of course if the box is sealed, a fan inside the box isn't going to help that much. It would even-out the temperature inside the box and possibly cool the hottest components slightly, and that might[ help a bit. But since a fan consumes energy, it generates heat and overall you'll have more heat inside the box with a fan inside.

Do you guys have any more recommendation of how to keep the heat from entering the box?

Given enough time, the inside of the box will always reach the outside temperature, plus any heat generated by the electronics (plus any absorbed heat radiation if the box ins't in the shade). With enough insulation, you slow-down the heat transfer to average-out the temperature for lower maximums and higher minimums. Bur realistically, I'd expect the internal temperature to come very close to the outside maximum, even with no electronics inside.

This is less practical than burying the box, but water has a high thermal mass (it changes temperature slowly). So, "surrounding" the box with water will tend to reduce the temperature extremes.

Evaporation also absorbs heat. If you have a continuous supply of water, keeping the heatsink wet also would reduce temperature.

8

aluminium foil

On the whole aluminium foil is not a good idea.

It tends to oxidize to a grey color even if you use the shiny side outside. And grey is not a good color to reflect heat.

White exterior grade paint is a better alternative, as in places like here in Australia the exterior grade paint, especially the light colors are designed to be reflective to stop them getting destroyed by the strength of the sun.

PS. I'd second burying the box.

Also for thermal mass. I use carbonated water bottles full of water. If you put them inside the box, they will increase the thermal mass.
Things like house bricks are also good for thermal mass, but used carbonated water bottles are essentially free, as is water.