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I am designing a prototype of an electronics package that

  • will sit out on a pole in the sun 24/7 with a

  • max ambient temperature of 50˚C and

  • at 30˚N latitude.

  • It produces 50W of internal heat and the electronics are rated to 70˚C.

Currently, it is all housed in a grey polycarbonate box (approx. 18"x12"x6" hwd). I have room for a 100W air-to-air heat exchanger (I need to keep the box watertight which also precludes most vents) but from back-of-the-envelope calculations I can already tell that the solar loading will likely overwhelm the exchanger.
So, from the outset I have planned on putting a solar shield around the enclosure, but now that I have reached that point in design I realize that my heat transfer skills are more than a bit rusty and I'm having trouble fitting the solar shield into my thermal model.

I am just wondering if any of you have any tips, suggestions, or resources on how to incorporate solar shields and what their actual effects are on the cooling of enclosures.
Do any of you have any experience with solar shields?
I have contacted the major enclosure companies for white papers and did some Googling but it all seems very surface-level with not much being given to the driving theory or secondary considerations such as sky temperature or ground reflections.

I am currently leaning towards some sort of aluminum "shell" with an air gap because of its high reflectivity and corresponding reduction of emissivity inwards towards the enclosure, but I have no idea how to translate this concept into harder numbers to make sure I can actually reduce the solar loading to the point that the exchanger can handle it.

I should note that I work at a startup so my resources are a bit limited (in terms of consulting and labor) so I've found myself straying outside of my skillset quite a bit.

I appreciate any responses, let me know if I need to include more info.

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  • $\begingroup$ Bullet points are your friends. Please show your heat calculations. Being in direct sunlight will result in a wattage heat input into box related to the reflectivity of the box and the intensity of the sun. Fully or partially shading the box will remove/reduce that input. $\endgroup$
    – Drew
    May 5, 2020 at 5:01
  • $\begingroup$ How did this work out? What did you end up doing? $\endgroup$
    – Russell McMahon
    Oct 3, 2020 at 9:39

2 Answers 2

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If you are worried about the solar radiation (which is understandable), a much more affordable solution might be to put the whole thing in a shade if its possible, with enough clearance from the actual shading material.

If you stick solar shields on the box, they are bound to get hot and, you won't be able to avoid conductive or convective heat transfer.

What I would use for the shading is a two layer material. One reflective (aluminimum) facing the sun, and a material which isn't conductive (wood or a ceramic). It's a simplistic approach, but it may be more cost-effective.

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Briefest start. Written from a hosp[ital bed just post operation :-) - better could follow if useful.

Solar loading will be peak in hours around noon.
Angles shift across year.
Find your site 0n Gaisma
There are more relevant sites but that's an excellent start.

Peak daily insolation is typically ABOUT 1 kW/m^2 BUT this is site specific and can be about 1.3+ kW/m^2 in some cases.
Gaisma clearness value gives some clue.

If you add reflectivity to a shield you reduce transferred radiation proportionately.
If shield heats it will reradiate at that temperature.
Finning will cool an inner conductive surface.

If you add insulation layer to a shield with an air space behind then you can use insulation R value to determine wattage transfer through the shield and layer.
At best inner side can be at Tambient without evaporative cooling.
So best case you can approach is an insulated shield with an inner face at ambient. Now you have to decide if you can cool your box with a 20C delta.
Note that forced air massively improves either delta T of a heatsink at a given power or power at a given delta T or some mix.

Note that heat pipes can be extremely simple (semi evacuated tube or container with some fluid in with pressure arranged to cause fluid boiling at some target temperature).
Maybe with a wick, maybe much more.

With a heatpipe you can get heat out of a sealed box into a larger than otherwise available heatsink (forced or free flow)

Evaporative cooling is marvellous but probably not an option.

A PV (solar) panel as the sun side of a heat shield serves double duty - and could drive an otherwise unsupportable fan.

What does the box do - knowing may help solution finding.
What location?

50C ambient is only a relatively few places other than Death Valley.

30N - El Paso might get almost that hot on a hot hot day in June? Gaisma El Paso

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