Effect of Solar Shield on Electronics Enclosure

[sheafromme]

Hello All,
Longtime reader but this is my first post.
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.
-Shea

 

[IRstuff]

> The sun doesn't travel in a circle around your box, so it seems to me that you can find a side that's mostly away from the sun
> That said, 50C background is also emitting at your exchanger
> How much margin you have dictates how much shielding you need
> Highly polished aluminum has low emissivity, but environment, dust, bird poop, corrosion, etc., will have a deleterious effect on its reflectivity/emissivity
> Note that absorbed visible light converts to long-wavelength infrared (LWIR) emission
> Multi-layer insulation (MLI) works very well for satellites and cryogenic instruments


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[sheafromme]

Thank you, IRstuff! For conduction/convection I imagine I would use the 50˚C ambient as you said, but what do I use for radiant transfer? The sky temperature? And that makes sense about the environmental effects, I imagine that there'd have to be some "depreciation factor" to account for reflectivity/emissivity being altered over time. When you say that visible light converts to LWIR emission, is that only for radiant emission or does it apply to reflection as well? I think multi-layer insulation would be great, or perhaps simply the concept itself to save cost. Do you have any suggestions for theoretical models for this sort of thing?
Thanks again,
-Shea

 

[EdStainless]

You need to make the shield large enough to protect from both mid-day sun and later afternoon sun.
The bought packages can be dual layer with an air gap between them.
And you have to make sure that they are mounted so that rain will rinse them off, including the inner one.
And make sure that they mounted well enough that wind will not tear them off.
I have seen them in both Al and SS.
The people that sell these systems provide calculators.

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P.E. Metallurgy

 

[sheafromme]

Hi EdStainless, thank you for the response! That makes sense to me, do you have any recommended suppliers for off-the-shelf shield packages? Do you know if the calculators are detailed enough to trust for field deployments?
Thanks again,
-Shea

 

[EdStainless]

Sorry, I wasn't the one responsible for that part.
We did have a coastal application (in a place with no rain) and ended up hand making one from Ti sheet.

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P.E. Metallurgy

 

[IRstuff]

"When you say that visible light converts to LWIR emission, is that only for radiant emission or does it apply to reflection as well?"

No, visible light is absorbed and converted into heat energy, i.e., temperature rise, which, in turn, results in thermal emission, mostly in the LWIR regime, since Wien's Law puts the peak wavelength at around 10 um, compared to 550 nm for visible, since its source is the Sun at ~5500 kelvins, so visible in, LWIR out. Reflection is almost purely a direct optical phenomenon, although there is an atomic interaction with the surface.

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