Correlating Temperature to Solar Load

  • Thread starter ddelaiarro
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BLUF: Is it possible to mimic a solar loading environment solely in a temperature chamber and, if so, how?

We have an 6061 Aluminum box with a known heat load inside. It needs to be subjected to a solar load IAW MIL-STD-810G, Method 505.4, Proc I. Essentially, it is a 24-hour exposure cycle with varying ranges of temperature and solid load. Our company has a thermal chamber (too small to put heat lamps in) that we'd like to pretest our units in to make sure we pass qualification testing. We can't create a solar load with our equipment, but what I'd like to do is create a temperature profile that would essentially mimic the total heat load our module will experience during solar load.

I know the following:
- Surface area of the module, A
- Reflectance level for the paint used
- Temperature and Thermal Loading (W/m2) profiles for the 24-hour cycle

What I'd like to do is convert the known Power from the solar load (W/m2 * A) at each temperature interval to a [tex]\Delta[/tex]T that I can add to the prescribed ambient temperature to essentially mimic the solar loading in my thermal chamber.

In scouring the internet, I have run into http://en.wikipedia.org/wiki/Sol-air_temperature" [Broken]. The concept makes sense, but I am having a lot of trouble with the [tex]\Delta[/tex]Qir value. The values for Fr, hr and [tex]\Delta[/tex]To-sky are eluding me.

Any suggestions?
 
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After talking it over with some others here, we've decided on a simplified method with some known issues to at least somewhat simulate this test. Here's our solution and thought process:

The convection heat transfer equation is as follows:

P = k⋅Aconv⋅ΔT where ΔT = Tcase - Tamb, Aconv is the area of the module used for convection and P = power to be dissipated

Rearranged, that equation works out to:

Tcase = Tamb + P/(h⋅Aconv)

Obviously, as the power dissipation requirement or ambient temperature rise, the case temperature will rise. The power dissipation requirements are defined by P/(h⋅Aconv). We know that, in a solar loading environment, this section of the equation will rise. However, due to our test limitations, we cannot adjust this part of the equation. Therefore, we will change the Tamb portion of the equation to mimic the change in the P/(h⋅Aconv) portion of the equation.

The new ambient temperature which will mimic solar exposure, Tsol, will be raised by the product of the solar load, W, and the exposure area, Aexp, which is defined as the area of the module exposed to the solar load. Therefore:

ΔTamb = (W⋅Aexp)/(h⋅Aconv)

and

Tsol = Tamb + ΔTamb

This solution has obvious shortcomings with the most glaring being localization of heat. In this solution we are taking a localized heat load (W⋅Aexp) and distributing it over the entire area used for convection (Aconv). This results in a more constant thermal gradient and less localized thermal stresses which may affect performance. Again, this test is being used as a 'pre-test' to a more rigid qualification test and is just a sanity check before going to the lab.

Thoughts on the approach?
 

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