How to calculate solar heat load on a roof?

[sabotage]

I am determining the amount of air required to cool a outdoor space with only fans, so the indoor and ambient temperatures will be very similar. For my purposes the roof load from the sun will be the only heat conducted to the space.

The only method I have found to determine solar load is the CLTD (Cooling Load Temperature Difference) method with Q = U * A * CLTD, although I am not sure it is appropriate for ONLY solar loads. Is there another method to determine solar loads suitable for manual calculations?

 

[IRstuff]

There have been many postings about solar heating in this and the HVAC forum; you should do a search

TTFN

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[MintJulep]

Could you define the "space" a bit better.

Is it only an area? Is there a canopy or other shade?

If you have no walls and the room under the roof is cooler than outside you have no conduction.

 

[sabotage]

IRstuff: You're correct that I did not perform any sort of search, I will definitely do one in addition to this thread. However, I would also appreciate any help you would have to offer on this subject.

MintJulep: It is a single story building with only one room and no windows. A standard construction of four walls and a roof with no shading.

 

[IRstuff]

1120W/m^2 in * absorptivity = convection + radiation

TTFN

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[MintJulep]

So then it's not an outdoor space at all? It's an indoor space?

 

[sabotage]

Yes, indoor space only with exterior exposures.

 

[IRstuff]

You mean like a gazebo?

TTFN

FAQ731-376
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[sabotage]

I just reread my first post and realized that it was causing the confusion, it is not an 'outdoor' space as I stated. It is just a building located outdoors, which now that I've typed it out sounds extremely redundant.

I hope this clears up any confusion I've caused. If it helps the building in question is a simple maintenance garage for vehicles with a flat built up roof.

 

[IRstuff]

Let me redo that equation:
1120W/m^2 in * absorptivity = passive convection up + radiation up + conduction down.

The interior surface then has:
conduction down = forced convection + radiation down

Given the boundary conditions, there will be a T.rooftop and T.ceiling that balances all the heat flows.

TTFN

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[MintJulep]

Well as long as it isn't really extremely redundant.

The CLTD calculation has some values for indoor and outdoor temperature assumed into it.

You can correct for non standard conditions such as yours. See here:

http://me.lsu.edu/~meniki/me4643/downloads/COOLING LOAD LECTURE OUTLINE.pdf

 

[sabotage]

IRstuff: During my search on the board I've seen you use the figure 1120W/m^2 in other replies, and you mention you took it from some MIL spec, is there an ASHRAE equivalent? I understand this is the peak solar radiation for something like 1% of the year?

MintJulep: Yes, that would just be ridiculous then! Also, thank you for the confirmation that the CLTD calculation is appropriate. I am using the 1997 ASHRAE Fundamentals (which was the last year CLTD tables were produced in ASHRAE) for guidance but I will also check out your link. I'm assuming you're not aware of any other method for this calculation (beside the one IRstuff has posted)?

 

[IRstuff]

That power flux is indeed a relatively extreme case. There are, somewhere on the web, sites or tables that give you insolation for specific locales.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize