Interior Shading = More cooling??? 2

[tolind]

I'm looking at the procedure for calculating space design cooling load in Ashrae. For Solar gain - q=A x SC x SHGF x CLF. I understand that and agree with it. My problem lies when I read that charts for interior shading. The Values for "With Interior Shading" are higher than that without. I've read through the section but I can't seem to grasp the concept.

If I install a blind system shouldn't the blinds lower the amount of cooling required within the space? The sunlight can't get in and make everything feel warmer.

 

[schola]

The blind storages heat, that later will be rejected to the space.

regards,
schola

 

[pablo2410]

(To schola)

But is not the heat gain from direct solar radiation trough the windows suposed to drop down?

 

[MintJulep]

ASHRAE has come up with a better calculation for solar gains then the SHGF method presented in older editions of the fundamentals handbook. If memory serves, it is called the radiant time series method. I think it is presented in the most recent edition of fundamentals. It overcomes the fundamental problems with SHGF (which is that the factors were all pretty much just SWAGs).

 

[stanlsimon]

Here is one way to look at it: There are two modes of heat transfer through the glazing, conduction and radiation. Without the shades, radiation travels through the glazing, and hits some solid heavy surface like a floor or wall where it causes a slow increase of surface temperature. The subsequent rise in space temperature is delayed. This tends to spread out the heat over a long enough time period to reduce it's effect. By the time the heat starts getting to the space the sun has moved to the south or west or simply gone down.

With shades the radiation is absorbed by the lightweight shades which warm up quickly and cause an almost immediate transfer of heat to the space air. Yes the shades do reradiate a fraction of the heat back out the window, but the windows are designed to keep heat in.

The computerized load programs use transfer functions that approximate the time delays.

 

[GMcD]

Exactly, Stanlsimon - this is an issue near and dear to me when designing low energy buildings. I have measured interior surface temperatures on interior window shades in the range of 90F to over 110F on a sunny day. This translates to over 45-60 btuh/SF of window shade area worth of radiant heat transfer to an occupant in the room, plus the convective heat from the bouyant plume off the hot blinds going into the room air side.

The right thing to do is to use exterior solar shades and tints in the glass, plus tuned Low-e coatings depending on your application for solar control. Keep the climate variations outside the building where they belong.

 

[imok2]

GmcD, Wouldn't you agree then, that interior shading is best used during the cold season on a sunny day, where we could take advantage of both radient heat and the insulating effect of air space between the winow and the blinds

 

[GMcD]

Imok- no, I would take the direct passive solar through the window, without internal shades to warm up the thermal mass of the room to take the best advantage of it, with a low-e coat on the inside piece of glass ro reflect infrared heat back into the room, to minimize the heat losses, and to maximize the free daylighting. It's a matter of designing the exterior shading and the glass design for the specific climate zone- keep the summer solar gains out, while allowing the winter solar heat gain inside at low sun angles, and to maximize winter daylighting.

 

[Ashereng]

GMcD, I guess the exterior shade is like the remote controlled awnings that I see on buildings? Or is that something else altogether?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[GMcD]

Ashereng: yes- very popular in Europe and you can see them on many buildings. Architects in North America have an aversion to them becuase of aesthetics. There are also maintenance concerns for the motorized type exterior shades. Passive fixed shades are, in my opinion, the best aporoach since they can be designed to keep out the summer sun while allowing the winter sun IN with a minimum amount of fuss.