Heat output and window dressings 1

[ChasBean1]

I'm wondering if anyone has any good data or resources on heat output from sun hitting a) aluminum miniblinds, b) vinyl miniblinds, c) ____ other type of window dressing, etc.

E.g., direct sun impact on type x will be ballpark __BTU/hr-ft2 and type y will be about __BTU/hr-ft2.

Not looking for exacts-I realize all the variables, just comparatives. Assuming all the same color (say white). I've googled such but didn't get too far.

Thanks for any insight... CB

 

[imok2]

Mr. Bean, ACCA Manual J says for single pane, vertical or horizontal with slats at a 45 degree angle has a default "U" of 0.98 at a 10-15 degree TD based on inside temp of 75*F and outside temp. However for more accureacy you need to take into consideration the direction of its location E.W. etc and shading coeff. and or solar heat gain coeff.. see table 3 for those and other information if you require it.

 

[MintJulep]

This looks relevant:


Solar absorption by each element in a glazing/shading layer array.
ASHRAE Transactions | July 1, 2006| Wright, John L.; Kotey, Nathan A. |

ABSTRACT

Window solar gain can strongly influence building energy consumption and peak cooling load. Shading devices such as venetian blinds, roller blinds, and drapes are routinely used to control solar gain. There is a strong need for models that allow shading layers to be included in glazing system analysis. This paper presents methods by which existing solar optical models for systems of specular glazing layers can be extended to include the effect of layers that create scattered, specifically diffuse, radiation in reflection and/or transmission.

 

[ChasBean1]

Thanks to you both! Definitely an article worth looking at, Mint. Imok, ACCA is what? I can look into this.

I might try this question in the heat transfer forum also. It could be important for a job I'm trying to help with - a "glass box" building with an overheating south face. It could probably be calculable somehow, with radiation heat transfer and Stephan Boltzman's constant and all that stuff. I'm just not proficient in that.

This might end with me using an IR thermometer, a couple miniblind purchases and a real set of data...

Thanks again, CB

 

[IRstuff]

South facing windows are pretty much fubar, without external screens, etc. While window treatments can help somewhat, they bottom line is that most of the sunlight gets in, but a large portion stays in. Much of that is due to the fact that window treatments change the angle of reflection of the incident sunlight, and shallower reflection angles trap the sunlight inside the room.

I think you need to have reflective window coverings applied directly to the glass to make a big impact, or to have external louvers, or somesuch.

TTFN

FAQ731-376

 

[GMcD]

My information is based on direct measurement of the interior face of typical office venetian blinds, of various colours, as well as light coloured 5% open weave roller blinds. On the south and west facing windows, it was typical to measure an average of 90F-95F interior surface temperature on the surface of the blinds as one stands in the room, even with white/light grey coloured blinds. Temps were slightly lower for shiny white coloured venetian blinds, but still above 85F. It's all about the surface emmissivity.

Translate that into a radiant and convective heat load to the room, and that's the amount of solar gain that's now inside the space and has to be treated. The biggest comfort effect is from the radiant heat- at a delta of 20F-25F room temp to surface temp of the blinds, that's a good 30-35 Btuh/SF of radiant heating for the blind area The convective heated air isn't much of a factor to the room comfort, as that warmed air is drawn off by a ceiling return air path, but that does impact the return air temp and hence your main cooling coil in the AHU.

You can try to calculate the heat effect from the interior blinds, but it's a transient load and you can only really estimate the peak load. I just cut to the chase and beat up the Architects to get exterior shading, reduced window areas, tuned for the climate and exposure. Keep the solar gain outside in the first place- don't let it get inside the space, unless you are in a heating dominated climate and want to harvest a bit of winter passive solar heat.

 

[ChasBean1]

IR and GMcD, thanks. GMcD, were these vinyl or aluminum? I get the general feeling that opinion is that there wouldn't be that much difference between types, given similar surfaces and colors of the two.

 

[GMcD]

Chas- the venetian blinds I measured were a mix of vinyl and aluminum depending on what building I was in with my surface temperature IR gun. The actual material of the blinds is not important, it's the "shiny-ness" of the finish- the shinier the finish on the outer surface facing the incoming sun, the more they'll reflect some solar IR gain back out through the window (and warm up the window surfaces), but based on my walking around on sunny days, the difference was not statistically significant except for the colour of the blind- the whites were not bad, but the natural aluminum, and dark colours were the worst in terms of temperature. The worst are the textile blinds with a fairly matte surface, those make great solar gain collectors, even in very light coloured finishes.

 

[IRstuff]

The sheen, I think, is only part of the equation. The actual colorant's spectral characterisitics makes a big difference. Your eye only sees between about 400 nm to 800 nm. This is the only part that needs to be "white" to the human eye. Everything in the UV and beyond NIR can be anything, and the manufacturers don't even necessarily care about or characterize the absorption in those wavelengths.

Additionally, most blinds are rarely designed to be constantly perpendicular to the sun, so only a tiny fraction of the day will have optimum retroreflectance, assuming the blind was even flat to start with. The blinds I have at home have a slight curvature, compounding the problem. But, even if the blind was perfectly flat, perfectly aligned in azimuth, the midday sun, in someplace like Los Angeles, is about 70 degrees high, so a large portion of what makes it through the glass will not escape due to the higher reflectance of the glass at high incidence angles.

TTFN

FAQ731-376

 

[ChasBean1]

Ok. Moral of the story, keep it on the outside wherever possible. A couple of nice tall pine trees may be in order. Thanks again for the responses. CB

 

[GMcD]

Chas- also beware of heavily tinted glass as a solar gain control method - besides the challenge of visible light (Tvis) and getting natural daylighting, the darkly tinted glass absorbs a ton of heat and actually passes the heat on to the inner glass on a double paned sealed window, so you still end up with a high component of infrared heat radiating from the warm inner piece of glass as the hot outer glass transmits the IR gain through the airspace to the inner glass. Again, direct on site measurements of a darkly tinted solar ban style glass showed exterior surface temps on the outer glass surface of over 118F at an ambient air temp of 72F on a clear sunny day, south window at a May solar elevation.

Nothing beats exterior shading devices, deciduous trees, eyebrows, exterior adjustable blinds, punched/recessed windows, etc.