Re: [EnergyPlus_Support] why blinds not reducing heat gain through window?

Jean,

Sorry for the delay in responding, mostly because I had to read your post several times to understand what you were driving at, and am still not exactly sure (I feel a bit like a defendant at a trial wondering whether the witness was speaking for or against me :-) ).

My memory is that long-wave radiation is around 40% of the solar spectrum, but I could be wrong. How much of it gets into a zone varies greatly depending on the optical properties of the glazing, esp. those with selective low-E coatings.

I was following the proposition that zone air temperatures might be increasing due to the blinds converting short-wave to long-wave that stays in the zone, thus heating up the air.

Thanks for pointing out that even long-wave radiation doesn't all stay in the zone in the case of a blind sitting right behind a window,
although in your fourth line "long wave... hits window (e=0.7)...", shouldn't that be 70% and not 30% "reflected back towards blind" ?

Without doing a Gauss-Seidel solution (:-)), I would guess the net amount of long-wave radiation remaining in the zone from your scenario between 50-60% on the window side. It would be 100% for the long-wave on the zone side of the blinds, but that radiation would be coupled to the floor slab as much as direct solar radiation.

So, the question for me would be whether 25-30% conversion to long-wave radiation would more than counteract the 80% rejection of
short-wave radiation due to the blinds?

Joe

On 10/9/2014 11:17 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:

I like the way you are breaking this down and would love to hear also how you consider the LONG wave radiation (which to my understanding carries the bigger "chunk" of the energy).

I was thinking roughly as follows:

Long wave hits glass (emmissivity 0.7) --> 30% gets through

Long wave hits slat (emmissivity highly variable... a "bad" blind may be cheaply painted and have the emmissivity degraded from 0.3 to 0.95) --> 5% hit floor and 95% is absorbed and reradiated equally in both directions.

Long wave from blind towards window --> hits window (e=0.7) --> 30% reflected back towards blind --> loop continues until dissipated (I'm assuming a gauss-seidel or similarely solved differential equation)

So now comparing to (in FullExterior mode) all the entering radiation hits floor and the reflected (0.5%) is area weightedly distributed to all zone surfaces, the windows amongst these recieve a significant portion to reradiate (question is if the energy is reproportioned appropriately into short wave and long wave reradiating from the floor).

One aspect which is not talked about is whether the model accounts for the locally increase air temperatures between blind and window and the heat transfer from this air to the outdoors. The surface temps of both blinds and glass pane may significantly change surface airflows (convection heat exchange from air to surface).

Mit freundlichen GrÃ¼Ã?en- Sent from my iPhone (excuse the brevity)

i. A.

Jean Marais

b.i.g. bechtold

Tel. +49 30 6706662-23

On 09.10.2014, at 20:17, "Joe Huang YJHuang@xxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
What you say is true, but it's not the only effect of window blinds. Of the solar radiation that's intercepted by window blinds, most of it is reflected back out of the window depending on its reflectance, a small fraction (< 5%) is transmitted, and the remainder absorbed and reradiated as long-wave radiation. So, when I hear that window blinds actually increase the indoor air temperature, this would imply that the reradiated long-wave is significantly greater than the reflected short-wave, which would happen only if the blinds are heavy and dark. However, all the blinds I've seen, especially those used for solar control, are either white or very light in color.

It's pretty well-known that external blinds are more effective than internal blinds for reducing unwanted solar gain. However, this is the first I've heard that internal blinds are not just ineffective, but counter-productive. In rereading the original post, I noticed it said "Zone mean air temperature" and "Surface inside face temperature" have both gone up. I'm not particularly surprised at the latter, because blinds tend to have higher absorptivity than glazing (although I would caution not to ignore the transmitted short-wave through the window in comparing surface temperatures), but I am surprised at the former. I don't want to speculate further on this until I know more how the blinds are being modeled, how the zone is being controlled, and how the HVAC, if there is one, is being modeled.

The following link is for a paper done in 2004 studying the same issue in a similar building (80% glazing on the facades) and comparable climate (Canada). That study concluded that internal blinds reduced cooling loads on a continuum depending on their reflectance, with the best (90% reflectance) approaching but still significantly less effective than external blinds.

http://sbrn.solarbuildings.ca/c/sbn/file_db/Doc_File_e/Simulation%20design%20study%20for%20the%20facade%20renovation.pdf

Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
On 10/8/2014 10:58 PM, Almofeez almofeez@xxxxxxxxx [EnergyPlus_Support] wrote:
It may be explained that, the blinds absorb short wave solar heat and radiate long wave heat. We know glass is opaque to short wave radiant heat. Therefore heat is trapped inside room.

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I can't point to a study, but the guys at trnsys have told me that studies have shown that internal blinds bring little to no positive effect on room heat gain.

However, as we're on the topic of blinds, could someone please draw up a table showing which controls are effected by which input parameters, e.g. Control type glare - does it go the the "fixed slat" position on trigger or does it change possition between min/max slat angles to maintain a setpoint?

There are a lot of control options...a table would be a great addition to the documentaion.

Jean

On 09.10.2014, at 03:04, "Joe Huang YJHuang@xxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
Hmmm, interesting. Wouldn't the blinds reflect a fair amount of the solar back out through the window? Seems counter-intuitive that deploying blinds behind a window will increase rather than decrease the mean air temperature of the space. Have we been wrong all these decades? I'd still
like to know the thermal/optical characteristics of the window and the blinds. I suppose having a black
blind behind a window with very low IR transmissivity might create problems.

Joe

On 10/8/2014 4:45 PM, 'Edward G. Lyon' eglyon@xxxxxxx [EnergyPlus_Support] wrote:
The low mass blinds will also absorb all the solar and transfer it to the interior air. Without blinds, the solar hits a more massive floor/walls with much less immediate transfer to the air. This could give you higher cooling peak demand.

Ned Lyon, P.E. (MA, WV)
Staff Consultant

SIMPSON GUMPERTZ & HEGER
781.907.9000 main
781.907.9350 direct
617.285.2162 mobile
781.907.9009 fax
www.sgh.com

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Wednesday, October 08, 2014 7:07 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] why blinds not reducing heat gain through window?
I suspect there's something wrong in the modeling of the blinds. The effect of night sky radiation is not so pronounced on a window, because (1) half of what it sees is ground, (2) the other half is the sky at low angles where the sky temperatures are much less depressed as they are looking straight up the sky.

Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
On 10/7/2014 11:00 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:

If you have a building with lots of glass, don't forget the radiant exchange with the night sky at -60 C. The "radiant night cooling" is possibly being blocked by the blinds, if the blinds are there during the night.

Mit freundlichen GrÃ¼Ã?en- Sent from my iPhone (excuse the brevity)

i. A.

Jean Marais

b.i.g. bechtold

Tel. +49 30 6706662-23

On 06.10.2014, at 21:53, "annavenediki@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Hello,

I've designed a south facing room with a floor- to-ceiling window and blinds covering the whole surface of the window. I run two simulations, one with no blinds on the window and one with blinds covering the window in order to compare the results. The building's location is in Eastern Europe. The run period is from July to August (sunny and hot period). Both output variables "Zone mean air temperature" and "Surface inside face temperature", have much higher values when the simulation is run with the blinds on the window. How can this be possible? Can it be explained in a way or is there something wrong with the data?

Thanks in advance,

Anna V.