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What assumptions do you make for modeling blinds?

asked 2014-10-02 09:06:52 -0500

CJ gravatar image

updated 2015-07-11 09:32:51 -0500

Hi everyone. Regardless of modeling software, I am curious to know what assumptions you make when modeling blinds, when all you know is that there are blinds.

This is with the consideration that the occupants will not be actively moving blinds up and down as required. Ideally, you would model using a schedule/s that makes blinds open / close with varying degrees in response to direct solar radiation, or another criteria pertinent to your case. In eQUEST, you can add a sun control probability %, which basically says how often your occupants will demonstrate conscious behavior. Other programs likely have similar input.

So, what is your usual basic input for the following, when detailed info is unavailable:

  1. Percent of occupied hours when blinds are down?
  2. When down, percent of blinds closed?
  3. Points 1 and 2 above, but for unoccupied hours.

I realize that my subject line is a bit more broad. You are welcome to make this a general discussion on modeling blinds / interior operable shades / good old window shutters etc, trade-offs, pros and cons of methods. Thank you.

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answered 2014-10-03 20:11:43 -0500

Joe Huang gravatar image

updated 2014-10-07 14:21:02 -0500

This question brings to mind an experience I had back in 2006 on a California Energy Commission project to translate their Certification Suite of 160+ building runs from DOE-2.1E to EnergyPlus that made me ponder the trade-off between technical rigor and practicality. Repeating what Neal said, blinds and drapes are almost always used, so that if one models a building without them, its cooling energy use will tend to be high. In DOE-2, drapes and blinds are accounted for by a SHADING-FRACTION that discounts the solar gain through the window, to which schedules of any complexity can be added to model their operation. Use of this keyword in DOE-2 is so prevalent that someone (not me) has even coined the name sill_sc.

So, on the Commission project, the person assigned the task of converting the window inputs asked me how to deal with the sill_sc, since shades in EnergyPlus are modeled more physically requiring 10 inputs for solar reflectance, visible transmittance, IR emissivity, conductivity, etc., none of which are knowable for these hypothetical models. Even if we were to venture guesses, there would still be the problem of getting functional correspondence to the DOE-2 sill_sc that were developed through trial-and-error.

In the end, my team decided to punt on this issue, modeling the buildings with no shade in both DOE-2 and E+, even though we knew that wasn't very realistic. This is why it left me pondering the trade-off between getting something right technically and getting something that is practically doable. I realize that it's now been 8 years, and maybe this situation has been ameliorated, but when I just checked the Energy+.idd V8.0, I still see the same inputs for WindowMaterial:Shade, and now an even more detailed WindowMaterial:ComplexShade. How about adding a WindowMaterial:SimpleShade for dummies like me?

BTW, I've seen sill_sc ranging from 0.60 in the commercial building prototypes I worked on the 90's to 0.80 in the Commission certification files.

To answer Neal's question on their operation, I've always modeled them as constant in commercial buildings, but in residential buildings, I've varied them between 0.80 in the winter (0.10 due to drapes + 0.10 due to dirt on the windows) and 0.70 in the summer (0.20 due to drapes + 0.10 due to dirt). Winter and summer are determined by keeping track of the number of cooling degree days over the past 4 days. For many years I had modeled residential drapes as 0.90 winter/0.63 summer, but the late Dariush Arasteh convinced me that people were not so diligent in their use of drapes.

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answered 2014-10-03 08:36:12 -0500

updated 2014-10-03 09:19:55 -0500

My experience is with BEopt running EnergyPlus where BEopt assumes the following:

  1. Blinds are always present - i.e. not a user variable
  2. Blind use is scheduled and either up or down (0 or 1 schedule)
  3. When down, solar gains are multiplied by a constant interior shading multiplier of 0.7 (or reduced by 30%)

See the WindowProperty:ShadingControl object in the EnergyPlus Input Output Reference or do a quick sim in BEopt and then look at the IDF for the assumed schedule.

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The assumptions used in BEopt for shading can be found in the Building America House Simulation Protocols.

Neal Kruis gravatar imageNeal Kruis ( 2014-10-03 09:22:30 -0500 )edit

Basically two lines...

4.2.2 Operation of Shading Devices Solar gains through windows shall be modified by a constant interior shading multiplier of 0.7, meaning that all solar gains are reduced by 30% through use of blinds, shades, or curtains."

MatthewSteen gravatar imageMatthewSteen ( 2014-10-03 11:16:33 -0500 )edit

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Asked: 2014-10-02 09:06:52 -0500

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Last updated: Oct 07 '14