[Bldg-sim] Modeling of Ventilated Facades with Cavity Blinds -Blind Parameters

[Timothy Moore]

Graham,

 

You can, within IES Virtual Environment, explicitly model blinds as you have described below: 

 

First either divide the vented façade into two zones (inner and outer air spaces) with a corrugated surface or add such a partition to one zone; every other facet of this corrugated surface will be a blind slat with specified material properties and tilt angle, etc., and the facets in between will be holes. Having completed this, if you made two zones to begin with, then combine them as one zone with the blinds as floating partition or planar surface elements. If you need to model the façade over the course of a year with the louvers at dynamically varied angles or in various states of deployment, I’d suggest multiple runs with post-processing to combine the data into a single year. For this you would combine selected results based upon incident solar gain for that hour, or whatever variable you choose to determine the control of the louver angles, etc.

 

It is important that you end up with a single combined zone between the inner and outer glazing to ensure proper calculation of transmitted direct and diffuse solar gain (however, you will still require separate vertically stacked zones for each story, etc. of the building façade as means of determining differences in buoyancy with MacroFlo). The thermal properties of the blind materials and their surfaces will also be important to determining the absorbed solar gain, re-radiation to the façade elements, and convective heat transfer to the air in the vented cavity. Lastly, because the theory applied in MacroFlo is based on the flow characteristics of openings that are small in relation to the spaces they connect—i.e., it’s set up to account for the flow resistance (aero drag coefficient) of operable windows, other punched openings, etc.—you will need to have a look at Section 7: Techniques for Modeling Flow in Façades and Flues in the MacroFlo Calculation Methods section of the user manual. This will tell you how to adjust the dimensions of your façade cavity model so that a MacroFlo opening at each floor height (or similar) will appropriately account for resistance from openings, mullion protrusions, catwalks, surface friction, and so forth.

 

Regards,

 

Timothy Moore,

Building Science Research and Performance Simulation

 

Center for the Built Environment (CBE)

University of California at Berkeley 

 <http://www.CBE.berkeley.edu> www.CBE.berkeley.edu

 

Sustainable Design Consultant, LEED AP

 

Whole Systems Design

Office: 510-525-4809

Mobile: 303-324-1044

eFax: 413-480-7252

 <mailto:tmoore at whole-systems-design.com> tmoore at whole-systems-design.com

 

 

 

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of hamnmegs at ozemail.com.au
Sent: Wednesday, April 02, 2008 1:15 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Modelling of Ventilated Facades with Cavity Blinds - Blind Parameters

 

Hi all,

We are revisiting ventilated facades with cavity blinds in an attempt to minimise the fabric heat loads on a building.  We are looking at internally and externally ventilated options.  My experience in this area is limited to use of bulk airflow modelling (IES Apache or use of Arup's in house tools during the time I worked for them).

 

We have done some preliminary digging around to source the best software to determine average annual performance, bulk temperatures etc recognising that CFD will always have a place in understanding airflows and temperatures in more detail.  Our current position is:

1) We explored the possibilities with Energy+ and the ventilated facade / cavity model 2-3 years ago and were unable to generate results that we had confidence in.  In skimming the version updates it does not appear that there has been further work on this module so we don't plan to go back to Energy+.

2) We use IES Apache extensively but the blind inputs are indirect (unknown) rather than direct (known) properties and thus lead to uncertainty (see below)

3) We were initially encouraged by the ICE software out of Sweden, but now understand that the blind inputs are similar to IES.

 

IES / ICE take as input for the blind the a SHGC modifier (1 - SHGC = the overall reflected component of the blind be it SW or LW radiation) along with the proportion of this which ends up as shortwave versus longwave radiation.  There is also a U-Value / conductance modifier.  The blind can be incident solar or time controlled like most programs.

 

We would prefer to input blind transmittance and absorptance (or reflectance) and let the software dynamically determine SHGC.  Until you have an understanding of the temperature of the blind which is a function of incident solar radiation, ambient temperatures, cavity ariflow etc you won't know exactly what the longwave radiation component is of the SHGC.  So the egg comes before the chicken ... .  

 

In short the longwave component of the blind SHGC is a strong function of other dynamic or design variables and as such cannot be pre-determined with confidence in our professional opinion.

 

Does TRNSYS or other overcome this limitation?  

 

Any suggestions? Any killer papers out there that have correlated longwave blind SHGC to different glazing & blind types and ventilation arrangements? for a variety of solar / ambient conditions ..... ?  Anyone considering a PhD in the near future in this area :)?

 

Regards,

Graham

 

Graham Carter | Manager Sustainable Design | Lend Lease design

T 61 2 9236 6347 | F 61 2 9383 8124 | M 0407 949 193

30 The Bond, 30 Hickson Road, Millers Point NSW 2000

 

graham.carter at lendlease.com.au

 

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