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One has to be careful about defining the shading
factor. The shaded portion of the window in PREBID is treated as the opaque part
of the window which may have a very different U-value than that of the
glass (glazing). The heat conducted across the window will be affected if a
glazing forms significantly large portion of the wall.
Let me explain by the following example (a
single glazed window with wooden frame).
If the shading coefficient is 0.5, beam and
diffuse radiation will pass through 50% of the area of the glass. The
remaining 50% of glass area would be treated as wood.
In reality, heat conduction will take place
through the remaining 50% of the glass area (it should not be treated as
wood). (Imagine the difference when the conductivity of glass = 2.88 kJ/m-K and the conductivity of wood = 0.54 kJ/m-K).
A better method to model shading would be to
treat the beam and diffuse radiation with the shading coefficients in IISiBat
before it is given as input to Type 56.
Caution: The modified radiation to
be given separately for windows, it should not affect the radiation incident on
the walls. This would require creating a separate surface to contain the windows
only. Same caution is required also when Types 34 and 68 are used as
suggested by David.
David: pl confirm the above
statement.
Anil Misra
----- Original Message -----
Sent: Monday, November 15, 2004 9:38
PM
Subject: [BLDG-SIM] Windows Shading
Coefficient
Dear Maurizio,
1) I'm
currently modelling in PREBID a residential building for which the montly
Shading Coefficients on the windows have been calculated with the software
ECOTECT. I insert the SC in the appropriate field ("external shading
coefficient") as an input and set the "view factor" to 0,5 (vertical
surfaces). It's the exact way? My doubt is that the solar incident radiation
is halved (view factor = 0,5) and then reduced of the
SC...
It is correct, provided that the shading
coefficient from Ecotect is the opaque fraction of the window (i.e. 1 =
completely shaded).
The view factor to the sky (FSky) is only used for
longwave radiation so it will not interfere with shading.
The one thing
to bear in mind about PREBID's external and internal shading coefficient is
that the entered fraction effects both beam and diffuse radiation equally.
Thus if you enter a value of 0.5 for the external shading factor, half of the
incident diffuse radiation and half of the incident beam radiation will enter
the space. If you want to treat beam and diffuse radiation separately, you can
use one of the external shading models (Type34, or Type68).
2) It's
possible to use old TESS libraries with TRNSYS 16 or are they already
comprised in the new version?
Because of the change in
coding standards for components between version 15 and 16, the TESS Libraries
v. 1.x are not directly useable with TRNSYS v 16. We have,
however, expanded the libraries and released version 2.0, which are not only
compatible with TRNSYS 16 but are also backwards compatible with TRNSYS 15. If
you are interested in upgrading, you will be credited the full amount that you
paid for your version 1.x libraries toward the purchase of version 2.x
libraries. If you do not wish to upgrade then you can follow the steps laid
out in the TRNSYS 16 documentation that show you how to modify a TRNSYS 15
Type to make it accessible from TRNSYS 16. I will send along a second email
containing a document that lists the contents of the 12 version 2.0 libraries
as well as an order form that contains the pricing information. I would be
happy to answer any questions that you might have about them.
Kind
regards,
David
****************************************************************************************
Thermal
Energy System Specialists (TESS), LLC
David
BRADLEY
2916 Marketplace Drive - Suite 104
Partner
Madison, WI 53719
Phone: (608) 274-2577 USA
Fax: (608) 278-1475
E-mail: bradley@xxxxxxxxxxxx
Web Pages: http://www.tess-inc.com
and http://www.trnsys.com
"Providing software
solutions for today's energy engineering projects"
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