Re: [TRNSYS-users] Types 68 and 34 (shading)

[Enertech <sidler@club-internet.fr>]

Dear Fernando and TRNSYS Users,
 
(b) If the upstream component is type68, 
 
  Sky diffuse on window = Shaded sky diffuse on window without
overhang
* (1-Fws)
 
In both cases (a) and (b) I suppose that the sky diffuse radiation
input
is always the input number 4 of type34. The label in the proforma is
somewhat error prone (it says ?sky diffuse on the horizontal?).
 
I think you?re right !! There?s an error in the TYPE 34 proforma :
input(4) has to be the diffuse radiation on surface and not the
horizontal diffuse radiation. You can check it in the TYPE34 source
code.

I?m a little bit angry to discover more and more errors in proforma
definitions. How can we trust in TRNSYS simulation results if there is
these kinds of error in models ?
 
I?m working on complex building projects with a lot of far-away masks
(TYPE68) and a lot of overhangs, with very short notice (of course!!).
I?ve no time to check that all inputs defined in proforma are the ones
needed by source codes?
 
I?m sorry to be so hard on this subject but modelling a building is not
an easy job and I need to trust in models I?m using otherwise it only
remains for me to say to my boss that I can?t do this job anymore,
there?s too much errors in the software.
 
Salutation,
C. Plantier

,At 11:38 16/08/2005, Fernando Domínguez Muñoz wrote:
> Dear TRNSYS users,
>
> I have some doubts about TYPES 34 (overhang and wingwall shading) and
> 68
> (shading by external object) that I would like to discuss with you.
> These problems are related to the diffuse radiation
> calculations.
>
> In order to model a vertical window with an overhang and a wall in
> front
> (of the window), I tried to use types 16, 68 and 34 connected in
> cascade. The first parameter of type34 changes the way in which the
> view
> factors are calculated, being offered the following two choices:
>
> (a) If the upstream component is type16, there is no problem:
>
>     Sky diffuse on window = Sky diffuse on horizontal *
> (0.5 ­ Fws)
>
> Where Fws = view factor between window and sky (takes account of the
> overhang and the windwalls)
>
> (b) If the upstream component is type68, 
>
>   Sky diffuse on window = Shaded sky diffuse on window without
> overhang
> * (1-Fws)
>
> In both cases (a) and (b) I suppose that the sky diffuse radiation
> input
> is always the input number 4 of type34. The label in the proforma is
> somewhat error prone (it says ?sky diffuse on the horizontal?).
>
> In case (b), locations behind the window were already excluded (by
> type16), so the first term is 1. I do not understand the second
> term,
> because the overhang is obstructing part of the sky over the window,
> so
> the involved radiation should be that coming from the vault of
> heaven,
>
> (b*)  Sky diffuse on window = 1*Shaded sky diffuse on window
> without
> overhang - Sky diffuse on horizontal*Fws
>
> In fact, suppose that type68 is used without obstacles defined (all
> obstruction height angles equal to zero). In this case
> shaded_view/full_view = 1, so
>
>   Shaded sky diffuse on window without overhang=0.5* Sky diffuse
> on  
> horizontal 
>
> The results of expressions (a) and (b) are inconsistent:
>
> (a) Sky diffuse on window = 0.5* Sky diffuse on horizontal ­ Fws*
> Sky
> diffuse on horizontal
>
> (b) Sky diffuse on window = 0.5* Sky diffuse on horizontal ­
> 0.5*Fws*
> Sky diffuse on horizontal
>
> A different and more difficult situation is when the overhang or a
> wingwall (or part of it) sees the external obstacle (that is, there
> are
> two obstacles in series). In this case, part of the shaded radiation
> is
> obstructed. So as to solve this problem, I would try to compose the
> geometry of the problem by combining several masks.
>
> Other questions are:
>
> Q1) The geometrical dimensions of the window can be comparable to
> those
> of the obstacle in some cases. In this situation strict view factor
> algebra should be used, but if we use type68 it is not clear where
> to
> place the measurement reference for the obstruction angles, because
> these angles are different for different points on the window. The
> best
> place seems to be the centre of the window.
>
> Q2) When type68 is used in conjunction with type1 (flat solar
> collector), I think that the description given in the proforma is
> error
> prone, because the total radiation given by type68 does not include
> the
> ground reflected radiation, so it should not be directly connected
> with
> the total radiation input of the solar collector type. This term can
> be
> difficult to calculate when the obstacle is close to the collector
> (in
> this case the radiation reflected by the obstacle can be
> significant),
> but shall be included anyway.
>
> I hope this helps.
>
> Fernando Domínguez Muñoz
> University of Málaga (Spain)
> _______________________________________________
> TRNSYS-users mailing list
> TRNSYS-users@engr.wisc.edu
> https://www.cae.wisc.edu/mailman/listinfo/trnsys-users

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